JP2008053756A - Optical fiber amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure stable operation of an excitation light source to efficiently utilize residual excitation optical power. <P>SOLUTION: An optical fiber amplifier is provided with an excitation light source 32, an optical circulator 33 having three or more ports connected with the excitation light source 32, a first optical coupler 34-1 for combining excitation lights coming through the excitation light source 32 and optical circulator 33 to incident into a rare earth dope fiber, a second optical coupler 34-2 for separating the residual excitation light arrived at the other end of the rare earth dope fiber, a reflection mirror 35 for reflecting the separated residual excitation light to return it again into the rare earth dope fiber through the second optical coupler 34-2, a detector 36 of the residual excitation light for detecting the residual excitation light which is returned by reflection and input to the optical circulator 33 through one end of the rare earth dope fiber and the first optical coupler 34-1, and a controller 37 for controlling the excitation light source 32 so that the detected residual excitation light becomes constant. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical fiber amplifier.

In recent years, research and development of optical communication systems have been energetically promoted, but optical amplification using a rare earth doped fiber such as an erbium (Er) doped fiber (hereinafter, erbium doped fiber may be referred to as “EDF”). The importance of technology-based booster amplifiers, repeaters, or preamplifiers has become clear.
In addition, with the advent of optical amplifiers, transmission systems that amplify optical amplifiers in multiple relays are attracting attention as they play a very important role in promoting the economics of communication systems in the multimedia society.

By the way, in the conventional rare earth doped fiber optical amplifier that specifically amplifies one signal wavelength, the length of the doped fiber is set to a value that maximizes the gain in order to increase the conversion efficiency from the pumping light power to the signal light power. ing.
In a wavelength division multiplexing (WDM) optical amplifier that performs multi-wavelength collective amplification, it is important to keep the wavelength dependence of gain as flat as possible. As a result, it is necessary to operate a rare earth doped fiber (discussed below with a typical EDF) with a shallow gain saturation. For this purpose, when the upper-level ion density is represented by N2, the total ion density is represented by N1, and N2 / N1 is defined as the pumping rate, the average pumping rate N2 / N1 of the doped fiber length is increased. In addition, it is necessary to shorten the length of the doped fiber.

However, when the doped fiber is shortened in this way, a lot of residual pumping power leaks from the other end of the doped fiber and the conversion efficiency deteriorates. Nevertheless, the number of signal wavelengths increases and the required pumping is increased. Since the power increases, it is necessary to increase the output power of the pumping semiconductor laser.
That is, the fact that the pumping power increases as the number of wavelengths increases is apparent from the energy conservation law, but the wavelength multiplexing optical amplifier cannot be used with a good conversion efficiency from the pumping light power to the signal light power. . Because, in order to make the gain wideband or flat, the rare earth doped fiber is intentionally shortened so that saturation does not occur, so that the pump light power that has not been converted into a signal leaks from the other end. Because it will be.

Therefore, compared with the conventional amplification of one signal wavelength, even if high pump light power is required, it must be used in a leaking state.
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 doped fiber and used again for optical amplification. The technique to do is disclosed by Unexamined-Japanese-Patent No. 3-25985 and Unexamined-Japanese-Patent No. 3-166782.

However, when the residual excitation light is reflected by the reflecting mirror in this way, the excitation light is returned not only to the doped fiber but also to the excitation light source, and this excitation light may cause unstable operation of the excitation light source such as interference. There is.
By the way, as mentioned above, with the advent of optical amplifiers, transmission systems that multi-amplify optical amplifiers are attracting attention as playing an extremely important role in the economics of communication systems in the multimedia society. Problems in such a transmission system include dispersion compensation, reduction of nonlinear effects (those that adversely affect transmission quality) in the optical fiber that is the transmission path, and economical broadband wavelength division multiplexing transmission.

In general, an optical fiber that is a transmission line has dispersion characteristics, and the amount of dispersion accumulates in proportion to the length of the fiber. Conventionally, there is no optical amplification repeater, etc. The amount of distribution was reset each time it was relayed, and there was no problem.
However, since optical amplification repeater is a kind of repeater based on analog amplification, the dispersion amount is accumulated. For this, the signal wavelength may be set to zero dispersion wavelength and transmitted. 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. On the other hand, in an optical amplifier that is expected to be put to practical use, the 1.55 μm band is used. Only the signal can be amplified.
(1-2) Recently, even if a new optical fiber with a zero dispersion wavelength of 1.55 μm is laid and transmitted with a signal of 1.55 μm, the nonlinear effect in the optical fiber is active. It became clear that this would occur. This means that an undesirable nonlinear effect becomes significant when the signal wavelength is made to coincide with the zero dispersion wavelength.

(1-3) In particular, in wavelength division multiplexing, since there are a plurality of different signal light wavelengths, the concept of matching the zero dispersion wavelength cannot be applied.
Therefore, recently, it has been proposed to intentionally shift the signal wavelength from the zero-dispersion wavelength intentionally, for example, to compensate the dispersion for each relay.
As described above, research on dispersion compensators has been actively carried out in recent years, but the one closest to practical use is a dispersion compensating fiber (hereinafter referred to as “DCF”). This is an abbreviation for Dispersion Compensation Fiber.) 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 a relay point that already exists. For this reason, research and development have been conducted to shorten the length of the dispersion compensating fiber.
(2-2) When a new fiber is laid, it is also conceivable that the dispersion compensating fiber is laid 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. The research and development of such a new dispersion compensating fiber has the dispersion used in the application (2-1) above. When combined with research and development of compensation fiber, it becomes a double development.

  That is, in summary, in WDM transmission, it is necessary to compensate for chromatic dispersion, and since it is most practical to use a dispersion compensating fiber, a method using a dispersion compensating fiber is an effective method. In addition, it can be said that the dispersion compensating fiber has been studied as a single component in the optical amplifying repeater, but in general, the mode field diameter of the dispersion compensating fiber (DCF) compensates for the dispersion. Therefore, a small non-linear effect is likely to occur, and the loss increases as the amount of dispersion to be compensated increases.

  For this purpose, a method of compensating for the loss of the dispersion compensating fiber with an optical amplifier is conceivable. However, self-phase modulation (SPM) or cross-phase modulation (XPM) occurring in the dispersion compensating fiber is conceivable. ), It is necessary to compensate for the loss so as not to be affected by the nonlinear effect that deteriorates the signal quality, and there is a problem that it is difficult to design a level diagram. Furthermore, a WDM optical amplifier is required to have a flat and wide optical amplification band, but the rare earth doped fiber optical amplifier also has a wavelength dependency of gain, and it is difficult to realize a flat and wide amplification band. There is also a problem that there is.

Moreover, in rare earth doped fiber optical amplifiers with high gain, unnecessary oscillation may occur when performing optical amplification. If such unnecessary oscillation occurs, the rare earth doped fiber optical amplifier operates in an unstable manner. Will do.
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, an erbium-doped fiber optical amplifier adjusted for multi-wavelength amplification (that is, an erbium-doped fiber optical amplifier with a high pumping rate) has a high gain near 1.53 μm, so unnecessary oscillation occurs at this wavelength. If an unnecessary oscillation occurs, the erbium-doped fiber optical amplifier operates in an unstable manner.

The present invention was devised in view of such a problem, and aims to efficiently use the residual pumping light power generated when the average pumping rate is increased while ensuring stable operation of the pumping light source, thereby converting the conversion efficiency. An object of the present invention is to provide an optical fiber amplifier capable of improving the above.
Further, the technique related to the present invention is based on Raman amplification by utilizing the fact that the dispersion compensation fiber uses a mode drop diameter of the dispersion compensation fiber so that 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 of the dispersion compensating fiber.

Further, the technique related to the present invention uses a silica optical fiber having a Raman amplification function in the same manner as the dispersion compensation fiber, and thus, as in the case of using the dispersion compensation fiber, the loss compensation of the silica optical fiber by Raman amplification. An object of the present invention is to provide an optical fiber amplifier capable of performing the above.
In addition, the technology related to the present invention provides 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 collective amplification. The purpose is to do.

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

Reference numeral 4 denotes 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. 5 is provided between the pumping light source 2 and the first optical coupler 3-1 to prevent the pumping light source 2 from performing an unstable operation due to interference by the residual pumping light returned into the rare earth doped fiber 1. Optical isolator.
That is, in the optical fiber amplifier provided with the rare earth doped fiber 1 shown in FIG. 1, the first means for making the pump light incident 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 entering the pumping light from one end of the rare earth doped fiber 1, the residual pumping light reaching the other end of the rare earth doped fiber 1 is separated by the second optical coupler 3-2, and the residual pumping light is reflected by the reflecting means (reflecting The second means reflected by the mirror 4 and returned into the rare earth-doped fiber 1, and the residual excitation light returned into the rare earth-doped fiber 1 by the second means is incident on the rare-earth doped fiber 1 by the first means. And a third means for preventing the pumping light source 2 for the pumping light 2 from performing an unstable operation by using an optical isolating means (optical isolator) 5. It becomes door.

Further, a Faraday rotating reflecting mirror can be used as the reflecting means (reflecting mirror) 4.
Further, the input signal light may be input through the optical circulator and the output signal light may be output through the optical circulator.
(2) Description of Second Aspect FIG. 2 is a principle block diagram showing a second aspect relating to a technique related to the present invention. In FIG. 2, reference numerals 11-1 and 11-2 denote optical fiber amplifiers. It is the 1st rare earth doped fiber and 2nd rare earth doped fiber arrange | positioned over 2 steps | paragraphs.

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

Reference numeral 14 denotes a reflecting mirror that reflects the residual pumping light separated by the second optical coupler 13-2 and returns it to the rare-earth doped fiber 11-1 through the second optical coupler 13-2.
Reference numeral 15 denotes 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.
In this case, the pumping light from the pumping light source 12 is incident from one end of one rare earth doped fiber 11-1 from the first optical coupler 13-1 via the optical circulator 15, and then the one rare earth doped fiber 11 is used. -1 is separated by the second optical coupler 13-2, and the residual excitation light is reflected by the reflecting mirror 14 and returned into one rare-earth doped fiber 11-1, and further the light. The optical path is changed by the circulator 15, and the third pump coupler 13-3 is configured to multiplex the residual pumping light into the other rare earth doped fiber 11-2.

  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. 2, an optical circulator having three or more ports of pumping light. The first means for causing light to enter from one end of one rare-earth doped fiber 11-1 from the first optical coupler 13-1 via the first optical coupler 13-1, and pumping light from one end of one rare-earth doped fiber 11-1 by this first means. As a result of the incidence, the residual pumping light that has reached the other end of one of the rare earth doped fibers 11-1 is separated by the second optical coupler 13-2, and the residual pumping light is reflected by the reflecting means 14 so that the one rare earth doped After returning the second excitation light reflected from the reflection means 14 into one of the rare earth doped fibers 11-1 by the second means for returning into the fiber 11-1, and the second means. In the third optical coupler 13-3 by changing the optical path to the other rare-earth doped fiber 11-2 optical circulator 15, it will have been configured to include a third means for combining the residual pump light.

In addition, the input port to which the input signal light is input, the output side of the second optical coupler 13-2 and the input side of the third optical coupler 13-3, and the output port from which the output signal light is output, respectively. It is possible to add an isolator (in the two-stage front and rear configuration, it is very effective to add an isolator in the front stage and the rear stage).
Also in this case, a Faraday rotating reflecting mirror can be used as the reflecting mirror 14.

Further, the input signal light can be input through the optical circulator and the output signal light can be output through the optical circulator.
(3) Description of Third Aspect FIG. 3 is a principle block diagram showing a third aspect according to the technique related to the present invention. In FIG. 3, 21-1 and 21-2 are optical fiber amplifiers, It is the 1st rare earth doped fiber and 2nd rare earth doped fiber arrange | positioned over 2 steps | paragraphs.

Reference numeral 22 denotes an excitation light source, and reference numeral 23 denotes an optical branching unit that branches the excitation light power from the excitation light source 22 to n: 1 (n is a real number of 1 or more).
24-1 multiplexes pumping light from one port of the optical branching unit 23 and one of the first rare earth doped fiber 21-1 and the second rare earth doped fiber 21-2. Is a first optical coupler to be incident on the first optical coupler.

Reference numeral 24-2 denotes a second optical coupler that extracts residual pumping power extracted from one rare earth-doped fiber 21-1.
24-3 combines the residual pumping power extracted by the second optical coupler 24-2, and the other rare earth doped fiber of the first rare earth doped fiber 21-1 and the second rare earth doped fiber 21-2. This is the third optical coupler incident on 21-2.

Reference numeral 24-4 denotes a fourth optical coupler which combines 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.
That is, in the optical fiber amplifier provided with 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. Branching into n: 1 (n is a real number of 1 or more), the pumping light of one port of the optical branching unit 23 is multiplexed by the first optical coupler 24-1, and enters one of the rare earth doped fibers 21-1. The first optical device 24 and the second optical coupler 24 connected to the other end of the rare earth doped fiber 21-1 after the pumping light is incident from one end of the rare earth doped fiber 21-1 by the first means. -2 takes out the residual pumping power, combines the residual pumping power with the third optical coupler 24-3, and makes it incident from one end of the other rare earth doped fiber 21-2; Branched light The pump power of other ports parts 23 so that the fourth is composed of the optical coupler 24-4 and a third means for multiplexing the other end of the other rare-earth doped fiber 21-2.

The input port to which the input signal light is input, between the excitation light source 22 and the optical branching unit 23, between the connection portions of the signal ports of the second optical coupler 24-2 and the third optical coupler 24-3, and the output signal An isolator may be added to each output port from which light is output.
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.

(4) Description of Fourth Aspect 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, 32 is an excitation light source, and 33 is an excitation light source. This is an optical circulator having three or more ports connected to one port 32.
Reference numeral 34-1 denotes a first optical coupler that combines pumping light from the pumping light source 32 via the optical circulator 33 and enters the rare-earth doped fiber 31.

Reference numeral 34-2 denotes a second optical coupler that separates residual pumping light that has reached 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.
Reference numeral 35 denotes a reflecting mirror that reflects the residual pumping light separated by the second optical coupler 34-2 and returns it to the rare-earth doped fiber 31 through the second optical coupler 34-2.

Reference numeral 36 denotes a residual excitation light detector that detects the residual excitation light that is returned to the rare earth doped fiber 31 by the reflecting mirror 35 and is input to the optical circulator 33 through one end of the rare earth doped fiber 31 and the first optical coupler 34-1. .
Reference numeral 37 denotes a controller that controls the excitation light source 32 so that the residual excitation light detected by the residual excitation light detector 36 is constant.

In this case as well, a Faraday rotating reflecting mirror can be used as the reflecting mirror 35.
In addition, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator. The input signal light is input to the input port and the output signal light is output to the output port. In addition, an isolator may be added respectively.
(5) Description of Fifth Aspect FIG. 5 is a principle block diagram showing a fifth aspect according to the technique related to the present invention. In FIG. 5, reference numerals 51 and 52 denote rare earth doped elements arranged in two stages. Fiber and 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 54-1 is pumping light from the first pumping light source 53-1 for the rare earth doped fiber 51. Is a first optical coupler incident on the.
53-2 is a second pumping light source that generates 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. Is a second optical coupler incident on.

The dispersion compensating fiber 52 is pumped with pumping light in the second wavelength band from the second pumping light source 53-2 to generate Raman amplification.
That is, a rare-earth-doped fiber optical amplifying unit composed of a rare-earth-doped fiber 51 and a Raman optical amplifying unit composed of a dispersion-compensating fiber 52 that generates Raman amplification when excited by a desired pumping light are cascaded in two stages. It has been done.

In this case, the wavelength band of the excitation light generated by the first excitation light source 53-1 is 0.98 μm, and the wavelength band of the excitation light generated by the second excitation light source 53-2 is 1.47 μm (1.45 to 1.49 μm). : Hereinafter, the 1.47 μm band is preferably 1.45 to 1.49 μm unless otherwise specified).
In addition, the Raman light amplifying unit may be disposed as a pre-stage amplifying unit, and the rare earth-doped fiber light amplifying unit may be disposed as a post-stage amplifying unit. In the case of being configured as an amplifying unit, the rare-earth-doped fiber optical amplifying unit may be disposed as a pre-stage amplifying unit, and the Raman light amplifying unit may be disposed as a post-stage amplifying unit.

  The second pumping light source 53-2 can also be configured by two pumping light sources and a polarization beam combiner that performs orthogonal polarization combining on the pumping light from these pumping light sources, and the second pumping light source 53-2. Can also be configured to depolarize the pumping light by combining the pumping light source and the depolarizer, and the second pumping light source 53-2 is configured to generate modulated pumping light. You can also

(6) Description of Sixth Aspect FIG. 6 is a principle block diagram showing a sixth aspect according to the technique related to the present invention. In FIG. 6, 61 and 62 are erbiums arranged in two stages in the front and rear. A doped fiber and a dispersion compensating fiber.
Reference numeral 63 denotes an excitation light source that generates excitation light in a 1.47 μm band, and reference numeral 64 denotes an optical coupler that makes the excitation light from the excitation light source 63 incident on the erbium-doped fiber 61.

In this case, the dispersion compensating fiber 62 is pumped by the residual pumping light from the erbium-doped fiber 61 to generate Raman amplification.
That is, a rare-earth-doped fiber optical amplifying unit composed of an erbium-doped fiber 61, which is a rare-earth-doped fiber, and a Raman optical amplifying unit that generates Raman amplification by being excited with desired pumping light that can excite the rare-earth doped fiber optical amplifying unit. (This Raman light amplifying unit is composed of a dispersion compensating fiber 62) is arranged in a cascade and is provided with pumping light for exciting the rare earth doped fiber light amplifying unit and the Raman light amplifying unit. A light source 63 is provided.

The pumping light source 63 can also be composed of two pumping light sources and a polarization beam combiner that performs orthogonal polarization combining on the pumping light from these pumping light sources. The pumping light source 63 includes the pumping light source and the depolarizer. In combination, the excitation light can be configured to be non-polarized, and the excitation light source 63 can be configured to generate modulated excitation light.
(7) Description of Seventh Aspect FIG. 7 is a principle block diagram showing a seventh aspect according to the technique related to the present invention. In FIG. 7, reference numerals 71 and 72 denote erbium doped elements arranged in two stages. Fiber and dispersion compensating fiber.

Reference numeral 73 denotes an excitation light source that generates excitation light in the 1.47 μm band, and reference numeral 74 denotes an optical coupler that makes the excitation light from the excitation light source 73 incident on the dispersion compensation fiber 72.
In this case, the erbium-doped fiber 71 is configured to be excited with residual pumping light from the dispersion compensating fiber 72.
(8) Description of Eighth Embodiment FIG. 8 is a principle block diagram showing an eighth embodiment according to the technique related to the present invention. In FIG. 8, 81 is a dispersion compensating fiber doped with a rare earth element, 82 is a pumping light source that generates pumping light for the dispersion compensating fiber 81 doped with the rare earth element, and 83 is a light that enters the pumping light from the pumping light source 82 into the dispersion compensating fiber 81 doped with the rare earth element. It is a coupler.

(9) Explanation of Ninth Embodiment FIG. 9 is a principle block diagram showing a ninth embodiment according to the technique related to the present invention. In FIG. 9, 91 and 92 are erbium doped elements arranged in two stages. Fiber and dispersion compensating fiber.
Reference numeral 93 denotes an excitation light source that generates 1.47 μm band excitation light for the erbium-doped fiber 91, and 94 is an optical coupler that makes the excitation light from the excitation light source 93 incident on the erbium-doped fiber 91.

Reference numeral 95 denotes an optical filter that is interposed between the erbium-doped fiber 91 and the dispersion compensating fiber 92 and blocks the residual excitation light in the 1.47 μm band that emerges from the erbium-doped fiber 91.
(10) Description of Tenth Aspect FIG. 10A is a principle block diagram showing a tenth aspect relating to a technique related to the present invention. In FIG. 10A, reference numeral 101 denotes a silica-based optical fiber (SOF). , 102 is an erbium-doped fiber (EDF), and the optical fiber amplifier shown in FIG. 10A includes the silica-based optical fiber 101 on the front side and the erbium-doped fiber 102 on the rear side.

103-1 is a silica-based optical fiber pumping light source that generates pumping light in the wavelength band for the silica-based optical fiber 101, and 104-1 is pumping light from the silica-based optical fiber pumping light source 103-1. This is an optical coupler that enters the silica-based optical fiber 101.
103-2 is an erbium-doped fiber pumping light source that generates pumping light in the wavelength band for the erbium-doped fiber 102, and 104-2 is an erbium-doped fiber pumping light from the erbium-doped fiber pumping light source 103-2. This is an optical coupler that is incident on 102.

In this case, the silica-based optical fiber 101 is pumped with pumping light in the wavelength band from the silica-based optical fiber pumping light source 103-1, thereby causing Raman amplification.
That is, in the optical fiber amplifier shown in FIG. 10 (a), a rare earth-doped fiber optical amplifying unit composed of an erbium-doped fiber 102, which is a rare earth-doped fiber, and a silica-based amplifier that causes Raman amplification when excited by desired pump light. The Raman light amplifying unit composed of the optical fiber 101 is connected in cascade in two stages, front and rear, the Raman light amplifying unit is disposed as a front-stage amplifying unit, and the rare earth-doped fiber light amplifying unit is disposed as a back-stage amplifying unit. It has been done.

In the case where the rare earth doped fiber optical amplifying unit is configured as an optical amplifying unit having a low noise figure, the rare earth doped fiber optical amplifying unit is disposed as a pre-stage amplifying unit, and the Raman light amplifying unit is a post-stage amplifying unit. You may arrange as.
Furthermore, an excitation light source that generates excitation light may be provided, and this excitation light source may also serve as the silica-based optical fiber excitation light source 103-1 and the erbium-doped fiber excitation light source 103-2.

(11) Description of Eleventh Embodiment FIG. 10 (b) is a principle block diagram showing an eleventh embodiment of the technique related to the present invention. In FIG. 10 (b), 111 is erbium having a low noise figure. A doped fiber (EDF), 112 is a silica-based optical fiber (SOF), and the optical fiber amplifier shown in FIG. 10B has an erbium-doped fiber 111 on the front side and a silica-based optical fiber 112 on the rear side. Each has it.

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

In this case, the silica-based optical fiber 112 is excited with excitation light in the wavelength band from the silica-based optical fiber excitation light source 113-2 to generate Raman amplification.
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.

(12) Description of Twelfth Embodiment FIG. 11 is a principle block diagram showing a twelfth embodiment of the technique related to the present invention. In FIG. 11, 121-1 is a first erbium-doped fiber having a low noise figure. 11 is a silica-based optical fiber (SOF), 121-2 is a second erbium-doped fiber (EDF), and the optical fiber amplifier shown in FIG. 11 is the first erbium-doped fiber 121-1. In the middle, the silica-based optical fiber 122 in the middle, and the second erbium-doped fiber 121-2 in the rear.

123-1 is a pumping light source for the first erbium-doped fiber that generates pumping light in the wavelength band for the first erbium-doped fiber 121-1, and 124-1 is a pumping light source for the first erbium-doped fiber 123-1. Is an optical coupler that makes the excitation light from the light incident on the first erbium-doped fiber 121-1.
123-2 is a silica-based optical fiber pumping light source that generates pumping light in the wavelength band for the silica-based optical fiber 122, and 124-2 is pumping light from the silica-based optical fiber pumping light source 123-2. This is an optical coupler that enters the silica-based optical fiber 122.

123-3 is a pumping light source for the second erbium-doped fiber that generates pumping light in the wavelength band for the second erbium-doped fiber 121-2, and 124-3 is a pumping light source for the second erbium-doped fiber 123-3. Is an optical coupler that makes the excitation light from the light incident on the second erbium-doped fiber 121-2.
In this case, the silica-based optical fiber 122 is pumped with pumping light in the wavelength band from the silica-based optical fiber pumping light source 123-2 to generate Raman amplification.

  That is, in the optical fiber amplifier shown in FIG. 11, a rare-earth-doped fiber optical amplifying unit composed of an erbium-doped fiber 121-1, which is a rare-earth-doped fiber, and having a low noise figure is arranged as a pre-amplifier and pumped with desired pumping light. As a result, a Raman optical amplifying part made of silica-based optical fiber 122 that causes Raman amplification is arranged as a middle stage amplifying part, and a rare earth doped fiber optical amplifying part made of erbium doped fiber 121-2, which is a rare earth doped fiber, is arranged in the subsequent stage. It is arranged as an amplifying part.

(13) Description of Thirteenth Embodiment FIG. 12 is a principle block diagram showing a thirteenth embodiment of the technique related to the present invention. In FIG. 12, 131-1 is a first erbium-doped fiber having a low noise figure. (EDF), 132 is a dispersion compensating fiber (DCF), 131-2 is a second erbium-doped fiber (EDF), and the optical fiber amplifier shown in FIG. 12 includes the first erbium-doped fiber 131-1. A dispersion compensating fiber 132 is provided in the middle stage, and a second erbium-doped fiber 131-2 is provided in the subsequent stage.

133-1 is a pump light source for the first erbium-doped fiber that generates pump light in the wavelength band for the first erbium-doped fiber 131-1, and 134-1 is a pump light source for the first erbium-doped fiber 133-1. Is an optical coupler that makes the excitation light from the light incident on the first erbium-doped fiber 131-1.
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 a pumping light source from the dispersion-compensating fiber pumping light source 133-2. This is an optical coupler that is incident on 132.

133-3 is a pumping light source for the second erbium-doped fiber that generates pumping light in the wavelength band for the second erbium-doped fiber 131-2, and 134-3 is a pumping light source for the second erbium-doped fiber 133-3. Is an optical coupler that makes the excitation light from the light incident on the second erbium-doped fiber 131-2.
In this case, the dispersion compensating fiber 132 is configured to be pumped with pumping light in the wavelength band from the pumping light source 133-2 for the dispersion compensating fiber to generate Raman amplification.

  That is, in the optical fiber amplifier shown in FIG. 12, a rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber 131-1 which is a rare-earth doped fiber and having a low noise figure is arranged as a pre-stage amplifying unit, and is pumped with desired pumping light. As a result, a Raman optical amplifying unit composed of the dispersion compensating fiber 132 that generates Raman amplification is arranged as a middle stage amplifying unit, and a rare earth doped fiber optical amplifying unit composed of an erbium doped fiber 131-2 that is a rare earth doped fiber is a post-stage amplifying unit. It is arranged as a part.

(14) Description of Fourteenth Aspect FIG. 13A is a principle block diagram showing a fourteenth aspect relating to a technique related to the present invention. In FIG. 13A, 141 is a dispersion compensating fiber (DCF). 142 is a pumping light source that generates pumping light, and 143 is an optical coupler that makes the pumping light from the pumping light source 142 incident on the dispersion compensating fiber 141, and the dispersion compensating fiber 141 is pumped from the pumping light source 142. It is configured to generate Raman amplification when excited by the.

Thus, the optical fiber amplifier includes a dispersion compensating fiber module having the dispersion compensating fiber 141 and the pumping light source 142 that pumps the dispersion compensating fiber 141 to cause Raman amplification.
Also in this case, the input signal light is input through the optical circulator and the output signal light is output through the optical circulator, or the input port to which the input signal light is input or the output from which the output signal light is output. An isolator may be added to each port.

(15) Description of Fifteenth Aspect FIG. 13B is a principle block diagram showing a fifteenth aspect relating to a technique related to the present invention. In FIG. 13B, 151 is a silica-based optical fiber (SOF). , 152 is an excitation light source that generates excitation light, 153 is an optical coupler that makes the excitation light from the excitation light source 152 incident on the silica-based optical fiber 151, and the silica-based optical fiber 151 is connected from the excitation light source 152. It is configured to generate Raman amplification by being excited by the excitation light.

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.
(16) Description of Sixteenth Aspect FIG. 14 is a principle block diagram showing a sixteenth aspect relating to a technique related to the present invention. In FIG. 14, reference numeral 154 denotes a rare-earth doped fiber optical amplification comprising a rare-earth doped fiber 61. 155 is an optical fiber attenuating unit made of an optical fiber to which an optical fiber or an optical isolator is added.

The optical fiber attenuating unit 155 suppresses unstable operation of the rare earth doped fiber optical amplifying unit 154.
The optical fiber attenuating unit 155 may also serve as a Raman light amplifying unit that generates Raman amplification when excited with desired pumping light.
In FIG. 14, reference numeral 63 denotes an excitation light source, and reference numeral 64 denotes an optical coupler that makes excitation light from the excitation light source 63 incident on the rare earth doped fiber 61.

(17) Description of Seventeenth Aspect FIG. 15 is a principle block diagram showing a seventeenth aspect relating to a technique related to the present invention. In FIG. 15, reference numerals 156-1 and 156-2 denote rare earth doped fibers 121-. These are a pre-stage optical amplifying section and a post-stage optical amplifying section configured as rare earth-doped fiber optical amplifying sections consisting of 1,121-2. An optical amplification unit is configured by including the front-stage optical amplification unit 156-1 and the rear-stage optical amplification unit 156-2.

Reference numeral 157 denotes an optical fiber attenuating unit composed of an optical fiber or an optical fiber to which an optical isolator is added. The optical fiber attenuating unit 157 includes a front-stage optical amplifying unit 156-1 and a rear-stage optical amplifying unit 156-2 in the optical amplifying unit. Between the two and suppresses the unstable operation of the optical amplification unit.
The optical fiber attenuating unit 157 may also serve as a Raman light amplifying unit that generates Raman amplification when excited with desired pumping light.

  In FIG. 15, 123-1 and 123-3 are excitation light sources, 124-1 is an optical coupler that makes the excitation light from the excitation light source 123-1 incident on the rare earth doped fiber 121-1, and 124 -3 is an optical coupler that makes the excitation light from the excitation light source 123-3 incident on the rare earth doped fiber 121-2.

  As described above in detail, according to the optical fiber amplifier of the technology related to the present invention, in the rare earth doped fiber optical amplifier, the residual pumping light that is incident from one end of the rare earth doped fiber through the first optical coupler and reaches the other end. Are separated by the second optical coupler, and when the pumping light is reflected by the reflecting mirror and reciprocated in the rare earth-doped fiber, the pumping light source is interfered by the residual pumping light returned into the rare earth-doped fiber. Is configured to provide an optical isolator between the pumping light source and the first optical coupler so that the pumping light power is increased while ensuring stable operation of the pumping light source. There is an advantage that it is possible to provide an optical fiber amplifier used for efficiency.

  In the optical fiber amplifier of the technology related to the present invention, an optical amplifier having two or more stages of rare earth-doped fibers is used, and an optical circulator having three or more ports is used. The first optical coupler separates the residual pumping light incident from one end of the rare-earth doped fiber at the front stage or the rear stage and reaching the other end by the second optical coupler, and reflects the pumping light by the reflecting mirror. Thus, after reciprocating in the rare earth doped fiber, the optical path is changed by the optical circulator, and the third optical coupler is configured to multiplex the pump light to the other rare earth doped fiber. There is an advantage that it is possible to provide an optical fiber amplifier having a two-stage configuration that efficiently utilizes the above.

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

In addition, a common pumping light source can be provided for both rare earth doped fibers, and in this way, the number of pumping light sources to be used can be reduced, which can contribute to simplification of the configuration and cost reduction.
Furthermore, in the optical fiber amplifier of the present invention, in the rare earth doped fiber optical amplifier, an optical circulator having three or more ports is used. First, after passing the pumping light through the optical circulator, the first optical coupler allows the rare earth doped fiber. The residual pumping light that has entered from one end of the optical fiber and reached the other end 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 first light. The residual pumping light power separated by the coupler is extracted by changing the optical path with an optical circulator and monitored to keep this residual pumping light power constant, so that the wavelength characteristics of the gain do not fluctuate with changes in the input level. There is an advantage that can greatly contribute to the realization of a multi-wavelength collective amplifier.

In addition, when a Faraday rotating mirror is used as the reflecting mirror, the polarization of the pumping light can be intentionally rotated, thereby eliminating the optical isolator provided between the pumping light source and the first optical coupler. Thus, it is possible to provide an optical fiber amplifier with a reduced PHB while simplifying the configuration.
Furthermore, an optical circulator can also be provided in the input / output unit. In this way, the number of isolators to be used can be reduced, which can contribute to cost reduction.

Also, an isolator can be provided at a required location of the optical fiber amplifier, and in this way, light interference can be prevented.
The optical fiber amplifier includes a rare-earth-doped fiber optical amplifying unit made of a rare-earth-doped fiber and a Raman optical amplifying unit that generates Raman amplification by being pumped with desired pumping light (the Raman optical amplifying unit is a dispersion compensating fiber or Is composed of silica-based optical fibers (the same applies hereinafter), and has the advantage of providing a two-stage optical fiber amplifier that utilizes pumping light power with high efficiency. is there.

Furthermore, by providing a common pumping light source that supplies pumping light for pumping these rare earth-doped fiber optical amplifier and Raman optical amplifier, the pumping light power can be used with high efficiency and used. It is possible to reduce the number of excitation light sources to be used, thereby contributing to simplification of the configuration and cost reduction.
Further, according to the optical fiber amplifier of the technology related to the present invention, the erbium doped fiber (the erbium doped fiber may have a low noise figure, and the same shall apply hereinafter) and the dispersion compensating fiber arranged in two stages. Or a silica-based optical fiber, a first pumping light source for generating pumping light in a first wavelength band for an erbium-doped fiber, and pumping light in a second wavelength band for a dispersion-compensating fiber or silica-based optical fiber. And the second pumping light source that is generated, and the dispersion compensating fiber or the silica-based optical fiber is pumped with the pumping light of the second wavelength band from the second pumping light source to generate Raman amplification. While performing optical amplification with a doped fiber, it is possible to perform loss compensation of a dispersion compensating fiber or silica optical fiber by Raman amplification. Kill.

  Further, the rare earth doped fiber is composed of an erbium doped fiber, the wavelength band of the pumping light generated by the first pumping light source is 0.98 μm band, and the wavelength band of the pumping light generated by the second pumping light source is 1.47 μm band. Therefore, loss compensation of the dispersion compensation fiber or silica-based optical fiber by Raman amplification can be performed while performing optical amplification by the erbium-doped fiber more effectively.

Also, it is possible to provide a common pumping light source for the rare earth doped fiber and the dispersion compensating fiber or the silica-based optical fiber. In this way, the number of pumping light sources to be used can be reduced to simplify the configuration and reduce the cost. Can contribute.
Furthermore, in the optical fiber amplifier of the technology related to the present invention, the dispersion compensation fiber is Raman-amplified using the residual pumping power generated when the erbium-doped fiber is pumped in the 1.47 μm band, or the dispersion compensation fiber is 1 Since the erbium-doped fiber is configured to be excited using the residual pumping power generated when pumping in the .47 μm band, the loss of the dispersion compensating fiber can be reduced. Furthermore, there is an advantage that the dispersion compensation fiber is Raman-amplified in a band of ˜1.44 μm or 1.47 μm, so that the amplification band of the erbium-doped fiber can be supplemented and further broadening or flattening can be promoted.

Further, in the optical fiber amplifier of the technology related to the present invention, by using a dispersion compensating fiber doped with a rare earth element, dispersion compensation is performed, and at the same time, the loss of the dispersion compensating fiber is reduced, and signal light is also transmitted. There is an advantage that an optical fiber amplifier with a dispersion compensation function capable of sufficiently amplifying light can be provided.
Furthermore, since the optical fiber amplifier of the technology related to the present invention is provided with an optical filter that blocks the pumping light in the 1.47 μm band from being input to the dispersion compensating fiber, the leakage pumping light power in the 1.47 μm band is provided. However, by performing Raman amplification on the dispersion compensating fiber, it is possible to prevent the optical fiber amplifier from performing unstable operation or changing the wavelength dependence of the amplification band.

  In the optical fiber amplifier of the technology related to the present invention, the first erbium-doped fiber having a low noise figure is provided in the front stage, the dispersion compensating fiber or silica-based optical fiber is provided in the middle stage, and the second erbium-doped fiber is provided in the rear 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 by the dispersion compensating fiber or the silica optical fiber can be increased, and a broadband optical amplifier can be realized while simplifying the structure and reducing the cost.

Furthermore, the excitation light source for exciting the Raman amplification unit can be composed of two excitation light sources and a polarization beam combiner that performs orthogonal polarization synthesis on the excitation light from these excitation light sources. For example, the Raman compensation can be effectively generated by the dispersion compensating fiber, and the loss compensation of the dispersion compensating fiber can be performed by this Raman amplification.
Further, the excitation light source for exciting the Raman amplifying unit may be configured to depolarize the excitation light by combining the excitation light source and the depolarizer, and in this way, it is made of a dispersion compensating fiber. The polarization dependency in the Raman optical amplifier can be reduced.

  Furthermore, the light from the excitation light source for exciting the Raman amplification unit may be modulated to generate excitation light having a spectrum broadened to several hundred kHz or more. While increasing the scattering threshold and suppressing harmful nonlinear effects, the dispersion compensation fiber can effectively generate Raman amplification, and the loss compensation of the dispersion compensation fiber can be performed by this Raman amplification.

Further, in the technology related to the present invention, since the optical fiber amplifier is configured by using a module in which the dispersion compensation fiber is excited to generate Raman amplification, there is an advantage that the loss of the dispersion compensation fiber can be reduced. is there.
In this case as well, if an optical circulator is provided in the input / output unit, the number of isolators to be used can be reduced, thereby contributing to cost reduction.

  In the optical fiber amplifier of the technology related to the present invention, an optical fiber or an optical isolator is added to suppress the unstable operation of the rare earth doped fiber optical amplifying unit and the rare earth doped fiber optical amplifying unit. Since the optical fiber attenuating portion made of an optical fiber is provided, the unstable operation of the rare earth-doped fiber optical amplifying portion can be suppressed and highly accurate optical amplification can be performed.

  Furthermore, in the optical fiber amplifier of the technology related to the present invention, an optical amplification unit having a front-stage optical amplification unit and a rear-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 An optical fiber attenuating section is provided between the front-stage optical amplifying section and the rear-stage optical amplifying section, and is composed of an optical fiber to which an optical fiber or an optical isolator is added in order to suppress unstable operation of the optical amplifying unit. Therefore, the unstable operation of the optical amplifying unit can be suppressed, and highly accurate optical amplification can be performed.

  Further, if the optical fiber attenuating part is also used as a Raman light amplifying part that generates Raman amplification when excited with desired pumping light, loss compensation of the optical fiber attenuating part can be performed.

Hereinafter, embodiments of the present invention and techniques related to the present invention will be described with reference to the drawings.
(1) Description of First Embodiment FIG. 16 is a block diagram showing a first embodiment according to a technique related to the present invention. The optical fiber amplifier shown in FIG. 16 includes two erbium-doped fibers as rare earth-doped fibers. (EDF) 11-1, 11-2, two pumping light sources 12-1, 12-2, four optical demultiplexers / multiplexers (WDM) 13-1 to 13-4 as first to fourth optical couplers, A reflecting mirror (reflecting means) 14, an optical circulator 15, three isolators (ISO) 16-1 to 16-3, and an optical filter 17 are provided.

That is, in this optical fiber amplifier, in order from the input side, an isolator 16-1, an optical demultiplexer / multiplexer 13-1, an erbium-doped fiber 11-1, an optical demultiplexer / multiplexer 13-2, an optical filter 17, and an isolator. 16-2, an optical demultiplexer / multiplexer 13-3, an erbium-doped fiber 11-2, an optical demultiplexer / multiplexer 13-4, and an isolator 16-3 are provided.
Further, the optical demultiplexer-multiplexer 13-1 is connected to the pumping light source 12-1 via the optical circulator 15, and the optical circulator 15 is configured such that the other ports in addition to the pumping light source 12-1 have optical branching. It is connected to a wave 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 of the erbium-doped fibers 11-1 and 11-2 is a part that functions as an optical amplifying unit, and the pumping light source 12-1 including a lens and an LD chip generates pumping light in a 0.98 μm band, for example. The excitation light source 12-2, which is an excitation light source and includes two lenses, an optical isolator (optical ISO), and an LD chip, for example, when referred to as a 1.47 μm band (1.47 μm band), is specifically described in the following embodiments. Unless otherwise specified, it means an excitation light source having a wavelength of 1.45 to 1.49 μm). The pumping light source 12-2 that generates pumping light in the 1.47 μm band incorporates an optical isolator (optical ISO) when the optical signal in the 1.55 μm band is amplified when the optical signal is erbium-doped fiber 11-2. This is to prevent the noise light in the 1.55 μm band generated in step 1 from returning to the excitation light source 12-2.

As described above, when the excitation light wavelengths of the respective excitation light sources 12-1 and 12-2 are selected, the optical demultiplexers 13-1 to 13-3 are used in the 0.98 μm band, and the optical demultiplexers are used. The multiplexer 13-4 is used in the 1.47 μm band.
Further, as the optical demultiplexers / multiplexers 13-1 to 13-4, a fusion type is used in the illustrated example, but a bulk (dielectric multilayer) type may of course be used. For example, when a bulk type is used as the optical demultiplexer-multiplexer 13-4, the optical isolator (optical ISO) built in the excitation light source 12-2 can be omitted. -2 is the same type of excitation light source (but 1.47 μm band) as the excitation light source 12-1 (the same applies to the following embodiments).

Further, a Faraday rotating reflecting mirror is used as the reflecting mirror 14, and using this reflecting mirror 14, the residual excitation light separated by the optical demultiplexing / multiplexing device 13-2 is reflected, and the optical demultiplexing / multiplexing is performed again. It can return to one erbium-doped fiber 11-1 through the vessel 13-2.
The optical circulator 15 is a three-port optical circulator, and is configured in the same manner as that in which a fiber is not connected to the port 4 in the four-port optical circulator shown in FIG.

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

The isolators 16-1 to 16-3 allow light to pass only in the direction of the arrows in the figure, and as shown in FIG. 54, lenses, birefringent prisms A and B, and polarization rotators (reciprocal). And a 45 ° Faraday rotator (non-reciprocal).
As shown in FIG. 54A, the isolators 16-1 to 16-3 receive an optical signal from the left fiber, and the optical signal reaches the right fiber, as shown in FIG. 54B. Thus, even if an optical signal is input from the right fiber, the optical signal is configured not to reach the left fiber (hereinafter, unless otherwise specified, the isolator is shown in FIG. 54). Have a structure).

In the optical fiber amplifier according to the present embodiment, isolators 16-1 and 16-2 are provided before and after the erbium-doped fiber 11-1, and isolators 16-2 and 16-3 are provided before and after the erbium-doped fiber 11-2. Is provided to prevent generation of noise light in the erbium-doped fibers 11-1 and 11-2.
In an optical amplifier having a plurality of optical amplifying units, it is possible to prevent the generation of noise light in the erbium-doped fiber 11-1 that is an amplifying unit positioned on the optical signal input side in order to perform optical amplification with low noise. Therefore, the isolator 16-3 at the rear stage of the erbium-doped fiber 11-2, which is an amplifying unit located on the output side of the optical signal, can be omitted (the same applies to the following embodiments).

  The optical filter 17 cuts a peak portion (for example, a 1.535 μm portion: see FIG. 46) of the ASE (Amplified Spontaneous Emission) of the output of the erbium-doped fiber 11-1 (crushes the mountain and flattens it). Or delete the wavelength shorter than ~ 1.538 μm) and is provided with a dielectric multilayer film, but the optical filter 17 may be omitted.

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

In this way, the residual pumping light leaking after reaching the other end of the erbium-doped fiber 11 is separated by the optical demultiplexer-multiplexer 13-2, and then reflected by the reflecting mirror 14 and returned.
Thereafter, the reflected residual pumping light is returned into the erbium-doped fiber 11, and further, the optical path is changed by the optical circulator 15 via the optical demultiplexer-multiplexer 13-1, and the optical demultiplexer-multiplexer 13-3. Then, together with the signal light amplified by the preceding erbium-doped fiber 11-1, the residual excitation light is combined and incident on the erbium-doped fiber 11-2.

Note that the latter-stage erbium-doped fiber 11-2 receives the excitation light from the excitation light source 12-2 and performs optical amplification.
That is, in the first embodiment, in a two-stage erbium-doped fiber optical amplifier using a three-port optical circulator 15, an optical demultiplexer 13-1 is passed from the input side of the front-stage erbium-doped fiber 11-1. Excitation light (for example, 0.98 μm) is incident, and an optical demultiplexer-multiplexer 13-2 is installed on the output side of the pre-stage erbium-doped fiber 11-1, so that the signal light is transmitted through the optical filter 17, optical isolator ( ISO) 16-2 is input to the latter-stage erbium-doped fiber 11-2.

On the other hand, the excitation light is separated from the signal light by the optical demultiplexer-multiplexer 13-2, reflected by the reflecting mirror 14, and reciprocates in the previous stage erbium-doped fiber 11-1. The excitation light separated by the 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 considering the interference between the pumping light sources 12-1 and 12-2, the pumping light source 12-1 is 0.98 μm and the pumping light source 12-2 are set to 1.47 μm, and the optical demultiplexer / multiplexer 13-4 suppresses 0.98 μm residual pumping light from entering the pumping light source 12-2.

  That is, in the first embodiment shown in FIG. 16, in an optical fiber amplifier having erbium-doped fibers 11-1 and 11-2 arranged in two stages, front and rear, the pumping light is separated through the optical circulator 15. As a result of the first light entering from one end of the erbium doped fiber 11-1 from the wave combiner 13-1 and the pumping light incident from one end of the erbium doped fiber 11-1 by this first means, this erbium doped fiber The residual pump light that has reached the other end of 11-1 is separated by the optical demultiplexer-multiplexer 13-2, and the residual pump light is reflected by the reflecting means 14 and returned into the erbium-doped fiber 11-1. Two means and the second means return the residual excitation light reflected from the reflecting means 14 into the erbium-doped fiber 11-1, and then change the optical path by the optical circulator 15. Ete erbium doped fiber 11-2 in the optical demultiplexer-multiplexer 13-3 it will have been configured to include a third means for combining the residual pump light.

As described above, in the first embodiment, the residual pumping light power generated when the average pumping rate is increased is reflected by the optical demultiplexer-multiplexer 13-2 and the reflecting mirror 14 which are newly prepared, and the erbium-doped fiber. By reciprocating within 11-1, it is possible to efficiently use the pumping light power, thereby improving the conversion efficiency.
At this time, since the optical circulator 15 is used, a loop can be formed so that the excitation light power does not become unstable.

Furthermore, since the Faraday rotating reflecting mirror is used as the reflecting mirror 14, the polarization of the excitation light can be rotated, thereby reducing PHB (Polarization Hole Burning).
(1-1) Description of First Modification of First Embodiment FIG. 17 is a block diagram showing a first modification of the first embodiment. The optical fiber amplifier shown in FIG. 17 is an isolator in order from the input side. 16-1, optical demultiplexer-multiplexer 13-1, erbium-doped fiber 11-1, optical demultiplexer-multiplexer 13-2, isolator 16-2, optical demultiplexer-multiplexer 13-3, erbium-doped fiber 11 -2, Isolator 16-3 is arranged.

Further, the optical demultiplexer-multiplexer 13-1 is connected to the pumping light source 12-1 via the optical circulator 15, and the optical circulator 15 is configured such that the other ports in addition to the pumping light source 12-1 have optical branching. It is connected to a wave multiplexer 13-3. Further, a reflecting mirror 14 is connected to the optical demultiplexer-multiplexer 13-2.
In FIG. 17, the same reference numerals as those in FIG. 16 denote the same parts.

As can be seen from this configuration, the optical fiber amplifier shown in FIG. 17 is configured to pump both the front and rear erbium-doped fibers 11-1 and 11-2 with a single pumping light source 12-1. . Although the optical filter 17 is omitted, the optical filter 17 may be provided at the position shown in FIG. 16 in this modified example.
Also in this case, the pumping light is incident from one end of the erbium-doped fiber 11-1 from the optical demultiplexer-multiplexer 13-1 via the optical circulator 15, and as a result of the pumping light being incident, the erbium-doped fiber 11- 1 is separated by the optical demultiplexer-multiplexer 13-2, and the residual pump light is reflected by the reflecting mirror 14, returned to the erbium-doped fiber 11-1, and further reflected. After the residual pumping light reflected from the mirror 14 is returned into the erbium-doped fiber 11-1, the optical path is changed by the optical circulator 15, and the residual pumping is performed on the erbium-doped fiber 11-2 by the optical demultiplexer-multiplexer 13-3. Since the lights are multiplexed, the same operation or effect as in the case of the first embodiment shown in FIG. 16 can be obtained, and the pumping light source 12-1 is made of both erbium-doped fibers 1. Because it is a common to -1,11-2, it can contribute to cost reduction of the simplification and cost of construction.

(1-2) Description of Second Modification of First Embodiment FIG. 18 is a block diagram showing a second modification of the first embodiment. The optical fiber amplifier shown in FIG. 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, erbium-doped fiber 11- In addition, the input signal light is input through the 4-port optical circulator 15-2, and the output signal light is also output through the optical circulator 15-2. Further, the optical demultiplexer-multiplexer 13-1 is connected to the pumping light source 12-1 via the optical circulator 15, and the optical circulator 15 is configured such that the other ports in addition to the pumping light source 12-1 have optical branching. It is connected to a 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. 18 has a configuration in which an optical circulator 15-2 is used instead of the isolator in the input / output unit.
Here, as shown in FIG. 53, the optical circulator 15-2 is composed of a PBS, a 45 ° Faraday rotator (non-reciprocal) and a 45 ° polarization rotator (reciprocal), and has four ports. Is an optical circulator.

  In this optical circulator 15-2, the optical signal input from port 1 is output to port 2 as shown in FIG. 53 (a), and the light input from port 2 as shown in FIG. 53 (b). The signal is configured to be output to port 3. 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 embodiment, unless otherwise specified, the optical circulator has the structure shown in FIG. 53).

An optical circulator 15-2 as shown in FIG. 18 is configured such that an optical signal is input from port 1 and an optical signal is output from port 4, and an optical demultiplexer-multiplexer 13- is connected to port 2. 1 is connected, and an erbium-doped fiber 11-2 is connected to the port 3.
The optical filter 17 may be omitted.

  Also in this case, the pumping light is incident from one end of the erbium-doped fiber 11-1 from the optical demultiplexer-multiplexer 13-1 via the optical circulator 15, and as a result of the pumping light being incident, the erbium-doped fiber 11- 1 is separated by the optical demultiplexer-multiplexer 13-2, and the residual pump light is reflected by the reflecting mirror 14, returned to the erbium-doped fiber 11-1, and further reflected. After the residual pumping light reflected from the mirror 14 is returned into the erbium-doped fiber 11-1, the optical path is changed by the optical circulator 15, and the residual pumping is performed on the erbium-doped fiber 11-2 by the optical demultiplexer-multiplexer 13-3. Since light is multiplexed, the same operation or effect as in the case of the first embodiment shown in FIG. 16 can be obtained, and an optical circulator 15-2 is provided in the input / output unit. Runode, it is possible to reduce the number of isolators to be used, there is an advantage that can contribute to cost reduction.

(1-3) Description of Third Modification of First Embodiment FIG. 19 is a block diagram showing a third modification of the first embodiment. The optical fiber amplifier shown in FIG. 16-1, optical demultiplexer / multiplexer (second optical coupler) 13-2 ', erbium-doped fiber 11-1, optical demultiplexer / multiplexer (first optical coupler) 13-1', optical filter 17, isolator 16-2, an optical demultiplexer-multiplexer 13-3, an erbium-doped fiber 11-2, an optical demultiplexer-multiplexer 13-4, and an isolator 16-3 are arranged, and the optical demultiplexer-multiplexer 13- 1 'is connected to a pumping light source 12-1 via an optical circulator 15. The optical circulator 15 is connected to the optical demultiplexer-multiplexer 13-3 at the other port in addition to the pumping light source 12-1. Has been. 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.

In FIG. 19, the same reference numerals as those in FIGS. 16 to 18 denote the same parts.
As can be seen from this configuration, the optical fiber amplifier shown in FIG. 19 has a configuration in which excitation light is incident from the output side of the erbium-doped fiber 11-1. Also in this case, the optical filter 17 can be omitted.
In this example, pump light is incident from the output end of the erbium-doped fiber 11-1 from the optical demultiplexer-multiplexer 13-1 'via the optical circulator 15, and as a result of the pump light being incident, the erbium-doped fiber The residual pump light reaching the input terminal 11-1 is separated by the optical demultiplexer-multiplexer 13-2 ', and the residual pump light is reflected by the reflecting mirror 14 and returned into the erbium-doped fiber 11-1. Further, after returning the residual excitation 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. The residual excitation light is multiplexed.

Thereby, 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 Modification of First Embodiment FIG. 20 is a block diagram showing a fourth modification of the first embodiment. The optical fiber amplifier shown in FIG. 16-1, 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 A fiber 11-2, an optical demultiplexer / multiplexer 13-4, an isolator 16-3, an optical filter 17-2, and a coupler 13-5 are provided. Further, as in the example shown in FIG. A pumping light source 12-1 is connected to the multiplexer 13-1 'via an optical circulator 15, and the optical circulator 15 has an optical demultiplexing multiplexer other than the pumping light source 12-1. 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 light detector 18 for detecting the output light from the coupler 13-5 is provided, and the output of the excitation light source 12-2 is controlled to be constant based on the detection result of the output light detector 18. A light output constant controller 19 is provided.
That is, as shown in FIG. 21, the output light detector 18 includes a photodiode 18A, and a detection value Vin1 detected by the photodiode 18A is a differential amplifier that constitutes the light output constant controller 19. It is input to 19A. The differential amplifier 19A supplies the difference G1 between the reference values Vref1 and Vin1 as a control signal Vcont1 to the laser diode that constitutes the excitation light source 12-2.

Here, the relationship among the output light power, the control signal Vcont1, and the output of the laser diode is shown in FIG.
In FIG. 20, the same reference numerals as those in FIGS. 16 to 19 denote the same parts.
As can be seen from this configuration, the optical fiber amplifier shown in FIG. 20 has a configuration that realizes constant optical output control (ALC).

  Also in this example, as in FIG. 19, the excitation light is made incident from the output end of the erbium-doped fiber 11-1 through the optical demultiplexer 13-1 ′ via the optical circulator 15 and this excitation light is made incident. As a result, the residual pumping light that has reached 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, so that an erbium-doped fiber is obtained. 11-1 and the residual excitation light reflected from the reflecting mirror 14 is returned to the erbium-doped fiber 11-1, and then the optical path is changed by the optical circulator 15, and the erbium is supplied by the optical demultiplexer-multiplexer 13-3. Residual pumping light is multiplexed into the doped fiber 11-2, but in addition, constant light output 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 the optical output constant control is performed. Therefore, the accumulation of ASE and the nonlinear effect in the optical fiber transmission line are performed. It is possible to set the optical output level so that the degradation of the signal-to-noise ratio (SNR) due to is small.
(1-5) Others In the above example, as the erbium-doped fiber that uses the reflected residual pumping light, the preceding one is shown. Of course, the reflected erbium-doped fiber is used as the erbium-doped fiber. Also good.

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

An optical signal line having an optical filter 26 and an isolator 25-3 and an excitation light line are provided in parallel between the optical demultiplexers / multiplexers 24-2 and 24-3.
The pumping light source 22 is connected to the optical demultiplexer-multiplexer 24-1 via the optical branching unit 23 and the isolator 25-2.
Here, the optical branching unit 23 branches the pumping light power (having a wavelength of, for example, 0.98 μm) from the pumping light source 22 to n: 1 (n is a real number of 1 or more). The branched light (for example, the smaller one) is supplied to the optical demultiplexer-multiplexer 24-1, and the light branched by the optical branching unit 23 (for example, the larger one) is the optical demultiplexer-multiplexer 24-4. To be supplied.

  That is, in the optical fiber amplifier having the erbium-doped fibers 21-1 and 21-2 arranged in two stages before and after shown in FIG. The first means for branching into the above real number), combining the pumping light of one port of the optical branching unit 23 with the optical demultiplexer-multiplexer 24-1, and entering the erbium-doped fiber 21-1; First, after the pump light is incident from one end of the erbium-doped fiber 21-1, the residual pump power is taken out by the optical demultiplexer-multiplexer 24-2 connected to the other end of the erbium-doped fiber 21-1. The second means for combining the residual pumping power with the optical demultiplexer-multiplexer 24-3 and making it incident from one end of the erbium-doped fiber 21-2, and the other of the optical branching unit 23 branched by the optical branching unit 23 Port excitation power It will have been configured to include a third means for multiplexing the other end of the erbium-doped fiber 21-2 in the optical demultiplexer-multiplexer 24-3.

In this embodiment, the same name as that of the first embodiment has the same function.
With such a configuration, in the optical fiber amplifier according to the second embodiment, the pumping light power is branched to n: 1 (n is a real number of 1 or more) by the optical branching unit 23, and one port of the optical branching unit 23 is provided. The excitation light is combined with the signal light by the optical demultiplexer-multiplexer 24-1, and is incident on the preceding erbium-doped fiber 21-1.

  After the pump light is incident from the input end of the erbium-doped fiber 21-1, the residual pump power is taken out by the optical demultiplexer-multiplexer 24-2 connected to the output end of the erbium-doped fiber 21-1. Further, the residual pumping power is multiplexed by the optical demultiplexer-multiplexer 24-3, and is incident from the input end of the subsequent erbium-doped fiber 21-2. The signal light passes through the optical filter 26 and the isolator 25-3, and is input to the subsequent erbium-doped fiber 21-2. At this time, the optical filter 26 causes a peak of the output of the erbium-doped fiber 21-1. The ASE portion (for example, a portion of 1.535 μm: see FIG. 46) is cut (crush the mountain to make it flat, or delete the shorter wavelength side than ˜1.538 μm).

Thereafter, the pumping power of other ports of the optical branching unit 23 branched by the optical branching unit 23 is multiplexed from the output end of the erbium-doped fiber 21-2 by the optical demultiplexing multiplexer 24-4.
As described above, 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 the pumping light source is made common and necessary. Accordingly, the distribution of the pumping light power to the first and second erbium-doped fibers 21-1 and 21-2 can be set, so that the pumping light power can be used more efficiently. The conversion efficiency can be greatly improved.

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 Modified Example of Second Embodiment FIG. 24 is a block diagram showing a modified example of the second embodiment according to the technique related to the present invention. The optical fiber amplifier shown in FIG. In this order, the isolator 25-1, the optical demultiplexer / multiplexer (second coupler) 24-2 ', the erbium-doped fiber 21-1, the optical demultiplexer / multiplexer (first coupler) 24-1', and the optical filter 26. , Isolator 25-3, optical demultiplexer / multiplexer (fourth coupler) 24-4 ′, erbium-doped fiber 21-2, optical demultiplexer / multiplexer (third coupler) 24-3 ′, and isolator 25-4. It is arranged.

An optical signal line including the optical filter 26 and the isolator 25-3 and an excitation light line are also provided in parallel between the optical demultiplexers / multiplexers 24-1 'and 24-4'.
The pumping light source 22 is connected to the optical demultiplexers / multiplexers 24-1 ′ and 24-4 ′ via the optical branching unit 23 and the isolator 25-2.
That is, also 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) by the optical branching unit 23, and the pumping light of one port of the optical branching unit 23 is optically transmitted. A first means for multiplexing by the demultiplexing multiplexer 24-1 'and entering the erbium-doped fiber 21-1, and after the excitation light is incident from one end of the erbium-doped fiber 21-1 by the first means The residual pumping power is taken out by the optical demultiplexer-multiplexer 24-2 'connected to the other end of the erbium-doped fiber 21-1, and the residual pumping power is multiplexed by the optical demultiplexer-multiplexer 24-3'. Then, the second means for making the light incident from one end of the erbium-doped fiber 21-2 and the pumping power of the other port of the optical branching unit 23 branched by the optical branching unit 23 are erbium-coupled by the optical demultiplexing multiplexer 24-3 '. Combined from the other end of the doped fiber 21-2 Third and means will have been configured that.

In this embodiment, the same name as that of the first embodiment has the same function.
With such a configuration, in the optical fiber amplifier shown in FIG. 24, the pumping light power is branched to n: 1 (n is a real number of 1 or more) by the optical branching unit 23, and the pumping light of one port of the optical branching unit 23 Are combined by the signal light and the optical demultiplexer-multiplexer 24-1 ′, and are incident from the output side of the erbium-doped fiber 21-1 at the preceding stage.

  After the pumping light is incident from the output end of the erbium-doped fiber 21-1, the residual pumping power 24-2 'is connected to the input end of the erbium-doped fiber 21-1. Then, the residual pumping power is further multiplexed by the optical demultiplexer-multiplexer 24-3 ′, and is incident from the output end of the subsequent erbium-doped fiber 21-2. 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.

Thereafter, the pumping power of the other port of the optical branching unit 23 branched by the optical branching unit 23 is multiplexed from the input end of the erbium-doped fiber 21-2 by the optical demultiplexing multiplexer 24-4 ′.
In this way, in this example as well, as 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. Since the light source is shared and the distribution of the pump light power to the erbium-doped fibers 21-1 and 21-2 at the front and rear stages can be set as necessary, more efficient use of the pump light power Therefore, the conversion efficiency can be greatly improved.

In the case of 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 technology related to the present invention. The optical fiber amplifier shown in FIG. 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 provided. Further, the pumping light source 2 is connected to the optical demultiplexer-multiplexer 3-1 through an isolator 5-2. In addition, 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 making the excitation light incident from the input end of the erbium-doped fiber 1 by the optical demultiplexer-multiplexer 3-1, and the first means are erbium-doped fiber. As a result of the excitation light entering from the input end of 1, the residual pumping light reaching the output end of the erbium-doped fiber 1 is separated by the optical demultiplexer-multiplexer 3-2, and the residual pumping light is reflected by a reflecting means (reflecting mirror). ) And the second means for reflecting the light back into the erbium-doped fiber 1.

As the reflecting mirror 4, a Faraday rotating reflecting mirror is used.
With such an arrangement, in the optical fiber amplifier shown in FIG. 25, after the pumping light is incident from one end of the erbium-doped fiber 1 by the optical demultiplexer-multiplexer 3-1, as a result of this incidence, the other of the erbium-doped fiber 1 is obtained. The residual pumping light reaching the 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.

Thereby, also in the third embodiment, the residual pumping light power generated when the average pumping rate is increased is reflected by the optical demultiplexing multiplexer 3-2 and the reflecting mirror 4 which are newly prepared, and the inside of the doped fiber 1 is reflected. By reciprocating, it is possible to efficiently use the pumping light power, thereby improving the conversion efficiency.
Furthermore, since the Faraday rotating reflecting mirror is used as the reflecting mirror 4, the polarization of the excitation light can be rotated, thereby reducing PHB.

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 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 according to the technique related to the present invention. The optical fiber amplifier shown in FIG. 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 provided with an isolator 5-3 and a pumping light line are provided in parallel between the optical demultiplexers / multiplexers 3-3 and 3-4.
Also in this example, the pumping light source 2 is connected to the optical demultiplexer-multiplexer 3-1 via the isolator 5-2. A reflecting mirror (Faraday rotating reflecting mirror) 4 is connected to the optical demultiplexer-multiplexer 3-5.

  With such an arrangement, in the optical fiber amplifier shown in FIG. 26, the pumping light is incident from the input end of the erbium-doped fiber 1-1 by the optical demultiplexer-multiplexer 3-1, and thus the erbium-doped fiber 1- After entering the pumping light from the input terminal 1, the residual pumping power is taken out by the optical demultiplexer-multiplexer 3-3 connected to the output terminal of the erbium-doped fiber 1-1, and the residual pumping power is further converted into the light. The signal is multiplexed by the demultiplexer-multiplexer 3-4, and is incident from the input end of the subsequent erbium-doped fiber 1-2. The signal light is input to the subsequent erbium-doped fiber 1-2 through the isolator 5-3.

Thereafter, the residual pumping light reaching the output end of the latter stage erbium-doped fiber 1-2 is separated by the optical demultiplexing multiplexer 3-5, and the residual pumping light is reflected by the reflecting mirror 4 to erbium-doped fiber. Return to 1-2 and 1-1.
Thus, also in the fourth embodiment, the residual pumping light power generated when the average pumping rate is increased is reflected by the optical demultiplexing multiplexer 3-5 and the reflecting mirror 4 which are newly prepared, and the doped fiber 1-2. , 1-1 can be reciprocated to efficiently use the pumping light power, thereby improving the conversion efficiency.

Also in this case, since the Faraday rotating reflecting mirror is used as the reflecting mirror 4, the polarization of the excitation light can be rotated, and 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 that uses the reflected residual pumping light is shown in the previous stage. Of course, the reflected erbium-doped fiber may be used in the rear-stage erbium-doped fiber.
(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 is also in order from the input side in the same manner as in the third embodiment. An 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 provided. In addition, an excitation light source 32 is connected to the optical demultiplexer-multiplexer 34-1 via a three-port optical circulator 33. In addition, a reflecting mirror (Faraday rotating reflecting mirror) 35 is connected to the optical demultiplexer-multiplexer 34-2.

Further, a residual pumping light detector 36 is connected to the optical circulator 33, and the residual pumping light detector 36 returns the erbium-doped fiber 31 to the erbium-doped fiber 31 by the reflecting mirror 35. The residual pumping light input to the optical circulator 33 through the wave combiner 34-1 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 is constant.

  That is, as shown in FIG. 28, the residual excitation light detector 36 is configured to include a photodiode 36A, and a detection value Vin2 detected by the photodiode 36A is applied to a differential amplifier 37A constituting the controller 37. It is designed to be entered. The differential amplifier 37A supplies the difference G2 between the reference values Vref2 and Vin2 as a control signal Vcont2 to the laser diode that constitutes the excitation light source 32.

Here, the relationship among the output light power, the control signal Vcont2 and the output of the laser diode is shown in FIG.
As can be seen from this configuration, the optical fiber amplifier shown in FIG. 27 is configured to realize the control of the pumping light source 32 so that the residual pumping light detected by the residual pumping light detector 36 is constant. It has become.

Thereby, also in this fifth embodiment, the pumping light power can be used efficiently, the conversion efficiency can be improved, and the average pumping rate can be kept constant by monitoring the residual pumping light power. Thus, the wavelength dependence of gain can be kept constant with respect to fluctuations in 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 27, the input signal light is input through the optical circulator 38 and the output signal light is output through the optical circulator 38. By doing so, the fifth embodiment described above is performed. In addition to the effects or advantages obtained by the configuration, since the optical circulator 38 is provided in the input / output unit, the number of isolators to be used can be reduced, and the advantages that can contribute to cost reduction can be obtained. .

In FIG. 30, the same reference numerals as those in FIG. 27 indicate 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 this order, an isolator 39-1, an optical demultiplexer / multiplexer 34-2 ', an erbium-doped fiber 31-1, an optical demultiplexer / multiplexer 34-1', an optical filter 40, an isolator 39-3, and an optical demultiplexer / multiplexer. 34-1 ″, erbium-doped fiber 31-2, optical demultiplexer / multiplexer 34-2 ″, and isolator 39-2 are provided.

The pumping light source 32 is connected to the optical demultiplexers / multiplexers 34-1 ′ and 34-1 ″ via a four-port optical circulator 33 ′. Also, reflecting mirrors (Faraday rotating reflecting mirrors) 35 ′ and 35 ″ are connected to the optical demultiplexers / multiplexers 34-2 ′ and 34-2 ″.
Further, a residual pumping light detector 36 is connected to the optical circulator 33 ′, and by this residual pumping light detector 36, erbium-doped fibers 31-1 and 31-2 are respectively reflected by reflecting mirrors 35 ′ and 35 ″. So that the residual pumping light input to the optical circulator 33 'through the erbium-doped fibers 31-1, 31-2 and the optical demultiplexers / multiplexers 34-1', 34-1 '' is detected. It has become.

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 is constant.
Also in this embodiment, the same name as that of the fifth embodiment has the same function.
As can be seen from this configuration, in the optical fiber amplifier shown in FIG. 31, the residual pump light from the erbium-doped fibers 31-1 and 31-2 detected by the residual pump light detector 36 is constant. The configuration is such that control of the excitation light source 32 is realized.

Therefore, also in this example, substantially the same effects or advantages as in the fifth embodiment described above can be obtained.
In the case of 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 according to the technique related to the present invention. The optical fiber amplifier shown in FIG. 32 includes an isolator 144 and dispersion compensation in order from the input side. A fiber 141 and an optical demultiplexer / multiplexer 143 are provided. The pumping light source 142 is connected to the optical demultiplexer / multiplexer 143.

Here, the excitation light source 142 is an excitation light source that generates excitation light in a band (for example, 1.44 to 1.49 μm) that can perform band compensation of erbium-doped fiber amplification by Raman amplification. The excitation light is incident from the output end of the dispersion compensating fiber 141 through the optical demultiplexer / multiplexer 143.
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 excited with the excitation light from the excitation light source 142 to cause Raman amplification. That is, in the dispersion compensating fiber 141, the mode field diameter is generally small, so the threshold value for Raman amplification is lowered, and thus Raman amplification is likely to occur.
By the way, the dispersion compensating fiber has the following characteristics.

  In other words, the dispersion compensating fiber (DCF) has a small core diameter and a mode field diameter which is about half of the normal size, and nonlinear effects (stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), four-photon mixing (FWM), self-phase). Modulation effects (SPM, etc.) are more likely to occur than fibers that are transmission lines. In addition, it is known from the usage mode that the dispersion compensating fiber is not as long as the fiber as the transmission line, and can be used by reducing the optical power when passing through the dispersion compensating fiber. This is because the influence of the nonlinear effect increases as the length increases.

In addition, it has been found that the attenuation (loss) of light in the dispersion compensating fiber is not negligible. For this reason, it is necessary to compensate for this loss with an optical amplifier.
On the other hand, the input power is limited to a small value as described above, which makes it difficult to design a level as an optical amplifier.
However, the non-linear effects described above can be harmful and useful for communication. Of these, Raman amplification is beneficial.

The points where this Raman amplification can be very useful are as follows. That is, if the dispersion compensation fiber is Raman-amplified, the dispersion compensation fiber itself becomes an optical amplifier and the loss can be compensated.
Note that Raman amplification is stimulated Raman scattering, that is, when intense monochromatic light is applied to an optical fiber, coherent Stokes light whose wavelength is shifted by a specific amount by interacting with the optical phonon of the optical fiber. 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 compensation fiber 141 is pumped by the pumping light in the band as described above from the pumping light source 142 to generate Raman amplification, whereby the loss compensation of the dispersion compensating fiber by this Raman amplification (erbium doped fiber). Flattening of the pits of the gain and compensating compensation for the decrease in the gain of the erbium-doped fiber can be performed.
In order to flatten the gain depression of the erbium-doped fiber in the 1.54 μm band, it is excited at ˜1.44 μm to cause Raman amplification.

As shown in FIG. 33, an isolator 144-2 can be added on the output side.
Furthermore, as shown in FIGS. 32 and 33, instead of providing an isolator in the input unit or the input / output unit, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator and the output signal It can also be configured such that light is output through this optical circulator.

Further, a silica-based optical fiber can be used instead of the dispersion compensating fiber 141.
(7) Description of Seventh Embodiment FIG. 34 is a block diagram showing a seventh embodiment according to the technology related to the present invention. The optical fiber amplifier shown in FIG. 34 includes isolators 55-1, in order from the input side. An optical demultiplexer / multiplexer 54-1, an erbium-doped fiber (rare earth doped fiber) 51, an isolator 55-2, a dispersion compensating fiber 52, an optical demultiplexer / multiplexer 54-2, and an isolator 55-3 are provided. . In addition, an excitation light source 53-1 is connected to the optical demultiplexing / multiplexing device 54-1, and an excitation light source 53-2 is connected to the optical demultiplexing / multiplexing device 54-2.

Here, the excitation light source 53-1 generates excitation light in the first wavelength band (for example, 0.98 μm band) for the erbium-doped fiber 51. The excitation light source 53-2 Therefore, excitation light in a second wavelength band (for example, a 1.47 μm band (1.45 to 1.49 μm) or a band up to 1.44 μm (˜1.44 μm)) is generated.
Thereby, the dispersion compensation fiber 52 can be excited with the excitation light from the excitation light source 53-2, and Raman amplification can be caused by the same principle as in the sixth embodiment. Therefore, also in this embodiment, the dispersion compensation fiber 52 is excited by excitation light in the 1.47 μm band or the band up to 1.44 μm from the excitation light source 53-2 to generate Raman amplification. The loss compensation of the dispersion compensating fiber can be performed.

  Furthermore, while the wavelength characteristics of the gain of rare-earth-doped fiber optical amplifiers are determined by rare-earth ions, the wavelength characteristics of the gain of Raman optical amplifiers are determined by the pump wavelength, and the peak value shifts if the pump wavelength is changed. In addition, it is possible to select a pump wavelength when performing Raman amplification so as to compensate the wavelength characteristic of the gain of the rare earth-doped fiber optical amplifier, and in this way, a broadband optical amplifier can be realized.

That is, 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 compensation fiber but also the amplification bandwidth of the erbium doped fiber can be compensated for and the bandwidth can be increased.
In other words, the wavelength characteristics of the erbium-doped fiber amplifier are not flat, as shown in FIGS. 46 and 47, and therefore, by performing Raman amplification using a dispersion compensating fiber, the unevenness of the wavelength characteristics of the erbium-doped fiber amplifier can be reduced. As a result, it is possible to realize a broadband optical amplifier, which is suitable when performing multi-wavelength amplification (see FIG. 47).

In addition, the rare earth doped fiber optical amplification part which consists of an erbium doped fiber which is a rare earth doped fiber may be comprised as an optical amplification part which has a low noise figure.
In the optical fiber amplifier shown in FIG. 34, a rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber is arranged as a pre-stage amplifying unit, and a Raman optical amplifying unit made of a dispersion compensating fiber is arranged as a post-stage amplifying unit. However, the present invention is not limited to this, and a Raman light amplifying unit made of a dispersion compensating fiber or a silica-based optical fiber is arranged as a pre-amplifier, and a rare-earth doped fiber optical amplifying unit made of an erbium-doped fiber is a post-amplifier (In the case where the Raman light amplifying section is made of a silica-based optical fiber, one pumping light source can be used both as a silica-based optical fiber pumping light source and an erbium-doped fiber pumping light source).

  Further, the excitation light source 53-2 includes, for example, two excitation light sources and excitations from these excitation light sources, similarly to the excitation light sources 53-2, 53-2 ′, 53-2 ″ shown in FIGS. It may be configured with a polarization synthesizer that synthesizes orthogonal polarization of light, or may be configured to depolarize pumping light by combining a pumping light source and a depolarizer, and modulated pumping. It may be configured to generate light.

In addition, about the excitation light sources 53-2, 53-2 ', and 53-2''shown in FIGS. 43-45, respectively, 14th Embodiment which concerns on the technique relevant to this invention, 1st modification of 14th Embodiment, respectively. An example and a second modification of the fourteenth embodiment will be described.
(8) Description of Eighth Embodiment FIG. 35 is a block diagram showing an eighth embodiment according to the technique related to the present invention. The optical fiber amplifier shown in FIG. 35 includes isolators 65-1, in order from the input side. An optical demultiplexer / multiplexer 64, an erbium doped fiber (rare earth doped fiber) 61, an isolator 65-2, a dispersion compensating fiber 62, and an isolator 65-3 are provided. An excitation light source 63 is connected to the optical demultiplexer / multiplexer 64.

Here, the excitation light source 63 generates excitation light in a 1.47 μm band (1.45 to 1.49 μm), for example.
With such a configuration, in the optical fiber amplifier shown in FIG. 35, excitation light is incident from one end of the erbium-doped fiber 61 by the optical demultiplexer / multiplexer 64 to excite and amplify the erbium-doped fiber 61. Residual excitation light arrives from the other end of the erbium-doped fiber 61. Thereafter, this residual pumping light is supplied to the dispersion compensating fiber 62 via the isolator 65-2 to cause Raman amplification.

In this way, amplification using both fibers can be performed by using a common pumping light source for the erbium-doped fiber and the dispersion compensating fiber.
That is, the excitation wavelength band when Raman-amplifying 1.55 μm band signal light is the 1.47 μm band (1.45-1.49 μm) which is the excitation wavelength band of the erbium-doped fiber (EDF), Therefore, Raman amplification can be performed using the residual pumping light power when the EDF is pumped with 1.47 μm band light. Thus, the loss of the dispersion compensating fiber 62 can be compensated 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, which is suitable for performing multi-wavelength collective amplification, etc. Since only one excitation light source is required, the structure can be simplified and the cost can be reduced.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator, and the output signal light is transmitted through the optical circulator. It can also be configured to output.

Further, the pumping light source 63 may be composed of two pumping light sources and a polarization beam combiner that performs orthogonal polarization combining on the pumping light from these pumping light sources, and combining the pumping light source and the depolarizer, May be configured to depolarize the light, and may be configured to generate modulated excitation light.
(8-1) Description of First Modification of Eighth Embodiment FIG. 36 is a block diagram showing a first modification of the eighth embodiment. The optical fiber amplifier shown in FIG. 36 includes isolators in order from the input side. 65-1, optical demultiplexer-multiplexer 64-1, erbium doped fiber (rare earth doped fiber) 61-1, isolator 65-2, dispersion compensating fiber 62, erbium doped fiber (rare earth doped fiber) 61-2, optical component A wave multiplexer 64-2 and an isolator 65-3 are provided. The excitation light source 63-1 is connected to the optical demultiplexer-multiplexer 64-1, and the excitation light source 63-2 is connected to the optical demultiplexer-multiplexer 64-2.

Here, both the excitation light sources 63-1 and 63-2 generate excitation light in a 1.47 μm band (1.45 to 1.49 μm), for example.
With such an arrangement, in the optical fiber amplifier shown in FIG. 36, the excitation light is incident from the input end of the erbium-doped fiber 61-1 by the optical demultiplexer-multiplexer 64-1, and the erbium-doped fiber 61-1 is made to enter. Excitation and amplification are performed. At this time, the residual excitation light arrives 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.

  The pumping light is incident from the output end of the erbium-doped fiber 61-2 by the optical demultiplexer-multiplexer 64-2, and the erbium-doped fiber 61-2 is excited and amplified. Residual excitation light also 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 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 can be increased. A broadband optical amplifier can be realized while simplifying the structure and reducing the cost.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator, and the output signal light is transmitted through the optical circulator. It can also be configured to output.

Further, an excitation light source and an optical demultiplexer / multiplexer for the dispersion compensating fiber 62 can be provided.
That is, in the same manner as in FIG. 12, an optical fiber amplifier may be configured using the 0.98 μm band pumping light sources 133-1 to 133-3 and the optical demultiplexers / multiplexers 134-1 to 134-3. Good.
Note that a silica-based optical fiber may be used in place of the dispersion compensating fiber 62.

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

Here, both the excitation light sources 63-1 and 63-2 generate excitation light in a 1.47 μm band (1.45 to 1.49 μm), for example.
An optical signal line having an optical filter 66 and an isolator 65-3 and an excitation light line are provided in parallel between the optical demultiplexers / multiplexers 64-3 and 64-4.
With this configuration, in the optical fiber amplifier shown in FIG. 37, the excitation light is incident from the input end of the erbium-doped fiber 61-1 by the optical demultiplexer-multiplexer 64-1, and the erbium-doped fiber 61-1 is made to enter. Excitation and amplification are performed. At this time, residual excitation light reaches 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.

  The pumping light is incident from the output end of the erbium-doped fiber 61-2 by the optical demultiplexer-multiplexer 64-5 to excite and amplify the erbium-doped fiber 61-2. Residual excitation light also 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 demultiplexers / multiplexers 64-4 and 64-3 to cause Raman amplification.

Also in this case, since the dispersion compensating fiber 62 is Raman-amplified 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 can be increased. A broadband optical amplifier can be realized while simplifying the structure and reducing the cost.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator, and the output signal light is transmitted through the optical circulator. It can also be configured to output.

Further, an excitation light source and an optical demultiplexer / multiplexer for the dispersion compensating fiber 62 can be provided.
That is, in the same manner as in FIG. 12, an optical fiber amplifier is configured using 0.98 μm band pumping light sources 133-1 to 133-3 and optical demultiplexers 134-1 to 134-3. Also good.
Note that a silica-based optical fiber may be used in place of the dispersion compensating fiber 62.

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

Here, the excitation light source 73 generates excitation light in a 1.47 μm band (1.45 to 1.49 μm), for example.
With this configuration, in the optical fiber amplifier shown in FIG. 38, the pumping light is 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. The residual pumping light from 72 enters from the output end of the erbium doped fiber 71 to excite the erbium doped fiber 71 and amplify the signal light.

  On the contrary, 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, and the broadband optical amplifier In addition to being suitable for performing multi-wavelength collective amplification, etc., since only one excitation 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 excitation 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 unit or the input / output unit, 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.

The pumping light source 73 may be composed of two pumping light sources and a polarization beam combiner that synthesizes orthogonally polarized light from the pumping light from these pumping light sources, and combines the pumping light source and the depolarizer to generate pumping light. May be configured to depolarize the light, and may be configured to generate modulated excitation light.
(10) Description of Tenth Embodiment FIG. 39 is a block diagram showing a tenth embodiment related to the technology related to the present invention. The optical fiber amplifier shown in FIG. 39 includes isolators 84-1, An optical demultiplexer / multiplexer 83, a dispersion compensating fiber doped with erbium (rare earth element) ions (hereinafter referred to as erbium doped dispersion compensating fiber) 81, and an isolator 84-2 are provided. The optical demultiplexer / multiplexer 83 is connected to a pumping light source 82 that generates pumping light in a 1.47 μm band (1.45 to 1.49 μm) or 0.98 μm, for example.

With such an arrangement, in the optical fiber amplifier shown in FIG. 39, pump light is incident from one end of the erbium-doped dispersion compensating fiber 81 by the optical demultiplexer / multiplexer 83 to excite the erbium-doped dispersion compensating fiber 81. Amplify the signal light.
If the core of the dispersion compensation fiber is doped with Er ions in this way, the excitation light is rapidly attenuated in the dispersion compensation fiber 81, so that no Raman amplification occurs, and the loss of the dispersion compensation fiber 81 is reduced in each minute section. It will be compensated and the signal-to-noise ratio can be kept good.

Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.
The pumping light source 82 may be composed of two pumping light sources and a polarization beam synthesizer that performs orthogonal polarization synthesis on the pumping light from these pumping light sources. The pumping light source 82 and the depolarizer combine the pumping light. May be configured to depolarize, and may be configured to generate modulated excitation light.

(11) Description of Eleventh Embodiment FIG. 40 is a block diagram showing an eleventh embodiment relating to a technique related to the present invention. The optical fiber amplifier shown in FIG. 40 includes isolators 96-1, in order from the input side. 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 compensating fiber 92 are disposed. An excitation light source 93 that generates excitation light in a 1.47 μm band (1.45 to 1.49 μm), for example, is connected to the optical demultiplexer / multiplexer 94.

The optical filter 95 blocks the residual excitation light in the 1.47 μm band that emerges from the erbium-doped fiber 91.
With this configuration, in the optical fiber amplifier shown in FIG. 40, excitation light is incident from one end of the erbium-doped fiber 91 by the optical demultiplexer / multiplexer 94 to excite the erbium-doped fiber 91 and amplify the signal light. However, at this time, residual excitation light arrives from the other end of the erbium-doped fiber 91. This residual excitation light is blocked by the optical filter 95.

If light in the 1.47 μm band is unnecessarily passed through the dispersion compensating fiber 92, the Raman amplification will disturb the level diagram design or the wavelength characteristics of the optical amplifier. In this case, the 1.47 μm band This light is blocked by the optical filter 95 from being input to the dispersion compensating fiber 92.
Therefore, the dispersion compensating fiber 92 is mainly used for compensating for dispersion in the transmission path.

Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.
The pumping light source 93 may be composed of two pumping light sources and a polarization beam combiner that performs orthogonal polarization synthesis on the pumping light from these pumping light sources. May be configured to depolarize, and may be configured to generate modulated excitation light.

(12) Description of Twelfth Embodiment FIG. 41 is a block diagram showing a twelfth embodiment according to the technique related to the present invention. The optical fiber amplifier shown in FIG. An optical demultiplexer-multiplexer 3-1, an erbium-doped fiber (rare earth-doped fiber) 1 that uses silica as a host, an optical demultiplexer-multiplexer 3-2, and an isolator 5-2 are disposed. The pumping light source 2-1 that generates, for example, 0.98 μm band pumping light is connected to the optical demultiplexing / multiplexing unit 3-1, and the optical demultiplexing / multiplexing unit 3-2 is connected to, for example, about 1.44 μm. Or an excitation light source 2-2 that generates excitation light of about 1.46 μm is connected.

  Here, a fusion type instead of a bulk type is used as the optical demultiplexer-multiplexer 3-1, and a type that does not incorporate an optical isolator (optical ISO) is used as the excitation light source 2-1. This is because the 1.55 μm band noise light generated in the erbium-doped fiber 1 when a 1.55 μm band optical signal is amplified does not return to the pumping light source 2-1 that generates 0.98 μm band pumping light. (The same applies to the following embodiments).

  With this configuration, in the optical fiber amplifier shown in FIG. 41, 0.98 μm band excitation light is incident from one end of the erbium-doped fiber 1 by the optical demultiplexer-multiplexer 3-1, and the erbium-doped fiber 1 is Excited and amplified signal light. Further, 1.44 μm excitation light or 1.46 μm excitation light is incident from the output end of the erbium-doped fiber 1 by the optical demultiplexer-multiplexer 3-2 and is Raman-amplified by the erbium-doped fiber 1.

In the erbium-doped fiber 1 as well, it is known that Raman amplification occurs when strong light is input.
In this way, erbium-doped fiber 1 having silica as a host is amplified at a general excitation wavelength [for example, 0.98 μm (or may be 1.47 μm)] and Raman-amplified at ˜1.44 μm. The gain depression of the 1.54 μm band of the fiber (see FIG. 46) can be flattened, and the gain reduction of the erbium-doped fiber around 1.57 μm (FIG. 46) by Raman amplification at ˜1.46 μm. Can be realized by flattening the characteristics to make a wider band.

Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.
(13) Description of Thirteenth Embodiment FIG. 42 is a block diagram showing a thirteenth embodiment according to the technique related to the present invention. The optical fiber amplifier shown in FIG. 42 includes isolators 144-1, in order from the input side. A dispersion compensating fiber 141, a deflection holding type optical demultiplexer / multiplexer 143, and an isolator 144-2 are provided. A polarization beam combining type excitation light source 142 is connected to the optical demultiplexer / multiplexer 143.

Here, the excitation light source 142 includes two excitation light sources 142A and 142B and a polarization beam combiner (PBS) 142C that performs orthogonal polarization synthesis on the excitation light from these excitation light sources 142A and 142B.
The excitation light sources 142A and 142B both have the same excitation light power, and both output excitation 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 light can be multiplexed or demultiplexed while maintaining the polarization state.
With such a configuration, in the optical fiber amplifier shown in FIG. 42, the excitation light combined with orthogonal polarization is incident from the output end of the dispersion compensating fiber 141 by the optical demultiplexer / multiplexer 143, and the dispersion compensating fiber 141 Effectively produces Raman amplification. The loss compensation of the dispersion compensating fiber can be performed by this Raman amplification.

Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.
Further, a silica-based optical fiber can be used instead of the dispersion compensating fiber 141.

Furthermore, the excitation light source 142 is configured to depolarize the excitation light by combining the excitation light source and the depolarizer in the same manner as the excitation light sources 53-2 ′ and 53-2 ″ shown in FIGS. 44 and 45, for example. It may be configured, and may be configured to generate modulated excitation light.
The excitation light sources 53-2 ′ and 53-2 ″ shown in FIGS. 44 and 45 will be described in a first modification of the fourteenth embodiment and a second modification of the fourteenth embodiment, respectively.

(14) Description of Fourteenth Embodiment FIG. 43 is a block diagram showing a fourteenth embodiment relating to a technique related to the present invention. The optical fiber amplifier shown in FIG. 43 includes isolators 55-1, in order from the input side. An optical demultiplexer / multiplexer 54-1, an erbium-doped fiber (rare earth doped fiber) 51, an isolator 55-2, a dispersion compensating fiber 52, a polarization maintaining optical demultiplexer / multiplexer 54-2, and an isolator 55-3 are provided. Has been. The pumping light source 53-1 is connected to the optical demultiplexing / multiplexing unit 54-1, and the polarization beam combining type pumping light source 53-2 is connected to the optical demultiplexing / multiplexing unit 54-2.

Here, the excitation light source 53-1 outputs, for example, 0.98 μm excitation light, and the excitation light source 53-2 includes two excitation light sources 53-2A and 53-2B and these excitation light sources 53-2A. , 53-2B is composed of a polarization beam combiner (PBS) 53-2C that synthesizes orthogonally polarized waves with respect to the excitation light.
Also in this case, both the excitation light sources 53-2A and 53-2B have the same excitation light power, and both output excitation light of, for example, 1.45 to 1.49 μm (or 1.45 to 1.48 μm). Is.

As the optical demultiplexing / multiplexing unit 54-1, a fusion type having no polarization maintaining function is used, while as the optical demultiplexing / multiplexing unit 54-2, an optical film type is used. It is used so that light can be multiplexed or demultiplexed while maintaining the polarization state.
With this configuration, in the optical fiber amplifier shown in FIG. 43, the pumping light from the pumping light source 53-1 is incident from the optical demultiplexing / multiplexing unit 54-1 along 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.

Further, the excitation light combined with the orthogonal polarization is incident from the output end of the dispersion compensating fiber 52 by the optical demultiplexer / multiplexer 54-2, and the Raman compensation is effectively generated in the dispersion compensating fiber 52. The loss compensation of the dispersion compensating fiber 52 is performed by this Raman amplification.
Even if it does in this way, the same effect thru | or advantage as above-mentioned 13th Embodiment is acquired.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.

Further, the rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber may be configured as an optical amplifying unit having a low noise figure, and the Raman optical amplifying unit made of dispersion-compensating fiber is arranged as a pre-amplifying unit. A rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber may be arranged as a subsequent-stage amplifying unit.
(14-1) Description of First Modification of Fourteenth Embodiment FIG. 44 is a block diagram showing a first modification of the fourteenth embodiment. The optical fiber amplifier shown in FIG. 44 includes isolators in order from the input side. 55-1, an optical demultiplexer / multiplexer 54-1, an erbium doped fiber (rare earth doped fiber) 51, an isolator 55-2, a dispersion compensating fiber 52, a polarization maintaining optical demultiplexer / multiplexer 54-2, an isolator 55- 3 is disposed. The pumping light source 53-1 is connected to the optical demultiplexing / multiplexing unit 54-1, and the non-polarization polarization combining type pumping light source 53-2 'is connected to the optical demultiplexing / multiplexing unit 54-2. ing.

Here, the excitation light source 53-1 outputs, for example, 0.98 μm excitation light, and the excitation light source 53-2 ′ is depolarized (non-polarized light) with one excitation light source 53-2 A ′. And a depolarizer 53-2B ′.
Here, the depolarizer 53-2B ′ reduces the polarization dependency in the Raman optical amplifier including the dispersion compensating fiber 52, and the polarization maintaining coupler 53- demultiplexes the pumping light from the pumping light source 53-2A ′. 2E ′, and a polarization beam combiner (PBS) 53-2C ′ that combines the polarization beams of the excitation light demultiplexed by the polarization maintaining coupler 53-2E ′ and the excitation light delayed by the delay line. ing.

Also in this case, the excitation light source 53-2A ′ outputs, for example, 1.45 to 1.49 μm (or 1.45 to 1.48 μm) of excitation light.
As the optical demultiplexer / multiplexer 54-1, a fusion type having no polarization maintaining function is used, while the optical demultiplexer / multiplexer 54-2 is also an optical film type. It is used so that light can be multiplexed or demultiplexed while maintaining the polarization state.

With such a configuration, 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 from one end of the erbium-doped fiber 51 together with the signal light. As a result, the signal light is amplified by the erbium-doped phi 51.
Further, the non-polarized pumping light is incident from the output end of the dispersion compensating fiber 52 by the optical demultiplexer / multiplexer 54-2, and the dispersion compensating fiber 52 effectively causes Raman amplification. The loss compensation of the dispersion compensating fiber 52 is performed by this Raman amplification.

By doing this, it is possible to obtain the same effects or advantages as those of the above-described fourteenth embodiment while reducing the polarization dependence in the dispersion compensating fiber 52.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.

Further, the rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber may be configured as an optical amplifying unit having a low noise figure, and the Raman optical amplifying unit made of dispersion-compensating fiber is arranged as a pre-amplifying unit. A rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber may be arranged as a subsequent-stage amplifying unit.
(14-2) Description of Second Modification of Fourteenth Embodiment FIG. 45 is a block diagram showing a second modification of the fourteenth embodiment. The optical fiber amplifier shown in FIG. 55-1, an optical demultiplexer / multiplexer 54-1, an erbium doped fiber (rare earth doped fiber) 51, an isolator 55-2, a dispersion compensating fiber 52, a polarization maintaining optical demultiplexer / multiplexer 54-2, an isolator 55- 3 is disposed. The pumping light source 53-1 is connected to the optical demultiplexing / multiplexing unit 54-1, and the modulation polarization combining type pumping light source 53-2 ″ is connected to the optical demultiplexing / multiplexing unit 54-2. ing.

  Here, the excitation light source 53-1 outputs, for example, 0.98 μm excitation light, and the excitation light source 53-2 ″ includes two excitation light sources 53-2 A ″ and 53-2 B ″, A polarization beam combiner (PBS) 53-2C ″ for combining the excitation light from these excitation light sources 53-2A ″ and 53-2B ″ with orthogonal polarization, and each excitation light source 53-2A ″, 53−. 2B ″ is composed of a modulator 53-2D ″ that modulates several hundred kHz to 1 MHz.

Also in this case, the pumping light sources 53-2A ″ and 53-2B ″ both have the same pumping light power, and both pumping of, for example, 1.45 to 1.49 μm (or 1.45 to 1.48 μm). It outputs light.
As the optical demultiplexer / multiplexer 54-1, a fusion type having no polarization maintaining function is used, while the optical demultiplexer / multiplexer 54-2 is also an optical film type. It is used so that light can be multiplexed or demultiplexed while maintaining the polarization state.

With such an arrangement, in the optical fiber amplifier shown in FIG. 45, the pumping light from the pumping light source 53-1 is incident from the optical demultiplexer-multiplexer 54-1 along 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 phi 51.
Also, the modulated light having a spectrum of several hundred kHz or higher and the orthogonal polarization combined (the spectral line width of the pump light can be widened) is generated by the optical demultiplexer / multiplexer 54-2. The light is incident from the output end of the dispersion compensating fiber 52, and the Raman compensation is effectively generated by the dispersion compensating fiber 52. The loss compensation of the dispersion compensating fiber is performed by this Raman amplification.

In this way, the same effects or advantages as those of the fourteenth embodiment described above can be obtained while increasing the threshold value of stimulated Brillouin scattering and suppressing harmful nonlinear effects.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.

Further, the rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber may be configured as an optical amplifying unit having a low noise figure, and the Raman optical amplifying unit made of dispersion-compensating fiber is arranged as a pre-amplifying unit. A rare-earth-doped fiber optical amplifying unit made of erbium-doped fiber may be arranged as a subsequent-stage amplifying unit.
(15) Description of Fifteenth Embodiment FIG. 48 is a block diagram showing a fifteenth embodiment of the technology related to the present invention. The optical fiber amplifier shown in FIG. Optical demultiplexer / multiplexer 124-1, erbium doped fiber (rare earth doped fiber) 121-1, isolator 125-2, silica-based optical fiber 122, erbium doped fiber (rare earth doped fiber) 121-2, optical demultiplexer / multiplexer A device 124-3 and an isolator 125-3 are provided. Then, pumping light sources 123-1 and 123-3 that generate pumping light in a 1.47 μ band (1.45 to 1.49 μm), for example, are connected to the optical demultiplexers / multiplexers 124-1 and 124-3. .

Here, the silica-based optical fiber 122 functions as a Raman optical amplifier whose amplification frequency band can be changed depending on the excitation wavelength, and the band characteristic is determined by the silica and core doping material and concentration of the host glass.
The erbium-doped fibers 121-1 and 121-2 function as rare-earth-doped fiber optical amplifiers whose amplification frequency band and band characteristics are determined by the host glass and the core doping material.

  The mode field diameter of the silica-based optical fiber 122 in the present embodiment is reduced, and the noise figure of the Raman optical amplifier composed of the silica-based optical fiber 122 is rare earth-doped fiber light composed of the erbium-doped fibers 121-1 and 121-2. If the amplifier is larger than the amplifier, a rare-earth-doped fiber optical amplifier is used for the pre-amplifier, a Raman optical amplifier is used for the middle-amplifier, and a rare-earth-doped fiber optical amplifier is used for the post-amplifier with high signal light power. By connecting them in cascade, 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 with 1.47 μm band excitation) is used for the pre-amplifier, and a minimal signal light is amplified in a low noise state. , Nonlinear effects that degrade the signal-to-noise ratio (SNR) (where nonlinear effects are self-phase modulation (SPM), four-wave mixing (FWM), cross-phase) In order to reduce the influence of modulation (Cross-Phase Modulation, XPM) and other effects on signal-to-noise ratio (SNR), a Raman optical amplifier using a silica-based optical fiber with low signal light power It is used for the amplification section.

  With this configuration, in the optical fiber amplifier shown in FIG. 48, the pumping light is incident from one end of the erbium-doped fiber 121-1 by the optical demultiplexer / multiplexer 124-1 to pump the erbium-doped fiber 121-1. Then, the signal light is amplified, and the silica-based optical fiber 122 is excited by the residual pumping light generated at this time, and the Raman amplification is performed in the same manner as the dispersion compensating fiber.

Further, the pump light is incident from the output end of the erbium-doped fiber 121-2 by the optical demultiplexer / multiplexer 124-3 to excite the erbium-doped fiber 121-2 to amplify the signal light, and the residual generated at this time The silica-based optical fiber 122 is excited by the excitation light and is Raman-amplified.
As described above, in the optical fiber amplifier shown in FIG. 48, any of the erbium-doped fibers 121-1 and 121-2 and the silica-based optical fiber 122 is obtained by using the pump light sources 123-1 and 123-3 in the 1.47 μm band. Thus, the pumping light source 123-2 in the optical fiber amplifier shown in FIG. 12 can be reduced, and the simplification of the optical fiber amplifier and the efficiency of the pumping light power can be achieved.

Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.
An isolator may be provided between the silica-based optical fiber 122 and the erbium-doped fiber 121-2.

An excitation light source and an optical demultiplexer / multiplexer for the silica-based optical fiber 122 can also be provided.
That is, in the same manner as in FIG. 11, an optical fiber amplifier is configured using the 0.98 μm band pumping light sources 123-1 to 123-3 and the optical demultiplexers / multiplexers 124-1 to 124-3. Also good.

Instead of the silica-based optical fiber 122, a dispersion compensating fiber may be used.
(15-1) Description of Modified Example of Fifteenth Embodiment FIG. 49 is a block diagram showing a modified example of the fifteenth embodiment. The optical fiber amplifier shown in FIG. 49 includes, in order from the input side, isolators 125-1, Optical demultiplexer / multiplexer 124-1, erbium doped fiber (rare earth doped fiber) 121-1, isolator 125-2, silica-based optical fiber 122, optical filter 126, erbium doped fiber (rare earth doped fiber) 121-2, light A demultiplexer / multiplexer 124-3 and an isolator 125-3 are provided. Then, polarization-combining pumping light sources 123-1 'and 123-3' are connected to the optical demultiplexers / multiplexers 124-1 and 124-3, respectively.

  Here, the excitation light source 123-1 ′ is a polarization that combines the two excitation light sources 123-1 A ′ and 123-1 B ′ and orthogonally polarized light for the excitation light from these excitation light sources 123-1 A ′ and 123-1 B ′. The pumping light sources 123-1A 'and 123-1B' both have the same pumping light power, and are both 1.45 to 1.49 [mu] m (or 1), for example. .45 to 1.48 μm) of excitation light.

  Further, the excitation light source 123-3 'is a polarized wave that combines two excitation light sources 123-3A' and 123-3B 'and orthogonally polarized light with respect to the excitation light from these excitation light sources 123-3A' and 123-3B '. Although it is composed of a synthesizer (PBS) 123-3C ', it is a pumping light source that is simply combined with orthogonal polarization in order to increase the pumping light power, so that the pumping light sources 123-3A' and 123-3B 'are pumped. The wavelength and pump light power may be different.

Further, the erbium-doped fiber 121-1 and the silica-based optical fiber 122 are firmly fixed to a bobbin or the like so that the non-polarized state of the excitation light combined with the orthogonal polarization is maintained in the silica-based optical fiber 122. By being housed in the case, it is not affected by outside air.
The isolators 125-1 to 125-3 are polarization-independent optical isolators, and the optical filter 126 removes or flattens the ASE peak near 1.535 μm generated in the erbium-doped fiber 121-1. It is an optical filter and can be omitted.

  With this configuration, in the optical fiber amplifier shown in FIG. 49, the 1.47 μm band excitation light is incident from one end of the erbium-doped fiber 121-1 by the optical demultiplexer / multiplexer 124-1, and the erbium-doped fiber. 121-1 is excited to amplify the signal light, and the silica-based optical fiber 122 is excited by the residual excitation light generated at this time to be Raman amplified.

Further, 1.47 μm excitation light is incident from the output end of the erbium-doped fiber 121-2 by the optical demultiplexer / multiplexer 124-3 to excite and amplify the erbium-doped fiber 121-2. The silica-based optical fiber 122 is excited by the residual excitation light, and Raman amplification is performed.
In this manner, in the optical fiber amplifier shown in FIG. 49, the erbium-doped fibers 121-1 and 121-2 and the silica-based optical fiber 122 are used by using the pump light sources 123-1 ′ and 123-3 ′ in the 1.47 μm band. 11 can be pumped, whereby the pumping light source 123-2 in the optical fiber amplifier shown in FIG. 11 can be reduced, and the optical fiber amplifier can be simplified and the pumping light power can be made more efficient. it can.

Also in this case, instead of providing an isolator in the input unit or the input / output unit, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator, and the output signal light is transmitted through the optical circulator. It can be configured to be output.
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, an optical fiber amplifier may be configured using the 0.98 μm band pumping light sources 123-1 to 123-3 and the optical demultiplexers / multiplexers 124-1 to 124-3. Good.
An isolator may be provided between the silica-based optical fiber 122 and the erbium-doped fiber 121-2.

Further, instead of the silica-based optical fiber 122, a dispersion compensating fiber may be used.
(16) Description of Sixteenth Embodiment FIG. 50 is a block diagram showing a sixteenth embodiment relating to the technique related to the present invention. The optical fiber amplifier shown in FIG. 50 includes isolators 115-1, in order from the input side. 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 114-2, and an isolator 115-3 are arranged. It is installed. The pumping light source 113-1 is connected to the optical demultiplexing / multiplexing device 114-1, and the polarization beam combining type pumping light source 113-2 is connected to the optical demultiplexing / multiplexing device 114-2.

  Therefore, in the optical fiber amplifier shown in FIG. 50, these rare earth doped fiber optical amplifier and Raman optical amplifier are compensated for each other so that a flatter band characteristic or a wide amplification frequency band can be obtained. A rare-earth-doped fiber optical amplifier having a low noise figure (such as an erbium-doped fiber optical amplifier with 0.98 μm band pumping or 1.47 μm band pumping) is used for the pre-amplifier and a silica-based optical fiber is used for the post-amplifier. By connecting them in series using a Raman optical amplifier, the optical fiber amplifier has a flat 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 former stage amplification section and the Raman optical amplifier is used in the subsequent stage amplification section. By connecting to, a low-noise optical fiber amplifier is realized.
Further, the excitation light source 113-1 outputs, for example, 0.98 μm excitation light, and the excitation light source 113-2 includes two excitation light sources 113-2 A and 113-2 B, and these excitation light sources 113-2 A, It comprises a polarization beam combiner (PBS) 113-2C that performs orthogonal polarization combining on the excitation light from 113-2B.

Also in this case, both the excitation light sources 113-2A and 113-2B have the same excitation light power, and both output excitation light of, for example, 1.45 to 1.49 μm (or 1.45 to 1.48 μm). Is.
As the optical demultiplexer / multiplexer 114-1, a fusion type without polarization maintaining function is used, while as the optical demultiplexer / multiplexer 114-2, an optical film type is used. It is used so that light can be multiplexed or demultiplexed while maintaining the polarization state.

With such a configuration, in the optical fiber amplifier shown in FIG. 50, the pumping light from the pumping light source 113-1 is incident along with the signal light from one end of the erbium-doped fiber 111 from the optical demultiplexing multiplexer 114-1. As a result, the signal light is amplified by the erbium-doped phi 111.
Further, the excitation light combined with the orthogonal polarization is incident from 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, loss compensation of the silica-based optical fiber 112 is performed by this Raman amplification.

Even if it does in this way, the same effect thru | or advantage as above-mentioned 14th Embodiment is acquired.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.
An excitation light source that generates excitation light in the 1.47 μm band may be provided, and this excitation light source may be used as both a silica-based optical fiber excitation light source and an erbium-doped fiber excitation light source.

When a high output cannot be obtained by the Raman optical amplifier, a Raman optical amplifier composed of a silica-based optical fiber or a dispersion compensating fiber is used for the input-side amplifier (pre-stage amplifier), and the output-side amplifier ( These are connected in cascade using a rare-earth doped fiber optical amplifier made of erbium-doped fiber in the subsequent stage amplification unit).
In particular, when the pump wavelength of the pump light source for the Raman optical amplifier is about 1.44 μm, the gain depression generated in the vicinity of about 1.54 μm in the rare-earth doped fiber optical amplifier can be compensated by Raman light amplification. When the pumping wavelength of the pumping light source for the amplifier is about 1.46 μm, the decrease in gain occurring on the longer wavelength side than about 1.57 μm in the rare earth-doped fiber optical amplifier can be compensated by Raman optical amplification. The bandwidth characteristics of the amplifier can be further flattened or increased.

  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-based optical fiber or the dispersion compensating fiber starts to be generated, the silica-based optical fiber having a reduced mode field diameter is used. In order to reduce the influence of the non-linear effect that has become larger because the mode field diameter has been reduced, a Raman optical amplifier comprising a silica-based optical fiber is used for the input-side amplification unit (pre-stage amplification unit) with low signal light power. In addition, a rare-earth doped fiber optical amplifier made of erbium-doped fiber is used for the output side amplifying part (the latter stage amplifying part) having a large signal light power, and these are connected in cascade, so that the optical fiber amplifier has a further flat band. It is also possible to have a 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. The optical fiber amplifier shown in FIG. 115-1, optical demultiplexer-multiplexer 114-1, erbium-doped fiber (rare earth-doped fiber) 111, isolator 115-2, silica-based optical fiber 112, polarization maintaining optical demultiplexer-multiplexer 114-2, isolator 115 -3 is arranged. The pumping light source 113-1 is connected to the optical demultiplexing / multiplexing device 114-1, and the non-polarization polarization combining type pumping light source 113-2 'is connected to the optical demultiplexing / multiplexing device 114-2. ing.

Here, the excitation light source 113-1 outputs, for example, 0.98 μm excitation light, and the excitation light source 113-2 ′ depolarizes the excitation light (non-polarized light) with one excitation light source 113-2A ′. And a depolarizer 113-2B ′.
The depolarizer 113-2B ′ is for reducing the polarization dependence in the Raman optical amplifier composed of the silica-based optical fiber 112. The polarization maintaining coupler 113- demultiplexes the pump light from the pump light source 113-2A ′. 2E ′ and a polarization beam combiner (PBS) 113-2C ′ that combines the polarization beams of the excitation light demultiplexed by the polarization maintaining coupler 113-2E ′ and the excitation light delayed by the delay line. ing.

Also in this case, the excitation light source 113-2A ′ outputs, for example, 1.45 to 1.49 μm (or 1.45 to 1.48 μm) excitation light.
As the optical demultiplexer / multiplexer 114-1, a fusion type that does not have the function of maintaining polarization is used, while the optical demultiplexer / multiplexer 114-2 is also an optical film type. It is used so that light can be multiplexed or demultiplexed while maintaining the polarization state.

With this configuration, in the optical fiber amplifier shown in FIG. 51, the pumping light from the pumping light source 113-1 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 signal light is amplified by the erbium-doped phi 111.
Further, the non-polarized pumping light is incident from the output end of the silica optical fiber 112 by the optical demultiplexer / multiplexer 114-2, and Raman amplification is effectively generated in the silica optical fiber 112. Then, loss compensation of the silica-based optical fiber 112 is performed by this Raman amplification.

In this way, the same effects or advantages as those of the sixteenth embodiment described above can be obtained while reducing the polarization dependence in the silica-based optical fiber 112.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.

An excitation light source that generates excitation light in the 1.47 μm band may be provided, and this excitation light source may be used as both a silica-based optical fiber excitation light source and an erbium-doped fiber excitation 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. The optical fiber amplifier shown in FIG. 115-1, optical demultiplexer-multiplexer 114-1, erbium-doped fiber (rare earth-doped fiber) 111, isolator 115-2, silica-based optical fiber 112, polarization maintaining optical demultiplexer-multiplexer 114-2, isolator 115 -3 is arranged. The pumping light source 113-1 is connected to the optical demultiplexing / multiplexing device 114-1, and the modulation polarization combining type pumping light source 113-2 ″ is connected to the optical demultiplexing / multiplexing device 114-2. ing.

  Here, the excitation light source 113-1 outputs, for example, 0.98 μm excitation light, and the excitation light source 113-2 ″ includes two excitation light sources 113-2 A ″ and 113-2 B ″, A polarization beam combiner (PBS) 113-2C ″ for combining the excitation light from these excitation light sources 113-2A ″ and 113-2B ″ with orthogonal polarization, and each excitation light source 113-2A ″, 113−. 2B ″ is composed of a modulator 113-2D ″ that modulates several hundred kHz to 1 MHz.

Also in this case, the pumping light sources 113-2A ″ and 113-2B ″ both have the same pumping light power, and both pumping of 1.45 to 1.49 μm (or 1.45 to 1.48 μm), for example. It outputs light.
As the optical demultiplexer / multiplexer 114-1, a fusion type that does not have the function of maintaining polarization is used, while the optical demultiplexer / multiplexer 114-2 is also an optical film type. It is used so that light can be multiplexed or demultiplexed while maintaining the polarization state.

With such a configuration, 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. As a result, amplification is performed by the erbium-doped phi 111.
Also, the modulated light having a spectrum of several hundred kHz or higher and the orthogonally polarized combined light (which can widen the spectral line width of this pumped light) is transmitted by the optical demultiplexer / multiplexer 114-2. The light is incident from the output end of the silica-based optical fiber 112 and the silica-based optical fiber 112 effectively causes Raman amplification. Then, loss compensation of the silica-based optical fiber 112 is performed by this Raman amplification.

In this way, the same effects or advantages as those of the sixteenth embodiment described above can be obtained while increasing the threshold value of stimulated Brillouin scattering and suppressing harmful nonlinear effects.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.

An excitation light source that generates excitation light in the 1.47 μm band may be provided, and this excitation light source may be used as both a silica-based optical fiber excitation light source and an erbium-doped fiber excitation light source.
(17) Description of Seventeenth Embodiment FIG. 55 is a block diagram showing a seventeenth embodiment relating to a technique related to the present invention. The optical fiber amplifier shown in FIG. An optical demultiplexer / multiplexer 64, an erbium doped fiber (rare earth doped fiber amplifier) 61, a dispersion compensating fiber 62 (optical fiber attenuator), and an isolator 65-3 are disposed. An excitation light source 63 is connected to the optical demultiplexer / multiplexer 64.

Here, the excitation light source 63 generates excitation light in a 1.47 μm band (1.45 to 1.49 μm), for example.
Moreover, in rare earth doped fiber optical amplifiers with high gain, unnecessary oscillation may occur when performing optical amplification. If 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, 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, an erbium-doped fiber optical amplifier adjusted for multi-wavelength simultaneous amplification (that is, an erbium-doped fiber optical amplifier with a high excitation rate) has a high gain in the vicinity of 1.53 μm. Therefore, unnecessary oscillation easily occurs at this wavelength. When such unnecessary oscillation occurs, the erbium-doped fiber optical amplifier operates in an unstable manner.

In order to suppress such unstable operation, it is effective to provide a medium for losing (attenuating) 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 pumped by the residual pumping light incident through the erbium doped fiber 61, and the loss (attenuation) of the signal light in the dispersion compensating fiber 62 is compensated. However, in practice, it is difficult to compensate for all losses, and a certain amount of loss remains, so that the dispersion compensating fiber 62 functions as a loss medium.

Here, the principle of suppressing unstable operation by providing a lossy medium will be described.
In general, the gain of the erbium-doped fiber is G, and the reflectances at both ends (front end and rear end) of the erbium-doped fiber are R1 and R2, respectively (where the reflectance R1 is all of the upstream stages of the erbium-doped fiber. The reflectance of the reflection from the component, and the reflectance R2 is the reflectance of the reflection from all components subsequent to the rear end of the erbium-doped fiber). ) ^1/2 ], GR can be used as a measure of the stability of the operation of the erbium-doped fiber. When the GR is large, the erbium-doped fiber operates in an unstable manner. In particular, when the GR is 1 or more, the erbium-doped fiber oscillates. For this reason, it is necessary to make GR small, and specifically, GR becomes 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 represented by η () on the output side of the signal light) after the erbium doped fiber 61 (the gain of the erbium doped fiber 61 is G). 0 ≦ η ≦ 1)] is provided, for example, by 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, and the reflectance R1 is the value of the erbium-doped fiber 61. (Reflectance R2 is the reflectivity of reflection from all components subsequent to the rear end of the dispersion compensating fiber 62). Further, when the reflectance of reflection caused by the difference in refractive index at the boundary A between the erbium-doped fiber 61 and the dispersion compensating fiber 62 is RA (RA << R1, R2; this condition is satisfied if the loss medium is an optical fiber). The parameter indicating the operational stability of the erbium-doped fiber is changed from GR to (Gη) R. That is, GR is considered as a one-way gain when the light makes a round. When a loss medium is provided, the net gain when the light makes a round is (R1 × G × η) × (R2 × η × G) = (Gη) ² R1R2, so the net gain in one way is G
η (R1R2) ^1/2 = (Gη) R. Since RA << R1, R2, reflectance R
The influence of A can be ignored. Here, since 0 ≦ η ≦ 1, GR is equivalently reduced.

Thus, by providing the loss medium, the parameter GR indicating the stability of the operation of the erbium-doped fiber is reduced, so that 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 at the subsequent stage of the erbium doped fiber 61, and this dispersion compensating fiber 62 is used with the residual pumping light from the erbium doped fiber 61. By performing excitation, the loss compensation of the dispersion compensating fiber 62 (including the flattening of the gain depression of the erbium-doped fiber 61 and the compensation compensation for the decrease in the gain of the erbium-doped fiber 61) is performed at the same time. 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 incident from one end of the erbium doped fiber 61 by the optical demultiplexer / multiplexer 64 to excite the erbium doped fiber 61, and the signal Although the light is amplified, the residual excitation light reaches the other end of the erbium-doped fiber 61. Thereafter, the residual pumping light is supplied to the dispersion compensating fiber 62 to cause Raman amplification.

In this way, amplification using both fibers can be performed by using a common pumping light source for the erbium-doped fiber and the dispersion compensating fiber.
That is, the excitation wavelength band when Raman-amplifying 1.55 μm band signal light is the 1.47 μm band (1.45-1.49 μm) which is the excitation wavelength band of the erbium-doped fiber (EDF), Therefore, Raman amplification can be performed using the residual pumping light power when the EDF is pumped with 1.47 μm band light. Thus, the loss of the dispersion compensating fiber 62 can be compensated 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, which is suitable for performing multi-wavelength collective amplification, etc. Since only one excitation light source is required, the structure can be simplified and the cost can be reduced.
Further, in this optical fiber amplifier, the unstable operation of the erbium-doped fiber 61 is simultaneously suppressed by the loss of the dispersion compensating fiber 62, whereby the rare earth adjusted for wavelength multiplexing (WDM). Unnecessary oscillation operation in the doped fiber optical amplifier can be prevented and stable optical amplification can be performed.
When the excitation light source 63 generates 0.98 μm excitation light, the dispersion compensation fiber 62 does not perform Raman amplification, and therefore the loss compensation of the dispersion compensation fiber 62 is not performed.

Further, in an optical fiber such as a dispersion compensating fiber, there is reflection due to Rayleigh backscattering. The reflectance of this reflection depends on the length of the optical fiber, and the reflectance of this reflection increases as the length of the optical fiber increases.
Therefore, when the influence of the reflectance due to Rayleigh backscattering of the dispersion compensation fiber itself is larger than the effect of reducing the reflectance due to the loss of the dispersion compensation fiber as described above, an optical isolator may be added to the dispersion compensation fiber. Conceivable.

For example, in FIG. 55, this optical isolator can be provided between the erbium-doped fiber 61 and the dispersion compensating fiber 62. In this way, when the influence of the reflectance due to Rayleigh backscattering appears, the reflectance is always maintained. Can be reduced.
Furthermore, in this case, instead of providing an isolator in the input unit or the input / output unit, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator, and the output signal light is transmitted through the optical circulator. It can also be configured to output.

The pumping light source 63 may be composed of two pumping light sources and a polarization beam synthesizer that performs orthogonal polarization synthesis on the pumping light from these pumping light sources. The pumping light source and the depolarizer combine the pumping light. May be configured to depolarize, and may be configured to generate modulated excitation light.
(17-1) Description of First Modification of Seventeenth Embodiment FIG. 56 is a block diagram showing a first modification of the seventeenth embodiment. The optical fiber amplifier shown in FIG. 115-1, an optical demultiplexer / multiplexer 114-1, an erbium doped fiber (rare earth doped fiber amplifier) 111, a silica-based optical fiber 112 (optical fiber attenuator), and an isolator 115-3 are provided. The pumping light source 113-1 is connected to the optical demultiplexer-multiplexer 114-1.

The pumping light source 113-1 outputs pumping light in a 1.47 μm band (1.45 to 1.49 μm), for example, and the optical demultiplexer / multiplexer 114-1 is, for example, a fusion type. Things are used.
As described in the seventeenth embodiment, in the rare earth doped fiber optical amplifier having a high gain, unnecessary oscillation may occur when performing optical amplification. Rare earth doped fiber optical amplifiers operate unstable.

  56, similarly to the case of the optical fiber amplifier shown in FIG. 55, a silica-based optical fiber 112 as a loss medium is provided after the erbium-doped fiber 111 as the rare earth-doped fiber optical amplifier. By providing this, the unstable operation of the erbium-doped fiber 111 is suppressed. Also in FIG. 56, R1, R2 and RA represent the reflectance, and A represents the boundary.

Similarly to the seventeenth embodiment described above, in the optical fiber amplifier shown in FIG. 56, the silica-based optical fiber 112 is provided at the rear stage of the erbium-doped fiber 111, and the silica-based optical fiber 112 is made to remain from the erbium-doped fiber 111. Simultaneously with the loss compensation of the silica-based optical fiber 112 (including the flattening of the gain depression of the erbium-doped fiber 111 and the compensation compensation for the decrease of the gain of the erbium-doped fiber 111) by performing excitation using the pumping light. The unstable operation of the erbium-doped fiber 111 is suppressed by the remaining loss.
With such a configuration, in the optical fiber amplifier shown in FIG. 56, the pumping light from the pumping light source 113-1 enters the one end of the erbium-doped fiber 111 from the optical demultiplexing multiplexer 114-1 together with the signal light. . As a result, the signal light is amplified by the erbium-doped phi 111.

Further, the silica-based optical fiber 112 is excited by using the residual pumping light generated at this time, so that Raman amplification is performed in the same manner as the dispersion compensating fiber, and loss compensation of the silica-based optical fiber 112 is performed by this Raman amplification.
As described above, in the optical fiber amplifier shown in FIG. 56, by using the pumping light source 113-1 in the 1.47 μm band, it is possible to pump both the erbium-doped fiber 111 and the silica-based optical fiber 112. The simplification of the optical fiber amplifier and the efficiency of the pumping light power can be achieved.

Further, in this optical fiber amplifier, unnecessary oscillation generated by the erbium-doped fiber 111 is simultaneously removed by the loss of the silica-based optical fiber 112, thereby adjusting for wavelength multiplexing (WDM). An unnecessary oscillation operation in the rare earth-doped fiber optical amplifier can be prevented and stable optical amplification can be performed.
Note that when the pumping light source 113-1 generates pumping light of 0.98 μm, the silica-based optical fiber 112 does not perform Raman amplification, and therefore the loss compensation of the silica-based optical fiber 112 is not performed.

In addition, reflection due to Rayleigh backscattering exists in optical fibers such as silica-based optical fibers. The reflectance of this reflection depends on the length of the optical fiber, and the reflectance of this reflection increases as the length of the optical fiber increases.
Therefore, an optical isolator is added to the silica optical fiber when the influence of the reflectance due to Rayleigh backscattering of the silica optical fiber itself is larger than the effect of reducing the reflectance due to the loss of the silica optical fiber as described above. It is possible to do.

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 optical isolator is always reflected. The rate can be reduced.
Also in this case, instead of providing an isolator in the input unit or the input / output unit, 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.

(17-2) Description of Second Modification of Seventeenth Embodiment FIG. 57 is a block diagram showing a second modification of the seventeenth embodiment. The optical fiber amplifier shown in FIG. 65-1, optical demultiplexer-multiplexer 64-1, erbium-doped fiber (pre-stage optical amplifying unit configured as a rare earth-doped fiber amplifying unit) 61-1, dispersion compensation fiber 62 (optical fiber attenuating unit), erbium-doped fiber (Second-stage optical amplifier configured as a rare earth-doped fiber amplifier) 61-2, an optical demultiplexer-multiplexer 64-2, and an isolator 65-3 are provided. The excitation light source 63-1 is connected to the optical demultiplexer-multiplexer 64-1, and the excitation light source 63-2 is connected to the optical demultiplexer-multiplexer 64-2.

Here, both the excitation light sources 63-1 and 63-2 generate excitation light in a 1.47 μm band (1.45 to 1.49 μm), for example.
As described in the seventeenth embodiment, in the rare earth-doped fiber optical amplifier having a high gain, unnecessary oscillation may occur when performing optical amplification. When such unnecessary oscillation occurs, Rare earth doped fiber optical amplifier operates unstable.

In the optical fiber amplifier according to the seventeenth embodiment shown in FIG. 55, by providing a dispersion compensating fiber 62 as a loss medium in the subsequent stage of the erbium doped fiber 61 as the rare earth doped fiber optical amplifier, the erbium doped fiber 61 is provided. Unstable operation 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 is large (because the gain G of the erbium-doped fiber 61 is very large, Although the influence of the reflectance RA cannot be ignored despite RA << R1 and R2, the effect of the loss η does not appear even if the dispersion compensating fiber 62 is provided in the subsequent stage, and the erbium-doped fiber 61 is not effective. Stable operation 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 front and rear erbium-doped fibers, and a dispersion compensating fiber 62 is disposed between them. The one 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, between the erbium doped fibers 61-1 and 61-2 (the gain of the erbium doped fibers 61-1 and 61-2 is set to G / 2, respectively), the dispersion compensating fiber 62 [this dispersion compensating If the loss of the fiber 62 is η (0 ≦ η ≦ 1), for example, by fusion splicing, a boundary A ′ is generated between the erbium-doped fiber 61-1 and the dispersion compensating fiber 62, A boundary B ′ is generated between the dispersion compensating fiber 62 and the erbium doped fiber 61-2.

  Further, the reflectance at the front end of the erbium-doped fiber 61-1 is R1 ', the reflectance at the rear end of the erbium-doped fiber 61-2 is R2', and the reflectance at the boundary A 'is RA' (RA '<< R1 ′, R2 ′), and the reflectance at the boundary B ′ is RB ′ (RB ′ << R1 ′, R2 ′). Here, the reflectivity R1 ′ is the reflectivity of reflection from all components preceding the front end of the erbium-doped fiber 61-1, and the reflectivity R2 ′ is subsequent to the rear end of the erbium-doped fiber 61-2. This is the reflectance of reflection from all parts. Further, 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 ′.

  Possible GR parameters at this time are: (1) GR parameter composed of the reflectance R1 ′, the gain G / 2 of the erbium-doped fiber 61-1 and the reflectance RA ′, and (2) the reflectance R1 ′, erbium-doped. GR parameter composed of gain G / 2 of fiber 61-1, loss η and reflectivity RB ', (3) reflectivity R1', gain G / 2 of erbium doped fiber 61-1, loss η, erbium doped fiber A GR parameter composed of a gain G / 2 of 61-2 and a reflectance R2 '; (4) composed of a reflectance RA', a loss η, a gain G / 2 of the erbium-doped fiber 61-2 and a reflectance R2 '. (5) the reflectance RB ′, the gain G / 2 of the erbium-doped fiber 61-2, and the reflectance R2 ′.

Here, regarding (1), the gain of the erbium-doped fiber 61 shown in FIG. 55 is G, so that GR = G (R1RA) ^1/2 , whereas the erbium-doped fiber 61 shown in FIG. −1, the gain is G / 2 and half the gain G of the erbium-doped fiber 61 shown in FIG. 55, so that GR = (G / 2) (R1RA) ^1/2 (R
A ′ = RA), which is half of the GR of the erbium-doped fiber 61 shown in FIG.

As for (2), in the erbium-doped fiber 61-1, since there is a loss η (0 ≦ η ≦ 1) in the subsequent stage, as in the case of the seventeenth embodiment, when the light makes a round, Since the gain is [R1 ′ × (G / 2) × η] × [RB ′ × η × (G / 2)] = [(G / 2) η] ² R1′RB ′, the net in one way Gain of (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 is further smaller than in the case of (1), and the GR at this time can be ignored.

As for (3), since there is a loss η (0 ≦ η ≦ 1) between the erbium-doped fibers 61-1 and 61-2, as in the case of the seventeenth embodiment, The net gain is [R1 ′ × (G / 2) × η] × [R2 ′ × η × (G / 2)] = [(G / 2) η] ² R1′R2 ′. The net gain of (G / 2) η (R1′R2 ′) ^1/2 = [(G / 2) η] R, indicating the stability of the operation of the erbium doped fibers 61-1 and 61-2. The parameter is changed from (G / 2) R to [(G / 2) η] R. Since RA ′ << R1 ′, 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 those for (2) and (1), respectively.
Therefore, when the gain G of the erbium-doped fiber 61 shown in FIG. 55 is very large, the erbium-doped fiber 61 operates in an unstable manner because the GR parameters of R1, G, and RA are large. 61 is divided into erbium-doped fibers 61-1 and 61-2 at the front and rear stages as shown in FIG. 57, and a dispersion compensating fiber 62 as a loss medium is disposed between them, so that (1) and With respect to (5), the GR parameter can be reduced, thereby suppressing the unstable operation of the erbium-doped fibers 61-1 and 61-2.

  For this reason, in the optical fiber amplifier shown in FIG. 57, the dispersion compensating fiber 62 is disposed between the erbium doped fibers 61-1 and 61-2, and the dispersion compensating fiber 62 is used as the erbium doped fibers 61-1 and 61. -2 is compensated by using the residual pumping light from -2 to compensate for the loss of the dispersion compensating fiber 62 (flattening of the gain depression of the erbium-doped fibers 61-1 and 61-2 and erbium-doped fibers 61-1 and 61 -2 gain reduction compensation is included), and the unstable operation of the erbium-doped fibers 61-1 and 61-2 is suppressed by the remaining loss.

  With this configuration, in the optical fiber amplifier shown in FIG. 57, the excitation light and the signal light are incident from the input end of the erbium-doped fiber 61-1 by the optical demultiplexer-multiplexer 64-1, and the erbium-doped fiber 61 is obtained. -1 is excited to amplify the signal light. At this time, the residual excitation light reaches the other end of the erbium-doped fiber 61-1. This residual pumping light is supplied to the dispersion compensating fiber 62 to cause Raman amplification.

  The pumping light is incident from the output end of the erbium-doped fiber 61-2 by the optical demultiplexer-multiplexer 64-2, to excite the erbium-doped fiber 61-2, and input from the input end of the erbium-doped fiber 61-2. The amplified signal light is also amplified. At this time, the residual pumping light still 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 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 can be increased. A broadband optical amplifier can be realized while simplifying the structure and reducing the cost.
Further, in this optical fiber amplifier, unnecessary oscillation generated by the erbium-doped fibers 61-1 and 61-2 is also simultaneously removed by the loss of the dispersion compensating fiber 62, whereby wavelength multiplexing ( Unnecessary oscillation operation in the rare earth-doped fiber optical amplifier adjusted for WDM) can be prevented, and stable optical amplification can be performed with little noise.

When the excitation light sources 63-1 and 63-2 generate excitation light of 0.98 μm, the dispersion compensation fiber 62 does not perform Raman amplification, and therefore the loss compensation of the dispersion compensation fiber 62 is not performed.
Further, in an optical fiber such as a dispersion compensating fiber, there is reflection due to Rayleigh backscattering. The reflectance of this reflection depends on the length of the optical fiber, and the reflectance of this reflection increases as the length of the optical fiber increases.

Therefore, when the influence of the reflectance due to Rayleigh backscattering of the dispersion compensation fiber itself is larger than the effect of reducing the reflectance due to the loss of the dispersion compensation fiber as described above, an optical isolator may be added to the dispersion compensation fiber. Conceivable.
For example, in FIG. 57, this optical isolator can be provided between the erbium-doped fiber 61-1 and the dispersion compensating fiber 62. In this way, whenever the influence of the reflectance due to Rayleigh backscattering appears, The reflectance can be reduced.

Furthermore, in this case, instead of providing an isolator in the input unit or the input / output unit, as shown in FIGS. 18 and 30, the input signal light is input through the optical circulator, and the output signal light is transmitted through the optical circulator. It can also be configured to output.
Also, an excitation light source and an optical demultiplexer / multiplexer for the dispersion compensating fiber 62 can be provided. That is, an optical fiber amplifier may be configured using the pumping light sources 133-1 to 133-3 and the optical demultiplexers / multiplexers 134-1 to 134-3 in the same manner as in FIG.

Further, a silica-based optical fiber may be used instead of the dispersion compensating fiber 62.
(17-3) Description of Third Modification of Seventeenth Embodiment FIG. 58 is a block diagram showing a third modification of the seventeenth embodiment. The optical fiber amplifier shown in FIG. 125-1, optical demultiplexer-multiplexer 124-1, erbium-doped fiber (pre-stage optical amplifying unit configured as a rare-earth doped fiber amplifying unit) 121-1, silica-based optical fiber 122 (optical fiber attenuating unit), erbium-doped A fiber (a rear-stage optical amplifying unit configured as a rare earth-doped fiber amplifying unit) 121-2, an optical demultiplexer / multiplexer 124-3, and an isolator 125-3 are provided. Then, pumping light sources 123-1 and 123-3 that generate pumping light in a 1.47 μ band (1.45 to 1.49 μm), for example, are connected to the optical demultiplexers / multiplexers 124-1 and 124-3. .

As described in the seventeenth embodiment, in the rare earth-doped fiber optical amplifier having a high gain, unnecessary oscillation may occur when performing optical amplification. When such unnecessary oscillation occurs, Rare earth doped fiber optical amplifier operates unstable.
In the optical fiber amplifier shown in FIG. 56, the 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 be.

However, when the gain G of the erbium-doped fiber 111 is very large, the GR parameter becomes large as in the case of the optical fiber amplifier shown in FIG. The effect of η does not appear, and the unstable operation of the erbium-doped fiber 111 cannot be suppressed.
Therefore, even in such a case, in order to suppress the unstable operation of the erbium-doped fiber 111, the erbium-doped fiber 111 is divided into front and rear erbium-doped fibers, and a silica-based optical fiber 122 is disposed between them. This is the optical fiber amplifier shown in FIG. The principle of suppressing unstable operation at this time is the same as that described in the second modification of the seventeenth embodiment, and R1 ′, R2 ′, RA ′, and RB ′ are also reflected in FIG. The rates A ′ and B ′ indicate the boundaries.

  For this reason, 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 is pumped using the residual pumping light from the erbium-doped fibers 121-1, 121-2. Thus, the loss compensation of the silica-based optical fiber 122 (compensation for flattening the dent of the gain of the erbium-doped fibers 121-1, 121-2 and compensating for the decrease of the gain of the erbium-doped fibers 121-1, 121-2 is performed. In addition, the unstable operation of the erbium-doped fibers 121-1 and 121-2 is suppressed by the remaining loss.

  With such a configuration, in the optical fiber amplifier shown in FIG. 58, the pumping light and the signal light are incident from one end of the erbium doped fiber 121-1 by the optical demultiplexer / multiplexer 124-1, and the erbium doped fiber 121- 1 is excited to amplify the signal light, and the silica-based optical fiber 122 is excited by the residual excitation light generated at this time, and the Raman amplification is performed in the same manner as the dispersion compensating fiber.

Further, excitation light is incident from the output end of the erbium-doped fiber 121-2 by the optical demultiplexer / multiplexer 124-3 to excite the erbium-doped fiber 121-2 and input from the input end of the erbium-doped fiber 121-2. The amplified signal light is amplified, and the silica-based optical fiber 122 is excited and Raman-amplified by the residual excitation light generated at this time.
As described above, in the optical fiber amplifier shown in FIG. 58, any of the erbium-doped fibers 121-1 and 121-2 and the silica-based optical fiber 122 is obtained by using the pump light sources 123-1 and 123-3 in the 1.47 μm band. Thus, the pumping light source 123-2 in the optical fiber amplifier shown in FIG. 12 can be reduced, and the simplification of the optical fiber amplifier and the efficiency of the pumping light power can be achieved.

  Further, in this optical fiber amplifier, unnecessary oscillation generated by the erbium-doped fibers 121-1 and 121-2 is simultaneously removed by the loss of the silica-based optical fiber 122, whereby wavelength multiplexing is performed. Unnecessary oscillation operation in the rare-earth doped fiber optical amplifier adjusted for (WDM) can be prevented, and stable optical amplification can be performed with little noise.

When the excitation light sources 123-1 and 123-3 generate excitation light of 0.98 μm, the silica-based optical fiber 122 does not perform Raman amplification, and therefore the loss compensation of the silica-based optical fiber 122 is not performed.
In addition, reflection due to Rayleigh backscattering exists in optical fibers such as silica-based optical fibers. The reflectance of this reflection depends on the length of the optical fiber, and the reflectance of this reflection increases as the length of the optical fiber increases.

Therefore, an optical isolator is added to the silica optical fiber when the influence of the reflectance due to Rayleigh backscattering of the silica optical fiber itself is larger than the effect of reducing the reflectance due to the loss of the silica optical fiber as described above. It is possible to do.
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, 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 unit or the input / output unit, 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.
Furthermore, an isolator may be provided between the silica-based optical fiber 122 and the erbium-doped fiber 121-2.

An excitation light source and an optical demultiplexer / multiplexer for the silica-based optical fiber 122 can also be provided. That is, an optical fiber amplifier may be configured using the pumping light sources 123-1 to 123-3 and the optical demultiplexers / multiplexers 124-1 to 124-3 in the same manner as in FIG.
Further, instead of the silica-based optical fiber 122, a dispersion compensating fiber may be used.
(18) Appendix (Appendix 1) In an optical fiber amplifier having a rare earth doped fiber,
First means for injecting excitation light from one end of the rare earth-doped fiber by a first optical coupler;
As a result of the pump light entering from one end of the rare earth-doped fiber by the first means, the residual pump light reaching the other end of the rare earth-doped fiber is separated by the second optical coupler, and the residual pump light is reflected. Second means reflected by the means back into the rare earth doped fiber;
The pumping light source for the pumping light incident on the rare earth-doped fiber by the first means is unstablely operated by the residual pumping light returned into the rare earth-doped fiber by the second means. An optical fiber amplifier comprising a third means for preventing by using an optical isolation means.

(Supplementary note 2) The optical fiber amplifier according to supplementary note 1, wherein the reflecting means is configured as a Faraday rotary reflecting mirror.
(Appendix 3) In an optical fiber amplifier having a rare earth doped fiber,
An excitation light source;
A first optical coupler that makes excitation light from the excitation light source incident from one end of the rare earth-doped fiber;
A second optical coupler that separates residual pumping light that has reached the other end of the rare earth-doped fiber as a result of entering pumping light from one end of the rare earth-doped fiber through the first optical coupler;
A reflecting mirror that reflects the residual pumping light separated by the second optical coupler and returns it to the rare earth-doped fiber through the second optical coupler;
An optical isolator provided between the pumping light source and the first optical coupler to prevent the pumping light source from performing unstable operation due to interference by the residual pumping light returned into the rare earth-doped fiber. An optical fiber amplifier characterized by comprising:

(Additional remark 4) In the optical fiber amplifier provided with the 1st rare earth doped fiber and the 2nd rare earth doped fiber which were arrange | positioned in two steps before and behind,
A first means for causing an incident from one end of one of the first rare earth doped fiber and the second rare earth doped fiber from the first optical coupler via an optical circulator having three or more ports of pumping light;
As a result of the pump light entering from one end of the one rare earth doped fiber by the first means, the residual pump light reaching the other end of the one rare earth doped fiber is separated by the second optical coupler, and the residual A second means for reflecting the excitation light by the reflecting means and returning it into the one rare earth-doped fiber;
The second means returns the residual excitation light reflected from the reflecting means into the one rare earth doped fiber, then changes the optical path with the optical circulator, and the third rare earth doped fiber with the third optical coupler. An optical fiber amplifier comprising a second rare earth doped fiber of the second rare earth doped fiber and a third means for multiplexing the residual pumping light.

(Additional remark 5) In the optical fiber amplifier provided with the 1st rare earth doped fiber and the 2nd rare earth doped fiber which were arrange | positioned in two steps before and behind,
An excitation light source;
A first optical coupler provided at one end of one of the first rare earth doped fiber and the second rare earth doped fiber;
A second optical coupler provided at the other end of the one rare earth doped fiber;
A third optical coupler provided at one end of the other rare earth doped fiber of the first rare earth doped fiber and the second rare earth doped fiber;
A reflecting mirror that reflects the residual pumping light separated by the second optical coupler and returns it to the one rare earth-doped fiber through the second optical coupler;
An optical circulator having three or more ports connected to the pumping light source, the first optical coupler, and the third optical coupler;
The pumping light from the pumping light source is incident from one end of the one rare earth doped fiber from the first optical coupler through the optical circulator, and then the residual pumping light that has reached the other end of the one rare earth doped fiber. Separated by the second optical coupler, and the residual pump light is reflected by the reflecting mirror and returned into the one rare earth doped fiber, and further the optical path is changed by the optical circulator, and the other optical coupler is used by the third optical coupler. An optical fiber amplifier configured to multiplex the residual pumping light into the rare earth-doped fiber.

(Supplementary Note 6) Isolators are respectively provided at the input port to which the input signal light is input, between the output side of the second optical coupler and the input side of the third optical coupler, and at the output port from which the output signal light is output. The optical fiber amplifier according to appendix 5, which is added.
(Additional remark 7) In the optical fiber amplifier provided with the 1st rare earth doped fiber and the 2nd rare earth doped fiber which were arrange | positioned in two steps before and behind,
The pumping light power is branched to n: 1 (n is a real number of 1 or more) at the optical branching unit, and the pumping light of one port of the optical branching unit is multiplexed by the first optical coupler, and the first rare earth First means for entering one of the doped fiber and the second rare earth doped fiber;
After the pumping light is incident from one end of the one rare earth doped fiber by the first means, the residual pumping power is taken out by the second optical coupler connected to the other end of the one rare earth doped fiber, and the residual pumping is performed. A second means for combining the power with a third optical coupler and causing the power to be incident 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 combining the pumping power of the other port of the optical branching unit branched by the optical branching unit from the other end of the other rare earth doped fiber by a fourth optical coupler. An optical fiber amplifier.

(Additional remark 8) In the optical fiber amplifier provided with the 1st rare earth doped fiber and the 2nd rare earth doped fiber which were arrange | positioned in two steps before and behind,
An excitation light source;
An optical branching unit for branching the pumping light power from the pumping light source to n: 1 (n is a real number of 1 or more);
A first optical coupler that multiplexes pumping light from one port of the optical branching unit and makes it incident on one of the first rare earth doped fiber and the second rare earth doped fiber;
A second optical coupler for extracting residual pumping power extracted from the one rare earth doped fiber;
A third optical coupler which combines the residual pumping power extracted by the second optical coupler and enters the other rare earth doped fiber of the first rare earth doped fiber and the second rare earth doped fiber;
And a fourth optical coupler which combines the pumping power from the other port of the optical branching unit branched by the optical branching unit and enters the other rare earth-doped fiber. , Fiber optic amplifier.

(Supplementary Note 9) Input port to which input signal light is input, between the pumping light source and the optical branching unit, between the second optical coupler and the signal port of the third optical coupler, and output from which the output signal light is output 9. The optical fiber amplifier according to appendix 8, wherein an isolator is added to each port.
(Supplementary Note 10) In an optical fiber amplifier provided with a rare earth-doped fiber,
An excitation light source;
An optical circulator having three or more ports connected to the excitation light source;
A first optical coupler that multiplexes the excitation light that has passed through the optical circulator from the excitation light source and enters the rare earth-doped fiber;
A second optical coupler that separates residual pumping light that has reached the other end of the rare earth-doped fiber as a result of pumping light entering one end of the rare earth-doped fiber through the first optical coupler;
A reflecting mirror that reflects the residual pumping light separated by the second optical coupler and returns it to the rare earth-doped fiber through the second optical coupler;
A residual excitation light detector for detecting the residual excitation light that is returned into the rare earth doped fiber by the reflecting mirror and input to the optical circulator through one end of the rare earth doped fiber and the first optical coupler;
An optical fiber amplifier comprising a controller for controlling the excitation light source so that the residual excitation light detected by the residual excitation light detector is constant.

(Supplementary note 11) The optical fiber amplifier according to any one of supplementary notes 3, 5, and 10, wherein a Faraday rotary reflection mirror is used as the reflection mirror.
(Additional remark 12) It is comprised so that input signal light may be input through an optical circulator, and output signal light may be output through this optical circulator. An optical fiber amplifier as described.

(Supplementary note 13) The optical fiber amplifier according to supplementary note 10, wherein an isolator is added to each of an input port to which input signal light is input and an output port from which output signal light is output.
(Additional remark 14) The rare earth doped fiber optical amplification part which consists of rare earth doped fibers, and the Raman optical amplification part which produces a Raman amplification by being pumped with desired excitation light are arrange | positioned in cascade. An optical fiber amplifier.

(Supplementary Note 15) A rare-earth-doped fiber optical amplifying unit made of a rare-earth-doped fiber, and a Raman optical amplifying unit that causes Raman amplification by being excited with desired pumping light that can excite the rare-earth-doped fiber optical amplifying unit, Arranged in tandem,
An optical fiber amplifier comprising a pumping light source that supplies pumping light for pumping the rare earth-doped fiber optical amplifier and the Raman amplifier.

(Supplementary Note 16) A rare-earth-doped fiber optical amplifying unit made of a rare-earth-doped fiber and a Raman optical amplifying unit made of a dispersion-compensating fiber that generates Raman amplification when excited by desired pumping light are cascaded over two stages. An optical fiber amplifier.
(Supplementary note 17) The optical fiber amplifier according to supplementary note 16, wherein the Raman light amplification unit is disposed as a pre-stage amplification unit, and the rare earth-doped fiber light amplification unit is disposed as a post-stage amplification unit.

(Supplementary Note 18) The rare earth-doped fiber optical amplifying unit is configured as an optical amplifying unit having a low noise figure, the rare earth-doped fiber optical amplifying unit is disposed as a pre-amplifier, and the Raman optical amplifying unit is a post-amplifier. The optical fiber amplifier according to appendix 16, wherein the optical fiber amplifier is disposed as a portion.
(Supplementary note 19) The pumping light source for pumping the Raman light amplifying unit is composed of two pumping light sources and a polarization beam combiner that performs orthogonal polarization combining on the pumping light from these pumping light sources. The optical fiber amplifier according to any one of appendices 14 to 16, which is characterized by the following.

(Supplementary note 20) Supplementary notes 14 to 16, wherein the excitation light source for exciting the Raman light amplification unit is configured to depolarize the excitation light by combining the excitation light source and the depolarizer. An optical fiber amplifier according to any one of the above.
(Supplementary note 21) The light according to any one of supplementary notes 14 to 16, wherein an excitation light source for exciting the Raman light amplification unit is configured to generate modulated excitation light. Fiber amplifier.

(Supplementary Note 22) A rare earth-doped fiber and a dispersion compensating fiber arranged in two stages before and after,
A first pumping light source for generating pumping light in a first wavelength band for the rare earth doped fiber;
A first optical coupler for injecting excitation light from the first excitation 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;
A second optical coupler that makes the excitation light from the second excitation light source incident on the dispersion compensating fiber;
An optical fiber amplifier, wherein the dispersion compensating fiber is pumped with pumping light of the second wavelength band from the second pumping light source to generate Raman amplification.

(Supplementary Note 23) The rare earth-doped fiber is composed of an erbium-doped fiber, the wavelength band of the pumping light generated by the first pumping light source is 0.98 μm band, and the wavelength band of the pumping light generated by the second pumping light source 23. The optical fiber amplifier according to appendix 22, wherein 1. is a 1.47 μm band.
(Supplementary Note 24) An erbium-doped fiber and a dispersion-compensating fiber arranged in two stages before and after,
An excitation light source that generates excitation light;
An optical coupler for injecting excitation light from the excitation light source into the erbium-doped fiber;
An optical fiber amplifier, wherein the dispersion compensating fiber is pumped with residual pumping light from the erbium-doped fiber to generate Raman amplification.

(Supplementary Note 25) An erbium-doped fiber and a dispersion compensating fiber arranged in two stages before and after,
An excitation light source that generates excitation light;
An optical coupler that makes the excitation light from the excitation light source incident on the dispersion compensating fiber;
An optical fiber amplifier characterized in that the erbium-doped fiber is pumped with residual pumping light from the dispersion compensating fiber.

(Supplementary Note 26) A dispersion compensating fiber doped with a rare earth element;
An excitation light source for generating excitation light for the dispersion compensating fiber doped with the rare earth element;
An optical fiber amplifier comprising: an optical coupler that makes excitation light from the excitation light source incident on a dispersion compensating fiber doped with the rare earth element.
(Supplementary note 27) An erbium-doped fiber and a dispersion compensating fiber arranged in two stages before and after,
An excitation light source for generating excitation light for the erbium-doped fiber;
An optical coupler for injecting excitation light from the excitation light source into the erbium-doped fiber;
An optical fiber amplifier comprising an optical filter interposed between the erbium-doped fiber and the dispersion compensating fiber and configured to block residual pumping light emitted from the erbium-doped fiber. .

(Supplementary note 28) A rare-earth-doped fiber optical amplifying unit composed of a rare-earth-doped fiber and a Raman optical amplifying unit composed of a silica-based optical fiber that generates Raman amplification when excited by desired pumping light are cascaded in two stages. An optical fiber amplifier characterized by being connected.
(Supplementary note 29) The optical fiber amplifier according to supplementary note 28, wherein the Raman light amplification unit is disposed as a front-stage amplification unit, and the rare earth-doped fiber light amplification unit is disposed as a rear-stage amplification unit.

(Supplementary Note 30) In the case where the rare earth doped fiber optical amplifying unit is configured as an optical amplifying unit having a low noise figure, the rare earth doped fiber optical amplifying unit is disposed as a pre-amplifier, and the Raman optical amplification 29. The optical fiber amplifier according to appendix 28, wherein the portion is arranged as a post-stage amplifier.
(Supplementary Note 31) A silica-based optical fiber is provided on the front side and an erbium-doped fiber is provided on the rear side.
A pumping light source for silica-based optical fiber that generates pumping light in a wavelength band for the silica-based optical fiber;
An optical coupler that makes excitation light from the excitation light source for the silica-based optical fiber incident on the silica-based optical fiber;
An excitation light source for an erbium-doped fiber that generates excitation light in the wavelength band for the erbium-doped fiber;
An optical coupler for injecting excitation light from the excitation light source for the erbium-doped fiber into the erbium-doped fiber;
An optical fiber amplifier characterized in that the silica-based optical fiber is pumped with pumping light in a wavelength band from the silica-based optical fiber pumping light source to generate Raman amplification.

(Supplementary Note 32) An erbium-doped fiber having a low noise figure is provided on the front side, and a silica-based optical fiber is provided on the rear side.
A pumping light source for silica-based optical fiber that generates pumping light in a wavelength band for the silica-based optical fiber;
An optical coupler that makes excitation light from the excitation light source for the silica-based optical fiber incident on the silica-based optical fiber;
An excitation light source for an erbium-doped fiber that generates excitation light in the wavelength band for the erbium-doped fiber;
An optical coupler for injecting excitation light from the excitation light source for the erbium-doped fiber into the erbium-doped fiber;
An optical fiber amplifier characterized in that the silica-based optical fiber is pumped with pumping light in a wavelength band from the silica-based optical fiber pumping light source to generate Raman amplification.

(Supplementary note 33) Supplementary note 31 or Supplementary note 32, characterized in that a pumping light source for generating pumping light is provided, and the pumping light source also serves as the silica-based optical fiber pumping light source and the erbium-doped fiber pumping light source. An optical fiber amplifier as described in 1.
(Supplementary Note 34) A rare-earth-doped fiber optical amplifying unit made of a rare-earth-doped fiber and having a low noise figure is arranged as a pre-amplifying unit,
A Raman light amplifying unit that causes Raman amplification by being excited with desired pumping light is disposed as a middle stage amplifying unit,
An optical fiber amplifier, characterized in that a rare earth doped fiber optical amplifying section made of a rare earth doped fiber is arranged as a subsequent stage amplifying section.

(Supplementary note 35) The optical fiber amplifier according to supplementary note 34, wherein the Raman light amplification unit is configured as an optical amplification unit made of a dispersion compensating fiber.
(Supplementary note 36) The optical fiber amplifier according to supplementary note 34, wherein the Raman light amplification unit is configured as an optical amplification unit made of a silica-based optical fiber.
(Supplementary Note 37) The first erbium-doped fiber having a low noise figure is provided in the front stage, the dispersion compensating fiber is provided in the middle stage, and the second erbium-doped fiber is provided in the rear stage.
An excitation light source for a first erbium-doped fiber that generates excitation light in the wavelength band for the first erbium-doped fiber;
An optical coupler for injecting pump light from the pump light source for the first erbium-doped fiber into the first erbium-doped fiber;
A pump light source for a dispersion compensating fiber that generates pump light in a wavelength band for the dispersion compensating fiber;
An optical coupler that makes excitation light from the excitation light source for the dispersion compensation fiber incident on the dispersion compensation fiber;
An excitation light source for a second erbium-doped fiber that generates excitation light in the wavelength band for the second erbium-doped fiber;
An optical coupler for injecting excitation light from the excitation light source for the second erbium-doped fiber into the second erbium-doped fiber;
An optical fiber amplifier characterized in that the dispersion compensating fiber is pumped with pumping light in a wavelength band from a pumping light source for the dispersion compensating fiber to generate Raman amplification.

(Supplementary Note 38) A first erbium-doped fiber having a low noise figure is provided in the front stage, a silica-based optical fiber is provided in the middle stage, and a second erbium-doped fiber is provided in the subsequent stage.
An excitation light source for a first erbium-doped fiber that generates excitation light in the wavelength band for the first erbium-doped fiber;
An optical coupler for injecting pump light from the pump light source for the first erbium-doped fiber 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;
An optical coupler that makes excitation light from the excitation light source for the silica-based optical fiber incident on the silica-based optical fiber;
An excitation light source for a second erbium-doped fiber that generates excitation light in the wavelength band for the second erbium-doped fiber;
An optical coupler for injecting excitation light from the excitation light source for the second erbium-doped fiber into the second erbium-doped fiber;
An optical fiber amplifier characterized in that the silica-based optical fiber is pumped with pumping light in a wavelength band from the silica-based optical fiber pumping light source to generate Raman amplification.

(Supplementary note 39) A dispersion compensating fiber module for an optical fiber amplifier, comprising: a dispersion compensating fiber; and a pumping light source for exciting the dispersion compensating fiber to cause Raman amplification.
(Supplementary Note 40) In an optical fiber amplifier having a dispersion compensating fiber,
An excitation light source;
An optical coupler that makes the excitation light from the excitation light source incident on the dispersion compensating fiber;
An optical fiber amplifier, wherein the dispersion compensating fiber is pumped with pumping light from the pumping light source to generate Raman amplification.

(Supplementary note 41) The optical fiber amplifier according to supplementary note 40, wherein the input signal light is input through the optical circulator and the output signal light is output through the optical circulator.
(Supplementary note 42) The optical fiber amplifier according to supplementary note 40, wherein an isolator is added to each of an input port to which input signal light is input or an output port from which output signal light is output.

(Supplementary Note 43) In an optical fiber amplifier having a silica-based optical fiber,
An excitation light source;
An optical coupler that makes the excitation light from the excitation light source incident on the silica-based optical fiber;
An optical fiber amplifier characterized in that the silica-based optical fiber is pumped with pumping light from the pumping light source to generate Raman amplification.

(Supplementary note 44) The optical fiber amplifier according to supplementary note 43, wherein the input signal light is input through the optical circulator and the output signal light is output through the optical circulator.
(Supplementary Note 45) A rare-earth-doped fiber optical amplifying unit comprising a rare-earth doped fiber
An optical fiber amplifier, comprising: an optical fiber attenuating unit comprising an optical fiber or an optical fiber to which an optical isolator is added in order to suppress unstable operation of the rare earth doped fiber optical amplifying unit.

(Supplementary Note 46) An optical amplification unit having a front-stage optical amplification section and a rear-stage optical amplification section each configured as a rare-earth-doped fiber optical amplification section made of a rare-earth doped fiber;
An optical fiber attenuation comprising an optical fiber or an optical fiber provided between the front-stage optical amplification unit and the rear-stage optical amplification unit in the optical amplification unit and added with an optical fiber or an optical isolator to suppress unstable operation of the optical amplification unit. An optical fiber amplifier comprising a plurality of sections.

  (Supplementary note 47) The optical fiber according to supplementary note 45 or supplementary note 46, wherein the optical fiber attenuating portion also serves as a Raman light amplifying portion that causes Raman amplification when excited by a desired excitation light. amplifier.

It is a principle block diagram which shows a 1st aspect. It is a principle block diagram which shows a 2nd aspect. It is a principle block diagram which shows a 3rd aspect. It is a principle block diagram which shows a 4th aspect. It is a principle block diagram which shows a 5th aspect. It is a principle block diagram which shows a 6th aspect. It is a principle block diagram which shows a 7th aspect. It is a principle block diagram which shows an 8th aspect. It is a principle block diagram which shows a 9th aspect. (A) is a principle block diagram which shows a 10th aspect, (b) is a principle block diagram which shows an 11th aspect. It is a principle block diagram which shows a 12th aspect. It is a principle block diagram which shows the 13th aspect. (A) is a principle block diagram which shows a 14th aspect, (b) is a principle block diagram which shows a 15th aspect. It is a principle block diagram which shows the 16th aspect. It is a principle block diagram which shows the 17th aspect. It is a block diagram which shows 1st Embodiment of the technique relevant to this invention. It is a block diagram which shows the 1st modification of 1st Embodiment. It is a block diagram which shows the 2nd modification of 1st Embodiment. It is a block diagram which shows the 3rd modification of 1st Embodiment. It is a block diagram which shows the 4th modification of 1st Embodiment. It is an electric circuit diagram which shows a light output constant control system. It is a figure explaining the effect | action of a light output fixed control system. It is a block diagram which shows 2nd Embodiment of the technique relevant to this invention. It is a block diagram which shows the modification of 2nd Embodiment. It is a block diagram which shows 3rd Embodiment of the technique relevant to this invention. It is a block diagram which shows 4th Embodiment of the technique relevant to this invention. It is a block diagram which shows 5th Embodiment of this invention. It is an electric circuit diagram showing a pumping light output constant control system. It is a figure explaining the effect | action of an excitation light output fixed control system. It is a block diagram which shows the 1st modification of 5th Embodiment of this invention. It is a block diagram which shows the 2nd modification of 5th Embodiment of this invention. It is a block diagram which shows 6th Embodiment of the technique relevant to this invention. It is a block diagram which shows the modification of 6th Embodiment. It is a block diagram which shows 7th Embodiment of the technique relevant to this invention. It is a block diagram which shows 8th Embodiment of the technique relevant to this invention. It is a block diagram which shows the 1st modification of 8th Embodiment. It is a block diagram which shows the 2nd modification of 8th Embodiment. It is a block diagram which shows 9th Embodiment of the technique relevant to this invention. It is a block diagram which shows 10th Embodiment of the technique relevant to this invention. It is a block diagram which shows 11th Embodiment of the technique relevant to this invention. It is a block diagram which shows 12th Embodiment of the technique relevant to this invention. It is a block diagram which shows 13th Embodiment of the technique relevant to this invention. It is a block diagram which shows 14th Embodiment of the technique relevant to this invention. It is a block diagram which shows the 1st modification of 14th Embodiment. It is a block diagram which shows the 2nd modification of 14th Embodiment. It is a figure explaining the wavelength characteristic of an optical fiber amplifier. It is a figure explaining the wavelength characteristic of an optical fiber amplifier. It is a block diagram which shows 15th Embodiment of the technique relevant to this invention. It is a block diagram which shows the modification of 15th Embodiment. It is a block diagram which shows 16th Embodiment of the technique relevant to this invention. It is a block diagram which shows the 1st modification of 16th Embodiment. It is a block diagram which shows the 2nd modification of 16th Embodiment. It is a figure which shows the structure of an optical circulator. It is a figure which shows the structure of an isolator. It is a block diagram which shows 17th Embodiment of the technique relevant to this invention. It is a block diagram which shows the 1st modification of 17th Embodiment. It is a block diagram which shows the 2nd modification of 17th Embodiment. It is a block diagram which shows the 3rd modification of 17th Embodiment.

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 (reflecting 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 Excitation light source 13-1 to 13-4, 13-1 ', 13-2' Optical demultiplexing 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 photodetector 18A Photo diode 19 Optical output constant 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 to 24-4, 24-1 'to 24-4' Optical demultiplexing 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 Excitation 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 light detector 36A Photo diode 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 compensating fiber 63, 63-1, 63-2 Excitation light source 64, 64-1 to 64-5 Optical demultiplexing multiplexer (Optical coupler)
65-1 to 65-4 Isolator 66 Optical filter 71 Erbium-doped fiber 72 Dispersion compensation fiber 73 Excitation light source 74 Optical demultiplexing multiplexer (optical coupler)
75-1, 75-2 Isolator 81 Rare earth doped dispersion compensating fiber 82 Excitation light source 83 Optical demultiplexing multiplexer (optical coupler)
84-1, 84-2 Isolator 91 Erbium-doped fiber 92 Dispersion compensating fiber 93 Excitation light source 94 Optical demultiplexer / multiplexer (optical coupler)
DESCRIPTION OF SYMBOLS 95 Optical filter 96-1,96-2 Isolator 101 Silica type optical fiber 102 Erbium dope fiber 103-1,103-2 Excitation light source 104-1,104-2 Optical demultiplexing multiplexer (optical coupler)
111 Erbium-doped fiber 112 Silica-based optical fiber 113-1, 113-2, 113-2A, 113-2B, 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 Light Demultiplexer / multiplexer (optical coupler)
115-1 to 115-3 Isolators 121-1, 121-2 Erbium-doped fiber 122 Silica-based optical fibers 123-1 to 123-3, 123-1 ', 123-1A', 123-1B ', 123-3' , 123-3A ′, 123-3B ′ Excitation light source 123-1C ′, 123-3C ′ Polarization combiner 124-1 to 124-3 Optical demultiplexing multiplexer (optical coupler)
125-1 to 125-3 Isolator 126 Optical filter 131-1, 131-2 Erbium-doped fiber 132 Dispersion compensation fiber 133-1 to 133-3 Excitation light source 134-1 to 134-3 Optical demultiplexing multiplexer (optical coupler) )
141 Dispersion compensating fiber 142, 142A, 142B Excitation light source 142C Polarization combiner 143 Optical demultiplexer / multiplexer (optical coupler)
144, 144-1, 144-2 Isolator 151 Silica-based optical fiber 152 Excitation light source 153 Optical demultiplexing multiplexer (optical coupler)
154 Rare earth doped fiber optical amplifying section 155 Optical fiber attenuating section 156-1 Pre-stage optical amplifying section 156-2 Post-stage optical amplifying section 157 Optical fiber attenuating section

Claims (4)

In an optical fiber amplifier with a rare earth doped fiber,
An excitation light source;
An optical circulator having three or more ports connected to the excitation light source;
A first optical coupler that multiplexes the excitation light that has passed through the optical circulator from the excitation light source and enters the rare earth-doped fiber;
A second optical coupler that separates residual pumping light that has reached the other end of the rare earth-doped fiber as a result of pumping light entering one end of the rare earth-doped fiber through the first optical coupler;
A reflecting mirror that reflects the residual pumping light separated by the second optical coupler and returns it to the rare earth-doped fiber through the second optical coupler;
A residual excitation light detector for detecting the residual excitation light that is returned into the rare earth doped fiber by the reflecting mirror and input to the optical circulator through one end of the rare earth doped fiber and the first optical coupler;
An optical fiber amplifier comprising a controller for controlling the excitation light source so that the residual excitation light detected by the residual excitation light detector is constant.   2. An optical fiber amplifier according to claim 1, wherein a Faraday rotating mirror is used as the reflecting mirror.   2. The optical fiber amplifier according to claim 1, wherein the input signal light is input through the optical circulator and the output signal light is output through the optical circulator.   2. The optical fiber amplifier according to claim 1, wherein an isolator is added to each of an input port to which input signal light is input and an output port from which output signal light is output.

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