JP2001159749A - Optical modulation device and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an optical modulation device capable of holding its operation point when the operation point of an optical modulator is set in a maximum value point or a minimum value point of a modulation curve. SOLUTION: A superposed circuit 23 mutually superposes a main modulation signal A1 and a low frequency signal A2 to output it to the optical modulator 2 through a capacitor 24. A low-pass filter 27 transmits the component of the low frequency signal A2 through from the signal that a branch beam B1 of a modulated beam is converted to an electric signal by a photoelectric conversion circuit 26 to output a low frequency component signal C1. A differential amplifier 30 amplifies a voltage of a difference between the signal C2 and the signal C3 subjected to sample-hold from the low frequency component signal C1 with pulse signals A3, A4 by sample-hole circuits 29a, 29b. An addition/drive circuit adds the amplified voltage to an initial bias voltage generated from a variable voltage generator to output to the optical modulator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator and a method for use as an optical switch in an optical communication field.

[0002]

2. Description of the Related Art In an optical communication system, an optical modulator having an external optical modulator for performing optical modulation outside a light source is used for an optical switch for switching by turning on / off output light. One of the external light modulators of the light modulation device is a waveguide type light intensity modulator (hereinafter, referred to as a light modulator), and a modulation signal and a bias for performing light modulation are applied to the light modulator. An optical modulation control circuit is used as a circuit that generates and outputs a voltage. FIG. 10 is a block diagram showing a configuration of an optical modulation device including the optical modulator 2 and a conventional optical modulation control circuit 100. In this figure, an optical modulation control circuit 100 includes a modulation signal generator 101 and a bias voltage generator 102. The light generated by the light source 1 is input to the optical modulator 2 and the modulated signal generator 1
The modulated light is output from the optical modulator 2 as modulated light whose intensity is modulated in accordance with the modulation signal generated by the signal generator 01 and the bias voltage generated by the bias voltage generator 102. This optical modulator 2
For example, when a Mach-Zehnder type optical modulator is used, the optical output is intensity-modulated as shown in the following equation. Iout / Iin = (1/2) × {1 + sin [(π / 2) × ((V + Vdc) / Vm) + Φ]｝ (0) where Iout is the output light intensity, Iin is the input light intensity, and V Is a modulation signal, Vdc is a bias voltage, Vm is a half-wavelength voltage of the optical modulator 2, and Φ is an optical phase angle (optical phase) of the optical modulator 2.

FIG. 11 is a waveform diagram showing an example of the optical output characteristic when the Mach-Zehnder type optical modulator 2 having the modulation characteristic shown in the equation (0) is used. In this figure, Wv1 is a modulation curve of the optical modulator 2, and the input light is intensity-modulated to an optical output shown on the vertical axis according to an applied voltage taken on the horizontal axis. Wv5 is a modulation signal generated by the modulation signal generator 101, and is a half-wavelength voltage having an amplitude of the modulation curve Wv1.
This is a Vm pulse signal. Wv3 is the modulation signal Wv
5 and the bias voltage generated by the bias voltage generator 102
9 is a waveform showing the optical output of the modulated light modulated according to Vx. In FIG. 11, the operating point of the optical modulator 2 is set to the middle point Pt1 of the light intensity of the modulation curve Wv1 by setting the bias voltage to Vx. The light output waveform Wv3 at this time is excellent because the extinction ratio, which is the ratio between the maximum value and the minimum value of the light intensity, is large.

In such an optical modulator 2, the optical phase Φ of the above equation (0) changes due to aging, environmental temperature change, DC (direct current) drift due to applied DC voltage, and the like. Change, for example, FIG.
Is shifted to the modulation curve Wv2. Due to such a shift, the operating point of the optical modulator 2 changes from the middle point Pt1 of the light intensity of the modulation curve Wv1 to the point Pt3 of the modulation curve Wv2. As a result, the optical output waveform Wv4 becomes a distorted waveform, and the extinction ratio of the optical output is small, which is not good. In such a case, it is necessary to change the bias voltage from Vx to Vy and set the operating point to the middle point Pt2 of the light intensity of the modulation curve Wv2. As a method for solving this problem, for example, Japanese Patent Application Laid-Open No.
A method described in JP-A-249418 is known.

[0005]

However, the above-mentioned conventional solution requires a complicated circuit such as a dither circuit. Further, the conventional solutions are methods in which the operating point of the optical modulator is set to the midpoint of the light intensity of the above-described modulation curve, and the operating point that has recently begun to be adopted is the maximum value of the modulation curve. Not applicable when setting to point or minimum point. The present invention has been made in view of such circumstances, and its purpose is to set the operating point of the optical modulator to the maximum value point or the minimum value point of the modulation curve, and to deal with environmental changes and aging. On the other hand, it is an object of the present invention to provide an optical modulation device and a method capable of stably maintaining its operating point. It is another object of the present invention to provide an optical modulator and a method which can be realized by a simpler circuit than the conventional solution described above.

[0006]

According to a first aspect of the present invention, there is provided an optical intensity modulator for changing an optical intensity in accordance with an input modulation signal and a bias voltage. In the optical modulation device, to generate a main modulation signal and a low frequency signal of a single frequency lower than the main modulation signal,
A low-frequency superimposed modulation signal generation unit that superimposes the main modulation signal and the low-frequency signal to generate the modulation signal, and a signal obtained by partially splitting the modulated light output by the light intensity modulator and performing photoelectric conversion, A low-frequency transmission unit that attenuates the main modulation signal component and generates a low-frequency component signal by transmitting the low-frequency signal component; and the low-frequency component signal at a predetermined positive polarity phase of the low-frequency signal. And controlling the increase / decrease of the bias voltage according to a difference between a first sample value sampled from the second sample value and a second sample value sampled from the low frequency component signal at a predetermined phase of a negative polarity of the low frequency signal. And a bias voltage control means.

According to a second aspect of the present invention, in the first aspect of the invention, the first sample value is sampled from the low frequency component signal for each predetermined positive polarity phase of the low frequency signal. And the second sampled value is a value sampled from the low-frequency component signal for each predetermined negative polarity phase of the low-frequency signal. According to a third aspect of the present invention, in the first or second aspect, the phase at which the first sample value is sampled is a phase of a positive extreme value of the low frequency signal. The phase at which the second sample value is sampled is a phase of a negative extreme value of the low frequency signal.

According to a fourth aspect of the present invention, there is provided an optical modulator having an optical intensity modulator for changing an optical intensity in accordance with an input modulation signal and a bias voltage, wherein the main modulation signal and the main modulation signal are compared with each other. A low-frequency superimposed modulation signal generating means for generating a low-frequency signal of a low single frequency and superimposing the main modulation signal and the low-frequency signal to generate the modulation signal; and A low-frequency transmission unit that attenuates the main modulation signal component from a signal obtained by partially splitting and outputting the modulated light to be output and transmitting the low-frequency signal component to generate a low-frequency component signal; A first sample value for sampling a predetermined value of the low-frequency component signal in a positive-polarity phase section of the frequency signal, and a second sample value for sampling a predetermined value of the low-frequency component signal in a negative-polarity phase section of the low-frequency signal. Ninosa According to a difference between the pull values are those formed by and a bias voltage control means for controlling the increase and decrease of the bias voltage.

According to a fifth aspect of the present invention, in the fourth aspect of the invention, the first sampled value is a predetermined value of the low frequency component signal in a positive polarity phase section of the low frequency signal. The second sampled value is a value sampled every cycle of the low-frequency signal, and the second sampled value samples a predetermined value of the low-frequency component signal in a negative-polarity phase section of the low-frequency signal every cycle of the low-frequency signal. Characteristic value. In the invention according to claim 6, in the invention according to claim 4 or 5, the predetermined value sampled as the first sample value is the low value in the positive polarity phase section of the low frequency signal. The predetermined value sampled as the second sample value is the maximum value or the minimum value of the low frequency component signal in the negative polarity phase section of the low frequency signal. There is a feature.

According to a seventh aspect of the present invention, in the sixth aspect, the bias voltage control means selects either the maximum value or the minimum value sampled as the first sample value. It is characterized by comprising a first sample value selecting means and a second sample value selecting means for selecting either the maximum value or the minimum value sampled as the second sample value. The invention according to claim 8 is the invention according to any one of claims 1 to 7, wherein the bias voltage control means includes bias voltage control inversion means for inverting the increase or decrease of the bias voltage. It is characterized by the following.

According to a ninth aspect of the present invention, there is provided an optical modulation apparatus having an optical intensity modulator for changing an optical intensity according to an input modulation signal and a bias voltage, wherein the main modulation signal and the main modulation signal are compared with each other. A low-frequency signal having a low single frequency, and superimposing the main modulation signal and the low-frequency signal to generate the modulation signal; A second process of generating a low frequency component signal by attenuating the main modulation signal component and transmitting the low frequency signal component from a signal obtained by partially splitting the modulated light and photoelectrically converting the low frequency signal; A difference between a first sample value sampled from the low frequency component signal at a predetermined phase of positive polarity and a second sample value sampled from the low frequency component signal at a predetermined phase of negative polarity of the low frequency signal According to the bias voltage And having a third step of controlling the increase and decrease.

According to a tenth aspect of the present invention, in a light modulation device having a light intensity modulator for changing light intensity in accordance with an input modulation signal and a bias voltage, a main modulation signal and a ratio between the main modulation signal and the main modulation signal are reduced. A low-frequency signal having a low single frequency, and superimposing the main modulation signal and the low-frequency signal to generate the modulation signal; From the signal obtained by partially splitting the modulated light and performing photoelectric conversion,
A second process of attenuating the main modulation signal component and generating a low frequency component signal by transmitting the low frequency signal component, and the low frequency component signal in a positive polarity phase section of the low frequency signal. In accordance with a difference between a first sample value that samples a predetermined value and a second sample value that samples a predetermined value of the low-frequency component signal in a negative-polarity phase section of the low-frequency signal, And a third step of controlling increase / decrease.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the light modulation device according to the first embodiment. In this figure, 1 is a light source, 2 is a Mach-Zehnder type optical modulator,
Reference numeral 3 denotes an optical modulation control circuit. First, the configuration of the optical modulator 2 will be described. The light output from the light source 1 is an optical fiber 4
The light is input to the input waveguide 11 of the optical modulator 2 via a. Light input to the input waveguide 11 is converted into a Y branch 12
As a result, the same amount of light is branched and output to the waveguide 14a and the waveguide 14b. The waveguide 14a and the waveguide 1
The optical phase of the light branched to 4b is modulated by the modulation electrodes 13a and 13b provided on the waveguides 14a and 14b in accordance with the applied modulation voltage V1. The light modulated by these modulation electrodes 13a and 13b is further subjected to Y phase after the change in the optical phase according to the bias voltage V2 is superimposed by the bias electrodes 15a and 15b arranged in series with the modulation electrodes 13a and 13b, respectively. Combined by multiplexing 16. The light combined by the Y multiplex 16 is output to the optical fiber 4b via the output waveguide 17 as modulated light whose light intensity has changed. The modulation electrode 1
For 3a and 13b, a traveling-wave type electrode is usually used, and a terminal resistor (not shown) is connected to the terminal.

Next, the configuration of the light modulation control circuit 3 will be described. In FIG. 1, 21 is a single frequency f, peak-t
A modulation signal generator for generating a main modulation signal A1 having an o-peak amplitude (hereinafter referred to as a pp amplitude) of 2 · Vm, and a modulation signal generator 22 having a frequency sufficiently lower than the frequency f of the main modulation signal A1 Low-frequency signal generator for generating a low-frequency signal A2 of
Is a superimposing circuit for superimposing the low-frequency signal A2 on the main modulation signal A1, and 24 is a modulation electrode 1
This is a capacitor to be applied to 3a and 13b. 25
Is an optical splitter that splits the modulated light input via the optical fiber 4b, and 26 is a photoelectric conversion circuit that converts the split light B1 split by the optical splitter 25 into an electric signal.
Reference numeral 27 denotes a low-frequency transmission band in which the main modulation signal A1 component is attenuated among the modulation signal components included in the output signal of the photoelectric conversion circuit 26, and the low-frequency signal A2 component transmits the signal component without distortion. It is a low-pass filter having. Reference numeral 28 denotes pulse signals A3 and A4 as shown in FIG.
Wherein the pulse signal A3 is a half cycle A2a having the maximum value of the low frequency signal A2.
The pulse signal A4 is generated for each pole P10a of the half cycle A2b having the minimum value of the low frequency signal A2.
Generated every time. Reference numerals 29a and 29b denote sample and hold circuits which sample and hold the low-frequency component signal C1, which is the output of the low-pass filter 27, at the timing of the input pulse signals A3 and A4. Reference numeral 30 denotes a differential amplifier for amplifying the voltage difference between the signals C2 and C3, which are the hold outputs of the sample and hold circuits 29a and 29b, by a factor of G, and 31 an inverting amplifier for inverting the sign of the signal C4 which is the output of the differential amplifier 30. . 32 is the initial bias voltage Vd
A variable voltage generator for generating c, a switch 33 for switching and selecting between the signal C4 and the signal C5 and outputting it as a signal C6, and a switch 34 for adding the initial bias voltage Vdc and the voltage of the signal C6 to obtain a bias voltage V2 This is an addition / drive circuit applied to the bias electrodes 15a and 15b.

Next, the operation of the optical modulator according to the first embodiment will be described with reference to FIGS.
First, referring to FIG. 3, the minimum value point P1 of the modulation curve W1
Next, the optical modulation when the operating point of the optical modulator 2 having the optical modulation characteristic of the modulation curve W1 is set will be described. First,
The initial state will be described. However, low frequency signal generator 2
No. 2 does not generate the low frequency signal A2, and the voltage of the signal C6 is also 0. In order to set the operating point to the minimum value point P1 of the modulation curve W1, the optical modulator 2 has a bias voltage V
2, the initial bias voltage Vdc is applied to the bias electrode 15a,
15b. In this state, when the voltage of the main modulation signal A1 generated by the modulation signal generator 21 is applied to the modulation electrodes 13a and 13b via the capacitor 24, the light input to the input waveguide 11 changes the modulation curve W1. Is modulated in accordance with the above, and light having the output characteristic of the optical output waveform W2 is output to the optical fiber 4b. This optical output waveform W2 has a frequency 2f which is twice the frequency of the main modulation signal A1.
RZ (Return-to-Zero) signal. When the operating point is set to the maximum value point P2 of the modulation curve W1, the initial bias voltage may be set to Vdca, and its output characteristics are shown in the optical output waveform W3.

Next, with reference to FIG. 4, a description will be given of a phenomenon in which the operating point of the optical modulator 2 having the optical modulation characteristic shown in FIG. 3 changes and the optical output waveform W2 is distorted. In the optical modulator 2, the optical phase Φ of the modulation curve W1 expressed by the above equation (0) changes due to a temperature change, an aging change, or a DC drift. As a result, the modulation curve W1 shifts to the modulation curve W1a, and the optical output waveform W2 becomes a distorted optical output waveform W2a by changing the operating point from the minimum value point P1 to the point P3. This shifted modulation curve W
The desired operating point in 1a is the minimum value point P1a,
The bias voltage V2 corresponding to the minimum value point P1a is Vc.

Therefore, referring to FIGS. 2, 4 and 5, even when the operating point changes as described above, the operating point is controlled by controlling the bias voltage V2 to change the operating point to the modulation curve W1.
The operation of correcting the value a to the minimum value point P1a will be described.
First, in order to control the bias voltage V2, the low frequency signal A2 is superimposed on the main modulation signal A1 by the superimposition circuit 23,
The modulation voltage V1 is applied to the modulation electrodes 13a and 13b. At this time, the bias voltage V2 = Vdc, the operating point is the minimum value point P1 of the modulation curve W1, and FIG. 5A is a waveform showing the optical output at this time. In FIG. 5A, W2
Indicates an optical output waveform of the RZ signal, W4a and W5 are envelopes of the optical output waveform W2, W4a is a modulation curve by the low frequency signal A2, and W5 is a waveform indicating a peak value of the optical output waveform W2. This modulation curve W4a is equal to 2 of the low-frequency signal A2.
Twice the frequency, and half periods A2a and A2 of the low-frequency signal A2.
The pp amplitudes Va and Vb corresponding to the poles P10a and P10b in 2b are equal. In FIGS. 5B and 5C, only the modulation curves W4b and W4c by the low frequency signal A2 are shown.

Next, in the same state where the bias voltage V2 = Vdc, when the optical phase Φ changes to become the modulation curve W1a and the operating point becomes the point P3, as shown in the modulation curve W4b of FIG. P- corresponding to the poles P10a and P10b
A difference occurs in the amplitude between the p amplitudes Va1 and Vb1, and Va1 becomes a value larger than Vb1. Therefore, the pp amplitudes Va1 and Vb1 are sampled and held, and the bias voltage V2 is controlled so that the difference between them is zero, that is, the bias point is V2 = Vc and the operating point is the minimum point P1a.

A specific operation of the light modulation control circuit 3 for controlling the bias voltage V2 will be described. First, the split light B1 split from the modulated light output from the optical modulator 2 is converted into an electric signal by the photoelectric conversion circuit 26, and then the low-frequency signal A2 is converted by the low-pass filter 27.
The low-frequency component signal C1 including only the component is input to the sample and hold circuits 29a and 29b. The sample and hold circuit 29a samples and holds the low frequency component signal C1 by the pulse signal A3. Since this sampling timing is the pole P10a of the half cycle A2a of the low frequency signal A2, the sampled value is the pp amplitude Va1. Further, the sample and hold circuit 29b samples and holds the low frequency component signal C1 with the pulse signal A4. Since this sampling timing is the pole P10b of the half cycle A2b of the low frequency signal A2, the sampled value is the pp amplitude Vb1. Next, the voltage difference between the signals C2 and C3, which are the hold outputs, is amplified G times by the differential amplifier 30 and input to the addition / drive circuit 34 via the switch 33 as the signal C6. Then add
The voltage of the signal C6, that is, the voltage obtained by amplifying the difference between the pp amplitude Va1 and the pp amplitude Vb1 by G times by the driving circuit 34;
Initial bias voltage Vdc generated by the variable voltage generation circuit 32
Are added to obtain a bias voltage V2.

As described above, when the modulation curve W1 in FIG. 4 changes to the modulation curve W1a, Va1 of the modulation curve W4b becomes a value larger than Vb1, and the bias voltage V2 is expressed by the following equation. Bias voltage V2 = Vc = Vdc + G × (Va1−Vb1) (1) From the equation (1), the magnitude (Va) corresponding to the change in the optical phase Φ
1, the feedback control to the bias voltage V2 is performed based on the absolute value of the difference between Vb1 and the direction (to be larger than Vdc).

As shown in FIG. 4, when the optical phase Φ changes in the opposite direction to the modulation curve W1a and the modulation curve W1 becomes the modulation curve W1b, the modulation curve by the low-frequency signal A2 is not shown. The modulation curve W4c is 5 (c). In the modulation curve W4c, the magnitude relationship between Va2 and Vb2 is reversed from that of the modulation curve W4b, and Va2 is a value smaller than Vb2. The bias voltage V2 in this case is given by the following equation. Bias voltage V2 = Vd = Vdc−G × abs (Va2−Vb2) (2) where abs () is an absolute value in parentheses. As shown in the equation (2), feedback control is performed on the bias voltage V2 in a direction opposite to the equation (1) (to be smaller than Vdc). As described above, when the operating point is set to the minimum value point P1 of the modulation curve W1, the sign of the signal C4 sampled and sampled by the sample and hold circuits 29a and 29b and the direction in which the feedback control is performed. Is consistent with

Next, an operation for controlling the bias voltage V2 when the operating point is set to the maximum value point P2 of the modulation curve W1 in FIG. 6 will be described with reference to FIGS. As shown in FIG. 6, the initial bias voltage is set to Vdca, and the maximum value point P2 of the modulation curve W1 becomes the operating point. The optical output modulated by the modulation voltage V1 on which the low-frequency signal A2 is superimposed has a waveform shown in FIG. In this figure, as in FIG. 5A, the waveform W6 indicating the minimum value of the optical output waveform W3 and the modulation curve W7a by the low frequency signal A2 are the envelope of the optical output waveform W3. In this modulation curve W7a, the half cycle A2 of the low frequency signal A2
The minimum values Vg and Vh corresponding to the extreme points P10a and P10b at a and A2b are equal values. Next, FIG. 7B shows an optical output waveform when the modulation curve becomes the modulation curve W1a due to the change of the optical phase Φ, and FIG. 7C shows an optical output waveform when the modulation curve becomes the modulation curve W1b. Show. FIG. 7 (b)
In (c), the modulation curve W7 due to the low frequency signal A2
b, only W7c are shown.

First, when the modulation curve becomes W1a,
It is necessary to set the bias voltage V2 to Ve which is larger than Vdca. However, since the minimum value Vg1 of the modulation curve W7b is smaller than Vh1, the bias voltage V2 is given by the following equation in order to set the bias voltage V2 to Ve larger than Vdca. Bias voltage V2 = Ve = Vdca + G × abs (Vg1-Vh1) (3) where abs () is an absolute value in parentheses.

When the modulation curve W1b is obtained,
The minimum value Vg2 of the modulation curve W7c is a value larger than Vh2. Therefore, in order to set the bias voltage V2 to Vf smaller than Vdca, the bias voltage V2 is given by the following equation. Bias voltage V2 = Vf = Vdca−G × (Vg2−Vh2) (4) (3) As shown in the equations (3), the operating point is changed to the modulation curve W.
When it is set to the maximum value point P2 of 1, the sign of the signal C4 sampled and sampled by the sample and hold circuits 29a and 29b and the direction in which the feedback control is performed is different. Therefore, in order to obtain the bias voltage V2 shown in the equations (3) and (4), the sign of the signal C4 may be inverted and added to the initial bias voltage Vdca. Therefore, the signal of the signal C4 is inverted by the inverting amplifier 31 to output the signal C5, and the switch 33 is switched so that the signal C5 is selected, so that the adding / driving circuit 34 initializes the voltage of the signal C5. The feedback control to the bias voltage V2 shown in the equations (3) and (4) is performed by adding to the bias voltage Vdca. The above is the configuration of the optical modulator according to the first embodiment, and in the optical modulator according to the first embodiment, when the operating point of the optical modulator 2 is set to the maximum value point or the minimum value point of the modulation curve. Is an explanation of the operation in which the desired operating point is held.

Next, FIG. 8 is a block diagram showing the configuration of the light modulation device according to the second embodiment. The light modulation device according to the second embodiment includes the light modulation control circuit 3 of the first embodiment and the light modulation control circuit 5 of the second embodiment.
In the above, the low-frequency component signal C1 is sampled and the signal C
Only the circuit that generates C2 and C3 has a different configuration from the optical modulation device according to the above-described first embodiment. The configuration and operation of a circuit that samples the low frequency component signal C1 and generates the signals C2 and C3 in the second embodiment will be described below with reference to FIGS. 3 and 5 to 9.

First, the case where the operating point is set to the minimum value point P1 of the modulation curve W1 in FIG. 3 will be described. First, in FIG. 8, the timing signal generation circuit 41 converts the input low-frequency signal A2 into the pulse signal S1 shown in FIG.
To S3. S1 is a pulse signal in which a pulse is generated in a section of a half cycle A2a which is a half-wave part of the positive sign of the low frequency signal A2. S2 is a pulse signal in which a pulse is generated in a section of a half cycle A2b which is a half-wave part of a negative sign of the low frequency signal A2. S3 is a pulse signal for generating a pulse corresponding to the zero-crossing point of the low-frequency signal A2. Next, the peak hold circuit 42a holds the peak value of the low frequency component signal C1 input in the pulse section of the pulse signal S1. This retained peak value is shown in FIG.
(A) Corresponds to pp amplitudes Va, Va1, Va2 of each half cycle A2a shown in (b) and (c). On the other hand, the peak hold circuit 42b holds the peak value of the low frequency component signal C1 input in the pulse section of the pulse signal S2. The held peak values correspond to the pp amplitudes Vb, Vb1, and Vb2 of each half cycle A2b shown in FIGS. 5A, 5B, and 5C.

Next, the peak hold circuits 42a, 42
The peak value held in b is input to the sample and hold circuits 44a and 44b as signals S8 and S9 via the switches 43a and 43b as signals S4 and S5, respectively.
The sample-and-hold circuits 44a and 44b sample and hold the input signals S8 and S9 at the timing of the pulse signal S3, and output the signals S2 and C3 as signals C2 and C3.
Output to 0. Next, the sample and hold circuit 44a,
44b outputs reset signals S10 and S11 to reset the peak hold circuits 42a and 42b, respectively.

Next, the case where the operating point is set to the maximum value point P2 of the modulation curve W1 in FIG. 3 will be described. When this operating point is set to the maximum value point P2, the minimum hold circuits 45a and 45b respectively hold the minimum value of the low frequency component signal C1 input in the pulse section of the pulse signals S1 and S2. The minimum values held for each of these are shown in FIG.
These correspond to the minimum values Vg to Vg2 and Vh to Vh2 of the half periods A2a and A2b shown in (a) to (c). Next, the minimum value held in the minimum hold circuits 45a and 45b is
The signals S6 and S7 are transmitted through the switches 43a and 43b, respectively, and the signals S8 and S9 are transmitted through the sample and hold circuit 44.
a, 44b. This sample hold circuit 4
4a and 44b sample and hold the input signals S8 and S9 at the timing of the pulse signal S3,
2, and output to the differential amplifier 30 as C3. Next, the sample and hold circuits 44a and 44b output the reset signal S
10 and S11, and output the minimum hold circuit 4 respectively.
5a and 45b are reset. The switch 43a,
43b is set to select the signals S4 and S5 respectively when the operating point is set to the minimum value point P1, while
When the operating point is set to the maximum value point P2, each signal S
6, it is set to select S7. The above is the description of the configuration and operation of the circuit that samples the low-frequency component signal C1 and generates the signals C2 and C3 in the optical modulation device according to the second embodiment.

In the optical modulator according to the above-described second embodiment, the configuration and operation of superimposing the modulation signal A1 and the low-frequency signal A2 and outputting the resultant signal to the optical modulator 2 and the operation of the low-frequency component signal C1 A signal C2 generated by sampling,
The configuration for controlling the increase and decrease of the bias voltage V2 using C3 and the operation thereof are the same as those of the optical modulation device according to the first embodiment. Further, similarly to the optical modulator according to the first embodiment, when the operating point of the optical modulator 2 is set to the maximum value point or the minimum value point of the modulation curve, the desired operating point is held.

In the first or second embodiment described above, the modulation electrodes 13a, 13b and the bias electrode 1
5a and 15b are provided independently of each other, but the modulation electrode 13a and the bias electrode 15a are constituted by one and the same electrode.
May be constituted by another same electrode. Further, the circuit configuration in which the low-frequency signal A2 is superimposed on the main modulation signal A1 has been described, but the circuit configuration may be such that the low-frequency signal A2 is superimposed on the signal of the bias voltage V2.

[0031]

As described above, according to the present invention,
In an optical modulator having an optical modulator that changes light intensity according to an input modulation signal and a bias voltage, a main modulation signal and a low-frequency signal of a single frequency lower than the main modulation signal are superimposed. The optical modulator modulates the light intensity according to the modulated signal. A low-frequency component signal is generated by attenuating a main modulation signal component and transmitting a low-frequency signal component from a signal obtained by partially splitting the modulated light and performing photoelectric conversion. A first sample value that is sampled at a predetermined phase of the positive polarity of the low frequency signal from the generated low frequency component signal, and a second sample value that is sampled at a predetermined phase of the negative polarity of the low frequency signal from the low frequency component signal Since the increase or decrease of the bias voltage is controlled in accordance with the difference from the sample value of the optical modulator, when the operating point of the optical modulator is set to the maximum value point or the minimum value point of the modulation curve, the desired optical modulation It is possible to follow the operating point, and the operating point can be stably maintained even when the environment changes or changes over time.
In addition, there is also obtained an effect that it can be realized with a simple circuit as compared with the conventional solution.

Further, the first sample value is a value sampled from the low frequency component signal for each predetermined phase of the positive polarity of the low frequency signal, and the second sample value is the negative polarity of the low frequency signal. Since the value is sampled from the low-frequency component signal for each predetermined phase, the change in the operating point can be immediately detected to control the increase or decrease of the bias voltage.
Furthermore, the phase at which the first sample value is sampled is the phase of the extreme value of the positive polarity of the low frequency signal, and the phase at which the second sample value is sampled is the phase of the extreme value of the negative polarity of the low frequency signal. , The bias voltage can be controlled with higher accuracy, and the effect that the operating point of light modulation can be held more stably can be obtained.
Furthermore, since the apparatus includes the bias voltage control inverting means for inverting the increase and decrease of the bias voltage, the operating point of the light modulation is easily set to either the maximum value point or the minimum value point of the modulation curve, and the light modulation is stabilized. An operating point can be obtained.

Further, in an optical modulator having an optical modulator for changing light intensity in accordance with an input modulation signal and a bias voltage, a main modulation signal and a low frequency signal having a single frequency lower than the main modulation signal are provided. The optical modulator modulates the light intensity by the modulation signal on which the frequency signal is superimposed. A low-frequency component signal is generated by attenuating a main modulation signal component and transmitting a low-frequency signal component from a signal obtained by partially splitting the modulated light and performing photoelectric conversion. A first sampled value which is a value of the generated low frequency component signal and which samples a predetermined value in a positive phase section of the low frequency signal, and a negative value of the low frequency signal which is the value of the low frequency component signal The increase / decrease of the bias voltage is controlled in accordance with the difference from the second sample value for sampling a predetermined value in the phase section of the characteristic, so that the operating point of the optical modulator is set to the maximum value point or the minimum value of the modulation curve. In the case of setting the point, it becomes possible to follow the operating point of the desired light modulation, and the operating point can be stably maintained even with environmental changes and aging. In addition, there is also obtained an effect that it can be realized with a simple circuit as compared with the conventional solution.

Further, the first sample value is a value for sampling a predetermined value of the low frequency component signal in the positive polarity phase section of the low frequency signal for each cycle of the low frequency signal, and the second sample value is Since the predetermined value of the low-frequency component signal in the negative-polarity phase section of the low-frequency signal is sampled in each cycle of the low-frequency signal, a change in the operating point is immediately detected to increase or decrease the bias voltage. Can be controlled. Further, the predetermined value sampled as the first sample value is the maximum value or the minimum value of the low frequency component signal in the positive polarity phase section of the low frequency signal, and the predetermined value sampled as the second sample value is Since the low-frequency component signal has the maximum value or the minimum value in the negative-polarity phase section of the low-frequency signal, the bias voltage can be more accurately controlled, and the operating point of the optical modulation can be further stabilized. Can be obtained.

Further, first sample value selecting means for selecting either the maximum value or the minimum value sampled as the first sample value, and the maximum value or the minimum value sampled as the second sample value And the second sample value selecting means for selecting either one of them. Therefore, when the operating point is set to either the maximum value point or the minimum value point of the modulation curve, the optimum value used for controlling the bias voltage is set. Sample values can be easily selected. Furthermore, since the apparatus includes the bias voltage control inverting means for inverting the increase and decrease of the bias voltage, the operating point of the light modulation is easily set to either the maximum value point or the minimum value point of the modulation curve, and the light modulation is stabilized. An operating point can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a light modulation device according to a first embodiment of the present invention.

FIG. 2 is a timing signal generation circuit 2 according to the first embodiment;
FIG. 8 is a waveform diagram illustrating pulse signals A3 and A4 generated by No. 8;

FIG. 3 is a first waveform diagram illustrating light modulation of the light modulation device according to the embodiment.

FIG. 4 is a second waveform diagram illustrating light modulation of the light modulation device according to the same embodiment.

FIG. 5 is a first waveform chart for explaining an operation of controlling the bias voltage of the light modulation device according to the same embodiment.

FIG. 6 is a third waveform diagram illustrating light modulation of the light modulation device according to the same embodiment.

FIG. 7 is a second waveform chart for explaining an operation of controlling the bias voltage of the light modulation device according to the same embodiment.

FIG. 8 is a block diagram illustrating a configuration of an optical modulation device according to a second embodiment of the present invention.

FIG. 9 is a timing signal generation circuit 4 according to the same embodiment;
FIG. 3 is a waveform diagram for explaining pulse signals S1 to S3 generated by a pulse signal 1;

FIG. 10 is a block diagram illustrating a configuration of a conventional light modulation device.

FIG. 11 is a waveform diagram illustrating light modulation of a conventional light modulation device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Optical modulator 3 Optical modulation control circuit 21 Modulation signal generator 22 Low frequency signal generator 23 Superposition circuit 24 Capacitor 25 Optical splitter 26 Photoelectric conversion circuit 27 Low-pass filter 28 Timing signal generation circuit 29a, 29b Sample hold circuit 30 Differential amplifier 31 Inverting amplifier 32 Variable voltage generator 33 Switch 34 Addition / drive circuit

Claims (10)

[Claims] An optical modulator having an optical intensity modulator for changing an optical intensity according to an input modulation signal and a bias voltage, comprising: a main modulation signal; and a single-frequency signal having a lower single frequency than the main modulation signal. A low-frequency superimposed modulation signal generating unit that generates a low-frequency signal, superimposes the main modulation signal and the low-frequency signal to generate the modulation signal, and partially modulates the modulated light output by the light intensity modulator. A low-frequency transmission unit that attenuates the main modulation signal component from the branched and photoelectrically converted signal and generates a low-frequency component signal by transmitting the low-frequency signal component; and a predetermined positive polarity of the low-frequency signal. According to the difference between the first sample value sampled from the low-frequency component signal at the phase and the second sample value sampled from the low-frequency component signal at a predetermined negative polarity phase of the low-frequency signal, The bias voltage And a bias voltage control means for controlling an increase or decrease of the light modulation device. 2. The method according to claim 1, wherein the first sampled value is a value sampled from the low-frequency component signal for each predetermined phase of a positive polarity of the low-frequency signal, and the second sampled value is the low-frequency signal. 2. The light modulation device according to claim 1, wherein the value is a value sampled from the low-frequency component signal for each predetermined negative polarity phase of the signal. 3. The phase at which the first sample value is sampled is a phase of an extreme value of a positive polarity of the low frequency signal, and the phase at which the second sample value is sampled is the low phase. 3. The optical modulation device according to claim 1, wherein the frequency signal has a phase of an extreme value of a negative polarity. 4. An optical modulation device having an optical intensity modulator for changing an optical intensity according to an input modulation signal and a bias voltage, comprising: a main modulation signal; and a single-frequency signal having a lower frequency than the main modulation signal. A low-frequency superimposed modulation signal generating unit that generates a low-frequency signal, superimposes the main modulation signal and the low-frequency signal to generate the modulation signal, and partially modulates the modulated light output by the light intensity modulator. A low-frequency transmission unit that attenuates the main modulation signal component from the branched and photoelectrically converted signal and generates a low-frequency component signal by transmitting the low-frequency signal component; and a positive polarity phase of the low-frequency signal. A first sample value for sampling a predetermined value of the low-frequency component signal in the interval,
Bias voltage control means for controlling increase or decrease of the bias voltage in accordance with a difference from a second sample value for sampling a predetermined value of the low frequency component signal in a negative polarity phase section of the low frequency signal. Light modulation device. 5. The first sample value is a value for sampling a predetermined value of the low frequency component signal in a positive polarity phase section of the low frequency signal for each cycle of the low frequency signal. 5. The light according to claim 4, wherein the sample value is a value for sampling a predetermined value of the low-frequency component signal in a negative-polarity phase section of the low-frequency signal for each cycle of the low-frequency signal. 6. Modulation device. 6. The predetermined value sampled as the first sample value is a maximum value or a minimum value of the low frequency component signal in a positive polarity phase section of the low frequency signal, and the second sample The light according to claim 4 or 5, wherein the predetermined value sampled as a value is a maximum value or a minimum value of the low frequency component signal in a negative polarity phase section of the low frequency signal. Modulation device. 7. The bias voltage control means includes: first sample value selection means for selecting either the maximum value or the minimum value sampled as the first sample value; and as the second sample value The light modulation device according to claim 6, further comprising: a second sample value selection unit that selects either the maximum value or the minimum value to be sampled. 8. The light modulation device according to claim 1, wherein the bias voltage control means includes a bias voltage control inversion means for inverting an increase and a decrease in the bias voltage. apparatus. 9. An optical modulation device having an optical intensity modulator for changing optical intensity according to an input modulation signal and a bias voltage, comprising: a main modulation signal; and a single-frequency signal having a lower single frequency than the main modulation signal. A first step of generating a low-frequency signal and superimposing the main modulation signal and the low-frequency signal to generate the modulation signal; and partially branching the modulated light output from the light intensity modulator to generate A second process of attenuating the main modulation signal component from the converted signal and generating a low frequency component signal by transmitting the low frequency signal component, and at a predetermined positive polarity phase of the low frequency signal The bias voltage according to a difference between a first sample value sampled from the low frequency component signal and a second sample value sampled from the low frequency component signal at a predetermined negative polarity phase of the low frequency signal. Third to control the increase or decrease of A light modulation method, comprising the steps of: 10. An optical modulation device having an optical intensity modulator for changing optical intensity according to an input modulation signal and a bias voltage, comprising: a main modulation signal; and a single-frequency signal having a lower single frequency than the main modulation signal. A first step of generating a low-frequency signal and superimposing the main modulation signal and the low-frequency signal to generate the modulation signal; and partially branching the modulated light output from the light intensity modulator to generate A second process of attenuating the main modulation signal component from the converted signal and generating a low frequency component signal by transmitting the low frequency signal component; and A first sample value for sampling a predetermined value of the low frequency component signal,
A third step of controlling the increase or decrease of the bias voltage in accordance with a difference from a second sample value for sampling a predetermined value of the low frequency component signal in a negative polarity phase section of the low frequency signal. A light modulation method, comprising: