JP3939322B2 - Method and apparatus for controlling an optical amplifier for optically amplifying wavelength multiplexed signals - Google Patents

Description

  The present invention relates to a method and apparatus for controlling an optical amplifier including a rare earth-doped fiber that optically amplifies wavelength multiplexed signals.

  Currently, an optical amplification system using an optical amplifier based on an erbium-doped fiber has been put into practical use, and in order to further increase the transmission speed, a multi-wavelength signal in which multiple wavelengths of light are multiplexed on an optical line including the optical amplifier is transmitted. The practical application of the wavelength batch amplification system is underway. Therefore, it is necessary to control gains of a plurality of signal wavelengths as an optical amplifier for that purpose. In the case of a single wave, this can be realized by controlling the pumping light. However, in the case of multi-wavelength amplification, in addition to this, the gain difference between multiple signals is controlled to control the gain of each signal. It is necessary to do.

  In Japanese Patent Application No. 6-229164, two excitation lights that give complementary gain characteristics (relationship between signal light wavelength and gain) of 0.98 μm band excitation light and 1.48 μm band excitation light are used simultaneously. Thus, it has been proposed to reduce the gain difference between the signal lights of two wavelengths by flattening the gain characteristics. However, no proposal has been made regarding the control of the gain difference and the control of the output of individual optical signals based thereon.

  Accordingly, it is an object of the present invention to provide a method and apparatus for controlling a gain difference between optical signals in an optical amplifier for optically amplifying a wavelength multiplexed signal and controlling the output of individual optical signals based thereon.

  According to the present invention, there is provided a method for controlling an optical amplifier including a rare earth doped fiber that optically amplifies a wavelength multiplexed signal including a plurality of optical signals having different wavelengths, wherein the rare earth doped fiber has an optical amplification function. There is provided a method comprising the steps of injecting control light having a predetermined wavelength into the rare earth-doped fiber, controlling the power or wavelength of the injected control light, and thereby controlling the gain difference between the plurality of optical signals. Is done.

  According to the present invention, an apparatus for controlling an optical amplifier including a rare earth-doped fiber that optically amplifies a wavelength multiplexed signal including a plurality of optical signals having different wavelengths, the wavelength region in which the rare earth-doped fiber exhibits an amplifying function. Means for injecting control light having a wavelength within the rare earth-doped optical fiber, and means for controlling the power or wavelength of the injected control light, thereby controlling the gain difference between the plurality of optical signals. An apparatus is also provided.

  According to the present invention, the output of the optical amplifier that optically amplifies the wavelength multiplexed signal can be controlled for each wavelength.

FIG. 1 shows a configuration of an optical amplifier to which control according to a first specific example of the present invention is applied. In FIG. 1, signal light having wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ) is combined with excitation light (for example, wavelength 0.98 μm) from a light source 14 in an optical multiplexer 10 and erbium-doped fiber 12. Introduced into. Excitation light having a wavelength λ ₃ (for example, λ ₃ = 1.48 μm) from the light source 16 is introduced into the erbium-doped fiber 12 by the optical multiplexer 18 in the direction opposite to the signal light. Part of the optical signal amplified by the erbium-doped fiber 12 is branched by the optical coupler 20 and demultiplexed into wavelengths λ ₁ and λ ₂ by the optical demultiplexer 22, and the respective optical powers are detected by the photodetectors 24 and 26. Is detected. The control circuit 28 controls the optical power by controlling the drive currents of the light sources 14 and 16 based on the detection outputs of the photodetectors 24 and 26.

  Of course, the direction of introduction of the excitation light is not limited to the illustrated example. Further, instead of controlling the pumping light injection power by controlling the drive currents of the light sources 14, 16, the variable attenuators 15, 17 are provided between the light sources 14, 16 and the optical multiplexers 10, 18 as shown in FIG. And the pumping light injection power may be controlled by controlling the attenuation of the variable attenuators 15 and 17.

As shown in FIG. 15 of Japanese Patent Application No. 6-229164, when 0.98 μm band excitation light is used, the gain on the short wavelength side is larger than that on the long wavelength side, and the 1.48 μm band excitation light is used. In this case, the gain on the long wavelength side becomes larger than that on the short wavelength side. The control circuit 28 calculates the light outputs of λ ₁ and λ ₂ from the outputs of the photodetectors 24 and 26, and the output power P ₁ of the light of λ ₁ on the short wavelength side is the output of the light on the long wavelength side. If it is larger than the power P ₂ , the 0.98 μm band pumping light power effective to increase the short wavelength side gain is decreased, and the 1.48 μm band pumping light power effective to increase the long wavelength side gain is increased. Conversely, if P ₂ is larger than P ₁ , the 0.98 μm band pumping light power is increased and the 1.48 μm band pumping light power is decreased. If the total optical output of each signal light is less than or equal to the desired output, both powers of 0.98 μm band / 1.48 μm band pumping light are increased. To obtain a desired light output.

As described above, in the first specific example of the present invention, 0.98 μm band / 1.48 μm band pumping light power is obtained so that a desired output can be obtained for each optical signal while monitoring the output of each optical signal. Is adjusted.
FIG. 3 shows the experimental results when the input signal light power is −8.1 dBm, and the input signal light wavelengths λ ₁ and λ ₂ are λ ₁ = 1535 nm and λ ₂ = 1558 nm. In FIG. 3, the condition that the gain difference ΔG (= P ₁₅₅₈ −P ₁₅₃₅ ) is constant at 0 and +1 dB, and the total output (= P ₁₅₅₈ + P ₁₅₃₅ ) is constant at 11, 12, 13, 13.5 and 14. The relationship between the 0.98 μm band pumping light power and the 1.48 μm band pumping power under the conditions is shown. For example, under ΔG = 0, if (0.98 μm band pumping light power, 1.48 μm band pumping light power) = (40 mW, 15 mW), a total output of 13 dBm can be obtained, and (30 mW, 35 mW). By changing the 0.98 μm band pumping light power and the 1.48 μm band pumping light power along the ΔG = 0 curve so as to obtain a total output of 13.5 dBm, the light output is changed while maintaining a gain difference of 0. Can do.

  FIG. 4 is a graph showing the wavelength dependence of the emission probability (probability of emission transition), the absorption probability (probability of absorption transition), and the ratio of the emission probability to the absorption probability of the erbium-doped fiber. In FIG. 4, it can be seen that radiation occurs at 1.48 μm (1480 nm), and the excitation rate (ratio of erbium atoms in the excited state) decreases. However, since the proportion of radiation is small, light of this wavelength is not amplified. On the other hand, it can be seen that the longer the wavelength, the greater the proportion of radiation and the lower the excitation rate.

FIG. 5 is a graph showing changes in the wavelength dependence of the gain coefficient at various excitation rates. According to FIG. 5, the gain on the short wavelength side (for example, λ ₁ = 1.54 μm) becomes relatively larger as the pumping rate is closer to 1.0, and the longer the wavelength side (for example, λ _It can be seen that the gain of ₂ = 1.55 μm is relatively large.
From the above, the reason why the gain difference can be controlled by the above-described 0.98 μm / 1.48 μm hybrid excitation is that radiation occurs at 1.48 μm, and the pumping rate decreases. It is estimated to be. However, as long as a wavelength that is not amplified in the erbium-doped fiber such as the 1.48 μm band is used, it can be seen that the control range is not widened because the pumping rate is limited.

  Therefore, in the second embodiment of the present invention, the wavelength in the wavelength region in which the rare earth-doped fiber exhibits an amplifying action, preferably the wavelength in the wavelength region in which the radiation probability is larger than the absorption probability, more preferably shorter than 1520 nm. A non-wavelength is used as the wavelength of the control light, and the pumping rate is controlled by controlling the optical power or wavelength of the control light, thereby controlling the gain difference between signals.

FIG. 6 shows a configuration of an optical amplifier to which the control according to the second specific example of the present invention is applied. In FIG. 6, signal light having wavelengths λ ₁ and λ ₂ (for example, λ ₁ = 1.54 μm, λ ₂ = 1.55 μm as shown in FIG. 5) is pumped from the light source 14 in the optical multiplexer 10 ( For example, a wavelength of 0.98 μm is combined and introduced into the erbium-doped fiber 12. Control light of wavelength λ ₃ from the light source 40 (for example, λ ₃ = 1.57 μm as shown in FIG. 5) is introduced into the erbium-doped fiber 12 by the optical multiplexer 18 in the direction opposite to the signal light. Part of the optical signal amplified by the erbium-doped fiber 12 is branched by the optical coupler 20 and demultiplexed into wavelengths λ ₁ and λ ₂ by the optical demultiplexer 22, and the respective optical powers are detected by the photodetectors 24 and 26. Is detected. The control circuit 28 controls the gain difference by controlling the light power or the light emission wavelength of the light source 40 based on the detection outputs of the photodetectors 24 and 26, and controls the light power of the light source 14 to control the entire light power. Controls the (average) gain. As a result, the output levels of the signal lights λ ₁ and λ ₂ can be controlled to be constant.

Of course, the direction of introduction of the excitation light and the control light is not limited to the illustrated example. Further, when it is necessary to remove the control light λ ₃ , it can be removed by providing an optical filter at the output. When controlling the optical power of the control light, for example, the drive current of the laser diode as the light source 40 is controlled. When controlling the wavelength of the control light, for example, a wavelength tunable laser is used as the light source 40.

  Instead of introducing the control light separately from the signal light, the power of the optical signal input to the optical amplifier 30 having the erbium-doped fiber is controlled using the variable attenuator 32 as shown in FIG. The gain difference can be controlled. Since the wavelength of the signal light is naturally set to a wavelength that exhibits a certain radiation probability, the gain difference can also be controlled by controlling the incident power of the signal light itself. As shown in FIG. 8, the power of the signal light incident on the optical amplifier 30 provided on the receiving side of the optical transmission line 34 may be controlled by a variable attenuator 32 provided on the transmitting side.

Furthermore, in FIG. 6, it is also possible to modulate the control light with an SV (supervision) signal for monitoring the transmission path and transmit it together with the signal lights λ ₁ and λ ₂ . In this case, the control light is preferably introduced into the erbium-doped fiber in the same direction as the signal light.

It is a block diagram which shows the 1st example of this invention. It is a block diagram which shows one modification of the 2nd example of this invention. It is a graph explaining operation | movement of the circuit of FIG. It is a graph which shows the wavelength dependence of the radiation probability of the erbium atom in an erbium doped fiber, the absorption probability, and the radiation probability / absorption probability. It is a graph which shows the wavelength dependence of the gain coefficient in various excitation rates. It is a block diagram which shows the 2nd example of this invention. It is a block diagram which shows one modification of the 2nd example of this invention. It is a block diagram which shows the other modification of the 2nd example of this invention.

Explanation of symbols

10, 18 Optical multiplexer 14, 16 Excitation light source 20 Optical coupler 22 Optical demultiplexer 24, 26 Photo detector 40 Control light source

Claims (18)

A method for controlling an optical amplifier including a rare earth-doped fiber that optically amplifies a wavelength multiplexed signal including a plurality of optical signals having different wavelengths,
Injecting control light having a wavelength in a wavelength region in which the rare earth-doped fiber exhibits an optical amplification action into the rare earth-doped fiber,
A method comprising the steps of controlling the injection power or wavelength of the control light and thereby controlling the gain difference between the plurality of optical signals.   The method according to claim 1, wherein a wavelength of the control light is a wavelength different from any of wavelengths of the plurality of optical signals.   The method according to claim 2, wherein the wavelength of the control light is a wavelength in a wavelength region in which a radiation probability in the rare earth-doped fiber is larger than an absorption probability.   4. The method of claim 3, wherein the rare earth doped fiber is an erbium doped fiber and the wavelength of the control light is equal to or longer than 1520 nm.   The method of claim 4, wherein the wavelengths of the plurality of optical signals are in a range of 1530 to 1565 nm, and the wavelength of the control light is equal to or longer than 1565 nm.   The method of claim 2, further comprising removing the control light at an output side of the optical amplifier.   The method according to claim 2, wherein the control light is modulated with a monitoring signal for monitoring a transmission line.   The method according to claim 1, wherein the control light is the wavelength-multiplexed signal itself. Detecting the power of each of the plurality of optical signals at the output of the optical amplifier,
The method according to claim 1, wherein in the controlling step, an injection power or a wavelength of the control light is controlled in accordance with the detected power of the optical signal. An apparatus for controlling an optical amplifier including a rare earth-doped fiber that optically amplifies a wavelength multiplexed signal including a plurality of optical signals having different wavelengths,
Means for injecting control light having a wavelength in a wavelength region in which the rare earth doped fiber exhibits an amplifying action into the rare earth doped optical fiber;
Means for controlling the injection power or wavelength of the control light and thereby controlling the gain difference between the plurality of optical signals.   The device according to claim 10, wherein a wavelength of the control light is different from any of wavelengths of the plurality of optical signals.   The apparatus according to claim 11, wherein the wavelength of the control light is a wavelength in a wavelength region in which a radiation probability in the rare earth-doped fiber is larger than an absorption probability.   13. The apparatus of claim 12, wherein the rare earth doped fiber is an erbium doped fiber and the wavelength of the control light is equal to or longer than 1520 nm.   The apparatus of claim 13, wherein the wavelengths of the plurality of optical signals are in a range of 1530 to 1565 nm, and the wavelength of the control light is equal to or longer than 1565 nm.   The apparatus of claim 11, further comprising means for removing the control light at an output side of the optical amplifier.   12. The apparatus according to claim 11, wherein the control light is modulated with a monitoring signal for monitoring a transmission line.   The apparatus according to claim 10, wherein the control light is the wavelength-multiplexed signal itself. Further comprising means for detecting the power of each of the plurality of optical signals at the output of the optical amplifier,
The apparatus according to claim 10, wherein the control unit controls the injection power or wavelength of the control light in accordance with the detected power of the optical signal.

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