JPH09326766A - Optical filter wavelength tracking control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extend the service life of an optical filter by suppressing deterioration in the mechanism of the optical filter by conducting intermittently the input of a control signal to the optical filter. SOLUTION: Input of an AC control signal 44 to an optical filter 110 is stopped when a center wavelength of the optical filter 110 is discriminated coincident with a wavelength of a transmission light of the optical filter 110. While the input is stopped, a level of the transmitted light of the optical filter 110 is periodically monitored and when the level is discriminated less than a prescribed level, the input of the AC control signal 344 to the optical filter 110 is restarted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system using an optical filter.

[0002]

2. Description of the Related Art In a wavelength division multiplexing (WDM) optical transmission system that multiplexes a plurality of signal lights having different wavelengths and transmits them collectively, only a signal light of a desired wavelength is selectively received from input signal lights. An optical filter is essential for this.
Further, also in an optical amplification repeater transmission system in which optical amplifiers are connected in multiple stages, an optical filter is widely used to remove broadband noise light generated by the optical amplifiers and improve transmission characteristics.

In an optical transmission system using these optical filters, when there is a deviation between the center wavelength of the transmitted signal light and the center wavelength of the optical filter, the received light level is lowered and adjacent optical crosstalk is caused. , Or the deterioration of various transmission characteristics such as the increase of waveform distortion due to the wavelength dependence of the transmittance of the optical filter. Therefore, it is necessary to perform wavelength tracking control that causes the center wavelength of the optical filter to follow the center wavelength of the signal light.

As the optical filter wavelength tracking control, a method of utilizing the intensity modulation component of the signal light obtained by modulating the central wavelength of the optical filter is widely used.
That is, an error signal is generated by inputting a modulated signal having a frequency of about several hundred Hz to several KHz to the optical filter and synchronously detecting the intensity modulation component of the signal light generated by the same modulated signal as that input to the optical filter. Then, the central wavelength of the optical filter is controlled so that this error signal becomes zero.

Regarding the optical filter wavelength tracking control using the above-mentioned method, for example, "Suppression of response speed of small adaptive wavelength tunable optical filter using piezoelectric actuator" by Suemura et al., 1994 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers B-
951 and the like.

[0006]

In the conventional optical filter wavelength tracking control method as described above, the central wavelength of the optical filter is constantly modulated.

Among the wavelength tunable optical filters currently used in optical transmission systems, interference film optical filters and the like realize wavelength tunable operation by a mechanical mechanism such as a motor and a shaft. It is considered that the life of the optical filter is determined by the wear of the mechanism.

Therefore, when the wavelength tracking control of a structure in which a modulation signal is input and the mechanical mechanism is constantly operated is applied to an interference film optical filter or the like, the life of the optical filter may be significantly impaired.

The present invention has been made in view of the problems of the above-mentioned conventional technique, and it is possible to suppress the deterioration of the mechanical mechanism of the optical filter and extend the life of the optical filter. An object is to provide a wavelength tracking control method.

[0010]

In order to achieve the above object, the present invention has an optical filter whose center wavelength is modulated by inputting a control signal, and transmits a plurality of signal lights having different wavelengths. In the optical transmission system, an optical filter wavelength tracking control method for tracking the center wavelength of the optical filter to the center wavelength of the signal light, wherein the control signal is input to the optical filter intermittently. And

The control signals are an AC control signal and a DC control signal, and an AC signal is generated and output from the oscillator that generates and outputs the AC control signal and the signal light that has passed through the optical filter. An optical receiving circuit, a synchronous detection circuit that generates and outputs an error signal by synchronously detecting the AC signal output from the optical receiving circuit with the AC control signal, and the error output from the synchronous detection circuit. Generating the DC control signal based on a signal, using a control system having a processing circuit for outputting, the DC control signal in the time zone in which the input of the AC control signal to the optical filter is stopped, It is characterized by holding the value at the time when the input to the optical filter is stopped.

Further, the time from the start of the input of the AC control signal to the optical filter to the stop of the input of the AC control signal to the optical filter is a predetermined value. Characterize.

Further, after the input of the AC control signal to the optical filter is started, at the time when the absolute value of the voltage of the error signal becomes equal to or smaller than a predetermined value, the AC control signal to the optical filter. It is characterized by stopping the input of.

Further, the optical receiving circuit has a function of outputting a reception level signal corresponding to the reception level of the signal light, and the reception is performed after the input of the AC control signal to the optical filter is started. It is characterized in that the input of the AC control signal to the optical filter is stopped when the voltage of the level signal exceeds a predetermined value.

Further, the optical receiving circuit has a function of outputting a reception level signal corresponding to the reception level of the signal light, and after the input of the AC control signal to the optical filter is started, the reception signal is received. It is characterized in that the input of the AC control signal to the optical filter is stopped when the absolute value of the voltage change of the level signal with time becomes equal to or less than a predetermined value.

Further, the input of the AC control signal to the optical filter is started at a predetermined cycle.

Further, the optical receiving circuit has a function of outputting a reception level signal corresponding to the reception level of the signal light, and after the input of the AC control signal to the optical filter is stopped, the reception signal is received. It is characterized in that the input of the AC control signal to the optical filter is started when the voltage of the level signal becomes equal to or lower than a predetermined value.

Further, the optical receiving circuit has a function of outputting a reception level signal corresponding to the reception level of the signal light, and the reception at the time when the input of the AC control signal to the optical filter is stopped. When the voltage of the level signal is stored as a reference voltage, and after the input of the AC control signal to the optical filter is stopped, the reception level signal becomes equal to or lower than a predetermined ratio with respect to the reference voltage. The input of the AC control signal to the optical filter is started.

(Operation) In the present invention configured as described above, since the control signal for modulating the center wavelength of the optical filter is intermittently input to the optical filter, the control signal is input to the optical filter. The optical filter is not modulated during the non-operating time, and the life of the optical filter is extended due to deterioration of the mechanical mechanism. Here, the change in the wavelength of the signal light or the optical filter is a change in environmental conditions such as temperature or the deterioration of the element, and is considered to be very slow in time.Therefore, the AC control signal is intermittently transmitted to the optical filter. Inputting to does not affect the transmission characteristics of the main signal at all.

If the time intervals at which the input of the AC control signal to the optical filter is started are set to predetermined values, it is possible to configure a circuit that extends the life of the optical filter with a simple circuit.

Further, after the input of the AC control signal to the optical filter is started, the input of the AC control signal to the optical filter is stopped when the absolute value of the voltage of the error signal becomes equal to or less than a predetermined value. When configured, the wavelength tracking control operation can be performed more reliably.

Further, in the case where the stop of the input of the control signal to the optical filter is judged by using the reception level signal,
The time for inputting the AC control signal to the optical filter is shortened as compared with the control method in which judgment is performed using the error signal.

Further, when the input start of the AC control signal to the optical filter is judged by using the reception level signal, the AC control signal is not wastefully input as compared with the method of inputting in a fixed cycle. It is possible to further extend the life of the optical filter.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the principle of the optical filter wavelength tracking control method of the present invention, showing a state in which the optical filter input voltage of an AC control signal changes with time.

A shaded portion in FIG. 1 represents a time region in which an AC control signal for modulating the central wavelength of the optical filter is input to the optical filter. It is performed intermittently according to a start command and a stop command. In the control method according to the present invention, the life of the optical filter is extended due to deterioration of the mechanical mechanism in proportion to the ratio of the operating time of the optical transmission system to the time of inputting the AC control signal to the optical filter.

(First Embodiment) FIG. 2 is a diagram showing a first embodiment of a control system in the optical filter wavelength tracking control method of the present invention.

In this embodiment, as shown in FIG. 2, a laser diode 10 for outputting a signal light 41 and a laser diode 1 are provided.
A temperature control circuit 20 that controls the temperature of 0, an optical filter wavelength tracking control circuit 100 that selects light of a desired wavelength from the signal light 41, and an optical receiving circuit that receives light that has passed through the optical filter wavelength tracking control circuit 100. 30, a laser diode 10, an optical filter wavelength tracking control circuit 100, and an optical fiber transmission line 50 that connects the optical filter wavelength tracking control circuit 100 and the optical receiving circuit 30 and transmits light. The filter wavelength tracking control circuit 100 includes an optical filter 110 that passes only the signal light having a desired wavelength of the input signal light 41, and an optical coupler that branches the signal light that has passed through the optical filter 110 into 9: 1. 130, an optical receiving circuit 150 that receives the signal light branched by the optical coupler 130, and outputs an AC signal 45 having a constant frequency component, and an AC control circuit. An oscillator 170 which generates a signal 44, and outputs, the timer circuit 1 for outputting an oscillator output control signal 43 for controlling the timing of the output of the oscillator 170
90 and the AC control signal 44 output from the oscillator 170
And a synchronous detection circuit 160 for performing synchronous detection of the AC signal 45 output from the optical receiving circuit 150 at
The low-pass filter 180 that outputs the error signal 46 by passing the low-pass component of the AC signal that has been subjected to the synchronous detection in the above, and a linear amplifier by an operational amplifier, an integrating circuit, and a differentiating circuit are provided. Is input to the optical filter 110 based on the voltage value of the error signal 46.
PI for generating and outputting the DC control signal 42 to be input to
D processing circuit 200, hold circuit 140 that holds and outputs DC control signal 42 output from PID processing circuit 200, DC control signal 42 input via hold circuit 140, and AC control signal output from oscillator 170 And an adder 120 for adding and outputting the optical filter 110 and the output from the adder 120.
And the center wavelength of the optical filter 110 is modulated.

The optical filter wavelength tracking control method in the control system configured as described above will be described below.

When the signal light 41 is output from the laser diode 10, the pass band width 1n provided in the optical filter wavelength tracking control circuit 100 through the optical fiber transmission line 50.
The signal light 41 is received by the m optical filter 110. Here, in the laser diode 10, the temperature control circuit 20 controls the signal light 41 having a wavelength of 1552.00 nm to be stably output.

When the signal light 41 passes through the optical filter 110, the signal light is split into 9: 1 by the optical coupler 130, 10% of which is sent to the optical receiver circuit 30 and 90% of which is received by the optical receiver circuit 1.
50 respectively.

On the other hand, the oscillator 170 outputs an AC control signal 44 having a frequency of 1 kHz based on the timing of the oscillator output control signal 43 output from the timer circuit 190.

Here, as the oscillator output control signal 43, the AC control signal 44 is output at a predetermined time interval,
It controls to be stopped.

Frequency 1 kHz output from oscillator 170
The AC control signal 44 of z is input to the optical filter 110 via the adder 120, and the center wavelength of the optical filter 110 is modulated with an amplitude of 0.1 nm.

As a result, the AC signal 45 having a fundamental frequency of 1 kHz is output from the optical receiving circuit 150, and the synchronous detection circuit 1
At 60, after the synchronous detection is performed with the AC control signal 44, the error signal 46 is obtained by passing through the low pass filter 180, and is input to the PID processing circuit 200.

Then, in the PID processing circuit 200,
The DC control signal 42 is generated and output so that the error signal 46 becomes zero.

The DC control signal 42 output from the PID processing circuit 200 is held and output in the hold circuit 140 at a timing based on the oscillator output control signal 43 output from the timer circuit 190, and is added from the oscillator 170 in the adder 120. It is added to the output AC control signal 44 and sent to the optical filter 110.

As a result, the central wavelength of the optical filter 110 is modulated.

The center wavelength of the optical filter 110 is actually set to 15
When swept in the range of 42 nm to 1562 nm, an error signal curve as shown in FIG. 3 was obtained.

FIG. 3 is a diagram showing the measurement result of the error signal voltage with respect to the deviation between the signal light wavelength and the optical filter center wavelength of the optical filter wavelength control circuit 100 shown in FIG.

Further, the signal light 4 having a wavelength of 1552.00 nm
The initial value of the center wavelength of the optical filter 110 is 15 for 1
When the error signal 46 was observed after the input of the AC control signal 44 to the optical filter 110 was started at 51.50 nm, the results shown in FIG. 4 were obtained.

FIG. 4 is a diagram showing the time change of the error signal 46 shown in FIG.

As shown in FIG. 4, the error signal in this embodiment converges in about 200 ms after the AC control signal 44 starts to be input to the optical filter 110.

In the present embodiment, the AC control signal 4
The input start cycle and the input time of No. 4 are fixed values predetermined by the oscillator output control signal 43 output from the timer circuit 190. The start and stop of the output of the AC control signal 44 from the oscillator 170 is controlled by the oscillator output control signal 43 output from the timer circuit 190 being input to the power supply terminal of the oscillator 170. Further, the oscillator output control signal 43 is also input to the hold circuit 140, and when the AC control signal 43 is output, the DC control signal 42 output from the PID processing circuit 200 passes through the hold circuit 140 as a through. However, when the AC control signal 43 is not output, processing is performed such that the voltage value of the DC control signal 42 at the time when the output is stopped is held and continues to be output.

In the present embodiment, the AC control signal 4 is actually used.
The period of starting the input to the optical filter 110 of No. 4 was set to 10 seconds, and the input time was set to 500 ms.

A transmission experiment of 10 Gbps between the laser diode 10 and the optical receiving circuit 30 was carried out by using the above-described optical filter wavelength tracking control system. As a result, transmission without error was carried out for 24 hours. It was confirmed that the control system was operating without problems.

(Second Embodiment) FIG. 5 is a diagram showing a second embodiment of the control system in the optical filter wavelength tracking control method of the present invention.

In this embodiment, as shown in FIG. 1, a laser diode 10 for outputting a signal light 41 and a laser diode 1 are provided.
The temperature control circuit 20 for controlling the temperature of 0, the optical filter wavelength tracking control circuit 300 for selecting light of a desired wavelength from the signal light 41, the laser diode 10 and the optical filter wavelength tracking control circuit 300 are connected to each other, and The optical filter wavelength tracking control circuit 300 is configured to transmit only the signal light of a desired wavelength out of the input signal light 41 to the optical filter wavelength tracking control circuit 300.
And an optical receiving circuit 150 that receives the signal light that has passed through the optical filter 110 and outputs an AC signal 45 and a reception level signal 47 having constant frequency components, and an AC control signal 44.
The optical receiving circuit that generates and outputs the light receiving circuit using the oscillator 170, the switch 340 that controls the output timing of the AC control signal 44 output from the oscillator 170, and the AC control signal 44 output from the oscillator 170 via the switch 340. 1
The synchronous detection circuit 160 that performs synchronous detection of the AC signal 45 output from 50 and the low-frequency component that outputs the error signal 46 by passing the low-frequency component of the AC signal 45 that has been synchronously detected in the synchronous detection circuit 160. Band pass filter 18
0, an A / D converter 320a that converts the error signal 46 into a digital signal, an A / D converter 320b that converts the reception level signal 47 output from the optical receiving circuit 150 into a digital signal, and an input error The DC control signal 42 and the switch 340 based on the signal 46 and the reception level signal 47.
CPU 330, which is a processing circuit for generating and outputting an oscillator output control signal 43 for controlling the operation of the
D / A converters 310a and 310b for converting the DC control signal 42 and the oscillator output control signal 43 output from 30 into analog signals, respectively, and the DC control signal 42 and the AC control signal 44 output from the oscillator 170 are added. And an adder 120 for outputting the
Is output to the optical filter 110, and the center wavelength of the optical filter 110 is modulated.

The optical filter wavelength tracking control method in the control system configured as described above will be described below.

When the signal light 41 is output from the laser diode 10, the optical fiber transmission line 50 and the optical filter 11 are provided.
The signal light 41 is received by the optical receiving circuit 150 via 0.

When the signal light 41 is received by the optical receiving circuit 150, an AC signal 45 and a reception level signal 47 are output.

On the other hand, in the oscillator 170, the switch 3
An AC control signal 44 is output via 40.

The operation timing of the switch 340 is controlled by the oscillator output control signal 43 output from the CPU 330.

The AC signal 45 output from the light receiving circuit 150 is subjected to synchronous detection in the synchronous detection circuit 160 with the AC control signal 44 output via the switch,
The error signal 46 is obtained by passing through the low pass filter 180.

The error signal 46 and the reception level signal 47 are converted into digital signals in the A / D converters 320a and 320b, respectively, and input to the CPU 330.

The CPU 330 generates and outputs the DC control signal 42 and the oscillator control signal 43 based on the input reception level signal 47 and error signal 46. In the D / A converters 310a and 310b,
The DC control signal 42 and the oscillator control signal 43 output from the CPU 330 are each converted into an analog signal.

After that, in the adder 120, the DC control signal 42 and the AC control signal 4 output from the oscillator 170 are output.
4 and 4 are added and sent to the optical filter 110.

As a result, the central wavelength of the optical filter 110 is modulated.

In this embodiment, the CPU 330 sets the PID control period to 1 ms and acquires the error signal 46 and the reception level signal 47 at 1 ms intervals. In addition, the optical filter 1 for the signal light 41 having a wavelength of 1552.00 nm
When the initial value of the center wavelength of 10 was set to 155.50 nm and the error signal 46 and the reception level signal 47 after starting the wavelength tracking control were observed, the results shown in FIG. 6 were obtained.

FIG. 6 is a diagram showing the changes over time of the error signal 46 and the reception level signal 47 shown in FIG.

In the present embodiment, the input of the AC control signal 44 to the optical filter 110 is stopped when it is determined that the control has converged. When judging by the value of the signal 46, the convergence condition is defined as an absolute value of 0.1 V or less, while when judging by the convergence of the reception level signal 47, the convergence condition is defined as a time change rate of 0.01 V / ms or less, The time required for control was compared. As a result, as shown in FIG. 6, it was found that the control converges in 60 ms when judged by the error signal 46 and 50 ms when judged by the reception level signal 47.

Further, when the center wavelength of the optical filter 110 after the control is measured, it is 1552.01 nm when the control convergence is judged by the error signal 46, and 155 when the control convergence is judged by the reception level signal 47.
It was 1.96 nm, and in both cases, it was confirmed that the shift between the center wavelength of the optical filter and the signal light wavelength after the control was completed was sufficiently smaller than the bandwidth of the optical filter.

Subsequently, an intermittent optical filter wavelength tracking control experiment was conducted for 24 hours using the oscillator output control signal 43.

In this embodiment, the error signal 46 is used to judge whether the AC control signal 44 is stopped. At this time, in order to avoid making an erroneous stop determination due to the oscillatory behavior of the error signal 46 immediately after the input of the AC control signal 44 is started, the time of 40 ms from the start of the input of the AC control signal 44 is It was set not to judge the stop.

The output of the AC control signal 44 is started when the voltage of the reception level signal 47 becomes 0.1 V or less. This 0.1V is applied to the optical receiving circuit 150.
Is the minimum light receiving level at −30d
Corresponds to when Bm is reached. In addition, the reception level signal 47 was acquired every one minute while the AC control signal 44 was stopped.

As a result of the experiment under the above conditions,
The characteristic of the reception level signal 47 as shown in FIG. 7 was obtained.

FIG. 7 shows the reception level signal 47 shown in FIG.
It is a figure which shows the time change of. Here, the reception level signal 4
The time when 7 changes discontinuously from 0.1 V to 0.2 V is the time zone in which the wavelength tracking control is performed.

As shown in FIG. 7, the wavelength tracking control was performed 5 times in 24 hours. Since the time required for one control is 60 ms, the optical filter 11 can be used within 24 hours.
The time at which the center wavelength of 0 was modulated was 300 ms, and the result expected to extend the life of the optical filter by about 300,000 times as compared with the case of always modulating was obtained.

In this embodiment, the AC control signal 4
No. 4 of the error signal 4
However, it is also possible to make this judgment using the reception level signal 47. Further, even when the reception level signal 47 is used, the input stop is judged when the voltage of the reception level signal 47 becomes a predetermined value or more, and the voltage of the reception level signal 47 converges to a constant value. Therefore, both of determining the input stop can be applied.

Actually, a 24-hour control experiment was conducted under the condition that the input stop is judged when the reception level signal 47 converges to a constant value, and the time required for convergence is 50 ms per one time. The expected result was to extend the life of the filter by about 350,000 times.

In the present embodiment, the AC control signal 4
4 is determined by the absolute value of the voltage of the reception level signal 47, but this determination is made by the relative value of the reception level signal 47 based on the voltage of the reception level signal 47 when the input is stopped. It is also possible to do so.

That is, the voltage of the reception level signal 47 at the time when the input of the AC control signal 44 to the optical filter 110 is stopped is stored as a reference voltage, and the input of the AC control signal 44 to the optical filter 110 is stopped. After that,
It is also possible to start the input of the AC control signal 44 to the optical filter 110 at the time when the reception level signal 47 becomes less than or equal to a predetermined ratio with respect to the stored reference voltage.

Further, in the CPU 330, it is possible to standardize the input error signal 46 with the reception level signal 47 and then process the PID. In this case, the signal light input level capable of wavelength tracking control can be expanded.

[0074]

Since the present invention is constructed as described above, it has the following effects.

Since the control signal for modulating the central wavelength of the optical filter is intermittently input to the optical filter in the first and second aspects, the control signal is input to the optical filter. The optical filter is not modulated during the non-operating time, and the life of the optical filter can be extended due to deterioration of the mechanical mechanism.

In the third and seventh aspects, since the time intervals at which the input of the AC control signal to the optical filter is started are predetermined values, the life of the optical filter can be reduced by a simple circuit. Can be configured.

According to the fourth aspect of the present invention, after the input of the AC control signal to the optical filter is started, when the absolute value of the voltage of the error signal becomes equal to or less than a predetermined value, the AC control signal Since the input to the optical filter is stopped, a more reliable wavelength tracking control operation becomes possible.

In the fifth and sixth aspects, since the stop of the input of the control signal to the optical filter is judged by using the reception level signal, the control method by the error signal is used. In comparison, it is possible to shorten the time for inputting the AC control signal to the optical filter.

In the eighth and ninth aspects, since the start of input of the AC control signal to the optical filter is determined by using the reception level signal, it is more wasteful than the method of inputting in a fixed cycle. It is possible to further extend the life of the optical filter without inputting an AC control signal to.

[Brief description of drawings]

FIG. 1 is a diagram showing the principle of an optical filter wavelength tracking control method of the present invention.

FIG. 2 is a diagram showing a first embodiment of a control system in the optical filter wavelength tracking control method of the present invention.

FIG. 3 is a diagram showing a measurement result of an error signal voltage with respect to a deviation between a signal light wavelength and an optical filter center wavelength of the optical filter wavelength control circuit shown in FIG.

FIG. 4 is a diagram showing a time change of the error signal shown in FIG.

FIG. 5 is a diagram showing a second embodiment of a control system in the optical filter wavelength tracking control method of the present invention.

FIG. 6 is a diagram showing a time change of the error signal and the reception level signal shown in FIG.

7 is a diagram showing a time change of the reception level signal 47 shown in FIG.

[Explanation of symbols]

 10 Laser diode 20 Temperature control circuit 30,150 Optical receiving circuit 41 Signal light 42 DC control signal 43 Oscillator output control signal 44 AC control signal 45 AC signal 46 Error signal 47 Reception level signal 50 Optical fiber transmission line 100,300 Optical filter wavelength Tracking control circuit 110 Optical filter 120 Adder 130 Optical coupler 140 Hold circuit 160 Synchronous detection circuit 170 Oscillator 180 Low pass filter 190 Timer circuit 200 PID processing circuit 310a, 310b D / A converter 320a, 320b A / D converter 330 CPU 340 switch

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04J 14/02

Claims (9)

[Claims] 1. An optical transmission system having an optical filter, the center wavelength of which is modulated when a control signal is input, for transmitting a plurality of signal lights of different wavelengths, wherein the center wavelength of the optical filter is the signal light. An optical filter wavelength tracking control method for tracking the center wavelength of the optical filter, wherein the control signal is intermittently input to the optical filter. 2. The optical filter wavelength tracking control method according to claim 1, wherein the control signal is an AC control signal and a DC control signal, and an oscillator that generates and outputs the AC control signal, and the optical filter. An optical receiving circuit that generates and outputs an AC signal from the signal light that has passed through, and a synchronization that generates and outputs an error signal by synchronously detecting the AC signal output from the optical receiving circuit with the AC control signal. A control system having a detection circuit and a processing circuit for generating and outputting the DC control signal based on the error signal output from the synchronous detection circuit is used, and the AC control signal is input to the optical filter. The DC control signal in the stopped time zone holds the value at the time when the input to the optical filter is stopped, and the optical filter wavelength tracking control is characterized. Your way. 3. The optical filter wavelength tracking control method according to claim 2, wherein the input of the AC control signal to the optical filter is stopped after the input of the AC control signal to the optical filter is started. The optical filter wavelength tracking control method, wherein the time until is a predetermined value. 4. The optical filter wavelength tracking control method according to claim 2, wherein the absolute value of the voltage of the error signal is a predetermined value after the input of the AC control signal to the optical filter is started. An optical filter wavelength tracking control method, characterized in that the input of the AC control signal to the optical filter is stopped at the following times. 5. The optical filter wavelength tracking control method according to claim 2, wherein the optical receiving circuit has a function of outputting a reception level signal corresponding to a reception level of the signal light, After the input to the optical filter is started, the input of the AC control signal to the optical filter is stopped when the voltage of the reception level signal becomes a predetermined value or more. Wavelength tracking control method. 6. The optical filter wavelength tracking control method according to claim 2, wherein the optical receiving circuit has a function of outputting a reception level signal corresponding to a reception level of the signal light, After the input to the optical filter is started, the input of the AC control signal to the optical filter is stopped when the absolute value of the time change of the voltage of the reception level signal becomes equal to or less than a predetermined value. An optical filter wavelength tracking control method characterized by: 7. The optical filter wavelength tracking control method according to claim 2, wherein the input of the AC control signal to the optical filter is started.
An optical filter wavelength tracking control method, which is performed in a predetermined cycle. 8. The optical filter wavelength tracking control method according to claim 2, wherein the optical receiving circuit has a function of outputting a reception level signal corresponding to a reception level of the signal light, After the input to the optical filter is stopped, the input of the AC control signal to the optical filter is started at a time when the voltage of the reception level signal becomes equal to or lower than a predetermined value. Wavelength tracking control method. 9. The optical filter wavelength tracking control method according to claim 2, wherein the optical receiving circuit has a function of outputting a reception level signal corresponding to a reception level of the signal light, The voltage of the reception level signal at the time when the input to the optical filter is stopped is stored as a reference voltage, and after the input of the AC control signal to the optical filter is stopped, the reception level signal is referred to as the reference voltage. An optical filter wavelength tracking control method, wherein the input of the AC control signal to the optical filter is started at a time when the ratio becomes a predetermined ratio or less with respect to the voltage.