JP2003057692A - Raman amplifier, method for controlling raman amplifier and optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Raman amplifier, etc., provided with a structure that keeps the flatness of power spectrum of Raman-amplified signal light that is subjected to Raman amplification. SOLUTION: This Raman amplifier comprises an optical fiber (11) for Raman- amplifying a plurality of signal channels of signal light having respective center optical frequencies different from each other, a pumping light supplying part (12) for supplying N (N being an integer of 2 or more) pumping channels of pumping light having respective center optical frequencies different from each other to the optical fiber, and feedbacking part (13) for dividing detected Raman- amplified signal light into N optical frequency ranges defined so as to include one Raman amplification peak as an optical frequency lower than respective center optical frequencies of the pumping channels by an optical frequency shift of about 15 THz, and controlling the pumping light supplying part such that the Raman-amplified signal light has a power fluctuation of 2 dB or less in each of thus divided N optional frequency ranges.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Raman amplifier which Raman-amplifies signal lights of a plurality of signal channels by utilizing Raman scattering effect, a control method of the Raman amplifier, and an optical communication system including the Raman amplifier. is there.

[0002]

2. Description of the Related Art Wavelength division multiplexing (WDM) of signal light is used for the purpose of improving the information transmission capability of an optical communication system.
on Multiplexing) transmission has spread widely, and in such an optical communication system, an optical amplifier that directly amplifies the signal light is used during transmission. Here, it is important that the optical amplifier obtains amplification in a wide wavelength range (optical frequency range) and that the power spectrum of the amplified signal light in the wavelength range (optical frequency range) becomes flat. As the optical amplifier, an optical fiber amplifier such as a rare earth element-doped optical fiber amplifier or a Raman amplifier is generally used.

A rare earth element-doped optical fiber amplifier, which is one type of optical fiber amplifier, is generally an erbium element-doped optical fiber amplifier in which, for example, erbium element is added. In the erbium element-doped optical fiber amplifier, the wavelength band of 980 nm (converted to optical frequency is 30
By introducing pumping light having a pumping channel such as a 6.1 THz band) or a 1480 nm wavelength band (corresponding to an optical frequency of 202.7 THz band) into an erbium-doped optical fiber. Inversion distribution is generated. When the signal light in the 1550 nm wavelength band (corresponding to the optical frequency of 193.5 THz band) is incident on the erbium-doped optical fiber in which the population inversion state is generated, the signal light is amplified. The amplification band of such a rare earth element-doped optical fiber amplifier is determined by the type of rare earth element added to the optical fiber. For example, in the case of an erbium-doped optical fiber amplifier, the wavelength range in which the power variation (flatness of power spectrum) of the amplified signal light is within 1 dB is 1540 nm.
(194.8 THz) to 1560 nm (192.3 TH
z) and by applying an equalization filter or the like for correcting the flatness of the power spectrum, the wavelength range is 1530 nm (196.1 THz) to 1560 nm (1
92.3 THz).

However, in response to the recent demand for improved information transmission capability of optical communication systems, high-density multiplexing of signal light in wavelength division multiplexing transmission for the purpose of improving the capability is approaching its limit. . Therefore, in order to further improve the information transmission capability of the optical communication system, it has become necessary to expand the amplification wavelength band (optical frequency band) in which the power spectrum of the amplified signal light becomes flat.

A Raman amplifier, which is the other type of optical fiber amplifier, constitutes at least a part of a transmission path through which signal light propagates, and is an optical fiber that Raman-amplifies the signal light.
For example, 1400 nm wavelength band (optical frequency 214.3 THz
By supplying pumping light of a plurality of pumping channels each having a central wavelength of (corresponding to a band), a wavelength band (light) which is 100 nm longer wavelength side (13 THz low optical frequency side) than the wavelength (optical frequency) of the pumping light. The Raman scattering effect is used to obtain an amplification peak in the frequency band.

Raman amplifiers are commonly used in erbium-doped optical fiber amplifiers, for example, as pumping light sources.
When an excitation light source in the 400 nm wavelength band (corresponding to the optical frequency 214.3 THz band) is used, signal light in the 1500 nm wavelength band (corresponding to the optical frequency 200.0 THz band) can be Raman-amplified. Then, this Raman amplifier has a wavelength band (optical frequency band) in which the power spectrum of the Raman amplified signal light after Raman amplification becomes flat, if pumping light of a plurality of pumping channels of different wavelengths (optical frequencies) is appropriately used, It can be greatly expanded compared to erbium-doped optical fiber amplifiers. Japanese Patent Laid-Open No. 2000-984
No. 33 publication discloses a pumping light generator for generating pumping light of pumping channels of different wavelengths (optical frequencies), a WDM coupler for multiplexing pumping light of each pumping channel, and output light of an optical amplifier (Raman). The optical output power control unit is provided for detecting the amplified signal light and controlling the pumping light generation unit based on the result. In order to reduce the wavelength dependence of the amplification to the extent that the amplification output equalization filter is not required, the interval between the central wavelengths of the excitation light is set to 6 nm or more and 35 nm or less.

[0007]

DISCLOSURE OF THE INVENTION As a result of detailed examination of the conventional Raman amplifier, the inventors have found the following problems. That is, in order to flatten the wavelength dependence of the output (optical power) of the optical amplifier, the conventional Raman amplifier uses about 100 nm at the center wavelength of each pumping channel of pumping light.
The output light of the added wavelength is monitored, and the power of the pumping light of each pumping channel is controlled so that the power of the output light is made uniform. However, when the signal light is multiplexed in the wavelength direction (optical frequency direction), it is Raman-amplified multiplexed signal light due to interference during Raman amplification between signal channels having different center wavelengths (center optical frequencies). It was insufficient for flattening the power spectrum of the Raman amplified signal light. Therefore, it has been difficult to cope with the improvement of the transmission capacity of the optical communication system.

The present invention has been made to solve the above-mentioned problems, and even in the case of Raman-amplifying multiplexed signal lights including a plurality of signal channels having different central optical frequencies, they are mutually amplified. Raman amplifier in which the power spectrum of Raman-amplified signal light Raman-amplified by pumping light including a plurality of pumping channels having different center optical frequencies is flat in the wavelength direction (optical frequency direction), a control method of the Raman amplifier, and An object is to provide an optical communication system including this Raman amplifier.

In this specification, the optical frequency is used in place of the wavelength of light. The conversion formula for the wavelength of light and the optical frequency is νλ = c. Where ν is the optical frequency (Hz), λ is the wavelength (m), and c is the speed of light, which is 3 × 1.
It becomes 0 ⁸ (m / s).

[0010]

In order to achieve the above object, a Raman amplifier according to the present invention comprises an optical fiber for Raman amplification, a pumping light supply section, and a feedback section. The Raman amplification optical fiber includes an optical fiber for Raman amplifying signal lights of a plurality of signal channels having different center optical frequencies. The pumping light supply section supplies pumping lights of N (integer of 2 or more) pumping channels having different center optical frequencies to the optical fiber. The feedback unit detects a part of the Raman amplified signal light that has been Raman-amplified in the optical fiber by supplying the pumping light, and the power spectrum of the Raman amplified signal light based on the detection result is the optical frequency. The excitation light supply unit is controlled so as to be substantially flat with respect to the direction.

In particular, in order to realize the flattening of the power spectrum of the Raman amplified signal light, the above-mentioned feedback unit outputs the detected Raman amplified signal light from the center optical frequency of each pumping channel of the pumping light from 13.5-15.7 THz. Is divided into N optical frequency ranges defined so as to include one Raman amplification peak having an optical frequency smaller by the optical frequency shift of, and the power of the Raman amplified signal light in each of the N divided optical frequency ranges. The excitation light supply unit is controlled so that the variation is 2 dB or less, preferably 1 dB or less. In this specification, the power variation of the Raman amplified signal light is given by the difference between the maximum power and the minimum power of the Raman amplified signal light in the Raman amplification band, and means the flatness of the power spectrum of the Raman amplified signal light.

The feedback section divides the detected Raman amplified signal light into the same number of optical frequency ranges as the number of pumping channels of the pumping light, and averages the power of the Raman amplified signal light contained in each of the divided optical frequency ranges. Value variation is 2
The pumping light supply unit may be controlled to be not more than dB, preferably not more than 1 dB.

According to the Raman amplifier, even if the signal light including a plurality of signal channels multiplexed in the optical frequency direction is Raman amplified, the power spectrum of the Raman amplified signal light can be easily flattened. it can.

The division of the N optical frequency ranges by the feedback section can be set as follows, separately for the case where the integer N = 2 and when the integer N is 3 or more.

That is, when the integer N = 2, the first optical frequency range among the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the N excitation channels. Of the intermediate optical frequency between the larger first central optical frequency and the smaller second central optical frequency, the lower limit is an optical frequency about 15 THz (optical frequency shift) smaller, while the first central optical frequency and the intermediate optical frequency are intermediate. The upper limit is set to an optical frequency that is about 15 THz lower than the optical frequency that is higher than the first central optical frequency by the difference from the frequency. Further, the second optical frequency range among the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the intermediate optical frequency between the first central optical frequency and the second central optical frequency. An optical frequency lower by about 15 THz is set as an upper limit, while an optical frequency smaller than the second central optical frequency by the difference between the second central optical frequency and the intermediate frequency is set as a lower limit by an optical frequency lower by about 15 THz. Preferably.

Further, the above-mentioned integer N is 3 or more, and the pumping channel having the largest center optical frequency among the N pumping channels in the pumping light is defined as the first pumping channel, and the nth ( (Integer of 2 or more) When the pumping channel having the center optical frequency is the nth pumping channel and the pumping channel having the smallest center optical frequency is the Nth pumping channel, the feedback unit detects the Raman amplified signal light. The first, the n-th (integer of 2 or more (N-1) or less), and the N-th optical frequency ranges of the N optical frequency ranges divided into are respectively set to the following ranges. Is preferred. That is, the first optical frequency range has a lower limit of an optical frequency that is about 15 THz (optical frequency shift) smaller than the intermediate optical frequency between the center optical frequency of the first excitation channel and the center optical frequency of the second excitation channel, The upper limit of the optical frequency is about 15 THz lower than the optical frequency higher than the central optical frequency of the first excitation channel by the difference between the central optical frequency of the first excitation channel and the intermediate frequency. The nth optical frequency range is about 15 from the intermediate optical frequency between the central optical frequency of the nth excitation channel and the central optical frequency of the (n + 1) th excitation channel.
The lower limit is an optical frequency lower than THz, and the upper limit is an optical frequency lower than about 15 THz from an intermediate optical frequency between the center optical frequency of the nth excitation channel and the center optical frequency of the (n-1) th excitation channel. To be done. The upper limit of the Nth optical frequency range is an optical frequency that is about 15 THz smaller than the intermediate optical frequency between the central optical frequency of the Nth excitation channel and the central optical frequency of the (N-1) th excitation channel. The lower limit is set to an optical frequency lower than the central optical frequency of the Nth excitation channel by a difference between the central optical frequency of the Nth excitation channel and the intermediate frequency, and an optical frequency lower than about 15 THz.

As described above, since the N optical frequency ranges are set as the optical frequency ranges including the Raman amplification peaks corresponding to the respective pumping channels, the central optical frequency of each pumping channel and the number of pumping channels vary. Also, the power spectrum flatness of the Raman amplified signal light is stably controlled.
Specifically, the case where the center optical frequency of each pumping channel fluctuates includes the case where the pumping light power of one of the pumping channels decreases. In this case, it is preferable that the feedback unit includes a light detection unit that monitors the excitation light and a control unit that controls the excitation light source that supplies the excitation light. The photodetector monitors the power of the Raman amplified signal light and the pump light power at each center optical frequency of the pump channel. When the Raman amplified signal light is divided into the same number of optical frequency ranges as the excitation channels, the control section determines the optical frequency indicating the boundary of each optical frequency range based on the detection result of the light detection section.
In particular, when the photodetector detects the occurrence of an excitation channel with a weak power to the extent that the photodetector does not effectively contribute to Raman amplification, it uses the remaining excitation channels excluding the excitation channel with the weak power. Then, the optical frequency indicating the boundary of each optical frequency range is changed. On the other hand, when an increase in the number of pumping channels is detected, that is, when the photodetector detects the occurrence of a pumping channel having a powerful power that can effectively contribute to Raman amplification, the control unit Utilizing all excitation channels, including the excitation channel of power,
The optical frequency indicating the boundary of each optical frequency range is changed. It is preferable that the photodetection unit includes an optical performance monitor that detects multiplexed monitor light in which the Raman amplified signal light of a plurality of signal channels and the pump light of a plurality of pump channels are multiplexed.

In the Raman amplifier according to the present invention, the interval between the center optical frequencies of the respective pump channels in the pump light is
It is preferably 4 THz or less. Since the Raman amplification bands by the pumping lights of the N pumping channels having different center optical frequencies are superimposed close to each other in the optical frequency direction, the power spectrum flatness of the Raman amplified signal light can be improved.

The feedback unit receives a part of the Raman amplified signal light and outputs an electric signal according to the power of the Raman amplified signal light, and a pump according to the electric signal output from the photodetector. And a control unit that controls the light supply unit. At this time, it is preferable that the light detection unit includes an optical performance monitor. This allows
The photodetector has high accuracy in receiving the Raman amplified signal light, can control the power spectrum flatness of the Raman amplified signal light with high accuracy, and can improve the power spectrum flatness of the Raman amplified signal light.

The photodetection section includes a demultiplexing section for demultiplexing the Raman amplified signal light into N optical frequency ranges, and a light receiving section for receiving the Raman amplified signal light demultiplexed by the demultiplexing section. You may prepare. At this time, it is preferable that the demultiplexing unit includes either an optical circuit having a dielectric optical filter or an optical circuit having an optical circulator and a Bragg diffraction grating. By configuring the light receiving unit with an optical filter made of a dielectric material, demultiplexing of Raman amplified signal light into N different optical frequency ranges can be realized at low cost. Also, by configuring the optical circuit of the light receiving section with an optical circulator and a Bragg diffraction grating, demultiplexing of Raman amplified signal light into N different optical frequency ranges can be realized at low cost, and further the light receiving accuracy can be improved. You can

The control method of the Raman amplifier according to the present invention flattens the power spectrum of the Raman amplified signal light in the optical frequency direction for Raman amplification having the above-mentioned structure. Specifically, the control method supplies pumping light to an optical fiber for Raman amplification, detects a part of Raman-amplified signal light (Raman-amplified signal light), and detects the power spectrum of the detected Raman-amplified signal light. Is flat in the optical frequency direction.

The pumping light includes N (integer of 2 or more) channels having different center optical frequencies. The Raman amplified signal light is flattened by detecting the detected Raman amplified signal light from the center optical frequency of each channel of the pumping light by 13.5 to 15.
By dividing into N optical frequency ranges defined so as to include one Raman amplification peak having an optical frequency smaller by the optical frequency shift of 7 THz, and controlling the pumping light supply unit, the divided N optical frequency ranges are obtained. The power variation of the Raman amplified signal light in each optical frequency range of 2 dB is suppressed to 2 dB or less, preferably 1 dB or less.

In the control method according to the present invention,
The flattening of the Raman amplified signal light is performed by dividing the detected Raman amplified signal light into the same number of optical frequency ranges as the number of pumping channels of the pumping light, and controlling the pumping light supply unit to control the divided optical frequency. The fluctuation of the average power value of the Raman amplified signal light included in each range is suppressed to 2 dB or less, preferably 1 dB or less.

According to the control method, the Raman amplified signal light is divided into a predetermined optical frequency range, and the signal light power variation between the divided optical frequency ranges is minimized. Even when Raman amplification is performed on the signal light multiplexed in the frequency direction, the power spectrum of the Raman amplified signal light can be easily flattened.

The division of the N optical frequency ranges is the integer N
= 2 and the integer N is 3 or more, and may be set as described above separately. The N optical frequency ranges are N
One Raman amplification peak having an optical frequency smaller by about 15 THz (optical frequency shift) from the central optical frequency of each of the pumping channels having different central optical frequencies is included. Further, each Raman amplification peak is located near the middle of the pre-divided optical frequency range. Therefore, the power spectrum flatness of the Raman amplified signal light can be stably controlled even if the center optical frequency of each pumping channel or the number of pumping channels changes. Specifically, the case where the center optical frequency of each pumping channel fluctuates includes the case where the pumping light power of one of the pumping channels decreases. In this case, in the control method, when the Raman amplified signal light is divided into the same number of optical frequency ranges as the excitation channels, the optical frequency range is determined based on the detection result of the excitation light power at each central optical frequency of the excitation channels. The optical frequency indicating each boundary is determined. In particular, when the occurrence of an excitation channel with a weak power that does not contribute effectively to Raman amplification is detected in the excitation channel,
It is preferable that the optical frequencies at the boundaries of the optical frequency ranges are changed using the remaining excitation channels except the weak power excitation channel. On the other hand, when an increase in the number of pumping channels is detected, that is, when the occurrence of a pumping channel having a powerful power enough to effectively contribute to Raman amplification is detected, all pumping including the pumping channel having the strong power is detected. The channels are preferably used to change the optical frequencies that mark the boundaries of each optical frequency range.

The optical communication system according to the present invention is an optical communication system for transmitting signal light to perform optical communication, and has a Raman structure having the above-described structure for amplifying signal light in each relay section. It includes one or more Raman amplifiers with a structure similar to the amplifier (Raman amplifier according to the invention).

According to the optical communication system, since the power spectrum of the Raman amplified signal light at the time of relay becomes flat in the optical frequency direction, malfunction can be effectively suppressed while enhancing the information transmission capability of the optical communication system. it can. In other words, it is possible to realize an optical communication system with stable communication operation while enhancing the information transmission capability.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a Raman amplifier and the like according to the present invention will be described below with reference to FIGS. In the description of the drawings, the same elements will be denoted by the same reference symbols and redundant description will be omitted.

First, the background of the completion of the present invention will be described. In FIG. 1, the center optical frequencies of the unmultiplexed signal light and the multiplexed signal light are 20
Raman amplification spectrum (power spectrum) when Raman amplification is performed with pumping light including a 1.3 THz pumping channel
Is.

The spectrum A is a Raman amplification spectrum when the signal light propagating through the optical fiber is not multiplexed in the optical frequency direction. The optical frequency at which the spectrum A has the maximum amplification is 188.3 THz, which is 13 THz lower than the central optical frequency of the excitation light.

On the other hand, in the spectrum B, 40 signal channels are 186.3 TH as signal light propagating in the optical fiber.
It is a Raman amplification spectrum at the time of Raman-amplifying the signal light multiplexed in the optical frequency direction in the range from z to 195.8 THz. The optical frequency at which the spectrum B has the maximum amplification is 186.3 THz, which is different from the case of signal light that is not multiplexed in the optical frequency direction. This optical frequency is 15 THz less than the center optical frequency of the excitation channel.

Further, FIGS. 2 to 5 show that as signal light propagating in the optical fiber, 40 signal channels are 186.3 TH.
Raman amplification spectrum in the case of Raman amplification of signal light multiplexed in the optical frequency direction in the range from z to 195.8 THz by supplying pumping light of a plurality of pumping channels having different central optical frequencies to the optical fiber. is there.

In FIG. 2, the center optical frequencies are 205.
It is a Raman amplification spectrum when using the excitation light of three excitation channels which are 5 THz, 208.3 THz, and 210.5 THz, and spectra a to c have center excitation frequencies of 205.5 THz and 208.3, respectively, in this order.
It is a Raman amplification spectrum with respect to the excitation channel of THz and 210.5 THz. Further, the spectrum C is a superimposed spectrum obtained by superimposing the spectra a to c. In FIG. 2, the optical frequency at which the maximum amplification is obtained for each of the spectra a to c is 15 THz smaller than the center optical frequency of each excitation channel. Also in the spectrum C, Raman amplification peaks C1 to C3 exist at optical frequencies 15 THz lower than the central optical frequencies of the three excitation channels.

Similarly, in FIG. 3, the center optical frequencies are 201.3 THz, 205.5 THz, and 208.3 TH, respectively.
z, 210.5 THz, where four pumping channels of pumping light are used, FIG. 4 shows that the center light frequencies are 201.3 THz, 205.5 THz, and 206.9 THz, respectively.
z, 209.1 THz, 210.5 THz, the case of using pumping light of five pumping channels, FIG. 5 shows that the center optical frequencies are 201.3 THz and 205.5 TH, respectively.
z, 206.9 THz, 209.1 THz, 210.5
It is Raman amplification spectrum af with respect to each pumping channel, and the superposition Raman amplification spectrum DF obtained by superposing these spectrums af in the case where the pumping light of 6 pumping channels which is THz is utilized. Similar to FIG. 2, in FIGS. 3 to 5 as well, the optical frequencies at which the maximum amplification is obtained for the spectra a to f are
It is about 15 THz smaller than the center optical frequency of each excitation channel. Also in the superposed spectra D to F, Raman amplification peaks D1 to D4 and E1 to E are obtained at optical frequencies 15 THz lower than the central optical frequencies of the respective excitation channels.
5, F1 to F6 are present.

As can be seen from FIGS. 1 to 5, in Raman amplification of signal light of a plurality of signal channels multiplexed in the optical frequency direction, maximum Raman amplification is obtained due to interference between signal channels having different central optical frequencies. The optical frequency is about 15 THz lower than the central optical frequency of the excitation channel. Furthermore, even in the superimposed Raman amplification spectrum obtained by superimposing Raman amplification spectra for a plurality of pumping channels having different center optical frequencies, the Raman amplification peak is obtained at an optical frequency that is about 15 THz smaller than the center optical frequency of each of the plurality of pumping channels. Exists.

When Raman amplification is performed on the signal lights of a plurality of signal channels multiplexed in the optical frequency direction by inputting pump lights of a plurality of pump channels having different central optical frequencies into an optical fiber for Raman amplification, Raman amplified signal lights. It can be understood that the following control can be performed in order to flatten the power spectrum of 1 in the optical frequency direction. That is, as shown in FIG. 6, each pumping channel (the center optical frequency interval of each pumping channel is preferably 4 THz or less).
The Raman amplification peak (which exists in the optical frequency smaller than the optical frequency shift of about 15 THz with respect to the center optical frequency of each pumping channel) in the superimposed Raman amplification spectrum by superimposing the Raman amplification spectrum for The range is divided, and the Raman amplification peak included in each optical frequency range is set so that the power variation of the Raman amplified signal light in the divided optical frequency range is set to a predetermined range, for example, 2 dB or less, preferably 1 dB or less. Controlling the output of the excitation light. As a result, the power spectrum flattening of the Raman amplified signal light is realized in a wide optical frequency band in which Raman amplification is performed by the pumping light of the multiple pumping channels. The present invention was made based on the findings of the inventors as described above.

Next, a Raman amplifier and a method for controlling the Raman amplifier according to the present invention will be described. FIG. 7 is a diagram showing the configuration of the first embodiment of the Raman amplifier according to the present invention. In FIG. 7, the Raman amplifier 10 includes an optical fiber 11 that Raman-amplifies the propagating signal light, a pumping light supply unit 12 that supplies pumping light to the optical fiber 11, and a Raman-amplified signal light (Raman amplified signal). Light),
A feedback unit 13 that controls the pumping light supply unit 12 is provided so as to flatten the power spectrum of the Raman amplified signal light in the optical frequency direction based on the detection result. The signal light is multiplexed signal light including 40 signal channels including a plurality of signal channels having different central optical frequencies, and the central optical frequencies are optical frequencies at 100 GHz intervals, for example.

The optical fiber 11 has an input end 11a and an output end 11b, and the signal light is transmitted from the input end 11a to the output end 11b.
While propagating to b, the signal light is Raman-amplified by the pump light supplied from the pump light supply unit 12.

The pumping light supply unit 12 multiplexes the pumping light source 12a for outputting the pumping light, the multiplexer / demultiplexer 12b for guiding the pumping light to the optical fiber 11, and the pumping light output from the pumping light source 12a. An optical fiber 12c that leads to the container 12b is provided.

The number of pumping light sources 12a is N (N is an integer and N ≧
2) Pumping lights (pumping channels) having different central light frequencies are output. The pumping light source 12a may include N light emitting elements that output pumping light having different central light frequencies, and an optical fiber and an optical multiplexer for multiplexing the light emitted from the light emitting elements into the optical fiber 11. . in this case,
Each of the N light emitting elements that output pumping light having different center optical frequencies may be, for example, a semiconductor light emitting element in which a light reflecting surface and a light emitting surface that face each other across an active region are formed. A drive signal is supplied from the outside of the excitation light source 12a for each light emitting element. Then, by appropriately changing the driving signal supplied to each light emitting element, the output power of the excitation light of each light emitting element is adjusted. Here, the pumping channel included in the pumping light is the n-th pumping channel (n is an integer of 1 or more and N or less) in order from the channel having a large center optical frequency.

The optical multiplexer / demultiplexer 12b is arranged between the input end 11a and the output end 11b of the optical fiber 11 and has the input end 1a.
The signal light propagating from the 1a side is passed to the output end 11b side, and at the same time from the pumping light source 12a to the optical fiber 12c.
The excitation light that has reached via the optical fiber is guided to the optical fiber 11. The pumping light guided into the optical fiber 11 propagates through the optical fiber 11 from the optical multiplexer / demultiplexer 12b toward the input end 11a, and contributes to Raman amplification of the signal light propagating from the input end 11a. . The signal light propagating from the input end 11a is Raman-amplified by supplying the pumping light from the pumping light source 12a, and the Raman-amplified signal light propagates toward the optical multiplexer / demultiplexer 12b. The Raman amplified signal light that has passed through the optical multiplexer / demultiplexer 12b further propagates toward the output end 11b.

The feedback unit 13 includes the optical fiber 11
A spectroscope 13a that disperses a part of the Raman-amplified signal light that has been Raman-amplified therein, a photodetector section 14 that detects the Raman-amplified signal light that is spectroscopically separated by the spectroscope 13a, and the photodetector section 14
The control unit 13e is provided for controlling the pumping light source 12a so that the power spectrum of the Raman amplified signal light is flattened in the optical frequency direction based on the result detected in (1). Further, the spectroscope 13a and the photodetection unit 14 are optically connected via the optical fiber 13c, the photodetection unit 14 and the control unit 13e are electrically connected via the electric signal line 13d, and the control unit 1
3e and the pumping light source 12a are electrically connected by an electric signal line 13f.

The spectroscope 13a is disposed between the optical multiplexer / demultiplexer 12b and the output end 11b, passes most of the Raman amplified signal light to the output end 11b side, and partially transmits the Raman amplified signal light to the optical fiber. Lead to 13c. Optical fiber 13c
The optical power of the Raman amplified signal light guided to is about 5% of the optical power of the Raman amplified signal light propagating through the optical fiber 11 from the optical multiplexer / demultiplexer 12b side and reaching the spectroscope 13a.

The photodetector 14 receives the Raman amplified signal light separated by the spectroscope 13a and outputs an electric signal corresponding to the optical power of the Raman amplified signal light to the controller 13e via the electric signal line 13d. To do.

The control section 13e is composed of a semiconductor integrated circuit element having an arithmetic processing function, etc., and flattens the power spectrum of the Raman amplified signal light in the optical frequency direction based on the electric signal supplied from the photodetection section 14. Drive signal level for Then, the control unit 13e supplies the drive signal of the calculated level to the excitation light source 12a via the line 13f.

Here, in the Raman amplifier and the control method of the Raman amplifier, the feedback section 13 makes the Raman amplified signal light have an optical frequency 15 THz smaller than the central optical frequencies of the N channels of which the central optical frequencies are different from each other. It is divided into N optical frequency ranges set so as to include one Raman amplification peak, and the detection values based on the signal lights in these optical frequency ranges are substantially equal to each other, for example, Raman amplification in each optical frequency range. The pumping light supply unit 12 is controlled so that the power variation of the signal light is 2 dB or less, preferably 1 dB or less. More specifically, the photo-detecting unit 14 outputs the signal light for each of N optical frequency ranges set to include one Raman amplification peak corresponding to N pumping channels having different center optical frequencies.
Receives Raman amplified signal light. Further, the light detection unit 14 is
The control unit 13 outputs an electric signal corresponding to the optical power of the received Raman amplified signal light in each of the N optical frequency ranges.
Output to e. The control unit 13e calculates the average value of the electric signals received for each of the N optical frequency ranges. Then, a drive signal level for controlling the pumping light source 12a is calculated so that the average value of the N electrical signals becomes substantially equal, for example, the power variation becomes 2 dB or less, preferably 1 dB or less. The drive signal of the calculated level is supplied from the control unit 13e to the excitation light source 12a by the electric signal line 1
It is supplied via 3f. Therefore, even when the signal light multiplexed in the optical frequency direction is Raman-amplified by the pumping lights of the N pumping channels having different central optical frequencies,
The power spectrum of the Raman amplified signal light can be flattened in the optical frequency direction. The light detection unit 14
Supplies an electric signal to the control unit 13e so that the correlation between the optical power of the received Raman amplified signal light and the optical frequency can be recognized. The control unit 13e sets the Raman amplified signal light to N optical frequency ranges set so as to include one Raman amplification peak seen at an optical frequency smaller by about 15 THz for N pumping channels having different center optical frequencies. You may make it divide for every.

The division of the N optical frequency ranges by the feedback section is set as follows depending on whether the integer N = 2 or when the integer N is 3 or more.

That is, when the integer N = 2, the first optical frequency range among the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the N excitation channels. Of the intermediate optical frequency between the larger first central optical frequency and the smaller second central optical frequency, the lower limit is an optical frequency about 15 THz (optical frequency shift) smaller, while the first central optical frequency and the intermediate optical frequency are intermediate. The upper limit is set to an optical frequency that is about 15 THz lower than the optical frequency that is higher than the first central optical frequency by the difference from the frequency. Further, the second optical frequency range among the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the intermediate optical frequency between the first central optical frequency and the second central optical frequency. An optical frequency lower by about 15 THz is set as an upper limit, while an optical frequency smaller than the second central optical frequency by the difference between the second central optical frequency and the intermediate frequency is set as a lower limit by an optical frequency lower by about 15 THz. To be done.

Further, the above-mentioned integer N is 3 or more, and the pumping channel having the largest center optical frequency among the N pumping channels in the pumping light is defined as the first pumping channel, and the nth ( (Integer of 2 or more) When the pumping channel having the center optical frequency is the nth pumping channel and the pumping channel having the smallest center optical frequency is the Nth pumping channel, the feedback unit detects the Raman amplified signal light. The first, the n-th (integer of 2 or more and (N-1) or less), and the N-th optical frequency ranges of the N optical frequency ranges classified into are respectively set to the following ranges. That is, the optical frequency range of the first surface is about 15 THz from the intermediate optical frequency between the central optical frequency of the first excitation channel and the central optical frequency of the second excitation channel.
(Optical frequency shift) While lower optical frequency is the lower limit,
The upper limit of the optical frequency is about 15 THz lower than the optical frequency higher than the central optical frequency of the first excitation channel by the difference between the central optical frequency of the first excitation channel and the intermediate frequency. The lower limit of the nth optical frequency range is an optical frequency that is smaller than the intermediate optical frequency between the central optical frequency of the nth excitation channel and the central optical frequency of the (n + 1) th excitation channel by about 15 THz, while the nth excitation channel is Is set to a range with an optical frequency smaller by about 15 THz from the intermediate optical frequency between the optical frequency of the center and the optical frequency of the (n-1) th excitation channel as an upper limit. Then, the Nth optical frequency range corresponds to the center optical frequency of the Nth excitation channel and the (N−
1) The upper limit is an optical frequency that is about 15 THz lower than the intermediate optical frequency with respect to the center optical frequency of the excitation channel, while
The lower limit is set to an optical frequency smaller than the central optical frequency of the Nth excitation channel by a difference between the central optical frequency of the excitation channel and the intermediate frequency, and an optical frequency smaller than approximately 15 THz.

As described above, the N optical frequency ranges each include one Raman amplification peak corresponding to each of the N excitation channels having different center optical frequencies, and these Raman amplification peaks are in the middle of each optical frequency range. It is located near and detected. Therefore, the power spectrum flatness of the Raman amplified signal light is stably controlled even if the center optical frequency of each pumping channel fluctuates due to changes in the environmental temperature.

Next, FIGS. 8 to 10 show that in the Raman amplifier 10 shown in FIG.
The signal light of 40 signal channels in the range of 6.3 THz to 195.8 THz has a center optical frequency of 20.
1.3 THz, 204.8 THz, 206.9 THz,
When Raman amplification is performed by pumping light of 5 pumping channels of 208.8 THz and 210.5 THz, 14 TH is obtained for each center optical frequency of the 5 pumping channels.
The pump light source 12a is divided into five optical frequency ranges so that the optical frequencies smaller by z, 15 THz, and 16 THz are near the middle, and the optical power detection result of the Raman amplified signal light in each of the divided optical frequency ranges is detected. It is a Raman amplification spectrum when controlling. The power of the signal light at the input end 11a was 8 dBm / ch, and the power of the Raman amplified signal light at the output end 11b was 0 dBm / ch. The initial values of the pumping light powers at the center optical frequencies of the five pumping channels, which are the backward pumping, are all 10
It is 0 mW. The length from the input end 11a of the optical fiber 11 to the optical multiplexer / demultiplexer 12b is 80 km.

The power variation of each signal channel included in the Raman amplified signal light is due to the optical frequency shift of 14T in FIG.
It was 1.23 dB in the case of Hz, and 2.75 dB in the case of the optical frequency shift of 16 THz in FIG. On the other hand, in the case where the optical frequency shift of FIG. 8 is 15 THz, it was 1.07 dB. From this, it is possible to divide the optical frequency range for Raman amplified signal light detection so that the optical frequency that is smaller than the center optical frequency of each pumping channel by about 15 THz is near the middle. It turns out to be advantageous to be flat with respect to direction. The power variation is given by the difference between the maximum power and the minimum power of the Raman amplified signal light in the Raman amplification band.

The inventors of the present invention have described the optical frequency shift and gain variation (power variation of Raman amplified signal light) as described above.
I examined the relationship. FIG. 11 is a graph showing the relationship between optical frequency shift and gain variation. As can be seen from this graph, in order to suppress the power variation of the Raman amplified signal light to 2 dB or less, the optical frequency shift is 1
It is preferably in the range of 3.5 to 15.7 THz, and in order to further suppress the power variation to 1 dB or less,
The optical frequency shift is preferably in the range 14.3 to 14.7 THz.

The photodetection section 14 may be an optical performance monitor such as an optical spectrum analyzer. As described above, when an optical performance monitor such as an optical spectrum analyzer is applied to the photodetection unit 14, the resolution accuracy of the optical frequency is increased, so that the Raman amplified signal light can be detected with high accuracy. Therefore, the power spectrum flattening of the Raman amplified signal light can be controlled with high accuracy, and the flatness of the power spectrum is improved. In addition, when an optical performance monitor such as an optical spectrum analyzer is applied to the light detection unit 14, the Raman amplified signal light separated by the spectroscope 13a can be detected without further separating or demultiplexing. The power of the Raman amplified signal light split by the spectroscope 13a may be small. In this case, in particular, the Raman amplified signal light can be efficiently taken in from the output end 11b, so that the amplification efficiency of the Raman amplifier can be improved.

The photo-detecting section 14 includes a spectroscopic section for spectroscopically dividing the signal light and a light receiving section for receiving the Raman-amplified signal light that has been spectrally separated and converting it into an electric signal. The spectroscopic unit may include an optical circuit including an optical filter formed of a dielectric material or the like, or an optical circuit including an optical circulator and a Bragg diffraction grating. In this case, a semiconductor light receiving element such as a photodiode can be applied to the light receiving section. Note that FIGS. 12A and 12B are diagrams showing another configuration of the photodetecting section 14. In FIG. 12A, the photodetector unit 1
Reference numeral 4 denotes an optical filter 140 formed of a dielectric material or the like for separating the Raman amplified signal light propagating through the optical fiber 13c into each signal channel, and the signal channel light output from the optical filter 140. A light receiving unit 141 is provided which receives light and outputs an electric signal corresponding to each optical power to the control unit 13e via the electric signal line 13d. On the other hand, in FIG. 12B, the photodetector unit 14 includes a circulator 142 for separating the Raman amplified signal light propagating through the optical fiber 13c into each signal channel, and each branch line separated from the circulator 142. The Bragg grating 143 arranged to pass only the unique optical frequency and the light of the signal channel passing through the Bragg grating 143 are received, and an electric signal corresponding to each optical power is sent to the control unit 13e via the electric signal line 13d. The light receiving unit 141 for outputting is provided.

If the spectroscopic section 13a is an optical circuit provided with an optical filter made of a dielectric material, the optical circuit can be constructed at low cost. On the other hand, when the spectroscopic unit 13a is an optical circuit including an optical circulator and a Bragg diffraction grating,
Compared with the optical circuit including the above optical filter, the sharp optical power cutoff characteristic of the Bragg diffraction grating in the optical frequency direction improves the resolution accuracy of the optical frequency, and the Raman amplified signal light can be detected with high accuracy. In addition, the spectroscopic unit 13a can be constructed at a lower cost than when an optical performance monitor such as the optical spectrum analyzer is used.

Further, it can be seen from FIG. 1 that the optical frequency band in which flat Raman amplification is obtained by the pumping channel having one central optical frequency is 4 THz. Therefore, if the center optical frequency intervals of the N pumping channels are set to 4 THz or less, the amplification bands Raman-amplified by the pumping light of the N pumping channels are superimposed close to each other in the optical frequency direction. The power spectrum flatness of the Raman amplified signal light is improved.

The Raman optical amplifier according to the present invention operates as follows. Excitation light source 12 in the excitation light supply unit 12
The pumping lights of the N pumping channels having different center optical frequencies output from a are supplied to the optical fiber 11 via the optical multiplexer / demultiplexer 12b and propagate toward the input end 11a.
On the other hand, the signal light is input from the input end 11a of the optical fiber 11 and Raman-amplified by the pump light (Raman-amplified signal light). The Raman amplified signal light passes through the optical multiplexer / demultiplexer 12b and reaches the spectroscope 13a. Most of the Raman amplified signal light input to the spectroscope 13a propagates through the optical fiber 11 and is output from the output end 11b. On the other hand, a part of the Raman amplified signal light propagates through the optical fiber 13c and reaches the photodetector 14.

Here, the feedback unit 13 sets the N optical frequencies of the Raman amplified signal light set so as to include one Raman amplification peak which is an optical frequency smaller by about 15 THz from each of the central optical frequencies of the N excitation channels. The pumping light is divided into ranges so that the detection values based on the Raman amplified signal light in each optical frequency range are equal to each other, for example, the power variation of the Raman amplified signal light in each optical frequency range is 2 dB or less, preferably 1 dB or less. Supply unit 12
To control.

As described above, the Raman amplifier 1
At 0, the photodetector 14 divides the Raman amplified signal light into N optical frequency ranges for detection. Then, the control unit 13e controls the excitation light supply unit 12 so that the detection values of the respective light frequency ranges detected by the light detection unit 14 become equal. Therefore, even when the signal light in which the respective signal channels are multiplexed in the optical frequency direction is Raman-amplified, the power spectrum of the Raman amplified signal light can be flattened in the optical frequency direction.

Further, FIG. 13 is a diagram showing the configuration of the second embodiment of the Raman amplifier according to the present invention. This second
In the Raman amplifier according to the embodiment, the variation in the detection value in each optical frequency range detected by the photodetector 14 (the power variation of the Raman amplified signal light) is 2 dB or less, preferably 1 dB or less even when the number of pumping channels changes. It is characterized in that a structure for controlling the excitation light source 12 is provided.

Specifically, the pumping light supply section 12 is a pumping light source 12 for outputting pumping light of a plurality of pumping channels.
a, an optical multiplexer / demultiplexer 12b for guiding the pumping light to the optical fiber 11, and an optical multiplexer / demultiplexer 12e for separating a part of the pumping light as monitor light. Excitation light source 12a
Is an LD 122 for outputting pumping light having mutually different center optical frequencies, a grating 121 for removing an unnecessary wavelength component for each pumping channel, and a multiplexer for multiplexing pumping light for each pumping channel. And a container 120.

On the other hand, the photo detector 1 of the feedback unit 13
Reference numeral 4 includes an optical performance monitor (OPM), and the OPM monitors not only the power of the Raman amplified signal light but also the power of the pumping light. Therefore, an optical multiplexer / demultiplexer for guiding a part of the pumping light to the optical fiber 13c. 130 is further provided, and the optical multiplexer / demultiplexers 12e and 130 are connected via an optical fiber 12d.

In the second embodiment, the controller 13e
Is the excitation light source 1 based on the detection result of the light detector 14.
2a is controlled via the optical fiber 13f, but when the Raman amplified signal light is divided into the same number of optical frequency ranges as the pumping channels, the optical frequency indicating the boundary of each optical frequency range is determined. In particular, when the control unit 13e detects the occurrence of an excitation channel having a weak power that does not effectively contribute to Raman amplification, based on the detection result of the photodetector, the control unit 13e excludes the excitation channel having the weak power. The excitation frequency of each of the optical frequency ranges is changed by using the excitation channel of.

FIG. 14A shows a Raman gain (power spectrum of Raman amplified signal light) when pumping lights of four pumping channels are supplied. Each center optical frequency of the supplied excitation channel is 202.3 THz (1481.9n).
m), 205.4 THz (1459.5 nm), 20
7.0 THz (1448.3 nm), 208.2 THz
(1439.9 nm). In FIG. 14 (a),
Spectra a to d are Raman amplification spectra by the excitation light of each of these excitation channels, and spectrum G is a superimposed spectrum obtained by superimposing these spectra a to d. In this superposed spectrum G, Raman amplification peaks (optical frequencies smaller than the central optical frequency of each excitation channel by about 15 THz) corresponding to each excitation channel are obtained.
There are G1 to G4. When the pumping light of four pumping channels is supplied in this way, the control unit 13e divides the Raman amplified signal light into the optical frequency ranges S1 and S2, and the power variation or the power average of the Raman amplified light in each optical frequency range. The excitation light source 122 is controlled so as to suppress the value variation.

On the other hand, FIG. 14B shows a center optical frequency of 207.0 THz (1448.3) among the above four excitation channels.
(nm) shows the control result of the control unit 13e when the LD that outputs the excitation channel has a failure. That is, each central optical frequency of the pump channel provided is 20
2.3 THz (1481.9 nm), 205.4 THz
(1459.5 nm), 208.2 THz (1439.
9 nm). In the case of FIG. 14B, since the spectrum c does not exist, the control unit 13e expands the optical frequency range S1 to perform re-segmentation, and the center optical frequency 208.2 THz.
LD1 to increase the pumping light power of the pumping channel of
22 is controlled. As a result, it can be seen that flattening of the superimposed spectrum G (power spectrum of Raman amplified signal light) obtained by superimposing the spectra a, b, and d is achieved. In the superposed spectrum G, the Raman amplification peak (optical frequency smaller than the central optical frequency of each excitation channel by about 15 THz) is associated with each excitation channel.
There are G1, G2, and G4.

As a case where the pumping light power is lowered in any of the pumping channels, for example, deterioration of the LD chip with time, failure of the temperature control circuit, fiber breakage, etc. can be assumed. In the example of FIGS. 14A and 14B described above, the pumping power of the pumping channel having the optical frequency of 207.0 THz is reduced to about 1/10 of the normal level, and the effective pumping power is 20%.
2.3 THz, 205.4 THz and 208.2 THz
Since the OPM 14 monitors each excitation channel when the remaining 3 excitation channels become, the signal light monitoring section is changed from the section shown in FIG.
The Raman gain can be flattened by changing to the section shown in (b) and readjusting each pump power of the remaining three pump channels.

On the other hand, in an optical network, there may be a case where the number of signal channels fluctuates due to changes in subscriber demand, network failures, and the like. If the demand for communication capacity does not start in earnest, the optical frequency is 192.5 to 194.5.
When the signal light of THz (corresponding to frequencies 1542 to 1558 nm) is unused, the excitation channel of optical frequency 208.2 THz may be turned off as shown in FIG. Conversely, if an increase in communication capacity occurs from the state shown in FIG. 15 (Raman amplification by 3 excitation channels), a signal for performing Raman amplification by 4 excitation channels as shown in FIG. By changing the optical monitor section, the flatness of Raman gain is maintained both before and after the increase of communication capacity.

The excitation light source for supplying the excitation light is turned on.
Even if the number of pumping channels does not change due to ON / OFF, the center optical frequency of each pumping channel changes due to the influence of the current supplied to the pumping light source and the strain applied to the fiber grating for wavelength stabilization. There is also a possibility.
Including the cases as described above, these fluctuations in the pump power may occur unexpectedly during system operation. Therefore, from the viewpoint of system maintenance, it is preferable that the control unit 13e perform pumping control while repeatedly monitoring all pumping channels and calculating signal light sections.

Next, the optical communication system 20 according to the present invention.
Will be described. FIG. 16 is a diagram showing the configuration of an embodiment of an optical communication system according to the present invention. The optical communication system 20 includes a relay station 23 between a transmitting station 21 and a receiving station 22.
Are arranged in the relay section between the transmitting station 21 and the relay station 23, and in the relay section between the relay station 23 and the receiving station 22,
Optical fibers 24 and 25 are laid respectively. Further, the Raman amplifier 10 having the above-described structure is arranged in the relay station 23.

In the optical communication system 20, the signal lights of a plurality of signal channels whose central optical frequencies are different from each other are transmitted by the transmitting station 2.
1 to the optical fiber 24. The transmitted signal light is Raman-amplified by the Raman amplifier 10 and then propagates through the optical fiber 25 to reach the receiving station 22. Since the optical communication system 20 according to this embodiment includes the Raman amplifier 10, the Raman amplification is performed by the receiving station 22 even in optical communication using signal lights (multiplexed signal lights) of a plurality of signal channels having different central optical frequencies. The signal light can be accurately received.

In the optical communication system 20 according to this embodiment, one relay station 23 is arranged between the transmitting station 21 and the receiving station 22, but the relay station 23 is the transmitting station 21 and the receiving station 22. A plurality of relay stations may be arranged depending on the distance between the relay stations. Further, the optical communication system 2 according to this embodiment
In No. 0, the Raman amplifier 10 is provided inside the relay station 23, but only the pumping light supply unit 12 and the feedback unit 13 are provided inside the relay station 23 so that the pumping light supplied from the pumping light supply unit 12 is supplied. The configuration may be such that it is supplied to the optical fiber 24. In this case, the optical fiber 24 functions as an optical fiber that Raman-amplifies the signal light.

[0073]

As described above, according to the present invention, an optical fiber for Raman-amplifying signal lights of a plurality of signal channels having different central optical frequencies, and N optical fibers having different central optical frequencies (an integer of 2 or more). ), A pumping light supply unit that supplies the pumping light of the pumping channel to the optical fiber, and a part of the Raman-amplified signal light that is Raman-amplified by the pumping light being supplied, and Raman amplification is performed based on the detection result. A Raman amplifier is configured with a feedback unit that controls the pumping light supply unit so that the power spectrum of the signal light is substantially flat in the optical frequency direction. In particular, the feedback section divides the detected Raman amplified signal light into N optical frequency ranges defined so as to include one Raman amplified peak having an optical frequency smaller by about 15 THz from the center optical frequency of each excitation channel, The pumping light supply unit is controlled so that the power variation of the Raman amplified signal light in each of the N divided optical frequency ranges is 2 dB or less. As a result, even when multiplexed signal lights of a plurality of signal channels having different center optical frequencies are transmitted,
The power spectrum of the Raman amplified signal light can be flattened in the optical frequency direction.

[Brief description of drawings]

FIG. 1 is a Raman amplification spectrum when Raman amplification is performed on unmultiplexed signal light and multiplexed signal light with pumping light including a pumping channel having a center optical frequency of 201.3 THz.

FIG. 2 is a Raman amplification spectrum when the multiplexed signal light is Raman-amplified by pumping lights of three pumping channels having different central light frequencies.

FIG. 3 is a Raman amplification spectrum when the multiplexed signal light is Raman-amplified by pumping lights of four pumping channels having different central light frequencies.

FIG. 4 is a Raman amplification spectrum when the multiplexed signal light is Raman-amplified by pumping lights of five pumping channels having different center light frequencies.

FIG. 5 is a Raman amplification spectrum when the multiplexed signal light is Raman-amplified with pumping lights of six pumping channels having different center light frequencies.

FIG. 6 is a diagram for explaining a method of setting an optical frequency range for monitoring multiplexed signal light.

FIG. 7 is a diagram showing a configuration of a first embodiment of a Raman amplifier according to the present invention.

FIG. 8 shows a case where an optical frequency range is set such that an optical frequency lower by 15 THz is intermediate with respect to a central optical frequency of each pumping channel included in the pumping light, the multiplexed signal lights are mutually centered at optical frequencies. 2 is a Raman amplification spectrum (power variation is 1.07 dB) when Raman amplification is performed by pumping light of five different excitation channels.

FIG. 9 is a diagram showing a case where an optical frequency range is set such that an optical frequency smaller by 14 THz is intermediate with respect to a central optical frequency of each pumping channel included in the pumping light; 2 is a Raman amplification spectrum (power variation is 1.23 dB) when Raman amplification is performed using pumping light of five different excitation channels.

FIG. 10 shows a case where an optical frequency range is set such that an optical frequency smaller by 16 THz is intermediate between central optical frequencies of respective pumping channels included in pumping light.
It is a Raman amplification spectrum (power variation is 2.77 dB) when the multiplexed signal light is Raman-amplified by pumping lights of five pumping channels having different central light frequencies.

FIG. 11 is a graph showing a relationship between optical frequency shift and gain variation (power variation of Raman amplified signal light).

FIG. 12 is a diagram showing another configuration of the photodetector section in the Raman amplifier according to the present invention.

FIG. 13 is a diagram showing a configuration of a Raman amplifier according to a second embodiment of the present invention.

FIG. 14 shows Raman amplification of multiplexed signal light,
It is a Raman amplification spectrum before the change in the number of channels (the number of channels is 4) and after the change (the number of channels is 3).

FIG. 15 shows Raman amplification of multiplexed signal light,
It is a Raman amplification spectrum before the number of channels changes (the number of channels is 3).

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

[Explanation of symbols]

10, 23a ... Raman amplifiers, 11, 12c, 13c,
24, 25 ... Optical fiber, 11a ... Input end, 11b ... Output end, 12 ... Excitation light supply section, 12a ... Excitation light source, 12
b ... Optical multiplexer / demultiplexer, 13 ... Feedback section, 13a ... Spectrometer, 13e ... Control section, 13d, 13f ... Electrical signal line, 14 ... Photodetection section, 21 ... Transmitting station 22 ... Receiving station, 23
… Relay station.

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2K002 AA02 AB30 BA04 CA15 DA10                       EB12 EB15 HA23                 5F072 AB07 AK06 HH02 PP07 QQ07                       YY17                 5K102 AA55 AD01 MA03 MB03 MB05                       MB06 MC03 MC14 MD01 MH04                       MH13 PH14

Claims (33)

[Claims] 1. An optical fiber for Raman-amplifying signal lights of a plurality of signal channels having different central optical frequencies, and pumping light of N (integer of 2 or more) excitation channels having different central optical frequencies. A pumping light supply unit for supplying the fiber, and detecting a part of the Raman amplified signal light Raman-amplified in the optical fiber by supplying the pumping light, and the Raman amplified signal light based on the detection result. And a feedback unit that controls the pumping light supply unit so that the power spectrum of the pumping light is substantially flat in the optical frequency direction, and the feedback unit sends the detected Raman amplified signal light to the pumping channel of the pumping light. One Raman amplification peak that has an optical frequency that is smaller by 13.5 to 15.7 THz optical frequency shift from each center optical frequency is included. A Raman amplifier which divides into N prescribed optical frequency ranges, and controls the pumping light supply unit so that the power variation of the Raman amplified signal light in each of the N divided optical frequency ranges becomes 2 dB or less. 2. The feedback section controls the pumping light supply section so that the power variation of the Raman amplified signal light in each of the N divided optical frequency ranges is 1 dB or less. The Raman amplifier according to Item 1. 3. The photodetection unit, wherein the feedback unit monitors the pumping light power at each central optical frequency of the pumping channel together with the power of the Raman amplified signal light,
A control unit that determines an optical frequency indicating a boundary of each of the optical frequency ranges when the Raman amplified signal light is divided into the same number of optical frequency ranges as the excitation channels based on the detection result of the photodetection unit. The Raman amplifier according to claim 1, wherein 4. When the control section detects, based on the detection result of the photodetection section, the occurrence of an excitation channel having a weak power that does not effectively contribute to Raman amplification, the excitation channel having the weak power is detected. The Raman amplifier according to claim 3, wherein the optical frequency indicating the boundary of each of the optical frequency ranges is changed by using the remaining pumping channels except for. 5. When the control unit detects, based on the detection result of the photodetection unit, the occurrence of a pumping channel having a power strong enough to effectively contribute to Raman amplification, the pumping of the strong power is performed. The Raman amplifier according to claim 3, wherein an optical frequency indicating a boundary of each of the optical frequency ranges is changed by using all pumping channels including channels. 6. The optical detection unit includes an optical performance monitor for detecting multiplexed monitor light in which the Raman amplified signal light of a plurality of signal channels and the pump light of a plurality of pump channels are multiplexed. The Raman amplifier according to claim 3. 7. When the integer N = 2, the first optical frequency range among the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the N optical frequency ranges. Of the excitation channels, the lower limit is an optical frequency that is smaller than the intermediate optical frequency between the larger first central optical frequency and the smaller second central optical frequency by the optical frequency shift, while the first central optical frequency is The upper limit is an optical frequency smaller than the first central optical frequency by the difference from the intermediate frequency and an optical frequency smaller by the optical frequency shift, and the feedback unit detects the Raman amplified signal light. The N to be classified when
The second optical frequency range of the optical frequency ranges has an upper limit of an optical frequency that is smaller than the intermediate optical frequency between the first central optical frequency and the second central optical frequency by the optical frequency shift. 2. A range in which the lower limit is an optical frequency smaller than the second central optical frequency by the optical frequency shift from the optical frequency smaller than the second central optical frequency by the difference between the second central optical frequency and the intermediate frequency. Raman amplifier described. 8. The pump channel having the largest center optical frequency among the N pump channels in the pumping light having the integer N of 3 or more is defined as a first pumping channel, and a pumping channel having the largest center optical frequency is selected from the nth pumping channel. When the pumping channel having an (integral number of 2 or more) th central optical frequency is the nth pumping channel and the pumping channel having the smallest central optical frequency is the Nth pumping channel, the feedback unit outputs the Raman amplified signal light. The first optical frequency range of the N optical frequency ranges to be divided when detecting is the optical frequency from the intermediate optical frequency between the central optical frequency of the first excitation channel and the central optical frequency of the second excitation channel. The lower limit is an optical frequency that is smaller than the frequency shift, while the central optical frequency of the first excitation channel is equal to the difference between the central optical frequency of the first excitation channel and the intermediate frequency. Is a range in which the upper limit is an optical frequency that is smaller than the optical frequency shift by an optical frequency shift from the higher optical frequency, and the nth optical frequency range is divided from the N optical frequency ranges that the feedback unit divides when detecting the Raman amplified signal light. (2
The (n-1) th or lesser integer optical frequency range is smaller than the intermediate optical frequency between the central optical frequency of the nth pumping channel and the central optical frequency of the (n + 1) th pumping channel by the optical frequency shift. A range in which the lower limit is the optical frequency, and the upper limit is the optical frequency that is smaller than the intermediate optical frequency between the central optical frequency of the nth excitation channel and the central optical frequency of the (n-1) th excitation channel by the optical frequency shift. The N-th optical frequency range of the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the center optical frequency of the N-th pumping channel and the (N-th) optical frequency range. -1) The upper limit is an optical frequency that is smaller than the intermediate optical frequency with the central optical frequency of the excitation channel by the optical frequency shift, while the central optical frequency of the Nth excitation channel and the intermediate frequency 2. The Raman amplifier according to claim 1, wherein the lower limit of the Raman amplifier is an optical frequency smaller than the central optical frequency of the N-th pumping channel by a difference from the optical frequency smaller than the central optical frequency by the optical frequency shift. 9. The Raman amplifier according to claim 1, wherein an interval between center optical frequencies of the respective pump channels in the pump light is 4 THz or less. 10. The feedback unit receives a part of the Raman amplified signal light and outputs an electric signal according to the power of the Raman amplified signal light, and the photodetector outputs the electric signal. The Raman amplifier according to claim 1, further comprising a control unit that controls the pumping light supply unit according to the electric signal. 11. The Raman amplifier according to claim 10, wherein the photodetector includes an optical performance monitor. 12. The photodetection unit receives a demultiplexing unit that demultiplexes the Raman amplified signal light into the N optical frequency ranges, and a Raman amplified signal light that is demultiplexed by the demultiplexing unit. The Raman amplifier according to claim 10, further comprising a light receiving unit. 13. The Raman according to claim 12, wherein the demultiplexing unit includes any one of an optical circuit including a dielectric optical filter and an optical circuit including an optical circulator and a Bragg diffraction grating. amplifier. 14. A Raman amplifier control method comprising at least an optical fiber for Raman-amplifying signal lights of a plurality of signal channels having different center optical frequencies, and a pumping light supplying section, wherein Pumping light of different N (integer of 2 or more) channels is supplied to the Raman amplification optical fiber; one of the Raman amplified signal lights Raman-amplified in the Raman amplification optical fiber by supplying the pumping light. And detects the Raman amplified signal light from the center optical frequency of each channel of the pumping light by 13.5-15.7 TH.
By dividing into N optical frequency ranges defined to include one Raman amplification peak having an optical frequency smaller by the optical frequency shift of z, and controlling the pumping light supply unit,
A Raman amplifier control method for suppressing the power variation of Raman amplified signal light in each of the divided N optical frequency ranges to 2 dB or less. 15. The pumping light supply unit is controlled so that the power variation of the Raman amplified signal light in each of the N divided optical frequency ranges is suppressed to 1 dB or less. Raman amplifier control method. 16. When the Raman amplified signal light is divided into the same number of optical frequency ranges as the excitation channels, each of the optical frequency ranges is detected based on the detection result of the excitation light power at each central optical frequency of the excitation channels. 15. The method of controlling a Raman amplifier according to claim 14, wherein an optical frequency indicating the boundary of is determined. 17. When the generation of an excitation channel having a weak power that does not contribute effectively to Raman amplification is detected among the excitation channels, the remaining excitation channels except the excitation channel having the weak power are used. 17. The Raman amplifier control method according to claim 16, wherein the optical frequency indicating the boundary of each of the optical frequency ranges is changed. 18. When it is detected that one of the excitation channels has such a powerful power as to effectively contribute to Raman amplification, all the excitation channels including the strong power excitation channel are utilized. 17. The method of controlling a Raman amplifier according to claim 16, wherein the optical frequency indicating the boundary of each of the optical frequency ranges is changed. 19. When the integer N = 2, the first optical frequency range among the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the N optical frequency ranges. Of the excitation channels, the lower limit is an optical frequency that is smaller than the intermediate optical frequency between the larger first central optical frequency and the smaller second central optical frequency by the optical frequency shift, while the first central optical frequency is When the feedback unit detects the Raman amplified signal light, the upper limit is an optical frequency that is smaller than the first center optical frequency by a difference from the intermediate frequency and is smaller by the optical frequency shift. The second optical frequency range of the N optical frequency ranges to be divided is smaller than the intermediate optical frequency between the first central optical frequency and the second central optical frequency by the optical frequency shift. While the frequency is the upper limit, the lower limit is an optical frequency smaller than the optical frequency shift smaller than the second central optical frequency by the difference between the second central optical frequency and the intermediate frequency. 15. The method of controlling a Raman amplifier according to claim 14, wherein: 20. A pumping channel having the largest center optical frequency among the N pumping channels in the pumping light having the integer N of 3 or more is defined as a first pumping channel, and a pumping channel having the largest center optical frequency is selected from the nth pumping channel. When the pumping channel having an (integral number of 2 or more) th central optical frequency is the nth pumping channel and the pumping channel having the smallest central optical frequency is the Nth pumping channel, the feedback unit outputs the Raman amplified signal light. The first optical frequency range of the N optical frequency ranges to be divided when detecting is the optical frequency from the intermediate optical frequency between the central optical frequency of the first excitation channel and the central optical frequency of the second excitation channel. The lower limit is an optical frequency that is smaller by the amount of frequency shift, while the central optical frequency of the first excitation channel is reduced by the difference between the central optical frequency of the first excitation channel and the intermediate frequency. Is a range in which an optical frequency smaller than the optical frequency larger than a number is smaller than the optical frequency by an amount corresponding to the optical frequency shift is an upper limit, and the feedback unit divides the N optical frequency ranges when the Raman amplified signal light is detected. n (2
The (n-1) th or lesser integer optical frequency range is smaller than the intermediate optical frequency between the central optical frequency of the nth pumping channel and the central optical frequency of the (n + 1) th pumping channel by the optical frequency shift. A range in which the lower limit is the optical frequency, and the upper limit is the optical frequency that is smaller than the intermediate optical frequency between the central optical frequency of the nth excitation channel and the central optical frequency of the (n-1) th excitation channel by the optical frequency shift. The N-th optical frequency range of the N optical frequency ranges divided when the feedback unit detects the Raman amplified signal light is the center optical frequency of the N-th pumping channel and the (N-th) optical frequency range. -1) The upper limit is an optical frequency that is smaller than the intermediate optical frequency with the central optical frequency of the excitation channel by the optical frequency shift, while the central optical frequency of the Nth excitation channel and the intermediate frequency 15. The control of the Raman amplifier according to claim 14, wherein the lower limit is a light frequency smaller than the center light frequency of the Nth pumping channel by a difference from the light frequency smaller than the light frequency shift by the light frequency shift. Method. 21. An optical communication system including the Raman amplifier according to claim 1. 22. An optical fiber for Raman-amplifying signal light of a plurality of signal channels having different central optical frequencies, and pumping light of N (integer of 2 or more) pumping channels having different central optical frequencies as the optical fibers. A pumping light supply unit for supplying the fiber, and detecting a part of the Raman amplified signal light Raman-amplified in the optical fiber by supplying the pumping light, and the Raman amplified signal light based on the detection result. And a feedback unit that controls the pumping light supply unit so that the power spectrum of the pumping light becomes substantially flat in the optical frequency direction, the feedback unit pumping the detected Raman amplified signal light into the pumping light. The optical frequency range is divided into the same number as the number of channels, and the power average value variation of the Raman amplified signal light included in each of the divided optical frequency ranges is 2 dB
A Raman amplifier for controlling the pumping light supply unit as follows. 23. The feedback unit controls the pumping light supply unit so that a power average value variation of Raman amplified signal light included in each of the divided optical frequency ranges is 1 dB or less. The Raman amplifier according to claim 22. 24. The feedback unit monitors the pumping light power at each center optical frequency of the pumping channel together with the power of the Raman amplified signal light, and the feedback unit based on a detection result of the photodetecting unit. 3. A control unit that determines an optical frequency indicating a boundary of each of the optical frequency ranges when the Raman amplified signal light is divided into the same number of optical frequency ranges as the excitation channels.
2. The Raman amplifier described in 2. 25. When the control section detects, based on the detection result of the photodetection section, the occurrence of an excitation channel having a weak power that does not effectively contribute to Raman amplification, the excitation channel having the weak power is detected. 25. The Raman amplifier according to claim 24, wherein the optical frequency indicating the boundary of each of the optical frequency ranges is changed by using the remaining pumping channels except for. 26. When the control unit detects, based on the detection result of the photodetection unit, the occurrence of a pumping channel having a power that is powerful enough to effectively contribute to Raman amplification, the pumping of the strong power is performed. 25. The Raman amplifier according to claim 24, wherein an optical frequency indicating a boundary of each of the optical frequency ranges is changed by using all excitation channels including channels. 27. The optical detection unit includes an optical performance monitor for detecting a multiplexed monitor light in which the Raman amplified signal light of a plurality of signal channels and the pump light of a plurality of pump channels are multiplexed. Claim 2
4. The Raman amplifier according to 4. 28. An optical communication system including the Raman amplifier according to claim 22. 29. A Raman amplifier control method comprising at least an optical fiber for Raman-amplifying signal lights of a plurality of signal channels having different center optical frequencies, and a pumping light supply unit, wherein Pumping light of different N (integer of 2 or more) channels is supplied to the Raman amplification optical fiber, and one of the Raman amplified signal lights Raman-amplified in the Raman amplification optical fiber by supplying the pumping light. A section, the Raman amplified signal light detected is divided into an optical frequency range of the same number as the number of pumping channels of the pumping light, by controlling the pumping light supply unit, each of the divided optical frequency range The control method of the Raman amplifier which suppresses the fluctuation of the average power value of the Raman amplified signal light included in the above to 2 dB or less. 30. The pumping light supply section is controlled so that a power average value variation of Raman amplified signal light included in each of the N divided optical frequency ranges is suppressed to 1 dB or less. The control method of the Raman amplifier according to claim 29. 31. When the Raman amplified signal light is divided into the same number of optical frequency ranges as the excitation channels, each of the optical frequency ranges is detected based on the detection result of the excitation light power at each central optical frequency of the excitation channels. 30. The Raman amplifier according to claim 29, wherein an optical frequency indicating a boundary of the Raman is determined. 32. When it is detected that a pumping channel having a weak power that does not contribute effectively to Raman amplification is detected among the pumping channels, the remaining pumping channels except the pumping channel having the weak power are used. 32. The method of controlling a Raman amplifier according to claim 31, wherein the optical frequency indicating the boundary of each of the optical frequency ranges is changed. 33. When it is detected that a pumping channel having a powerful enough power to effectively contribute to Raman amplification among the pumping channels is detected, all pumping channels including the pumping channel having the strong power are utilized. 32. The Raman amplifier control method according to claim 31, wherein the optical frequency indicating the boundary of each of the optical frequency ranges is changed.