JP2000223764A - Optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber amplifier which controls gain to be constant at high speed and permit the control without generating level fluctuation of existing channels, in the case that change of the number of channels is generated which is caused by switchover of optical paths, increase of systems, troubles, etc., in a WDM transmission system. SOLUTION: This optical fiber amplifier is provided with a first feedback loop which so controls quantity of pumping light of a light source for pumping that the output value from an output signal light detecting means follows the output value from an input signal light detecting means, and an observing device 12 which estimates all the input signal light level of input signal light from a driving signal of the light source for pumping and an output value of the output signal light detecting means. In the optical fiber amplifier, a second feedback loop which feedbacks an output of the observing device 12 to the driving signal of the light source for pumping is constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier for directly amplifying a signal light using an amplifying optical fiber, and more particularly to a constant control (AGC) for controlling a ratio between an input signal light and an output signal light to be constant. Optical fiber amplifier.

[0002]

2. Description of the Related Art Conventionally, an optical fiber amplifier for amplifying an optical signal has been used. At this time, the present applicant has proposed an optical fiber amplifier capable of controlling an input signal and an output signal at a constant ratio.

As shown in FIG. 6, input light 31 passes through an optical splitter 32, an EDF (erbium-doped fiber) 33 as an optical fiber for amplification, an optical multiplexer 34, and an optical splitter 35. The amplified output light 36 is obtained. On the other hand, the pumping light output from the pumping semiconductor laser 39, which is the pumping light source, is guided to the EDF 33 via the optical multiplexer 34, and pumps and amplifies the input light 31.

Here, the degree of amplification of the input light 31 is determined by the wavelength of the input light 31, the light amount level (light power), the power of the semiconductor laser 39 for excitation (excitation power), and the like. In this method, the power of the output light 36 is controlled by changing the excitation power of the semiconductor laser 39.

[0005] The optical splitters 32 and 35 respectively receive the input light 3
1. Each is branched by several percent to monitor the output light 36, and each output is input light monitor 3
8. Connected to the output light monitor 41 and converted into an electric signal.

The respective outputs converted into electric signals are compared by a comparator 45, and the error signal thereof controls a semiconductor laser 39 for excitation via a phase compensator 44 and a drive circuit 43. Further, the output of the input light monitor 38 is added to the drive circuit 43 via the feedforward compensation 46.

With this configuration, the output fluctuation of the optical fiber amplifier due to a large temperature change (for example, -25.degree. C. to + 80.degree. C.) or a change over time is compensated, and a stable constant gain with very little long-term output fluctuation. Optical fiber amplifiers that can be controlled have been developed.

FIG. 7 shows the gain (calculated value) of the optical fiber amplifier when the input signal power and the pump power are changed, and the gain depends on the input signal light level and the pump power. Also, when the pumping power is kept constant, the smaller the input signal level, the larger the gain.

[0009]

However, in such an optical fiber amplifier, in the case of a WDM transmission system using a multiplexed signal light including a plurality of wavelengths as the input light 31, there is a problem that control becomes difficult. Was.

That is, in a WDM transmission system using a plurality of transmission channels, the number of channels fluctuates due to optical path switching, system addition, failure, and the like. When the number of channels fluctuates, that is, when the number of channels is lost or increased, the gain of the surviving channel fluctuates in the optical fiber amplifier shown in FIG. 6, thereby causing a fluctuation in the signal level of the output light 36, resulting in transient vibration. There is a problem that bit errors (transmission errors) on the receiving side are increased by propagating through the optical repeater system.

This is because the optical fiber amplifier shown in FIG.
Although steady stability is sufficient, it is due to the fact that responsiveness to transient characteristics such as a change in the number of channels occurring in WDM transmission is not taken into account. Even in the feedforward compensation 46, the compensator was a constant, and the transient characteristics were oscillating, resulting in insufficient performance.

In order to solve the above problem, the above-mentioned gain constant control (AGC) is performed at high speed when the number of channels fluctuates.
Action is required.

The response speed of this control is 10 μm which is higher than the transient characteristic of the amplification gain in the amplification optical fiber.
It is necessary to operate stably in less than sec (Y. Sun et a
l., "Dynamic Effects in Amplified networks", OSA TOP
S on OAA '97, Vol. XVI, pp. 333-353, 1997). That is,
The time constant of the transient response of the gain is not about 110 to 340 μsec as conventionally known, and the time constant of the transient response of the gain is 10 μsec because a fiber amplifier having a higher output than the conventional one is required in WDM transmission. It is required that:

[0014] The dynamic characteristic of the gain referred to here means that, when the pumping power is kept constant and, for example, one wavelength light is dropped or added out of the multiplexed signal light of the two wavelength lights, the other characteristic is obtained. This is a dynamic characteristic in which gain fluctuation of one wavelength light occurs. By changing the pump power, it is possible to suppress this gain variation, and such gain characteristics are the background to which constant gain control is important.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical fiber amplifier which does not generate a level fluctuation of a surviving channel even when the number of channels fluctuates in a WDM transmission system using a multiplexed signal light of a plurality of wavelengths. I do.

[0016]

According to a first aspect of the present invention, an input signal light comprising a multiplexed signal light and a pump light from a pump light source are supplied to an amplifying optical fiber. An optical fiber amplifier for amplifying light to produce an output signal light, comprising: an input signal light detection means for detecting the input signal light; and an output signal light detection means for detecting the amplified output signal light. Forming a first feedback loop for controlling the amount of excitation light of the excitation light source so that the output value from the output signal light detection means follows the output value from the signal light detection means; And an observer for estimating the total input signal light level of the input signal light from the output value of the output signal detecting means, and a second feedback circuit for feeding back the output of the observer to the drive signal of the excitation light source. And wherein the configuring the croup.

According to a second aspect of the present invention, there is provided an optical fiber amplifier which supplies input signal light composed of multiplexed signal light and excitation light from an excitation light source to an amplification optical fiber, and amplifies the input signal light to produce output signal light. An input signal light detecting means for detecting the input signal light, and an output signal light detecting means for detecting the amplified output signal light. The output signal light detecting means Forming a first feedback loop for controlling the amount of excitation light of the excitation light source so that the output value from the optical fiber follows the output value of the excitation light source, and changing the output value of the input signal light detection means to the inverse system of the amplification gain characteristic of the amplification optical fiber. And feed-forward control for adding to the drive signal of the excitation light source through

Further, in the first and second inventions, the input signal detecting means and the output signal detecting means in the first feedback loop may be configured such that only the signal light of a specific wavelength is included in the input / output signal light composed of the combined signal light. Is detected.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, FIG. 1 shows an embodiment in which only the signal light of a certain specific wavelength is monitored in the first invention.

An input signal light 1 comprising a multiplexed signal light of eight wavelengths of wavelengths λ1 to λ8 is provided with an optical splitter 2, an EDF (erbium-doped fiber) 3, which is an optical amplification fiber, and an optical multiplexer. 4. The output signal light 6 is amplified via the optical splitter 5. On the other hand, the output of the semiconductor laser 9 for excitation, which is a light source for excitation, passes through the optical multiplexer 4 to the EDF 3
To excite the input signal light 1, and the input signal light 1 is amplified to become the output signal light 6.

The degree to which the input signal light 1 is amplified is determined by the wavelength of the input signal light 1, the light quantity level (light power), the power of the pumping semiconductor laser 9 (pumping power), and the like. In this method, the power of the output signal light 6 is controlled.

Each of the optical splitters 2 and 5 splits about 4% for monitoring the input signal light 1 and the output signal light 2, and outputs the wavelength selection filters 7 and 10 respectively.
Of the combined signal light (8 wavelength lights of λ1 to λ8), for example, only the wavelength light of the wavelength λ8 is selected, connected to the input light monitor 8 and the output light monitor 11, respectively, and converted into an electric signal. .

Each output converted into an electric signal is compared by a comparator 15, and the error signal is controlled by controlling a semiconductor laser 9 for excitation via a phase compensator 14 and a laser driving circuit 13, thereby providing a first feedback signal. A loop is configured.

With this configuration, the ratio of the input signal light 1 to the output signal light 6 (amplification gain of the signal light) is controlled. Here, the phase compensator 14 is used for stabilizing the feedback loop from the output light monitor 11, and is particularly for securing the loop gain in the low frequency region. A response speed of 10 μsec or less required by the present invention cannot be realized by using only the above.

For example, if the signals of the wavelengths λ1 to λ4 are lost in the multiplexed signal light, the gains of the wavelengths λ5 to λ8 increase and the output power fluctuates. The above-described first feedback loop operates so as to eliminate the output fluctuation, but the output fluctuation occurs because the response speed is about 1 msec.

Therefore, in the present invention, an observer 12 designed to operate at a high speed of 10 μsec or less is provided, and the observer 12 estimates a level change of the input signal light 1 and compensates for a gain change due to the change. Then, the semiconductor laser 9 for excitation is operated.

Here, the observer 12 is a disturbance observer used in control theory. In the optical fiber amplifier of the present embodiment, the target value is the level of the input signal light 1,
The control signal is a control system for the excitation power of the semiconductor laser 9 for excitation. As shown in FIG. 7, when the level of the input signal light 1 is low, the levels of the input signal light 1 and the output signal light 6 are in a proportional relationship. Saturates. From these, the input signal light 1 can be regarded as a feedforward signal or a disturbance signal from the viewpoint of the control system. Here, the input signal light 1
Is regarded as a disturbance signal, is estimated using a disturbance observer, and the value is added to the laser drive circuit 13.

The observer 12 is provided with an inverse system G ＾ of the amplification gain characteristic G (s) of the EDF 3 which is an amplification optical fiber.
The signal from the output monitor 11 is passed through (s), and this is compared with the laser drive signal passed through the phase compensator 14 by a comparator. The error signal is Q (s) which is a low-pass filter with a cutoff of 3 MHz. A second feedback loop is formed by passing the signal through a filter and adding to the laser drive signal.

FIG. 2 shows another embodiment of the first invention in which all signal lights are monitored as monitor signals for performing gain constant control.

This embodiment is the same as that shown in FIG. 1, but does not include the wavelength selection filters 7 and 10 as shown in FIG. Is monitored, thereby forming a first feedback loop similar to the above, and is provided with an observer 12.

In the optical fiber amplifier shown in FIGS. 1 and 2 above, by providing the second feedback loop including the observer 12 in addition to the first feedback loop, constant gain control can be performed at a response speed of 10 μsec or less. It can be performed.

Next, FIG. 3 shows an embodiment in which only the signal light of a certain specific wavelength is monitored in the second invention.

The optical fiber amplifier shown in FIG. 3 is the same as that shown in FIG.
Instead, a new light splitter 18 and input light monitor 19
The total light amount of the input signal light 1 is monitored via the control signal and passed through the feedforward compensator 16 and added to the laser drive signal.

Here, the feedforward compensator 16 is E
This is a low-pass pseudo-inverse system having a configuration that simulates the inverse transient characteristic of the amplification gain of the DF3 and that is a low-pass filter that does not amplify high frequency component noise. The specific feedforward compensation is as shown in Expression 1, for example, T
_It suffices to set ₁ = 0.000001 and T ₂ = 0.00000025.

[0035]

(Equation 1)

Compared with the case where the feedforward compensator of the prior art shown in FIG. 6 is a constant, the optical fiber amplifier of the present embodiment takes into account the characteristics of the EDF 3 to be controlled and can perform high-speed constant control. .

FIG. 4 shows another embodiment of the second invention in which all signal lights are monitored as monitor signals for performing gain constant control.

This embodiment is the same as that shown in FIG. 3, but does not include the wavelength selection filters 7 and 10 as shown in FIG. 3, and all of the input signal light 1 and the output signal light 6 Is monitored, thereby forming a first feedback loop similar to the above, and the signal of the input monitor 8 is passed through the feedforward compensator 16 and added to the laser drive signal.

In the embodiment shown in FIGS.
By performing feedforward compensation that simulates the reverse transient characteristic of the amplification of F3, the transient characteristic is improved so that the output follows the sudden fluctuation of the input signal light 1.

[0040]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical fiber amplifier of the present invention shown in FIG.
With respect to the optical fiber amplifier of FIG. 6 which is a comparative example, the gain fluctuation when the number of channels fluctuated was measured. As a result, as shown in FIG. 5, the optical fiber amplifier of the present invention has a very small gain variation with respect to the variation in the number of channels, and can perform high-speed constant gain control without causing a level variation of the surviving channel.

[0041]

As described above, according to the present invention, in the WDM transmission system, even if the number of channels fluctuates due to optical path switching, system addition, failure, etc.
It is possible to perform the constant gain control at a high speed, and to perform the control without causing the level fluctuation of the surviving channel. Therefore, an extremely stable optical fiber amplifier suitable for a WDM transmission relay system can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an optical fiber amplifier of the present invention.

FIG. 2 is a block diagram showing an optical fiber amplifier according to another embodiment of the present invention.

FIG. 3 is a block diagram showing an optical fiber amplifier according to another embodiment of the present invention.

FIG. 4 is a block diagram showing an optical fiber amplifier according to another embodiment of the present invention.

FIG. 5 is a graph showing gain fluctuation at the time of channel fluctuation in the optical fiber amplifiers of the present invention and a comparative example.

FIG. 6 is a block diagram showing a conventional optical fiber amplifier.

FIG. 7 is a graph showing the relationship between pump power and gain in an optical fiber amplifier.

[Description of Signs] 1: Input signal light 2, 5: Optical splitter 3: EDF 4: Optical multiplexer 6: Output signal light 7, 10: Wavelength selection filter 8: Input light monitor 9: Semiconductor laser for excitation 11: Output light monitor 12: Observer 13: Laser drive circuit 14: Phase compensator 15: Comparator 16: Feed forward compensator

Claims (3)

[Claims] An optical fiber amplifier for supplying an input signal light comprising a multiplexed signal light and an excitation light from an excitation light source to an amplification optical fiber, and amplifying the input signal light to produce an output signal light. An input signal light detecting means for detecting the input signal light, and an output signal light detecting means for detecting the amplified output signal light, wherein an output value from the input signal light detecting means is output from the output signal light detecting means. Forming a first feedback loop for controlling the amount of excitation light of the excitation light source such that the value follows, and calculating the total input signal of the input signal light from the drive signal of the excitation light source and the output value of the output signal detection means. An optical fiber amplifier comprising an observer for estimating a light level, and comprising a second feedback loop for feeding back the output of the observer to the drive signal of the excitation light source. . 2. An optical fiber amplifier for supplying an input signal light comprising a multiplexed signal light and an excitation light from an excitation light source to an amplification optical fiber, and amplifying the input signal light to produce an output signal light. An input signal light detecting means for detecting the input signal light, and an output signal light detecting means for detecting the amplified output signal light, wherein an output value from the input signal light detecting means is output from the output signal light detecting means. Forming a first feedback loop for controlling the amount of pumping light of the pumping light source so as to follow the value, and controlling the output value of the input signal light detecting means through the inverse system of the amplification gain characteristic of the amplifying optical fiber; An optical fiber amplifier which performs feedforward control for adding to a drive signal of a light source for use. 3. An input signal detecting means and an output signal detecting means in the first feedback loop each detect only a signal light of a specific wavelength out of input / output signal light composed of a combined signal light. Claim 1 characterized by the following:
Or the optical fiber amplifier according to 2.