JP2713396B2 - Optical fiber amplifier and optical fiber transmission system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier capable of amplifying a plurality of signal light so that a plurality of signal light gains become equal in an optical transmission system using an optical fiber as a transmission line.

[0002]

2. Description of the Related Art Conventional multi-wavelength optical fiber amplifiers and bidirectional optical fiber amplifiers include at least a rare earth-doped optical fiber, a semiconductor laser light source for exciting rare earth ions, and a pumping light coupled to a rare earth-doped optical fiber. Optical coupler. When the signal light is input to the optical fiber amplifier, the stimulated emission of the rare earth ions in a population inversion state due to the excitation light causes the signal light to be amplified. When a plurality of signal lights are simultaneously amplified by the same rare-earth-doped optical fiber, the gain shows a wavelength characteristic because the cross-sectional area of absorption and emission of rare-earth ions is reflected.

As a method for suppressing the gain difference due to such a wavelength, there are the following reports. The wavelength dependence of the absorption and emission cross-sections of rare earth-doped optical fibers, which is the reason for the gain difference, is found to decrease with the co-doping of aluminum.Journal of Lightwave Technology Vol.9, No.9, ( (1991) pages 1105 to 1112
Page (Journal of Lightwave Technology, Vol.9, No.9,
pp. 1105-1112, 1991.).

Further, a method using an acousto-optical element is described in the literature, OFC / IOC '93, ThD2, 1993, (OFC / IOOC'93, paper ThD2, 199).
3), a method using a fiber grating is described in OFC '91, PD20-1, 1991, (OFC '
91, paper PD20-1), a method of connecting optical fiber amplifiers in multiple stages so that the gain-wavelength characteristics cancel each other is described in OhE'93, SuE3-1, 19
1993, (OAA'93, paper SuE3-1, pp.70-73, 1993).

[0005] In any case, the gain characteristics are made uniform only when the input light amount is uniform, and a method of gain equalization when the input light amount is different has not been studied.

[0006]

In an optical fiber amplifier, large input light is input when an analog signal is amplified, and small signal light is input when a digital signal is amplified. Therefore, when these signals are amplified simultaneously, as in a bidirectional optical amplifier or the like, the input light level of each signal is greatly different, so that the gain is basically non-uniform. On the other hand, even in the case of using a multi-stage cascade connection in a distribution system, since the distribution loss received by each signal is equal, the non-uniform gain causes a shortage of light amount at the light receiving unit, and the transmission characteristics deteriorate. Occurs.

The present invention provides an optical fiber amplifier capable of obtaining an equal gain for a plurality of signal light wavelengths having different input light amounts,
And a bidirectional optical fiber amplifier.

[0008]

An optical fiber amplifier according to the present invention comprises a plurality of rare earth-doped optical fibers connected in series, and a pump light for exciting the plurality of rare earth-doped optical fibers connected in series. And an optical fiber amplifier that optically amplifies a plurality of signal lights having different input wavelengths, wherein a first signal light of the plurality of signal lights is Propagating through some of the rare earth-doped optical fibers connected in series, the second signal light of the plurality of signal lights is connected to the plurality of serially-connected light fibers. The rare-earth-doped optical fiber propagates through all of the rare-earth-doped optical fibers, thereby achieving the above object.

[0009] A branch optical fiber may be provided in the middle of the plurality of rare earth-doped optical fibers connected in series, and the first signal light may be output through the branch optical fiber.

[0010] A branch optical fiber may be provided in the middle of the plurality of rare earth-doped optical fibers connected in series, and the first signal light may be input via the branch optical fiber.

[0011] In a preferred embodiment, each of the plurality of rare-earth-doped optical fibers is pumped into a state in which a gain higher than a gain for the second signal light is given to the first signal light.

[0012] In a preferred embodiment, the at least one excitation light source includes a semiconductor laser.

In a preferred embodiment, the multiplexer / demultiplexer for multiplexing the pump light generated by the pump light source and the plurality of signal lights includes a plurality of rare earth-doped optical fibers connected in series. Are connected to any of the input units.

In a preferred embodiment, the multiplexer / demultiplexer that multiplexes the pump light generated by the pump light source and the plurality of signal lights includes a plurality of rare earth-doped optical fibers connected in series. Are connected to any of the output units.

[0015] In a preferred embodiment, both the light belonging to the first wavelength band and the light belonging to the second wavelength band are selectively transmitted, and the light not belonging to the first and second wavelength bands is branched. Means for splitting into optical fibers is provided in the middle of the plurality of rare earth-doped optical fibers connected in series.

In a preferred embodiment, the wavelength of the first signal light is not included in the first wavelength band and the second wavelength band, and the wavelength of the second signal light is equal to the first wavelength. Band or the second wavelength band.

In a preferred embodiment, the wavelength of the excitation light is included in the first wavelength band or the second wavelength band.

In a preferred embodiment, the wavelength of the first signal light is longer than the wavelength of the second signal light, the first signal light is analog-modulated, and the wavelength of the second signal light is Is digitally modulated.

The bidirectional optical amplifier according to the present invention comprises a first input / output portion for outputting at least a first signal light and for inputting a second signal light, and for inputting at least the first signal light and the second signal light. A bi-directional optical amplifier comprising: a second input / output section for outputting the signal light of the first type; and an optical fiber amplifier connected to the first input / output section and the second input / output section. The amplification unit includes a plurality of rare earth-doped optical fibers connected in series, and at least one pump light source that generates pump light for exciting the plurality of rare earth-doped optical fibers connected in series. An optical fiber amplifier for optically amplifying a plurality of signal lights having different wavelengths,
In addition, the first signal light of the plurality of signal lights propagates through some of the rare earth-doped optical fibers connected in series and rare earth-doped optical fibers. The second signal light is an optical fiber amplifier that propagates through all of the rare earth-doped optical fibers connected in series, and the first signal received from the optical fiber amplifier is The apparatus further includes a demultiplexing means for selectively transmitting light to the first input / output portion, thereby achieving the above object.

In a preferred embodiment, the second signal light input from the first input / output part and the first signal light input to the second input / output part are multiplexed, and the second signal light is multiplexed. A multiplexing means is provided for inputting both the first signal light and the second signal light to the input portions of the plurality of rare earth-doped optical fibers connected in series.

In a preferred embodiment, there is provided a multiplexing means for inputting the first signal light inputted from the second input / output portion to a midway of the plurality of rare earth-doped optical fibers connected in series. ing.

The first input / output section may include a first optical circulator, and the second input / output section may include a second optical circulator.

The first input / output portion may include first multiplexing / demultiplexing means, and the second input / output portion may include second multiplexing / demultiplexing means.

The optical fiber transmission system according to the present invention comprises:
A transmitting station including a first signal light source that emits the first signal light, and a first light receiver that detects the second signal light; a second light receiver that detects the first signal light; A receiving station including a second signal light source for emitting the second signal light, an optical fiber transmission line connecting the transmitting station and the receiving station, and an optical fiber amplifier provided in the middle of the optical fiber transmission line. An optical fiber transmission system comprising:
A plurality of rare earth-doped optical fibers connected in series, and at least one pump light source that generates pump light for exciting the plurality of rare earth-doped optical fibers connected in series, different input wavelengths An optical fiber amplifier for optically amplifying a plurality of signal lights having the following formula: wherein the first signal light of the plurality of signal lights is one of the rare earth-doped optical fibers connected in series. The second signal light of the plurality of signal lights propagates through all the rare earth-doped optical fibers of the plurality of rare earth-doped optical fibers connected in series. This achieves the above object.

In a preferred embodiment, at least one analog-modulated signal light is used as the first signal light, and a plurality of digitally-modulated signal lights are used as the second signal light.

[0026] The bidirectional optical fiber transmission system of the present invention comprises a first signal light source for emitting a first signal light, and a second signal light source.
A transmitting station including a first optical receiver for detecting the first signal light, a second optical receiver for detecting the first signal light, and a receiving station including a second signal light source for emitting the second signal light A station, an optical fiber transmission line connecting the transmitting station and the bidirectional optical amplifier,
A bidirectional optical fiber transmission system comprising: a bidirectional optical amplifier provided in the middle of the optical fiber transmission line, wherein the bidirectional optical amplifier comprises an output of the first signal light and a second signal. A second input / output section for inputting light; a second input / output section for inputting the first signal light and outputting the second signal light; a first input / output section and the second input / output And an optical fiber amplifier connected to the portion, the optical fiber amplifier pumping the plurality of rare earth-doped optical fibers connected in series and the plurality of rare earth-doped optical fibers connected in series. And at least one pumping light source for generating pumping light for performing an optical fiber amplification unit that optically amplifies a plurality of signal lights having different input wavelengths, and furthermore, The first signal light is a plurality of the serially connected signal lights. Propagating through some of the rare earth-doped optical fibers of the rare earth-doped optical fibers, the second signal light of the plurality of signal lights is all the rare earth-doped optical fibers of the plurality of rare earth-doped optical fibers connected in series. An optical fiber amplifying unit that propagates the optical fiber, further comprising a demultiplexing unit that selectively propagates the first signal light received from the optical fiber amplifying unit to the first input / output unit; Objective is achieved.

In a preferred embodiment, at least one analog-modulated signal light is used as the first signal light, and a plurality of digitally-modulated signal lights are used as the second signal light.

As described above, in the present invention, gain compensation is performed by additionally amplifying signal light having a small gain, and an equal gain is obtained for a plurality of signal lights having different wavelengths and input light amounts. Can be given.

When pumping light is input from one of the rare-earth-doped optical fibers, the transmitted light is used effectively to pump the rare-earth-doped optical fiber for additional amplification. When the pumping light is input from both ends of the rare earth-doped optical fiber, gain compensation can be performed over a wider input light amount range.

By pumping the rare earth-doped optical fiber with 0.98 μm light at the signal light incident portion, noise characteristics can be improved even when the total input light amount of a plurality of signal lights is large.

Further, by using an optical fiber amplifier that performs such gain compensation in the amplifying section, a bidirectional optical amplifier that amplifies each signal light without generating a gain difference depending on the transmission direction is provided.

[0032]

Embodiments of the present invention will be described below.

First, the gain wavelength characteristics of the erbium-doped optical fiber are shown in FIG. As shown in the figure, the gain characteristic of the optical fiber has wavelength dependence. In the case of simultaneous amplification of two signals of an analog signal and a digital signal, each signal must be 1.53 in consideration of the wavelength separation characteristics of a commercially available optical filter.
It is appropriate to set the gain peak wavelengths to μm and 1.56 μm.

An analog signal requires a higher signal-to-noise ratio than a digital signal, and therefore operates in a high input state. In such a case, 1.56 μm
By allocating a high input analog signal to the band and a low input digital signal to the 1.53 μm band, it becomes possible to minimize the characteristic fluctuation of the analog signal with respect to the fluctuation of the digital signal. However, in such a case, the long wavelength side has a high gain, but the short wavelength side is deprived of the gain and has a low gain.

In the following description of the embodiment, 1.56 μm light modulated by an analog signal is referred to as “first signal light”, and 1.53 μm light modulated by a digital signal is referred to as “second signal light”. Light. " Further, the input light amount of the first signal light is set to 0.
dBm, and the input light amount of the second signal light is -30 dBm.

(Embodiment 1) Hereinafter, referring to FIG.
An embodiment of the optical fiber amplifier according to the present invention will be described in detail.

The optical fiber amplifier of FIG. 1 includes a single input terminal to which a plurality of signal lights having different wavelengths are input, an amplifying unit for optically amplifying the plurality of signal lights input to the input terminals, A plurality of output terminals for respectively outputting a plurality of amplified signal lights. In this example, a signal light having a wavelength of 1.56 μm modulated by an analog signal (first signal light 5
1) and 1.53 μm wavelength modulated by digital signal
Signal light (second signal light 53) is input to the input terminal,
Amplified.

The amplifying section includes a first erbium-doped optical fiber 32 for amplifying both the first and second signal lights 51 and 53, and a second erbium-doped optical fiber 33 for amplifying only the second signal light 53. have. The two erbium-doped optical fibers 32 and 33 are connected via a multiplexer / demultiplexer 24. This multiplexer / demultiplexer 24
Transmits the second signal light 53 to the second erbium-doped optical fiber 33 among the plurality of signal lights that have propagated through the first erbium-doped optical fiber 32, while selectively separating the first signal light 51. Input to the optical fiber 35.

The first signal light 51 is supplied to the optical isolator 26
Is output to another output terminal via the inserted optical fiber 35. The output first signal light is as shown in FIG.
Indicated by “52”. The second signal light 53 amplified by the second erbium-doped optical fiber 33 is output to the output terminal via the optical fiber 36 into which the optical isolator 27 is inserted. The output second signal light is, as shown in FIG.
Indicated by “54”.

The amplifying section is a pump light (wavelength: 1.4) for pumping the first and second erbium-doped optical fibers 32 and 33.
(8 μm). The pumping light in the 1.48 μm band is transmitted to the multiplexer / demultiplexer 22.
Accordingly, the signal light is multiplexed with the signal light in the 1.55 μm band, and is input to the first erbium-doped optical fiber 32. The first and second signal lights 51 and 53 are input to the multiplexer / demultiplexer 22 via the optical fiber 34 into which the optical isolator 25 is inserted. The excitation light in the 1.48 μm band is transmitted to the multiplexer / demultiplexer 2.
4 and is input to the second erbium-doped optical fiber 33 to excite the second erbium-doped optical fiber 33 as well. The operation when signal light enters the optical fiber amplifier will be described in more detail.

The 1.56 μm signal light as the first signal light 51 and the 1.53 μm signal light as the second signal light 53 are:
The light enters the input end of the optical fiber 34. The 1.48 μm pump light is multiplexed with the signal lights 51 and 53 by the multiplexing / demultiplexing device 22 and input to the erbium-doped optical fiber 32.
Here, the pump light 56 amplifies the signal lights 51 and 53, and the attenuated transmission component is input to the erbium-doped optical fiber 33 via the multiplexer / demultiplexer 24. 1.56 μm
The signal light 51 is demultiplexed by the multiplexer / demultiplexer 24 and transmitted through the optical isolator 26 to be output. On the other hand, 1.53μ
The m signal light 53 is input to the erbium-doped optical fiber 33 after passing through the multiplexer / demultiplexer 24. In the erbium-doped optical fiber 33, the 1.53 μm signal light 53
Are amplified by the transmitted excitation light and output through the optical isolator 27.

FIG. 3 shows the amount of transmission excitation light (vertical axis) with respect to the input light quantity (horizontal axis) of the first and second signal lights. From FIG. 3, when only the first signal light of 1.56 μm is incident,
The excitation light transmitted through the erbium-doped optical fiber 32 is 1
It turns out that there is also 0-30 mW. Further, when the input light amount of the first signal light is several dBm (for example, +5 dBm in FIG. 3),
Even when the second signal light is changed from -20 to -3 dBm, about 10 mW of pumping light is transmitted through the erbium-doped optical fiber 32, and this pumping light is a loss in the conventional configuration. You can see that. In the present embodiment, the transmission excitation light, which has conventionally been a loss, is incident on the second erbium-doped optical fiber 33, and the first
Only the second signal light excluding the above signal light is incident on the second erbium-doped optical fiber 33. By doing this,
The second signal light will be additionally amplified. That is, in the present embodiment, the transmission pumping light that is a loss is positively used, and as a result, it is possible to give the second signal light a gain equivalent to the gain for the first signal light. .

The fact that the gains of the first and second signal lights can be made equal by the configuration of this embodiment will be described with reference to FIG. The horizontal axis of FIG.
The input light quantity of the signal of 53 μm is shown, and the vertical axis indicates the gain of the signal light of 1.53 μm. In this figure, □ indicates the gain of the signal light of 1.53 μm when the doped optical fiber 33 is not provided, and ○ indicates the gain when the fiber 33 is provided.
In FIG. 4, the gain of the signal light of 1.56 μm is shown as ● in FIG. 4 so that it can be compared with the signal light of 1.56 μm. Here, the input light amount of the 1.56 μm signal light is +5 dB.
m is constant.

From this figure, it can be seen that a gain ○ of 1.53 μm,
The gain ● of 1.56 μm means that the input light quantity of 1.53 μm is −
It can be seen that they are almost the same in the range of 40 to -20 dBm. This is 1.53 as described in the embodiment.
This is because the μm signal light was amplified by the erbium-doped optical fiber 32 and then further amplified by the optical fiber 33. From this result, it was clearly confirmed that the gain of the signal light of 1.53 μm and 1.56 μm having different input wavelengths and input light amounts can be made substantially equal to each other in the effect of the present embodiment.

(Embodiment 2) Hereinafter, referring to FIG.
Another embodiment of the optical fiber amplifier according to the present invention will be described in detail. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The difference between the optical fiber amplifier of FIG. 5 and the optical fiber amplifier of FIG. 1 is that the optical fiber amplifier of FIG. 5 further includes a 1.48 μm pumping semiconductor laser 13 as a second pumping light source. The multiplexing / demultiplexing device 23 for inputting the pumping light emitted from the semiconductor laser 13 to the erbium-doped optical fiber 33 is provided between the erbium-doped optical fiber 33 and the optical isolator 27. The multiplexer / demultiplexer 23 has a 1.48 μm light and a 1.48 μm light.
The multiplexing / demultiplexing is performed on the 55 μm light.

The case where two light beams are made incident on the optical fiber amplifier will be described.

The 1.56 μm signal light as the first signal light 51 and the 1.53 μm signal light as the second signal light 53 are:
The light enters from the optical fiber 34. The pumping light 56 is multiplexed with the signal lights 51 and 53 by the multiplexing / demultiplexing device 22 and input to the erbium-doped fiber 32. Here, the excitation light 56
After amplifying the signal lights 51 and 53, the attenuated transmission component is input to the erbium-doped optical fiber 33 via the multiplexer / demultiplexer 24. The 1.56 μm signal light 51 is multiplexed.
The light is split by the splitter 24 and transmitted through the optical isolator 26 to be output. The 1.53 μm signal light 53 is transmitted through the multiplexer / demultiplexer 24 and then input to the erbium-doped optical fiber 33. On the other hand, the pumping light 57 is multiplexed with the signal light 53 by the multiplexer / demultiplexer 23 and is input to the erbium-doped fiber 33. In the erbium-doped optical fiber 33, the 1.53 μm signal light 53 is amplified by the transmitted pump light 56 and pump light 57, and output through the optical isolator 27.

FIG. 3 shows that the input light quantity of the first signal light is several d.
It can be seen that, when it is as large as Bm or more, the amount of transmitted excitation light becomes very small, and it becomes insufficient to additionally pump the second signal light. Even in such a case, in order to compensate for the gain of each signal light wavelength, a configuration in which pump light is obtained from behind the erbium-doped optical fiber 33 as in this embodiment is effective.

(Embodiment 3) Hereinafter, referring to FIG.
Another embodiment of the optical fiber amplifier according to the present invention will be described in detail. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The difference between the optical fiber amplifier of FIG. 6 and the optical fiber amplifier of FIG. 5 is that the optical fiber amplifier of FIG. 6 further includes an erbium-doped optical fiber 31, a pumping semiconductor laser 11, And a wave device 21. Here, the semiconductor laser 11 for excitation has a wavelength of 0.98.
Emit excitation light of μm. The multiplexer / demultiplexer 21 is 0.98
The multiplexing / demultiplexing of the μm light and the 1.55 μm light is performed. In FIG. 6, reference numeral 55 denotes 0.98 μm excitation light.

A case where two signal lights are incident on the optical fiber amplifier will be described.

The 1.56 μm signal light that is the first signal light 51 and the 1.53 μm signal light that is the second signal light 53 are:
The light enters from the optical fiber 34. 0.98 μm excitation light 5
5 is multiplexed with the signal lights 51 and 53 by the multiplexing / demultiplexing device 21 and input to the erbium-doped fiber 31. Here, the signal lights 51 and 53 amplified by the pump light 55
Is input to the erbium-doped optical fiber 32 via the multiplexer / demultiplexer 22. The pumping light 56 is multiplexed with the signal lights 51 and 53 by the multiplexing / demultiplexing device 22 and input to the erbium-doped fiber 32. Here, the pump light 56 is the signal light 5
After amplifying 1 and 53, the attenuated transmission components combine and
The light is input to the erbium-doped optical fiber 33 via the splitter 24. The 1.56 μm signal light 51 is applied to the multiplexer / demultiplexer 24.
And output through the optical isolator 26. On the other hand, the 1.53 μm signal light 53 is
, And is input to the erbium-doped optical fiber 33. In this erbium-doped optical fiber 33,
The 1.53 μm signal light 53 is amplified by the transmitted excitation light,
The light is output through the optical isolator 27 as the signal light 54.

When signal light of a plurality of wavelengths is input, noise characteristics deteriorate due to an increase in the total input light quantity. Also in this case, according to this configuration, the noise can be reduced by exciting the signal light input unit by 0.98 μm. In addition, when amplification is performed simultaneously with 0.98 μm pump light and 1.48 μm pump light, noise characteristics are deteriorated.
In this configuration, such a problem does not occur because 1.48 μm light does not enter the 0.98 μm excitation region.

(Embodiment 4) Hereinafter, referring to FIG.
Another embodiment of the optical fiber amplifier according to the present invention will be described in detail. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The optical fiber amplifier shown in FIG. 7 has two input terminals to which a plurality of signal lights having different wavelengths are inputted, an amplifying unit for optically amplifying the plurality of signal lights inputted to the input terminals, A single output terminal for outputting a plurality of amplified signal lights, respectively. In this example, a signal light having a wavelength of 1.56 μm modulated by an analog signal (first signal light 5
1) and 1.53 μm wavelength modulated by digital signal
(The second signal light 53) are input to separate input terminals and amplified.

The amplifying unit includes a first erbium-doped optical fiber 132 for amplifying only the second signal light 53 and a second erbium-doped optical fiber 133 for amplifying both the first and second signal lights 51 and 53. Have. The two erbium-doped optical fibers 1132 and 133 are connected via a multiplexing / demultiplexing 148. The multiplexer / demultiplexer 148 multiplexes the first signal light 51 and the second signal light 53 propagated through the first erbium-doped optical fiber 132 and inputs the multiplexed signal to the second erbium-doped optical fiber 133. . The first signal light 51 passes through an optical fiber 135 into which an optical isolator 126 is inserted, and is coupled to a multiplexer / demultiplexer 148.
Is input to

The first and second signal lights 51 and 53 amplified by the second erbium-doped optical fiber 133 are output to a common output terminal via the optical fiber 36 in which the optical isolator 27 is inserted.

The amplifying section has an excitation semiconductor laser 12 for emitting excitation light (wavelength 1.48 μm) for exciting the first and second erbium-doped optical fibers 132 and 133. The second signal light 53 is input to the multiplexer 22 via the optical fiber 34 into which the optical isolator 25 has been inserted. The pumping light in the 1.48 μm band is transmitted to the multiplexer / demultiplexer 22.
Multiplexed with the 1.53 μm second signal light,
The erbium-doped optical fiber 132 is input. First
1.48 transmitted through erbium-doped optical fiber 132
The pump light in the μm band further passes through the multiplexer / demultiplexer 148, is input to the second erbium-doped optical fiber 133, and also excites the second erbium-doped optical fiber 133.

(Embodiment 5) Hereinafter, referring to FIG.
Another embodiment of the optical fiber amplifier according to the present invention will be described in detail. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The difference between the optical fiber amplifier of FIG. 8 and the optical fiber amplifier of FIG. 7 is that the optical fiber amplifier of FIG. 8 further includes a 1.48 μm pumping semiconductor laser 13 as a second pumping light source. The multiplexing / demultiplexing device 23 for inputting the pumping light emitted from the semiconductor laser 13 to the erbium-doped optical fiber 133 is provided between the erbium-doped optical fiber 133 and the optical isolator 27. The multiplexer / demultiplexer 23 multiplexes / demultiplexes the 1.48 μm light and the 1.55 μm light.

Embodiment 6 Hereinafter, referring to FIG.
Another embodiment of the optical fiber amplifier according to the present invention will be described in detail. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The difference between the optical fiber amplifier of FIG. 9 and the optical fiber amplifier of FIG. 8 is that the optical fiber amplifier of FIG. 9 further includes an erbium-doped optical fiber 131, a pumping semiconductor laser 11, And a wave device 21. Here, the semiconductor laser 11 for excitation has a wavelength of 0.9.
Emit 8 μm excitation light. The multiplexer / demultiplexer 21 is 0.9
8 μm light and 1.55 μm light are combined and demultiplexed.

(Embodiment 7) Hereinafter, an embodiment of a bidirectional optical amplifier according to the present invention will be described in detail with reference to FIG. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The optical amplifying section of the present embodiment comprises a pumping semiconductor laser 12 having a wavelength of 1.48 μm, similar to the embodiment of FIG.
1.48 μm light / 1.55 μm light multiplexer / demultiplexer 22, wavelength band 1.55 μm or less / 1.55 μm light multiplexer / demultiplexer 24, optical isolators 25, 26 and 27, erbium-doped optical fiber 32 and 33 are provided. Further, by using the optical circulators 46 and 47, the multiplexer / demultiplexer 48, and the optical fibers 37 and 38, a multiplexer / demultiplexer for signal light from both directions is configured.

Reference numeral 51 denotes a first signal light input to the optical fiber 37, 52 denotes an amplified first signal light output from the optical fiber 38, and 53 denotes a second signal light input to the optical fiber 38. , 54 are amplified second signal lights output from the optical fiber 37.

Next, a case where two signal lights enter this bidirectional optical amplifier will be described.

Now, the first signal light 51 is input from the right side of the optical fiber 37. When this signal light 51 is input to the optical circulator 46, only the signal light from the optical fiber 37 is multiplexed / demultiplexed by the operation of the optical circulator.
After being multiplexed with the second signal light 53, the light is input to the erbium-doped optical fiber 32 via the optical isolator 25.

The amplified first signal light 51 is demultiplexed by the multiplexing / demultiplexing unit 24 in the optical amplifying unit, and is emitted only in the direction of the optical circulator 47 but not in the direction of the optical circulator 46. When the amplified first signal light 51 is input to the optical circulator 47, the amplified first signal light 51 is transmitted toward the optical fiber 38 by the action of the optical circulator and output. The second signal light 53 having a wavelength of 1.53 μm input from the left side of the optical fiber 38 enters the optical circulator 47 and is multiplexed with the 1.56 μm signal light 51 by the multiplexer / demultiplexer 48. It is amplified by erbium-doped optical fibers 32 and 33. When the amplified signal light 53 is input to the optical circulator 46, it is transmitted to the right side of the optical fiber 37 by the function of the optical circulator, and is output as an amplified signal light 54.

According to this configuration, since the optical fiber amplifier of FIG. 1 is used for optical amplification, it is possible to provide a bidirectional optical amplifier in which the gain of the signal light that is bidirectionally amplified is equal.

Embodiment 8 Hereinafter, another embodiment of the bidirectional optical amplifier according to the present invention will be described in detail with reference to FIG. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The optical amplifying section of this embodiment is similar to the embodiment of FIG. 5 and has the pumping semiconductor lasers 12 and 1 having a wavelength of 1.48 μm.
3, a multiplexer / demultiplexer 22 for 1.48 μm light and 1.55 μm light
, 23, a wavelength band of 1.55 μm or less and a 1.55 μm light multiplexer / demultiplexer 24, optical isolators 25, 26 and 27, and erbium-doped optical fibers 32 and 33. Further, optical circulators 46 and 47, a multiplexer / demultiplexer 48,
The optical fibers 37 and 38 are used to combine and demultiplex the two-way signal light.

Reference numeral 51 denotes a first signal light input to the optical fiber 37, 52 denotes an amplified first signal light output from the optical fiber 38, and 53 denotes a second signal light input to the optical fiber 38. , 54 are amplified second signal lights output from the optical fiber 37.

Next, a case where two signal lights enter this optical fiber amplifier will be described.

Now, the first signal light 51 is input from the right side of the optical fiber 37. When this signal light 51 is input to the optical circulator 46, only the signal light from the optical fiber 37 is multiplexed / demultiplexed by the operation of the optical circulator.
After being multiplexed with the second signal light 53, it is input to the erbium-doped optical fiber 32.

The amplified first signal light 51 is demultiplexed by the multiplexing / demultiplexing device 24 in the optical amplifier, and is emitted only in the direction of the optical circulator 47 but not in the direction of the optical circulator 46. When the amplified first signal light 52 is input to the optical circulator 47, it is transmitted through the optical circulator in the direction of the optical fiber 38 and output. The second signal light 53 having a wavelength of 1.53 μm input from the left side of the optical fiber 38 enters the optical circulator 47 and is multiplexed with the 1.56 μm signal light 51 by the multiplexer / demultiplexer 48. It is amplified by erbium-doped optical fibers 32 and 33. When the amplified signal light 53 is input to the optical circulator 46, it is transmitted to the right side of the optical fiber 37 by the function of the optical circulator, and is output as an amplified signal light 54.

According to this configuration, since the optical fiber amplifier of FIG. 5 is used for the optical amplifier, even if the input light amount is large, the gain of the signal light that is bidirectionally amplified is uniform. Can be provided.

Embodiment 9 Hereinafter, still another embodiment of the bidirectional optical amplifier according to the present invention will be described in detail with reference to FIG. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

As in the optical fiber amplifier shown in FIG. 6, the optical amplifying unit of this embodiment comprises a pumping semiconductor laser 11 having a wavelength of 0.98 μm, pumping semiconductor lasers 12 and 13 having a wavelength of 1.48 μm, and a 0.98 μm light. And 1.55μm light multiplexing / demultiplexing device 2
1, a multiplexer / demultiplexer 22 for 1.48 μm light and 1.55 μm light
, A wavelength band 1.55 μm or less and a 1.55 μm light multiplexer / demultiplexer 24, optical isolators 25, 26 and 27, and erbium-doped optical fibers 31, 32 and 33. Further, by using the optical circulators 46 and 47, the multiplexer / demultiplexer 48, and the optical fibers 37 and 38, a multiplexer / demultiplexer for signal light from both directions is configured.

Reference numeral 51 denotes a first signal light input to the optical fiber 37, 52 denotes an amplified first signal light output from the optical fiber 38, and 53 denotes a second signal light input to the optical fiber 38. , 54 are amplified second signal lights output from the optical fiber 37.

Next, the case where signal light enters this optical fiber amplifier will be described.

Now, the first signal light 51 is input from the right side of the optical fiber 37. When this signal light 51 is input to the optical circulator 46, only the signal light from the optical fiber 37 is multiplexed / demultiplexed by the operation of the optical circulator.
And is multiplexed with the second signal light 53, and then input to the erbium-doped optical fiber 31.

The amplified first signal light 51 is demultiplexed by the multiplexing / demultiplexing device in the optical amplifying unit.
, But not in the direction of the optical circulator 46. When the signal light 51 is input to the optical circulator 47, the signal light 51 is transmitted toward the optical fiber 38 by the function of the optical circulator and is output.

Similarly, the second signal light 53 having a wavelength of 1.53 μm input from the left side of the optical fiber 38 enters the optical circulator 47,
It is multiplexed with the 6 μm signal light 51 and amplified by the erbium-doped optical fibers 31, 32 and 33. When the amplified signal light 53 is input to the optical circulator 46, it is transmitted to the right side of the optical fiber 37 by the function of the optical circulator, and is output as an amplified signal light 54.

According to this configuration, since the optical amplifier shown in FIG. 6 is used for the optical amplifier, even if the total input light quantity of all the signal lights is large, the gains of the signal lights amplified bidirectionally are uniform. ,
Further, a bidirectional optical amplifier having good noise characteristics can be provided.

(Embodiment 10) Hereinafter, still another embodiment of the bidirectional optical amplifier according to the present invention will be described in detail with reference to FIG. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The difference between the bidirectional optical amplifier of FIG. 13 and the bidirectional optical amplifier of FIG. 10 is that the bidirectional optical amplifier of FIG. 13 is different from the bidirectional optical amplifier of FIG. 147 is used. As the demultiplexer / combiner, a WDM coupler is preferable.

In the bidirectional optical amplifiers of FIGS. 11 and 12, demultiplexers / combiners 146 and 147 may be used instead of the optical circulators 46 and 47.

Embodiment 11 Hereinafter, still another embodiment of the bidirectional optical amplifier according to the present invention will be described in detail with reference to FIG. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The optical amplifying section of the present embodiment is similar to the embodiment of FIG. 7 in that the pumping semiconductor laser 12 having a wavelength of 1.48 μm,
A multiplexer / demultiplexer 22 for 1.48 μm light and 1.53 μm light, multiplexers / demultiplexers 124 and 148 for wavelength bands 1.55 μm or less and 1.55 μm light, optical isolators 25, 126, and 12.
7 and 227, and erbium-doped optical fibers 132 and 133. Further, by using the optical circulators 46 and 47 and the optical fibers 37 and 38, a multiplexing / demultiplexing unit for bidirectional signal light is formed.

Reference numeral 51 denotes a first signal light input to the optical fiber 37, 52 denotes an amplified first signal light output from the optical fiber 38, and 53 denotes a second signal light input to the optical fiber 37. , 54 are amplified second signal lights output from the optical fiber 38.

Next, the case where two signal lights enter this bidirectional optical amplifier will be described.

Now, the first signal light 51 is input from the right side of the optical fiber 37. When the signal light 51 is input to the optical circulator 46, only the signal light from the optical fiber 37 enters the multiplexer / demultiplexer 148 by the operation of the optical circulator 46, and is combined with the second signal light 53. After that, the light is input to the erbium-doped optical fiber 133.

The first signal light 51 amplified by the erbium-doped optical fiber 133 is supplied to the multiplexer / demultiplexer 1 in the optical amplifier.
The light is split by the optical waveguide 24 and propagates through the optical isolator 127 to the optical circulator 47, but does not propagate to the optical circulator 46. When the amplified first signal light 51 is input to the optical circulator 47, the amplified first signal light 51 is transmitted toward the optical fiber 38 by the action of the optical circulator and output.

Similarly, the second signal light 53 having a wavelength of 1.53 μm input from the left side of the optical fiber 38 enters the optical circulator 47, and is thereafter
, And is input to the erbium-doped optical fiber 132. Erbium-doped optical fiber 13
The second signal light 53 amplified at 2 is coupled to the multiplexer / demultiplexer 14
At 8, it is multiplexed with the 1.56 μm signal light 51 and further amplified by the erbium-doped optical fiber 133. The amplified second signal light 53 is separated to the right side in the figure by a multiplexer 124, passes through an optical isolator 227, and passes through an optical circulator 4.
6 is input. The amplified second signal light 53 is transmitted to the right side of the optical fiber 37 by the operation of the optical circulator 46 and is output as a signal light 54.

Embodiment 12 Hereinafter, still another embodiment of the bidirectional optical amplifier according to the present invention will be described in detail with reference to FIG. In the following description, the same reference numerals are given to the corresponding parts of the embodiments already described.

The difference between the bidirectional optical amplifier of FIG. 15 and the bidirectional optical amplifier of FIG. 14 is that the bidirectional optical amplifier of FIG. 15 further includes a 1.48 μm pumping semiconductor laser 13 as a second pumping light source. And a multiplexing / demultiplexing device 23 for inputting the pumping light emitted from the pumping semiconductor laser 13 to the erbium-doped optical fiber 32 is provided between the erbium-doped optical fiber 32 and the optical isolator 27. is there. The multiplexer / demultiplexer 23 has a 1.48 μm light and a 1.48 μm light.
The multiplexing / demultiplexing is performed on the 55 μm light.

Embodiment 13 Hereinafter, an embodiment of an optical fiber transmission system according to the present invention will be described in detail with reference to FIG.

The transmitting station 101 has a signal light source 102, and the subscriber 105 has a light receiver 107. The transmitting station 101 and the subscriber 105 are connected by a transmission line 131, and on the way, the optical fiber amplifier 111 and the 1 × 8 optical splitter 1 described in the first embodiment.
21 is inserted.

Next, a case where an analog optical signal is transmitted and received in this optical transmission system will be described.

In the transmitting station 101, the output light from the 1.56 μm semiconductor laser as the signal light source 102, the intensity of which is modulated by the analog signal, is amplified by the optical fiber amplifier 111, and then transmitted as a single transmission line 131 having a length of 10 km. Transmitted to mode optical fiber. This analog signal light is transmitted to the subscriber 105 by the pin-
Received by PD.

As described above, any optical fiber amplifier according to the present invention may be used as the optical fiber amplifier 111. According to the present embodiment, since the gain of each signal light is equal, even when the number of connection stages of the optical amplifier is increased, there is no variation in the amount of received light between the signal lights, and stable characteristics can be obtained. In particular, if the optical fiber amplifier of FIG. 5 is used, the gain of each signal light can be equalized even when the input light amount is large in order to obtain the signal-to-noise ratio of the analog signal light. Accordingly, even when the number of connection stages of the optical amplifier is increased, there is no deviation between channels in the amount of light received by the light receiver, and stable characteristics can be obtained. Also, if the optical fiber amplifier of FIG. 6 is used, the signal light incident part is pumped by 0.98 μm,
Good noise characteristics can be obtained even when the input light quantity of all signal lights is large.

Embodiment 14 Hereinafter, an embodiment of an optical fiber transmission system according to the present invention will be described in detail with reference to FIG.

The transmitting station 101 includes the signal light source 102 and the photodetector 1
03 and a multiplexer / demultiplexer 104, and a subscriber 105
Is a signal light source 106, a light receiver 107, and a multiplexer / demultiplexer 108
And The transmitting station 101 and the subscriber 105 are connected by a transmission line 131, and one of the above-described bidirectional optical amplifiers 114 and the 1 × 8 optical splitter 121 is inserted on the way.

Next, a case where an analog optical signal and a digital optical signal are transmitted and received in this optical transmission system will be described first for a downstream analog signal from the transmitting station to the subscriber side, and then for an upstream digital.

In the transmitting station 101, the output light from the 1.56 μm semiconductor laser as the signal light source 102, the intensity of which is modulated by the analog signal, is output through the multiplexer / demultiplexer 104. After being amplified by an analog / digital bidirectional optical amplifier 114, the transmission line 131 has a length of one.
It is transmitted to a 0 km single mode optical fiber. This analog signal light is transmitted to the subscriber 105 by the multiplexer / demultiplexer 1.
After 08, the light is received by the pin-PD as the light receiver 107.

At the subscriber 105, the signal light source 106 intensity-modulated by the digital signal is 1.53 μm.
The output light from the semiconductor laser is multiplexed with the 1.56 μm light in the multiplexing / demultiplexing device 108 and sent out to a single-mode optical fiber having a length of 10 km as the transmission line 131. This digital signal light is transmitted to the bidirectional optical fiber amplifier 1.
After being amplified by 15, it reaches the transmitting station 101. In the transmitting station 101, the multiplexing / demultiplexing device 104 uses 1.56 μm.
After being demultiplexed with the m light, the light is received by a light receiver 103 made of Ge-APD.

As described above, the present optical fiber transmission system includes:
Since the bidirectional optical amplifier according to the present invention is used, the gain becomes equal to the signal light propagating in both directions. Therefore, when the number of connection stages of the optical amplifier is increased or when the amount of signal from the subscriber side increases or decreases, the amount of light received by the light receiver does not change, so that stable characteristics can be obtained.

As described above, since the bidirectional optical amplifier of the present invention is used, the gain of the signal light propagating in both directions can be equalized even when the input light amount is large in order to obtain the signal-to-noise ratio of the analog signal light. Therefore, when the number of connection stages of the optical amplifier is increased or when the amount of signal from the subscriber side increases or decreases, the amount of light received by the light receiver does not change, so that stable characteristics can be obtained.

FIG. 18 shows the difference in 1.53 μm digital signal transmission characteristics in the present embodiment depending on the presence or absence of the erbium-doped optical fiber 33 in the bidirectional optical amplifier 115. The transmission speed is 200 Mbps. When the erbium-doped optical fiber 33 is not provided, the minimum amount of received light that gives a code error rate of 10 ^-9 is -26 dBm, but when it is present, it is -35 dBm, and the characteristic is improved by 9 dB. In other words, it is clear that adopting this configuration in a bidirectional optical transmission system has a great effect on improving transmission characteristics.

As described above, when the bidirectional optical amplifier according to the present invention is used, since the gain is equal to the signal light propagating in both directions, even when the number of connection stages of the optical amplifier is increased, the signal light amount from the subscriber side is increased. Is increased or decreased, the amount of light received by the light receiver does not fluctuate, so that stable characteristics can be obtained. Further, since the signal light incident portion is excited by 0.98 μm, good noise characteristics can be obtained even when the input light amount of all signal lights is large.

In this embodiment, the bidirectional optical amplifier is installed at a position near the transmitting station, and operates as a post-amplifier for analog signal light and as a preamplifier for digital signal light. Depending on the installation position of the bidirectional optical amplifier, it can operate as a post amplifier, an in-line amplifier or a preamplifier.

Further, in this embodiment, only the analog signal light having one wavelength is shown as the signal light of the first wavelength band. A plurality of analog or digital signal lights can be transmitted. Similarly, it goes without saying that signal light of a plurality of wavelengths can be transmitted as signal light of the second wavelength band.

Further, a case where three or more wavelength bands are transmitted by the multiplexer / demultiplexer is also included in the concept of the present invention.

In this embodiment, the optical amplifier in the 1.5 μm band doped with rare earth erbium has been described.
In the present invention, the same effects can be obtained for optical amplifiers of different wavelength bands using other rare earth elements such as neodymium and praseodymium, and there is no limitation on the constituent materials.

[0116]

As described above, according to the present invention,
It is possible to provide an optical fiber amplifier in which signal gains of multiple wavelengths are equal. Further, it is possible to provide a bidirectional optical fiber amplifier for amplifying multi-wavelength signal light input from two different directions with equal gain. Furthermore, even when the optical fiber amplifiers are connected in multiple stages, the amount of light received by the light receiving section becomes equal between the channels, so that it is possible to provide an optical transmission system capable of obtaining stable characteristics.

Further, according to the present invention, even when bidirectional optical amplifiers are connected in multiple stages, the bidirectional gain becomes uniform, so that there is an effect that it is possible to provide an optical transmission system capable of obtaining stable characteristics.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing wavelength dependence of gain in an erbium-doped optical fiber used in an example of the present invention.

FIG. 3 is a diagram showing the input light quantity dependency of the excitation light transmission light quantity in the erbium-doped optical fiber used in the embodiment of the present invention.

FIG. 4 is a diagram comparing gains of a signal light of 1.53 μm and a signal light of 1.56 μm used in an embodiment of the present invention.

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

FIG. 6 is a diagram showing a configuration of still another embodiment of the optical fiber amplifier according to the present invention.

FIG. 7 is a diagram showing a configuration of still another embodiment of the optical fiber amplifier according to the present invention.

FIG. 8 is a diagram showing a configuration of still another embodiment of the optical fiber amplifier according to the present invention.

FIG. 9 is a diagram showing a configuration of still another embodiment of the optical fiber amplifier according to the present invention.

FIG. 10 is a diagram showing a configuration of an embodiment of a bidirectional optical amplifier according to the present invention.

FIG. 11 is a diagram showing a configuration of another embodiment of the bidirectional optical amplifier according to the present invention.

FIG. 12 is a diagram showing a configuration of still another embodiment of the bidirectional optical amplifier according to the present invention.

FIG. 13 is a diagram showing a configuration of still another embodiment of the bidirectional optical amplifier according to the present invention.

FIG. 14 is a diagram showing a configuration of still another embodiment of the bidirectional optical amplifier according to the present invention.

FIG. 15 is a diagram showing a configuration of still another embodiment of the bidirectional optical amplifier according to the present invention.

FIG. 16 is a diagram showing a configuration of an embodiment of an optical fiber transmission system according to the present invention.

FIG. 17 is a diagram showing a configuration of another embodiment of the optical fiber transmission system according to the present invention.

FIG. 18 is a graph showing a difference in 1.53 μm digital signal transmission characteristics depending on the presence or absence of the second erbium-doped optical fiber in the bidirectional optical amplifier according to the present invention.

[Explanation of symbols]

11 Excitation semiconductor laser of wavelength 0.98 μm 12, 13 Excitation semiconductor laser of wavelength 1.48 μm 21 Multiplexer / demultiplexer of 0.98 μm light and 1.55 μm light 22, 23 1.48 μm light and 1.55 μm Optical multiplexer / demultiplexer 24 Wavelength band 1.55 μm or less and 1.5
5 μm light multiplexer / demultiplexer 25, 26, 27 Optical isolator 31, 32, 33 Erbium-doped optical fiber 34, 35, 36 Optical fiber 46, 47 Optical circulator 48 Multiplexer / demultiplexer 51 Input to optical fiber 34 The first signal light 52 to be output 52 The amplified first signal light 53 output from the optical fiber 35 The second signal light 54 input to the optical fiber 34 The amplified second signal output from the optical fiber 36 Signal light 55 0.98 μm pump light 56, 57 1.48 μm pump light 101 Transmitting station 102 Signal light source 103 Optical receiver 104 Multiplexer / demultiplexer 105 Subscriber 106 Signal light source 107 Optical receiver 108 Multiplexer / demultiplexer 111 Light Fiber amplifier 114 Bidirectional optical amplifier 121 1x8 optical splitter 131 Optical fiber

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication H04B 10/17

Claims (20)

(57) [Claims] A plurality of rare earth-doped optical fibers connected in series, and at least one pump light source for generating pump light for exciting the plurality of rare earth-doped optical fibers connected in series; An optical fiber amplifier for optically amplifying a plurality of signal lights having different input wavelengths, wherein a first signal light of the plurality of signal lights is a plurality of rare earth-doped lights connected in series. Propagating through some of the rare earth-doped optical fibers of the fiber, the second signal light of the plurality of signal lights transmits all the rare-earth-doped optical fibers of the plurality of rare earth-doped optical fibers connected in series. Propagating fiber optic amplifier. 2. A branch optical fiber connected in the middle of the plurality of rare earth-doped optical fibers connected in series, wherein the first signal light is output via the branch optical fiber. The optical fiber amplifier according to claim 1. 3. A branch optical fiber connected in the middle of the plurality of rare earth-doped optical fibers connected in series, wherein the first signal light is input through the branch optical fiber. The optical fiber amplifier according to claim 1. 4. The light according to claim 1, wherein each of the plurality of rare-earth-doped optical fibers is pumped to a state in which a gain higher than a gain for the second signal light is given to the first signal light. Fiber amplifier. 5. The optical fiber amplifier according to claim 1, wherein said at least one pumping light source includes a semiconductor laser. 6. A multiplexer / demultiplexer for multiplexing the pump light generated by the pump light source and the plurality of signal lights,
The optical fiber amplifier according to claim 1, wherein the optical fiber amplifier is connected to any one of the input portions of the plurality of rare earth-doped optical fibers connected in series. 7. A multiplexer / demultiplexer for multiplexing the pump light generated by the pump light source and the plurality of signal lights,
The optical fiber amplifier according to claim 1, wherein the optical fiber amplifier is connected to any one of the output units of the plurality of rare earth-doped optical fibers connected in series. 8. A branch optical fiber which selectively transmits both light belonging to a first wavelength band and light belonging to a second wavelength band and transmits light not belonging to the first and second wavelength bands to the branch optical fiber. 4. The optical fiber amplifier according to claim 2, wherein the separating means is provided in the middle of the plurality of rare earth-doped optical fibers connected in series. 9. The wavelength of the first signal light is not included in the first wavelength band and the second wavelength band, and the wavelength of the second signal light is equal to the first wavelength band or the second wavelength band. The optical fiber amplifier according to claim 8, which is included in the second wavelength band. 10. The optical fiber amplifier according to claim 8, wherein the wavelength of the pump light is included in the first wavelength band or the second wavelength band. 11. The wavelength of the first signal light is longer than the wavelength of the second signal light, the first signal light is analog-modulated, and the wavelength of the second signal light is digitally modulated. The optical fiber amplifier according to claim 8, wherein: 12. A first input / output unit for outputting at least a first signal light and inputting a second signal light, and for inputting at least the first signal light and outputting the second signal light. A bidirectional optical amplifier comprising: a second input / output section; and an optical fiber amplifier connected to the first input / output section and the second input / output section, wherein the optical fiber amplifier is connected in series. A plurality of rare earth-doped optical fibers connected to the at least one, and at least one excitation light source for generating excitation light for exciting the plurality of rare earth-doped optical fibers connected in series, having different input wavelengths An optical fiber amplifier for optically amplifying a plurality of signal lights, and a first signal light of the plurality of signal lights is a light source of the plurality of rare earth-doped optical fibers connected in series. Propagating through some rare earth-doped optical fibers, The second of the plurality of signal lights
Is an optical fiber amplifier that propagates through all of the rare earth-doped optical fibers connected in series, and converts the first signal light received from the optical fiber amplifier into the first signal light. A bidirectional optical amplifier, further comprising a demultiplexing unit that selectively propagates to the first input / output unit. 13. The first signal light which is multiplexed with the second signal light input from the first input / output unit and the first signal light input to the second input / output unit. Multiplexing means for inputting both light and the second signal light to input portions of the plurality of rare earth-doped optical fibers connected in series.
The bidirectional optical amplifier according to claim 12. 14. A multiplexing means for inputting the first signal light input from the second input / output portion to a midway of the plurality of rare earth-doped optical fibers connected in series, The bidirectional optical amplifier according to claim 12. 15. The bidirectional optical amplifier according to claim 12, wherein the first input / output section includes a first optical circulator, and the second input / output section includes a second optical circulator. 16. The apparatus according to claim 12, wherein the first input / output portion includes a first multiplexing / demultiplexing device, and the second input / output portion includes a second multiplexing / demultiplexing device. Bidirectional optical amplifier. 17. A transmitting station including a first signal light source that emits a first signal light, and a first light receiver that detects a second signal light, and a second station that detects the first signal light. And a receiving station including a second signal light source that emits the second signal light; an optical fiber transmission line connecting the transmitting station and the receiving station; An optical fiber transmission system comprising: a plurality of rare earth-doped optical fibers connected in series; and a plurality of rare earth-doped optical fibers connected in series. An optical fiber amplifier comprising: at least one pumping light source that generates pumping light for pumping a fiber; and an optical fiber amplifier that optically amplifies a plurality of signal lights having different input wavelengths. The first signal light is connected in series. Of the plurality of rare earth-doped optical fibers, and the second signal light of the plurality of signal lights is transmitted by the plurality of rare earth-doped optical fibers connected in series. An optical fiber transmission system that propagates all rare earth doped optical fibers. 18. The method according to claim 18, wherein at least one of the first signal light
The optical fiber transmission system according to claim 17, wherein one analog-modulated signal light is used, and a plurality of digitally-modulated signal lights are used as the second signal light. 19. A transmitting station including a first signal light source for emitting a first signal light, and a first photodetector for detecting a second signal light, and a second station for detecting the first signal light. And a receiving station including a second signal light source that emits the second signal light; an optical fiber transmission line connecting the transmitting station and the bidirectional optical amplifier; A bidirectional optical fiber transmission system comprising: a bidirectional optical amplifier provided on the way, wherein the bidirectional optical amplifier performs output of the first signal light and input of the second signal light. A second input / output portion, a second input / output portion for inputting the first signal light and outputting the second signal light, and being connected to the first input / output portion and the second input / output portion. An optical fiber amplifying section, wherein the optical fiber amplifying section includes a plurality of rare earth-doped optical fibers connected in series. And at least one pumping light source that generates pumping light for pumping the plurality of rare-earth-doped optical fibers connected in series, and optically amplifies a plurality of signal lights having different input wavelengths. An optical fiber amplifier, and a first signal light of the plurality of signal lights propagates through some of the rare earth-doped optical fibers connected in series; , A second one of the plurality of signal lights
Is an optical fiber amplifier that propagates through all of the rare earth-doped optical fibers connected in series, and converts the first signal light received from the optical fiber amplifier into the first signal light. A bidirectional optical fiber transmission system, further comprising a demultiplexing unit that selectively propagates to the first input / output unit. 20. At least one of the first signal light beams
20. The bidirectional optical fiber transmission system according to claim 19, wherein one analog-modulated signal light is used, and a plurality of digitally-modulated signal lights are used as the second signal light.