JPH09179152A - Optical fiber amplifier and dispersion compensation fiber module for optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the conversion efficiency of an optical fiber amplifier by efficiently utilizing residual exciting light power generated at the time of increasing an average excitation rate while assuring the stable operation of an exciting light source. SOLUTION: The exciting light is made incident on the optical fiber amplifier provided with a rare earth doped fiber 1 from one end of this rare earth doped fiber 1 by a first optical coupler 3-1. The residual exciting light arriving at the other end of the rare earth doped fiber 1 is separated by a second optical coupler 3-2 as a result of the incidence of the exciting light from the one end of the rare earth doped fiber 1. In addition, the residual exciting light is reflected by a reflecting means 4 and is returned into the rare earth doped fiber 1. The unstable operation that the exciting light source 2 for the exciting light incident on the rare earth doped fiber 1 makes by the residual exciting light returned into the rare earth doped fiber 1 is prevented by using an optical isolating means 5.

Description

Detailed Description of the Invention

(Technical Field of the Invention) [Technical Field of the Invention] [Prior Art] [Problems to be Solved by the Invention] Means for Solving the Problems (FIGS. 1 to 15) Embodiments of the Invention • Description of the First Embodiment (FIG. 16) , FIG. 53, FIG. 54) ・ Explanation of the first modification of the first embodiment (FIG. 17) ・ Explanation of the second modification of the first embodiment (FIG. 18) ・ Measurement of the third modification of the first embodiment Description (FIG. 19) -Explanation of the fourth modification of the first embodiment (FIGS. 20-22) -Explanation of the second embodiment (FIG. 23) -Explanation of the modification of the second embodiment (FIG. 24)- Description of Third Embodiment (FIG. 25) Description of Fourth Embodiment (FIG. 26) Description of Fifth Embodiment (FIGS. 27 to 29) Description of First Modification of Fifth Embodiment (FIG. 30) ) -Explanation of the second modification of the fifth embodiment (Fig. 31) -Explanation of the sixth embodiment (Fig. 32) -Modification of the sixth embodiment Description of Example (FIG. 33) • Description of Seventh Embodiment (FIG. 34) • Description of Eighth Embodiment (FIG. 35) • Description of First Modification of Eighth Embodiment (FIG. 36) • Eighth Embodiment Of the second modified example (FIG. 37) of the ninth embodiment (FIG. 38) of the tenth embodiment (FIG. 39) of the eleventh embodiment (FIG. 40) of the twelfth embodiment Description (FIG. 41) Description of thirteenth embodiment (FIG. 42) Description of fourteenth embodiment (FIG. 43) Description of first modified example of fourteenth embodiment (FIG. 44) 2 Description of Modification (FIG. 45) Description of wavelength characteristics of optical fiber amplifier (FIGS. 46 and 4)
7) Description of fifteenth embodiment (FIG. 48) Description of modification of fifteenth embodiment (FIG. 49) Description of sixteenth embodiment (FIG. 50) Description of first modification of sixteenth embodiment (FIG. 51) Description of second modified example of 16th embodiment (FIG. 52) Description of 17th embodiment (FIG. 55) Description of first modified example of 17th embodiment (FIG. 56) 17th Description of Second Modification of Embodiment (FIG. 57) Description of Third Modification of Seventeenth Embodiment (FIG. 58) Effect of Invention

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier and a dispersion compensating fiber module used in the optical fiber amplifier.

[0003]

2. Description of the Related Art In recent years, research and development of optical communication systems have been vigorously pursued, but rare earth-doped fibers such as erbium (Er) -doped fiber (hereinafter, erbium-doped fiber may be referred to as "EDF") have been used. The importance of booster amplifiers, repeaters, or preamplifiers using the optical amplification technology used has been clarified.

With the advent of optical amplifiers, a transmission system that multiplies and amplifies optical amplifiers has been attracting attention because it plays a very important role in making a communication system economical in a multimedia society.

[0005]

In the conventional rare-earth-doped fiber optical amplifier for amplifying one signal wavelength, the length of the doped fiber is increased in order to increase the conversion efficiency from pumping light power to signal light power. It is set to a value that maximizes the gain. Further, in an optical amplifier for wavelength division multiplexing (WDM) that performs multi-wavelength batch amplification, it is important to keep the wavelength dependence of gain as flat as possible. As a result, it is necessary to operate the rare earth-doped fiber (discussed below with a typical EDF) with shallow gain saturation. To do this, the ion density of the upper level is set to N2.
When the total ion density is represented by N1 and N2 / N1 is defined as the pumping rate, it is necessary to shorten the length of the doped fiber in order to increase the average pumping rate N2 / N1 of the doped fiber length. is there.

However, if the length of the doped fiber is shortened in this way, a large amount of residual pumping power leaks out from the other end of the doped fiber and the conversion efficiency deteriorates. Nevertheless, the number of signal wavelengths increases. At the same time, the required pumping power also increases, so it is necessary to increase the output power of the pumping semiconductor laser. That is, although it is clear from the energy conservation law that the pump power increases as the number of wavelengths increases, the wavelength-multiplexing optical amplifier cannot be used in a state in which the pump light power is efficiently converted into the signal light power. . This is because in order to make the gain wide or flat, the rare-earth-doped fiber is intentionally shortened to prevent saturation, and the pumping light power that has not been converted to a signal leaks out from the other end. Because it will be.

Therefore, as compared with the conventional amplification of one signal wavelength, it is necessary to use it in a leaking state even when a high pumping light power is required. Therefore, in order to effectively use the leaked residual pumping light, a reflecting mirror is provided at the other end of the doped fiber, and the residual pumping light is reflected by the reflecting mirror to be returned to the inside of the doped fiber and used again for optical amplification. A technique for doing so is disclosed in Japanese Patent Laid-Open No. 3-2598.
No. 5 and Japanese Patent Laid-Open No. 3-166782.

However, when the residual pumping light is reflected by the reflecting mirror in this way, the pumping light is returned not only to the doped fiber but also to the pumping light source, and this pumping light causes unstable operation of the pumping light source such as interference. May cause it.
By the way, as mentioned above, with the advent of optical amplifiers, a transmission system that multiplies and amplifies optical amplifiers has been attracting attention because it plays a very important role in making a communication system economical in a multimedia society. Problems in such a transmission system include dispersion compensation, reduction of non-linear effects (those that adversely affect transmission quality) in an optical fiber that is a transmission path, and economical wide-band WDM transmission.

Generally, an optical fiber which is a transmission line has a dispersion characteristic, and a dispersion amount is accumulated in proportion to the length of the fiber. However, conventionally, there is no optical amplification repeater and the regenerative repeater is used. Therefore, the amount of dispersion was reset each time I relayed, and it was not a problem. However, since the optical amplification relay is a relay by a kind of analog amplification, the amount of dispersion is accumulated. Therefore, the signal wavelength may be set to the zero dispersion wavelength for transmission. However, this has the following problems.

(1-1) A large amount of optical fibers have already been laid, and the zero dispersion wavelength of the optical fibers is unfortunately 1.3 μm. Only signals in the .55 μm band can be amplified. (1-2) In a recent report, even if a new optical fiber with a zero dispersion wavelength of 1.55 μm is installed,
It has been revealed that the nonlinear effect is actively generated in the optical fiber even when the signal of m is transmitted. This means that if the signal wavelength is made to coincide with the zero-dispersion wavelength for transmission, the undesirable non-linear effect becomes noticeable.

(1-3) In particular, in wavelength division multiplexing, the concept of matching the zero dispersion wavelength cannot be applied because there are a plurality of different signal light wavelengths. Therefore, recently, it has been proposed to intentionally shift the signal wavelength appropriately from the zero-dispersion wavelength, for example, to compensate the dispersion for each relay. As described above, research on the dispersion compensator has been actively conducted in recent years, but the one that is closest to practical use is a dispersion compensation fiber (hereinafter, the dispersion compensation fiber is referred to as “DCF”. , Dispersion Compens
Abbreviation for ation Fiber. ), But in this case, there are the following problems.

(2-1) When using an existing fiber (transmission line), it is necessary to interpose a dispersion compensating fiber as a device in order to perform dispersion compensation intensively at already existing relay points. For this reason, research and development has been conducted to shorten the length of the dispersion compensating fiber. (2-2) When newly laying a fiber, it is possible to lay the dispersion compensating fiber as a part of the transmission line instead of interposing the dispersion compensating fiber as a device. For example, it is conceivable to construct a 40-km transmission line with a 20-km fiber and a 20-km dispersion-compensating fiber. Research and development of such a novel dispersion-compensating fiber is based on the dispersion used in the above (2-1). Combined with the research and development of compensating fiber, it will be a double development.

That is, to summarize the above, in wavelength division multiplexing transmission, it is necessary to compensate for chromatic dispersion, and it is the closest to practical use to use dispersion compensation fiber. Therefore, one method is to use dispersion compensation fiber. It can be said that the dispersion compensating fiber is put into an optical amplifier repeater as one component, and the mode field diameter of the dispersion compensating fiber (DCF) is generally The small non-linear effect is likely to occur in order to compensate for the above, and the loss increases as the amount of dispersion to be compensated increases.

Therefore, a method of compensating for the loss of the dispersion compensating fiber by an optical amplifier can be considered. However, self-phase modulation (SPM: Self-Phase M) generated in the dispersion compensating fiber is considered.
odulation) and cross phase modulation (XPM: Cross-Phase Mo
It is necessary to compensate the loss so as not to be affected by the non-linear effect that deteriorates the signal quality such as dulation), and there is a problem that it is difficult to design the level diagram. further,
An optical amplifier for WDM requires a flat and wide optical amplification band, but it is difficult to realize a flat and wide amplification band because a rare earth-doped fiber optical amplifier also has wavelength dependence of gain. There are also challenges.

Further, in a rare earth-doped fiber optical amplifier having a high gain, unnecessary oscillation may occur during optical amplification. When such unnecessary oscillation occurs, the rare earth-doped fiber optical amplifier fails. It will operate stably. For example, in an erbium-doped fiber optical amplifier, spontaneous emission light (ASE) of 1.53 to 1.57 μm is generated during optical amplification, and this ASE is reflected at a reflection point in the erbium-doped fiber optical amplifier. Since it repeats, unnecessary oscillation may occur. In particular, in an erbium-doped fiber optical amplifier adjusted for multi-wavelength batch amplification (that is, an erbium-doped fiber optical amplifier with a high pumping rate), the gain is high near 1.53 μm.
Unwanted oscillation is likely to occur at this wavelength, and when such unwanted oscillation occurs, the erbium-doped fiber optical amplifier operates in an unstable manner.

The present invention was devised in view of the above problems, and aims to efficiently use the residual pumping light power generated when increasing the average pumping rate while ensuring stable operation of the pumping light source. It is an object of the present invention to provide an optical fiber amplifier which can improve the conversion efficiency. Further, according to the present invention, when a dispersion compensating fiber is used, the fact that the mode field diameter of this dispersion compensating fiber is small and therefore the threshold of Raman amplification is lowered, An object of the present invention is to provide an optical fiber amplifier and a dispersion compensating fiber module for the optical fiber amplifier, which can perform loss compensation.

Further, according to the present invention, by using a silica optical fiber having a Raman amplification function like the dispersion compensating fiber, the loss compensation of the silica optical fiber by Raman amplification is performed as in the case of using the dispersion compensating fiber. An object of the present invention is to provide an optical fiber amplifier which can be performed. Another object of the present invention is to provide an optical fiber amplifier that suppresses unstable operation of a rare earth-doped fiber optical amplifier having a high gain and a rare earth-doped fiber optical amplifier adjusted for multi-wavelength batch amplification. And

[0018]

[Means for Solving the Problems]

(1) Description of the First Aspect of the Present Invention FIG. 1 is a principle block diagram showing the first aspect of the present invention. In FIG. 1, 1 is a rare earth-doped fiber, and 2
Is an excitation light source. Reference numeral 3-1 is a first optical coupler which makes pumping light from the pumping light source 2 incident from one end of the rare earth-doped fiber 1, and 3-2 is pumping light from one end of the rare earth-doped fiber 1 through the first optical coupler 3-1. As a result,
It is a second optical coupler that separates the residual pumping light that has reached the other end of the rare earth-doped fiber 11.

Reference numeral 4 is a reflecting mirror that reflects the residual pumping light separated by the second optical coupler 3-2 and returns it to the rare earth-doped fiber 1 through the second optical coupler 3-2 again. Reference numeral 5 is provided between the pumping light source 2 and the first optical coupler 3-1 in order to prevent the pumping light source 2 from performing unstable operation due to interference by the residual pumping light returned into the rare earth-doped fiber 1. Optical isolator (the above are the constituent features of claim 3).

That is, in the optical fiber amplifier having the rare earth-doped fiber 1 shown in FIG. 1, first means for injecting pumping light from one end of the rare earth-doped fiber 1 by the first optical coupler 3-1 and the first means. As a result of injecting the pumping light from one end of the rare earth-doped fiber 1 by one means, the residual pumping light reaching the other end of the rare earth-doped fiber 1 is
A second means for separating the residual pumping light by the optical coupler 3-2 and reflecting it by the reflecting means (reflecting mirror) 4 back to the rare earth-doped fiber 1; and the second means to the rare earth-doped fiber 1 The use of the optical isolation means (optical isolator) 5 prevents the pumping light source 2 for the pumping light that is incident on the rare earth-doped fiber 1 by the first means from being unstable due to the returned residual pumping light. Prevent third
It is configured to have means (claim 1).

A Faraday rotation reflecting mirror can be used as the reflecting means (reflecting mirror) 4.
1). Further, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator (claim 1).
2).

(2) Description of Second Aspect of the Present Invention FIG. 2 is a principle block diagram showing a second aspect of the present invention. In FIG. 2, 11-1 and 11-2 are optical fiber amplifiers. , A first rare-earth-doped fiber and a second rare-earth-doped fiber that are arranged in two stages before and after.

Reference numeral 12 is an excitation light source, 13-1 is a first rare earth-doped fiber 11-1 and a second rare earth-doped fiber 1
The first optical coupler is provided at one end of one rare earth-doped fiber 11-1 of 1-2. 13-2 is a second optical coupler provided at the other end of the one rare earth-doped fiber 11-1, and 13-3 is one of the first rare earth-doped fiber 11-1 and the second rare earth-doped fiber 11-2. It is a third optical coupler provided at one end of the other rare earth-doped fiber 11-2.

Reference numeral 14 is a reflecting mirror that reflects the residual pumping light separated by the second optical coupler 13-2 and returns it to one rare earth-doped fiber 11-1 through the second optical coupler 13-2 again. Reference numeral 15 is an optical circulator having three or more ports connected to the pumping light source 12, the first optical coupler 13-1, and the third optical coupler 13-3.

Then, in this case, the pumping light from the pumping light source 12 is passed through the optical circulator 15 to the first optical coupler 1
After being incident from one end of one rare earth-doped fiber 11-1 from 3-1 and then one rare earth-doped fiber 11-1.
−1 to the second pump coupler 12-
2 and the residual pumping light is reflected by the reflecting mirror 14 and returned to the rare earth-doped fiber 11-1 and the optical path is changed by the optical circulator 15 to change the third optical coupler 13-.
3 is configured to combine the residual pumping light with the other rare-earth-doped fiber 11-2 (the above are the requirements of claim 5).

That is, in the optical fiber amplifier having the first rare earth-doped fiber 11-1 and the second rare earth-doped fiber 11-2 arranged in two stages before and after shown in FIG. The first optical coupler 13-1 allows the light to enter from one end of one rare earth-doped fiber 11-1 through the optical circulator 15 that the first optical coupler 13-1 has.
Means and this first means, as a result of pumping light entering from one end of one rare earth-doped fiber 11-1, residual pumping light reaching the other end of one rare earth-doped fiber 11-1 is supplied to the second optical coupler 13 -2, and the residual pumping light is reflected by the reflecting means 14 so that one of the rare earth-doped fibers 1
The second means for returning to inside 1-1, and the residual pumping light reflected from the reflecting means 14 by this second means is returned to inside one rare earth-doped fiber 11-1 and then the optical circulator 15
In the third optical coupler 13-3, the third optical coupler 13-3 is provided with a third means for multiplexing the residual pumping light on the other rare earth-doped fiber 11-2 (claim 4).

The input port to which the input signal light is input, the output side of the second optical coupler 13-2 and the third optical coupler 13
-3 can be added to the input side and to the output port from which the output signal light is output (in addition, in the front and rear two-stage configuration, the isolator must be added to the front stage and the rear stage). Is very effective) (Claim 6).

Also in this case, a Faraday rotation reflecting mirror can be used as the reflecting mirror 14 (claim 1).
1). Further, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator (claim 12).

(3) Description of Third Aspect of the Present Invention FIG. 3 is a principle block diagram showing a third aspect of the present invention. In FIG. 3, 21-1 and 21-2 are optical fiber amplifiers. , A first rare-earth-doped fiber and a second rare-earth-doped fiber that are arranged in two stages before and after.
Reference numeral 22 is an excitation light source, and 23 is an optical branching unit that branches the excitation light power from the excitation light source 22 into n: 1 (n is a real number of 1 or more).

The reference numeral 24-1 multiplexes the pumping light from one port of the optical branching section 23 into the first rare earth-doped fiber 21-.
It is a first optical coupler that is incident on one rare earth-doped fiber 21-1 of the first and second rare earth-doped fibers 21-2. 24-2 is a second optical coupler for extracting the residual pumping power extracted from the rare earth-doped fiber 21-1.

24-3 multiplexes the residual pumping powers extracted by the second optical coupler 24-2 to form a first rare earth-doped fiber 21-1 and a second rare earth-doped fiber 21-.
It is a third optical coupler which is incident on the other rare-earth-doped fiber 21-2 of 2. Reference numeral 24-4 is a fourth optical coupler that multiplexes the pumping power from the other port of the optical branching unit 23 branched by the optical branching unit 23 and enters the other rare earth-doped fiber 21-2 (the above is (Requirement of claim 8).

That is, in the optical fiber amplifier having the first rare earth-doped fiber 21-1 and the second rare earth-doped fiber 21-2 arranged in two stages before and after shown in FIG. 3, the pumping light power is optically branched. N: 1 in section 23
(N is a real number equal to or greater than 1), the pumping light of one port of the optical branching section 23 is multiplexed by the first optical coupler 24-1, and is incident on one rare earth-doped fiber 21-1. Means and the first means, one of the rare earth-doped fibers 21
-1 after inputting the pumping light from one end, the second optical coupler 24-2 connected to the other end of the one rare-earth-doped fiber 21-1 extracts the residual pumping power and outputs the residual pumping power to the third optical coupler. 24-3, and the pumping power of the other port of the optical branching section 23 branched by the optical branching section 23 and the second light which is incident from one end of the other rare earth-doped fiber 21-2 and the fourth light. The coupler 24-4 is provided with a third means for multiplexing from the other end of the other rare earth-doped fiber 21-2 (claim 7).

An input port for inputting the input signal light, between the pumping light source 22 and the optical branching section 23, and between the connection sections of the signal ports of the second optical coupler 24-2 and the third optical coupler 24-3. Also, an isolator may be added to each of the output ports for outputting the output signal light (claim 9). Also in this case, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator (claim 1).
2).

(4) Description of Fourth Aspect of the Present Invention FIG. 4 is a principle block diagram showing a fourth aspect of the present invention. In FIG. 4, 31 is a rare earth-doped fiber,
Reference numeral 32 is an excitation light source, and 33 is an optical circulator having three or more ports in which one port is connected to the excitation light source 32. 34-1 denotes the optical circulator 3 from the excitation light source 32.
3 is a first optical coupler that multiplexes the excitation light that has passed through 3 and enters the rare earth-doped fiber 31.

The second optical coupler 34-2 separates the residual pumping light reaching the other end of the rare earth-doped fiber 31 as a result of the pumping light entering one end of the rare earth-doped fiber 31 through the first optical coupler 34-1. Is. Reference numeral 35 reflects the residual pumping light separated by the second optical coupler 34-2 and again passes through the second optical coupler 34-2 to transmit the rare earth-doped fiber 3
It is a reflecting mirror that returns to inside 1.

Numeral 36 indicates a residual pumping light detection for detecting the residual pumping light returned to the rare earth doped fiber 31 by the reflecting mirror 35 and inputted to the optical circulator 33 through one end of the rare earth doped fiber 31 and the first optical coupler 34-1. It is a vessel. Reference numeral 37 is a controller that controls the excitation light source 32 so that the residual excitation light detected by the residual excitation light detector 36 becomes constant (claim 10).

In this case as well, a Faraday rotation reflecting mirror can be used as the reflecting mirror 35 (claim 1).
1). Further, the input signal light is inputted through the optical circulator and the output signal light is outputted through the optical circulator (claim 12), and the input port and the output signal light to which the input signal light is inputted are outputted. Isolators may be added to the respective output ports (claim 13).

(5) Description of the Fifth Aspect of the Present Invention FIG. 5 is a block diagram showing the principle of the fifth aspect of the present invention. In FIG. 5, 51 and 52 are rare earth elements arranged in front and rear two stages. A doped fiber and a dispersion compensating fiber. 53-1 is a first pumping light source that generates pumping light in the first wavelength band for the rare earth-doped fiber 51, and 5-3
Reference numeral 4-1 is a first optical coupler that makes the pumping light from the first pumping light source 53-1 enter the rare earth-doped fiber 51.

53-2 is a second pumping light source for generating pumping light in the second wavelength band for the dispersion compensating fiber 52, and 54-2 is a pumping light from the second pumping light source 53-2 dispersed. It is a second optical coupler which is incident on the compensation fiber 52.
Then, the dispersion compensating fiber 52 is connected to the second pumping light source 53-2.
It is configured to be excited by the excitation light of the second wavelength band from to generate Raman amplification (claim 22).

That is, a rare earth-doped fiber optical amplification section composed of a rare earth-doped fiber 51 and a Raman optical amplification section composed of a dispersion compensating fiber 52 that causes Raman amplification by being pumped with desired pumping light are provided in front and rear two stages. They are connected in cascade over (claims 14 and 1).
6).

In this case, the wavelength band of the pumping light generated by the first pumping light source 53-1 is 0.98 μm, and the wavelength band of the pumping light generated by the second pumping light source 53-2 is 1.47 μm.
(1.45 to 1.49 μm: Hereinafter, when referring to a 1.47 μm band, 1.45 to 1.49 μm unless otherwise specified.
It is preferable that it is m (claim 23). The Raman light amplifying unit may be arranged as the front-stage amplifying unit and the rare earth-doped fiber light amplifying unit may be arranged as the latter-stage amplifying unit (claim 17). In the case of being configured as an optical amplification section having a noise figure, a rare earth-doped fiber optical amplification section is provided as a pre-stage amplification section,
The Raman light amplifying section may be arranged as a post-stage amplifying section (claim 18).

The second pumping light source 53-2 may be composed of two pumping light sources and a polarization combiner for combining the polarized lights of the pumping lights from these pumping light sources (claim 19). The second pumping light source 53-2 may be configured to depolarize the pumping light by combining the pumping light source and the depolarizer (claim 20), and the second pumping light source 53-2 is modulated. It can also be configured to generate excitation light that has been subjected to the above (claim 21).

(6) Description of the Sixth Aspect of the Present Invention FIG. 6 is a principle block diagram showing the sixth aspect of the present invention. In FIG. 6, 61 and 62 are arranged in two stages, front and rear. Erbium-doped fiber and dispersion compensating fiber. Reference numeral 63 is a pumping light source that generates pumping light in the 1.47 μm band, and 64 is an optical coupler that causes the pumping light from the pumping light source 63 to enter the erbium-doped fiber 61.

In this case, the dispersion compensating fiber 6
2 is excited by residual pumping light from the erbium-doped fiber 61 to cause Raman amplification (claim 24). That is, a rare-earth-doped fiber optical amplification section including an erbium-doped fiber 61 that is a rare-earth-doped fiber, and a Raman optical amplification section that causes Raman amplification by being excited by desired excitation light that can excite the rare-earth-doped fiber optical amplification section. (This Raman optical amplifier is
(Comprising the dispersion compensating fiber 62) are arranged in cascade, and a pumping light source 63 that supplies pumping light for pumping the rare earth-doped fiber light amplifying section and the Raman light amplifying section is provided. Yes (Claims 14, 1
5).

The pumping light source 63 may be composed of two pumping light sources and a polarization combiner for combining the polarized lights of the pumping lights from these pumping light sources (Claim 1).
9), the pumping light source 63 may be configured to combine the pumping light source and the depolarizer so as to depolarize the pumping light (claim 20). Can also be configured to generate (claim 21).

(7) Description of the Seventh Aspect of the Present Invention FIG. 7 is a principle block diagram showing the seventh aspect of the present invention. In FIG. 7, 71 and 72 are erbium arranged in two stages in front and rear. A doped fiber and a dispersion compensating fiber.

Reference numeral 73 is a pumping light source for generating pumping light in the 1.47 μm band, and 74 is an optical coupler for making the pumping light from the pumping light source 73 enter the dispersion compensating fiber 72. In this case, the erbium-doped fiber 71 is configured to be excited by the residual excitation light from the dispersion compensating fiber 72 (claim 25). (8) Description of Eighth Aspect of the Present Invention FIG. 8 is a principle block diagram showing an eighth aspect of the present invention. In FIG. 8, 81 is a dispersion compensating fiber doped with a rare earth element, and 82 is , A pumping light source that generates pumping light for the rare earth element-doped dispersion compensating fiber 81, and 83 is an optical coupler that causes the pumping light from the pumping light source 82 to enter the rare earth element-doped dispersion compensating fiber 81. There is (claim 26).

(9) Description of Ninth Aspect of the Present Invention FIG. 9 is a principle block diagram showing a ninth aspect of the present invention. In FIG. 9, 91 and 92 are erbium arranged in two stages in front and rear. A doped fiber and a dispersion compensating fiber. 93 is a pumping light source for generating pumping light in the 1.47 μm band for the erbium-doped fiber 91, and 9
Reference numeral 4 is an optical coupler for making the excitation light from the excitation light source 93 incident on the erbium-doped fiber 91.

Reference numeral 95 denotes an optical filter which is interposed between the erbium-doped fiber 91 and the dispersion compensating fiber 92 to block the 1.47 μm band residual pump light emitted from the erbium-doped fiber 91. 27). (10) Description of the 10th Aspect of the Present Invention FIG. 10A is a principle block diagram showing the 10th aspect of the present invention. In FIG. 10A, 101 is a silica optical fiber (SOF). Yes, 102 is an erbium-doped fiber (EDF), and the optical fiber amplifier shown in FIG. 10A has a silica optical fiber 101 on the front side and an erbium-doped fiber 102 on the rear side.

103-1 is a silica optical fiber 101.
Is a silica-based optical fiber pumping light source that generates pumping light in a wavelength band for, and 104-1 is an optical coupler that inputs pumping light from the silica-based optical fiber pumping light source 103-1 to the silica-based optical fiber 101. Is. Reference numeral 103-2 is an erbium-doped fiber pumping light source that generates pumping light in a wavelength band for the erbium-doped fiber 102, and 104-2 is pumping light from the erbium-doped fiber pumping light source 103-2. It is an optical coupler which is incident on 102.

In this case, the silica-based optical fiber 101 is configured to be pumped by the pumping light in the wavelength band from the pumping light source 103-1 for silica-based optical fiber to cause Raman amplification (claim 31). . That is, in the optical fiber amplifier shown in FIG. 10 (a), a rare earth-doped fiber optical amplifying section composed of an erbium-doped fiber 102 which is a rare earth-doped fiber, and a silica-based optical system that causes Raman amplification by being pumped with desired pumping light. Optical fiber 101
And a Raman light amplifying section consisting of 2 are cascaded in two stages before and after (Claim 28), the Raman light amplifying section is provided as a pre-stage amplifying section, and the rare earth-doped fiber light amplifying section is provided as a post-stage amplifying section. It is provided (Claim 29).

When the rare earth-doped fiber optical amplifier is configured as an optical amplifier having a low noise figure, the rare earth-doped fiber optical amplifier is provided as the pre-stage amplifier and the Raman optical amplifier is It may be provided as a post-stage amplification section (claim 30). Further, a pumping light source for generating pumping light may be provided, and this pumping light source may also serve as the above-mentioned silica-based optical fiber pumping light source 103-1 and erbium-doped fiber pumping light source 103-2. 33).

(11) Description of Eleventh Aspect of the Present Invention FIG. 10 (b) is a principle block diagram showing the eleventh aspect of the present invention. In FIG. 10 (b), 111 has a low noise figure. An erbium-doped fiber (EDF) 112 is a silica-based optical fiber (SOF). The optical fiber amplifier shown in FIG. 10B has the erbium-doped fiber 111 on the front side and the silica-based optical fiber 11.
2 is provided on the rear side.

Reference numeral 113-2 is a silica-based optical fiber 112.
Is a pumping light source for silica-based optical fiber that generates pumping light in the wavelength band for, and 114-2 is an optical coupler that makes pumping light from the pumping light source 113-2 for silica-based optical fiber enter the silica-based optical fiber 112. Is. 113-1 is an erbium-doped fiber pumping light source that generates pumping light in the wavelength band for the erbium-doped fiber 111, and 114-1 is pumping light from the erbium-doped fiber pumping light source 113-1. It is an optical coupler which is incident on 111.

In this case, the silica-based optical fiber 112 is configured to be pumped with the pumping light in the wavelength band from the silica-based optical fiber pumping light source 113-2 to cause Raman amplification (claim 32). . An excitation light source that generates excitation light in the 1.47 μm band may be provided so that the excitation light source also serves as the silica-based optical fiber excitation light source 113-2 and the erbium-doped fiber excitation light source 113-1. Good (Claim 33).

(12) Description of the twelfth aspect of the present invention FIG. 11 is a principle block diagram showing the twelfth aspect of the present invention. In FIG. 11, 121-1 is the first erbium-doped layer having a low noise figure. Fiber (EDF),
122 is a silica optical fiber (SOF),
2 is a second erbium-doped fiber (EDF), and the optical fiber amplifier shown in FIG. 11 has a first erbium-doped fiber 121-1 in the front stage, a silica-based optical fiber 122 in the middle stage, and a second erbium-doped fiber 12 in the middle stage.
1-2 are provided at the latter stage, respectively.

Reference numeral 123-1 denotes the first pumping light in the wavelength band for the first erbium-doped fiber 121-1.
An excitation light source for an erbium-doped fiber,
1 is a pumping light source 123 for the first erbium-doped fiber
-1 from the excitation light from the first erbium-doped fiber 12
It is an optical coupler which is incident on 1-1. Reference numeral 123-2 is a silica-based optical fiber pumping light source that generates pumping light in a wavelength band for the silica-based optical fiber 122.
Is an optical coupler that causes the excitation light from the excitation light source 123-2 for silica-based optical fiber to enter the silica-based optical fiber 122.

The reference numeral 123-3 designates the second pumping light in the wavelength band for the second erbium-doped fiber 121-2.
An excitation light source for an erbium-doped fiber,
3 is a pumping light source 123 for the second erbium-doped fiber
-3 from the excitation light from the second erbium-doped fiber 12
It is an optical coupler which is incident on 1-2. In this case, the silica-based optical fiber 122 is replaced by the silica-based optical fiber pumping light source 1
It is configured to generate Raman amplification by exciting with pumping light in the wavelength band from 23-2 (claim 38).

That is, in the optical fiber amplifier shown in FIG. 11, a rare earth-doped fiber optical amplifier section having a low noise figure, which is composed of an erbium-doped fiber 121-1 which is a rare earth-doped fiber, is arranged as a pre-stage amplifier section, and a desired pumping is performed. A Raman light amplifying section composed of a silica-based optical fiber 122 that is excited by light to cause Raman amplification is arranged as a middle-stage amplifying section, and a rare earth-doped fiber optical amplification consisting of an erbium-doped fiber 121-2 which is a rare earth-doped fiber. The section is arranged as a post-stage amplification section (claims 34 and 36).

(13) Description of the thirteenth aspect of the present invention FIG. 12 is a principle block diagram showing the thirteenth aspect of the present invention. In FIG. 12, 131-1 is the first erbium-doped layer having a low noise figure. Fiber (EDF),
Reference numeral 132 is a dispersion compensating fiber (DCF), and 131-
2 is a second erbium-doped fiber (EDF),
The optical fiber amplifier shown in FIG. 12 includes the first erbium-doped fiber 131-1 in the front stage and the dispersion compensation fiber 13
2 in the middle, the second erbium-doped fiber 131-2
Are provided in the subsequent stages.

133-1 is a first for generating pumping light in the wavelength band for the first erbium-doped fiber 131-1.
An excitation light source for an erbium-doped fiber,
1 is a pumping light source 133 for the first erbium-doped fiber
Pumping light from -1 to the first erbium-doped fiber 13
It is an optical coupler which is incident on 1-1. 133-2 is a dispersion compensating fiber pumping light source that generates pumping light in the wavelength band for the dispersion compensating fiber 132, and 134-2 is pumping light from the dispersion compensating fiber pumping light source 133-2. It is an optical coupler that is incident on the optical fiber 132.

133-3 is a second source for generating pumping light in the wavelength band for the second erbium-doped fiber 131-2.
An excitation light source for an erbium-doped fiber,
3 is a pumping light source 133 for the second erbium-doped fiber
-3 from the excitation light from the second erbium-doped fiber 13
It is an optical coupler which is incident on 1-2. In this case, the dispersion compensating fiber 132 is set to the pump light source 133 for dispersion compensating fiber.
It is configured to be excited by pumping light in the wavelength band from -2 to cause Raman amplification (claim 37).

That is, in the optical fiber amplifier shown in FIG. 12, a rare earth-doped fiber optical amplifier section having a low noise figure, which is composed of an erbium-doped fiber 131-1 which is a rare earth-doped fiber, is arranged as a pre-stage amplifier section, and desired pumping is performed. A Raman light amplifying unit including a dispersion compensating fiber 132 that causes Raman amplification when excited by light is provided as a middle-stage amplifying unit, and a rare earth-doped fiber light amplifying unit including an erbium-doped fiber 131-2 that is a rare-earth-doped fiber. Is provided as a post-stage amplification section (claims 34 and 35).

(14) Description of Fourteenth Aspect of the Present Invention FIG. 13A is a block diagram showing the principle of the fourteenth aspect of the present invention. In FIG. 13A, 141 is a dispersion compensating fiber (DCF). ), 142 is a pumping light source that generates pumping light, 143 is an optical coupler that inputs the pumping light from the pumping light source 142 to the dispersion compensating fiber 141, and pumps the dispersion compensating fiber 141 from the pumping light source 142. It is configured to be excited by light to cause Raman amplification (claim 40).

As a result, this optical fiber amplifier includes the dispersion compensating fiber 141 and the dispersion compensating fiber 141.
And a pumping light source 142 for pumping to generate Raman amplification (claim 39). In this case as well, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator (claim 41), or the input port or the output signal light to which the input signal light is input. An isolator may be added to each of the output ports for outputting (claim 42).

(15) Description of Fifteenth Aspect of the Present Invention FIG. 13B is a principle block diagram showing the fifteenth aspect of the present invention. In FIG. 13B, 151 is a silica optical fiber ( SOF), 152 is a pumping light source that generates pumping light, 153 is an optical coupler that causes the pumping light from the pumping light source 152 to enter the silica-based optical fiber 151, and the silica-based optical fiber 151 is used as the pumping light source 152. It is configured to generate Raman amplification by being excited by the excitation light from (claim 43).

In this case as well, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator (claim 44). (16) Description of Sixteenth Aspect of the Present Invention FIG. 14 is a principle block diagram showing a sixteenth aspect of the present invention. In FIG. 14, reference numeral 154 is a rare earth-doped fiber optical amplifying section composed of a rare earth-doped fiber 61. Yes, reference numeral 155 is an optical fiber attenuating unit formed of an optical fiber or an optical fiber to which an optical isolator is added.

The optical fiber attenuator 155 suppresses the unstable operation of the rare earth-doped fiber optical amplifier 154 (claim 45). The optical fiber attenuator 155 may also serve as a Raman optical amplifier that causes Raman amplification by being excited with desired pump light (claim 47). In FIG. 14, 63 is an excitation light source, and 64 is an optical coupler for making the excitation light from the excitation light source 63 incident on the rare earth-doped fiber 61.

(17) Description of the 17th Aspect of the Present Invention FIG. 15 is a principle block diagram showing the 17th aspect of the present invention. In FIG. 15, 156-1 and 156-2 are
A pre-stage optical amplifying unit and a post-stage optical amplifying unit which are respectively configured as rare earth-doped fiber optical amplifying units including rare earth-doped fibers 121-1 and 121-2. Having the front-stage optical amplification section 156-1 and the rear-stage optical amplification section 156-2,
An optical amplification unit is configured.

Reference numeral 157 denotes an optical fiber attenuating section composed of an optical fiber or an optical fiber to which an optical isolator is added. The optical fiber attenuating section 157 includes the pre-stage optical amplifying section 156-1 and the post-stage optical amplifying section in the optical amplifying unit. 156-
It is provided between the optical amplification unit and the optical amplification unit and suppresses unstable operation of the optical amplification unit (claim 46). The optical fiber attenuator 157 may also serve as a Raman optical amplifier that causes Raman amplification by being excited with desired pump light (claim 47).

It should be noted that, in FIG.
3-3 is an excitation light source, 124-1 is an excitation light source 1
23-1 to the excitation light from the rare-earth doped fiber 121-
1 is an optical coupler which is incident on the optical fiber 1, and 124-3 is a rare earth-doped fiber 121 for the pumping light from the pumping light source 123-3
It is an optical coupler which is incident on -2.

[0072]

Embodiments of the present invention will be described below with reference to the drawings. (1) Description of First Embodiment FIG. 16 is a block diagram showing a first embodiment of the present invention. The optical fiber amplifier shown in FIG. 16 includes two erbium-doped fibers (ED) as rare-earth-doped fibers.
F) 11-1, 11-2, two pump light sources 12-1, 1
2-2, four optical demultiplexer-multiplexers (WDM) 13-1 to 13-4 as the first to fourth optical couplers, a reflecting mirror (reflecting means) 1
4, optical circulator 15, three isolators (IS
O) 16-1 to 16-3 and an optical filter 17.

That is, in this optical fiber amplifier, the isolator 16-1 and the optical demultiplexer-multiplexer 13-in order from the input side.
1, erbium-doped fiber 11-1, optical demultiplexer-multiplexer 13-2, optical filter 17, isolator 16-2, optical demultiplexer-multiplexer 13-3, erbium-doped fiber 11-
2, an optical demultiplexer-multiplexer 13-4, and an isolator 16-3 are arranged.

Further, the pumping light source 12-1 is connected to the optical demultiplexer-multiplexer 13-1 via the optical circulator 15, and the optical circulator 15 has a port other than the pumping light source 12-1. Is connected to the optical demultiplexer-multiplexer 13-3. Further, a reflecting mirror 14 is connected to the optical demultiplexer-multiplexer 13-2. The pumping light source 12-2 is connected to the optical demultiplexer-multiplexer 13-4.

Here, each erbium-doped fiber 11
Reference numerals -1, 11-2 are portions that function as an optical amplification unit, and are a pumping light source 12-1 including a lens and an LD chip.
Is an excitation light source that generates excitation light in the 0.98 μm band, and the excitation light source 12-2 including two lenses, an optical isolator (optical ISO), and an LD chip is, for example, 1.
47 μm band (When the 1.47 μm band is called,
In the embodiment, unless otherwise specified, 1.45 to 1.
It is an excitation light source that generates excitation light having a wavelength of 49 μm). The pumping light source 12-2 that generates pumping light in the 1.47 μm band has an optical isolator (optical ISO) built in because it is used for amplifying an optical signal in the 1.55 μm band. Generated at 1.55 μm
This is to prevent noise light in the band from returning to the excitation light source 12-2.

As described above, each pumping light source 12-1, 12
-2 pump light wavelength is selected, the optical demultiplexer-multiplexer 13-1
.About.13-3 are in the 0.98 .mu.m band, and the optical demultiplexer-multiplexer 13-4 is in the 1.47 .mu.m band.
Further, as the optical demultiplexer-multiplexers 13-1 to 13-4, the fusion type is used in the example of the figure, but of course, a bulk (dielectric multilayer film) type may be used. When a bulk type optical demultiplexer-multiplexer 13-4 is used, for example, the optical isolator (optical ISO) built in the pumping light source 12-2 can be omitted, and thus the pumping light source 12 can be omitted. As -2, the same type of pumping light source as the pumping light source 12-1 (however, 1.47 μm band) is used (the same applies to the following embodiments).

Further, as the reflecting mirror 14, a Faraday rotation reflecting mirror is used, and the reflecting pump 14 is used to reflect the residual pumping light separated by the optical demultiplexer-multiplexer 13-2 and split it again. It can be returned to one erbium-doped fiber 11-1 through the wave multiplexer 13-2. The optical circulator 15 is a 3-port type optical circulator, and is configured in the same manner as the 4-port type optical circulator shown in FIG. 53 in which no fiber is connected to the port 4.

As shown in FIG. 16, the optical circulator 1
A pump light source 12-1 is connected to the port 1 of 5
The optical demultiplexer-multiplexer 13-1 is connected to the port 2,
An optical demultiplexer-multiplexer 13-3 is connected to the port 3.
The optical circulator 15 may be configured as an optical circulator having 3 or more ports.

Further, the isolators 16-1 to 16-3
Is to pass light only in the direction of the arrow in the figure,
As shown in FIG. 54, the lens and the birefringent prisms A, B,
It consists of a polarization rotator (reciprocal) and a 45 ° Faraday rotator (non-reciprocal). This isolator 16-1 to 16-3
As shown in FIG. 54 (a), when an optical signal is input from the left fiber, this optical signal reaches the right fiber, but as shown in FIG. 54 (b), the optical signal is input from the right fiber. Is configured so as not to reach the fiber on the left side (hereinafter, in the embodiments, the isolator has the structure shown in FIG. 54 unless otherwise specified).

In the optical fiber amplifier according to the present embodiment, the isolators 16-1 and 16-2 are provided before and after the erbium-doped fiber 11-1, and the isolator 1 is provided before and after the erbium-doped fiber 11-2.
By providing 6-2 and 16-3, it is possible to prevent noise light from being generated in the erbium-doped fibers 11-1 and 11-2.

In an optical amplifier having a plurality of optical amplifiers, it is possible to prevent optical noise from being generated in the erbium-doped fiber 11-1 which is an amplifier located on the input side of the optical signal, thereby reducing optical amplification. It is particularly important for the purpose of performing the above-mentioned operation, so that the isolator 16-3 at the latter stage of the erbium-doped fiber 11-2, which is the amplifier located on the output side of the optical signal, is used.
Can be omitted (the same applies to the following embodiments).

The optical filter 17 is an ASE (Amplified Spontaneous) output of the erbium-doped fiber 11-1.
The mountainous part of Emission (eg 1.53)
5 μm portion (see FIG. 46) is cut (the peaks are crushed to be flat, or the shorter wavelength side than ˜1.538 μm is deleted), and the dielectric multilayer film is provided. The optical filter 17 may be omitted.

With such a configuration, the excitation light source 12-1
The pumping light from (1) enters the optical demultiplexer / multiplexer 13-1 through the optical circulator 15 together with the signal light from one end of the erbium-doped fiber 11-1. Thereby, optical amplification is performed by the erbium-doped fiber 11-1. At this time, this erbium-doped fiber 11
Since -1 is shortened to increase the average pumping rate, residual pumping light power leaks from the other end of the erbium-doped fiber 11-1.

In this way, the residual pumping light that has reached the other end of the erbium-doped fiber 11 and leaked out is separated by the optical demultiplexer-multiplexer 13-2 and then reflected by the reflecting mirror 14 and returned. Come on. After that, the reflected residual pumping light is returned to the erbium-doped fiber 11, and further, via the optical demultiplexer-multiplexer 13-1, the optical path is changed by the optical circulator 15, and the optical demultiplexer-multiplexer 13-3. Then, the residual pump light is multiplexed with the signal light amplified by the erbium-doped fiber 11-1 in the previous stage, and the erbium-doped fiber 11-2 is combined.
Incident on.

The latter erbium-doped fiber 11-2 receives the pumping light from the pumping light source 12-2 and performs optical amplification. That is, in this first embodiment, 3
In the two-stage erbium-doped fiber optical amplifier using the optical circulator 15 of the port, the optical demultiplexer-multiplexer 1 from the input side of the front-stage erbium-doped fiber 11-1
Excitation light (for example, 0.98 μm) is made incident through 3-1 and the pre-stage erbium-doped fiber 11-1
An optical demultiplexer-multiplexer 13-2 is installed on the output side of the optical fiber to output the signal light from the optical filter 17 and the optical isolator (ISO) 16-2.
It is adapted to be input to the latter stage erbium-doped fiber 11-2 through the.

On the other hand, the pumping light is separated from the signal light by the optical demultiplexer-multiplexer 13-2, reflected by the reflecting mirror 14, and reciprocates in the front-stage erbium-doped fiber 11-2. The pumping light separated by the optical demultiplexer-multiplexer 13-1 is input to the post-stage erbium-doped fiber 11-2 by the optical circulator 15 via the optical demultiplexer-multiplexer 13-3. .

In this figure, the pumping light source 12-2 is also installed on the output side of the latter-stage erbium-doped fiber 11-2, and the pumping light source 12 is considered in consideration of interference between the pumping light sources 12-1 and 12-2. -1 to 0.98 μm, excitation light source 12-
2 is set to 1.47 μm, and the optical demultiplexer-multiplexer 13-4 sets 0.
The residual excitation light of 98 μm is suppressed from being incident on the excitation light source 12-2.

That is, in the first embodiment shown in FIG. 16, in the optical fiber amplifier including the erbium-doped fibers 11-1 and 11-2 arranged in two stages, the pumping light is passed through the optical circulator 15. From the optical demultiplexer-multiplexer 13-1 to the erbium-doped fiber 11-1
Means for injecting the pumping light from one end of the erbium-doped fiber 11-1 by the first means, and the residual pumping light reaching the other end of the erbium-doped fiber 11-1 Second means for separating the demultiplexer-multiplexer 13-2, reflecting the residual pumping light by the reflecting means 14, and returning it into the erbium-doped fiber 11-1;
With this second means, the residual pumping light reflected from the reflecting means 14 is returned to the erbium-doped fiber 11-1 and then the optical path is changed by the optical circulator 15 to cause the optical demultiplexer-multiplexer 1 to operate.
In 3-3, the erbium-doped fiber 11-2 is provided with a third means for multiplexing the residual pumping light.

Thus, in the first embodiment,
The residual pumping light power generated when increasing the average pumping rate is reflected by the newly prepared optical demultiplexer-multiplexer 13-2 and the reflecting mirror 14 and reciprocates in the erbium-doped fiber 11-1 to generate pumping light. Efficient use of power can be achieved, which can improve conversion efficiency.

At this time, since the optical circulator 15 is used, a loop can be formed to prevent the pumping light power from becoming unstable. Furthermore, the reflector 1
Since the Faraday rotating mirror is used for 4, the polarization of the pumping light can be rotated, which allows PHB (Po
larization hole burning) can be reduced.

(1-1) Description of First Modification of First Embodiment FIG. 17 is a block diagram showing a first modification of the first embodiment of the present invention. The optical fiber amplifier shown in FIG. In order from the input side, the isolator 16-1, the optical demultiplexer-multiplexer 13
-1, Erbium-doped fiber 11-1, Optical demultiplexer-multiplexer 13-2, Isolator 16-2, Optical demultiplexer-multiplexer 13
-3, an erbium-doped fiber 11-2, and an isolator 16-3 are arranged.

Further, the pumping light source 12-1 is connected to the optical demultiplexer-multiplexer 13-1 via the optical circulator 15, and the optical circulator 15 has a port other than the pumping light source 12-1. Is connected to the optical demultiplexer-multiplexer 13-3. Further, a reflecting mirror 14 is connected to the optical demultiplexer-multiplexer 13-1. Note that, in FIG. 17, the same reference numerals as those in FIG. 16 denote the same parts.

As can be seen from this structure, this FIG.
In the optical fiber amplifier shown in FIG. 7, one pump light source 12-
1 in the front and rear erbium-doped fibers 11-1,
11-2 is configured to be excited together. Although the optical filter 17 is omitted, the optical filter 17 may be provided at the position shown in FIG. 16 also in this modification.

Also in this case, the excitation light is transmitted to the optical circulator 1
5 from the optical demultiplexer-multiplexer 13-1 into one end of the erbium-doped fiber 11-1 and the excitation light is incident on the other end of the erbium-doped fiber 11-1. Optical demultiplexer-multiplexer 1
3-2, the residual excitation light is reflected by the reflecting mirror 14 and returned to the erbium-doped fiber 11-1, and the residual excitation light reflected from the reflecting mirror 14 is further reflected in the erbium-doped fiber 11-1. After that, the optical path is changed by the optical circulator 15 and the residual pumping light is combined with the erbium-doped fiber 11-2 by the optical demultiplexer-multiplexer 13-3. Therefore, the first embodiment shown in FIG. In addition to the same operation or effect as in the case of the embodiment, the pumping light source 12-1 is common to both erbium-doped fibers 11-1 and 11-2, which can contribute to simplification of the configuration and cost reduction. .

(1-2) Description of Second Modification of First Embodiment FIG. 18 is a block diagram showing a second modification of the first embodiment of the present invention. The optical fiber amplifier shown in FIG. Optical demultiplexer-multiplexer 13-1, erbium-doped fiber 11-1, optical demultiplexer-multiplexer 13-2, optical filter 17, isolator 16-2, optical demultiplexer-multiplexer 13-3 in order from the input side. ，
An erbium-doped fiber 11-2 is provided,
Further, the input signal light is a 4-port type optical circulator 15-2.
And the output signal light is also output through the optical circulator 15-2. Further, the pumping light source 12-1 is connected to the optical demultiplexer-multiplexer 13-1 via the optical circulator 15, and the optical circulator 15 uses the pumping light source 12-1 and other ports to split the light. It is connected to the wave multiplexer 13-3. Further, a reflecting mirror 14 is connected to the optical demultiplexer-multiplexer 13-2.

In FIG. 18, the same reference numerals as those in FIGS. 16 and 17 denote the same parts. As can be seen from this configuration, the optical fiber amplifier shown in FIG.
An optical circulator 15-2 is used instead of the isolator in the input / output section. Here, the optical circulator 15-2, as shown in FIG.
It is a 4-port type optical circulator including a 5 ° Faraday rotator (non-reciprocal) and a 45 ° polarization rotator (reciprocal) and having ports 1 to 4.

The optical circulator 15-2 shown in FIG.
3 (a), the optical signal input from the port 1 is output to the port 2, and the optical signal input from the port 2 is output to the port 3 as illustrated in FIG. 53 (b). Is configured. Although not shown, the optical signal input from the port 3 of the optical circulator 15-2 is output to the port 4, and the optical signal input from the port 4 is output to the port 1 ( Hereinafter, in the embodiments, the optical circulator has the structure shown in FIG. 53 unless otherwise specified).

Optical circulator 15 as shown in FIG.
-2 is configured so that an optical signal is input from the port 1 and an optical signal is output from the port 4, the optical demultiplexer-multiplexer 13-1 is connected to the port 2, and the port 3 An erbium-doped fiber 11-2 is connected to the. The optical filter 17 may be omitted.

Also in this case, the excitation light is transmitted to the optical circulator 1.
5 from the optical demultiplexer-multiplexer 13-1 into one end of the erbium-doped fiber 11-1 and the excitation light is incident on the other end of the erbium-doped fiber 11-1. Optical demultiplexer-multiplexer 1
3-2, the residual excitation light is reflected by the reflecting mirror 14 and returned to the erbium-doped fiber 11-1, and the residual excitation light reflected from the reflecting mirror 14 is further reflected in the erbium-doped fiber 11-1. After that, the optical path is changed by the optical circulator 15 and the residual pumping light is combined with the erbium-doped fiber 11-2 by the optical demultiplexer-multiplexer 13-3. Therefore, the first embodiment shown in FIG. In addition to the same operation or effect as in the case of the embodiment, since the optical circulator 17 is provided in the input / output unit, there is an advantage that the number of isolators used can be reduced and the cost can be reduced.

(1-3) Description of Third Modification of First Embodiment FIG. 19 is a block diagram showing a third modification of the first embodiment of the present invention. The optical fiber amplifier shown in FIG. In order from the input side, an isolator 16-1, an optical demultiplexer-multiplexer (second optical coupler) 13-2 ', an erbium-doped fiber 1
1-1, optical demultiplexer-multiplexer (first optical coupler) 13-1 ', optical filter 17, isolator 16-2, optical demultiplexer-multiplexer 1
3-3, an erbium-doped fiber 11-2, an optical demultiplexer-multiplexer 13-4, and an isolator 16-3 are provided. The optical demultiplexer-multiplexer 13-1 'includes an optical circulator 1
The pumping light source 12-1 is connected via 5 and the optical circulator 15 has other ports connected to the optical demultiplexer-multiplexer 13-3 in addition to the pumping light source 12-1. A reflecting mirror 14 is connected to the optical demultiplexer-multiplexer 13-2 '. The optical demultiplexer-multiplexer 13-4 includes a pumping light source 12-
2 are connected.

In FIG. 19, the same symbols as those in FIGS. 16 to 18 indicate the same parts. As can be seen from this configuration, in the optical fiber amplifier shown in FIG.
The pump light is configured to enter from the output side of the erbium-doped fiber 11-1. Again,
The optical filter 17 can be omitted. In this example, the excitation light is made incident from the output terminal of the erbium-doped fiber 11-1 from the optical demultiplexer-multiplexer 13-1 'through the optical circulator 15, and as a result of making the excitation light incident, the erbium-doped fiber is obtained. The residual pumping light that has reached the input end of 11-1 is separated by the optical demultiplexer-multiplexer 13-2 ', and the residual pumping light is reflected by the reflecting mirror 14 and returned to the erbium-doped fiber 11-1. After returning the residual pumping light reflected from the reflecting mirror 14 into the erbium-doped fiber 11-1, the optical path is changed by the optical circulator 15 and the erbium-doped fiber 11-2 is changed by the optical demultiplexer-multiplexer 13-3.
Then, the residual excitation light is multiplexed.

As a result, the same operation or effect as in the case of the first embodiment shown in FIG. 16 can be obtained. (1-4) Description of Fourth Modified Example of First Embodiment FIG. 20 is a block diagram showing a fourth modified example of the first embodiment of the present invention. The optical fiber amplifier shown in FIG. In order, the isolator 16-1 and the optical demultiplexer-multiplexer 13
-2 ', erbium-doped fiber 11-1, optical demultiplexer-multiplexer 13-1', isolator 16-2, optical demultiplexer-multiplexer 13-3, erbium-doped fiber 11-2, optical demultiplexer-multiplexer 13-4, isolator 16-3, optical filter 1
7-2 and a coupler 13-5 are provided, and further, FIG.
Similarly to the example shown in (1), the pump light source 12-1 is connected to the optical demultiplexer-multiplexer 13-1 'via the optical circulator 15, and the optical circulator 15 includes the pump light source 12-1 as well as the pump light source 12-1. The other port is connected to the optical demultiplexer-multiplexer 13-3. A reflecting mirror 14 is connected to the optical demultiplexer-multiplexer 13-2 '. The pumping light source 12-2 is connected to the optical demultiplexer-multiplexer 13-4.

An output photodetector 18 for detecting the output light from the coupler 13-5 is provided. Further, based on the detection result of the output photodetector 18, the pump light source 12-
An optical output constant controller 19 for controlling the output of the optical fiber 2 is provided. That is, as shown in FIG. 21, the output photodetector 18 is configured to include a photodiode 18A, and the detection value Vin1 at this photodiode 18A.
Is a differential amplifier 19A that constitutes the constant optical output controller 19
To be entered. And the differential amplifier 1
9A supplies the difference G1 between the reference values Vref1 and Vin1 as a control signal Vcont1 to the laser diode forming the pumping light source 12-2.

Here, the output light power and the control signal Vcon
The relationship between t1 and the output of the laser diode is shown in FIG.
It looks like 2. Note that, in FIG. 20, FIGS.
The same reference numerals as in FIG. As can be seen from this configuration, the optical fiber amplifier shown in FIG. 20 is configured to realize the constant optical output control (ALC).

Also in this example, as in the case of FIG. 19, the excitation light is made incident from the output end of the erbium-doped fiber 11-1 from the optical demultiplexer-multiplexer 13-1 'through the optical circulator 15, and the excitation light is also emitted. As a result, the residual pumping light reaching the input end of the erbium-doped fiber 11-1 is separated by the optical demultiplexer-multiplexer 13-2 ′, and the residual pumping light is reflected by the reflecting mirror 14, After returning to the erbium-doped fiber 11-1 and further returning the residual pumping light reflected from the reflecting mirror 14 to the erbium-doped fiber 11-1, the optical circulator 15 changes the optical path and the optical demultiplexer-multiplexer 13- Erbium-doped fiber 11-2 with 3
In addition, the residual pumping light is multiplexed, but in addition, the optical output constant control is performed.

As a result, the same operation or effect as in the case of the first embodiment shown in FIG. 16 can be obtained, and
Since the optical output constant control is performed, the signal-to-noise ratio (S
It is possible to set an optical output level with which deterioration of NR) is small. (1-5) Others In the above example, as the erbium-doped fiber that uses the reflected residual excitation light, the one in the previous stage is shown, but of course, in the erbium-doped fiber in the subsequent stage, the reflected residual excitation light is used. Good.

(2) Description of Second Embodiment FIG. 23 is a block diagram showing a second embodiment of the present invention. The optical fiber amplifier shown in FIG.
Isolator 25-1, optical demultiplexer-multiplexer (first coupler) 2
4-1, Erbium-doped fiber (rare earth-doped fiber) 21-1, Optical demultiplexer-multiplexer (second coupler) 24-
2, optical filter 26, isolator 25-3, optical demultiplexer-multiplexer (third coupler) 24-3, erbium-doped fiber (rare earth-doped fiber) 21-2, optical demultiplexer-multiplexer (fourth coupler) 24 -4 and an isolator 25-4 are provided.

Between the optical demultiplexer-multiplexers 24-2 and 24-3, an optical signal line having the optical filter 26 and the isolator 25-3 and a pumping optical line are provided in parallel. In addition, the optical demultiplexer-multiplexer 24-1 includes an optical branching unit 2
3, the excitation light source 22 is connected via the isolator 25-2.

Here, the optical branching section 23 is for branching the pumping light power (whose wavelength is, for example, 0.98 μm) from the pumping light source 22 into n: 1 (n is a real number of 1 or more). The light branched by the unit 23 (for example, the smaller one) is supplied to the optical demultiplexer-multiplexer 24-1, and the optical branching unit 2
The light branched by 3 (for example, the one with the most light) is the optical demultiplexer-multiplexer 24.
-4 is supplied.

That is, the erbium-doped fibers 21-1, 21-21 arranged in two stages before and after shown in FIG.
In the optical fiber amplifier having 2, the pumping light power is branched by the optical branching unit 23 into n: 1 (n is a real number of 1 or more), and the pumping light of one port of the optical branching unit 23 is optically demultiplexed. The erbium-doped fiber 21-
1 means for making the light incident on the optical fiber 1 and an optical demultiplexer-multiplexer connected to the other end of the erbium-doped fiber 21-1 after the excitation light is made incident from one end of the erbium-doped fiber 21-1 by the first means. A second means for extracting the residual pumping power at 24-2, multiplexing the residual pumping power at the optical demultiplexer-multiplexer 24-3, and making it incident from one end of the erbium-doped fiber 21-2, and the optical branching section 23. The pump power of the other port of the optical branching unit 23 branched by the optical demultiplexer-multiplexer 24-3
And the third means for multiplexing from the other end of the erbium-doped fiber 21-2.

Also in this embodiment, the same names as those in the first embodiment have the same functions. With such a configuration, in the optical fiber amplifier according to the second embodiment, the pumping light power is branched by the optical branching unit 23 into n: 1 (n is a real number of 1 or more), and one port of the optical branching unit 23 The pumping light is combined with the signal light by the optical demultiplexer-multiplexer 24-1,
It is incident on the erbium-doped fiber 21-1 in the front stage.

Then, the erbium-doped fiber 21-
After inputting the pumping light from the input end of No. 1, the residual pumping power is extracted by the optical demultiplexer-multiplexer 24-2 connected to the output end of the erbium-doped fiber 21-1, The beams are multiplexed by the demultiplexer-multiplexer 24-3 and made incident from the input end of the erbium-doped fiber 21-2 at the subsequent stage. The signal light passes through the optical filter 26 and the isolator 25-3, and then passes through the erbium-doped fiber 2 in the subsequent stage.
It is input to 1-2, but at this time, the optical filter 26
Of the output of the erbium-doped fiber 21-1, the peak ASE portion (for example, a portion of 1.535 μm: see FIG. 46) is cut (the peak is flattened or a wavelength shorter than 1.538 μm). Will be deleted).

After that, the pumping power of the other port of the optical branching unit 23 branched by the optical branching unit 23 is converted into the optical demultiplexer-multiplexer 24.
At -4, the waves are combined from the output end of the erbium-doped fiber 21-2. In this way, in the second embodiment, the residual pumping light power generated when the average pumping rate is increased can be supplied to other erbium-doped fibers, and further, the pumping light source is made common and necessary. The distribution of the pumping light power to the erbium-doped fibers 21-1 and 21-2 in the front and rear stages can be set according to the above, so that the pumping light power can be used more efficiently. The conversion efficiency can be greatly improved.

Also in this case, the input signal light can be input through the optical circulator and the output signal light can be output through the optical circulator in the same manner as in FIG. (2-1) Description of Modification of Second Embodiment FIG. 24 is a block diagram showing a modification of the second embodiment of the present invention. The optical fiber amplifier shown in FIG. 24 has an isolator 25 in order from the input side. -1, Optical demultiplexer-multiplexer (second coupler) 24-2 ', Erbium-doped fiber 21-
1, optical demultiplexer-multiplexer (first coupler) 24-1 ', optical filter 26, isolator 25-3, optical demultiplexer-multiplexer (fourth coupler) 24-4', erbium-doped fiber 21-
2, an optical demultiplexer-multiplexer (third coupler) 24-3 ', and an isolator 25-4 are provided.

The optical demultiplexer-multiplexers 24-1 'and 24-
The optical filter 26 and the isolator 25-
An optical signal line having 3 and a pumping light line are provided in parallel. In addition, the optical demultiplexer-multiplexers 24-1 ', 24
A pumping light source 22 is connected to -4 'through an optical branching unit 23 and an isolator 25-2.

That is, also in the optical fiber amplifier shown in FIG. 24, the pumping light power is n: 1 (n
Is a real number greater than or equal to 1), and the pumping light from one port of the optical branching unit 23 is multiplexed by the optical demultiplexer-multiplexer 24-1 'and made incident on the erbium-doped fiber 21-1. When,
The first means causes the pumping light to enter from one end of the erbium-doped fiber 21-1, and then the optical demultiplexer-multiplexer 24-connected to the other end of the erbium-doped fiber 21-1.
A second means for extracting the residual pumping power at 2 ', multiplexing the residual pumping power at the optical demultiplexer-multiplexer 24-3', and making it incident from one end of the erbium-doped fiber 21-2, and the optical branching section 23. And a third means for multiplexing the pumping power of the other port of the optical branching unit 23 branched by the optical demultiplexer-multiplexer 24-3 'from the other end of the erbium-doped fiber 21-2. Will be there.

Also in this embodiment, the same names as those in the first embodiment have the same functions. With such a configuration, in the optical fiber amplifier shown in FIG. 24, the pumping light power is branched into n: 1 (n is a real number of 1 or more) in the optical branching unit 23, and the pumping light of one port of the optical branching unit 23 is pumped. Is combined with the signal light by the optical demultiplexer-multiplexer 24-1 ', and is incident from the output side of the erbium-doped fiber 21-1 at the preceding stage.

Thus, the erbium-doped fiber 21
After inputting the pumping light from the output end of -1, the residual pumping power is taken out by the optical demultiplexer-multiplexer 24-2 'connected to the input end of the erbium-doped fiber 21-1, and the residual pumping power is further extracted. Are multiplexed by an optical demultiplexer-multiplexer 24-3 'and made incident from the output end of the erbium-doped fiber 21-2 at the subsequent stage. The signal light is input to the erbium-doped fiber 21-2 at the subsequent stage via the optical filter 26 and the isolator 25-3.

After that, the pump power of the other port of the optical branching unit 23 branched by the optical branching unit 23 is converted into the optical demultiplexer-multiplexer 24.
At -4 ', the light is multiplexed from the input end of the erbium-doped fiber 21-2. Thus, also in this example, as in the second embodiment described above, the residual pumping light power generated when the average pumping rate is increased can be supplied to other erbium-doped fibers, and further pumping can be performed. Common light source,
In addition, since it is possible to set the distribution of the pumping light power to the erbium-doped fibers 21-1 and 21-2 in the former stage and the latter stage, it is possible to use the pumping light power more efficiently. Therefore, the conversion efficiency can be greatly improved.

Also in this example, the input signal light can be input through the optical circulator and the output signal light can be output through the optical circulator in the same manner as in FIG. (3) Description of Third Embodiment FIG. 25 is a block diagram showing a third embodiment of the present invention. The optical fiber amplifier shown in FIG.
An isolator 5-1, an optical demultiplexer-multiplexer 3-1, an erbium-doped fiber (rare earth doped fiber) 1, an optical demultiplexer-multiplexer 3-2, and an isolator 5-3 are arranged. The pumping light source 2 is connected to the optical demultiplexer-multiplexer 3-1 via an isolator 5-2. A reflecting mirror (reflecting means) 4 is connected to the optical demultiplexer-multiplexer 3-2.

That is, in the optical fiber amplifier shown in FIG. 25, the first means for injecting the pumping light from the input end of the erbium-doped fiber 1 by the optical demultiplexer-multiplexer 3-1 and the first means, As a result of the pumping light incident from the input end of the erbium-doped fiber 1, the residual pumping light reaching the output end of the erbium-doped fiber 1 is converted into the optical demultiplexer-multiplexer 3
-2, and the residual excitation light is separated by a reflection means (reflecting mirror)
The second light reflected by 4 and returned to the erbium-doped fiber 1
It will be configured with means.

A Faraday rotation reflecting mirror is used as the reflecting mirror 4. With such a configuration, the optical fiber amplifier shown in FIG.
1 from one end of the erbium-doped fiber 1 and, as a result of this incidence, the erbium-doped fiber 1
The residual pumping light that has reached the other end is separated by the optical demultiplexer-multiplexer 3-2, and the residual pumping light is reflected by the reflecting mirror 4 and returned into the erbium-doped fiber.

As a result, also in the third embodiment, the residual pumping light power generated when the average pumping rate is increased is reflected by the newly prepared optical demultiplexer-multiplexer 3-2 and the reflecting mirror 4, and the doped fiber is used. By reciprocating within 1, the pumping light power can be used efficiently, and thus the conversion efficiency can be improved. Furthermore, since the Faraday rotation reflecting mirror is used as the reflecting mirror 4, the polarization of the pumping light can be rotated, and thus PHB can be reduced.

Also in this example, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator in the same manner as in FIG. The excitation light may be made incident from the output end of the erbium-doped fiber 1.

(4) Description of Fourth Embodiment FIG. 26 is a block diagram showing a fourth embodiment of the present invention. The optical fiber amplifier shown in FIG.
Isolator 5-1, Optical demultiplexer-multiplexer 3-1, Erbium-doped fiber (rare earth doped fiber) 1-1, Optical demultiplexer-multiplexer 3-3, Isolator 5-3, Optical demultiplexer-multiplexer 3
-4, Erbium-doped fiber (rare earth doped fiber) 1-2, Optical demultiplexer-multiplexer 3-5, Isolator 5-4
Are arranged.

An optical signal line having an isolator 5-3 and a pumping optical line are provided in parallel between the optical demultiplexer-multiplexers 3-3 and 3-4. In this example also, the pumping light source 2 is connected to the optical demultiplexer-multiplexer 3-1 via the isolator 5-2. A reflecting mirror (Faraday rotation reflecting mirror) 4 is connected to the optical demultiplexer-multiplexer 3-5.

With such a configuration, in the optical fiber amplifier shown in FIG. 26, the pumping light is input from the input end of the erbium-doped fiber 1-1 by the optical demultiplexer-multiplexer 3-1. After inputting the pumping light from the input end of the fiber 1-1, the optical demultiplexer-multiplexer 3-connected to the output end of the erbium-doped fiber 1-1.
The residual pumping power is taken out at 3, the residual pumping power is further multiplexed at the optical demultiplexer-multiplexer 3-4, and is input from the input end of the erbium-doped fiber 1-2 at the subsequent stage. The signal light is input to the erbium-doped fiber 1-2 in the subsequent stage via the isolator 5-3.

After that, the residual pumping light that has reached the output end of the erbium-doped fiber 1-2 in the subsequent stage is separated by the optical demultiplexer-multiplexer 3-5, and the residual pumping light is reflected by the reflecting mirror 4. Return to the erbium-doped fibers 1-2 and 1-1. As a result, also in the fourth embodiment, the residual pumping light power generated when the average pumping rate is increased is reflected by the newly prepared optical demultiplexer-multiplexer 3-5 and the reflecting mirror 4 and the doped fiber 1 is used.
By reciprocating the inside of -2 and 1-1, the pumping light power can be efficiently used, and thereby the conversion efficiency can be improved.

Also in this case, since the Faraday rotation reflecting mirror is used as the reflecting mirror 4, the polarization of the pumping light can be rotated, and thus PHB can be reduced. Also in this example, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator in the same manner as in FIG.

In the above example, the erbium-doped fiber utilizing the reflected residual pumping light is shown in the former stage, but of course, the reflected residual pumping light may be utilized in the latter stage erbium-doped fiber. Good. (5) Description of Fifth Embodiment FIG. 27 is a block diagram showing a fifth embodiment of the present invention. The optical fiber amplifier shown in FIG. 27 also has a configuration similar to that of the third embodiment described above, in order from the input side. Isolator 39-1,
An optical demultiplexer-multiplexer 34-1, an erbium-doped fiber (rare earth-doped fiber) 31, an optical demultiplexer-multiplexer 34-2, and an isolator 39-2 are arranged. The pumping light source 32 is connected to the optical demultiplexer-multiplexer 34-1 via a 3-port type optical circulator 33. Further, the optical demultiplexer-multiplexer 34-2 includes a reflecting mirror (Faraday rotation reflecting mirror) 3
5 is connected.

Further, a residual excitation light detector 36 is connected to the optical circulator 33, and the residual excitation light detector 36 returns the light into the erbium-doped fiber 31 by the reflecting mirror 35, and the erbium-doped fiber 3
1, optical circulator 3 through optical demultiplexer-multiplexer 34-1
The residual excitation light input to 3 is detected.

Further, a controller 37 is provided for controlling the excitation light source 32 so that the residual excitation light detected by the residual excitation light detector 36 becomes constant. That is, as shown in FIG. 28, the residual excitation light detector 36 has the photodiode 36.
It is configured with A, and this photodiode 3
The detected value Vin2 at 6A is input to the differential amplifier 37A that constitutes the controller 37. And
The differential amplifier 37A supplies the difference G2 between the reference values Vref2 and Vin2 as a control signal Vcont2 to the laser diode forming the pumping light source 32.

Here, the output light power and the control signal Vcon
The relationship between t2 and the output of the laser diode is shown in FIG.
It looks like 9. As can be seen from this configuration, the optical fiber amplifier shown in FIG. 27 has a configuration that realizes control of the pumping light source 32 such that the residual pumping light detected by the residual pumping light detector 36 becomes constant. Has become.

As a result, in the fifth embodiment as well, the pumping light power can be used efficiently, the conversion efficiency can be improved, and the average pumping power can be obtained by monitoring the residual pumping light power. Keep the rate constant,
The wavelength dependence of the gain can be kept constant with respect to variations in the input power. (5-1) Description of First Modification of Fifth Embodiment FIG. 30 is a block diagram showing a first modification of the fifth embodiment of the present invention. The optical fiber amplifier shown in FIG. In the configuration of FIG. 27, the input signal light is input through the optical circulator 38 and the output signal light is output through the optical circulator 38. In addition to the effects and advantages obtained in the form, since the optical circulator 38 is provided in the input / output section, it is possible to reduce the number of isolators to be used and obtain an advantage that can contribute to cost reduction. .

In FIG. 30, the same reference numerals as those in FIG. 27 denote the same parts. (5-2) Description of Second Modification of Fifth Embodiment FIG. 31 is a block diagram showing a second modification of the fifth embodiment of the present invention. The optical fiber amplifier shown in FIG. In order, the isolator 39-1, the optical demultiplexer-multiplexer 34
-2 ', erbium-doped fiber 31-1, optical demultiplexer-multiplexer 34-1', optical filter 40, isolator 39-
3, an optical demultiplexer-multiplexer 34-1 ″, an erbium-doped fiber 31-2, an optical demultiplexer-multiplexer 34-2 ″, and an isolator 39-2 are provided.

Further, the optical demultiplexer-multiplexers 34-1 ', 34-
An excitation light source 32 is connected to 1 ″ through a 4-port optical circulator 33 ′. Reflecting mirrors (Faraday rotation reflecting mirrors) 35 'and 35 "are connected to the optical demultiplexer-multiplexers 34-2' and 34-2". Further, the optical circulator 33 'includes a residual excitation light detector 3
6 is connected to the erbium-doped fibers 31-2 and 31-2 by the residual excitation light detector 36 and returned to the erbium-doped fibers 31-2 and 31-2 by the reflecting mirrors 35 'and 35 ", respectively. 2, optical demultiplexer-multiplexer 34
Optical circulator 33 'through -1' and 34-1 ''
The residual excitation light input to is detected.

Further, a controller 37 is provided for controlling the excitation light source 32 so that the residual excitation light detected by the residual excitation light detector 36 becomes constant. Also in this embodiment, the same names as those in the fifth embodiment have the same functions. As can be seen from this configuration, this FIG.
In the optical fiber amplifier shown in FIG. 3, each erbium-doped fiber 31-1, 31 detected by the residual pumping light detector 36 is used.
Pumping light source 32 such that the residual pumping light from -2 becomes constant
It is configured to realize the control of.

Therefore, also in this example, substantially the same effects and advantages as the above-mentioned fifth embodiment can be obtained. Also in this example, the input signal light can be input through the optical circulator and the output signal light can be output through the optical circulator in the same manner as in FIG.

(6) Description of Sixth Embodiment FIG. 32 is a block diagram showing a sixth embodiment of the present invention. The optical fiber amplifier shown in FIG.
An isolator 144, a dispersion compensation fiber 141, and an optical demultiplexer-multiplexer 143 are arranged. Also, the optical demultiplexer-multiplexer 1
An excitation light source 142 is connected to 43.

Here, the pumping light source 142 has a band (for example, 1.44 to 1.49 μm) in which band compensation of erbium-doped fiber amplification by Raman amplification can be performed.
And a pumping light source for generating the pumping light of
The pumping light from (1) enters through the optical demultiplexer-multiplexer 143 from the output end of the dispersion compensating fiber 141.

Therefore, this optical fiber amplifier has a dispersion compensating fiber module having the dispersion compensating fiber 141 and the pumping light source 142.

With such a configuration, the dispersion compensating fiber 141 can be pumped by the pumping light from the pumping light source 142 to cause Raman amplification. That is, in the dispersion compensating fiber 141, since the mode field diameter is generally small, the threshold value of Raman amplification is lowered, which easily causes Raman amplification.

The dispersion compensating fiber has the following characteristics. That is, dispersion compensating fiber (DCF)
Has a small core diameter and a mode field diameter of about half of the normal diameter, and nonlinear effects (stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), four-photon mixing (FWM), self-phase modulation effect (SPM), etc.) It is more likely to occur than the fiber that is the transmission line. It is known that the dispersion compensating fiber is not as long as the fiber which is the transmission line, and can be used if the optical power when passing through the dispersion compensating fiber is made small from the usage aspect. This is because the influence of the nonlinear effect also increases as the length becomes longer.

It has also been found that the attenuation (loss) of light in the dispersion compensating fiber cannot be ignored.
Therefore, the optical amplifier needs to compensate for this loss. On the other hand, the input power is limited to a small value as described above, which makes it difficult to design the level as an optical amplifier. However, some of the above-mentioned nonlinear effects are harmful and useful in communication. Of these, Raman amplification is beneficial.

The point that this Raman amplification can be very useful is as follows. That is, if the dispersion compensating fiber is Raman-amplified, the dispersion compensating fiber itself becomes an optical amplifier and the loss can be compensated. Raman amplification means stimulated Raman scattering, that is, coherent Stokes light whose wavelength is shifted by a specific amount by interacting with the optical phonon of the optical fiber when irradiating the optical fiber with strong monochromatic light. By applying the phenomenon generated by stimulated emission, the wavelength of monochromatic light is set so that the Stokes light has the same wavelength as the signal light, and the signal light is amplified by stimulated emission.

Therefore, as described above, the dispersion compensating fiber 1
By pumping 41 with pumping light in the above band from the pumping light source 142 to cause Raman amplification, loss compensation of the dispersion compensating fiber due to this Raman amplification (flattening of gain cavities of erbium-doped fiber and erbium-doped fiber) is performed. (Including compensation for the decrease in gain of the doped fiber).

The erbium-doped fiber 1.5
To flatten the gain depression in the 4 μm band, ~ 1.44
Excitation at μm causes Raman amplification.

Further, as shown in FIG. 33, an isolator 144-2 can be added on the output side.

Further, instead of providing an isolator in the input section or the input / output section as shown in FIGS. 32 and 33, the input signal light is inputted through the optical circulator as shown in FIGS. 18 and 30. The output signal light may be output through this optical circulator. Further, instead of the dispersion compensating fiber 141, a silica optical fiber can be used.

(7) Description of Seventh Embodiment FIG. 34 is a block diagram showing a seventh embodiment of the present invention. The optical fiber amplifier shown in FIG.
The isolator 55-1, the optical demultiplexer-multiplexer 54-1, the erbium-doped fiber (rare earth-doped fiber) 51, the isolator 55-2, the dispersion compensation fiber 52, the optical demultiplexer-multiplexer 54-2, and the isolator 55-3 are included. It is arranged. In addition, the optical demultiplexer-multiplexer 54-1 includes a pump light source 53-
1 is connected to the optical demultiplexer-multiplexer 54-2,
The excitation light source 53-2 is connected.

Here, the pumping light source 53-1 is for generating pumping light in the first wavelength band (for example, 0.98 μm band) for the erbium-doped fiber 51, and the pumping light source 53-2 is for dispersion compensation. A second wavelength band for the fiber 52 (e.g. 1.47 [mu] m band (1.45 to 1.49).
μm) or band up to 1.44 μm (up to 1.44 μm
m)) to generate the excitation light.

As a result, the dispersion compensating fiber 52 can be excited by the pumping light from the pumping light source 53-2, and Raman amplification can be generated according to the same principle as the sixth embodiment. Therefore, also in this embodiment, the dispersion compensating fiber 52 is pumped by the pumping light in the 1.47 μm band or the band up to 1.44 μm from the pumping light source 53-2 to cause Raman amplification, whereby Raman amplification is performed. The loss of the dispersion compensating fiber can be compensated.

Further, the wavelength characteristic of the gain of the rare earth-doped fiber optical amplifier is determined by the rare earth ions, whereas the wavelength characteristic of the gain of the Raman optical amplifier is determined by the pumping wavelength, and its peak value changes when the pumping wavelength is changed. Because of the shift, the pumping wavelength for Raman amplification can be selected so as to compensate the gain wavelength characteristic of the rare earth-doped fiber optical amplifier, and thus, a wideband optical amplifier can be realized.

That is, even in the case of Raman amplification, there is an amplification bandwidth, and if the wavelength dependence of this gain is used, not only the loss compensation of the dispersion compensating fiber but also the amplification band of the erbium-doped fiber can be supplemented to achieve a wider band. is there. In other words, the wavelength characteristic of the erbium-doped fiber amplifier is not flat as shown in FIGS. 46 and 47, and therefore Raman amplification is performed using the dispersion compensating fiber.
The unevenness of the wavelength characteristic of the erbium-doped fiber amplifier can be flattened, and as a result, a wideband optical amplifier can be realized, which is suitable for multi-wavelength collective amplification (see FIG. 47). .

The rare earth-doped fiber optical amplifying section made of an erbium-doped fiber which is a rare earth-doped fiber may be configured as an optical amplifying section having a low noise figure. Further, in the optical fiber amplifier shown in FIG. 34, a rare earth-doped fiber optical amplification section made of an erbium-doped fiber is provided as a pre-stage amplification section, and a Raman optical amplification section made of a dispersion compensation fiber is provided as a post-stage amplification section. However, the present invention is not limited to this, and a Raman optical amplification unit made of a dispersion compensating fiber or a silica optical fiber is provided as a pre-stage amplification unit, and a rare earth-doped fiber optical amplification unit made of erbium-doped fiber is provided as a post-stage amplification unit. (If the Raman optical amplifier is made of silica optical fiber, one pumping light source
It can be used both as a pumping light source for silica optical fibers and as a pumping light source for erbium-doped fibers).

Further, the excitation light source 53-2 is, for example, as shown in FIG.
3-Excitation light sources 53-2, 53-2 ', 53 shown in FIG.
-2 ″, it may be composed of two pumping light sources and a polarization combiner that performs orthogonal polarization combining of the pumping light from these pumping light sources, and the pumping light source and the depolarizer are combined to perform pumping. It may be configured to depolarize light, or may be configured to generate modulated excitation light.

The excitation light source 53 shown in FIGS.
Regarding -2, 53-2 ', 53-2 ", the fourteenth embodiment of the present invention, the first modified example of the fourteenth embodiment of the present invention, and the second modified example of the fourteenth embodiment of the present invention, respectively. Will be explained.

(8) Description of Eighth Embodiment FIG. 35 is a block diagram showing an eighth embodiment of the present invention. The optical fiber amplifier shown in FIG.
An isolator 65-1, an optical demultiplexer-multiplexer 64, an erbium-doped fiber (rare earth-doped fiber) 61, an isolator 65-2, a dispersion compensation fiber 62, and an isolator 65-3 are arranged. Then, the optical demultiplexer-multiplexer 64
An excitation light source 63 is connected to the.

Here, the excitation light source 63 is, for example, 1.47.
The excitation light in the μm band (1.45 to 1.49 μm) is generated. With such a configuration, in the optical fiber amplifier shown in FIG. 35, pumping light is made incident on one end of the erbium-doped fiber 61 by the optical demultiplexer-multiplexer 64,
The erbium-doped fiber 61 is excited and amplified, but the residual pump light reaches from the other end of the erbium-doped fiber 61. After that, the residual pumping light is supplied to the dispersion compensating fiber 62 via the isolator 65-2,
Raman amplification occurs.

As described above, amplification can be performed in both fibers by using a common pumping light source for the erbium-doped fiber and the dispersion compensating fiber. That is, 1.
This is because the pumping wavelength band for Raman amplification of the 55 μm band signal light is the 1.47 μm band (1.45 to 1.49 μm) which is the pumping wavelength band of the erbium-doped fiber (EDF). Raman amplification can be performed by using the residual pumping light power when pumping with light in the 1.47 μm band. This makes it possible to compensate for the loss of the dispersion compensating fiber 62 while performing optical amplification with the erbium-doped fiber 61.

As a result, similar to the seventh embodiment described above, the unevenness of the wavelength characteristics of the erbium-doped fiber amplifier can be flattened, and a wideband optical amplifier can be realized.
In addition to being suitable for multi-wavelength batch amplification, since only one pump light source is required, it is possible to contribute to simplification of the structure and cost reduction. Also in this case, instead of providing an isolator in the input section or the input / output section,
As shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator.

Further, the pumping light source 63 may be composed of two pumping light sources and a polarization combiner for synthesizing orthogonal polarizations of the pumping lights from these pumping light sources, and combining the pumping light source and the depolarizer. The excitation light may be depolarized, or the modulated excitation light may be generated.

(8-1) Description of First Modification of Eighth Embodiment FIG. 36 is a block diagram showing a first modification of the eighth embodiment of the present invention. The optical fiber amplifier shown in FIG. In order from the input side, the isolator 65-1, the optical demultiplexer-multiplexer 64
-1, Erbium-doped fiber (rare earth doped fiber) 61-1, Isolator 65-2, Dispersion compensation fiber 62, Erbium doped fiber (rare earth doped fiber) 61-2, Optical demultiplexer-multiplexer 64-2, Isolator 65- 3 are provided. Then, the optical demultiplexer-multiplexer 64
The excitation light source 63-1 is connected to -1, and the excitation light source 63-2 is connected to the optical demultiplexer-multiplexer 64-2.

Here, the excitation light sources 63-1 and 63-2 are
Both are, for example, 1.47 μm band (1.45 to 1.49 μ
It produces the excitation light of m). With such a configuration, in the optical fiber amplifier shown in FIG. 36, the pumping light is incident on the erbium-doped fiber 61-1 from the input end of the erbium-doped fiber 61-1 by the optical demultiplexer-multiplexer 64-1. It is excited and amplified, but at this time, the residual pump light reaches from the other end of the erbium-doped fiber 61-1. Further, this residual pumping light is supplied to the dispersion compensating fiber 62 via the isolator 65-2 to cause Raman amplification.

Further, pumping light is made incident from the output end of the erbium-doped fiber 61-2 by the optical demultiplexer-multiplexer 64-2 to pump the erbium-doped fiber 61-2,
Although amplification is also performed, at this time, the residual pumping light still arrives from the input end of the erbium-doped fiber 61-2. Further, this residual pumping light is also supplied to the dispersion compensating fiber 62 to cause Raman amplification.

In this case, since the dispersion compensating fiber 62 is Raman-amplified by using the residual pumping light from the front and rear erbium-doped fibers 61-1 and 61-2, the compensation effect by the dispersion compensating fiber 62 should be increased. While simplifying the structure and reducing the cost,
A broadband optical amplifier can be realized. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. It can also be configured to be output.

Further, an excitation light source and an optical demultiplexer-multiplexer for the dispersion compensating fiber 62 can be provided. That is, FIG.
In the same manner as in 2, the excitation light source 133-of the 0.98 μm band
1-133-3 and optical demultiplexer-multiplexers 134-1 to 134-
3 may be used to form an optical fiber amplifier.

In place of the dispersion compensating fiber 62,
A silica-based optical fiber may be used.

(8-2) Description of Second Modification of Eighth Embodiment FIG. 37 is a block diagram showing a second modification of the eighth embodiment of the present invention. The optical fiber amplifier shown in FIG. In order from the input side, the isolator 65-1, the optical demultiplexer-multiplexer 64
-1, Erbium-doped fiber 61-1, isolator 65-2, dispersion compensating fiber 62, optical demultiplexer-multiplexer 64
-3, optical filter 66, isolator 65-3, optical demultiplexer-multiplexer 64-4, erbium-doped fiber 61-2,
An optical demultiplexer-multiplexer 64-5 and an isolator 65-4 are provided. Then, the pumping light source 63-1 is connected to the optical demultiplexer-multiplexer 64-1, and the optical demultiplexer-multiplexer 64-5 is connected.
The excitation light source 63-2 is connected to the.

Here, the pumping light sources 63-1 and 63-2 are
Both are, for example, 1.47 μm band (1.45 to 1.49 μ
It produces the excitation light of m). Further, between the optical demultiplexer-multiplexers 64-3 and 64-4, an optical signal line having an optical filter 66 and an isolator 65-3 and a pumping optical line are provided in parallel.

With such a configuration, in the optical fiber amplifier shown in FIG. 37, the pump light is converted into the optical demultiplexer-multiplexer 64-1.
Causes the erbium-doped fiber 61-1 to enter from the input end to excite and amplify the erbium-doped fiber 61-1. At this time, residual pump light reaches from the other end of the erbium-doped fiber 61-1. Further, this residual pumping light is supplied to the dispersion compensating fiber 62 via the isolator 65-2 to cause Raman amplification.

Further, the pumping light is input from the output end of the erbium-doped fiber 61-2 by the optical demultiplexer-multiplexer 64-5 to pump the erbium-doped fiber 61-2,
Although amplification is also performed, at this time, the residual pumping light still arrives from the input end of the erbium-doped fiber 61-2. Further, this residual pumping light is also supplied to the dispersion compensating fiber 62 via the optical demultiplexer-multiplexers 64-4 and 64-3 to cause Raman amplification.

Also in this case, since the dispersion compensating fiber 62 is Raman-amplified by using the residual pumping light from the front and rear erbium-doped fibers 61-1 and 61-2, the compensation effect by the dispersion compensating fiber 62 should be increased. While simplifying the structure and reducing the cost,
A broadband optical amplifier can be realized. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. It can also be configured to be output.

Further, an excitation light source and an optical demultiplexer-multiplexer for the dispersion compensating fiber 62 can be provided. That is, FIG.
In the same manner as in 2, the excitation light source 133-of the 0.98 μm band
1-133-3 and optical demultiplexer-multiplexers 134-1 to 134-
3 may be used to configure an optical fiber amplifier.

In place of the dispersion compensating fiber 62,
A silica-based optical fiber may be used.

(9) Description of Ninth Embodiment FIG. 38 is a block diagram showing a ninth embodiment of the present invention. The optical fiber amplifier shown in FIG.
An isolator 75-1, an erbium doped fiber (rare earth doped fiber) 71, a dispersion compensation fiber 72, an optical demultiplexer / multiplexer 74, and an isolator 75-2 are arranged. The excitation light source 73 is connected to the optical demultiplexer-multiplexer 74.

Here, the excitation light source 73 is, for example, 1.47.
The excitation light in the μm band (1.45 to 1.49 μm) is generated. With such a configuration, in the optical fiber amplifier shown in FIG. 38, pumping light is made incident from the output side of the dispersion compensating fiber 72 by the optical demultiplexer-multiplexer 74 to cause Raman amplification, and the dispersion compensating fiber is also generated. 72
The residual pumping light from is incident from the output end of the erbium-doped fiber 71 to pump the erbium-doped fiber 71 and amplify the signal light.

As described above, conversely, by exciting the erbium-doped fiber 71 with the residual pumping light at the time of Raman amplification, the unevenness of the wavelength characteristic of the erbium-doped fiber is flattened as in the seventh embodiment. A wide-band optical amplifier can be realized, which is suitable for performing multi-wavelength collective amplification, and since only one pumping light source is required, the structure can be simplified and the cost can be reduced.

The reason why amplification can be performed in both fibers by using a common pumping light source for the erbium-doped fiber and the dispersion compensating fiber is the same as described above. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. Can be configured to.

Further, the pumping light source 73 may be composed of two pumping light sources and a polarization combiner for synthesizing orthogonal polarizations of the pumping lights from these pumping light sources, and combining the pumping light source and the depolarizer. The excitation light may be depolarized, or the modulated excitation light may be generated.

(10) Description of Tenth Embodiment FIG. 39 is a block diagram showing a tenth embodiment of the present invention.
The optical fiber amplifier shown in FIG. 39 has an isolator 84-1, an optical demultiplexer-multiplexer 83, and a dispersion compensation fiber doped with erbium (rare earth element) ions (hereinafter referred to as erbium-doped dispersion compensation fiber) in this order from the input side. ) 81 and an isolator 84-2. The optical demultiplexer-multiplexer 83 is connected to a pumping light source 82 that generates pumping light in the 1.47 μm band (1.45 to 1.49 μm) or 0.98 μm, for example.

With such a structure, in the optical fiber amplifier shown in FIG. 39, the pumping light is made incident on one end of the erbium-doped dispersion compensation fiber 81 by the optical demultiplexer-multiplexer 83, and the erbium-doped dispersion compensation fiber 81 is made. Are excited to amplify the signal light. When Er ions are doped in the core of the dispersion compensating fiber in this way, the pumping light is rapidly attenuated in the dispersion compensating fiber 81, so that Raman amplification does not occur and the loss of the dispersion compensating fiber 81 is reduced in each minute section. As a result, the signal-to-noise ratio can be kept good.

Also in this case, the input section or the input / output section is
Instead of providing the isolator, as shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator. The pumping light source 82 may be composed of two pumping light sources and a polarization combiner that performs orthogonal polarization combining of the pumping lights from these pumping light sources, and the pumping light source and the depolarizer are combined to form the pumping light. May be depolarized, or may be configured to generate modulated excitation light.

(11) Description of Eleventh Embodiment FIG. 40 is a block diagram showing the eleventh embodiment of the present invention.
The optical fiber amplifier shown in FIG. 40 has an isolator 96-1, an optical demultiplexer-multiplexer 94, an erbium-doped fiber (rare earth-doped fiber) 91, an isolator 96-2, an optical filter 95, and a dispersion compensation fiber in this order from the input side. 92 is provided. Then, the optical demultiplexer-multiplexer 94
In addition, for example, a 1.47 μm band (1.45 to 1.49 μm
An excitation light source 93 for generating the excitation light of m) is connected.

The optical filter 95 blocks the residual excitation light in the 1.47 μm band emitted from the erbium-doped fiber 91. With such a configuration, in the optical fiber amplifier shown in FIG. 40, pumping light is made incident on one end of the erbium-doped fiber 91 by the optical demultiplexer-multiplexer 94 to pump the erbium-doped fiber 91 and amplify the signal light. However, at this time, the residual excitation light reaches from the other end of the erbium-doped fiber 91. Then, this residual excitation light is blocked by the optical filter 95.

If light in the 1.47 μm band is passed through the dispersion compensating fiber 92 unnecessarily, Raman amplification will disturb the level diagram design or the wavelength characteristics of the optical amplifier. The optical filter 95 blocks the light in the .47 μm band from being input to the dispersion compensating fiber 92. Therefore, the dispersion compensating fiber 92
Will be used primarily to compensate for dispersion in the transmission line.

Also in this case, the input section or the input / output section is
Instead of providing the isolator, as shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator. The pumping light source 93 may be composed of two pumping light sources and a polarization combiner that performs orthogonal polarization combining of the pumping light from these pumping light sources, and the pumping light source and the depolarizer are combined to generate the pumping light. May be depolarized, or may be configured to generate modulated excitation light.

(12) Description of Twelfth Embodiment FIG. 41 is a block diagram showing a twelfth embodiment of the present invention.
The optical fiber amplifier shown in FIG. 41 has an isolator 5-1, an optical demultiplexer-multiplexer 3-1, an erbium-doped fiber (rare earth-doped fiber) 1 having silica as a host, and an optical demultiplexer-multiplexer in order from the input side. Vessel 3-2, isolator 5
-2 is provided. Then, the optical demultiplexer-multiplexer 3-1
To the optical demultiplexer-multiplexer 3-2, while being connected to a pumping light source 2-1 that generates pumping light in the 0.98 μm band, for example.
For example, about 1.44 μm excitation light or about 1.46 μm
A pumping light source 2-2 that generates the pumping light is connected.

Here, the optical demultiplexer-multiplexer 3-1 is not a bulk type but a fusion type, and the pumping light source 2-1 is a type without an optical isolator (optical ISO). That is, the noise light in the 1.55 μm band generated in the erbium-doped fiber 1 when amplifying the optical signal in the 1.55 μm band is not generated in the pumping light source 2-1 that generates the pumping light in the 0.98 μm band. This is because it does not return (the same applies to the following embodiments).

With such a configuration, in the optical fiber amplifier shown in FIG. 41, the pump light in the 0.98 μm band is made incident on one end of the erbium-doped fiber 1 by the optical demultiplexer-multiplexer 3-1 to be erbium-doped. The fiber 1 is excited to amplify the signal light. Further, the optical demultiplexer-multiplexer 3-of the 1.44 μm excitation light or the 1.46 μm excitation light is used.
The light enters from the output end of the erbium-doped fiber 1 by 2 and is Raman-amplified by the erbium-doped fiber 1.

It is known that even in the erbium-doped fiber 1, Raman amplification occurs when strong light is input. Thus, by amplifying the erbium-doped fiber 1 having silica as a host at a general excitation wavelength [for example, 0.98 μm (1.47 μm may be used)] and Raman amplification at ˜1.44 μm,
It is possible to flatten the gain depression (see FIG. 46) in the 1.54 μm band of the erbium-doped fiber, and
Raman amplification at 1.46μm yields 1.57μ
Gain reduction of erbium-doped fiber near m (Fig. 46)
It is possible to realize a wider band by further flattening the characteristics by compensating for (see).

In this case also, the input section or the input / output section is
Instead of providing the isolator, as shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator. (13) Description of 13th Embodiment FIG. 42 is a block diagram showing a 13th embodiment of the present invention.
The optical fiber amplifier shown in FIG. 42 includes an isolator 144-1 and a dispersion compensating fiber 14 in order from the input side.
1, a polarization-maintaining optical demultiplexer-multiplexer 143, an isolator 14
4-2 is provided. And the optical demultiplexer-multiplexer 143
A polarization beam combiner type pumping light source 142 is connected to.

Here, the pumping light source 142 includes two pumping light sources 142A and 142B, and these pumping light sources 142A and 142A.
And a polarization beam combiner (PBS) 142C that performs orthogonal polarization wave combination on the pumping light from the 142B. And
The pumping light sources 142A and 142B both have the same pumping light power and both output pumping light of, for example, 1.45 to 1.49 μm (or 1.45 to 1.48 μm).

As the optical demultiplexer-multiplexer 143, an optical film type is used, and it is possible to combine or demultiplex light while maintaining the polarization state. With such a configuration, in the optical fiber amplifier shown in FIG. 42, the orthogonal polarization-combined pumping light is input from the output end of the dispersion compensation fiber 141 by the optical demultiplexer-multiplexer 143, and the dispersion compensation fiber 141 is used. Effectively produces Raman amplification. Then, this Raman amplification enables the loss compensation of the dispersion compensating fiber.

Also in this case, the input section or the input / output section is
Instead of providing the isolator, as shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator. Further, instead of the dispersion compensating fiber 141, a silica optical fiber can be used.

Further, the excitation light source 142 is, for example, as shown in FIG.
4, like the pumping light sources 53-2 ′ and 53-2 ″ shown in FIG. 45, the pumping light source and the depolarizer may be combined so as to depolarize the pumping light. It may be configured to generate the excited light.
The excitation light sources 53-2 'and 53 shown in FIGS.
-2 ″ will be described in the first modification of the fourteenth embodiment of the present invention and the second modification of the fourteenth embodiment of the present invention, respectively.

(14) Description of 14th Embodiment FIG. 43 is a block diagram showing a 14th embodiment of the present invention.
The optical fiber amplifier shown in FIG. 43 includes an isolator 55-1, an optical demultiplexer-multiplexer 54-1 and an erbium-doped fiber (rare earth-doped fiber) 5 in order from the input side.
1, an isolator 55-2, a dispersion compensating fiber 52, a polarization maintaining type optical demultiplexer-multiplexer 54-2, an isolator 55-3
Are arranged. Then, in the optical demultiplexer-multiplexer 54-1,
The excitation light source 53-1 is connected, and the polarization combining pump light source 53-2 is connected to the optical demultiplexer-multiplexer 54-2.

Here, the excitation light source 53-1 is, for example, 0.9.
The excitation light source 53-outputs excitation light of 8 μm.
Reference numeral 2 denotes two pump light sources 53-2A and 53-2B and a polarization combiner (PBS) 53-2C that performs orthogonal polarization combination of the pump lights from the pump light sources 53-2A and 53-2B.
It is composed of In this case also, the excitation light source 5
3-2A and 53-2B have the same pumping light power, and both are, for example, 1.45 to 1.49 μm (or 1.4
5 to 1.48 μm) of excitation light is output.

As the optical demultiplexer-multiplexer 54-1, a fusion splicer having no polarization maintaining function is used, while as the optical demultiplexer-multiplexer 54-2, an optical film type is used. The one used is that it can combine or demultiplex light while maintaining the polarization state. With such a configuration, as shown in FIG.
In the optical fiber amplifier shown in (1), the pumping light from the pumping light source 53-1 is incident from the optical demultiplexer-multiplexer 54-1 together with the signal light from one end of the erbium-doped fiber 51. As a result, the signal light is amplified by the erbium-doped fiber 51.

[0200] Further, the orthogonally polarized wave-combined pump light enters from the output end of the dispersion compensation fiber 52 by the optical demultiplexer-multiplexer 54-2, and Raman amplification is effectively generated in the dispersion compensation fiber 52. . And by this Raman amplification,
Loss compensation of the dispersion compensating fiber 52 is performed. Even in this case, the same effect or advantage as the thirteenth embodiment can be obtained.

Also in this case, the input section or the input / output section is
Instead of providing the isolator, as shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator. further,
The rare earth-doped fiber optical amplification section made of an erbium-doped fiber may be configured as an optical amplification section having a low noise figure, and the Raman optical amplification section made of a dispersion-compensating fiber is arranged as a pre-stage amplification section and the erbium A rare earth-doped fiber optical amplification section made of a doped fiber may be arranged as the post-stage amplification section.

(14-1) Description of First Modification of Fourteenth Embodiment FIG. 44 is a block diagram showing a first modification of the fourteenth embodiment of the present invention. The optical fiber amplifier shown in FIG. In order from the input side, the isolator 55-1 and the optical demultiplexer-multiplexer 5
4-1, an erbium-doped fiber (rare earth-doped fiber) 51, an isolator 55-2, a dispersion compensation fiber 52, a polarization-maintaining optical demultiplexer-multiplexer 54-2, and an isolator 55-3 are arranged. Then, the optical demultiplexer-multiplexer 54
−1 is connected to the pumping light source 53-1, and the optical demultiplexer-multiplexer 54-2 is connected to the non-polarized polarization combining pumping light source 53−.
2'is connected.

Here, the excitation light source 53-1 is, for example, 0.9.
The excitation light source 53-outputs excitation light of 8 μm.
Reference numeral 2'denotes one excitation light source 53-2A 'and a depolarizer 53-2 for depolarizing the excitation light.
And B '. Here, the depolarizer 53-
2B 'reduces the polarization dependence in the Raman optical amplifier including the dispersion compensating fiber 52, and includes a polarization maintaining coupler 53-2E' for demultiplexing the pumping light from the pumping light source 53-2A 'and a polarization maintaining coupler 53-2E'. A polarization combiner (PBS) 53-2 that performs orthogonal polarization combination of the pumping light demultiplexed by the holding coupler 53-2E ′ and the pumping light delayed by the delay line.
It is composed of C'and.

In this case also, the excitation light source 53-2
A ′ is, for example, 1.45 to 1.49 μm (or 1.4
5 to 1.48 μm) of excitation light is output. As the optical demultiplexer-multiplexer 54-1 as well, a fusion splicer without polarization maintaining function is used, while the optical demultiplexer-multiplexer 54-1 is used.
As -2, an optical film type is also used, and it is possible to combine or demultiplex light while maintaining the polarization state.

With such a structure, in the optical fiber amplifier shown in FIG. 44, the pumping light from the pumping light source 53-1 enters from the optical demultiplexer-multiplexer 54-1 together with the signal light from one end of the erbium-doped fiber 51. To be done. As a result, the erbium-doped phi 51 amplifies the signal light. In addition, the depolarized pumping light is used as the optical demultiplexer-multiplexer 5
The light enters from the output end of the dispersion compensating fiber 52 by 4-2, and Raman amplification is effectively generated in the dispersion compensating fiber 52. The loss of the dispersion compensating fiber 52 is compensated by this Raman amplification.

In this way, the dispersion compensating fiber 52
It is possible to obtain the same effects or advantages as those of the above-described fourteenth embodiment while reducing the polarization dependence in. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. Can be configured to.

Furthermore, the rare earth-doped fiber optical amplification section made of erbium-doped fiber may be configured as an optical amplification section having a low noise figure, and the Raman optical amplification section made of dispersion compensation fiber is provided as a pre-stage amplification section. At the same time, a rare earth-doped fiber optical amplification section made of an erbium-doped fiber may be provided as a post-stage amplification section.

(14-2) Description of Second Modification of Fourteenth Embodiment FIG. 45 is a block diagram showing a second modification of the fourteenth embodiment of the present invention. The optical fiber amplifier shown in FIG. In order from the input side, the isolator 55-1 and the optical demultiplexer-multiplexer 5
4-1, an erbium-doped fiber (rare earth-doped fiber) 51, an isolator 55-2, a dispersion compensation fiber 52, a polarization-maintaining optical demultiplexer-multiplexer 54-2, and an isolator 55-3 are arranged. Then, the optical demultiplexer-multiplexer 54
-1, the pumping light source 53-1 is connected, and the optical demultiplexer-multiplexer 54-2 is connected to the modulated polarization combining pumping light source 53-.
2 ″ is connected.

Here, the excitation light source 53-1 is, for example, 0.9.
The excitation light source 53-outputs excitation light of 8 μm.
2 ″ denotes two pump light sources 53-2A ″ and 53-2.
B ″ and these excitation light sources 53-2A ″, 53-2
A polarization combiner (PBS) 53-2C ″ for combining orthogonal polarizations of the pump light from B ″, and each pump light source 53-2.
Several hundred kHz to 1 MHz for A ″ and 53-2B ″
And a modulator 53-2D ″ for performing the modulation of the above.

In this case also, the excitation light source 53-2
A ″ and 53-2B ″ both have the same pumping light power, and both are, for example, 1.45 to 1.49 μm (or 1.4
5 to 1.48 μm) of excitation light is output. As the optical demultiplexer-multiplexer 54-1 as well, a fusion splicer without polarization maintaining function is used, while the optical demultiplexer-multiplexer 54-1 is used.
As -2, an optical film type is also used, and it is possible to combine or demultiplex light while maintaining the polarization state.

With such a configuration, in the optical fiber amplifier shown in FIG. 45, the pumping light from the pumping light source 53-1 enters from the optical demultiplexer-multiplexer 54-1 together with the signal light from one end of the erbium-doped fiber 51. To be done. As a result, the erbium-doped phi 51 amplifies the signal light. In addition, the excitation light that has been modulated and has a spectrum of several hundreds of kHz or more and that has been orthogonally polarized and combined (the spectral line width of this excitation light can be widened) is generated by the optical demultiplexer-multiplexer 54-2. The light enters from the output end of the dispersion compensating fiber 52 and effectively causes Raman amplification in the dispersion compensating fiber 52. Then, the Raman amplification compensates the loss of the dispersion compensating fiber.

With this arrangement, the threshold value of stimulated Brillouin scattering can be increased and the harmful non-linear effect can be suppressed, and the same effects or advantages as those of the fourteenth embodiment can be obtained. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. Can be configured to.

Further, the rare earth-doped fiber optical amplification section made of an erbium-doped fiber may be configured as an optical amplification section having a low noise figure, and the Raman optical amplification section made of a dispersion compensation fiber is arranged as a pre-stage amplification section. At the same time, a rare earth-doped fiber optical amplification section made of an erbium-doped fiber may be provided as a post-stage amplification section.

(15) Description of Fifteenth Embodiment FIG. 48 is a block diagram showing a fifteenth embodiment of the present invention.
The optical fiber amplifier shown in FIG. 48 has an isolator 125-1 and an optical demultiplexer-multiplexer 124-in order from the input side.
1, erbium doped fiber (rare earth doped fiber) 121-1, isolator 125-2, silica optical fiber 122, erbium doped fiber (rare earth doped fiber) 121-2, optical demultiplexer-multiplexer 124-3,
An isolator 125-3 is provided. Then, for example, 1.47 is added to the optical demultiplexer-multiplexers 124-1 and 124-3.
Excitation light sources 123-1 and 123-3 that generate excitation light in the μ band (1.45 to 1.49 μm) are connected.

Here, the silica-based optical fiber 122 functions as a Raman optical amplifier whose amplification frequency band can be changed by the pumping wavelength, and its band characteristic depends on the silica of the host glass and the doping material and concentration of the core. Decided. In addition, the erbium-doped fiber 121-
1, 121-2 function as a rare earth-doped fiber optical amplifier whose amplification frequency band and its band characteristic are determined by the host glass and the doped material of the core.

The silica-based optical fiber 1 in this embodiment
The mode field diameter of 22 is small, and the noise figure of the Raman optical amplifier composed of the silica-based optical fiber 122 is erbium-doped fibers 121-1 and 121-2.
If it is larger than the rare-earth-doped fiber optical amplifier consisting of, a rare-earth-doped fiber optical amplifier is used for the front-stage amplification section, a Raman optical amplifier is used for the middle-stage amplification section, and a rare-earth-doped fiber optical amplifier is used for the rear-stage amplification section with a large signal light power. By using a fiber optical amplifier and connecting them in series, an optical fiber amplifier having low noise and a flatter band characteristic or a wide amplification frequency band is realized.

That is, a rare earth-doped fiber optical amplifier having a low noise figure (such as an erbium-doped fiber optical amplifier pumped by a 1.47 μm band) is used as a pre-amplifier.
A non-linear effect that amplifies a very small signal light in a low noise state and deteriorates the signal-to-noise ratio (SNR) (here, the non-linear effect means self-phase modulation (Se) of the signal light.
lf-Phase Modulation, SPM), four-photon mixing (Four Wave
Mixing, FWM), Cross-Phase Modulatio
In order to reduce the effect of the signal-to-noise ratio (SNR) such as (n, XPM)), a Raman optical amplifier using a silica optical fiber with a small signal light power is used in the middle-stage amplifying section. Is there.

With such a configuration, in the optical fiber amplifier shown in FIG. 48, the pumping light is sent to the optical demultiplexer-multiplexer 124-
1 from one end of the erbium-doped fiber 121-1 to excite the erbium-doped fiber 121-1 to amplify the signal light, and the residual pumping light generated at this time excites the silica-based optical fiber 122 to disperse the light. Raman amplification is performed similarly to the compensation fiber.

Further, the pumping light is combined with the optical demultiplexer-multiplexer 124-3.
Incident from the output end of the erbium-doped fiber 121-2 to excite the erbium-doped fiber 121-2 to amplify the signal light, and the residual pumping light generated at this time excites the silica-based optical fiber 122 to cause Raman Amplify. As described above, in the optical fiber amplifier shown in FIG.
By using 3-1 and 123-3, both of the erbium-doped fibers 121-1 and 121-2 and the silica-based optical fiber 122 can be pumped, whereby pumping in the optical fiber amplifier shown in FIG. It is possible to reduce the number of light sources 123-2, simplify the optical fiber amplifier, and improve the efficiency of pumping light power.

Also in this case, the input section or the input / output section is
Instead of providing the isolator, as shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator. The silica-based optical fiber 122 and the erbium-doped fiber 1
You may make it provide an isolator with 21-2.

An excitation light source and an optical demultiplexer-multiplexer for the silica optical fiber 122 may be provided. That is, in the same manner as in FIG. 11, the excitation light source 123 in the 0.98 μm band
-1 to 123-3 and optical demultiplexer-multiplexer 124-1 to 124
-3 may be used to configure the optical fiber amplifier.

A dispersion compensating fiber may be used instead of the silica optical fiber 122.

(15-1) Description of Modification of Fifteenth Embodiment FIG. 49 is a block diagram showing a modification of the fifteenth embodiment of the present invention. The optical fiber amplifier shown in FIG. 49 has an input side in order. , Isolator 125-1, optical demultiplexer / multiplexer 12
4-1, Erbium Doped Fiber (Rare Earth Doped Fiber) 121-1, Isolator 125-2, Silica Optical Fiber 122, Optical Filter 126, Erbium Doped Fiber (Rare Earth Doped Fiber) 121-2, Optical Demultiplexer-Multiplexer 124 -3, an isolator 125-3 is provided. Then, the optical demultiplexer-multiplexers 124-1 and 124-
3, the polarization combining type pump light sources 123-1 'and 12
3-3 'is connected.

Here, the pumping light source 123-1 'has two pumping light sources 123-1A' and 123-1B 'and orthogonal polarizations of the pumping light from these pumping light sources 123-1A' and 123-1B '. Polarization combiner (PBS) 123 for combining
-1C 'and a pump light source 123-1
A ′ and 123-1B ′ both have the same pumping light power, and both are, for example, 1.45 to 1.49 μm (or 1.4
5 to 1.48 μm) of excitation light is output.

Further, the pumping light source 123-3 'combines the two pumping light sources 123-3A', 123-3B 'and the pumping lights from these pumping light sources 123-3A', 123-3B 'with orthogonal polarization. Polarization combiner (PBS) 123-
3C ', but the pumping light sources 123-3A' and 123-3B 'are different in pumping wavelength and pumping light power because they are pumping light sources that are orthogonally polarized wave combined in order to simply increase the pumping light power. May be.

Furthermore, the erbium-doped fiber 121-1 and the silica-based optical fiber 122 are firmly fixed to the bobbin or the like so that the unpolarized state of the excitation light obtained by the orthogonal polarization synthesis is maintained in the silica-based optical fiber 122. Or, by being housed in the housing, it is not affected by the outside air. The isolator 12
5-1 to 125-3 are polarization-independent optical isolators, and the optical filter 126 is an optical filter that removes or flattens the ASE peak near 1.535 μm generated in the erbium-doped fiber 121-1. Yes, it can be omitted.

With such a configuration, in the optical fiber amplifier shown in FIG. 49, pumping light in the 1.47 μm band is made incident on one end of the erbium-doped fiber 121-1 by the optical demultiplexer-multiplexer 124-1. The erbium-doped fiber 121-1 is excited to amplify the signal light, and the residual pump light generated at this time excites the silica-based optical fiber 122 to perform Raman amplification.

Further, the pumping light of 1.47 μm is sent to the erbium-doped fiber 121 by the optical demultiplexer-multiplexer 124-3.
The erbium-doped fiber 121-2 is excited by the incident light from the output terminal of −2 to be amplified, and the residual optical pumping light generated at this time excites the silica optical fiber 122 to perform Raman amplification. As described above, in the optical fiber amplifier shown in FIG. 49, by using the 1.47 μm band pumping light sources 123-1 ′ and 123-3 ′, the erbium-doped fibers 121-1 and 121-2 and the silica-based optical fiber 122 are used. It is possible to excite any of the above, thereby reducing the number of pumping light sources 123-2 in the optical fiber amplifier shown in FIG. 11, and simplifying the optical fiber amplifier and improving the efficiency of pumping light power. it can.

Also in this case, instead of providing an isolator in the input section or the input / output section, the configuration shown in FIG.
As shown in, the input signal light can be input through the optical circulator and the output signal light can be output through the optical circulator. Furthermore, an excitation light source and an optical demultiplexer-multiplexer for the silica-based optical fiber 122 can be provided.

That is, in the same manner as in FIG. 11, 0.98 μ
An optical fiber amplifier may be configured using the m-band pumping light sources 123-1 to 123-3 and the optical demultiplexer-multiplexers 124-1 to 124-3. In addition, silica-based optical fiber 1
An isolator may be provided between the optical fiber 22 and the erbium-doped fiber 121-2.

A dispersion compensating fiber may be used instead of the silica optical fiber 122. (16) Description of Sixteenth Embodiment FIG. 50 is a block diagram showing a sixteenth embodiment of the present invention.
The optical fiber amplifier shown in FIG. 50 has an isolator 115-1 and an optical demultiplexer-multiplexer 114-in order from the input side.
1, an erbium-doped fiber (rare earth-doped fiber) 111, an isolator 115-2, a silica-based optical fiber 112, a polarization-maintaining optical demultiplexer-multiplexer 114-2, and an isolator 115-3 are arranged. The pumping light source 113-1 is connected to the optical demultiplexer-multiplexer 114-1, and the polarization combining pumping light source 113-2 is connected to the optical demultiplexer-multiplexer 114-2.

Therefore, in the optical fiber amplifier shown in FIG. 50, it is possible to obtain a flatter band characteristic or a wider amplification frequency band by compensating each other using these rare earth-doped fiber optical amplifier and Raman optical amplifier. Therefore, a rare-earth-doped fiber optical amplifier (0.98 μm band pump or 1.47 μm) having a low noise figure is used.
erbium-doped fiber optical amplifiers with μm band pumping) are used in the pre-amplifier, and Raman optical amplifiers made of silica-based optical fibers are used in the post-amplifier, and these are connected in series to form an optical fiber amplifier. It has a flatter band characteristic having a low noise characteristic or a wide amplification frequency band.

That is, when the noise figure of the Raman optical amplifier is larger than that of the rare earth-doped fiber optical amplifier, the rare-earth-doped fiber optical amplifier is used in the pre-stage amplifying section and the Raman optical amplifier is used in the post-stage amplifying section. By connecting the two in series, a low-noise optical fiber amplifier is realized.

Further, the excitation light source 113-1 may be, for example, 0.
The pumping light source 11 outputs a pumping light of 98 μm.
3-2 is two pump light sources 113-2A and 113-2B
And a polarization combiner (PB) that performs orthogonal polarization combination on the pump lights from the pump light sources 113-2A and 113-2B.
S) 113-2C. Also in this case, the pumping light sources 113-2A and 113-2B both have the same pumping light power, and both are, for example, 1.45 to 1.49.
It outputs the excitation light of μm (or 1.45 to 1.48 μm).

As the optical demultiplexer-multiplexer 114-1,
While a fusion splicer without polarization maintaining function is used,
As the optical demultiplexer-multiplexer 114-2, an optical film type is used, and it is possible to combine or demultiplex light while maintaining the polarization state. With such a configuration, the optical fiber amplifier shown in FIG.
The pumping light from is incident from the optical demultiplexer-multiplexer 114-1 from one end of the erbium-doped fiber 111 together with the signal light. As a result, the erbium-doped phi 111 amplifies the signal light.

Further, the pumping light that has been subjected to the orthogonal polarization synthesis is passed through the optical demultiplexer-multiplexer 114-2 to the silica optical fiber 112.
The light is incident from the output end of the optical fiber and the silica optical fiber 112 effectively causes Raman amplification. Then, the loss of the silica optical fiber 112 is compensated for by this Raman amplification. Even in this case, the same effects and advantages as those of the above-described fourteenth embodiment can be obtained.

Also in this case, the input section or the input / output section is
Instead of providing the isolator, as shown in FIGS. 18 and 30, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator. In addition,
By providing an excitation light source that generates excitation light in the 1.47 μm band,
The pumping light source may be used as both the silica-based optical fiber pumping light source and the erbium-doped fiber pumping light source.

When a high output cannot be obtained by the Raman optical amplifier, a Raman optical amplifier made of silica optical fiber or dispersion compensating fiber is used for the input side amplifying section (previous stage amplifying section), and the output side A rare earth-doped fiber optical amplifier made of an erbium-doped fiber is used for the amplification unit (post-stage amplification unit), and these are connected in cascade.

In particular, when the pumping wavelength of the pumping light source for Raman optical amplifier is set to about 1.44 μm, the depression of the gain generated in the vicinity of about 1.54 μm in the rare earth-doped fiber optical amplifier can be compensated by Raman optical amplification. If the pumping wavelength of the pumping light source for the Raman optical amplifier is about 1.46 μm, then about 1.
The decrease in gain that occurs on the wavelength side longer than 57 μm can be compensated for by Raman optical amplification, which makes it possible to further flatten the band characteristic or increase the band of the optical fiber amplifier.

Furthermore, in order to reduce the pumping light power (threshold pumping light power) at which the gain of the Raman optical amplifier using the silica type optical fiber or the dispersion compensating fiber begins to be reduced, the silica type optical fiber having a small mode field diameter is used. In order to reduce the influence of the non-linear effect, which is increased by reducing the mode field diameter, the Raman light composed of silica-based optical fiber is used for the input amplifier (pre-amplifier) with low signal light power. In addition to using the amplifiers, a rare earth-doped fiber optical amplifier made of an erbium-doped fiber is used in the output side amplification section (post-stage amplification section) where the signal light power is large, and these are cascaded to further improve the optical fiber amplifier. It is possible to have a flat band characteristic or a wide amplification frequency band.

(16-1) Description of First Modification of Sixteenth Embodiment FIG. 51 is a block diagram showing a first modification of the sixteenth embodiment of the present invention. The optical fiber amplifier shown in FIG. An isolator 115-1, an optical demultiplexer-multiplexer 114-1, an erbium-doped fiber (rare earth-doped fiber) 111, an isolator 115-2, a silica-based optical fiber 112, a polarization-maintaining optical demultiplexer-multiplexer in order from the input side. 114-
2, the isolator 115-3 is arranged. Then, the pumping light source 113-1 is connected to the optical demultiplexer-multiplexer 114-1, and the non-polarized polarization combining pumping light source 113-2 'is connected to the optical demultiplexer-multiplexer 114-2. ing.

Here, the excitation light source 113-1 is, for example, 0.
The pumping light source 11 outputs a pumping light of 98 μm.
3-2 'is one pumping light source 113-2A' and a depolarizer 11 for depolarizing this pumping light.
3-2B '. Also, depolarizer 1
13-2B 'reduces the polarization dependence in the Raman optical amplifier including the silica optical fiber 112, and is a polarization maintaining coupler 113-2E' for demultiplexing the pumping light from the pumping light source 113-2A '. , Polarization maintaining coupler 113
-2E 'and a polarization combiner (PBS) 113-2C' that performs orthogonal polarization combination of the pumping light delayed by the delay line and the pumping light delayed by the delay line.

In this case also, the excitation light source 113-2
A ′ is, for example, 1.45 to 1.49 μm (or 1.4
5 to 1.48 μm) of excitation light is output. As the optical demultiplexer-multiplexer 114-1, a fusion splicer having no polarization maintaining function is used, while the optical demultiplexer-multiplexer 1 is used.
An optical film type 14-2 is also used as 14-2 so that light can be combined or demultiplexed while maintaining the polarization state.

With such a structure, in the optical fiber amplifier shown in FIG. 51, the pumping light from the pumping light source 113-1 enters from the optical demultiplexer-multiplexer 114-1 together with the signal light from one end of the erbium-doped fiber 111. To be done. As a result, the erbium-doped phi 111 amplifies the signal light. Further, the depolarized pumping light is incident on the output end of the silica-based optical fiber 112 by the optical demultiplexer-multiplexer 114-2, and Raman amplification is effectively generated in the silica-based optical fiber 112. Then, the loss of the silica optical fiber 112 is compensated for by this Raman amplification.

In this way, the silica optical fiber 1
While reducing the polarization dependence in 12, the above 16th
The same effects and advantages as those of the embodiment can be obtained.
Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. Can be configured to.

A pumping light source for generating pumping light in the 1.47 μm band may be provided so that this pumping light source also serves as a silica-based optical fiber pumping light source and an erbium-doped fiber pumping light source. (16-2) Description of Second Modification of Sixteenth Embodiment FIG. 52 is a block diagram showing a second modification of the sixteenth embodiment of the present invention. The optical fiber amplifier shown in FIG. An isolator 115-1, an optical demultiplexer-multiplexer 114-1, an erbium-doped fiber (rare earth-doped fiber) 111, an isolator 115-2, a silica-based optical fiber 112, a polarization-maintaining optical demultiplexer-multiplexer 114-, in that order.
2, the isolator 115-3 is arranged. Then, the pumping light source 113-1 is connected to the optical demultiplexer-multiplexer 114-1, and the modulated polarization combining pumping light source 113-2 ″ is connected to the optical demultiplexer-multiplexer 114-2. ing.

Here, the excitation light source 113-1 is, for example, 0.
The pumping light source 11 outputs a pumping light of 98 μm.
3-2 ″ is two pump light sources 113-2A ″, 11
3-2B ″ and these pump light sources 113-2A ″,
A polarization combiner (PBS) 113-2C ″ that performs orthogonal polarization combination on the pump light from the 113-2B ″, and several hundreds k for each pump light source 113-2A ″ and 113-2B ″.
It is composed of a modulator 113-2D ″ that performs modulation of Hz to 1 MHz.

In this case also, the excitation light source 113-2
A ″ and 113-2B ″ both have the same pumping light power, and both are, for example, 1.45 to 1.49 μm (or 1.
45 to 1.48 μm) of excitation light is output.
As the optical demultiplexer-multiplexer 114-1, a fusion splicer without polarization maintaining function is used, and as the optical demultiplexer-multiplexer 114-2, an optical film type is also used. It is used to combine or split light while maintaining the polarization state.

With such a structure, in the optical fiber amplifier shown in FIG. 52, the pumping light from the pumping light source 113-1 enters from the optical demultiplexer-multiplexer 114-1 from one end of the erbium-doped fiber 111 together with the signal light. To be done. As a result, amplification is performed by the erbium-doped phi 111. In addition, the excitation light that has been modulated and has a spectrum of several hundreds of kHz or more and that is orthogonal polarization combined (the spectral line width of this excitation light can be widened) is generated by the optical demultiplexer-multiplexer 114-2. The light enters from the output end of the silica-based optical fiber 112 and effectively causes Raman amplification in the silica-based optical fiber 112. Then, the loss of the silica optical fiber 112 is compensated for by this Raman amplification.

In this way, the threshold value of the stimulated Brillouin scattering can be increased and the harmful non-linear effect can be suppressed, and the same effects or advantages as those of the 16th embodiment can be obtained. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. Can be configured to.

A pumping light source for generating pumping light in the 1.47 μm band may be provided so that this pumping light source serves as both the silica-based optical fiber pumping light source and the erbium-doped fiber pumping light source. (17) Description of the seventeenth embodiment

FIG. 55 is a block diagram showing a seventeenth embodiment of the present invention. The optical fiber amplifier shown in FIG. 55 has an isolator 65-1 and an optical demultiplexer-multiplexer 6 in order from the input side.
4, an erbium-doped fiber (rare earth-doped fiber amplifier) 61, a dispersion compensation fiber 62 (optical fiber attenuator), and an isolator 65-3 are arranged. The excitation light source 63 is connected to the optical demultiplexer-multiplexer 64.

Here, the excitation light source 63 is, for example, 1.47.
The excitation light in the μm band (1.45 to 1.49 μm) is generated. Further, in a rare earth-doped fiber optical amplifier having a high gain, unnecessary oscillation may occur during optical amplification. When such unnecessary oscillation occurs, the rare earth-doped fiber optical amplifier operates in an unstable manner. To do.

For example, in an erbium-doped fiber optical amplifier, it is 1.53 to 1.57 μm when performing optical amplification.
Spontaneous emission light (ASE) is generated, but since this ASE is repeatedly reflected at a reflection point in the erbium-doped fiber optical amplifier, unnecessary oscillation may occur. Particularly, in an erbium-doped fiber optical amplifier adjusted for multi-wavelength batch amplification (that is, an erbium-doped fiber optical amplifier with a high pumping rate), since the gain near 1.53 μm is high, unnecessary oscillation easily occurs at this wavelength. When such unnecessary oscillation occurs, the erbium-doped fiber optical amplifier operates unstable.

In order to suppress such an unstable operation, it is effective to provide a medium that loses (attenuates) the signal light (this is called a loss medium) (this principle will be described later). In the optical fiber amplifier as shown in FIG. 55, the dispersion compensating fiber 62 is excited by the residual pumping light incident through the erbium-doped fiber 61, and the loss (attenuation) of the signal light in this dispersion compensating fiber 62 is compensated. However, in practice, it is difficult to compensate for all losses, and some loss remains, so
The dispersion compensating fiber 62 will function as a loss medium.

Here, the principle of suppressing the unstable operation by providing the loss medium will be described. In general, the gain of an erbium-doped fiber is G, and the reflectance at both ends (front end and rear end) of the erbium-doped fiber is R, respectively.
1, R2 (where reflectance R1 is the reflectance of reflection from all the components in the front stage of the erbium-doped fiber, and reflectance R2 is the reflectance of all components in the rear stage of the erbium-doped fiber). The geometric mean of R1 and R2 is R [R = (R1R
2) ^1/2 ], GR can be used as a measure of the stability of the operation of the erbium-doped fiber. When GR is large, the erbium-doped fiber operates instable, and especially when GR is 1 or more, oscillation occurs in the erbium-doped fiber. Therefore, it is necessary to reduce GR, and specifically, GR is 0.02 or less as a standard.

As shown in FIG. 55, the dispersion compensating fiber 62 [the loss of the dispersion compensating fiber 62 is provided at the subsequent stage (output side of the signal light) of the erbium-doped fiber 61 (the gain of the erbium-doped fiber 61 is G). Is η (0
<.Ltoreq..ltoreq.1)] is provided by, for example, fusion splicing, a boundary A is generated between the erbium-doped fiber 61 and the dispersion compensating fiber 62.

At this time, as shown in FIG. 55, the reflectance at the rear end of the erbium-doped fiber 61 is R1, the reflectance at the front end of the dispersion compensating fiber 62 is R2 (here, the reflectance R1 is erbium-doped). The reflectance of the reflection from all the components in the front stage of the fiber 61, and the reflectance R2 is the reflectance of the reflection of all the components in the rear stage from the rear end of the dispersion compensating fiber 62). Further, the reflectance of the reflection caused by the refractive index difference at the boundary A between the erbium-doped fiber 61 and the dispersion compensating fiber 62 is RA.
(RA << R1, R2; if the loss medium is an optical fiber, this condition is satisfied), the parameter indicating the operation stability of the erbium-doped fiber is GR (Gη).
It becomes R. That is, GR is considered to be a one-way gain when the light makes one round, and when a loss medium is provided, the net gain when the light makes one round is (R1 × G × η) × (R2 × η ×
Since G) = (Gη) ² R1R2, the one-way net gain is Gη (R1R2) ^1/2 = (Gη) R. Since RA << R1 and R2, the influence of the reflectance RA can be ignored. Where 0 ≦ η ≦ 1
Therefore, GR is equivalently reduced.

As described above, by providing the loss medium, the parameter GR indicating the stability of the operation of the erbium-doped fiber becomes small, so that the unstable operation of the erbium-doped fiber 61 can be suppressed. In the optical fiber amplifier according to the present embodiment, as shown in FIG. 55, a dispersion compensating fiber 62 is provided in a subsequent stage of the erbium-doped fiber 61, and the dispersion compensating fiber 62 uses residual pump light from the erbium-doped fiber 61. The loss compensation of the dispersion compensating fiber 62 (including the flattening of the cavities in the gain of the erbium-doped fiber 61 and the compensation compensation of the decrease in the gain of the erbium-doped fiber 61) is performed at the same time as the remaining loss component. Thus, the unstable operation of the erbium-doped fiber 61 is suppressed.

With such a configuration, in the optical fiber amplifier shown in FIG. 55, the pumping light and the signal light are made incident from one end of the erbium-doped fiber 61 by the optical demultiplexer-multiplexer 64 to pump the erbium-doped fiber 61. Then, the signal light is amplified, but the residual pumping light reaches the other end of the erbium-doped fiber 61. After that, the residual pumping light is supplied to the dispersion compensating fiber 62 to cause Raman amplification.

As described above, amplification can be performed in both fibers by using a common pumping light source for the erbium-doped fiber and the dispersion compensating fiber. That is, 1.
This is because the pumping wavelength band for Raman amplification of the 55 μm band signal light is the 1.47 μm band (1.45 to 1.49 μm) which is the pumping wavelength band of the erbium-doped fiber (EDF). Raman amplification can be performed by using the residual pumping light power when pumping with light in the 1.47 μm band. This makes it possible to compensate for the loss of the dispersion compensating fiber 62 while performing optical amplification with the erbium-doped fiber 61.

As a result, similar to the seventh embodiment described above, the unevenness of the wavelength characteristics of the erbium-doped fiber amplifier can be flattened to realize a broadband optical amplifier.
In addition to being suitable for multi-wavelength batch amplification, since only one pump light source is required, it is possible to contribute to simplification of the structure and cost reduction. Further, in this optical fiber amplifier, the unstable operation of the erbium-doped fiber 61 is also suppressed at the same time by the loss of the dispersion compensating fiber 62.
Unnecessary oscillation operation in the rare earth-doped fiber optical amplifier adjusted for M) can be prevented and stable optical amplification can be performed.

When the pumping light source 63 produces pumping light of 0.98 μm, the dispersion compensating fiber 62 does not perform Raman amplification, and therefore the loss compensation of the dispersion compensating fiber 62 is not carried out. Further, in optical fibers such as dispersion compensating fibers, there is reflection due to Rayleigh backscattering.
The reflectance of this reflection depends on the length of the optical fiber, and as the length of the optical fiber increases, the reflectance of this reflection also increases.

Therefore, when the effect of the reflectance due to Rayleigh backscattering of the dispersion compensating fiber itself is greater than the effect of reducing the reflectance due to the loss of the dispersion compensating fiber as described above, an optical isolator is added to the dispersion compensating fiber. It is possible to do it. For example, in FIG. 55, this optical isolator can be provided between the erbium-doped fiber 61 and the dispersion compensating fiber 62. By doing so, the reflectivity is always maintained when the effect of the reflectivity due to Rayleigh backscattering appears. Can be reduced.

Further, in this case as well, instead of providing an isolator in the input section or the input / output section, FIG.
As shown in 0, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator.

Further, the pumping light source 63 may be composed of two pumping light sources and a polarization combiner for synthesizing orthogonal polarizations of the pumping lights from these pumping light sources, and by combining the pumping light source and the depolarizer. The excitation light may be depolarized, or the modulated excitation light may be generated. (17-1) Description of First Modification of Seventeenth Embodiment FIG. 56 is a block diagram showing a first modification of the seventeenth embodiment of the present invention. The optical fiber amplifier shown in FIG. In order, an isolator 115-1, an optical demultiplexer-multiplexer 114-1, an erbium-doped fiber (rare earth-doped fiber amplifying section) 111, and a silica-based optical fiber 112.
(Optical fiber attenuator) and isolator 115-3 are arranged. The pump light source 113-1 is connected to the optical demultiplexer-multiplexer 114-1.

The excitation light source 113-1 is, for example, 1.
The pumping light in the 47 μm band (1.45 to 1.49 μm) is output, and as the optical demultiplexer-multiplexer 114-1,
For example, a fusion type is used. As described in the seventeenth embodiment, in the rare earth-doped fiber optical amplifier having a high gain, unnecessary oscillation may occur during optical amplification. If such unnecessary oscillation occurs, Rare earth doped fiber optical amplifiers operate unstable.

Therefore, also in the optical fiber amplifier shown in FIG. 56, as in the case of the optical fiber amplifier shown in FIG. 55, a silica-based silica as a loss medium is provided after the erbium-doped fiber 111 as the rare earth-doped fiber optical amplifier. By providing the optical fiber 112,
The unstable operation of the erbium-doped fiber 111 is suppressed. Also in FIG. 56, R1,
R2 and RA are reflectances, and A is a boundary.

As with the seventeenth embodiment described above, FIG.
In the optical fiber amplifier shown in FIG.
2 is provided after the erbium-doped fiber 111,
The silica-based optical fiber 112 is excited by using the residual pumping light from the erbium-doped fiber 111 to compensate for the loss of the silica-based optical fiber 112 (flattening the gain pit of the erbium-doped fiber 111 and erbium-doped fiber 111). At the same time that the compensation for the reduction of the gain of 111 is included),
The unstable operation of the erbium-doped fiber 111 is suppressed.

With such a configuration, in the optical fiber amplifier shown in FIG. 56, the pumping light from the pumping light source 113-1 is sent from the optical demultiplexer-multiplexer 114-1 to one end of the erbium-doped fiber 111 together with the signal light. It is incident. As a result, the erbium-doped phi 111 amplifies the signal light.

The residual pumping light generated at this time is used to pump the silica optical fiber 112 to perform Raman amplification in the same manner as the dispersion compensating fiber, and the Raman amplification is used to compensate the loss of the silica optical fiber 112. Do. As described above, in the optical fiber amplifier shown in FIG. 56, the pumping light source 113-1 in the 1.47 μm band is used.
By using the erbium-doped fiber 111
It is possible to excite both the silica-based optical fiber 112 and the silica-based optical fiber 112, thereby simplifying the optical fiber amplifier and improving the efficiency of pumping light power.

Further, in this optical fiber amplifier, unnecessary oscillation caused by the erbium-doped fiber 111 is also removed at the same time by the loss of the silica optical fiber 112, which results in wavelength multiplexing (W
It is possible to prevent an unnecessary oscillation operation in the rare earth-doped fiber optical amplifier adjusted for DM) and perform stable optical amplification.

The excitation light source 113-1 is 0.98 μm.
The silica-based optical fiber 112 does not perform Raman amplification when the excitation light of the silica-based optical fiber 1 is generated.
12 loss compensations are not performed.

Optical fibers such as silica optical fibers have reflections due to Rayleigh backscattering. The reflectance of this reflection depends on the length of the optical fiber, and as the length of the optical fiber increases, the reflectance of this reflection also increases.
Therefore, when the effect of the reflectance due to Rayleigh backscattering of the silica-based optical fiber itself is greater than the effect of reducing the reflectance due to the loss of the silica-based optical fiber as described above, an optical isolator is added to the silica-based optical fiber. It is possible to do it.

For example, in FIG. 56, this optical isolator can be provided between the erbium-doped fiber 111 and the silica-based optical fiber 112. In this way, when the influence of the reflectance due to Rayleigh backscattering appears. The reflectance can always be reduced. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. Can be configured to.

(17-2) Description of Second Modification of Seventeenth Embodiment

FIG. 57 is a block diagram showing a second modification of the seventeenth embodiment of the present invention. The optical fiber amplifier shown in FIG. 57 has an isolator 65-1 and an optical demultiplexer-multiplexer in order from the input side. 64-1, erbium-doped fiber (pre-stage optical amplifier configured as a rare earth-doped fiber amplifier) 61-1, dispersion compensation fiber 62 (optical fiber attenuator), erbium-doped fiber (rare earth-doped fiber amplifier configured A post-stage optical amplifier section 61-2, an optical demultiplexer-multiplexer 64-2, and an isolator 65-3 are arranged. Then, the pump light source 6 is added to the optical demultiplexer-multiplexer 64-1.
3-1 is connected to the optical demultiplexer-multiplexer 64-2.
The excitation light source 63-2 is connected to the.

Here, the excitation light sources 63-1 and 63-2 are
Both are, for example, 1.47 μm band (1.45 to 1.49 μ
It produces the excitation light of m). As described in the seventeenth embodiment, in the rare earth-doped fiber optical amplifier having a high gain, unnecessary oscillation may occur during optical amplification. If such unnecessary oscillation occurs, The rare earth-doped fiber optical amplifier operates unstable.

In the optical fiber amplifier according to the seventeenth embodiment shown in FIG. 55, the dispersion compensating fiber 62 as a loss medium is provided at the subsequent stage of the erbium-doped fiber 61 as the rare earth-doped fiber optical amplifier, so that the erbium-doped fiber is provided. The unstable operation of the fiber 61 is suppressed. However, when the gain G of the erbium-doped fiber 61 is very large, the GR parameter constituted by the reflectance R1, the gain G and the reflectance RA becomes large (the erbium-doped fiber 6
Since the gain G of 1 is very large, the influence of the reflectance RA cannot be ignored even though RA << R1 and R2). Even if the dispersion compensating fiber 62 is provided in the subsequent stage,
The effect of the loss η does not appear, and unstable operation of the erbium-doped fiber 61 may not be suppressed.

Therefore, even in such a case, in order to suppress the unstable operation of the erbium-doped fiber 61, the erbium-doped fiber 61 is divided into erbium-doped fibers in the front and rear stages, and the dispersion compensating fiber 62 is provided between them. What is provided is the optical fiber amplifier shown in FIG. The principle of suppressing the unstable operation at this time will be described with reference to FIG.

As shown in FIG. 57, a dispersion compensating fiber 62 [is provided between the erbium-doped fibers 61-1 and 61-2 (the gains of the erbium-doped fibers 61-1 and 61-2 are respectively set to G / 2). This dispersion compensating fiber 6
2 is assumed to be η (0 ≦ η ≦ 1)] by, for example, fusion splicing, a boundary A ′ is generated between the erbium-doped fiber 61-1 and the dispersion compensating fiber 62, and dispersion is caused. A boundary B ′ occurs between the compensation fiber 62 and the erbium-doped fiber 61-2.

Also, the erbium-doped fiber 61-1
Of the erbium-doped fiber 61-2 at the front end is R1 ', the reflectance at the rear end of the erbium-doped fiber 61-2 is R2', the reflectance at the boundary A'is RA '(RA'<< R1 ', R2'), and the boundary B is The reflectance at ′ is RB ′ (RB ′ << R1 ′, R2 ′). Here, the reflectance R1 'is the erbium-doped fiber 61-
1 is the reflectance of the reflection from all the components in the front stage from the front end, and the reflectance R2 'is the erbium-doped fiber 61.
-2 is the reflectance of the reflection from all the components located after the rear end. The reflectance RA 'is the reflectance of reflection caused by the difference in refractive index at the boundary A', and the reflectance RB 'is the reflectance of reflection caused by the difference in refractive index at the boundary B'.

GR parameters considered at this time include (1) reflectance R1 ′ and erbium-doped fiber 6
G having a gain G / 2 of 1-1 and a reflectance RA '
R parameter, (2) reflectance R1 ′, gain G / 2, loss η and reflectance R of the erbium-doped fiber 61-1
GR parameter composed of B ′, (3) reflectance R
1 ', gain G / of the erbium-doped fiber 61-1
2, loss η, GR parameter composed of gain G / 2 of erbium-doped fiber 61-2 and reflectance R2 ′, (4) reflectance RA ′, loss η, gain G / 2 of erbium-doped fiber 61-2 And a reflectance parameter R2 ', (5) reflectance RB', gain G / 2 and reflectance R of the erbium-doped fiber 61-2.
There is a GR parameter composed of 2 '.

Here, regarding (1), in the erbium-doped fiber 61 shown in FIG. 55, since the gain is G, GR = G (R1RA) ^1/2 , whereas
Since the gain of the erbium-doped fiber 61-1 shown in FIG. 57 is G / 2 and half the gain G of the erbium-doped fiber 61 shown in FIG. 55, GR = (G /
2) (R1RA) ^1/2 becomes (RA '= RA), and FIG.
It is half the GR of the erbium-doped fiber 61 shown in FIG.

Looking at (2), in the erbium-doped fiber 61-1, the loss η (0 ≦ η
.Ltoreq.1), the net gain when the light makes one round is [R1 '* (G / 2), as in the seventeenth embodiment.
× η] × [RB ′ × η × (G / 2)] = [(G / 2)
Since η] ² R1′RB ′, the net gain in one way is (G / 2) η (R1′RB ′) ^1/2 . Here, since 0 ≦ η ≦ 1 and RB ′ = RB, GR is equivalently reduced. Further, since RA'≈RB ', the GR parameter becomes smaller than in the case of (1), and the GR at this time can be ignored.

Looking at (3), the loss η (0 ≦ η is measured between the erbium-doped fibers 61-1 and 61-2.
.Ltoreq.1), the net gain when the light makes one round is [R1 '* (G / 2), as in the seventeenth embodiment.
× η] × [R2 ′ × η × (G / 2)] = [(G / 2)
η] ² R1′R2 ′, so the net gain in one way is (G / 2) η (R1′R2 ′) ^1/2 = [(G / 2)
η] R, and erbium-doped fibers 61-1 and 6
The parameter indicating the stability of the operation of 1-2 is (G / 2)
From R becomes [(G / 2) η] R. RA '<< R
Since 1 ', R2' and RB '<<R1', R2 ', the influence of the reflectance RA' and the reflectance RB 'can be ignored. Here, since 0 ≦ η ≦ 1, GR is equivalently reduced.

Note that (4) and (5) are the same as in (2) and (1), respectively. Therefore, when the gain G of the erbium-doped fiber 61 shown in FIG.
Since the R parameter is large, the erbium-doped fiber 61 operates in an unstable manner. However, the erbium-doped fiber 61 is divided into erbium-doped fibers 61-1 and 61-2 at the front and rear stages as shown in FIG.
By arranging the dispersion compensating fiber 62 as a loss medium between them, GR (1) and (5) can be obtained.
The parameter can be reduced, and thus the unstable operation of the erbium-doped fibers 61-1 and 61-2 can be suppressed.

Therefore, in the optical fiber amplifier shown in FIG. 57, the dispersion compensating fiber 62 is arranged between the erbium-doped fibers 61-1 and 61-2, and the dispersion compensating fiber 62 is connected to the erbium-doped fiber 61-. 1,6
By exciting with the residual excitation light from 1-2,
When the loss compensation of the dispersion compensating fiber 62 (including the compensation of the flatness of the dent of the gain of the erbium-doped fibers 61-1 and 61-2 and the reduction of the gain of the erbium-doped fibers 61-1 and 61-2) is performed. At the same time, due to the remaining loss, the erbium-doped fibers 61-1 and 6-1
The unstable operation of 1-2 is suppressed.

With such a configuration, in the optical fiber amplifier shown in FIG. 57, the pumping light and the signal light are erbium-doped fiber 61-1 by the optical demultiplexer-multiplexer 64-1.
Incident from the input end of the erbium-doped fiber 61
-1 is excited and the signal light is amplified, but at this time, the residual pump light reaches the other end of the erbium-doped fiber 61-1. The residual pumping light is supplied to the dispersion compensating fiber 62 to cause Raman amplification.

[0290] Further, the pumping light is input from the output end of the erbium-doped fiber 61-2 by the optical demultiplexer-multiplexer 64-2 to pump the erbium-doped fiber 61-2,
The signal light input from the input end of the erbium-doped fiber 61-2 is also amplified, but at this time, the residual pump light also reaches the input end of the erbium-doped fiber 61-2. This residual pumping light is also supplied to the dispersion compensating fiber 62 to cause Raman amplification.

In this case, since the dispersion compensating fiber 62 is Raman-amplified by using the residual pumping light from the front and rear erbium-doped fibers 61-1 and 61-2, the compensation effect by the dispersion compensating fiber 62 should be increased. While simplifying the structure and reducing the cost,
A broadband optical amplifier can be realized. Further, in this optical fiber amplifier, due to the loss of the dispersion compensating fiber 62, the erbium-doped fibers 61-1 and 61-2 are used.
Unnecessary oscillation caused by is also performed at the same time, which prevents unnecessary oscillation operation in a rare earth-doped fiber optical amplifier adjusted for wavelength division multiplexing (WDM) and stabilizes in a state with less noise. Optical amplification can be performed.

The excitation light sources 63-1 and 63-2 are 0.
When the pumping light of 98 μm is generated, the dispersion compensating fiber 6
No. 2 does not perform Raman amplification, and therefore the loss compensation of the dispersion compensating fiber 62 is not performed. Further, in optical fibers such as dispersion compensating fibers, there is reflection due to Rayleigh backscattering. The reflectance of this reflection depends on the length of the optical fiber, and as the length of the optical fiber increases, the reflectance of this reflection also increases.

Therefore, when the effect of the reflectance due to Rayleigh backscattering of the dispersion compensating fiber itself is greater than the effect of reducing the reflectance due to the loss of the dispersion compensating fiber as described above, an optical isolator is added to the dispersion compensating fiber. It is possible to do it. For example, in FIG. 57, this optical isolator can be provided between the erbium-doped fiber 61-1 and the dispersion compensating fiber 62, and in this case, when the influence of the reflectance due to Rayleigh backscattering appears, The reflectance can be reduced.

Further, in this case as well, instead of providing an isolator in the input section or the input / output section, the configuration shown in FIG.
As shown in 0, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator.

An excitation light source and an optical demultiplexer-multiplexer for the dispersion compensating fiber 62 may be provided. That is, FIG.
An optical fiber amplifier may be configured using the pumping light sources 133-1 to 133-3 and the optical demultiplexer-multiplexers 134-1 to 134-3 in the same manner as. Further, instead of the dispersion compensating fiber 62, a silica optical fiber may be used.

(17-3) Description of Third Modification of Seventeenth Embodiment

FIG. 58 is a block diagram showing a third modification of the seventeenth embodiment of the present invention. The optical fiber amplifier shown in FIG. 58 has isolator 125-1 and isolator 125-1 in order from the input side.
Optical demultiplexer-multiplexer 124-1, erbium-doped fiber (pre-stage optical amplifier configured as a rare-earth-doped fiber amplifier) 121-1, silica-based optical fiber 122 (optical fiber attenuator), erbium-doped fiber (rare-earth-doped) (Post-stage optical amplification section configured as a fiber amplification section) 12
1-2, optical demultiplexer-multiplexer 124-3, isolator 125
-3 is provided. Then, the optical demultiplexer-multiplexer 124-
1,124-3, for example, 1.47μ band (1.45-
Pumping light source 123-1 for generating pumping light of 1.49 μm),
123-3 is connected.

As described in the seventeenth embodiment above, in the rare earth-doped fiber optical amplifier having a high gain,
Unwanted oscillations may occur during optical amplification, and when such unwanted oscillations occur, the rare earth-doped fiber optical amplifier operates in an unstable manner. In the optical fiber amplifier shown in FIG. 56, an unstable operation of the erbium-doped fiber 111 is suppressed by providing a silica-based optical fiber 122 as a loss medium after the erbium-doped fiber 111 as the rare-earth-doped fiber optical amplifier. It is supposed to do.

However, the erbium-doped fiber 11
When the gain G of 1 is very large, the GR parameter becomes large as in the case of the optical fiber amplifier shown in FIG. 55. Therefore, even if the silica optical fiber 122 is provided at the subsequent stage, the effect of the loss η appears. Therefore, the unstable operation of the erbium-doped fiber 111 cannot be suppressed. Therefore, even in such a case, the erbium-doped fiber 1
In order to suppress the unstable operation of the optical fiber 11, the erbium-doped fiber 111 is divided into front and rear erbium-doped fibers, and the silica-based optical fiber 1 is provided between them.
The optical fiber amplifier shown in FIG. 58 is provided with 22. The principle of suppressing unstable operation at this time is as follows.
This is similar to that described in the second modified example of the seventeenth embodiment, and in FIG. 58 also, R1 ′, R2 ′, RA ′, R.
B'denotes reflectance and A ', B'denotes boundaries.

Therefore, in the optical fiber amplifier shown in FIG. 58, the silica-based optical fiber 122 is provided in the middle stage, and the silica-based optical fiber 122 receives residual pumping light from the erbium-doped fibers 121-1 and 121-2. By exciting the silica-based optical fiber 122, the loss compensation (erbium-doped fibers 121-1 and 121-1) is performed.
-2 in the gain cavities of the erbium-doped fibers 121-1 and 121-2, and the compensation of the decrease in the gains of the erbium-doped fibers 121-1 and 121-2). The unstable operation of No. 2 is suppressed.

With such a configuration, in the optical fiber amplifier shown in FIG. 58, the pumping light and the signal light are sent to the erbium-doped fiber 121 by the optical demultiplexer-multiplexer 124-1.
-1 from one end of the erbium-doped fiber 1
21-1 is excited to amplify the signal light, and the residual pump light generated at this time causes the silica-based optical fiber 1
22 is excited, and Raman amplification is performed similarly to the dispersion compensating fiber.

Further, the pumping light is combined with the optical demultiplexer-multiplexer 124-3.
Is incident from the output end of the erbium-doped fiber 121-2 to excite the erbium-doped fiber 121-2, amplify the signal light input from the input end of the erbium-doped fiber 121-2, and cause residual excitation at this time. The light excites the silica-based optical fiber 122 for Raman amplification.

As described above, in the optical fiber amplifier shown in FIG. 58, 1.47 μm band pumping light sources 123-1 and 1
By using 23-3, the erbium-doped fibers 121-1 and 121-2 and the silica-based optical fiber 12
Any of the two can be excited, which results in
Pumping light source 123-2 in the optical fiber amplifier shown in FIG.
Can be reduced, and the optical fiber amplifier can be simplified and the pumping light power can be made more efficient.

Also, in this optical fiber amplifier, unnecessary oscillation caused by the erbium-doped fibers 121-1 and 121-2 is simultaneously removed by the loss of the silica optical fiber 122. It is possible to prevent unnecessary oscillation operation in a rare earth-doped fiber optical amplifier adjusted for wavelength division multiplexing (WDM), and perform stable optical amplification in a state with less noise.

When the pumping light sources 123-1 and 123-3 generate 0.98 μm pumping light, the silica-based optical fiber 122 does not perform Raman amplification, and therefore the loss compensation of the silica-based optical fiber 122 is performed. I can't.

Optical fibers such as silica optical fibers have reflections due to Rayleigh backscattering. The reflectance of this reflection depends on the length of the optical fiber, and as the length of the optical fiber increases, the reflectance of this reflection also increases.
Therefore, when the effect of the reflectance due to Rayleigh backscattering of the silica-based optical fiber itself is greater than the effect of reducing the reflectance due to the loss of the silica-based optical fiber as described above, an optical isolator is added to the silica-based optical fiber. It is possible to do it.

For example, in FIG. 58, this optical isolator can be provided between the erbium-doped fiber 121-1 and the silica-based optical fiber 122. In this way, the influence of the reflectance due to Rayleigh backscattering appears. In this case, the reflectance can always be reduced. Also in this case, instead of providing an isolator in the input section or the input / output section, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. Can be configured to.

Further, an isolator may be provided between the silica optical fiber 122 and the erbium-doped fiber 121-2.

Further, an excitation light source and an optical demultiplexer-multiplexer for the silica optical fiber 122 can be provided. That is, in the same manner as in FIG. 12, the excitation light sources 123-1 to 123-3
Alternatively, the optical fiber amplifier may be configured by using the optical demultiplexer-multiplexers 124-1 to 124-3. Further, a dispersion compensating fiber may be used instead of the silica-based optical fiber 122.

[0310]

As described in detail above, according to the optical fiber amplifier of the present invention, in the rare-earth-doped fiber optical amplifier, residual pumping is made incident from one end of the rare-earth-doped fiber through the first optical coupler and reaches the other end. When the light is separated by the second optical coupler and the pumping light is reflected by the reflecting mirror to reciprocate in the rare earth-doped fiber, it is excited by the interference of the residual pumping light returned to the rare earth-doped fiber. In order to prevent unstable operation of the light source, an optical isolator is provided between the pumping light source and the first optical coupler, so that the pumping light power can be controlled while ensuring stable operation of the pumping light source. There is an advantage that an optical fiber amplifier used with high efficiency can be provided.

Further, in the optical fiber amplifier of the present invention, an optical circulator having three or more ports is used in an optical amplifier having two stages of rare earth-doped fibers.
First, after passing the pumping light through this optical circulator, the first optical coupler separates the residual pumping light that has entered the other end of the rare-earth-doped fiber in the front stage or the rear stage and reached the other end by the second optical coupler. And, after the excitation light is reflected by a reflecting mirror and reciprocated in the rare earth-doped fiber, the optical path is changed by an optical circulator and the third optical coupler combines the excitation light with the other rare earth-doped fiber. Therefore, there is an advantage that it is possible to provide an optical fiber amplifier of a two-stage configuration that uses pumping light power with high efficiency.

Furthermore, in the optical fiber amplifier of the present invention,
In a rare-earth-doped fiber optical amplifier having a two-stage configuration, the pumping light power is branched to n: 1, the pumping light of one port thereof is multiplexed by the first optical coupler, and the incident pumping light is made incident on the rare-earth-doped fiber at the front stage or the rear stage. Then, the residual pumping power is taken out by the second optical coupler connected to the other end of this rare earth-doped fiber, this residual pumping power is combined by the third optical coupler, and is incident from one end of the other rare earth-doped fiber. , And the pump power of the other branched port
Since the optical coupler is configured to multiplex from the other end of the other rare earth-doped fiber, there is an advantage that it is possible to provide a two-stage optical fiber amplifier that uses pumping light power with high efficiency.

Further, it is possible to provide a common pumping light source for both the rare earth-doped fibers, which can reduce the number of pumping light sources to be used and contribute to simplification of the construction and cost reduction.

Furthermore, in the optical fiber amplifier of the present invention,
In a rare earth-doped fiber optical amplifier, an excitation light is first passed through this optical circulator using an optical circulator having three or more ports, and then the first optical coupler causes one end of the rare earth-doped fiber to enter and reach the other end. The residual pumping light thus separated is separated by the second optical coupler, and the pumping light is reflected by the reflecting mirror to reciprocate in the rare earth-doped fiber, and then the residual pumping light power separated by the first optical coupler is , The optical path is changed by the optical circulator, the light is extracted and monitored, and this residual pumping light power is kept constant, so that the wavelength characteristics of the gain can be prevented from fluctuating with respect to changes in the input level, and a multi-wavelength collective amplifier can be realized. There are advantages that can contribute to the canopy.

Further, when the Faraday rotation reflecting mirror is used as the reflecting mirror, the polarization of the pumping light can be intentionally rotated, whereby the optical isolator provided between the pumping light source and the first optical coupler. It is possible to provide an optical fiber amplifier in which PHB is reduced while omitting the above and simplifying the configuration.
Further, an optical circulator can be provided in the input / output section, which can reduce the number of isolators to be used and contribute to cost reduction.

Also, an isolator can be provided at a required position of the optical fiber amplifier, and by doing so, light interference can be prevented. Further, the optical fiber amplifier is a rare earth-doped fiber optical amplification section made of a rare earth-doped fiber, and a Raman optical amplification section that causes Raman amplification by being excited by a desired pump light (the Raman optical amplification section is a dispersion compensation fiber or (Since it is composed of a silica-based optical fiber. The same applies hereinafter), it is possible to provide an optical fiber amplifier of a two-stage structure in which pumping light power is used with high efficiency. is there.

Further, since the common pumping light source for supplying the pumping light for pumping the rare earth-doped fiber optical amplifier and the Raman optical amplifier is provided, the pumping light power can be used with high efficiency. At the same time, the number of pumping light sources used can be reduced, which can contribute to simplification of the configuration and cost reduction. Further, according to the optical fiber amplifier of the present invention, the erbium-doped fiber (the erbium-doped fiber may have a low noise figure; the same applies hereinafter) and the dispersion compensating fiber or the silica-based light arranged in two stages before and after. A first pumping light source having a fiber for generating pumping light in a first wavelength band for an erbium-doped fiber, and a second pumping light for generating a pumping light in a second wavelength band for a dispersion compensating fiber or a silica optical fiber Since the Raman amplification is produced by pumping the dispersion compensating fiber or the silica optical fiber with the pumping light of the second wavelength band from the second pumping light source, the light by the erbium-doped fiber is provided. It is possible to perform loss compensation of the dispersion compensating fiber or the silica optical fiber by Raman amplification while performing the amplification.

Further, the rare earth-doped fiber is composed of an erbium-doped fiber, and the wavelength band of the pumping light generated by the first pumping light source is 0.98 μm band,
The wavelength band of the excitation light generated by the second excitation light source is 1.47 μm
Due to the band, it is possible to perform the loss compensation of the dispersion compensating fiber or the silica optical fiber by Raman amplification while performing the optical amplification by the erbium-doped fiber more effectively.

Further, it is possible to provide a common pumping light source for the rare-earth-doped fiber and the dispersion compensating fiber or the silica optical fiber. In this case, the number of pumping light sources to be used can be reduced to simplify the structure and reduce the cost. It can contribute to cost reduction.

Furthermore, in the optical fiber amplifier of the present invention,
Raman amplification of the dispersion compensating fiber or 1.47 of the dispersion compensating fiber is performed by using the residual pumping power generated when the erbium-doped fiber is pumped in the 1.47 μm band.
Since the erbium-doped fiber is pumped using the residual pumping power generated when pumping in the μm band, the loss of the dispersion compensating fiber can be reduced. Furthermore, the dispersion compensating fiber is ~ 1.44μ.
Raman amplification in the m or 1.47 μm band has an advantage that the amplification band of the erbium-doped fiber can be supplemented and further broadening or flattening can be promoted.

In the optical fiber amplifier of the present invention, the dispersion compensating fiber doped with a rare earth element is used to perform the dispersion compensation and at the same time reduce the loss of the dispersion compensating fiber and to sufficiently suppress the signal light. Further, there is an advantage that an optical fiber amplifier with a dispersion compensation function capable of optical amplification can be provided. Furthermore, in the optical fiber amplifier of the present invention,
An optical filter is provided to prevent the pumping light in the 1.47 μm band from being input to the dispersion compensating fiber.
Raman amplification of the leakage pumping light power in the μm band in the dispersion compensating fiber can prevent the optical fiber amplifier from unstable operation or changing the wavelength dependence of the amplification band.

In the optical fiber amplifier of the present invention, the first erbium-doped fiber having a low noise figure is provided in the front stage, the dispersion compensating fiber or the silica-based optical fiber is provided in the middle stage, and the second erbium-doped fiber is provided in the latter stage. Therefore, the dispersion-compensating fiber or the silica-based optical fiber is Raman-amplified using the residual pumping light from the first and second erbium-doped fibers located before and after the dispersion-compensating fiber or the silica-based optical fiber. The compensation effect of the compensation fiber or the silica-based optical fiber can be increased, and the broadband optical amplifier can be realized while simplifying the structure and reducing the cost.

Further, the pumping light source for pumping the Raman amplifying section may be composed of two pumping light sources and a polarization combiner for combining orthogonal polarizations of the pumping lights from these pumping light sources. By doing so, Raman amplification can be effectively generated in the dispersion compensating fiber, and loss compensation of the dispersion compensating fiber can be performed by this Raman amplification.

Further, the pumping light source for pumping the Raman amplifying unit may be configured to combine the pumping light source and the depolarizer to depolarize the pumping light. By doing so, dispersion compensation is performed. It is possible to reduce the polarization dependence in the Raman optical amplifier made of fiber. further,
It may be configured to generate a pumping light whose spectrum is broadened to several hundreds kHz or more by modulating light from a pumping light source for pumping the Raman amplifying part. In this case, stimulated Brillouin scattering Raman amplification can be effectively generated in the dispersion compensating fiber while increasing the threshold value and suppressing harmful nonlinear effects, and loss compensation of the dispersion compensating fiber can be performed by this Raman amplification.

Further, according to the present invention, since the optical fiber amplifier is constructed by using the module for exciting the dispersion compensating fiber to generate Raman amplification, there is an advantage that the loss of the dispersion compensating fiber can be reduced. .

Also in this case, if an optical circulator is provided in the input / output section, the number of isolators used can be reduced and the cost can be reduced.
Further, the optical fiber amplifier of the present invention comprises a rare earth-doped fiber optical amplification section made of a rare earth-doped fiber and an optical fiber to which an optical fiber or an optical isolator is added to suppress unstable operation of the rare earth-doped fiber optical amplification section. Since the optical fiber attenuator is provided, it is possible to suppress unstable operation of the rare earth-doped fiber optical amplifier and perform highly accurate optical amplification.

Furthermore, in the optical fiber amplifier of the present invention,
An optical amplification unit having a pre-stage optical amplification unit and a post-stage optical amplification unit each configured as a rare earth-doped fiber optical amplification unit made of a rare earth-doped fiber, and an optical amplification unit in the optical amplification unit. Was set up,
Since it has an optical fiber attenuator made of an optical fiber or an optical fiber to which an optical isolator is added to suppress the unstable operation of the optical amplifying unit, the unstable operation of the optical amplifying unit is suppressed and the accuracy is high. Optical amplification can be performed.

Further, if the optical fiber attenuating section also serves as the Raman optical amplifying section which causes Raman amplification by being excited by the desired pumping light, the loss compensation of the optical fiber attenuating section can be performed. .

[Brief description of the drawings]

FIG. 1 is a principle block diagram showing a first aspect of the present invention.

FIG. 2 is a principle block diagram showing a second aspect of the present invention.

FIG. 3 is a principle block diagram showing a third aspect of the present invention.

FIG. 4 is a principle block diagram showing a fourth aspect of the present invention.

FIG. 5 is a principle block diagram showing a fifth aspect of the present invention.

FIG. 6 is a principle block diagram showing a sixth aspect of the present invention.

FIG. 7 is a principle block diagram showing a seventh aspect of the present invention.

FIG. 8 is a principle block diagram showing an eighth aspect of the present invention.

FIG. 9 is a principle block diagram showing a ninth aspect of the present invention.

10A is a principle block diagram showing a tenth aspect of the present invention, and FIG. 10B is a principle block diagram showing an eleventh aspect of the present invention.

FIG. 11 is a principle block diagram showing a twelfth aspect of the present invention.

FIG. 12 is a principle block diagram showing a thirteenth aspect of the present invention.

FIG. 13A is a principle block diagram showing a fourteenth aspect of the present invention, and FIG. 13B is a principle block diagram showing a fifteenth aspect of the present invention.

FIG. 14 is a principle block diagram showing a sixteenth aspect of the present invention.

FIG. 15 is a principle block diagram showing a seventeenth aspect of the present invention.

FIG. 16 is a block diagram showing a first embodiment of the present invention.

FIG. 17 is a block diagram showing a first modification of the first embodiment of the present invention.

FIG. 18 is a block diagram showing a second modification of the first embodiment of the present invention.

FIG. 19 is a block diagram showing a third modification of the first embodiment of the present invention.

FIG. 20 is a block diagram showing a fourth modified example of the first embodiment of the present invention.

FIG. 21 is an electric circuit diagram showing a constant light output control system.

FIG. 22 is a diagram for explaining the operation of the constant light output control system.

FIG. 23 is a block diagram showing a second embodiment of the present invention.

FIG. 24 is a block diagram showing a modified example of the second embodiment of the present invention.

FIG. 25 is a block diagram showing a third embodiment of the present invention.

FIG. 26 is a block diagram showing a fourth embodiment of the present invention.

FIG. 27 is a block diagram showing a fifth embodiment of the present invention.

FIG. 28 is an electric circuit diagram showing a pumping light output constant control system.

FIG. 29 is a diagram for explaining the operation of the pumping light output constant control system.

FIG. 30 is a block diagram showing a first modified example of the fifth embodiment of the present invention.

FIG. 31 is a block diagram showing a second modification of the fifth embodiment of the present invention.

FIG. 32 is a block diagram showing a sixth embodiment of the present invention.

FIG. 33 is a block diagram showing a modification of the sixth embodiment of the present invention.

FIG. 34 is a block diagram showing a seventh embodiment of the present invention.

FIG. 35 is a block diagram showing an eighth embodiment of the present invention.

FIG. 36 is a block diagram showing a first modified example of the eighth embodiment of the present invention.

FIG. 37 is a block diagram showing a second modified example of the eighth embodiment of the present invention.

FIG. 38 is a block diagram showing a ninth embodiment of the present invention.

FIG. 39 is a block diagram showing a tenth embodiment of the invention.

FIG. 40 is a block diagram showing an eleventh embodiment of the invention.

FIG. 41 is a block diagram showing a twelfth embodiment of the present invention.

FIG. 42 is a block diagram showing a thirteenth embodiment of the present invention.

FIG. 43 is a block diagram showing a fourteenth embodiment of the present invention.

FIG. 44 is a block diagram showing a first modified example of the fourteenth embodiment of the present invention.

FIG. 45 is a block diagram showing a second modified example of the fourteenth embodiment of the present invention.

FIG. 46 is a diagram illustrating wavelength characteristics of an optical fiber amplifier.

FIG. 47 is a diagram illustrating wavelength characteristics of an optical fiber amplifier.

FIG. 48 is a block diagram showing a fifteenth embodiment of the present invention.

FIG. 49 is a block diagram showing a modified example of the fifteenth embodiment of the present invention.

FIG. 50 is a block diagram showing a sixteenth embodiment of the present invention.

FIG. 51 is a block diagram showing a first modified example of the sixteenth embodiment of the present invention.

FIG. 52 is a block diagram showing a second modification of the sixteenth embodiment of the present invention.

FIG. 53 is a diagram showing a configuration of an optical circulator.

FIG. 54 is a diagram showing a configuration of an isolator.

FIG. 55 is a block diagram showing a seventeenth embodiment of the present invention.

FIG. 56 is a block diagram showing a first modified example of the seventeenth embodiment of the present invention.

FIG. 57 is a block diagram showing a second modified example of the seventeenth embodiment of the present invention.

FIG. 58 is a block diagram showing a third modified example of the seventeenth embodiment of the present invention.

[Explanation of symbols]

1,1-1,1-2 Erbium-doped fiber (rare earth-doped fiber) 2,2-1,2-2 Excitation light source 3-1 to 3-5 Optical demultiplexer-multiplexer (optical coupler) 4 Reflector (reflection) Means) 5,5-1 to 5-4 Isolator (optical isolator) 11-1, 11-2 Erbium-doped fiber (rare earth-doped fiber) 12, 12-1, 12-2 Pumping light source 13-1 to 13-4, 13-1 ', 13-2' Optical demultiplexer-multiplexer (optical coupler) 13-5 Coupler 14 Reflector 15, 15-2 Optical circulator 16-1 to 16-3 Isolator 17, 17-2 Filter 18 Output light Detector 18A Photodiode 19 Constant optical output controller 19A Differential amplifier 21-1, 21-2 Erbium-doped fiber (rare earth-doped fiber) 22 Excitation light source 23 Optical branching section 24-1 -24-4, 24-1 'to 24-4' Optical demultiplexer-multiplexer (optical coupler) 25-1 to 25-4 Isolator 26 Optical filter 31, 31-1, 31-2 Erbium-doped fiber (rare earth doped) Fiber) 32 pumping light source 33, 33 'optical circulator 34-1, 34-2, 34-1', 34-2 ', 34-
1 ″, 34-2 ″ Optical demultiplexer-multiplexer (optical coupler) 35, 35 ′, 35 ″ Reflector 36 Residual excitation photodetector 36A Photodiode 37 Controller 37A Differential amplifier 38 Optical circulator 39- 1 to 39-3 Isolator 40 Optical Filter 51 Erbium Doped Fiber 52 Dispersion Compensating Fiber 53-1, 53-2, 53-2A, 53-2B, 53-
2 ′, 53-2A ′, 53-2 ″, 53-2A ″,
53-2B ″ Excitation light source 53-2B ′ Depolarizer 53-2C, 53-2C ′, 53-2C ″ Polarization combiner 53-2D ″ Modulator 53-2E ′ Polarization maintaining coupler 54-1, 54 -2 optical demultiplexer-multiplexer (optical coupler) 55-1 to 55-3 isolator 61,61-1, 61-2 erbium-doped fiber 62 dispersion compensation fiber 63,63-1,63-2 pump light source 64,64 -1 to 64-5 Optical demultiplexer-multiplexer (optical coupler) 65-1 to 65-4 Isolator 66 Optical filter 71 Erbium-doped fiber 72 Dispersion compensation fiber 73 Pumping light source 74 Optical demultiplexer-multiplexer (optical coupler) 75 -1,75-2 Isolator 81 Rare earth doped dispersion compensating fiber 82 Excitation light source 83 Optical demultiplexer-multiplexer (optical coupler) 84-1-84-2 Isolator 91 Erbi Mu-doped fiber 92 Dispersion compensation fiber 93 Excitation light source 94 Optical demultiplexer-multiplexer (optical coupler) 95 Optical filter 96-1, 96-2 Isolator 101 Silica-based optical fiber 102 Erbium-doped fiber 103-1, 103-2 Excitation light source 104-1, 104-2 Optical demultiplexer-multiplexer (optical coupler) 111 Erbium-doped fiber 112 Silica-based optical fiber 113-1, 113-2, 113-2A, 113-2
B, 113-2 ', 113-2A', 113-2 ",
113-2A ″, 113-2B ″ Excitation light source 113-2B ′ Depolarizer 113-2C, 113-2C ′, 113-2C ″
Polarization combiner 113-2D "Modulator 113-2E 'Polarization maintaining coupler 114-1, 114-2 Optical demultiplexer-multiplexer (optical coupler) 115-1 to 115-3 Isolator 121-1, 121- 2 Erbium-doped fiber 122 Silica-based optical fiber 123-1 to 123-3, 123-1 ', 123-1
A ', 123-1B', 123-3 ', 123-3
A ', 123-3B' Excitation light source 123-1C ', 123-3C' Polarization combiner 124-1 to 124-3 Optical demultiplexer-multiplexer (optical coupler) 125-1 to 125-3 Isolator 126 Optical filter 131-1, 131-2 Erbium-doped fiber 132 Dispersion compensating fiber 133-1 to 133-3 Pumping light source 134-1 to 134-3 Optical demultiplexer-multiplexer (optical coupler) 141 Dispersion compensating fiber 142, 142A, 142B Pumping Light source 142C Polarization combiner 143 Optical demultiplexer-multiplexer (optical coupler) 144, 144-1, 144-2 Isolator 151 Silica optical fiber 152 Excitation light source 153 Optical demultiplexer-multiplexer (optical coupler) 154 Rare earth doped fiber Optical amplifier 155 Optical fiber attenuator 156-1 Pre-stage optical amplifier 156-2 Post-stage optical amplifier 157 Optical fiber Damping part

Claims (47)

[Claims] 1. An optical fiber amplifier having a rare earth-doped fiber, wherein a first means for injecting pumping light from one end of the rare earth-doped fiber by a first optical coupler, and one end of the rare earth-doped fiber by the first means. As a result of injecting the excitation light from the other end, the residual excitation light that has reached the other end of the rare earth-doped fiber is separated by the second optical coupler, and the residual excitation light is reflected by the reflecting means and returned into the rare earth-doped fiber. The second means, and the pumping light source for the pumping light incident on the rare earth-doped fiber by the first means, due to the residual pumping light returned into the rare earth-doped fiber by the second means, has an unstable operation. An optical fiber amplifier, characterized in that it is provided with a third means for preventing the above by using an optical isolation means. 2. The optical fiber amplifier according to claim 1, wherein said reflecting means is constructed as a Faraday rotation reflecting mirror. 3. An optical fiber amplifier having a rare earth-doped fiber, a pumping light source, a first optical coupler which receives pumping light from the pumping light source from one end of the rare earth-doped fiber, and the first optical coupler. A second optical coupler that separates the residual excitation light that has reached the other end of the rare earth-doped fiber as a result of the excitation light being incident from one end of the rare earth-doped fiber, and the residual excitation light that has been separated by the second optical coupler is reflected. Then, the pumping light source is prevented from performing an unstable operation due to the interference between the reflecting mirror that returns to the inside of the rare earth-doped fiber through the second optical coupler and the residual pumping light that returns to the inside of the rare earth-doped fiber. Therefore, the optical fiber amplifier is configured to include an optical isolator provided between the pumping light source and the first optical coupler. 4. An optical fiber amplifier comprising a first rare-earth-doped fiber and a second rare-earth-doped fiber arranged in front and rear two stages, wherein the first optical coupler is connected to the first optical coupler via an optical circulator having three or more pump lights. Of the first rare earth-doped fiber and the second rare earth-doped fiber, the first means for injecting the light from one end of the rare earth-doped fiber, and the first means for injecting the excitation light from one end of the one rare earth-doped fiber. As a result, the residual pumping light that has reached the other end of the one rare earth-doped fiber is separated by the second optical coupler, and the residual pumping light is reflected by the reflecting means to be returned into the one rare earth-doped fiber. Means for returning the residual pumping light reflected from the reflecting means into the one rare earth-doped fiber by the second means, And a third means for multiplexing the residual pumping light on the other rare earth-doped fiber of the first rare earth-doped fiber and the second rare earth-doped fiber by the third optical coupler by changing the optical path by the curator. An optical fiber amplifier characterized in that 5. An optical fiber amplifier comprising a first rare-earth-doped fiber and a second rare-earth-doped fiber arranged in front and rear two stages, wherein a pumping light source and the first rare-earth-doped fiber and the second rare-earth-doped fiber are provided. A first optical coupler provided at one end of one of the rare earth-doped fibers, a second optical coupler provided at the other end of the one rare earth-doped fiber, and a first rare earth-doped fiber and a second rare earth-doped fiber A third optical coupler provided at one end of the other rare earth-doped fiber of the fiber, and the residual pumping light separated by the second optical coupler are reflected, and the one rare earth-doped fiber is again passed through the second optical coupler. A reflecting mirror returning to the inside of the fiber, and an optical circulator having three or more ports connected to the pumping light source, the first optical coupler and the third optical coupler. The excitation light from the excitation light source from the first optical coupler through the optical circulator from one end of the one rare earth-doped fiber, and then to the other end of the one rare earth-doped fiber. The residual pumping light that has arrived is separated by the second optical coupler, the residual pumping light is reflected by the reflecting mirror and returned into the rare earth-doped fiber, and the optical path is changed by the optical circulator. An optical fiber amplifier, characterized in that the residual pumping light is combined with the other rare earth-doped fiber by a three-optical coupler. 6. An isolator is provided at each of an input port to which an input signal light is input, between an output side of the second optical coupler and an input side of the third optical coupler, and an output port from which an output signal light is output. 6. The method according to claim 5, wherein
An optical fiber amplifier as described. 7. An optical fiber amplifier comprising a first rare-earth-doped fiber and a second rare-earth-doped fiber arranged in front and rear two stages, wherein pumping light power is n: 1 (n is a real number not less than 1) at an optical branching portion. )
And the pumping light of one port of the optical branching portion is multiplexed by the first optical coupler and is incident on one of the first rare earth-doped fiber and the second rare earth-doped fiber. First means, and after the first means inputs pumping light from one end of the one rare earth-doped fiber, a second optical coupler connected to the other end of the one rare earth-doped fiber extracts the residual pumping power. A second means for multiplexing the residual pumping power by a third optical coupler and allowing the residual pumping power to enter from one end of the other rare earth-doped fiber of the first rare earth-doped fiber and the second rare earth-doped fiber; And a third means for multiplexing the pumping power of the other port of the optical branching part branched at the branching part from the other end of the other rare earth-doped fiber by the fourth optical coupler. Characterized Rukoto, optical fiber amplifier. 8. An optical fiber amplifier comprising a first rare earth-doped fiber and a second rare earth-doped fiber arranged in two stages, front and rear, wherein a pumping light source and pumping light power from the pumping light source are n: 1 (n Is a real number greater than or equal to 1), and the pumping light from one port of the light branching unit is multiplexed to obtain one rare earth element of the first rare earth-doped fiber and the second rare earth-doped fiber. A first optical coupler to be incident on the doped fiber, a second optical coupler to extract the residual pumping power extracted from the one rare earth-doped fiber, and a multiplexing of the residual pumping power extracted to the second optical coupler, A third optical coupler that is incident on the other rare earth-doped fiber of the first rare earth-doped fiber and the second rare earth-doped fiber, and is branched by the optical branching unit. An optical fiber amplifier, comprising: a fourth optical coupler that multiplexes pumping power from another port of the optical branching unit and enters the other rare earth-doped fiber. 9. An input port to which input signal light is input, between the pumping light source and the optical branching portion, between the signal ports of the second optical coupler and the third optical coupler, and output signal light is output. 9. The optical fiber amplifier according to claim 8, wherein an isolator is added to each output port. 10. An optical fiber amplifier having a rare earth-doped fiber, an excitation light source, an optical circulator having three or more ports in which one port is connected to the excitation light source, and pumping from the excitation light source via the optical circulator. First, the light is multiplexed and is incident on the rare earth-doped fiber.
An optical coupler, a second optical coupler for separating residual pumping light reaching the other end of the rare earth-doped fiber as a result of the pumping light being incident on one end of the rare earth-doped fiber through the first optical coupler, and the second optical coupler A reflector that reflects the residual pumping light separated by the coupler and returns it again into the rare earth-doped fiber through the second optical coupler; and one end of the rare earth-doped fiber that is returned into the rare earth-doped fiber by the reflector. A residual pumping light detector for detecting the residual pumping light input to the optical circulator through the first optical coupler, and the pumping light source so that the residual pumping light detected by the residual pumping light detector is constant. An optical fiber amplifier comprising a controller for controlling the optical fiber. 11. A Faraday rotation reflecting mirror is used as the reflecting mirror.
The optical fiber amplifier according to any one of 1. 12. The optical system according to claim 3, wherein the input signal light is inputted through the optical circulator and the output signal light is outputted through the optical circulator. An optical fiber amplifier according to claim 1. 13. The optical fiber amplifier according to claim 10, wherein an isolator is added to each of the input port for inputting the input signal light and the output port for outputting the output signal light. 14. A rare-earth-doped fiber optical amplification section made of a rare-earth-doped fiber and a Raman optical amplification section that causes Raman amplification by being excited by desired excitation light are arranged in cascade. A characteristic is an optical fiber amplifier. 15. A rare-earth-doped fiber optical amplification section made of a rare-earth-doped fiber, and a Raman optical amplification section that causes Raman amplification by being excited with desired pumping light that can excite the rare-earth-doped fiber optical amplification section. An optical fiber amplifier, which is arranged in tandem and is provided with a pumping light source for supplying pumping light for pumping the rare earth-doped fiber optical amplifier and the Raman optical amplifier. 16. A rare earth-doped fiber optical amplifying section made of a rare earth-doped fiber and a Raman optical amplifying section made of a dispersion compensating fiber that causes Raman amplification by being excited by desired pump light are cascaded in two stages before and after. Optical fiber amplifier characterized by being connected. 17. The optical fiber according to claim 16, wherein the Raman light amplifying section is provided as a pre-stage amplifying section, and the rare earth-doped fiber light amplifying section is provided as a post-stage amplifying section. amplifier. 18. The rare earth-doped fiber optical amplifier is configured as an optical amplifier having a low noise figure, the rare earth-doped fiber optical amplifier is provided as a pre-amplifier, and the Raman optical amplifier is post-stage. The optical fiber amplifier according to claim 16, wherein the optical fiber amplifier is provided as an amplifier. 19. The pumping light source for pumping the Raman light amplifying section is composed of two pumping light sources and a polarization combiner that performs orthogonal polarization combining on the pumping lights from these pumping light sources. The optical fiber amplifier according to any one of claims 14 to 16. 20. A pumping light source for pumping the Raman light amplifying section is a combination of a pumping light source and a depolarizer,
The optical fiber amplifier according to any one of claims 14 to 16, wherein the optical fiber amplifier is configured to depolarize the pumping light. 21. The pumping light source for pumping the Raman light amplifying section is configured to generate modulated pumping light. Fiber optic amplifier. 22. A first pumping light source, comprising a rare earth-doped fiber and a dispersion compensating fiber arranged in front and rear two stages, for generating pumping light in a first wavelength band for the rare earth-doped fiber; and the first pumping source. A first optical coupler for injecting pumping light from a light source into the rare earth-doped fiber, a second pumping light source for generating pumping light in a second wavelength band for the dispersion compensating fiber, and pumping from the second pumping light source A second optical coupler for injecting light into the dispersion compensating fiber is provided, and the dispersion compensating fiber is excited by pumping light in the second wavelength band from the second pumping light source to cause Raman amplification. An optical fiber amplifier characterized by the above. 23. The rare earth-doped fiber is an erbium-doped fiber, and the wavelength band of pumping light generated by the first pumping light source is 0.98 μm band,
The wavelength band of the excitation light generated by the second excitation light source is 1.47μ
23. The optical fiber amplifier according to claim 22, which is in the m band. 24. An erbium-doped fiber and a dispersion compensating fiber arranged in front and rear two stages, and an excitation light source for generating an excitation light, and an optical coupler for injecting the excitation light from the excitation light source into the erbium-doped fiber. An optical fiber amplifier, characterized in that the dispersion compensating fiber is pumped with residual pumping light from the erbium-doped fiber to generate Raman amplification. 25. An excitation light source, comprising an erbium-doped fiber and a dispersion compensating fiber arranged in front and rear two stages, for generating excitation light; and an optical coupler for injecting the excitation light from the excitation light source into the dispersion compensation fiber. An optical fiber amplifier, characterized in that the erbium-doped fiber is pumped by residual pumping light from the dispersion compensating fiber. 26. A dispersion compensating fiber doped with a rare earth element, a pumping light source for generating pumping light for the dispersion compensating fiber doped with the rare earth element, and a pumping light from the pumping light source doped with the rare earth element An optical fiber amplifier, comprising: an optical coupler which is incident on the dispersion compensation fiber. 27. An excitation light source comprising an erbium-doped fiber and a dispersion compensating fiber arranged in front and rear two stages, and an excitation light source for generating an excitation light for the erbium-doped fiber; and an excitation light from the excitation light source, the erbium-doped fiber. And an optical filter that is interposed between the erbium-doped fiber and the dispersion compensating fiber to block residual excitation light emitted from the erbium-doped fiber. A characteristic is an optical fiber amplifier. 28. A rare earth-doped fiber optical amplifying section made of a rare earth-doped fiber and a Raman optical amplifying section made of a silica-based optical fiber that causes Raman amplification by being excited by desired pumping light are provided in two stages before and after. An optical fiber amplifier characterized by being cascaded. 29. The optical fiber according to claim 28, wherein the Raman optical amplification section is provided as a pre-stage amplification section, and the rare earth-doped fiber optical amplification section is provided as a post-stage amplification section. amplifier. 30. When the rare earth-doped fiber optical amplifier section is configured as an optical amplifier section having a low noise figure, the rare earth-doped fiber optical amplifier section is provided as a pre-stage amplifier section, and the Raman light is provided. 29. The optical fiber amplifier according to claim 28, wherein the amplification section is provided as a post-stage amplification section. 31. A silica-based optical fiber is provided on the front stage side, and an erbium-doped fiber is provided on the rear stage side, and a silica-based optical fiber pumping light source for generating pumping light in a wavelength band for the silica-based optical fiber, An optical coupler for making pumping light from a pumping light source for silica-based optical fiber incident on the silica-based optical fiber, a pumping light source for erbium-doped fiber that generates pumping light in a wavelength band for the erbium-doped fiber, and the erbium-doped fiber Raman amplification by exciting the silica-based optical fiber with the pumping light in the wavelength band from the silica-based optical fiber pumping light source An optical fiber amplifier, characterized in that 32. A silica-based optical fiber, which comprises an erbium-doped fiber having a low noise figure on the front side and a silica-based optical fiber on the rear side, and produces pumping light in a wavelength band for the silica-based optical fiber. A pumping light source, an optical coupler that inputs pumping light from the silica-based optical fiber pumping light source to the silica-based optical fiber, and an erbium-doped fiber pumping light source that generates pumping light in a wavelength band for the erbium-doped fiber. An optical coupler for making pumping light from the pumping light source for the erbium-doped fiber incident on the erbium-doped fiber, and pumping the silica-based optical fiber with pumping light in a wavelength band from the pumping light source for the silica-based optical fiber. An optical fiber amplifier, characterized in that Raman amplification is generated. 33. An excitation light source for generating excitation light is provided,
33. The optical fiber amplifier according to claim 31 or 32, wherein the pumping light source serves as both the silica-based optical fiber pumping light source and the erbium-doped fiber pumping light source. 34. A rare earth-doped fiber optical amplifying section made of a rare earth-doped fiber and having a low noise figure is provided as a pre-stage amplifying section, and a Raman optical amplifying section for causing Raman amplification by being excited by desired pumping light is provided. An optical fiber amplifier, characterized in that a rare earth-doped fiber optical amplifying section made of a rare earth-doped fiber is provided as a post-stage amplifying section. 35. The optical fiber amplifier according to claim 34, wherein the Raman optical amplifier is configured as an optical amplifier composed of a dispersion compensating fiber. 36. The optical fiber amplifier according to claim 34, wherein the Raman optical amplification section is configured as an optical amplification section made of a silica optical fiber. 37. A first erbium-doped fiber having a low noise figure is provided in a front stage, a dispersion compensating fiber is provided in a middle stage, and a second erbium-doped fiber is provided in a rear stage, and a wavelength band for the first erbium-doped fiber is provided. A first erbium-doped fiber pumping light source that generates pumping light, an optical coupler that inputs pumping light from the first erbium-doped fiber pumping light source to the first erbium-doped fiber, and a wavelength band for the dispersion compensating fiber Pumping light source for the dispersion compensating fiber that generates the pumping light, an optical coupler that inputs the pumping light from the pumping light source for the dispersion compensating fiber to the dispersion compensating fiber, and a pumping light in the wavelength band for the second erbium-doped fiber And a second erbium-doped fiber pumping light source for producing a second erbium-doped fiber An optical coupler for injecting the pumping light from the pumping light source for the B into the second erbium-doped fiber, and pumping the dispersion compensating fiber with the pumping light in the wavelength band from the pumping light source for the dispersion compensating fiber to perform Raman amplification. An optical fiber amplifier characterized by being produced. 38. A first erbium-doped fiber having a low noise figure in the front stage, and a silica-based optical fiber in the middle stage,
A second erbium-doped fiber is provided in each subsequent stage, and a pumping light source for the first erbium-doped fiber that generates pumping light in a wavelength band for the first erbium-doped fiber, and pumping from the pumping light source for the first erbium-doped fiber An optical coupler for injecting light into the first erbium-doped fiber, a pumping light source for silica-based optical fiber that generates pumping light in a wavelength band for the silica-based optical fiber, and pumping from the pumping light source for silica-based optical fiber From an optical coupler for injecting light into the silica-based optical fiber, a second erbium-doped fiber pumping light source for generating pumping light in a wavelength band for the second erbium-doped fiber, and a second erbium-doped fiber pumping light source And an optical coupler for injecting the pumping light into the second erbium-doped fiber, An optical fiber amplifier, characterized in that a silica-based optical fiber is pumped with pumping light in a wavelength band from a pumping light source for the silica-based optical fiber to generate Raman amplification. 39. A dispersion compensating fiber module for an optical fiber amplifier, comprising: a dispersion compensating fiber; and a pumping light source that pumps the dispersion compensating fiber to generate Raman amplification. 40. An optical fiber amplifier having a dispersion compensating fiber, comprising: a pumping light source; and an optical coupler for making pumping light from the pumping light source incident on the dispersion compensating fiber. An optical fiber amplifier characterized in that Raman amplification is caused by pumping with the pumping light. 41. The optical fiber amplifier according to claim 40, wherein the input signal light is input through an optical circulator and the output signal light is output through the optical circulator. 42. The optical fiber amplifier according to claim 40, wherein an isolator is added to each of the input port into which the input signal light is input and the output port from which the output signal light is output. 43. An optical fiber amplifier having a silica-based optical fiber, comprising: an excitation light source; and an optical coupler for injecting excitation light from the excitation light source into the silica-based optical fiber. An optical fiber amplifier characterized by being excited by pumping light from a pumping light source to generate Raman amplification. 44. The optical fiber amplifier according to claim 43, wherein the input signal light is inputted through the optical circulator and the output signal light is outputted through the optical circulator. 45. A rare earth-doped fiber optical amplification section made of a rare earth-doped fiber, and an optical fiber attenuating section made of an optical fiber or an optical fiber added with an optical isolator for suppressing unstable operation of the rare earth-doped fiber optical amplification section. An optical fiber amplifier, characterized in that it is configured with. 46. An optical amplifying unit having a pre-stage optical amplifying unit and a post-stage optical amplifying unit each configured as a rare earth-doped fiber optical amplifying unit made of a rare earth-doped fiber, and a pre-stage optical amplifying unit and a post-stage optical amplifying unit in the optical amplifying unit. And an optical fiber attenuating section formed of an optical fiber to which an optical fiber or an optical isolator is added in order to suppress unstable operation of the optical amplification unit. , Fiber optic amplifier. 47. The optical fiber attenuating section also serves as a Raman optical amplifying section that causes Raman amplification by being excited by desired pumping light, and is also used as a Raman optical amplifying section. Fiber optic amplifier.