JP2728157B2 - Optical amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier required in an optical transmission system, optical signal processing, and the like.

[0002]

2. Description of the Related Art As an optical amplifier used to amplify an input optical signal as it is to obtain an optical signal having a large signal output, there are a semiconductor laser amplifier, a rare earth-doped optical fiber amplifier and the like. In all of these optical amplifiers, the signal light gain decreases as the input optical signal power increases. Therefore, when trying to increase the operating input level, the input / output proportional relationship cannot be maintained. Therefore, the input signal and the output signal to the optical amplifier are detected, and the driving current is
In the rare earth doped optical fiber amplifier, a control system has been studied in which the gain is constant by feeding back the pump light source.

[0003]

As described above, in the conventional gain control circuit, the input signal and the output signal are simultaneously detected and the input light and the output light are compared. There was a disadvantage that control was not possible. That is, since information is superimposed on the input signal, the signal always becomes zero in the intensity modulation method, and the time constant of the control system must be carefully set in consideration of the statistical properties of the input signal. there were. Further, since the output optical signal contains amplified spontaneous emission light, an amplifier having a large gain has a drawback that a control error increases.

Furthermore, in an optical amplifier used in an optical repeater transmission system, the input signal power to the repeater fluctuates due to loss fluctuation (temperature characteristics and aging characteristics) in the optical fiber transmission line. When the optical fiber loss decreases, the incident power to the repeater increases, and if the amplifier gain of the repeater is constant, the amplifier output becomes the input limit (non-linear) of the optical fiber on the output side (later stage) of the repeater. Effect, etc.). Also, when the optical fiber loss increases, the power incident on the repeater decreases, and when the amplifier gain of the repeater is constant, the output of the amplifier also decreases, which may degrade the S / N of the transmission system. is there. Therefore, it is necessary to set the signal output of the amplifier to a reference value. For this reason,
Although the application of a limiter amplifier is conceivable, the linearity of input and output cannot be maintained, so that in the light intensity modulation method, the distortion rate increases in an analog transmission system, and the error rate deteriorates in a digital transmission system. Also, due to the system configuration, the output of the optical amplifier is +1.
Although a high output of about 0 dBm is required, this value is in the saturation region of the optical amplifier, and if the constant gain control is not performed, the transmission characteristics deteriorate. That is, in the optical amplifier used in the optical repeater transmission system, it is necessary to simultaneously solve these two seemingly contradictory problems (constant output and constant gain).

[0005] The present invention solves these problems,
A first object is to provide an optical amplifier whose gain is controlled to be constant without detecting an input / output optical signal.

It is a second object of the present invention to provide an optical amplifier in which the output and the gain are simultaneously controlled to be constant.

[0007]

In order to achieve the above-mentioned object, an optical amplifier according to the present invention comprises a rare earth-doped optical fiber for amplifying signal light by optical pumping, and a pump which is respectively incident on the rare earth-doped optical fiber. Means for detecting optical power and output pumping light power passing through the rare-earth-doped optical fiber; and detecting a ratio between the incident pumping light power and the output pumping light power, wherein the ratio matches a preset reference value.
Characterized in that comprises means for controlling the light source of the excitation light to.

Here, a back light monitor of the semiconductor laser for photoexcitation may be used as a means for detecting the power of the incident excitation light. A log amplifier may be used as a means for obtaining the ratio between the incident pump light and the output pump light. The ratio of the input output pump light power, comparison with the reference value, and calculation of the amount of feedback to the pump light source may be performed by digital processing to control the pump light source.
Output the feedback amount after the / A conversion as a pulse train subjected to pulse width modulation or pulse density modulation (pulse number modulation),
The excitation light source may be driven by this pulse train.

The output optical signal power is calculated from the input / output pumping light power ratio and the incident pumping light power, integrated with an appropriate time constant, and the reference value of the input / output pumping light power ratio is set so that this value becomes constant. Or, after monitoring the output signal light and integrating with an appropriate time constant, change the reference value of the input / output pumping light power ratio so that this value becomes a constant value. A feedback system may be provided.

[0010]

The operation of the present invention will be described below.

It is known that the gain characteristic of a rare-earth-doped fiber amplifier is described by a partial differential equation (rate equation) using pump power, input optical signal power, and the like as parameters. However, conventionally, the rate equation was solved by numerical analysis, and it was not known that there was a simple relationship between these parameters as shown below.

The loss characteristic α of the pumping light power in the rare earth doped fiber is

[0013]

[Mathematical formula-see original document] α = (output power of pump light from rare earth doped fiber) / (incident power of pump light) (1)
By analytically solving the rate equation, it can be seen that there is a relationship between the pumping light power loss characteristic α and the following equation.

[0014]

[Number 2] G = exp [(Logα + ρη p σ p a L) η s (σ s e + σ s a) / {η p (σ p e + σ p a)} - ρη s σ s a L] (2) [rho: erbium doped density sigma _s ^a: absorption cross section at the signal wavelength sigma _s ^e: induction in the signal wavelength emission cross section sigma _p ^a: the absorption cross section at the excitation wavelength sigma _p ^e: induction at the excitation wavelength of the emission cross section eta _s: Signal wavelength overlap factor η _p : represented by pump wavelength overlap factor. That is, the gain G is described by the loss α of the pump light passing through the rare earth doped fiber defined by the equation (1). The other parameters constituting the equation (2) are constants independent of the pump light intensity and the signal light intensity. Therefore, the gain can be kept constant by controlling the power of the excitation light source so that α is constant.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing the basic configuration of the present invention.

The input signal light is passed through an isolator 1 and a wavelength-multiplexed optical device 2 to be incident on an optical fiber (EDF) 3 to which a rare earth element such as erbium is added, amplified by optical pumping, and combined with a multiplexer 4 and an isolator 5. And is extracted as output signal light. A semiconductor laser, which is a pump light source 6 for pumping the erbium-doped optical fiber, is driven by a drive circuit 7, and its output light (pump light) passes through a splitter 8 and a multiplexer 4 to the erbium-doped optical fiber 3. Incident. A part of the incident pump light is branched by the demultiplexer 8, and its optical power is detected by the pump light monitor 9, amplified (or attenuated) by the amplifier (or attenuator) 10, and input to the comparator 11. You. on the other hand,
The output pump light that has passed through the erbium-doped optical fiber 3 is extracted through the multiplexer 2 and its optical power is supplied to the pump light monitor 1.
2 after detection by the amplifier (or attenuator) 13
Is amplified (or attenuated) and input to the comparator 11. The output of the comparator 11 is fed back to the drive circuit 7. At this time, the power ratio between the incident pump light and the output pump light is kept constant by the amplifiers (or attenuators) 10 and 13.

That is, in the present embodiment, the power of the pumping light incident on the rare-earth-doped fiber such as the erbium-doped fiber and the power of the pumping light passing through the rare-earth-doped fiber are detected, and the ratio between the two (that is, the pump light) is detected. Is calculated, and the pumping light source is controlled so that this value is constant, so that the gain G can be set to a constant value, as is apparent from equation (2).

FIG. 2 shows another embodiment of the present invention. In this example, an excitation light monitor 9 which is a means for detecting incident excitation light power is used.
The configuration uses a back light monitor of a semiconductor laser for light excitation. Therefore, there is no need to provide an optical branch for detecting the incident power of the pump light, which simplifies the configuration of the device, and also eliminates the loss of the pump light due to the optical branch, thereby simplifying the optical amplifier and improving the performance. Contributes to

FIG. 3 shows a third embodiment of the present invention. A log amplifier 1 is used as a means for obtaining the ratio between the incident pump light and the output pump light.
4, the comparator 11 compares the ratio value with a preset reference value, which contributes to simplification of the optical amplifier.

FIG. 4 shows a fourth embodiment of the present invention. The optical powers of the input and output pump lights detected by the pump light monitors 9 and 12 are digitized by A / D converters 16 and 17, and the ratio of the input and output pump light powers, comparison with a reference value, and feedback to the pump light source Since the calculation of the amount is performed by a processing unit (CPU) 18 by digital processing, and the feedback amount is converted into an analog signal by the D / A converter 19 to control the excitation light source, highly accurate gain control is possible.

Next, FIG. 5 shows a fifth embodiment of the present invention. In the configuration shown in FIG. 4, after the feedback amount is D / A converted, it is output as a pulse train modulated by pulse width modulation or pulse density modulation (pulse number modulation) by the pulse amplifier 20, and the excitation light source 6 is driven by this pulse train. doing.
The gain of the erbium-doped optical fiber optical amplifier has a time constant on the order of milliseconds, and the pulse train pump light that changes at a high speed is integrated by the time constant, so that it becomes equivalent to the CW pump light. Therefore, everything from the D / A converter 19 to the drive circuit of the excitation light source 6 can be configured by a pulse operation circuit, and a high-precision analog amplifier and a low-pass filter are not required. Therefore, high-precision gain control is possible despite the simplification of the adjustment work that was essential in an analog circuit.
It is also expected that manufacturing costs will be reduced.

Next, a method of stabilizing the output simultaneously with the gain according to the sixth embodiment will be described.

The output optical power of the rare-earth-doped optical amplifier has the following relationship with the pump light loss, gain, and incident pump light power.

[0025]

(Equation 3)

## EQU1 ##

Accordingly, the output optical signal power can be calculated from the ratio of the input / output pumping light power and the incident pumping light power by the equation (3), so that the signal is output without branching to monitor the output optical signal. Output power.

Specifically, C in the block diagram shown in FIG.
According to the flowchart of FIG. 1. Calculation of input and output pumping light powers and their ratios; 2. Calculation of output signal power; 3. Calculation of average value of output signal power; 4. comparison with reference output signal power and The change of the reference pumping light loss is executed.

In this case, in particular, the output optical signal power calculated from the input / output pumping light power ratio and the incident pumping light power is calculated from the time constant τ (integration time during which the time variation of the integrated value is eliminated) based on the statistical properties of the signal. Is also integrated over a long period of time, and this integrated value is compared with the reference value of the signal output to change the reference value of the input / output pumping light power ratio of the optical amplifier so that the integrated value of the signal output power becomes constant. Good to do. Furthermore, since the optical amplifier controls the pumping light source for a time sufficiently shorter than the spontaneous emission time τ of the rare-earth-doped fiber so that the input / output pumping light power ratio matches the reference value, the amplification factor is constant. As a result, the input / output linearity of the amplifier is maintained, so that the transmission characteristics do not deteriorate. Next, since the output level is also kept constant (average equal to or longer than the time constant τ), it is possible to avoid deterioration of characteristics and deterioration of S / N due to input limitation of the fiber.

Next, a seventh embodiment of the present invention will be described with reference to FIG. In the configuration of the sixth embodiment, the output signal light is obtained by calculation. However, in the configuration of the present embodiment, the output signal light is branched by the demultiplexer 21 and monitored by the signal light monitor 22. The output signal light power is digitized by the A / D converter 23 and
Is being entered. As a result, the configuration of the signal processing system of the control system is simplified, but it is necessary to provide an optical component for splitting the signal light at the output of the optical amplifier. Since the effect achieved by this embodiment is equivalent to that of the sixth embodiment, the configuration may be selected in consideration of circuit manufacturability, cost, and the like.

FIG. 8 shows an eighth embodiment of the present invention. The block configuration of this embodiment is almost the same as the configuration of FIG.
The CPU 18 makes the CPU 18 execute an operation according to the flowchart shown in FIG. Further, when the reference loss α cannot be maintained, the CPU 18 outputs an alarm output 24 to the monitor. That is, in this embodiment, the power of the pumping light incident on the rare-earth-doped fiber and the power of the pumping light passing through the rare-earth-doped fiber are detected, and the ratio between the two (that is, the loss α of the pumping light) is calculated. Is controlled to keep the pump light source constant, the gain G is controlled to a constant value, and there is an arithmetic circuit for calculating the output optical signal power from the incident pump light power, and the output signal light is directly monitored. As a result, the configuration can be fed back to the control system so that the operation state can be output to the outside.

With this configuration, it is possible to cope with either constant gain control or constant output control as required.

In the constant gain control, an alarm can be issued if the set reference loss α cannot be maintained even when the pumping light source is controlled, so that it is determined whether the optical amplifier is operating normally. be able to. Further, it is possible to compare the signal output value obtained by the calculation with the monitor value.
Since the initial value is constant, if there is a difference between them,
This is due to deterioration of the optical amplifier (change in the pumping light wavelength, change in the internal loss, etc.), and it is possible to monitor the aging of the optical amplifier.

[0034]

According to the optical amplifier of the present invention, the gain of the amplifier can be controlled to be constant without detecting the signal light. Therefore, it is possible to completely suppress the gain fluctuation due to the signal pattern effect or the like. Further, it is possible to make the signal output power constant while controlling the gain of the amplifier to be constant, and a highly stable optical repeater transmission system can be constituted by the present amplifier.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of an optical amplifier according to the present invention.

FIG. 2 is a block diagram showing an embodiment of the present invention.

FIG. 3 is a block diagram showing an embodiment of the present invention.

FIG. 4 is a block diagram showing an embodiment of the present invention.

FIG. 5 is a block diagram showing an embodiment of the present invention.

FIG. 6 is a flowchart illustrating an example of an operation for stabilizing an output signal.

FIG. 7 is a block diagram showing an embodiment of the present invention.

FIG. 8 is a block diagram showing an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1,5 Isolator 2,4 Wavelength multi-wavelength device 3 Rare earth-doped fiber such as erbium-doped fiber 6 Excitation light source 7 Drive circuit 8 Demultiplexer 9,12 Excitation light monitor 10,13 Electric amplifier or attenuator 11 Comparator 14 Log amp 15 Setting reference value 16, 17, 23 A / D converter 18 Arithmetic unit 19 D / A converter 20 Pulse amplifier 21 Duplexer 22 Signal light monitor 24 Alarm monitor output

Claims (7)

(57) [Claims] 1. A rare earth-doped optical fiber for amplifying signal light by optical pumping, and means for detecting a pump light power incident on the rare earth-doped optical fiber and an output pump light power passing through the rare earth-doped optical fiber, respectively. And detecting the ratio of the incident pump light power to the output pump light power
An optical amplifier comprising means for controlling a light source of the excitation light such that the ratio matches a preset reference value . 2. The optical amplifier according to claim 1, wherein the means for detecting the power of the incident pump light is a back light monitor of a semiconductor laser for light pump. 3. An optical amplifier according to claim 1, wherein the means for controlling the light source of the pumping light includes a log amplifier for obtaining a ratio between the power of the incident pumping light and the power of the output pumping light. . 4. A means for controlling the light source of the pumping light, means for A / D converting the detected values of the incident pumping light and the output pumping light, calculation of the ratio between the input and output pumping light powers, and calculation of the ratio obtained. Means for digitally processing the value, the comparison with the reference value, and the calculation of the amount of feedback to the excitation light source and the amount of feedback as D
3. The optical amplifier according to claim 1, further comprising means for performing / A conversion. 5. A pump for driving an excitation light source by outputting the feedback amount subjected to the D / A conversion as a pulse train subjected to pulse width modulation or pulse density modulation (pulse number modulation). 5. The optical amplifier according to 4. 6. An output optical signal power is calculated from a ratio between the input and output pump light powers and the incident pump light power.
6. The light according to claim 1, further comprising a feedback system that changes a reference value of the input / output pumping light power ratio so that the value becomes a constant value after integration with an appropriate time constant. amplifier. 7. A feedback system for monitoring an output signal light, integrating it with an appropriate time constant, and then changing a reference value of an input / output pumping light power ratio so that this value becomes a constant value. The optical amplifier according to claim 1.