JP2001264713A - Bias control circuit for light modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent erroneous operation caused by unstable operation at the time of switching on a power source in a bias control circuit for an interferometer type light modulator having a periodical input-output characteristic. SOLUTION: A bias value of an LN-MZ light modulator 1 is monitored via an optical branching device 4 and a photo-electric converter 10 from output light of the light modulator 1; an error of an obtained monitor value to a target value is detected by an error detector 11 and a feedback loop is formed for feedback control; a loop open/close switch 13 inserted in this feedback loop is brought into an open state immediately after the power source 18 of a bias control circuit 9 is switched on by a switch control circuit 14, and the loop is brought into a closed state after the power source 18 has been stabilized; and immediately after the loop is closed, such a bias signal value as is larger than the minimum bias signal value by a value corresponding to a half period of the periodic input-output characteristic of the light modulator 1 or more and smaller than the maximum bias signal value by a value corresponding to a half period or more, is supplied to the light modulator 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a bias control circuit of an optical interferometer type optical modulator used for optical communication and the like.

[0002]

2. Description of the Related Art An optical interferometer type optical modulator, for example, a Mach-Zehnder (MZ) optical modulator (hereinafter referred to as an LN-MZ optical modulator) formed on a lithium niobate (LN) substrate.
Are often used as external optical modulators for high-speed optical communication. An optical interferometer-type optical modulator splits input light evenly into two paths and changes the optical path length difference between the two paths according to the magnitude of the signal input to the input end of the electric signal. The basic configuration is such that the transmittance is changed and the light passing through the two paths is combined and output.

If the optical path length difference between the two paths is 0, the light transmittance is 100% except for the excess loss due to imperfect elements, and
If the optical path length difference is half the wavelength (λ) of the input light,
The light transmittance is 0%. Such a characteristic also occurs when the optical path length difference is an integral multiple of the wavelength of the input light and an integral multiple of the input light plus a half wavelength. That is, the relationship between the optical path length difference between the two paths and the light transmittance has a periodic characteristic as shown in FIG. In other words, an optical interferometer-type optical modulator such as an LN-MZ optical modulator has a periodic input / output characteristic in which the intensity of output light periodically changes in response to a change in the magnitude of an input electric signal. Have.

When such an optical interferometer-type optical modulator is used for optical communication, a point of a certain light transmittance is generally biased, and modulation is performed with a data signal to be transmitted centered on the bias point. Is performed. For example, the steepness of the input / output characteristic in FIG.
%) Is set as a bias point, and modulation is performed around this point.

However, a bias signal, that is, a DC signal is applied to an electric signal input terminal of the optical interferometer type optical modulator,
Even if the magnitude of the applied signal is kept constant, the optical path length difference between the two paths fluctuates due to various factors during use,
In some cases, a fixed optical path length difference cannot always be obtained for the magnitude of the applied signal. For example, in the LN-MZ optical modulator, the change characteristic of the light transmittance with respect to the magnitude of the applied signal changes every moment due to a phenomenon called DC drift due to the movement of electric charge.

[0006] Such an optical interferometer type optical modulator, for example, L
In the N-MZ optical modulator, it is difficult to keep the bias point stable only by applying a constant DC voltage as a bias signal, and control for fixing the light transmittance to a certain bias point is required. Generally, feedback control is used for this bias control. Specifically, a part of the light output from the LN-MZ optical modulator is branched, and the light intensity is detected by the photoelectric converter, whereby the actual bias value of the LN-MZ optical modulator (at the bias point). And the bias signal (DC voltage) applied to the electric signal input terminal of the LN-MZ optical modulator is controlled so that the error of the monitor value with respect to the target value is minimized.

A bias control circuit for performing such control is constituted by an electronic circuit, and operates by receiving power supply from the outside. Generally, the power supply does not operate stably immediately after the power is turned on.
It stabilizes after a lapse of about 0 ms, and outputs a specified power supply voltage. The operation state of the bias control circuit fluctuates in an unstable manner until the power supply is stabilized and a specified power supply voltage can be output.

A case is considered in which a bias control circuit mainly composed of an operational amplifier circuit that can be simply and inexpensively configured is used to perform feedback control so that a bias value reaches a target value. If the input / output characteristics of the controlled object are monotonically decreasing or monotonically increasing, the operation of the controlled object when the power is turned on is slightly unstable, and even if the control start point of the bias value is unexpected, Can make the bias value reach the target value. However, when the LN-MZ optical modulator having the periodic input / output characteristic as shown in FIG. 4 is a control target, the bias value may not be able to reach the target value depending on the control start point.

This problem will be described with reference to FIG. In FIG. 5, the horizontal axis represents the applied voltage to the electric signal input terminal of the LN-MZ optical modulator, and the vertical axis represents the light transmittance. Target value of bias value (bias value corresponding to target light transmittance)
Is the point A, if the control start point of the bias value (the initial value of the bias voltage) is between the points C and B around the point A (within the pull-in range), the bias The voltage can reach point A along the direction of the arrow. On the other hand, when the control start point is outside the pull-in range, that is, when the control voltage is smaller than the point C or larger than the point B, the bias voltage does not go to the point A but to a point shifted by one cycle from the point A. Would.

In such a case, if the bias voltage can reach a point shifted by one cycle from the target value, the same light transmittance as that of the target value A at that point can be obtained. Although the operation of the bias control circuit is not a problem, the maximum or minimum value of the bias voltage that can be output by the bias control circuit is limited by the power supply voltage of the bias control circuit, so that a bias voltage shifted by one cycle from the point A is generated. You may not be able to. Therefore, if the control start point is out of the pull-in range due to the unstable operation until the power supply voltage of the bias control circuit is stabilized, the bias value cannot reach the target value and stops short of it.

[0011]

As described above, when a bias control circuit of an optical interferometer optical modulator having periodic input / output characteristics is constructed based on the conventional technique, the power supply of the bias control circuit is turned on. There is a problem that the bias value cannot reach the target value due to unstable operation at the time.

According to the present invention, a bias value of an optical modulator having a periodic input / output characteristic is ensured with respect to a target value even in an unstable condition at the time of power supply by using a configuration mainly including a relatively inexpensive analog circuit. It is an object of the present invention to provide a bias control circuit capable of causing the bias control signal to reach the threshold voltage.

[0013]

In order to solve the above-mentioned problems, the present invention has a periodic input / output characteristic in which the intensity of output light periodically changes with a change in the magnitude of an input electric signal. In a bias control circuit that controls a bias value of an optical modulator using a feedback loop, a loop open / close switch that opens and closes a feedback loop, and the loop open / close switch is set to a loop open state immediately after the bias control circuit is turned on, There are switch control means for closing the loop after the power is stabilized, and bias signal generation means provided in the feedback loop for generating a bias signal for controlling a bias value. The bias signal generating means is configured such that the value of the bias signal immediately after the loop opening / closing switch is in the loop closed state is larger than the minimum value of the bias signal by a value corresponding to a half cycle of the periodic input / output characteristic of the optical modulator, and A bias signal smaller than the maximum value of the bias signal by a value corresponding to a half cycle or more is generated.

In the bias control circuit configured as described above, the feedback control for controlling the bias value of the optical modulator is open immediately after the power is turned on, so that the feedback control is not performed. Then, for example, when a certain period of time elapses after the power is turned on and the power is stabilized, the feedback loop is closed and the feedback control is started.

Here, in order to guarantee that the value of the bias signal (control start point) immediately after the feedback loop is closed, the control start point is set to the bias signal value in the present invention. Smaller than the value corresponding to the half cycle of the periodic input / output characteristic of the optical modulator from the maximum value of
In addition, it is set to be larger than a value corresponding to a half cycle than the minimum value of the bias signal. In this way, a half-cycle section of the periodic characteristic of the optical modulator is secured between the control start point and the maximum value and the minimum value of the bias signal. Since the value exists, the bias signal can reliably reach the target value from the control start point, and malfunction at power-on is prevented.

According to one embodiment, the bias signal generating means monitors the bias value of the optical modulator to obtain a monitor value, and outputs an error signal corresponding to the error of the monitor value with respect to the target bias value. And a first low-pass filter to which the error signal is input via a loop open / close switch. The first low-pass filter forms a loop filter that determines a time constant of the bias control, and the bias signal is gradually controlled from the control start point toward the target value of the bias value by the filter. Further, in the bias signal generating means, an offset circuit is arranged at a stage subsequent to the loop on / off switch, for example, at the last stage, and the value of the bias signal immediately after the loop on / off switch is brought into the loop closed state by this offset circuit, that is, Is defined.

On the other hand, the switch control means converts the voltage of the power supply of the bias control circuit or a voltage obtained by dividing the voltage into a second low voltage having a time constant longer than the time required until the power supply is stabilized after the power is turned on. Input to the bandpass filter, this second
The loop open / close switch is controlled by the output of the low pass filter.

As described above, in the bias control circuit of the present invention, the bias value of the optical interferometer type optical modulator having periodic input / output characteristics and whose input / output characteristics fluctuate with time is supplied when the power is turned on. It is possible to reliably reach the target value without causing a malfunction due to unstable operation of.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. (System Configuration) FIG. 1 is a block diagram showing the configuration of the entire system according to an embodiment of the present invention. In the present embodiment, a bias control circuit for controlling a bias of an LN-MZ optical modulator, which is one of the optical interferometer type optical modulators, will be described.

The LN-MZ optical modulator 1 is a Mach-Zehnder (MZ) optical modulator formed on a lithium niobate (LN) substrate as described above. The LN-MZ optical modulator 1 has a light input / output end and an electric signal input end, and input light enters the light incident end via an input optical fiber 2. L
Output light from the light emitting end of the N-MZ optical modulator 1 is split by an optical splitter 4 via an optical fiber 3, and one of the splitters is guided to an output optical fiber 5. As the optical splitter 4, an optical coupler, a beam splitter, or the like can be used.

For example, a data signal to be transmitted is input to the signal input terminal 6, and after the bias signal from the bias control circuit 9 is superimposed by the bias T7, the LN-MZ
The signal is supplied to an electric signal input terminal of the optical modulator 1. The bias value of the LN-MZ optical modulator 1 is feedback-controlled by the bias control circuit 9. The bias control circuit 9 outputs the light 8 emitted from the light emitting end of the LN-MZ optical modulator 1 and split by the optical splitter 4 to monitor the intensity of the output light from the LN-MZ optical modulator 1. Is input to the bias control circuit 9.

(Regarding Bias Control Circuit) Next, the bias control circuit 9 will be described in detail. The bias control circuit 9 includes a photoelectric converter 10, an error detector 11, a loop open / close switch 13, a switch control circuit 14, a low-pass filter 15, and an offset circuit 16, and forms a feedback loop. The configuration of the bias control circuit 9 shown in FIG. 1 shows only a portion relevant to the gist of the present invention, and shows an amplifier for providing a loop gain necessary for actually configuring a circuit, a bias value, and the like. Circuits and the like for appropriately adjusting the target value are not shown. The bias control circuit 9 operates by receiving power supply from the power supply 18. The photoelectric converter 10 converts the monitor light 8 into an electric signal and detects the intensity of the output light from the LN-MZ optical modulator 1, and the output light intensity of the LN-MZ optical modulator 1 Since the light transmittance depends on the light transmittance and the light transmittance changes depending on the bias value, the photoelectric converter 10 outputs the monitor value of the bias value.

The error detector 11 compares the monitor value of the bias value of the LN-MZ optical modulator 1 from the photoelectric converter 10 with the target value of the bias value input from the target value input terminal 12. The method of generating the target value differs depending on the design of the device. For example, the target value is generated using a constant voltage generation circuit provided in the bias control circuit 9, or the target value is given from outside the bias control circuit 9.

The error detector 11 detects an error between the target value and the monitor value by comparing the target value and the monitor value, and outputs an error signal. Since the output of the photoelectric converter 10 is modulated at high speed by the data signal input from the signal input terminal 6, when the error detector 11 compares the output with the target value, the average value is used as the monitor value.
The averaging may be performed any time after the photoelectric conversion until the comparison is performed. Specifically, a low-pass filter for averaging may be provided inside the error detector 11, The photoelectric converter 10 may be configured to exhibit only a sufficiently slow response, or
An averaging circuit such as a low-pass filter may be inserted at any position between the error detector 11 and the error detector 11.

An error signal output from the error detector 11 is input to one end of a loop open / close switch 13. The loop opening / closing switch 13 is a switch for opening and closing a feedback loop formed by the bias control circuit 9.
Controlled off. Details of the switch control circuit 14 will be described later. A signal output from the other end of the loop opening / closing switch 13 is input to a low-pass filter 15 serving as a loop filter, and only low-frequency components determined by the frequency characteristics of the low-pass filter 15 are extracted and output. You. The output signal of the low-pass filter 15 is applied to an offset circuit 1
A predetermined DC level is added by 6 to form a bias signal, which is output from a bias signal output terminal 17. This bias signal is applied to the electrical signal input terminal of the LN-MZ type optical modulator 1 via the bias T7.

(Operation of Bias Control Circuit) Next, the operation of the bias control circuit 9 according to the present embodiment will be described. The bias control circuit 9 operates by receiving power supply from the power supply 18 and generates a bias signal. The loop open / close switch 13 is controlled by the switch control circuit 14, and is turned off immediately after the power supply 18 is turned on, that is, the feedback loop is opened. In this loop open state, the output of the error detector 11 is not transmitted to the low-pass filter 15.

The voltage of the bias signal in the open state of the loop, that is, the output voltage of the bias control circuit 9 is the control start point of the bias signal when the feedback loop is closed, and the voltage of the low-pass filter 15 This is the sum of the output voltage at the time of input and the voltage determined by the offset circuit 16. The bias control circuit 9 of FIG. 1 shows a block configuration divided for each function, and the low-pass filter 15
Is almost zero when there is no input, and the output voltage of the bias control circuit 9 in the loop open state is
6 shall be given only. Of course, due to the actual circuit configuration, the low-pass filter 15 and the offset circuit 16
Can be configured using one operational amplifier.

The output voltage of the bias control circuit 9 is applied as a bias signal to an electric signal input terminal of the LN-MZ optical modulator 1 via a bias T7. In the LN-MZ optical modulator 1, the voltage of the bias signal applied according to the input / output characteristics shown in FIG. 2 (the relationship between the voltage applied to the electric signal input terminal of the LN-MZ optical modulator 1 and the light transmittance). (Bias value)
The light is transmitted at a light transmittance corresponding to. Output light from the LN-MZ optical modulator 1 is converted into an electric signal by the photoelectric converter 10, and the average value of the electric signal is compared with a target value by the error detector 11 as a monitor value of the bias value. Error detector 1
1 outputs an error signal indicating the error of the monitor value with respect to the target value as described above. However, when the feedback loop is open, the feedback control for bringing the monitor value close to the target value is not performed. A very large error signal is output.

Next, after a certain period of time has elapsed since the power supply 18 was turned on and the voltage of the power supply 18 has stabilized, the switch control circuit 14 turns on the loop open / close switch 13 to close the feedback loop. When the loop is closed, a large error signal output from the error detector 11 is input to the low-pass filter 15 at once. The low-pass filter 15 gradually passes a large error signal by a time constant determined by its characteristics.

Immediately after the loop open / close switch 13 is turned on and the feedback loop is closed, the voltage of the bias signal output from the bias control circuit 9 starts to change. Since the signal passes through the band-pass filter 15, the voltage of the bias signal gradually changes from the control start point determined by the offset circuit 16. In this way, the feedback control of the bias signal is started from a desired control start point.

(Regarding the Control Start Point of the Bias Control Circuit) The bias control circuit 9 controls the voltage of the bias signal at the moment when the loop open / close switch 13 enters the closed state.
That is, the control of the bias value is started, but the control start point, which is the output voltage of the bias control circuit 9 immediately after the loop is closed, is required to be within a range that can converge to the target value, that is, within the pull-in range. However, as described in the related art, the input / output characteristics of the LN-MZ optical modulator 1 to be controlled are shifted in the horizontal axis direction because the input / output characteristics shown in FIG. Even if the control start point is adjusted to a point on a certain horizontal axis, there is no guarantee that the control start point will always be within the pull-in range.

On the other hand, in the present invention, the control start point of the bias signal by the bias control circuit 9 is set as follows, so that the control always starts even if the input / output characteristics of the LN-MZ optical modulator 1 change. Points can be made to fall within the pull-in range. Hereinafter, a method of setting the control start point will be described with reference to FIG.

In FIG. 2, Vmax and Vmin are the maximum value and the minimum value of the output voltage of the bias control circuit 9, and are determined mainly by the output voltage of the power supply 18 (power supply voltage) and the like. On the other hand, a change range of the applied voltage corresponding to a half cycle of the periodic input / output characteristic of the LN-MZ optical modulator 1 is defined as Vπ.

Here, in the present invention, the control start point Vb of the bias control circuit 9, that is, the initial value of the voltage of the bias signal output from the bias control circuit 9 at the moment when the feedback loop is closed, is (Vmin + Vπ) < Vb <
(Vmax−Vπ). That is, the control start point Vb is set to a value V corresponding to a half cycle of the periodic input / output characteristic from the minimum value Vmin of the output voltage of the bias control circuit 9.
It is set to be larger than π and smaller than the value Vπ corresponding to a half cycle from the maximum value Vmax of the output voltage of the bias control circuit 9.

When the control start point Vb is set within this range, Vb and the maximum value Vm of the output voltage of the bias control circuit 9 are set.
Since the change ranges Vπ1 and Vπ2 of the bias signal voltage for each half cycle are secured between ax and the minimum value Vmin, the bias signal voltage (bias value) is always included in any of the change ranges of the bias signal voltage. There is a target value that becomes the convergence point of.

That is, when the feedback loop is closed, the monitor value of the bias value obtained corresponding to the control start point Vb at that moment is larger or smaller than the target value, that is, the error detector In accordance with the polarity of the error signal output from 11, the bias value is controlled within one of the ranges of Vπ1 and Vπ2.
Since (Vπ1 + Vπ2) is one cycle of the periodic input / output characteristic of the LN-MZ optical modulator 1, these Vπ1, Vπ2
There is always a target value in any of the control ranges including. In the example of FIG. 2, target values exist in both of these ranges Vπ1 and Vπ2. Therefore, after the feedback loop is closed, the bias value is controlled so as to always reach the target value, and malfunction at power-on can be prevented.

(Regarding the Arrangement Order of Each Block in the Bias Control Circuit) In this embodiment, the control start point Vb
Described above, (Vmin + Vπ) <Vb <(Vmax−V
π), the error detector 11
It is important to arrange the loop open / close switch 13 and the low-pass filter 15 in the order shown in FIG.
Also, the offset circuit 16 for giving the control start point Vb is arranged at the output stage of the bias control circuit 9 as shown in FIGS. 1 and 2, or at a position near the output stage, at least after the loop open / close switch 13. It is necessary to. The reason is as follows.

When the feedback loop is open, the error detector 11 generally outputs a large error signal.
Here, if the loop open / close switch 13 is assumed to be the error detector 11
If provided at an earlier stage, the output of the error detector 11 is the control target L even when the feedback loop is open.
The signal is transmitted to the electric signal input terminal of the N-MZ optical modulator 1, and the bias control circuit 9 applies a large bias signal obtained by amplifying the large error signal to the optical modulator 1. As a result, in a situation where the loop gain of the feedback loop is sufficiently high, the voltage of the maximum value Vmax or the minimum value Vmin of the bias signal that can be generated in the bias control circuit 9 is applied to the LN-MZ modulator 1, so The start point Vb becomes the maximum value Vmax or the minimum value Vmin of the bias value control range, and the above-mentioned (Vmi
(n + Vπ) <Vb <(Vmax−Vπ). Therefore, by inserting the loop opening / closing switch 13 after the error detector 11 as shown in FIG. 1, the error signal is amplified in a state where the feedback loop is open and is not applied to the LN-MZ optical modulator 1. There is a need.

On the other hand, the low-pass filter 15 is used as a loop filter for determining the time constant of the feedback control. By the operation of the filter 15, the bias value of the LN-MZ optical modulator 1 is changed from the control start point Vb to the target value. Controlled gradually. Here, in the present invention, the loop open / close switch 13 is turned on and the feedback loop is closed after a predetermined time has elapsed after the power supply 18 is turned on.
5, the large error signal output from the error detector 11 is filtered by the filter 15 immediately before the loop is closed.
Is likely to be passed. Therefore, when the loop opening / closing switch 13 is turned on, a large error signal
The bias signal is applied to the N-MZ optical modulator 1, and the control start point Vb becomes the maximum value Vmax or the minimum value Vmin of the bias value control range. For such a reason, in order to gradually control the bias value of the LN-MZ optical modulator 1 from a desired control start point Vb, it is necessary to insert the loop opening / closing switch 13 before the low-pass filter 15. .

Further, when the low-pass filter 15 is disposed before the error detector 11, the error signal which has been increased by opening the feedback loop closes the LN-MZ optical modulator after the loop is closed. 1 cannot be obtained, so that the control start point Vb becomes the maximum value Vmax or the minimum value Vmin of the bias control range regardless of the position of the loop open / close switch 13 at any position.

From the above, in order to start the control of the bias value from the control start point Vb which is more than a certain magnitude (Vπ) away from the maximum value Vmax or the minimum value Vmin of the bias control range, the control start point An offset circuit 16 for giving a bias value of Vb is arranged at least at a stage subsequent to the loop opening / closing switch 13, and the error detector 11, the loop opening / closing switch 13 and the low-pass filter 15 are sequentially arranged along the signal transmission direction. There is a need.

With this configuration, according to the present embodiment, the bias value of the LN-MZ optical modulator 1 having periodic input / output characteristics and whose input / output characteristics fluctuate with time is supplied to the power supply. It is possible to control to a desired value without causing a malfunction due to an unstable operation at the time of closing.

Although the LN-MZ optical modulator is more frequently used in high-speed optical communication than in the past, at present, the problem of DC drift described in the prior art has not been solved.
Further, the voltage corresponding to the period of the periodic input / output characteristic is relatively large. Therefore, the bias control circuit according to the present invention is LN-
This is particularly effective as a bias control circuit for an MZ optical modulator.

(Specific circuit of bias control circuit)
FIG. 3 shows a specific circuit example of the bias control circuit 9. This circuit is mainly composed of an operational amplifier. The photoelectric converter 10 includes a photodiode PD and a preamplifier A1, and a part (monitor light) 8 of the output light of the LN-MZ optical modulator 1 that is branched by the optical branching unit 4 is converted into a current by the photodiode PD. , And is further converted and converted into a voltage by the preamplifier A1.

From the output of the photoelectric converter 10, LN-MZ
A monitor value obtained by determining the bias value of the optical modulator 1 from the light intensity of the monitor light 8 is obtained. This monitor value is output to the error detector 1
1 is input. The error detector 11 utilizes the differential amplification function of the operational amplifier A2, and has a target value input terminal 1
The difference between the target value of the bias value input from 2 and the monitor value (error of the monitor value with respect to the target value) is detected, and an error signal having a polarity and a voltage level corresponding to the error is output.

The output from the error detector 11 is connected to one end of a loop open / close switch 13. In this example, the loop opening / closing switch 13 is a relay switch having an exciting coil 13a and a contact 13b. The contact 13b is always off, and is turned on when a control current flows through the exciting coil 13a. The other end of the loop open / close switch 13 is connected to the input of the low-pass filter 15. Low pass filter 15
In this example, the amplifier comprises an operational amplifier A3, a capacitor C1, and a resistor R1, and has an amplifying function to provide an appropriate loop gain to a feedback loop. The gain of the low-pass filter 15 can be adjusted by a variable resistor VR1.

The output of the low-pass filter 15 is input to an offset circuit 16, and the output of the offset circuit 16 is connected to a bias signal output terminal 17 of the bias control circuit 9. The offset circuit 16 is mainly composed of an operational amplifier A4, and outputs an output voltage by a variable resistor VR2 connected between a positive power supply voltage + V and a negative power supply voltage -V provided from the power supply 18 in FIG. Can be adjusted. That is, the output voltage of the offset circuit 16 is (Vmin + Vπ) <Vb <(Vmax)
−Vπ) is adjusted to give a control start point Vb within the range.

The switch control circuit 14 for controlling the loop open / close switch 13 formed of a relay includes a voltage dividing circuit 21 for dividing the power supply voltage + V supplied from the power supply 18 in FIG.
It comprises a low-pass filter 22 (second low-pass filter) to which the output of the voltage dividing circuit 21 is input, and a relay drive transistor 23 which is turned on / off by the output of the low-pass filter 22. Note that the voltage dividing circuit 21 may be removed, and the power supply voltage + V may be directly input to the low-pass filter 22.
The relay drive transistor 23 forms a grounded-emitter amplifier, inputs the output of the low-pass filter 22 to the base, and supplies a drive current to the excitation coil 13b of the loop open / close switch 13 from the collector.

The low-pass filter 22 is composed of a resistor R2 and a capacitor C2, and its time constant R2 · C2 is relatively large, for example, about 1 to 10 msec. When the current of the power supply 18 in FIG. 1 is particularly large, for example, 100 msec.
に 1 sec. That is, after a sufficient time has elapsed after the power supply from the power supply 18 to the bias control circuit 9 is started and the power supply voltage is stabilized, a voltage for turning on the relay drive transistor 23 is output from the low-pass filter 22. , The time constant is set.

When the relay drive transistor 23 is turned on, a current flows through the exciting coil 13a of the relay which is the loop open / close switch 13, the contact 13b is turned on, and the feedback loop is closed. As a result, the voltage of the bias signal output from the bias control circuit 9 starts to change from the above-described control start point Vb, and thereafter, LN- is adjusted so that the voltage level of the error signal output from the error detector 11 becomes minimum. The bias value of the MZ optical modulator 1 is feedback-controlled.

The present invention is not limited to the above embodiment, but can be implemented with various modifications as follows. (1) In the above embodiment, the optical interferometer type optical modulator is L
The case where the N-MZ optical modulator is used has been described.
The present invention can be applied to a case where a Michelson interferometer type optical modulator formed on a waveguide or an MZ optical modulator formed on a semiconductor substrate is used. (2) In the above embodiment, a voltage is applied as a bias signal to the optical interferometer type optical modulator, but a current may be applied depending on the configuration of the optical interferometer type optical modulator. . (3) In the above embodiment, the optical splitter 4 is used to monitor the output light intensity of the LN-MZ optical modulator 1, but the optical interferometer type optical modulator usually has two outputs (a positive output and a positive output). Since only one of the two outputs is taken out and used normally, such an optical splitter is not used, and the other of the two outputs that is not used is photoelectrically converted as monitor light. You may make it input into the container 10. (4) In the above embodiment, the loop open / close switch 1
3 is in a loop open state immediately after the power supply 18 is turned on.
In order to close the loop after the power supply becomes stable, a low-pass filter 22 is provided in the switch control circuit 14 so that the switch 13 is turned on when a certain period of time has elapsed after the power supply 18 is turned on. The voltage or a voltage obtained by dividing the voltage is monitored by a voltage comparator. When the voltage reaches a predetermined value obtained when the power supply 18 is stabilized, the switch 13 is turned on to close the loop. It may be. (5) In the above embodiment, a relay is used as the loop open / close switch 13, but it goes without saying that an electronic switch such as a MOSFET switch can be used.

[0052]

As described above, according to the present invention, in a bias control circuit of an optical interferometer type optical modulator having periodic input / output characteristics and having input / output characteristics fluctuating with time, Malfunction due to unstable operation at power-on can be prevented. Further, the bias control circuit of the present invention can be realized at a low cost because it can be realized by a simple analog circuit mainly composed of an operational amplifier.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an LN-MZ optical modulator and a bias control circuit thereof according to an embodiment of the present invention.

FIG. 2 is a view for explaining a control start point of bias control for explaining the operation of the embodiment;

FIG. 3 is a circuit diagram showing a more specific configuration example of the bias control circuit according to the embodiment;

FIG. 4 is a diagram showing input / output characteristics of an optical interferometer type optical modulator;

FIG. 5 is a diagram for explaining an operation of a bias control circuit of an optical interferometer type optical modulator according to the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... LN-MZ optical modulator 2 ... Input optical fiber 3 ... Optical fiber 4 ... Optical splitter 5 ... Output optical fiber 6 ... Data signal input terminal 7 ... Bias T 8 ... Light intensity monitor light 9 ... Bias control circuit 10 ... Photoelectric converter 11 error detector 12 target value input terminal 13 loop open / close switch 14 switch control circuit 15 first low-pass filter 16 offset circuit 17 bias control signal output terminal 18 bias control circuit Power supply 21: voltage dividing circuit 22: second low-pass filter 23: relay driving transistor

Continued on the front page (72) Inventor Masayuki Fujise 3-4 Hikarinooka, Yokosuka City, Kanagawa Prefecture F-term (reference) 2H079 AA02 AA12 BA01 CA04 DA03 EA05 FA04 HA11 KA11 KA20

Claims (5)

[Claims] 1. A bias for controlling, using a feedback loop, a bias value of an optical modulator having a periodic input / output characteristic in which the intensity of output light periodically changes in response to a change in the magnitude of an input electric signal. In the control circuit, a loop open / close switch for opening / closing the feedback loop, and a switch control for setting the loop open / close switch to a loop open state immediately after the power supply of the bias control circuit is turned on, and a loop closed state after the power supply is stabilized. Means, provided in the feedback loop, and a bias signal generating means for generating a bias signal for controlling the bias value, wherein the bias signal generating means has the loop open / close switch in a loop closed state. The value of the immediately following bias signal corresponds to a half cycle of the periodic input / output characteristic from the minimum value of the bias signal. A bias control circuit for an optical modulator, which generates a bias signal that is larger than a threshold value and smaller than a value corresponding to a half cycle of the maximum value of the bias signal. A bias signal generator for monitoring a bias value of the optical modulator to obtain a monitor value; and outputting an error signal corresponding to an error of the monitor value with respect to a target value of the bias value. An error detector, and a first signal to which the error signal is input via the loop open / close switch.
2. The bias control circuit for an optical modulator according to claim 1, further comprising: a low-pass filter. 3. The apparatus according to claim 2, wherein said bias signal generating means has an offset circuit at a stage subsequent to said loop on / off switch for defining a value of a bias signal immediately after said loop on / off switch enters a loop closed state. 1 or 2
3. A bias control circuit for an optical modulator according to claim 1. 4. The switch control means includes a second low-pass filter having a time constant longer than a time required for the power supply to stabilize, which receives a voltage of the power supply or a voltage obtained by dividing the voltage. 4. A bias control circuit for an optical modulator according to claim 1, wherein said loop open / close switch is controlled by an output of said second low-pass filter. 5. The optical modulator according to claim 1, wherein the optical modulator is a Mach-Zehnder type optical modulator formed on a lithium niobate substrate. Bias control circuit.