JP2005079532A - Optical repeater and optical relay/transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an optical repeater and an optical relay/transmission system capable of improving the amplification characteristics of a signal light of a C band and suppressing the occurrence of a gain tilt to a deterioration in the power of the signal light. <P>SOLUTION: This device is constituted in such a manner that EDFs 17a and 17b use excitation light to optically amplify the signal light of a C band and transmit a part of the excitation light for the Raman amplification of the signal light of a C band in optical fibers 11a and 11b. Thereby, the amplification characteristics of the signal light of a C band can be improved, and the occurrence of a gain tilt to a deterioration in the power of the signal light can be suppressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical repeater and an optical repeater transmission system for optically amplifying signal light transmitted through an optical fiber.

At present, as an optical amplification repeater for realizing a long-distance optical transmission system, an optical amplification using an erbium-doped fiber (EDF: Erbium-DopEDFiber) is widely used.
In the optical amplification repeater system using EDF, a stable optical amplification characteristic of signal light can be obtained by using a semiconductor laser that outputs excitation light having a wavelength of either 980 nm or 1480 nm as an excitation light source. Further, since the amplification efficiency of the signal light is good, there is an advantage that an optical repeater with a simple configuration and low power consumption can be realized.

In addition, the optical amplifying and relaying method using EDF has a characteristic that, when the power of the signal light is reduced for some reason, the gain is increased without increasing the power of the pumping light, and the power reduction amount of the signal light is reduced. At a point where the signal light has passed through several stages of the optical repeater, it has a characteristic of returning to almost normal signal light power.
In other words, the lower the power of the signal light input to the optical repeater, the higher the gain saturation characteristic. Thus, the point which is excellent in fault tolerance becomes an important advantage at the time of operation of a real system.

When large-capacity transmission is performed by wavelength multiplexing optical transmission, if the wavelength band of 1530 to 1560 nm, which is called C band (C band), is selected as the signal wavelength, the amplification efficiency of EDF can be maximized, and light can be consumed with low power consumption. A relay device can be realized.
To increase the capacity by increasing the number of wavelength multiplexing, it is necessary to increase the output of the optical repeater according to the number of wavelengths of the signal light, but the wavelength band other than the C band with low amplification efficiency. If the amplification band is widened, the efficiency of the optical repeater will be reduced, which will hinder high output. Therefore, it is desirable to increase the C-band utilization efficiency and increase the number of wavelength multiplexing by narrowing the wavelength interval of the signal light.

In order to realize an optical transmission system that suppresses power consumption and cost of the entire system, it is effective to extend the relay interval and reduce the number of optical repeaters. For this purpose, it is effective to increase the output of the optical repeater and relay the signal light with a higher signal-to-noise ratio (SNR).
However, when the power of the signal light incident on the optical fiber is excessively increased, degradation of transmission characteristics due to a nonlinear phenomenon in the optical fiber that transmits the signal light becomes a problem. Although it depends on the effective cross-sectional area of the optical fiber, if the upper limit of the signal light power per wave allowed as a guide is 0 dBm and the multiplexing of 128 wave signal lights is assumed, the output of the optical repeater is 21 dBm. Become.

As a method of extending the repeat interval without further increasing the output of the optical repeater, it is conceivable to repeat with a higher SNR of signal light by improving the noise characteristics of the optical repeater.
In order to improve noise characteristics, it is effective to use Raman amplification in an optical fiber that transmits signal light. For example, an optical amplifier disclosed in Patent Document 1 below improves noise characteristics by using Raman amplification in combination with an L band having relatively poor EDF noise characteristics.

JP 2001-102666 (paragraph numbers [0021] to [0027], FIG. 1)

Since the conventional optical repeater is configured as described above, L-band signal light having relatively poor EDF noise characteristics can be Raman-amplified, but C-band signal light amplification characteristics can be improved. There was a problem that could not be done.
In addition, when the power of the signal light is reduced due to the failure of the optical fiber that transmits the signal light, due to the inherent characteristics of the EDF, the gains are not uniform among the wavelength multiplexed signal lights, and the power varies. There was also a problem that ended up.

  The present invention has been made in order to solve the above-described problems, and can improve the amplification characteristics of C-band signal light and suppress the occurrence of gain tilt with respect to a decrease in signal light power. An object of the present invention is to obtain an optical repeater and an optical repeater transmission system.

  In the optical repeater according to the present invention, the rare earth-doped optical fiber amplifies the C-band signal light using the pumping light introduced into the optical fiber and transmits a part of the pumping light to In the fiber, the C-band signal light is Raman-amplified.

  According to the present invention, the rare earth-doped optical fiber amplifies the C-band signal light using the pumping light introduced into the optical fiber, and transmits a part of the pumping light to transmit the C-band signal light in the optical fiber. Since the band-band signal light is amplified by Raman, the amplification characteristic of the C-band signal light can be improved, and the effect of suppressing the occurrence of gain tilt with respect to the decrease in the power of the signal light There is.

Embodiment 1 FIG.
1 is a block diagram showing an optical repeater according to Embodiment 1 of the present invention. FIG. 2 is a system block diagram showing an optical repeater transmission system according to Embodiment 1 of the present invention.
In the figure, an optical transmitter 1 converts an electrical signal into a C-band (C-band) signal light and transmits it, and an optical repeater 2 receives signal light transmitted from the optical transmitter 1 or the preceding optical repeater 2. When received, the signal light is amplified and transmitted. When receiving the signal light transmitted from the optical repeater 2, the optical receiver 3 converts the signal light into an electrical signal.

The optical fibers 11a and 11b transmit the C-band signal light transmitted from the optical transmitter 1 or the preceding optical repeater 2 to the optical repeater 2, and the optical fibers 12a and 12b are signals amplified by the optical repeater 2. The light is transmitted to the subsequent optical repeater 2 or the optical receiver 3.
The excitation light source 13 includes a semiconductor laser that oscillates excitation light having a wavelength of 1450 to 1470 nm. The pumping light source 13 may reflect a part of the oscillated pumping light by a fiber grating and return it to the semiconductor laser. In this case, the wavelength and power of the oscillating excitation light can be stabilized.
The polarization beam combiners 14 a and 14 b synthesize pump light oscillated from the semiconductor laser of the pump light source 13 and output the synthesized light to the 3 dB coupler 15. By synthesizing the excitation light by the polarization beam combiners 14a and 14b, high output excitation light can be obtained and the excitation light can be depolarized. Thereby, it is possible to reduce the polarization dependency in which the Raman amplification gain varies depending on the polarization state of the signal light.
The 3 dB coupler 15 multiplexes the excitation lights output from the polarization beam combiners 14a and 14b, distributes the combined excitation lights, and outputs them to the circulators 16a and 16b. The circulators 16a and 16b transmit the pumping light so that the transmission direction of the pumping light output from the 3 dB coupler 15 is opposite to the transmission direction of the C-band signal light transmitted through the optical fibers 11a and 11b. Install in the fiber.
The polarization beam combiners 14a and 14b, the 3 dB coupler 15 and the circulators 16a and 16b constitute excitation light introducing means.

EDFs 17a and 17b, which are rare-earth-doped optical fibers, are inserted in the middle of the optical fiber, amplify the C-band signal light using the excitation light introduced by the circulators 16a and 16b, and a part of the excitation light. The light is transmitted and given to the optical fibers 11a and 11b.
The gain equalizers 18a and 18b correct the power deviation of the wavelength-multiplexed signal light, have a loss characteristic depending on the wavelength, gain obtained by Raman amplification in the optical fibers 11a and 11b, and EDFs 17a and 17b. It is designed to correct the wavelength characteristic of the gain obtained in the above.

  The monitor couplers 19a and 19b branch a part of the power of the signal light in order to monitor the power of the signal light output from the optical repeater 2. The excitation light removal filters 20a and 20b remove the excitation light contained in the branch signals of the monitor couplers 19a and 19b. The monitor PDs 21a and 21b monitor the power of the signal light output from the optical repeater 2. The OTDR path 22 is an optical path connecting the monitor coupler 19a and the monitor coupler 19b. The OTDR path 22 is for monitoring the state of the optical fiber that transmits the signal light from the land station and the optical repeater 2 and identifying the failure point. This is a path for transmitting the monitoring light.

The excitation light bypass optical isolators 23a and 23b have a bypass path for supplying the excitation light transmitted through the EDFs 17a and 17b to the optical fibers 11a and 11b. It has a function to block signal light.
The monitor couplers 24a and 24b branch a part of the power of the signal light in order to monitor the power of the signal light input to the optical repeater 2. The excitation light removal filters 25a and 25b remove the excitation light (excitation light reflected by the optical fibers 11a and 11b) included in the branch signals of the monitor couplers 24a and 24b. The monitor PDs 26a and 26b monitor the power of the signal light input to the optical repeater 2.

The polarization beam combiners 14a and 14b, the gain equalizers 18a and 18b, the monitor couplers 19a and 19b, the excitation light removal filters 20a and 20b, the monitor PDs 21a and 21b, the excitation light bypass optical isolators 23a and 23b, the monitor coupler 24a, 24b, excitation light removal filters 25a and 25b, and monitor PDs 26a and 26b are used as necessary, and may not include all of them.
Moreover, you may comprise so that excitation light may be guide | induced to EDF17a, 17b using a wavelength multiplexer instead of circulators 16a, 16b.

Next, the operation will be described.
The signal light transmitted from the optical transmitter 1 or the preceding optical repeater 2 propagates through the optical fibers 11 a and 11 b and is input to the optical repeater 2. At that time, the signal light is attenuated and attenuated in the optical fibers 11a and 11b.
As a result, signal light is input from one end of the EDFs 17 a and 17 b of the optical repeater 2.

On the other hand, the semiconductor laser of the pumping light source 13 oscillates pumping light having a wavelength of 1450 to 1470 nm, and the polarization beam combiners 14 a and 14 b synthesize the pumping light oscillated from the semiconductor laser of the pumping light source 13 and output it to the 3 dB coupler 15. To do.
When receiving the excitation lights from the polarization beam combiners 14a and 14b, the 3 dB coupler 15 combines the excitation lights, distributes the combined excitation lights, and outputs them to the circulators 16a and 16b.
The circulators 16a and 16b transmit the excitation light so that the transmission direction of the excitation light output from the 3 dB coupler 15 is opposite to the transmission direction of the C-band signal light transmitted through the optical fibers 11a and 11b. Introduce into optical fiber.
As a result, excitation light is input from the other ends of the EDFs 17a and 17b of the optical repeater 2.

The signal light input from one end of the EDFs 17a and 17b and the pumping light input from the other end of the EDFs 17a and 17b are configured to be backward pumped, and the EDFs 17a and 17b use the pumping light. The C band signal light is optically amplified.
Thereby, a part of the excitation light input from the other ends of the EDFs 17a and 17b is consumed in the EDFs 17a and 17b, but a part of them passes through the EDFs 17a and 17b and reaches the optical fibers 11a and 11b.
In the optical fibers 11a and 11b, when receiving the excitation light transmitted through the EDFs 17a and 17b, the signal light being transmitted is Raman-amplified.

The Raman amplification in the optical fibers 11a and 11b has an effect of amplifying the signal light before being input to the optical repeater 2 and increasing the input power to the optical repeater 2.
As described above, the signal light input to the optical repeater 2 is amplified by the EDFs 17a and 17b. Although the SNR deteriorates to some extent by the noise light generated from the EDFs 17a and 17b, increasing the input power has an effect of suppressing the deterioration amount of the SNR.
Note that the power of the pumping light reaching the optical fibers 11a and 11b through the pumping light path 27 needs to be 50 mW or more in order to obtain the effect of Raman amplification. Assuming a silica-based optical fiber of about 20 μm ² with good Raman amplification efficiency, a gain of Raman amplification of about 3 dB can be obtained with 50 mW pumping light.
When the pump light power is higher than that, the gain [dB] of Raman amplification increases in proportion to the power [mW], so that the effect of improving the optical SNR is great, and low-noise optical amplification is possible.

Here, the wavelength of the excitation light will be described.
The graph of FIG. 3 is a characteristic example of EDF in the wavelength range of 1450 nm to 1500 nm.
The vertical axis represents the gain coefficient or absorption coefficient per meter of EDF. The absorption coefficient represents the efficiency absorbed by the EDF, and the gain coefficient represents the efficiency released to the contrary.
In the wavelength range shown in FIG. 3, the absorption efficiency greatly exceeds the emission efficiency, so that it can be seen that the absorption is efficiently performed.

In order to perform amplification most efficiently with the EDF, the signal light is set to a C band of 1530 to 1560 nm, and the excitation light is selected from the 1470 to 1490 nm band or the 980 nm band that efficiently absorbs the excitation light.
However, in the first embodiment, as described later, in consideration of the efficiency of Raman amplification, the wavelength band of 1450 to 1470 nm where the absorption rate of EDF is slightly low is preferably used as the excitation wavelength.
As shown in FIG. 3, even in the range of 1450 to 1470 nm, the absorption rate of EDF does not extremely decrease, so that signal light can be sufficiently amplified by EDF. Further, since the absorption rate of the excitation light in the EDF is low, the excitation light power that passes through the EDF increases. As a result, the gain due to Raman amplification is increased, which contributes to the improvement of noise characteristics, which is advantageous for the present invention.

The graph of FIG. 4 shows an example of Raman amplification gain for signal light of 1530 to 1560 nm for four types of pumping light wavelengths of 1440 nm, 1450 nm, 1460 nm, and 1470 nm. However, the Raman amplification gain here is a ratio at which the signal light is increased by inputting the excitation light. For example, the Raman amplification gain of 5 dB means that the power of the signal light output from the optical fiber is increased by 5 dB by inputting the pumping light as compared with the case where the pumping light is not input to the optical fiber. To do.
However, here, a silica-based optical fiber having an effective cross-sectional area of 20 μm ² is assumed as an optical fiber for performing Raman amplification, and the power of pumping light input to the optical fiber is 100 mW.
When 1450 nm and 1460 nm are used as the pumping light wavelengths, a high Raman amplification gain is obtained. On the other hand, when 1440 nm is used as the pumping light wavelength, a sufficient Raman gain cannot be obtained for the signal light. In the region where the excitation wavelength is longer than 1470 nm, the Raman amplification gain decreases monotonously. Therefore, by setting the excitation wavelength in the range of 1450 to 1470 nm, the effect of improving noise characteristics by Raman amplification can be greatly obtained.

Next, the design of the EDF will be described.
The graph of FIG. 5 shows the EDF absorption amount on the horizontal axis. This EDF absorption amount indicates the amount absorbed when weak light having a wavelength of 1550 nm is input to the EDF, and is a value determined by the design of the erbium ion doping concentration in the EDF and the length of the EDF. . The vertical axis shows the values calculated for the characteristics assuming that the wavelength of the excitation light input to the EDF is 1460 nm and the power is 250 mW.

The “EDFA housing gain (1)” is a gain by which the signal light is amplified in the optical repeater 2 incorporating the EDF. It can be seen that a higher value can be obtained as the EDF absorption amount is designed to be larger.
On the other hand, the “housing output pumping light power”, which is the power of the pumping light output from the optical repeater 2 and reaching the optical fibers 11a and 11b, becomes larger as the EDF absorption amount is smaller. “Raman gain (2)”, which is the gain of Raman amplification obtained by the fibers 11a and 11b, also increases.
“Repeater Equivalent NF”, which is obtained by adding the noise characteristic improvement effect by Raman amplification to the noise figure (NF) of the optical repeater 2, becomes smaller corresponding to the increase of “Raman gain”, and the noise characteristic is improved. I understand that.

“Span gain (1) + (2)”, which is the sum of “EDFA housing gain (1)” and “Raman gain (2)”, is the signal light obtained by the optical repeater 2 of the first embodiment. Although the total gain is shown, the larger the EDF absorption amount, the larger the gain, and the gain can be obtained efficiently. Conversely, the smaller the EDF absorption amount, the smaller the “repeater equivalent NF”, indicating that the effect of improving noise characteristics by Raman amplification is higher.
Further, by increasing the output of the pumping light source, the “housing output pumping light power” can be increased, and the “repeater equivalent NF” can be decreased. Although it is difficult to improve the noise characteristics by optical amplification using only the EDF, the optical repeater 2 of the first embodiment is designed to have an appropriate EDF absorption amount and pumping light power in this manner, thereby reducing the noise characteristics. Reduction design is possible.

Next, the fault tolerance of the optical repeater 2 will be described.
A case is assumed in which the power of the signal light is reduced due to a failure such as deterioration or repair of the optical fiber that transmits the signal light or the optical repeater 2 connected to the optical fiber.
The graph of FIG. 6 is a calculated value indicating the amount of change in the wavelength characteristic of the gain when the power of the signal light input to the optical fibers 11a and 11b decreases for some reason.
When signal light multiplexed with equal power in the wavelength range of 1540 to 1560 nm is input with normal power, all signal lights are designed to obtain equal gain by appropriate gain equalizers 18a and 18b. I am thinking.

The output power of the excitation light source 13 is constant regardless of the input power of the signal light. When the power of the input signal light is decreased by 3, 6, 10 dB, the gain received by the signal light of each wavelength is plotted. Here, the vertical axis of the graph is based on the gain of the wavelength of 1550 nm. Relative value.
As the power of the signal light is increased to 3, 6, and 10 dB, the amount of fluctuation in the gain increases, and gain tilts of −0.025, −0.044, and −0.064 dB / nm are generated. I understand that.

On the other hand, the graph of FIG. 7 shows a characteristic example in the case where the optical repeater 2 is configured only by EDF without using Raman amplification with respect to the power reduction of the signal light.
The gain slopes of −0.05, −0.09, and −0.13 dB / nm are generated with respect to the decrease amounts of 3, 6, and 10 dB, which are the same as those in the first embodiment. The amount is about twice as large. As described above, according to the first embodiment, an effect of suppressing the magnitude of the gain tilt generated when the power of the signal light is reduced due to the failure can be obtained.

Next, in the first embodiment, the principle that the occurrence of the gain tilt is suppressed to be described.
When the power of the signal light decreases, the pumping light consumed for amplification of the signal light decreases in the EDFs 17a and 17b, and the pumping light output to the optical fibers 11a and 11b increases. As a result, the gain of Raman amplification is increased and the amount of power reduction of the signal light input to the EDFs 17a and 17b is suppressed, so that the occurrence of gain tilt can be suppressed to a small level.
As shown in FIG. 7, the occurrence of gain tilt in the EDFs 17a and 17b in accordance with the power reduction amount of the signal light is an essential characteristic of the EDF. As described above, the effect of suppressing the occurrence of the gain tilt is obtained by the configuration of the first embodiment in which the Raman amplification is performed using the pumping light transmitted through the EDFs 17a and 17b propagating in the opposite direction to the signal light.

Further, in the first embodiment, the Raman amplification gain has a higher characteristic as the wavelength is longer as shown in FIG. 4, but this tendency is strong due to the increase of the pumping light output to the optical fibers 11a and 11b. Become.
For example, FIG. 8 shows the Raman amplification gain obtained when the pumping light wavelength reaches 1460 nm and the power of the pumping light reaching the optical fibers 11a and 11b is 100, 150, and 200 mW.
The gain increases as the pumping light power increases from 100 mW to 150,200 mW, and the gain slope increases from 0.11 dB / nm to 0.16, 0.22 db / nm. This tendency is in the direction of canceling the gain gradient generated in the EDF as shown in FIG. That is, when the power of the signal light decreases, the power of the pumping light reaching the optical fibers 11a and 11b increases, so that the gain tilt of the gain of Raman amplification has an effect of compensating for the gain tilt generated in the EDFs 17a and 17b. Therefore, the amount of gain tilt generated as a whole of the optical repeater can be reduced. Here, the case where the excitation wavelength is 1460 nm has been described as an example, but the same effect can be obtained if the wavelength of the excitation light is in the range of 1450 to 1470 nm.

  The gain gradient generated in the wavelength division multiplex repeater system results in variations in power between the signal lights, and for some signal lights, deviating from the normal power leads to deterioration of transmission characteristics. At the time of system design, it is necessary to secure a margin that anticipates degradation that occurs during operation due to an increase in optical fiber loss, etc., but when using the optical repeater 2 of the first embodiment with a small amount of gain tilt generation, Therefore, a design with a small margin and a lean system performance is possible.

Next, the failure of the excitation light source 13 will be described.
Since the pumping light source 13 is a pumping light source shared with the Raman amplification and the EDFs 17a and 17b, even when the output of some of the semiconductor lasers is reduced, the influence can be suppressed to a small level. This configuration provides high reliability without increasing the number of pumping light sources as compared with a conventional optical repeater.

Next, the excitation light bypass isolators 23a and 23b will be described.
The pumping light bypass isolators 23a and 23b are optical isolators that bypass the pumping light and prevent the backflow of only the signal light, but do not prevent the pumping light from being output from the EDFs 17a and 17b and reaching the optical fibers 11a and 11b. Therefore, it has a function of bypassing the excitation light.
For example, when the pumping light bypass isolators 23a and 23b are configured as shown in FIG. 9, the path 36 → 34 through which the pumping light bypasses the isolator body 33 by the wavelength demultiplexer 32 and the wavelength multiplexer 31 that separate the pumping light wavelengths. → Go through 35. The signal light in the forward direction passes from the path 35 through the isolator body 33 to the path 36, but the signal light that flows backward enters the isolator body 33 from the path 36 and is blocked.

The pumping light bypass isolators 23a and 23b are required because the signal light slightly reflected at the input ends of the circulators 16a and 16b is amplified by the EDFs 17a and 17b, reflected by the optical fibers 11a and 11b, and then again the EDF 17a. , 17b is in an oscillation state and the amplification characteristics become unstable.
In the first embodiment in which Raman amplification is performed using the optical fibers 11a and 11b, the power of the reflected light from the optical fibers 11a and 11b becomes relatively strong due to the Raman amplification. Insertion of the optical bypass isolators 23a and 23b is effective.

As is apparent from the above, according to the first embodiment, the EDFs 17a and 17b optically amplify the C-band signal light using the pumping light and transmit a part of the pumping light so as to transmit the optical fiber. Since the C-band signal light is Raman-amplified in 11a and 11b, the amplification characteristic of the C-band signal light can be improved, and the occurrence of a gain tilt can be suppressed against a decrease in signal light power. The effect which can be done is produced.
That is, the optical repeater 2 according to the first embodiment has a characteristic that combines the high efficiency and fault tolerance of the EDFs 17a and 17b with the low noise characteristic of Raman amplification, and has the effect of extending the repeat interval. In addition, since it has the characteristic of suppressing the occurrence of gain tilt with respect to the decrease in power of signal light, a system with excellent reliability can be realized.

As the EDFs 17a and 17b, the effect of the first embodiment can be obtained even if an EDF in which Al (aluminum), P (phosphorus) or the like is doped with erbium to improve the gain wavelength characteristics and amplification efficiency is used. It is clear that is obtained.
Although the case where only one wavelength in the range of 1450 to 1470 nm is used as the wavelength of the excitation light has been described above, the effect of the first embodiment can be obtained even when a plurality of wavelengths are used as the excitation light. It is clear.

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram which shows the optical repeater by Embodiment 1 of this invention. 1 is a system configuration diagram showing an optical repeater transmission system according to Embodiment 1 of the present invention; FIG. It is a graph which shows the example of a characteristic of EDF. It is a graph which shows an example of the Raman amplification gain with respect to signal light. It is a graph which shows the relationship between the absorption amount of EDF, and a gain. It is a graph which shows the variation | change_quantity of the wavelength characteristic of the gain in Embodiment 1 of this invention. It is a graph which shows the variation | change_quantity of the wavelength characteristic of a gain in the case of only EDF. It is a graph which shows the gain of Raman amplification. It is a block diagram which shows an excitation light bypass isolator.

Explanation of symbols

  1 optical transmitter, 2 optical repeater, 3 optical receiver, 11a, 11b optical fiber, 12a, 12b optical fiber, 13 excitation light source, 14a, 14b polarization beam combiner (excitation light introducing means), 15 3 dB coupler (excitation) Light introduction means), 16a, 16b circulator (excitation light introduction means), 17a, 17b EDF (rare earth doped optical fiber), 18a, 18b gain equalizer, 19a, 19b monitor coupler, 20a, 20b excitation light removal filter, 21a , 21b Monitor PD, 22 OTDR path, 23a, 23b Excitation light bypass optical isolator, 24a, 24b Monitor coupler, 25a, 25b Excitation light removal filter, 26a, 26b Monitor PD, 27 Excitation light path, 31 Wavelength multiplexer, 32 Wavelength demultiplexer, 33 isolator body, 34 to 36 paths.

Claims (6)

  An excitation light source that oscillates the excitation light, and the excitation light transmitted so that the transmission direction of the excitation light oscillated by the excitation light source is opposite to the transmission direction of the C-band signal light transmitted through the optical fiber. Excitation light introduction means for introducing into the optical fiber, and optical amplification of the C-band signal light using the excitation light inserted in the middle of the optical fiber and introduced by the excitation light introduction means. An optical repeater comprising: a rare earth-doped optical fiber that transmits a part of light and gives the optical fiber to the optical fiber.   2. The optical repeater according to claim 1, wherein the pumping light source oscillates pumping light having a wavelength of 1.4 [mu] m.   2. The optical repeater according to claim 1, wherein the pumping light source oscillates pumping light having a wavelength of 1450 to 1470 nm and power of pumping light transmitted through the rare earth-doped optical fiber of 50 mW or more.   4. The optical repeater according to claim 3, wherein the power of the signal light optically amplified by the rare earth doped optical fiber is 21 dBm or less.   An optical isolator that blocks the signal light reflected by the pumping light introducing means and flowing backward through the rare earth-doped optical fiber while giving the pumping light transmitted through the rare earth doped optical fiber to the optical fiber is provided as a signal of the rare earth doped optical fiber. 5. The optical repeater according to claim 1, wherein the optical repeater is provided on an optical input side.   An optical transmitter that converts an electrical signal into C-band signal light and transmits it, and is connected to the optical transmitter via an optical fiber. When the signal light transmitted from the optical transmitter is received from the optical fiber, An optical repeater that amplifies and transmits signal light, and the optical repeater connected to the optical repeater via an optical fiber. When the signal light transmitted from the optical repeater is received from the optical fiber, the signal light is converted into an electrical signal. In an optical repeater transmission system including an optical receiver for conversion, the optical repeater transmits an excitation light source that oscillates excitation light and a transmission direction of the excitation light oscillated by the excitation light source through an optical fiber. Pumping light introducing means for introducing the pumping light into the optical fiber so as to be opposite to the transmission direction of the C band signal light, and being inserted in the middle of the optical fiber and introduced by the pumping light introducing means Excited With optically amplify signal light in C band by utilizing, by transmitting part of the excitation light, an optical relay transmission system characterized by comprising a rare earth-doped optical fiber to be supplied to the optical fiber.

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