JPH1022555A - Optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To flatten the wavelength property of gain, covering a wide range of wavelength, by connoting the amplifying means of a rare earth doped optical fiber where excitation light is inputted, and the loss medium of a rare earth doped optical fiber of nonexcitation which passes only the signal light, to a cascade. SOLUTION: An ER doped optical fiber 10 as a first rate earth doped optical fiber (amplifying medium) is excited in rear by the excitation lights 13-1, 13-2, and 13-3 from a semiconductor laser 13. Likewise, an Er doped optical fiber 14 as a second rare earth doped optical fiber (amplifying medium) is excited in front by the excitation lights 16-1, 16-2, and 16-3 from a semiconductor laser 16. A loss medium has an Er doped optical fiber 17 as a third rare earth doped optical fiber, and it passes only the signal lights 1-2 and 1-3, and the excitation lights 13-3 and 16-3 are ones of nonexcitation with no passage. Then, the wavelength property of the gain is flattened by the combination of three wavelength properties each by combining a plurality of amplifying media and loss media with a cascade.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide-band optical fiber amplifier for amplifying signal light by transmitting signal light to a rare-earth-doped optical fiber and inputting pump light from a pump light source.

[0002]

2. Description of the Related Art In recent years, optical fiber amplifiers in which a rare earth element such as Er (erbium), Pr (praseodymium), or Nd (neodymium) is added to the core of an optical fiber have reached a practical level. In particular, since an optical fiber amplifier doped with Er has a high gain and a high saturation output in a 1.55 μm band, application to various systems is considered. Among them, 1.53 μm to 1.56 μm
Attention has been focused on high-speed, large-capacity, long-distance transmission and optical CATV systems by wavelength multiplexing transmission using several or more signal lights in the above wavelength band. For the use of an Er-doped optical fiber amplifier in this type of system, the gain characteristic of the Er-doped optical fiber amplifier in the above wavelength band is flat in order to suppress deterioration of the optical S / N characteristics and crosstalk characteristics. It is important that

In order to achieve such high gain and flattening (broadbanding), the present inventors have proposed an Er-doped multi-core fiber and an optical amplifier using the same. FIG. 7 is a sectional view of an Er-doped multi-core optical fiber proposed by the present inventors.

The Er-doped multi-core optical fiber 100 shown in FIG. 1 has a plurality of glass rods (seven in FIG. 1) having a core 102 doped with a rare earth element, for example, Er and Al, in a primary clad 101. Are not limited), and the glass rods are covered with a cladding 103. By using such an optical fiber, high gain and flattening of gain wavelength characteristics are achieved. be able to.

The first reason is that the Er-doped multi-core optical fiber 100 can have an Al doping concentration sufficiently higher than that of a conventional Er-doped optical fiber having one core.

[0006] The second reason is that in the conventional device, when the power of the pumping light in the core is reduced, the wavelength becomes 1.535.
The peak of the gain around μm decreases, and the gain becomes gradually flat.
As the power becomes lower, the gain in the short wavelength region on the 1.53 μm side decreases, and the wavelength characteristic becomes 1.56 μm.
The gain in the long wavelength region on the m side increases. Since the gain-wavelength characteristic rises to the right from the short wavelength side to the long wavelength side, it has been found that the gain becomes extremely low when the pumping light is reduced, and that the Er cannot be used as an optical amplifier. Conversely, the doped multi-core optical fiber 100 actively utilizes this principle.

That is, as shown in the drawing, the core diameter D and the core interval S are optimized so that the pump light and the signal light propagate in the respective cores 102 to which Er is added substantially uniformly. As a result, although the amplification gain of the signal light propagating in each core 102 decreases, the wavelength characteristic thereof becomes substantially flat, and after propagating the desired length, the signal amplified in each core 102 is superimposed. This is because the gain has a substantially flat wavelength characteristic.

[0008] In order to demonstrate the above-described principle, the results of evaluating the wavelength characteristics of gain using the optical fiber amplifier shown in FIG. 8 will be described. FIG. 8 is a block diagram of an optical fiber amplifier on which the present invention is based.

The Er-doped multi-core optical fiber 100 has a core interval S of 1.3 μm and a core diameter D of about 2 μm.
m, the cladding diameter is 125 μm, the relative refractive index difference Δ between the core 102 and the primary cladding 101 is 1.45%, the mode field diameter is about 8.8 μm (value at a wavelength of 1.55 μm), and each core 102 has 400 pp Er
m, the addition amount of Al is 8,500 ppm, the fiber length is about 45 m, the relative refractive index difference Δ is 2.19%, the mode field diameter is about 5.2 μm (the value at a wavelength of 1.55 μm). Amount of Er in the core 102 is 400 pp
Two kinds of multi-core optical fibers having an addition amount of m and Al of 17,000 ppm and a fiber length of about 20 m were used.

[0010] WDM couplers 104 and 105 are provided at both ends of the Er-doped multi-core optical fiber 100, and pump lights 106-1 and 107-1 emitted from semiconductor lasers 106 and 107 for pumping use the WDM couplers 104 and 105.
The excitation lights 106-2 and 107-2 after passing through
The multi-core optical fiber 100 is coupled and propagated.

Incidentally, the semiconductor lasers 106 and 107 for excitation are used.
The wavelengths of the pump lights 106-1 and 107-1 emitted from the laser are 0.98 μm, the power of the pump light 106-1 is 70 mW from the semiconductor laser 106, and the power of the pump light 107-1 is 80 mW from the semiconductor laser 107. Since the value to be adjusted was a value suitable for flattening the wavelength characteristic of the gain, the above-described value was adopted. Optical isolator 108,
Reference numeral 109 is inserted in order to suppress propagation of the signal lights 1-1, 1-2, 1-3, and 1-4 in the reverse direction.

FIGS. 9 (a) and 9 (b) show the results of measuring the wavelength characteristics of the gain with the above configuration. Of the two types of multi-core optical fibers described above, FIG. 9A shows the measurement results when the addition amount of Al was 8,500 ppm, and FIG. 9A shows the measurement results when the addition amount of Al was 17,000 ppm. 9 (b). 9 (a) and 9
(B) is a diagram showing the result of measuring the wavelength characteristic of the gain using the signal light power Sp as a parameter, where the horizontal axis represents the wavelength and the vertical axis represents the gain.

FIG. 9A and FIG. 9B show that the wavelength 154
It has been found that there is a problem that the gain characteristic becomes flat only in the range from 0 nm to 1560 nm.

One solution to this problem is disclosed in
Japanese Patent Application Publication No. 7561 discloses a configuration shown in FIG.
FIG. 10 is a block diagram of a conventional optical fiber amplifier.

The optical amplifier shown in FIG. 1 is a rare earth ion-doped optical fiber (Er ^{+ 3} ion-doped optical fiber) 110
, A WDM coupler 104, an optical amplifying means including a semiconductor laser 106, and an Er ⁺³ ion-doped optical fiber 11
1 are connected via an optical isolator 109 and a band-pass filter 112. That is, Er ⁺³
An optical circuit composed of an Er ^{+ 3} ion-doped optical fiber 111 containing the same rare earth ion without being pumped is connected to the output end of the optical amplification unit using the ion-doped optical fiber 110, and the saturable absorption characteristic of this optical circuit is used. Thus, the waveform distortion generated in the optical amplifier is flattened.

[0016]

By the way, in the optical fiber amplifier shown in FIG. 10, the waveform generated in the optical amplifier section by connecting the non-excited Er ⁺³ ion-doped optical fiber 111 to the output terminal of the optical amplifier section. Although the distortion is flattened, it is difficult to flatten the gain wavelength characteristic over a wavelength range from 1530 nm to 1560 nm.

This is because the desired high gain (35 to 40 d
Wavelength characteristic curve of gain of optical fiber amplifier using Er ⁺³ ion-doped optical fiber having characteristics of B) and non-excitation E
It is mathematically difficult to connect the loss characteristic curve (for example, the characteristic curve shown in FIG. 11) of the r ⁺³ ion-doped optical fiber in a cascade to flatten the gain within the above-mentioned wavelength range, and this is realized. In order to achieve this, the gain of the Er-doped fiber amplifier is greatly sacrificed, or a certain additive is added to the non-pumped Er-doped fiber, and the wavelength characteristic of the loss is flattened. Must be operated, but it is still difficult at present. FIG. 11 is a diagram showing the loss wavelength characteristics of the Er-doped multi-core fiber shown in FIG. 7 when the fiber is not pumped. The horizontal axis indicates the wavelength, and the vertical axis indicates the loss. From the figure, it can be seen that the loss decreases as the wavelength increases.

In any case, it is difficult to realize a flat gain characteristic curve only with the two characteristic curves of the wavelength characteristic curve and the loss wavelength characteristic.

It is an object of the present invention to solve the above-mentioned problems and to provide an optical fiber amplifier having a flat gain wavelength characteristic over a wide wavelength range.

[0020]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an optical fiber which propagates a signal light to a rare earth-doped optical fiber and amplifies the signal light by inputting a pumping light from a pumping light source. In the amplifier, a plurality of amplifying means using a rare earth-doped optical fiber having different gain wavelength characteristics and pump light from a pump light source as an amplification medium, and a non-pumped rare earth-doped optical fiber passing only signal light as a loss medium. And a cascade connection of these media.

According to the present invention, in addition to the above configuration, the amplifying means in the preceding stage comprises: a first rare-earth-doped optical fiber; a first WDM coupler having one input end connected to the first optical fiber; A second WDM coupler having an end connected to one output end of the first WDM coupler, and a first pumping light source connected to one output end of the second WDM coupler; The means comprises a second rare earth-doped optical fiber, a third WDM coupler having one output end connected to the input end of the second rare earth-doped optical fiber, and a third WD coupler having the other output end.
A second WDM coupler connected to one input end of the M coupler; and a second pump light source connected to the other input end of the second WDM coupler, wherein the loss medium is a first WDM coupler. A third rare earth-doped optical fiber may be connected between the other output end of the coupler and the other input end of the third WDM coupler.

In addition to the above-described structure, the present invention provides an amplifier comprising: a first amplifying means; a first rare-earth-doped optical fiber; a first WDM coupler having one input end connected to the output end of the first optical fiber; A second WDM coupler having one input terminal connected to one output terminal of the first WDM coupler, and a first excitation light source connected to one output terminal of the second WDM coupler; Amplifying means at the subsequent stage includes a second rare-earth-doped optical fiber, a second WDM coupler having the other output terminal connected to the input terminal of the second rare-earth-doped optical fiber, and one output terminal connected to the second WDM coupler. A first WDM coupler connected to one input end of the coupler, and a second excitation light source connected to the other input end of the first WDM coupler, wherein the loss medium is a first WDM coupler.
And a third rare earth-doped optical fiber connected between the other output end of the WDM coupler and the other input end of the second WDM coupler.

According to the present invention, in addition to the above configuration, the amplifying means in the preceding stage includes a first pumping light source, a first WDM coupler having one input terminal connected to the first pumping light source, and an input terminal connected to the first pumping light source. W
A first rare-earth-doped optical fiber connected to one output terminal of the DM coupler; amplifying means at a subsequent stage connected to a second pump light source and one input terminal connected to the second pump light source; A second WDM coupler, a third WDM coupler having one input terminal connected to one output terminal of the second WDM coupler,
An input end having a second rare earth doped optical fiber connected to one output end of the third WDM coupler;
A third rare earth-doped optical fiber may be connected between the other output terminal of the second WDM coupler and the other input terminal of the third WDM coupler.

According to the present invention, in addition to the above configuration, the preceding amplification medium includes a first pumping light source, a first WDM coupler having one input terminal connected to the first pumping light source, and an input terminal connected to the first pumping light source. W
A first rare earth-doped optical fiber connected to one output end of the DM coupler, and a second WDM coupler having one input end connected to the output end of the first rare earth doped optical fiber; Amplifying medium having a second pumping light source, a third WDM coupler having one output terminal connected to the second pumping light source, and an output terminal connected to one input terminal of the third WDM coupler. A second rare-earth-doped optical fiber and a fourth WDM having one output connected to the input of the second rare-earth-doped optical fiber and one input connected to one output of the second WDM coupler; And a loss medium having a second WD.
A third rare earth-doped optical fiber may be connected between the other output terminal of the M coupler and the other input terminal of the fourth WDM coupler.

According to the present invention, in addition to the above configuration, the preceding amplification medium includes a first pumping light source, a first WDM coupler having one input terminal connected to the first pumping light source, and an input terminal connected to the first pumping light source. W
A first rare earth-doped optical fiber connected to one output end of the DM coupler, a second WDM coupler having one input end connected to the output end of the first rare earth doped optical fiber, and one input end Has a third WDM coupler connected to one output terminal of the second WDM coupler, and a second pumping light source connected to one output terminal of the third WDM coupler, Has a third excitation light source and the other
A second WDM coupler connected to the pump light source, a third WDM coupler having one input terminal connected to one output terminal of the second WDM coupler, and an input terminal connected to the other of the third WDM coupler. A second rare earth-doped optical fiber connected to the output end of the second WDM coupler, a fourth WDM coupler having one input end connected to the output end of the second rare earth-doped optical fiber, and a fourth WDM
A fourth pumping light source connected to one output of the coupler, wherein a lossy medium is connected between the other output of the second WDM coupler and the other input of the third WDM coupler. And a third rare earth-doped optical fiber.

In addition to the above configuration, the present invention provides a wavelength λ _P1 of light demultiplexed by each WDM coupler in the 980 nm band or 14 μm.
An 80 nm band may be used, and the wavelength λ _P2 of light passing through each WDM coupler may be a 1480 nm band or a 980 nm band.

In addition to the above configuration, in the present invention, an optical isolator may be connected to the input side of the preceding stage amplifying means and the output side of the succeeding stage amplifying means.

Two amplifying media having different gain wavelength characteristics are arranged in cascade, and two loss media having an attenuation peak near 1530 nm and having the attenuation wavelength characteristic are cascaded. The wavelength characteristics of the gain include the length of the rare earth-doped fiber, the power of the pumping light, and the doping materials (Er, Al, etc.) added to the Er-doped fiber.
Can be controlled by Also, since one loss medium can be controlled by the length of the rare earth-doped fiber and the additive material (Er, Al, etc.) added to the fiber, the wavelength characteristic is higher than that of an optical amplifier using a conventional rare earth-doped optical fiber. With more parameters, the degree of freedom for flattening increases, and as a result, a flat gain wavelength characteristic over a wide wavelength band can be realized.

That is, according to the present invention, the wavelength gain characteristics of a plurality of amplifying means using a rare-earth-doped optical fiber as an amplifying medium are attenuated with an increase in wavelength, and the wavelength gain characteristics of the rare-earth-doped optical fiber as a loss medium have a wavelength-gain characteristic. Since the wavelength gain increases with the increase, by connecting the wavelength gain of each amplifying means and the wavelength loss of the loss medium in a cascade, the wavelength gain of the entire optical fiber amplifier is flattened.

Further, three WDM couplers having the same structure,
With a simple configuration using two excitation light sources, excitation light can be supplied to two rare earth-doped optical fibers as amplification media.

When one amplification medium is pumped backward and the other amplification medium is pumped forward, three WDM couplers and two pumping light sources are concentrated in one place, so that mounting, maintenance and management are easy. Become. In addition, by mounting two excitation light sources on a common radiator plate, cost and size can be reduced.

In the case of using two WDM couplers and two pumping light sources, the cost can be reduced and simplified, and these optical components can be concentrated in one place. As described above, mounting, maintenance and management become easy.

Since the wavelengths of the pumping light of the two amplifying media are different, there is no fear of fluctuation or deterioration of the power or wavelength of the pumping light due to interference between the pumping light sources.

When the two amplification media are respectively excited by forward excitation, low noise figure characteristics can be realized. Since the wavelengths of the pump light are different, there is no fear of fluctuation or deterioration of the power or wavelength of the pump light due to interference between the pump light sources.

By using four identically structured WDM couplers and two pumping light sources of the same wavelength, the respective powers of the pumping light sources can be supplied to the two amplifying media and used effectively, so that a high gain can be obtained. , Wideband characteristics can be realized.

By using four identically structured WDM couplers and four pumping light sources, a broadband optical fiber amplifier having higher gain and higher saturation output can be realized. Further, since a large light power is obtained in total of the four pump light sources without using a pump light source having a large optical power, a stable and low-cost pump light source can be used.

980 that can obtain low noise figure characteristics
nm wavelength band light, stable optical power and wavelength 14
By using the light having the wavelength in the 80 nm band together, an optical amplifier with higher performance and higher reliability can be realized.

[0038]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. The same members as those in the conventional example are denoted by the same reference numerals.

FIG. 1 is a block diagram showing one embodiment of the optical fiber amplifier of the present invention.

The optical fiber amplifier shown in FIG. 3 includes a preceding amplification means having an amplification medium, a loss means having a loss medium, and a later amplification means having an amplification medium, and each of the two amplification media has The wavelength characteristic of the gain is combined with the loss characteristic of the loss medium to flatten the wavelength characteristic of the gain over a wider wavelength range.

The amplifying means in the preceding stage (left side in the figure) connects an Er-doped optical fiber 10 as a first rare-earth-doped optical fiber (amplifying medium), and one input end 11a is connected to an output end of the Er-doped optical fiber 10. First WDM coupler 11
And a second WDM coupler 12 having one input terminal 12a connected to one output terminal 11c of the first WDM coupler 11.
And a first pumping light source (semiconductor laser) 13 connected to one output end 12 d of the second WDM coupler 12, and pumping lights 13-1 and 13-1 from the semiconductor laser 13.
The Er-doped optical fiber 10 is backward-pumped by -2 and 13-3.

The latter amplifying means includes an Er-doped optical fiber 14 as a second rare-earth-doped optical fiber (amplifying medium).
A third WDM coupler 15 having one output terminal 15a connected to the input terminal of the Er-doped optical fiber 14, and a second output terminal 12b having one input terminal 1 of the third WDM coupler 15.
5c and a second WDM coupler 12c.
The second input terminal 12c of the DM coupler 12
And a semiconductor laser 16 as an excitation light source of
Excitation light 16-1, 16-2, 1 from semiconductor laser 16
By 6-3, the Er-doped optical fiber 14 is pumped forward.

The loss medium is Er-doped as a third rare-earth-doped optical fiber connected between the other output end 11b of the first WDM coupler 11 and the other input end 15a of the third WDM coupler 15. An optical fiber 17 is provided, and only the signal lights 1-2 and 1-3 pass therethrough, and the pumping lights 13-3 and 16 are provided.
-3 is not excited without passing through.

Isolators 108 and 109 are connected to the input end of the front-stage Er-doped optical fiber 10 and the output end of the rear-stage Er-doped optical fiber 14, respectively, to prevent reflected light from returning.

In such an optical fiber amplifier, the signal light 1-1 is usually a wavelength-multiplexed signal light.
Signal light having a wavelength of several to several tens of wavelengths is wavelength-multiplexed in a wavelength band from 30 nm to 1560 nm. The signal light 1-1 passes through the optical isolator 108 and passes through the Er-doped optical fiber 10.
To enter. This Er-doped optical fiber 10 has a length L
_1, when diverted Er-doped multi-core fiber shown in FIG. 7, for example, the length L ₁ is 10 m, Er
The addition amount is 400 ppm and the Al addition amount is 17,000 pp.
m, each core interval is 1.3 μm, and each core diameter is 1.
9 μm, a cladding diameter of 125 μm, and a relative refractive index difference Δ between the core and the cladding of about 2.1% are used.

The outgoing light (excitation light) 13-1 of the semiconductor laser 13 having a wavelength of 980 nm is output from the output end side of the Er-doped optical fiber 10 via the WDM coupler 12 and the WDM coupler 11 as indicated by arrows 13-2 and 13-3. Thus, the light propagates in the Er-doped optical fiber 10 (propagates in the direction opposite to the propagation direction of the signal light 1-1). Thus the signal light 1-1 is amplified propagates as indicated by an arrow 1-2, and enters the Er doped optical fiber 17 of a length L _2.

Here, the power of the pumping light 13-3 is 100
The wavelength characteristic of the gain of the signal light amplified as shown by the arrow 1-2 in the case of mW is shown by a characteristic curve La as shown in FIG. 2 and has a gain peak near 1530 nm and a longer wavelength side than that. There is a characteristic that the gain decreases downward to the right. FIG. 2 is a diagram showing the wavelength gain characteristics of the optical fiber amplifier shown in FIG. 1. The horizontal axis indicates the wavelength, and the vertical axis indicates the gain or the loss.

[0048] The use of a Er-doped multi-core fiber having the characteristics shown in FIG. 11 in the Er-doped optical fiber 17, and the length L ₂ and 2m. Since neither pump light 13-1 or 16-1 of the semiconductor lasers 13 and 16 propagates in the Er-doped optical fiber 17, the Er-doped optical fiber 17 functions as a loss medium, and is indicated by a broken line Lb in FIG.
The characteristics are as shown in FIG.

Therefore, the amplified signal light 1-2 is generated by the E
The characteristic when the light propagates through the r-doped optical fiber 17 and completes the passage as indicated by the arrow 1-3 is represented by a solid line Lc as shown in FIG.
The characteristics were as shown by. That is, 1530n
The characteristic is such that the gain on the short wavelength side on the m side is reduced. This characteristic is not a flat gain characteristic over a wavelength of 1530 nm to 1560 nm. Amplified and attenuated signal light 1
-3 enters the Er doped optical fiber 14 of length L ₃ passes through the WDM coupler 15 as indicated by an arrow 1-4. Also,
The pumping light 16-1 from the semiconductor laser 16 is coupled to the Er-doped optical fiber 14 through the WDM couplers 12 and 15, and propagates as indicated by arrows 16-2 and 16-3. In addition, W
The DM couplers 11, 12, and 15 have the same structure, split the pump light in the 980 nm band, and convert the
This is an optical demultiplexing circuit having an optical fiber structure having a characteristic of passing signal light in the nm band to 1560 nm band as it is. E
The signal light 1-4 and the pump light 16-
The signal light 1-4 is further amplified by the propagation of 3, and is extracted through the optical isolator 109 as shown by an arrow 1-5.

Here, the length L of the Er-doped optical fiber 14
_{3 is set} to 8 m, and the pump power of the pump light 16-3 is set to 80 mW.
As a result, the wavelength characteristic of the gain of the subsequent amplification means itself becomes the wavelength characteristic of the gain indicated by the characteristic curve Ld in FIG.

The gain wavelength characteristic of the amplified signal light 1-5 can realize a flat characteristic as shown by the characteristic curve Le in FIG. That is, although the gain wavelength characteristics could not be flattened by combining only one gain medium and one loss medium, the combination of three gain characteristics was achieved by combining a plurality of gain media and loss media in a cascade. Together, the wavelength characteristics of the gain can be flattened. In FIG. 1, the optical fiber lengths L ₁ , L
₃ is a length (about 30 m, E
(depending on the amount of r or Al added and the optical fiber structure).

As described above, the noise figure of a plurality of amplifying media is suppressed to be small, and a wideband optical fiber amplifier is realized. As the WDM couplers 11, 12, and 15, besides the optical fiber type, an individual component type optical duplexer using an interference film filter, a lens, and an optical fiber may be used.

FIG. 3 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention.

The difference from the embodiment shown in FIG.
The point is that the front-stage rare-earth-doped optical fiber is pumped backward using two WDM couplers, and the rear-stage rare-earth-doped optical fiber is pumped forward. That is, the number of WDM couplers is one less than the configuration of FIG.

The amplifying means at the preceding stage comprises the Er-doped optical fiber 1
0, one input end 20a is connected to an output end of the Er-doped optical fiber 10, and one input end 21c is connected to one output end 20d of the first WDM coupler 20 by an optical fiber. A second WDM coupler 21 connected via the second WDM coupler 21 and one output terminal 2 of the second WDM coupler 21.
1d, and a semiconductor laser (wavelength 980 nm band) 13 as a first excitation light source.

The first WDM coupler 20 splits the 980 nm band pump light into a 1480 nm band pump light and 1530 nm.
An optical demultiplexer having a characteristic of transmitting signal light in the nm to 1560 nm band is used. Also, the second WDM coupler 21
An optical demultiplexer having a characteristic of demultiplexing the excitation light in the 1480 nm band and passing the excitation light in the 980 nm band and the signal light in the 1530 nm to 1560 nm band is used.

The amplifying means at the subsequent stage includes the Er-doped optical fiber 1
4, a second WDM coupler 21 having the other output end 21b connected to the input end of the Er-doped optical fiber 14, and an optical fiber having one output end 20d connected to one input end 21c of the second WDM coupler 21. And the other input terminal 2 of the first WDM coupler 20.
And a semiconductor laser (wavelength band of 1480 nm) 16 as a second excitation light source connected to the first excitation light source 0c.

The loss medium is the Er-doped optical fiber 17 connected between the other output end 20b of the first WDM coupler 20 and the other input end 21a of the second WDM coupler 21.
And only the signal lights 1-2 and 1-3 pass and the pump light 13
-3 and 16-3 are not excited without passing through.

With the above configuration, the wavelength of 980 nm from the semiconductor laser 13 is contained in the Er-doped optical fiber 10.
The pump light 13-1 of the band passes through the WDM
The light is propagated as shown in 3-2, and the pump light is coupled into the Er-doped optical fiber 10 and propagated through the WDM coupler 20 as shown in 13-3.

The Er-doped optical fiber 14 has a wavelength of 1480 n from the semiconductor laser 16 as the second pumping light source.
The m-band pump light 16-1 is propagated through the WDM coupler 20 as shown by the arrow 16-2, and is coupled through the WDM coupler 21 into the Er-doped optical fiber 14 like the pump light 16-3 and propagates.

Incidentally, a configuration opposite to the above-mentioned configuration, that is,
A 980 nm band semiconductor laser is used for the semiconductor laser 16, a 1480 nm band semiconductor laser is used for the semiconductor laser 13, an optical demultiplexer for demultiplexing the 1480 nm band excitation light is used for the WDM coupler 20, and 980 n is used for the WDM coupler 21.
A configuration using an optical splitter that splits the m-band excitation light may be used.

The characteristics of the optical fiber amplifier shown in FIG.
Since the number of WDM couplers is one less than that of the optical fiber amplifier shown in FIG. 1, the configuration is simplified, cost and loss are reduced, and higher gain and lower noise figure can be realized. Further, since the wavelength of the excitation light source 16 and the wavelength of the excitation light source 13 are different, the oscillation wavelength and the oscillation output do not fluctuate due to mutual interference. That is, it is a stable optical amplifier.

FIG. 4 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention.

The difference from the embodiment shown in FIG. 1 is that both the front and rear rare earth doped optical fibers are pumped forward.

The amplifying means at the preceding stage includes a semiconductor laser 13 as a first pumping light source, a first WDM coupler 30 having one input terminal 30c connected to the semiconductor laser 13, and an input terminal having a first WDM coupler. 30 and an Er-doped optical fiber 10 connected to one output end 30b.

The latter amplifying means includes a semiconductor laser 16 as a second pumping light source, a second WDM coupler 31 having one input terminal 31c connected to the semiconductor laser 16, and a second input terminal 32c having the second input terminal 32c connected to the second laser. WDM coupler 31 connected to one output end 31d of WDM coupler 31 via an optical fiber.
It has a coupler 32 and an Er-doped optical fiber 14 whose input terminal is connected to one output terminal 32b of the third WDM coupler 32.

The loss medium is supplied to the Er-doped optical fiber 17 connected between the other output terminal 31b of the second WDM coupler 31 and the other input terminal 32a of the third WDM coupler 32.
And only the signal lights 1-3 and 1-4 pass, and the pump lights 13-2 and 16-3 do not pass.

The Er-doped optical fiber 10 is excited by pumping light 13-1 from the semiconductor laser 13 in the 980 nm band (or 1480 nm band). Excitation light 13-1 is 980n
WD for splitting m-band (or 1480 nm band) excitation light
After passing through the M coupler 30 as shown by the arrow 13-2, it is absorbed in the Er-doped optical fiber 10. If not completely absorbed in the Er-doped optical fiber 10, the remaining pump light is split by the WDM coupler 31 for splitting the pump light in the 980 nm band (or 1480 nm band), as indicated by an arrow 13-3. To be propagated.

On the other hand, the Er-doped optical fiber 14
It is excited by the excitation light of the semiconductor laser 16 in the 0 nm band (or 980 nm band). The pump light 16-1 passes through the WDM coupler 31 and propagates as indicated by an arrow 16-2.
After passing through the WDM coupler 32 for splitting the pumping light in the 80 nm band (or 980 nm band), the Er-doped optical fiber 1
4 is propagated as indicated by an arrow 16-3.

The optical fiber amplifier shown in FIG. 10 pumps both the Er-doped optical fibers 10 and 14 by forward pumping.
Not only can it be amplified over a wide band, but also low noise figure characteristics can be realized.

FIG. 5 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention. The difference from the embodiment shown in FIG. 1 is that the former rare-earth-doped optical fiber is pumped by forward pumping, the latter rare-earth-doped optical fiber is pumped by backward pumping, and the remaining fiber is left during forward pumping. The point is that the pumping light is propagated to the rare-earth-doped optical fiber at the subsequent stage, and the pumping light remaining in the backward pumping is propagated to the rare-earth-doped optical fiber at the preceding stage.

The preceding amplification medium includes a semiconductor laser 13 and
A first WDM coupler 40 having one input terminal 40c connected to the semiconductor laser 13; an Er-doped optical fiber 10 having an input terminal connected to one output terminal 40b of the first WDM coupler 40; The end 41 a has the second WDM coupler 41 connected to the output end of the Er-doped optical fiber 10.

The amplification medium at the subsequent stage includes the semiconductor laser 16 and
A third WDM coupler 43 having one output terminal 43c connected to the semiconductor laser 16, an Er-doped optical fiber 14 having an output terminal connected to one input terminal 43a of the third WDM coupler 43, and one output terminal The end 42b is connected to the input end of the Er-doped optical fiber 14, and one input end 42c is connected to the second W
It has a fourth WDM coupler 42 connected to one output end 41c of the DM coupler 41 via an optical fiber.

The loss medium is supplied to the Er-doped optical fiber 17 connected between the other output end 41b of the second WDM coupler 41 and the other input end 42a of the fourth WDM coupler 42.
, And only the signal lights 1-3 and 1-4 pass, and the pump lights 13-3 and 16-3 do not pass.

The semiconductor lasers 13 and 16 have a wavelength of 980 nm.
The pump light 13-1 from the semiconductor laser 13 is demultiplexed by the WDM coupler 40 and then propagates through the Er-doped optical fiber 10 as indicated by an arrow 13-2 using a semiconductor laser of a band (or a 1480 nm band). Absorbed. If the pump light remains without being absorbed by the Er-doped optical fiber 10, W
The light is split by the DM coupler 41 as indicated by an arrow 13-3, and further propagates through the Er-doped optical fiber 14 by the WDM coupler 42 as indicated by an arrow 13-4.

The pump light 16-1 from the semiconductor laser 16 propagates through the Er-doped optical fiber 14 via the WDM coupler 43 as shown by the arrow 16-2 and is absorbed. If the pump light remains without being absorbed by the Er-doped optical fiber 14, W
The light is split by the DM coupler 42 and propagates as indicated by an arrow 16-3, and is further split by the WDM coupler 41 and propagates as indicated by an arrow 16-4.

Here, the WDM couplers 40, 41, 42,
Reference numeral 44 denotes an optical demultiplexer having a characteristic of demultiplexing pump light having a wavelength of 980 nm (or 1480 nm) and transmitting signal light having a wavelength of 1530 nm to 1560 nm. Er
Any semiconductor laser 13,
The configuration is such that the pumping light from 16 also does not propagate.

FIG. 6 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention.

The difference from the embodiment shown in FIG. 1 is that the rare-earth-doped optical fiber of the preceding stage is bidirectionally pumped,
The point is that the latter rare-earth-doped optical fiber is bidirectionally pumped.

The preceding amplification medium includes a semiconductor laser 50 as a first pumping light source, a first WDM coupler 55 having one input terminal 55c connected to the semiconductor laser 50, and a first WDM coupler 55 having an input terminal of the first WDM coupler 55. Of the Er-doped optical fiber 10 connected to one output end 55b of the
Is connected to the output end of the Er-doped optical fiber 10, and one input end 57c is connected to the second WD.
A third WDM coupler 57 connected to one output end 56d of the M coupler 56 via an optical fiber;
A semiconductor laser 52 as a second pumping light source connected to one output terminal 57d of the coupler 57;

The semiconductor lasers 50 and 52 have a wavelength of 980 n.
An m-band (or 1480 nm band) semiconductor laser is used.

The subsequent amplification medium includes a semiconductor laser 51 as a third pumping light source, a second WDM coupler 56 having the other input terminal 56c connected to the semiconductor laser 51, and a second input terminal 57c having the second input terminal 57c connected to the second laser diode 51. A third WDM connected to one output end 56d of the WDM coupler 56 via an optical fiber
A coupler 57 and an Er-doped optical fiber 14 whose input end is connected to the other output end 57b of the third WDM coupler 57
A fourth WDM coupler 58 having one input terminal connected to the output terminal of the Er-doped optical fiber 14, and a fourth pumping light source connected to one output terminal 58c of the fourth WDM coupler 58. And a semiconductor laser 53.

The semiconductor lasers 51 and 53 have a wavelength of 1480.
Semiconductor lasers in the nm band (or 980 nm band) are used.

The WDM couplers 55 and 56 have a wavelength of 980 n.
An optical demultiplexer that demultiplexes the m-band (or 1480 nm band) excitation light and passes the signal light is used.
7 and 58 have a wavelength of 1480 nm (or 980 nm).
An optical demultiplexer that demultiplexes the excitation light of the band and passes the signal light is used.

The loss medium is the Er-doped optical fiber 17 connected between the other output end 56b of the second WDM coupler 56 and the other input end 57a of the third WDM coupler 57.
And only the signal lights 1-3 and 1-4 pass, and the pump lights 50-3, 51-2, 52-2, and 53-3 do not pass and are not pumped.

The pumping light propagating in the Er-doped optical fiber 10 is a pumping light 50-2 from the semiconductor laser 50 and a pumping light 52-3 from the semiconductor laser 52. The pump light propagating in the Er-doped optical fiber 14 is a pump light 51-3 from the semiconductor laser 51 and a pump light 53-2 from the semiconductor laser 53.

The propagation path of the excitation light from each of the semiconductor lasers 50 to 53 is as follows.

First, the excitation light 50 from the semiconductor laser 50 is
-1 is coupled into the Er-doped optical fiber 10 by the WDM coupler 55 and propagates as indicated by an arrow 50-2.
It is absorbed by Er ions in the r-doped optical fiber 10 to form a population inversion state and contributes to amplification of the signal light 1-1. If the fiber length L ₁ when the power is too large or Er doped optical fiber 10 of the excitation light 50-1 is too short, when more was too small Er amount of addition, the excitation light 50-
A part of 2 passes through the Er-doped optical fiber 10, is demultiplexed by the WDM coupler 56, and propagates as indicated by an arrow 50-3.

Excitation light 52-1 from semiconductor laser 52
Is transmitted through the WDM coupler 57 and propagates as indicated by an arrow 52-2, is demultiplexed by the WDM coupler 56, is coupled into the Er-doped optical fiber 10, propagates as indicated by an arrow 52-3, and is converted into Er ions. It is absorbed and forms a population inversion state, contributing to amplification of the signal light 1-1.

Excitation light 51-1 from semiconductor laser 51
Is transmitted through the WDM coupler 56 and propagates as indicated by the arrow 51-2, is demultiplexed by the WDM coupler 57, is coupled into the Er-doped optical fiber 14, propagates as indicated by the arrow 51-3, and It is absorbed by the Er ions in the fiber 14 to form a population inversion state and contributes to the amplification of the signal light 1-5.

Excitation light 53-1 from semiconductor laser 53
Is the Er-doped optical fiber 14 by the WDM coupler 58.
After propagating in the Er-doped optical fiber 14 as shown by the arrow 53-2, the light propagates in the Er-doped optical fiber 14 as shown by the arrow 53-2, and is absorbed by Er ions in the Er-doped optical fiber 14 to form a population inversion state. It contributes to the amplification of the signal light 1-5. If the pumping light 53-2 remains without being completely absorbed by the Er ions in the Er-doped optical fiber 14, the remaining pumping light is demultiplexed by the WDM coupler 57 as shown by an arrow 53-3. Propagate.

As described above, in each of the Er-doped optical fibers 10 and 14, two semiconductor lasers (5
0, 52 and 51, 53) are provided so that the semiconductor laser is twice as large as the optical fiber amplifier shown in FIGS. 1, 3, 4 and 5. Thereby, high gain and high saturation output characteristics are provided.

[0093] With respect to a wider band, E
The fiber length L ₂ of the r-doped optical fiber 17 is shown in FIGS.
This can be achieved by making the length longer than in the case of the optical fiber amplifier shown in FIGS.

The present invention is not limited to the above embodiment.
First, instead of the Er-doped optical fiber 17, an interference film filter that strongly attenuates the signal light in the 1530 nm band may be inserted, or a grating that reflects the signal light in the 1530 nm band may be inserted. Further, instead of using the optical isolators 108 and 109, an optical circulator may be used. Er-doped optical fiber 10, 14, 1
In addition to the Er-doped multi-core fiber, ordinary Er
An Er-doped fiber having one doping core may be used. Further, the Er-doped optical fibers 10, 14, 17 are E
At least one other rare earth element such as Yb (ytterbium) or Ce (cerium) may be contained in addition to r and Al. For controlling the refractive index of Ge (germanium), P (phosphorus), F (fluorine), etc. Additives may be included.

As described above, according to the present invention, (1) two amplifying media having different gains and wavelength characteristics thereof;
One loss medium having an attenuation peak near 1530 nm and having wavelength characteristics of the attenuation is connected in cascade, and the gain and wavelength characteristics of each amplification medium are determined by the length of the Er-doped optical fiber and the pump light. Power and its E
The additive material (Er,
Al, etc.), and the loss medium can be controlled by the length of the Er-doped optical fiber and the additive material (Er, Al, etc.) added to the optical fiber. The degree of freedom in flattening the gain wavelength characteristic by using a larger number of parameters than in the optical fiber amplifier is increased, and as a result, a flat gain wavelength characteristic can be realized over a wide band.

(2) Three WDM couplers having the same structure,
The pump light can be supplied to the amplification medium at a low cost with a simple configuration using two pump light sources. When the amplification medium is pumped by the backward pumping and the amplification medium is pumped by the forward pumping, mounting and maintenance are easy because three WDM couplers and two pumping light sources are concentrated in one place.

(3) With a simple configuration using two WDM couplers and two pumping light sources and a low-cost configuration, and since these optical components can be arranged in one place,
Easy to implement, maintain and operate. In addition, since the wavelengths of the pump light of the amplification medium are different, the fluctuation and deterioration of the power and wavelength of the pump light due to the interference between the pump light sources are small.

(4) Since the amplification media are excited by forward excitation, low noise figure characteristics can be realized. Further, since the wavelengths of the pumping light are different, there is no fear of fluctuation or deterioration of the power or wavelength of the pumping light due to interference between the pumping light sources.

(5) By using four identically structured WDM couplers and two pumping light sources having the same wavelength, the power of each pumping light source can be supplied to the amplifying medium and used effectively, so that high gain and wide band characteristics can be obtained. It can be realized more efficiently.

(6) By using four WDM couplers and four pumping light sources, it is possible to realize a wide-band optical fiber amplifier having a high gain, a higher saturation output, and a higher output.

[0101]

In summary, according to the present invention, the following excellent effects are exhibited.

A plurality of amplifying means using a rare-earth-doped optical fiber to which the pump light from the pump light source is input as the amplifying medium having different wavelength characteristics of gain, Having a loss medium and cascading these media,
It is possible to provide an optical fiber amplifier having a flat gain wavelength characteristic over a wide wavelength range.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of an optical fiber amplifier according to the present invention.

FIG. 2 is a diagram showing a wavelength gain characteristic of the optical fiber amplifier shown in FIG.

FIG. 3 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention.

FIG. 4 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention.

FIG. 5 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention.

FIG. 6 is a block diagram showing another embodiment of the optical fiber amplifier of the present invention.

FIG. 7 is a cross-sectional view of an Er-doped multi-core optical fiber proposed by the present inventors.

FIG. 8 is a block diagram of an optical fiber amplifier on which the present invention is based.

FIGS. 9A and 9B are diagrams showing the results of measuring the wavelength characteristics of gain using the signal light power Sp as a parameter.

FIG. 10 is a block diagram of a conventional optical fiber amplifier.

11 is a diagram illustrating a loss wavelength characteristic of the Er-doped multi-core optical fiber illustrated in FIG. 7;

[Explanation of symbols]

10, 14, 17 Rare earth doped optical fiber (Er doped optical fiber) 13, 16 Pump light source (semiconductor laser) 11, 12, 15 WDM coupler 108, 109 Optical isolator

Claims (8)

[Claims] 1. An optical fiber amplifier for amplifying a signal light by propagating the signal light into a rare-earth-doped optical fiber and inputting the pumping light from a pumping light source, wherein the pumping light from the pumping light source has different wavelength characteristics of gain. Having a plurality of amplifying means using a rare-earth-doped optical fiber to which the input is input as an amplifying medium, and a loss means using a non-excited rare-earth-doped optical fiber that passes only signal light as a loss medium, and cascade-connecting these media. An optical fiber amplifier characterized by the above-mentioned. 2. An amplifying means in a preceding stage includes: a first rare-earth-doped optical fiber; a first WDM coupler having one input terminal connected to the first optical fiber; and a first WD coupler having one input terminal connected to the first WD.
A second WDM coupler connected to one output terminal of the M coupler; and a first pumping light source connected to one output terminal of the second WDM coupler. , A third WDM coupler having one output terminal connected to the input terminal of the second rare earth doped optical fiber, and the other output terminal connected to one input terminal of the third WDM coupler. A second WDM coupler, and a second pumping light source connected to the other input terminal of the second WDM coupler, wherein the loss medium is connected to the other output terminal of the first WDM coupler and to the third output terminal. 2. The optical fiber amplifier according to claim 1, further comprising a third rare earth-doped optical fiber connected between the other input end of the WDM coupler and the third input terminal. 3. An amplifying means in a preceding stage includes a first rare earth-doped optical fiber, a first WDM coupler having one input terminal connected to an output terminal of the first optical fiber, and one input terminal connected to a first WDM coupler. A second WDM connected to one output terminal of one WDM coupler.
A first pumping light source connected to one output terminal of the second WDM coupler and a DM pump;
A second rare earth-doped optical fiber and a second WD having the other output end connected to the input end of the second rare earth-doped optical fiber
An M coupler; a first WDM coupler having one output terminal connected to one input terminal of the second WDM coupler;
A second pump light source connected to the other input of the DM coupler, wherein a lossy medium is provided between the other output of the first WDM coupler and the other input of the second WDM coupler. The optical fiber amplifier according to claim 1, further comprising a third rare earth-doped optical fiber connected thereto. 4. An amplification means in a preceding stage comprises: a first excitation light source;
A first WD having one input connected to a first excitation light source
An M coupler, and a first rare earth-doped optical fiber having an input terminal connected to one output terminal of the first WDM coupler;
Amplifying means at the subsequent stage includes a second pumping light source, a second WDM coupler having one input terminal connected to the second pumping light source, and one input terminal connected to one output terminal of the second WDM coupler. A third WDM coupler connected thereto, and a second rare-earth-doped optical fiber having an input terminal connected to one output terminal of the third WDM coupler; 2. The optical fiber amplifier according to claim 1, further comprising a third rare earth doped optical fiber connected between the other output terminal and the other input terminal of the third WDM coupler. 5. The pre-amplification medium comprises: a first excitation light source;
A first WD having one input connected to a first excitation light source
An M coupler, a first rare earth-doped optical fiber having an input terminal connected to one output terminal of the first WDM coupler, and a second rare earth doped optical fiber having one input terminal connected to the output terminal of the first rare earth doped optical fiber. 2 WDM couplers, and the subsequent amplification medium has
A second pumping light source, a third WDM coupler having one output terminal connected to the second pumping light source, and a third WDM coupler having an output terminal.
A second rare earth-doped optical fiber connected to one input end of the coupler, and one output end connected to the input end of the second rare earth doped optical fiber and one input end of one end of the second WDM coupler; A fourth WDM coupler connected to the output end, wherein a lossy medium is connected between the other output end of the second WDM coupler and the other input end of the fourth WDM coupler. The optical fiber amplifier according to claim 1, further comprising a rare earth doped optical fiber. 6. The first amplification medium comprises: a first excitation light source;
A first WD having one input connected to a first excitation light source
An M coupler, a first rare earth-doped optical fiber having an input terminal connected to one output terminal of the first WDM coupler, and a second rare earth doped optical fiber having one input terminal connected to the output terminal of the first rare earth doped optical fiber. 2 WDM couplers, and one input terminal is connected to the second WD coupler.
A third WDM coupler connected to one output terminal of the M coupler; and a second pumping light source connected to one output terminal of the third WDM coupler. Pump light source, a second WDM coupler having the other input terminal connected to the third pump light source, and a third WDM coupler having one input terminal connected to one output terminal of the second WDM coupler. A second rare earth-doped optical fiber having an input end connected to the other output end of the third WDM coupler, and a fourth rare earth-doped optical fiber having one input end connected to the output end of the second rare earth-doped optical fiber.
And a fourth excitation light source connected to one output terminal of the fourth WDM coupler, wherein the loss medium is the second excitation light source.
2. The optical fiber amplifier according to claim 1, further comprising a third rare-earth-doped optical fiber connected between the other output terminal of the WDM coupler and the other input terminal of the third WDM coupler. 7. The wavelength λ of light split by each WDM coupler.
_{For P1} , a 980 nm band or a 1480 nm band is used.
7. The optical fiber amplifier according to claim 2, wherein the wavelength λ _P2 of the light passing through the DM coupler is in a 1480 nm band or a 980 nm band. 8. The optical fiber amplifier according to claim 2, wherein an optical isolator is connected to each of the input side of the first-stage amplifying unit and the output side of the second-stage amplifying unit.