JP2002261362A - Optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber amplifier which can realize reduction in the number of parts, reduction of its cost, stabilization of a gain in its large- current input operation. SOLUTION: The amplifier includes a circulator 60 for inputting and outputting signal light, an active optical fiber 10 for amplifying the signal light, a mirror 70 for reflecting the signal light, and an excitation light source for exciting the active optical fiber 10. An unsaturated small signal in the optical fiber 10 has a gain of 25 dB or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical fiber amplifier. For example, the present invention is applied to an optical fiber communication system that performs communication using an optical fiber.

[0002]

2. Description of the Related Art A conventional optical fiber amplifier is shown in FIGS. FIGS. 9A, 9B, and 9C show first to third configurations of the prior art, respectively, and FIGS. 10A and 10B show fourth and fifth configurations of the conventional technology, respectively, and FIGS. FIG. 12 shows sixth and seventh configurations of the prior art. As shown in FIG. 9 (a), the first configuration of the prior art employs two rare earth-doped fibers 11 as gain media to obtain a high gain.
And 12 are two-stage amplification type rare earth doped fiber amplifiers (H. Masuda et al., Electron. Lett., Vol. 3).
4, pp.567-568, 1998).

The signal light is supplied to an isolator 80, a multiplexer 5
1. The rare-earth-doped fiber 11, the multiplexer 52, the rare-earth-doped fiber 12, and the isolator 80 pass in this order. At this time, the excitation light is transmitted from the excitation light sources 31 and 32 to the multiplexers 51 and 52.
To excite the rare earth doped fibers 11 and 12.
The gain (in dB) of the amplifier is the sum of the gains (in dB) of the rare earth doped fibers 11 and 12.

A second prior art configuration is a two-stage amplification type rare earth doped fiber amplifier using a gain equalizer 21 as shown in FIG. 9B (H. Masuda et al., Electro.
n. Lett., Vol. 34, pp. 567-568, 1998). The gain equalizer 21 is used between the two rare-earth doped fibers 11 and 12 in order to obtain a low-noise and high-output characteristic, which is used for broadening the gain spectrum.

As shown in FIG. 9 (c), the third configuration of the prior art is a configuration for further enhancing the gain equalization of the second configuration of the prior art, and includes rare-earth-doped fibers 11, 12, and 1.
3. The gain equalizers 21 and 22 are used, and the gain equalization amount (loss amount) is larger in the third configuration. In general, the third configuration has a larger gain flat bandwidth.

As shown in FIG. 10A, a fourth configuration of the prior art is a reflection type configuration (double-path type configuration) using a circulator 60 and a mirror 70, and a signal light is a rare earth doped fiber which is a gain medium. It is amplified back and forth in 10. That is, the signal light input from the circulator 60 passes through the multiplexer 50 and the rare-earth-doped fiber 10 in this order, is reflected by the mirror 70, passes through the rare-earth-doped fiber 10 and the multiplexer 50 again, and then passes through the circulator. The excitation light is output from the excitation light source 60 and passes through the multiplexer 50 from the excitation light source 30 to excite the rare-earth-doped fiber 10. Therefore, the amplifier gain is twice the single transmission gain (in dB).
It is.

In general, the gain at a constant pump power has the advantage that the reflective configuration is higher than the single transmission configuration. In the fourth configuration of the prior art, the gain when the input power to the amplifier is small, so-called small signal gain (Gu
s) is less than 25 dB (M. Yamada et al., Electr
on, Lett., Vol. 29, pp. 1950-1951, 1993; S. Nishi
et al., ECOC, Vol. l, pp. 99-l02, 1990).

In the fifth configuration of the prior art, as shown in FIG. 10B, the gain spectrum is flattened using the gain equalizer 20 as compared with the fourth configuration of the prior art. Is different from the fourth configuration of the related art.

A sixth configuration of the prior art is a configuration devised to obtain a wide-band gain spectrum by using rare-earth-doped fibers 11, 12 and a Raman fiber 14 for Raman amplification, as shown in FIG. (S. Kawai et al., E
lectron. Lett., Vol. 34, pp. 897-898, 1998). Rare-earth-doped fiber 1 that has been confirmed to have low noise and high output
1 and 12 are arranged before and after the Raman fiber 14 to ensure low noise and high output of the amplifier.

In the seventh configuration of the prior art, as shown in FIG. 12, in order to obtain a high gain, pump light is pumped from pump light sources 31 and 32 through multiplexers 51 and 52 from before and after the rare earth doped fiber 10. This is an optical fiber amplifier that is bidirectionally pumped by passing through. The wavelengths of the excitation light emitted from the front and rear excitation light sources 31 and 32 are λ1 and λ, respectively.
It is 2. These wavelengths may coincide.

[0011]

In the above prior art,
The following problems have occurred. First, the first to first prior arts described above.
The three configurations have the disadvantage that the number of components of the amplifier is large and the amplifier is expensive. Next, in the above-described fourth configuration of the related art, there is a disadvantage that a gain is reduced and a sufficient output power cannot be obtained during a so-called large signal input operation in which the input signal / optical power is large.

In the fifth configuration of the prior art, since the gain equalizer 20 is provided at a stage subsequent to the rare earth doped fiber 20 on the output side of the amplifier, the signal light output power is reduced by the loss value of the gain equalizer 20. Is reduced. The sixth configuration of the prior art has the disadvantage that the number of components of the amplifier is large and the amplifier is expensive.

Further, in the seventh configuration of the prior art, a part of the pumping light from the pumping light source 31 at the former stage passes through the rare-earth-doped fiber 10 and enters the pumping light source 32 at the latter stage.
When the excitation light source 32 does not include the isolator, the laser in the excitation light source 32 becomes unstable, and the power of the excitation light from the excitation light source 32 fluctuates. Similarly, there is a disadvantage that the excitation light from the excitation light source 32 in the subsequent stage enters the excitation light source 31 in the previous stage, the laser in the excitation light source 31 becomes unstable, and the power of the excitation light from the excitation light source 31 fluctuates.

[0014]

According to a first aspect of the present invention, there is provided an optical fiber amplifier comprising: a circulator for inputting / outputting a signal light; an active optical fiber for amplifying the signal light; The active optical fiber has a reflecting mirror and an excitation light source for exciting the active optical fiber, and the active optical fiber has an unsaturated small signal gain of 25 dB or more.

According to a second aspect of the present invention, there is provided an optical fiber amplifier comprising: a circulator for inputting / outputting a signal light; an active optical fiber for amplifying the signal light; a mirror for reflecting the signal light; An excitation light source for exciting an optical fiber, and a gain equalizer provided between the active optical fiber and the mirror are provided.

According to a third aspect of the present invention, there is provided an optical fiber amplifier comprising: a circulator for inputting and outputting signal light; a plurality of active optical fibers for amplifying the signal light; a mirror for reflecting the signal light; A gain equalizer is provided between the plurality of active optical fibers and between the active optical fiber adjacent to the mirror and the mirror, and a pump light source for exciting the active optical fiber.

According to a fourth aspect of the present invention, there is provided an optical fiber amplifier comprising: a circulator for inputting and outputting signal light; a plurality of active optical fibers for amplifying the signal light; a mirror for reflecting the signal light; A gain equalizer is provided between the plurality of active optical fibers and an excitation light source for exciting the active optical fibers.

According to a fifth aspect of the present invention, there is provided an optical fiber amplifier having a plurality of amplifying sections, wherein at least one of the plurality of amplifying sections inputs and outputs signal light. A circulator, an active optical fiber that amplifies the signal light, and a mirror that reflects the signal light,
An excitation light source for exciting the active optical fiber is provided.

An optical fiber amplifier according to a sixth aspect of the present invention for solving the above-mentioned problems is an optical fiber amplifier having a plurality of amplifying sections, wherein at least one of the plurality of amplifying sections is the same as in the first, second, and third aspects. 3. The optical fiber amplifier according to item 3 or 4.

According to a seventh aspect of the present invention, there is provided an optical fiber amplifier comprising two amplifying sections, wherein the amplifying section on the signal light input side inputs and outputs signal light. An active optical fiber that amplifies the signal light, a mirror that reflects the signal light, and an excitation light source that excites the active optical fiber, wherein the amplification section on the signal light output side amplifies the signal light. And an excitation light source for exciting the active optical fiber.

According to another aspect of the present invention, there is provided an optical fiber amplifier having a plurality of amplifying units and a circulator having four or more ports. Each has an active optical fiber for amplifying the signal light, a mirror for reflecting the signal light, and an excitation light source for exciting the active optical fiber.

According to a ninth aspect of the present invention, there is provided an optical fiber amplifier for solving the above problems.
9. The optical fiber amplifier according to 6, 7, or 8, wherein the active optical fiber is a rare earth doped fiber or a Raman fiber.

An optical fiber amplifier according to a tenth aspect of the present invention for solving the above-mentioned problems is the first, second, third, fourth, fifth, and fifth aspects.
9. The optical fiber amplifier according to 6, 7, or 8, wherein the active optical fiber is a Pr-doped fiber or a Tm-doped fiber.

According to another aspect of the present invention, there is provided an optical fiber amplifier comprising: a circulator for inputting / outputting a signal light; a rare earth doped fiber for amplifying the signal light; a Raman fiber for amplifying the signal light; It is characterized by having a mirror for reflecting light and an excitation light source for exciting the rare-earth-doped fiber and the Raman fiber.

According to a twelfth aspect of the present invention, there is provided an optical fiber amplifier for amplifying signal light, and a first optical fiber for exciting the active optical fiber and emitting pump lights having different wavelengths from each other. , A second excitation light source, installed between the second excitation light source and the active optical fiber,
A band-pass filter that removes excitation light from the first excitation light source, and a band-pass filter that is installed between the first excitation light source and the active optical fiber and that removes excitation light from the second excitation light source. It is characterized by having a filter.

[0026]

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] FIG. 1A shows the configuration of an optical fiber amplifier according to a first embodiment of the present invention. As shown in the figure, the signal light input from the circulator 60 is
0, after passing through the rare-earth-doped fiber 10 in order, reflected by the mirror 70, again passing through the rare-earth-doped fiber 10, the multiplexer 50, and output from the circulator 60;
At that time, the excitation light passes through the multiplexer 50 from the excitation light source 30 to excite the rare-earth-doped fiber 10.

This embodiment is different from the fourth configuration of the prior art in FIG. 10A mainly in that the gain when the input power to the amplifier is small, that is, the so-called small signal gain (Gus) is 25 dB or more. Different. This is realized by means such as increasing the length of the rare-earth-doped fiber 10, increasing the rare-earth-doped concentration, or increasing the degree of confinement of light intensity.

The measurement condition of the signal gain is, for example, that the total input signal light power (the total value regarding the multi-channel or single-channel wavelength) is -50 dBm. The rare earth-doped fiber 10 is a Pr (praseodymium) -doped fiber, a Tm (thulium) -doped fiber, or an Er (erbium) -doped fiber. Also, instead of the rare earth-doped fiber 10,
A Raman fiber for Raman amplification may be used.

In this specification, an optical fiber having an amplifying action, such as a rare earth-doped fiber and a Raman fiber, is called an active optical fiber. In many cases, the operating condition of the optical fiber amplifier is a large signal input, and an example of the total input signal light power is -20 dBm to +10 dBm. For example, in the Pr-doped fiber amplifier, the output power when the total input signal light power is 0 dBm is +10 dBm in the fourth configuration of the related art at the same pump light power.
However, in this embodiment, it is +12 dBm, and the output power can be improved by this embodiment. An example of the length of the Pr-doped fiber at this time is 28 m in the fourth configuration of the related art, and 50 m in the present embodiment.

When this embodiment is compared with the first configuration of the prior art shown in FIG. 9A, both have the same gain and signal light output power. The number is small and inexpensive. In this embodiment, the configuration in which the rare-earth-doped fiber 10 is pumped from the left side of the drawing (referred to as forward pumping) is shown. The same holds for the configuration (assuming bidirectional excitation). However, the manner in which the excitation light is reflected by the mirror 70 differs depending on the configuration, but this is not essential in the above operation. This also applies to the following second embodiment and thereafter.

[Second Embodiment] FIG. 1B shows the configuration of an optical fiber amplifier according to a second embodiment of the present invention. This embodiment is different from the fifth configuration of the prior art shown in FIG.
In the fifth configuration of the related art, the gain equalizer 20 is provided on the output side of the amplifier. In the present embodiment, the gain equalizer 20 is provided between the rare-earth-doped fiber 10 and the mirror 70. Is mainly different.

Therefore, in this embodiment, the loss of signal light power is small, and a large signal light output power can be obtained. However, the influence of the loss of the pump light power due to the gain equalizer 20 was assumed to be small. It is easy to obtain the operating conditions of such a gain equalizer 20 or an amplifier. Also,
In the fifth embodiment of the prior art and the present embodiment, when the amplifier gain is the same, the spectral peak value of the gain from the multiplexer 50 to the mirror 70 is smaller in the present embodiment, so that the high gain and the residual The present embodiment has an advantage that instability related to laser oscillation due to reflection or the like is smaller in the present embodiment.

Comparing the present embodiment with the second configuration of the prior art shown in FIG. 9B, both have the same gain and signal light output power. The number is small and inexpensive. However, the peak loss value (in dB) of the gain equalizer 20 of the present embodiment may be half the peak loss value of the gain equalizer 20 of the second configuration of the related art.

Third Embodiment FIG. 2A shows a configuration of an optical fiber amplifier according to a third embodiment of the present invention. As shown in the figure, the signal light input from the circulator 60 passes through the multiplexer 51, the rare-earth-doped fiber 11, the gain equalizer 21, the multiplexer 52, and the rare-earth-doped fiber 12 in that order, and then passes through the mirror 70. Again, passes through the rare-earth-doped fiber 12, the multiplexer 52, the gain equalizer 21, the rare-earth-doped fiber 11, and the multiplexer 51 in this order, and passes through the circulator 6 again.
0, at which time the excitation light is pumped by the excitation light sources 31, 32
Pass through the multiplexers 51 and 52 from the
Excite 1,12.

Therefore, the amplification by the gain medium and the arrangement of the losses by the gain equalizers 21 and 22 are equivalent to the third configuration of the prior art shown in FIG. The following points are mainly different from the third configuration.

For example, if the gain saturation due to the signal light is neglected for simplicity, the length L1 ^* of the rare earth-doped fiber 11 of the present embodiment is changed to the length L1 of the rare earth-doped fiber 11 of the third prior art. Length L3 of doped fiber 13
And the length L2 ^* of the rare-earth-doped fiber 12 of the present embodiment is made equal to half the length L2 of the rare-earth-doped fiber 12 of the third configuration of the prior art. 9 (c) has the same gain and signal light output power in the third configuration of the prior art, but obviously this embodiment has fewer components and is inexpensive.

[Fourth Embodiment] FIG. 2B shows the configuration of an optical fiber amplifier according to a fourth embodiment of the present invention. This embodiment is different from the second embodiment in that, in the second embodiment, one rare-earth-doped fiber 10 and one gain equalizer 20 are used, and an equivalent two-stage amplification configuration with a gain equalizer is used. On the other hand, in this embodiment, two rare-earth-doped fibers 1 are used.
The main difference is that a four-stage amplification configuration with a gain equalizer is equivalently realized by using gain equalizers 21 and 22 and two gain equalizers 21 and 22. Therefore, compared to the conventional four-stage amplification configuration,
Obviously, this embodiment has a smaller number of components and is less expensive.

[Fifth Embodiment] FIG. 2C shows the configuration of an optical fiber amplifier according to a fifth embodiment of the present invention. This embodiment is different from the third embodiment in that, in the third embodiment, two rare earth-doped fibers 10 and one gain equalizer 20 are used, and an equivalent three-stage amplification configuration with a gain equalizer is used. In contrast to this, in this embodiment, three rare-earth-doped fibers 1 are used.
The main difference is that a five-stage amplification configuration with a gain equalizer is realized equivalently using 0 and two gain equalizers 20.
Obviously, this embodiment has fewer components and is less expensive than the five-stage amplification configuration according to the prior art.

Sixth Embodiment FIG. 3 shows the configuration of an optical fiber amplifier according to a sixth embodiment of the present invention. The present embodiment has a configuration in which two optical fiber amplifiers of the first embodiment are connected in tandem to form a first and a second amplifier, respectively. When the required gain of the optical fiber amplifier is large (for example, 35d
B) In one optical fiber amplifier of the first embodiment, unstable operation such as laser oscillation and excessive noise occur due to high gain.

Therefore, when the two optical fiber amplifiers of the first embodiment are used, the gain per optical fiber amplifier of the first embodiment is small, so that unstable operation and excessive noise can be eliminated. In a Pr-doped fiber amplifier, a Raman amplifier, or the like, the higher the input signal light power to the amplifier, the higher the power conversion efficiency from pump light to signal light. It is valid.

Seventh Embodiment FIG. 4 shows the configuration of an optical fiber amplifier according to a seventh embodiment of the present invention. The present embodiment differs from the sixth embodiment mainly in that the second amplifying unit is a single-transmission type optical fiber amplifier. This is because in an Er-doped fiber amplifier or the like, when the input signal light power is large to some extent, the difference between the gain and the output power between the reflection type and the single transmission type is sufficiently small. Further, it is not necessary to provide an isolator in a stage preceding the second amplifying unit. Therefore, in the case of an Er-doped fiber amplifier or the like, a single transmission type amplifier may be less expensive, and the configuration of this embodiment is effective.

[Eighth Embodiment] FIG. 5 shows the configuration of an optical fiber amplifier according to an eighth embodiment of the present invention. This embodiment is different from the sixth embodiment mainly in that one 4-port circulator 64 is used instead of the two 3-port circulators 61 and 62 used in the sixth embodiment. . This embodiment is effective because one 4-port circulator 64 may be less expensive than two 3-port circulators 61 and 62. Similarly, using a 5-port circulator, the amplification unit
A three-stage amplification configuration and a multi-stage amplification configuration of four or more stages are also possible.

Ninth Embodiment FIG. 6 shows the configuration of an optical fiber amplifier according to a ninth embodiment of the present invention. The present embodiment is different from the sixth configuration of the prior art in FIG. 11 in that the signal light passing through the rare earth-doped fiber 10 and the Raman fiber 14 in this order is:
The main difference is that the light is reflected by the mirror 70 and passes again through the Raman fiber 14 and the rare-earth-doped fiber 10 in this order. Therefore, gain characteristics equivalent to the sixth configuration of the related art can be obtained.
Therefore, in comparison with the prior art, the present embodiment clearly has a smaller number of components and is inexpensive.

[Tenth Embodiment] FIG. 7 shows the configuration of an optical fiber amplifier according to a tenth embodiment of the present invention. This embodiment is different from the seventh embodiment of the prior art shown in FIG.
1 and 32 and the multiplexer 50, λ1 and λ2, respectively.
The main difference is that band pass filters 41 and 42 are provided to transmit the excitation light of λ2 and λ1 and remove the excitation lights of λ2 and λ1, respectively. An example of the excitation light source 30 is a laser diode with a fiber grating. Examples of the band-pass filters 41 and 42 include fiber couplers.

As an example of the rare-earth-doped fiber 11, E
r-doped fiber (or Pr-doped fiber), examples of λ2 and λ1 are 975 nm and 985 nm (1015n
m and 1025 nm). Since there is no pumping light from the other pumping light source 30, the laser in the pumping light source is stabilized. At present, the isolator in the excitation light wavelength band has problems such as high cost and low reliability, and it is not easy to incorporate the isolator in the excitation light source 30.

[Eleventh Embodiment] FIG. 8 shows a configuration of an optical fiber amplifier according to an eleventh embodiment of the present invention. This embodiment is a specific example of the fourth embodiment shown in FIG. That is, the long-period fiber grating 90 is used as the gain equalizer 20.
And the loss of the long-period fiber grating 90 at the pumping light wavelength can be set small. Therefore, three rare-earth doped fibers 10 can be sequentially excited by one excitation light source 30. Also, the number of components is small,
An inexpensive amplifier can be realized.

[0047]

As described above, according to the present invention,
The number of component parts of the amplifier, which was a problem with the prior art, is large,
The amplifier is expensive, the gain is reduced during large signal input operation, the output power cannot be obtained sufficiently, the signal light output power is reduced by the loss value of the gain equalizer, and the pump light power is low. It is possible to solve the drawback that the amplifier fluctuates and an unstable operation such as a gain fluctuation occurs.

[Brief description of the drawings]

FIG. 1 (a) is a configuration diagram of an optical fiber amplifier according to a first embodiment of the present invention, and FIG. 1 (b) is a configuration diagram of an optical fiber amplifier according to a second embodiment of the present invention. is there.

FIG. 2A is a configuration diagram of an optical fiber amplifier according to a third embodiment of the present invention, FIG. 2B is a configuration diagram of an optical fiber amplifier according to a fourth embodiment of the present invention, FIG. 2 (c)
FIG. 13 is a configuration diagram of an optical fiber amplifier according to a fifth embodiment of the present invention.

FIG. 3 is a configuration diagram of an optical fiber amplifier according to a sixth embodiment of the present invention.

FIG. 4 is a configuration diagram of an optical fiber amplifier according to a seventh embodiment of the present invention.

FIG. 5 is a configuration diagram of an optical fiber amplifier according to an eighth embodiment of the present invention.

FIG. 6 is a configuration diagram of an optical fiber amplifier according to a ninth embodiment of the present invention.

FIG. 7 is a configuration diagram of an optical fiber amplifier according to a tenth embodiment of the present invention.

FIG. 8 is a configuration diagram of an optical fiber amplifier according to a tenth embodiment of the present invention.

9A is a configuration diagram of an optical fiber amplifier according to a first configuration of the related art, and FIG. 9B is a configuration diagram of the optical fiber amplifier according to the second configuration of the related art.
FIG. 2 is a configuration diagram of an optical fiber amplifier according to the configuration.

FIG. 10A is a configuration diagram of an optical fiber amplifier according to a third configuration of the related art, and FIG. 10B is a configuration diagram of an optical fiber amplifier according to a fourth configuration of the related art;
(C) is a configuration diagram of an optical fiber amplifier according to a fifth configuration of the related art.

FIG. 11 is a configuration diagram of an optical fiber amplifier according to a sixth configuration of the related art.

FIG. 12 is a configuration diagram of an optical fiber amplifier according to a seventh configuration of the related art.

[Explanation of symbols]

 10, 11, 12 Rare earth-doped fiber 14 Raman fiber 20, 21, 22 Gain equalizer 30, 31, 32 Excitation light source 41, 42 Band-pass filter 50, 51, 52 Combiner 60, 61, 62 Circulator 70, 71 , 72 mirror 80 isolator 90 long period fiber grating

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shinichi Aozasa 2-3-1 Otemachi, Chiyoda-ku, Tokyo Within Nippon Telegraph and Telephone Corporation (72) Inventor Tadashi Sakamoto 2-3-1 Otemachi, Chiyoda-ku, Tokyo No. 1 Nippon Telegraph and Telephone Corporation (72) Inventor Makoto Shimizu 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Yoshiki Nishida 1-chome Dogenzaka, Shibuya-ku, Tokyo No. 12 No. 1 F-term in NTT Electronics Corporation (reference) 5F072 AB09 AK06 JJ02 JJ05 JJ08 KK06 KK07 KK30 QQ07 YY17

Claims (12)

[Claims] A circulator for inputting and outputting signal light;
An active optical fiber for amplifying the signal light, a mirror for reflecting the signal light, and an excitation light source for exciting the active optical fiber, wherein the unsaturated small signal gain in the active optical fiber is 2
An optical fiber amplifier characterized by being at least 5 dB. A circulator for inputting and outputting signal light;
An active optical fiber that amplifies the signal light, a mirror that reflects the signal light, and an excitation light source that excites the active optical fiber,
An optical fiber amplifier comprising a gain equalizer between the active optical fiber and the mirror. A circulator for inputting and outputting signal light;
A plurality of active optical fibers for amplifying the signal light, a mirror for reflecting the signal light, an excitation light source for exciting the active optical fiber, and the active optical fiber between the plurality of active optical fibers and adjacent to the mirror. An optical fiber amplifier comprising a gain equalizer between the mirrors. A circulator for inputting and outputting signal light;
A plurality of active optical fibers for amplifying the signal light, a mirror for reflecting the signal light, an excitation light source for exciting the active optical fiber, and a gain equalizer between the plurality of active optical fibers, Optical fiber amplifier. 5. An optical fiber amplifier having a plurality of amplifying sections, wherein at least one of the plurality of amplifying sections includes a circulator for inputting / outputting a signal light, an active optical fiber for amplifying the signal light, and a An optical fiber amplifier comprising: a reflecting mirror; and an excitation light source for exciting the active optical fiber. 6. An optical fiber amplifier having a plurality of amplifying sections, wherein at least one of the plurality of amplifying sections is the optical fiber amplifier according to claim 1, 2, 3, or 4. Fiber amplifier. 7. An optical fiber amplifier having two amplifying sections, wherein the amplifying section on the signal light input side includes a circulator for inputting / outputting the signal light, an active optical fiber for amplifying the signal light, and reflecting the signal light. And an excitation light source that excites the active optical fiber, wherein the amplification section on the signal light output side has an active optical fiber that amplifies the signal light and an excitation light source that excites the active optical fiber. Optical fiber amplifier. 8. An optical fiber amplifier having a plurality of amplifying sections and a circulator having four or more ports, wherein each of the plurality of amplifying sections includes an active optical fiber for amplifying signal light, and an active optical fiber for amplifying signal light. An optical fiber amplifier comprising: a reflecting mirror; and an excitation light source for exciting the active optical fiber. 9. The optical fiber amplifier according to claim 1, wherein said active optical fiber is a rare earth doped fiber or a Raman fiber. Fiber amplifier. 10. The optical fiber amplifier according to claim 1, wherein said active optical fiber is a Pr-doped fiber or a Tm-doped fiber. Optical fiber amplifier. 11. A circulator for inputting / outputting a signal light, a rare-earth-doped fiber for amplifying the signal light, a Raman fiber for amplifying the signal light, a mirror for reflecting the signal light, and exciting the rare-earth-doped fiber and the Raman fiber. An optical fiber amplifier, comprising: a pump light source. 12. An active optical fiber for amplifying a signal light,
Exciting the active optical fiber, first and second excitation light sources that emit excitation light having different wavelengths, and the first excitation light source installed between the second excitation light source and the active optical fiber A band-pass filter that removes pump light from the first pump light source and a band-pass filter that removes pump light from the second pump light source, which is provided between the first pump light source and the active optical fiber. Optical fiber amplifier.