JP2003298162A - Optical amplifier - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To suppress fluctuation in gain of distributed Raman optical amplification due to a change in transmission line optical fiber loss and the like. The gain of distributed Raman amplification is controlled by means provided in an optical amplifier based on the light intensities of signal light and reference light. Specifically, the reference light (wavelength λr) is multiplexed with the signal light (wavelength λs) and transmitted, and the optical repeater controls the Raman optical amplifier based on the intensity information of the signal light and the reference light. The monitoring light of the optical repeater can be shared as the reference light. Even if the characteristics of the transmission line optical fiber change, the gain of Raman optical amplification can be kept constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplification device using distributed Raman optical amplification.

[0002]

2. Description of the Related Art In an optical transmission system, large capacity and long distance transmission are achieved by using an optical amplifier. To construct such an optical amplifier, EDFA (Erbium
-There is a configuration that uses optical amplification using a rare-earth element-doped optical fiber such as a Doped Fiber Amplifier), a configuration that uses Raman optical amplification that uses stimulated Raman scattering, or a configuration that uses both.

FIG. 2 shows an example of the configuration of a conventional optical repeater amplification transmission system using an optical amplification device using only an EDFA. This example is a transmission system that performs transmission from the first optical transmission terminal station device 123-1 to the second optical transmission terminal station device 123-2. 2 in between
One optical repeater 125 performs the repeater, and the optical repeater 125 includes an optical amplifying device 12 including only the EDFA.
2 is included. In this way, the optical amplifier using the EDFA amplifies the light intensity only in the optical repeater.

On the other hand, in Raman optical amplification, optical amplification is performed by an optical fiber that transmits an optical signal (transmission path optical fiber 124 shown in FIG. 2). This type of optical amplification is called distributed optical amplification because the gain of optical amplification is distributed in the transmission line.
It is known that the distributed optical amplification can reduce the effective loss of the optical transmission line and the light intensity in the transmission line fiber, and thus can reduce the influence of the nonlinear optical effect.

When this distributed Raman optical amplification is used practically, it is important to stabilize its gain. However, in the actual optical transmission system, the characteristics of the optical fiber and the pumping light source for Raman optical amplification are used. Temperature dependence of characteristics and deterioration over time may cause fluctuations in the gain of Raman amplification.

Japanese Unexamined Patent Publication No. 2001-235772 "Raman amplification control device and optical transmission system" (Reference 1) discloses a Raman optical amplification gain control method for solving this problem. The present invention introduces a reference light into an optical fiber where Raman amplification is performed and detects the backscattered light of the reference light generated in the optical fiber to obtain the effective loss distribution of the optical transmission line and obtain the Raman amplification gain. It controls.

[0007]

SUMMARY OF THE INVENTION The present invention provides an optical amplifying device which is capable of controlling the gain to a desired value and which is constituted by distributed Raman optical amplification.

In distributed Raman optical amplification, since the amplification medium is the transmission line optical fiber itself, the entrance and exit of the amplification medium exist at different locations. This is different from the case where the rare earth-doped optical fiber amplifier is built into the optical repeater, and it is necessary to control the gain of Raman optical amplification only with information on either the entrance or exit side of the optical amplifier. Become.

Reference 1 makes it possible to control the gain of Raman optical amplification. In this method, in order to obtain the effective loss distribution of the optical fiber, pulsed light is introduced into the optical fiber as the reference light and the change over time in the backscattered light intensity is observed. Therefore, the pulse light source must be prepared as the reference light, and the temporal change information of the light intensity must be processed, which complicates the control of the Raman optical amplifier.

Further, since the reference light is attenuated by the transmission line optical fiber, the intensity of the backscattered light generated at a distant place is smaller than that of the backscattered light generated at a position near the reference light introduction position, and the signal-to-noise ratio is small. turn into. as a result,
As a result, the sensitivity to changes in the effective loss at a position far from the reference light introduction position is reduced.

[0011]

The optical amplifying device of the present invention comprises:
This enables simple and highly sensitive control of the gain of an optical device configured using distributed Raman amplification. Here, the outline and principle of the present invention will be described.

According to the embodiment of the present invention, at least one signal light propagating in the optical fiber by multiplexing at least one pumping light at the exit of the optical fiber of the transmission line into the optical fiber of the transmission line serving as a Raman optical amplification medium. A Raman light amplification unit for amplifying Raman light, and combining at least one reference light having a difference in optical frequency with at least one of the excitation light of 15 THz or more or 3 THz or less at an entrance of an optical fiber, At the exit of the optical fiber, part of the signal light and part or all of the reference light are demultiplexed, and the gain of the entire optical amplifier is controlled based on the ratio of the average optical intensities of the demultiplexed signal light and reference light. It is a characteristic optical amplifier.

The gain of Raman light amplification shows a spectrum shape as shown in FIG. The horizontal axis represents the deviation of the signal light frequency from the pumping light frequency (optical frequency shift), and the vertical axis represents the gain. The pumping light frequency is determined so that the signal light frequency shift is around 12 THz so that the gain is maximized. As can be seen from FIG. 10, Raman optical amplification can obtain a large gain in a limited optical frequency range.
That is, by setting the optical frequency of the reference light to be outside the gain band, typically, the optical frequency shift is 15 THz or more or 3 THz or less, the influence of Raman light amplification on the reference light is exerted on the signal light. Can be kept small compared to.

The reference light as described above is multiplexed with the signal light from the entrance of the transmission path fiber and transmitted, and the ratio of the average light intensities of the signal light and the reference light is obtained at the exit of the transmission path optical fiber.
It is possible to obtain information about the gain of Raman light amplification. For example, if the average light intensities of the signal light and the reference light at the entrance of the transmission line optical fiber are equal and the losses received from the transmission line optical fiber are also equal, the intensities of the signal light and the reference light at the exit of the transmission line optical fiber are equal. The ratio represents the gain of Raman optical amplification.
Even if the average light intensities of the signal light and the reference light at the entrance of the transmission line optical fiber are not equal, or even if the losses caused by the transmission line optical fiber are not equal, by introducing the corresponding compensation coefficient, Raman optical amplification It is possible to get a gain.

Thus, the gain of Raman optical amplification can be obtained only on the exit side of the transmission line optical fiber based on the average light intensity information of the signal light and the reference light. Based on this information, the optical amplifier device can be obtained. Can be controlled. In particular, if the pumping light for Raman light amplification is introduced from the exit side of the transmission line optical fiber, the gain of the Raman light amplifying section can be controlled by controlling the light intensity of this pumping light. As a result, it is possible to control the gain of the entire optical amplification device.

In the method of the above-mentioned reference 1, pulsed light is required for the reference light and it is necessary to process information on the temporal change of scattered light, but the present invention eliminates such need. Further, in Document 1, the sensitivity to changes in the effective loss that occurs at a location farther than the reference light source decreases, but in the method of the present invention, the signal light and the reference light affected by the effective loss over the entire transmission line optical fiber. Since the light intensity information of is used, the sensitivity does not change depending on the location of the optical fiber.

The reference light used in the present invention is continuous wave (CW).
It doesn't have to be light. Therefore, the monitoring light used to transfer the monitoring information of the optical repeater can be shared with the reference light of the present invention, and the complexity of the configuration of the optical transmission device and the optical amplifier can be solved.

In the present invention, the signal light or the reference light for detecting the light intensity may be single or plural. In particular, if there are a plurality of signal lights to be detected, the gains at a plurality of wavelengths can be known. Therefore, for example, it is possible to perform control in consideration of a gain spectrum shape of Raman light amplification that is not flat. Especially when multiple pumping lights are introduced to flatten the gain shape of Raman amplification,
By obtaining the gain information at a plurality of wavelengths, it is possible to control the pumping light intensity so that each observed gain is constant, and the gain can be flattened. Therefore, the present invention is effective. Is.

Further, according to the present invention, the light intensity of the reference light output from the reference light source is controlled by the light intensity information of the signal light at the entrance of the transmission line optical fiber, thereby improving the accuracy of gain control of the optical amplifier. You can The optical intensity of the signal light at the entrance of the transmission line optical fiber may change due to the fact that the gain shape of the optical amplifier is not completely flat, the temperature dependence of the optical transmission device, and aging.
This leads to uncertainty in gain observation of Raman optical amplification. However, by controlling the output light intensity of the reference light according to the signal light intensity as described above, this uncertainty can be eliminated.

[0020]

1 is a block diagram showing a first embodiment of the present invention. This example shows the configuration of an optical repeater to which the present invention is applied. Raman light amplification pumping light (wavelength λp) and reference light (wavelength λp) are used for main signal light (wavelength λs).
This is an example when r) goes backward.

The optical repeater 110 includes a pumping light source 107 for Raman light amplification, and pumping light is wavelength-multiplexed optical coupler 104.
-2 and is introduced into the transmission path optical fiber 111. The signal light amplified by Raman light is input to the optical repeater 110 from the input optical fiber 101. The signal light may be a single information signal or a plurality of information signals may be wavelength-multiplexed.

A part of the signal light and a part of the reference light are optical couplers 1.
03-1 and the wavelength division multiplexing optical coupler 104-1, and is detected by the photodetectors 106-1 and 106-2, respectively. Although a wavelength division multiplexing optical coupler and an ordinary optical coupler are used as the branching means for the signal light and the reference light here, another branching means capable of branching the signal light and the reference light may be used. For example, the reference light can be branched by using a fiber Bragg grating and a circulator.

The present invention controls the gain of Raman light amplification according to the detected light intensities of the signal light and the reference light. The control circuit 109 controls the gain of Raman light amplification based on the light intensities of the signal light and the reference light. Typically, as in this example, the output light intensity of the pumping light source 107 for Raman light amplification is controlled. If the ratio between the detected signal light intensity and the reference light intensity deviates from the reference value, the output light intensity of the pumping light source 107 for Raman amplification is adjusted accordingly, and the desired gain value is maintained.

The optical repeater 110 is equipped with an optical amplifier 105, which functions in a complementary manner with Raman optical amplification in the transmission line optical fiber to compensate for the loss in the transmission line optical fiber. Examples of the optical amplifying device 105 include a device using a rare earth element-doped fiber represented by EDFA and a Raman optical amplifying device, but any device that amplifies light intensity may be used. Or, if the loss of the transmission line optical fiber can be compensated only by the gain of distributed Raman optical amplification,
It is not necessary to include the optical amplification device 105.

The signal light is output from the optical repeater 110 through the output optical fiber 102, but before the output, the reference light source 1
The reference light output from 08 is the wavelength multiplexing optical coupler 103-
Combined through 2. Here, the control means for keeping the output light intensity of the reference light source constant is not specified, but if the control for keeping the light intensity constant is applied, the control accuracy is improved. In this example, the wavelength multiplexing optical coupler is used to combine the reference lights, but the combining means may be different. For example, it is possible to combine the reference light using a circulator and a fiber Bragg grating.

When the present invention is applied to the optical transmission terminal station, it is sufficient to provide only the reference light multiplexing section (103-2, 108). Further, when the present invention is applied to the optical receiving terminal station, the reference light combining section (103-2, 108) is not necessary.

Further, in the optical repeater 110 of this example, the Raman light amplification pumping light introducing section (104-2) is arranged immediately after the signal light and reference light branching section (103-1). The arrangement order of both may be reversed.

FIG. 3 shows a second embodiment of the present invention.
This example shows a part of the optical multi-repeater transmission system, and represents an example of connection between the first optical transmission device and the second optical transmission device. In optical multi-repeater transmission, such optical transmission systems are connected in multiple stages.

The optical repeater 112 shows the main part of the output section of the optical repeater. Although an example of an optical repeater is shown here, this can be similarly applied to an optical transmission terminal station device. Similarly, the present invention can be applied to general devices for transmitting reference light.

At the output of the optical repeater 112, the reference light (wavelength λr) output from the reference light source 108-1 is combined with the signal light (wavelength λs) and sent to the downstream optical repeater 110. . If the wavelength of the reference light is properly selected, the reference light will not receive the gain of Raman light amplification, and the loss received by the signal light and the reference light will be approximately the same or the ratio will be constant, so that the optical repeater If the light intensity of the signal light and the reference light is detected by the device 110, the gain of Raman light amplification can be detected.

It is desirable that the wavelength of the reference light is outside the gain band of Raman light amplification. For example, when considering not the wavelength but the optical frequency, the difference between the optical frequency of the reference light and the optical frequency of the pumping light for Raman light amplification is 3 THz or less or 15 TH.
It is better to have z or more. If the wavelength of the monitoring light of the optical repeater satisfies this condition, the monitoring light can be shared as the reference light.

In the optical repeater 112, the optical coupler 103-2
The signal light and the reference light are guided to the photodetectors 106-1 and 106-2 using the wavelength multiplexing optical coupler 104-1. The control circuit 109 receives information on the detected light intensity, and controls the output light intensity of the Raman light amplification pumping light source based on the information.
This makes it possible to keep the gain of Raman light amplification constant.

FIG. 4 shows a configuration example of the control circuit 109 according to the second embodiment. The intensity signal 140 of the signal light and the intensity signal 141 of the reference light are input to the division circuit 143-1,
An intensity ratio signal between the signal light and the reference light is output. The intensity ratio signal is input to the division circuit 143-2 and compared with the preset reference power source 144 in magnitude. According to this result, the conversion circuit 145 outputs the Raman light amplification pumping light source control signal 142.

Transmission line fiber 1 for receiving the signal light and the reference light
When the loss of 11 and the loss of the wavelength division multiplexing optical coupler are different, it is possible to correct by changing the set value of the reference power supply 144 in consideration of the influence. The configuration of the control circuit 109 is not limited to this example. For example, a configuration using a digital circuit or a control program is also possible.

Further, the optical repeater 110 according to the second embodiment is provided with a reference light source 108-2, which combines the reference light with the output light through the wavelength multiplexing optical coupler 103-3 and transmits the reference light as a signal. To the downstream side.

FIG. 5 shows a third embodiment of the present invention,
In the WDM optical transmission, the photodetector 106 detects the optical intensities of two signal lights (wavelengths λs1 and λs2) having different wavelengths.
This is an example in which control is performed by detecting -1, 106-2. When Raman optical amplification is applied in wavelength-multiplexed optical transmission, the gain for signal light of each wavelength may not be constant. According to this embodiment, for example, 2
It is possible to control the average gain of Raman light amplification in one signal light to a desired value. Note that the number of signal lights to be detected is not limited to two, and it is possible to use the light intensity of more signal lights for control.

FIG. 6 shows a fourth embodiment of the present invention,
This is an example of using two pumping light sources for pumping Raman light amplification.

When Raman optical amplification is applied in wavelength division multiplexed optical transmission, the gain for signal light of each wavelength may not be constant. To improve this gain deviation, it is effective to use a plurality of Raman light amplification pumping lights having different wavelengths. At this time, the present invention can be applied as a means for controlling the light intensity of a plurality of excitation lights.

The Raman light amplification pumping lights having different wavelengths are pumping light sources 107-1 and 107-2 (wavelengths λp1,
It is output from λp2) and is introduced into the transmission line fiber 111. 2 detected by photodetectors 106-1 and 106-2
The two signal lights have different wavelengths (wavelengths λs1, λs2)
Choose to have. 2 by the excitation light of wavelength λp1
The gains generated in the two signal lights are different, but the ratio is constant. The same applies to the excitation light having the wavelength λp2. Therefore, if the gains generated in the two signal lights can be observed, the gain received by the signal lights can be divided into a gain due to the pump light of wavelength λp1 and a gain due to the pump light of wavelength λp2. Based on this information, the excitation light sources 107-1, 1
By controlling the output light intensity of 07-2, the gain of Raman light amplification can be stabilized. The number of pumping light sources is not limited to two, and the present invention is effective for controlling more pumping light sources.

FIG. 7 shows a fifth embodiment of the present invention,
This is an example of controlling the gain of the optical amplifying device 105 incorporated in the optical repeater 110 as the gain controlling means of the optical amplifying device. The only difference from FIG. 1 is that the target controlled by the control circuit 109 is the Raman light amplification pumping light source 107 to the optical amplification device 105. in this way,
By means of controlling the light intensity other than Raman light amplification,
It is possible to control the gain of the entire optical amplifier.

FIG. 8 shows a sixth embodiment of the present invention.
This is an example of controlling the gain of the variable optical attenuator 114 built in the optical repeater 110 as the gain control means of the optical amplifier. In this way, the gain of the optical amplifier can be controlled by the variable optical attenuator.

FIG. 9 shows a seventh embodiment of the present invention,
In this example, the light intensity of the reference light source is controlled by the output light intensity of the signal light from the optical repeater. At the output of the optical repeater, part of the signal light (wavelength λs) is branched by the optical coupler 103-3, and the light intensity is detected by the photodetector 106-3. A part of the output of the reference light source is branched by the optical coupler 103-4, and the photodetector 10
The light intensity is detected at 6-4. The control circuit 113 controls the output light intensity of the reference light source based on the light intensity information of the signal light and the reference light. When the output signal light intensity from the optical repeater or the optical amplifier is not constant, the control circuit 109
The gain of Raman optical amplification recognized in Section 1 is not stable, which may cause unstable transmission characteristics. However, according to the present embodiment, the relationship between the light intensities of the signal light and the reference light can be kept constant, so that the control circuit 109 can stably obtain the gain of Raman light amplification. That is, it leads to stabilization of gain control of Raman optical amplification.

[0043]

According to the embodiments of the present invention, it is possible to control the gain of the distributed Raman optical amplifier, and it is possible to provide an optical amplifying device having a stable gain. Furthermore, by using the optical amplifier of the embodiment of the present invention, it is possible to provide an optical transmission system having good transmission characteristics.

Further, it is possible to stabilize the gain for the Raman light amplifying device provided with a plurality of pumping light sources.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing a conventional optical repeater transmission system.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a diagram showing an example of a control circuit.

FIG. 5 is a configuration diagram showing a third embodiment of the present invention.

FIG. 6 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 8 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 9 is a configuration diagram showing a seventh embodiment of the present invention.

FIG. 10 is a diagram showing a gain spectrum of Raman light amplification.

[Explanation of symbols]

101 Input optical fiber 102 output optical fiber 103 Optical coupler 104 WDM optical coupler 105 Optical amplifier 106 photo detector 107 Pumping light source for Raman amplification 108 Reference light source 109 control circuit 110 Optical repeater 111 Transmission line optical fiber 112 Output part of optical repeater 113 control circuit 114 Variable optical attenuator 120 optical transmitter 121 Optical multiplexer 122 Optical amplifier 123 Optical Transmission Terminal Equipment 124 Transmission line optical fiber 125 optical repeater 126 Optical demultiplexer 127 Optical receiver 140 signal light intensity signal 141 Reference light intensity signal 142 Pumping Light Source Control Signal for Raman Light Amplification 143 division circuit 144 Reference power supply 145 conversion circuit.

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Claims (9)

[Claims] 1. A Raman optical amplification of at least one signal light propagating in an optical fiber by multiplexing at least one pumping light with a signal light at an entrance or an exit of a transmission line optical fiber which serves as a Raman optical amplification medium. At least one reference light having a Raman optical amplifier and having an optical frequency difference from at least one of the pumping lights of 15 THz or more or 3 THz or less is multiplexed at the optical fiber inlet, and at the optical fiber outlet. A part of the signal light and a part or all of the reference light are demultiplexed, and the gain of the entire optical amplifier is controlled based on the ratio of the average light intensity of the demultiplexed signal light and the reference light. An optical amplification device characterized by the above. 2. The optical amplifying device according to claim 1, wherein the reference light is used for signal transfer of a control signal or the like. 3. The optical amplifying device according to claim 1, wherein the ratio of the average light intensity of the demultiplexed signal light and the reference light is controlled to a constant value. 4. The optical amplifying device according to claim 2, wherein the ratio of the average light intensity of the demultiplexed signal light to the reference light is controlled to a constant value. 5. The optical amplifying device according to claim 1, wherein the control of the optical amplification gain is performed by controlling the light intensity of the pumping light. 6. The optical amplifying device has at least one different second optical amplifying section, and the control of the optical amplifying gain is performed by the second optical amplifying section.
The optical amplifying device according to claim 1, wherein the optical amplifying unit is provided by controlling the gain of the optical amplifying unit. 7. The optical amplifying device according to claim 6, wherein the second optical amplifying section is composed of a rare earth element-doped optical fiber. 8. The optical amplifying device has at least one optical attenuator, and the control of the optical amplification gain is performed by controlling an attenuation amount of the optical attenuator. The optical amplification device according to. 9. The light according to claim 1, wherein the light intensity of the reference light included in the optical amplifying device is controlled based on the information of the light intensity of the signal light at the entrance of the transmission line optical fiber. Amplification device.