JP2760482B2 - Optical fiber amplifier and optical fiber transmission system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a fiber amplifier and an optical fiber transmission system. More specifically,
The present invention relates to an optical fiber amplifier that amplifies a plurality of signal lights having different wavelengths by wavelength multiplexing with a uniform gain, and an optical fiber transmission system in which a plurality of the optical fiber amplifiers are connected.

[0002]

2. Description of the Related Art An optical fiber amplifier includes a rare earth-doped optical fiber, a semiconductor laser light source for exciting a rare earth element in the optical fiber, and excitation light (laser light) from the semiconductor laser for excitation. An optical multiplexer coupled to the rare earth-doped optical fiber. When the input signal and the pump light are input to a rare earth-doped optical fiber such as erbium,
Inversion distribution due to the pumping light is formed in the rare-earth-doped optical fiber, and as a result, the signal light undergoes induced amplification in the fiber.

As one type of optical fiber amplifier using such a principle, a reflection type optical fiber amplifier is disclosed in Reference 1 (Kobayashi, Ishihara, Goto, "Study on low noise of reflection type optical amplifier",
Proceedings of the 1993 IEICE Autumn Conference B-883, p4-
124, 1993.) and Reference 2 (Asai, Okuma, "Proof of Proposal of Improvement of Noise Characteristics in Prototype EDFA Prototype", Koshin Shin Kogyo Co., Ltd., December, No. 49, pp.23 -25, 1993.).

[0004] Such a conventional optical fiber amplifier is referred to as the seventh.
Shown in the figure. The optical fiber amplifier shown in FIG. 7 includes a signal light inputting optical fiber 711 and an optical fiber 7 connected to the signal light inputting optical fiber 711 via an optical circulator 713.
14 and a signal output optical fiber 712 connected to the optical fiber 714 via an optical circulator 713. The optical fiber 714 is connected to an erbium-doped optical fiber 715 via a wavelength combining coupler 717. The pumping light emitted from the pumping semiconductor laser 718 is input to one end of the erbium-doped optical fiber 715 via the wavelength combining coupler 717. At the other end of the erbium-doped optical fiber 715, a reflector 716 is provided.

Hereinafter, the operation of the optical fiber amplifier shown in FIG. 7 will be described.

The signal light propagating through the signal light input optical fiber 711 is input to the optical fiber 714 by the optical circulator 713 by changing the path in the direction of the arrow in FIG. Thereafter, the signal light is combined with the pumping light from the pumping semiconductor laser 718 by the wavelength combining coupler 717 and guided to the erbium-doped optical fiber 715. The signal light is amplified while propagating through the erbium-doped optical fiber 715, and the amplified signal light is reflected by the reflector 716 to change the course. The signal light reflected by the reflector 716 is
The light is amplified again while propagating through the erbium-doped optical fiber 715 and returns to the optical fiber 714. Thereafter, the signal light changes its path in the direction of the arrow by the action of the optical circulator 713 and is output to the signal light output optical fiber 712.

[0007]

When a plurality of signal lights having different wavelengths are simultaneously input to the above-mentioned optical fiber amplifier, there is a problem that the gain differs for each wavelength. FIGS. 8A and 8B are graphs respectively showing an input signal intensity and an amplified optical output intensity spectrum when signal light having eight different wavelengths is input to the optical fiber amplifier.

As can be seen from FIGS. 8 (a) and 8 (b), since a plurality of signal lights receive gains of different magnitudes for each wavelength, even after the input light signal intensities are set to be equal, after amplification. However, the light output intensity greatly differs for each wavelength. This is because, as shown in FIG. 9, the absorption cross section and the emission cross section of erbium, which is a rare earth element added to the optical fiber, change depending on the wavelength, and occur regardless of the configuration of the amplifier. In general, the gain G (λs) per unit length of the rare-earth element erbium with respect to the signal light wavelength λs is represented by an emission cross section σe (λs), an absorption cross section σa (λs), a lower level ion number No, and an upper level. The gain is not uniform with respect to the wavelength, which is represented by the following equation, depending on the number Ne of ions excited at each position.

G (λs) = σe (λs) · Ne−σa (λs) · No This is a problem in an optical transmission system for transmitting a plurality of wavelength-multiplexed optical signals using an optical fiber and an optical fiber amplifier. Is a serious problem. For example, Reference 3 (ELGold
stein, AF Elrefaie, N. Jackman, and S. Zaidi, "M
ultiwavelengthfiber-amplifier cascades in unidirec
nation interoffice ring networks ", Technical digest
of conference on Optical Fiber Communication (OFC /
IOOC'93), TuJ3, pp. 42-44, 1993.) states that there is a non-uniform gain for each wavelength when 14 wavelength multiplexed signal lights pass through 14 optical fiber amplifiers connected by optical fibers. Has been reported.

In order to solve such a problem, a method of compensating for a gain difference with respect to wavelength by using a loss characteristic with respect to wavelength has been attempted. For example, reference 4 (M. Wilkinson, A. B.
eddington, SA Cassidy and P. McKee, "D-fibre filt
er for Erbium gain spectrum flattering ", Electroni
cs Letters, vol. 28, No. 2, pp. 131-132, 1992.), it is proposed to use the wavelength loss characteristics of a diffraction grating in a fiber, and reference 5 (SF Su, R. Olshansky, G. .
Joyce, DA Smith, JE Baran, "Use of acoustoopt
ic tunable filters as equalizers in WDM lightwave
systems ", Technical digest of Conference on Optica
l Fiber Communication (OFC'92), ThC4, pp203-204, 199
In 2.), a method using the wavelength loss characteristics of an optical modulator using ultrasonic waves is proposed, but the gain has not been completely made uniform.

When the level difference between signal lights having different wavelengths is large, see Reference 6 (Mitsuta, Oya, Uno, "Analog Transmission Characteristics of Bidirectional Optical Fiber Amplifier", Proceedings of the 1994 IEICE Spring Conference, C-398). , 1994.), a large signal light input on the long wavelength side causes a gain crosstalk phenomenon in which the gain on the short wavelength side decreases. Reference 5
It has been experimentally investigated that almost no gain on the short wavelength side can be obtained. In such a case, it is impossible to use the method of compensating for the gain difference with respect to the wavelength using the loss characteristic with respect to the wavelength as described in References 4 and 5, which is a major problem in wavelength division multiplexing optical transmission.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and an object of the present invention is to provide a method for transmitting a plurality of signal lights having different wavelengths when a gain per unit length has wavelength dependency. An object of the present invention is to provide an optical fiber amplifier and an optical fiber transmission system that can provide a uniform gain.

[0013]

An optical fiber amplifier according to the present invention is an optical fiber amplifier comprising a rare earth-doped optical fiber and a pump light generator for optically pumping the rare earth-doped optical fiber. The signal light input from the input end of the rare-earth-doped optical fiber is further provided with reflecting means for selectively reflecting the signal light to the input end, thereby providing a gain to the signal light, thereby achieving the above object. Is done.

Another optical fiber amplifier according to the present invention includes a rare-earth-doped optical fiber and a pump light generator for optically exciting the rare-earth-doped optical fiber, and a gain per unit length of the rare-earth-doped optical fiber. Is an optical fiber amplifier having wavelength dependence, comprising a plurality of reflectors, each of the plurality of reflectors, among a plurality of signal lights input from the input end of the rare earth doped optical fiber, A reflector that selectively reflects signal light in a predetermined wavelength band, and as a result,
The gain given to the plurality of signal lights is adjusted to be substantially equal, thereby achieving the above object.

[0015] In one embodiment, the plurality of reflectors include:
At least a first gain provided to the signal light having the first wavelength is substantially equal to a second gain provided to the signal light having the second wavelength. The distance from the input end of the rare earth doped optical fiber to the first reflector and the distance from the input end of the rare earth doped optical fiber to the second reflector are adjusted to be equal.

In one embodiment, the reflectivity of each of the plurality of reflectors is adjusted, so that the gains given to the plurality of signal lights are substantially equal.

[0017] An excitation light reflector for selectively reflecting the excitation light may be further provided.

The reflector may have a fiber grating portion having a grating inside, or may have a dielectric multilayer structure.

In a preferred embodiment, an optical circulator connected to the input end of the rare-earth-doped optical fiber, an optical fiber for signal light input connected to the optical circulator and supplying the signal light to the input end, A signal light output optical fiber connected to the optical circulator for receiving the signal light from the input end.

An optical fiber transmission system according to the present invention is an optical fiber transmission system to which a plurality of optical fiber amplifiers are connected, wherein each of the plurality of optical fiber amplifiers includes a rare earth-doped optical fiber and the rare earth-doped optical fiber. A pump light generator that optically pumps the optical fiber, and selectively reflects the signal light input from the input end of the rare-earth-doped optical fiber to the input end, whereby: Further, there is provided a reflection means for giving a gain to the signal light, whereby the object is achieved.

Another optical fiber transmission system according to the present invention comprises:
An optical fiber transmission system to which a plurality of optical fiber amplifiers are connected, wherein each of the plurality of optical fiber amplifiers is
An optical fiber amplifier comprising: a rare-earth-doped optical fiber; and a pumping light generator for optically pumping the rare-earth-doped optical fiber, wherein the gain per unit length of the rare-earth-doped optical fiber has wavelength dependence. Further, each of the plurality of optical fiber amplifiers includes a plurality of reflections for selectively reflecting signal light in a predetermined wavelength band among the plurality of signal lights input from the input end of the rare earth-doped optical fiber. And the gain given to the plurality of signal lights is substantially equal, thereby achieving the above object.

Still another optical fiber transmission system according to the present invention is an optical fiber transmission system in which a plurality of optical fiber amplifiers are connected in a ring via an optical fiber, wherein each of the plurality of optical fiber amplifiers comprises: A rare earth-doped optical fiber connected to the optical fiber via one end, an excitation light generator for optically exciting the rare earth-doped optical fiber, and input from the one end to the rare earth-doped optical fiber Reflector means for selectively reflecting, to the one end, signal light in a predetermined wavelength band of the plurality of signal lights, wherein the reflecting means comprises a light source for each of the plurality of signal lights. The signal light belonging to the wavelength band specific to the fiber amplifier is transmitted and output from the other end of the rare-earth-doped optical fiber, thereby achieving the above object.

When a plurality of signal lights are used, in the present invention, in order to correct different gain characteristics for each wavelength, a reflection type optical fiber amplifier is provided at a specific position of a rare earth doped optical fiber with respect to each optical signal wavelength. By arranging a specific reflector, it is possible to receive gain for each signal wavelength range even though only one rare earth-doped optical fiber as an optical gain medium is used. Is changed so that the gain for each wavelength is constant.

[0024]

Embodiments of the present invention will be described below.

First, a first embodiment of an optical fiber amplifier according to the present invention will be described with reference to FIG.

The optical fiber amplifier shown in FIG. 1 comprises a signal light inputting optical fiber 11 and an optical fiber 14 connected to the signal light inputting optical fiber 11 via an optical circulator 13.
And a signal output optical fiber 12 connected to the optical fiber 14 via the optical circulator 13. The optical fiber 14 is connected to an erbium-doped optical fiber 15 via a wavelength combining coupler 17. Excitation light (laser light having a wavelength of 1.48 μm or 0.98 μm) emitted from the excitation semiconductor laser 18 is input to one end of the erbium-doped optical fiber 15 via the wavelength combining coupler 17.

In these respects, the optical fiber amplifier of FIG. 1 is not different from the optical fiber amplifier of FIG. A characteristic of the optical fiber amplifier of FIG. 1 is that a wavelength selective reflector 26 for selectively reflecting light in a predetermined wavelength band is provided at the other end of the erbium-doped optical fiber 15. Light having a wavelength outside the predetermined wavelength band is not reflected by the wavelength selective reflector 26. In the present embodiment, the wavelength selectivity is adjusted so that the signal light is reflected by the wavelength selective reflector 26.

Hereinafter, the operation of the optical fiber amplifier will be described.

The signal light propagating through the signal light input optical fiber 11 is input to the optical fiber 14 by the optical circulator 13, changing its path in the direction of the arrow in FIG. The signal light is then combined with the pumping light from the pumping semiconductor laser 18 by the wavelength combining coupler 17 and guided to the erbium-doped optical fiber 15. The signal light is amplified while propagating through the erbium-doped optical fiber 15, and the amplified signal light is reflected by the wavelength selective reflector 26 to change its course, and the signal light reflected by the wavelength selective reflector 16 is The light is amplified again while propagating through the erbium-doped optical fiber 15 and returns to the optical fiber 14. Thereafter, the signal light changes its path in the direction of the arrow by the action of the optical circulator 13 and is output to the signal light output optical fiber 12.

In the erbium-doped optical fiber, spontaneous emission light is generated, and the amplified spontaneous emission light forms noise.
In the graph of FIG. 8B, the curve that changes gradually shows the output intensity distribution of the spontaneous emission light. In the present embodiment, the wavelength selectivity of the wavelength selective reflector 26 is adjusted so that the wavelength selective reflector 26 transmits most of the spontaneous emission light and reflects only a narrow wavelength band component including the signal light. ing. Therefore, the output light includes only the band component of the spontaneous emission light that is reflected by the wavelength-selective reflector 26, in addition to the signal light. Therefore, the signal light can be amplified while removing the unnecessary spontaneous emission light component, and the signal / noise (spontaneous emission light component) ratio included in the amplified signal light is improved.

Next, a specific configuration of the wavelength selective reflector 26 will be described with reference to FIGS. 2 (a) and 2 (b). FIG. 2A shows a reflector made of a dielectric multilayer film. By stacking dielectric layers having different refractive indices, desired low-pass characteristics, high-pass characteristics, or band-pass characteristics can be obtained. It is connected to the optical fiber 15 by an optical system using a lens.

FIG. 2B shows a reflector called a fiber grating. By irradiating a part of the optical fiber 15 with ultraviolet laser light such as an excimer laser from the outside, a grating is formed directly on a part of the optical fiber 15. A reflector for a specific wavelength range can be formed at an arbitrary position in the length direction of the optical fiber 15 without cutting the optical fiber 15. The reflected wavelength is controlled by the period of the grating. The reflectance is
It is adjusted by controlling the amount of laser exposure and the amount of change in the refractive index in the optical fiber. Such a fiber grating basically has band-pass transmission characteristics, and has a large reflectance only in a specific wavelength band. In addition, since it is not necessary to cut the optical fiber at the time of manufacture, characteristics of low loss and high reflectivity can be obtained, and it is stable even at high temperatures.

According to the wavelength selective reflector shown in FIG. 2A or 2B, for example, 1.550 to 1.560 μm
A reflectance of about 95% or more can be given to light in the wavelength band of m, and a reflectance of about 0.001% or less can be given to light outside the band.

FIG. 2C schematically shows the wavelength dependence of the reflectance of the reflector 716 in FIG. 7, and FIG.
4 schematically shows the wavelength dependence of the reflectance used in the present invention.

As shown in FIG. 2D, the width of the reflectance profile of the reflector of the present invention is preferably smaller than the width of the gain profile. If a reflector having such a reflectance is used, most of the spontaneous emission light in FIG. 8B can be removed from the output light.

Next, a second embodiment of the optical fiber amplifier according to the present invention will be described with reference to FIG. In FIG. 3, components corresponding to the components of the optical fiber amplifier of FIG. 1 are given the same reference numerals.

In this embodiment, three signal lights (wavelength λ)
1, λ2, λ3) are input and amplified. The optical fiber amplifier according to the present embodiment includes a plurality of wavelength selective reflectors 26-
1, 26-2 and 26-3, each of which selectively reflects light in a different wavelength selection band. More specifically, reflector 26-1 reflects light of wavelength λ1, reflector 26-2 reflects light of wavelength λ2, and reflector 26-3 reflects light of wavelength λ3. The reflector shown in FIG. 2A or 2B can be used as the reflector.

From the optical circulator 13 to the wavelength selective reflector 26-3, a first optical fiber 14-1, a first erbium-doped optical fiber (length L1) 25-1, and a second erbium-doped optical fiber ( The length L2) 25-2 and the second optical fiber 14-2 are connected in this order.

When using a frequency-multiplexed analog signal light of λ1 = 1.56 μm, a baseband digital signal light of λ2 = 1.535 μm, and a baseband digital signal light of λ3 = 1.538 μm, three wavelength signals are obtained under the following conditions. On the other hand, an equal gain of 18 dB was obtained. Condition: Excitation light output 1
00mW, erbium ion concentration of rare earth doped optical fiber 250ppm, L1 = 50m, L2 = 20m, analog signal light input intensity 0dBm, other digital signal light intensity -20dB
m.

The gain per unit length of the rare-earth-doped optical fiber used in this embodiment is highest at λ1 = 1.56 μm, and somewhat lower at λ2 = 1.535 μm and λ3 = 1.538 μm. λ2 = 1.535 μm and λ3 = 1.538 μm
At m, there is no significant difference in gain per unit length.

In the present embodiment, the length of the rare earth-doped optical fiber through which the signal light of wavelength λ1 propagates is L1. On the other hand, the length of the rare earth-doped optical fiber through which the signal lights of the wavelengths λ2 and λ3 propagate are both L1 + L2. Since erpium is not added to the optical fiber 14-2, the signal light having the wavelength λ3 is not amplified by the optical fiber 14-2.

The reason why the lengths of the rare earth-doped optical fibers through which the signal lights of the wavelengths λ2 and λ3 propagate are made equal because the input intensities of both signal lights are small and the wavelengths λ2 and λ3 are relatively close.

Note that a rare earth-doped optical fiber may be used instead of the optical fiber 14-2. Then, the gains given to the three signal lights can be controlled independently. In the present embodiment, the explanation has been made by taking an example of analog signal light and digital signal light, but the present invention does not limit the modulation method of the signal light at all.

When a large number of signal lights are to be amplified, an optimal selection wavelength band is set for the number of wavelength selective reflectors in consideration of the wavelength of the signal light to be used and the wavelength dependence of the gain.

Next, a third embodiment of the optical fiber amplifier according to the present invention will be described with reference to FIG. The difference between this embodiment and the embodiment of FIG. 3 is that an excitation light reflector 36 that selectively reflects excitation light (wavelength λp) is provided at the end of the second optical fiber 14-2. On the point.

Here, in the case of inputting a frequency-multiplexed analog signal light of λ1 = 1.56 μm and a baseband digital signal light of λ2 = 1.535 μm, a gain of 19 dB is equal to two signal lights under the following conditions. Given the.

Conditions: pump light output 100 mW, erbium ion concentration of rare earth doped optical fiber 250 ppm, L1 = 5
0 m, L2 = 20 m, analog signal light input intensity 0 dBm, digital signal light intensity -20 dBm.

In this embodiment, the rare-earth-doped optical fiber 2
Excitation light components that have passed through these optical fibers without being absorbed by 5-1 and 25-2 are reflected by the excitation light reflector 36 to re-excite the rare earth-doped optical fibers 25-1 and 25-2 again. . For this reason, the amplification gain can be improved as compared with the case where the excitation light reflector is not used.

When a laser beam having a wavelength λp = 1.48 μm is used as the excitation light, for example, the reflectance is about 98% or more for light in a wavelength band of 1.460 to 1.485 μm, and the reflectance is not less than the other wavelength bands. It is preferable to use a reflector having a ratio of about 0.1% or less.

Next, an embodiment of an optical fiber transmission system according to the present invention will be described with reference to FIGS.

In the present optical fiber transmission system, as shown in FIG.
1, 100-2, 100-3 and 100-4 and optical fibers 110-1, 110-2, 110-3 and 110-
4 are connected alternately. Optical fiber amplifier 10
Each of 0-1, 100-2, 100-3, and 100-4 has a configuration similar to that of the optical fiber amplifier of FIG. 3, and has substantially the same gain for eight different wavelengths of signal light. Designed to give.

FIG. 5 (b) shows the spectrum of the input optical signal 120, and FIG. 5 (c) shows the spectrum of the optical output 130 after transmission. No change in signal intensity for each signal wavelength due to non-uniform gain as seen in conventional optical transmission is observed. In the output after transmission, a large spontaneous emission light intensity due to accumulation of spontaneous emission light components as in the related art was not observed, and a high signal / noise intensity ratio was obtained.

Finally, another embodiment of the optical fiber transmission system according to the present invention will be described with reference to FIG.

In FIG. 6, 200-1, 200-2,
Reference numerals 200-3 and 200-4 denote optical fiber amplifiers similar to those in the embodiment shown in FIG. Reference numeral 210 denotes a circular optical fiber for connecting each optical fiber amplifier.

A plurality of signal lights (λi, λj, λk, λl,...) Having different wavelengths are transmitted through the optical fiber transmission line. Since the optical fiber amplifier 200-1 has no wavelength selective reflector for the wavelength of λi among the plurality of signal lights, the signal light of λi is connected to the optical circulator of the rare earth doped optical fiber of the optical fiber amplifier 200-1. Is taken out from a different end. Similarly, in the optical fiber amplifier 200-2, the signal light of the wavelength λj is output, and in the optical fiber amplifier 200-3, the signal light of the wavelength λk is output.
At 00-4, the signal light having the wavelength λl is extracted outside.
In the optical fiber amplifier 200-3, the signal light having the wavelength λk is input from a different end from that connected to the optical circulator of the rare earth-doped optical fiber.
In 4, the signal light having the wavelength λ1 is input from an end different from that connected to the optical circulator of the rare-earth-doped optical fiber. In this transmission system, signal light of the same wavelength circulates around the circulating optical fiber 2100, so that signal overlap can be prevented. Further, since the gain is made uniform with respect to the wavelength, the effect of large spontaneous emission light was not observed, and a high signal / noise intensity ratio was obtained. By using this transmission system, it is possible to construct a system such as a local area network that can freely transmit different signals from multiple users to the other user using a physically common optical fiber transmission line. It is possible.

The signal wavelength, signal light intensity, pumping light condition, rare earth-doped optical fiber and other conditions are set in accordance with the use conditions, and are not limited to the values shown in the above embodiments. Absent.

Also, an optical amplifier in a 1.5 μm wavelength band to which erbium as a rare earth material is added has been described.
The present invention is also applicable to optical amplifiers using other rare earth materials such as neodymium and praseodymium.

[0058]

As described above, according to the present invention, it is possible to amplify signal light while removing unnecessary spontaneous emission light components. Further, it is possible to amplify so that the gain is constant for a plurality of signal light wavelengths. As a result, long-distance or distribution systems or LANs (Local Area Net
work) to greatly contribute to the construction of optical fiber transmission systems.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber amplifier according to a first embodiment of the present invention.

FIGS. 2A and 2B are configuration diagrams of a reflector used in the present invention, FIG. 2C is a graph showing the reflectance of the reflector in FIG. 7, and FIG. 2D is used in the present invention. 4 is a graph showing an example of the reflectance of a wavelength selective reflector.

FIG. 3 is a configuration diagram of a second embodiment of the optical fiber amplifier of the present invention.

FIG. 4 is a configuration diagram of a third embodiment of the optical fiber amplifier of the present invention.

5A is a configuration diagram of an optical fiber transmission system according to an embodiment of the present invention, FIG. 5B is a graph showing an input signal spectrum, and FIG. 5C is a graph showing an output signal spectrum after transmission.

FIG. 6 is a configuration diagram of another embodiment of the optical fiber transmission system of the present invention.

FIG. 7 is a configuration diagram of a conventional reflection type optical fiber amplifier.

FIGS. 8A and 8B are diagrams showing transmission characteristics of a conventional optical fiber amplifier.

FIG. 9 is a graph showing the wavelength dependence of the absorption / issue cross-section of an erbium-doped optical fiber.

[Description of Signs] 11 ... Optical fiber for signal light input 12 ... Optical fiber for signal light output 13 ... Optical circulator 14 ... Optical fiber 15 ... Erbium-doped optical fiber 17 ... Wavelength Synthetic coupler 18 ... Pumping semiconductor laser 26 ... Wavelength selective reflector 36 ... Pumping light reflector

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-226560 (JP, A) JP-A-6-177467 (JP, A) Proceedings of the 1993 Fall Meeting of the Institute of Electronics, Information and Communication Engineers B-883 (1993) , Kobayashi, Ishihara, Goto, "Study of noise reduction in reflection-type optical amplifier". 4-124 (58) Field surveyed (Int. Cl. ⁶ , DB name) H01S 3/07 H01S 3/10 H01S 3/17

Claims (10)

(57) [Claims] And 1. A rare earth-doped optical fiber, and a pumping light generator for optically exciting said rare earth-doped optical fiber, light having a gain wavelength dependency of the per unit length of the rare earth-doped optical fiber A fiber amplifier comprising a plurality of reflectors, each of the plurality of reflectors comprising:
A reflector that selectively reflects signal light in a predetermined wavelength band among a plurality of signal lights input from an input end of the rare-earth-doped optical fiber, and as a result, a gain given to the plurality of signal lights An optical fiber amplifier, wherein the optical fiber amplifiers are adjusted substantially equally. 2. The signal processing device according to claim 1, wherein the plurality of reflectors include at least first and second reflectors, and wherein the first gain given to the signal light having the first wavelength is a signal having the second wavelength. to be substantially equal to the second gain given to light, the rare earth distance from the input end of the doped optical fiber said to the first reflector and said rare earth doped optical the second from the input end of the fiber The optical fiber amplifier according to claim 1 , wherein the distance to the reflector is adjusted. 3. being adjusted reflectivity of the plurality of reflectors are each a result, the plurality of gain provided to the signal light becomes substantially equal, the optical fiber amplifier according to claim 1 . Wherein said excitation light includes selectively further excitation light reflector for reflecting, optical fiber amplifier according to any one of claims 1 to 3. Wherein said reflector comprises a fiber grating having a grating therein, an optical fiber amplifier according to any one of claims 1 to 4. Wherein said reflector has a dielectric multilayer structure, an optical fiber amplifier according to any one of claims 1 to 4. 7. An optical circulator connected to the input end of the rare-earth-doped optical fiber, an optical fiber for signal light input connected to the optical circulator and supplying the signal light to the input end, and the optical circulator The optical fiber amplifier according to any one of claims 1 to 6 , further comprising: a signal light output optical fiber that is connected to the optical fiber and receives the signal light from the input end. 8. An optical fiber transmission system to which a plurality of optical fiber amplifiers are connected, wherein each of the plurality of optical fiber amplifiers includes a rare earth-doped optical fiber and a pump for optically exciting the rare earth-doped optical fiber. A light generator, the signal light being input from the input end of the rare earth doped optical fiber being selectively reflected to the input end, thereby providing a gain to the signal light. An optical fiber transmission system further comprising a reflecting means. 9. An optical fiber transmission system to which a plurality of optical fiber amplifiers are connected, wherein each of the plurality of optical fiber amplifiers includes a rare earth-doped optical fiber and a pump for optically exciting the rare earth-doped optical fiber. a light generator comprises a, an optical fiber amplifier gain per unit length of the rare earth-doped optical fiber has a wavelength dependency, further, each of the plurality of optical fiber amplifiers, the rare earth-doped A plurality of reflectors for selectively reflecting signal light in a predetermined wavelength band among a plurality of signal lights input from an input end of the optical fiber are provided, and a gain given to the plurality of signal lights is substantially Optical fiber transmission system. 10. An optical fiber transmission system in which a plurality of optical fiber amplifiers are connected in a ring via an optical fiber, wherein each of the plurality of optical fiber amplifiers is connected to the optical fiber via one end. A connected rare earth-doped optical fiber, an excitation light generator for optically exciting the rare earth-doped optical fiber, and a predetermined wavelength of a plurality of signal lights input to the rare earth-doped optical fiber from the one end. Reflecting means for selectively reflecting signal light in a band to the one end, wherein the reflecting means comprises a signal light belonging to a wavelength band specific to each optical fiber amplifier among the plurality of signal lights. An optical fiber transmission system that transmits light through the other end and outputs the light from the other end of the rare earth-doped optical fiber.