JPH08201742A - Mach-zehnder type light intensity modulator bias voltage control circuit - Google Patents

Abstract

PURPOSE: To always limit the bias voltage to light emission logic corresponding to an α parameter positive/negative code conversion signal and to stably control a light intensity modulator output light by using a limit circuit suppressing a bias voltage control circuit output value to a specified range for a Mach- Zehnder half-cycle voltage. CONSTITUTION: The limit circuit 1 setting an output range of a bias voltage value Vb applied to a light intensity modulator 5 to ±1/2 or below of the half- cycle voltage Vπ around a light emission logic decision voltage selection circuit output value is provided between a logic selection circuit 2 and a bias voltage application circuit 3. Thus, a mean light output detection circuit output voltage Vb is controlled to one setting solution Vb respectively always like Vb=0 (v) before light emission logic inversion, Vb=-Vπ(v) after the light emission inversion so as to be stabilized to a target value C, and it is controlled by the light emission logic matching with the α parameter positive/negative code conversion signal even when the change, etc., in the quenching characteristics of the light intensity modulator 5 occurs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention can be used in an optical output intensity modulation optical transmitter using a Mach-Zehnder type optical intensity modulator (hereinafter, simply optical intensity modulator) as an external modulator. Even when the transmission signal is logically inverted due to the positive / negative sign conversion, one control solution is always selected from the plural control solutions for the optical output that is the control target so that the emission logic always corresponds to the α parameter positive / negative sign conversion input. The present invention relates to a bias voltage control circuit of an optical intensity modulator, which is suitable for stably controlling the output light of the optical intensity modulator.

[0002]

2. Description of the Related Art In a light output intensity modulation optical transmitter using a light intensity modulator as an external modulator, its extinction characteristic fluctuates due to changes in the light intensity modulator over time and temperature, and Changes occur.

An example of a control circuit configuration of a conventional solution to this phenomenon is shown in FIG. In the figure, the light intensity modulator 5
The average output value of the intensity-modulated light output from the detector is detected as the average voltage value by the photodetector 10, the deviation between the value and the target value is amplified by the bias voltage control circuit 6, and the bias control voltage is output so that negative feedback is applied. To do. This output voltage is automatically inverted in the positive / negative control direction of the bias control voltage output by the logic selection circuit 2 in accordance with the light emission logic determined by the α parameter positive / negative sign conversion signal input, and the bias voltage Vb is applied to the light intensity modulator 5. Is applied via the bias voltage application circuit 3 to obtain a stable optical output value. An example of adopting a similar control format is Japanese Patent Laid-Open No. 4-140712.
There is a control circuit described in the publication.

However, in the above-mentioned conventional technique, the light intensity modulation output value of the light intensity modulator changes significantly due to the inversion of the emission signal of the transmission signal and the change of the extinction characteristic of the light intensity modulator accompanying the sign conversion of the α parameter. Occurs, there are a plurality of control solutions in the output range of the bias voltage Vb that controls the light output of the light intensity modulator. This causes a possibility that the logic inversion and the stable control of the light output cannot be performed because the control solution is controlled to be different from the target control solution.

[0005]

Generally, a light intensity modulator for applying a modulation signal and a bias voltage to different waveguides has a DC extinction characteristic of P0 approximated by the equation (1). Here, Vm 'is a DC voltage applied to the modulation signal input side, Vb is a bias voltage applied to a waveguide different from that to which the modulation signal is applied, and A is a light intensity when Vm' and Vb are applied. It represents the difference between the maximum and minimum output light levels of the modulator.

[0006]

[Equation 1]

Po: Light intensity modulator output light Vb; Bias voltage A: Light intensity modulator light output amplitude Vm '; Signal input side DC voltage Further, the modulation signal amplitude Vm applied to the light intensity modulator is:
When Vm = Vπ (vpp), the branched voltage of the output light of the light intensity modulator is converted into a voltage by the light receiver, and the output voltage V0 obtained by averaging the voltage value by the average light output detection circuit is approximated by Equation 2. To be done.

[0008]

[Equation 2]

V0: average light output detection circuit output voltage value B: average light output detection circuit output voltage amplitude Vm; input signal amplitude C; output voltage value of average light output detection circuit when Vb = 0, Vm = Vπ The DC extinction characteristic of the light intensity modulator, which is approximated by Equation 1 to 3,
FIG. 4 shows an optical output waveform when the optical intensity modulator having the DC extinction characteristic of FIG. 3 is modulated with the modulation signal amplitude Vm. Now, it is assumed that the input signal having the modulation signal amplitude Vm and the bias voltage Vb are applied to the light intensity modulator 5. At this time, (a) Vb = 0 (v), (b) Vb = −Vπ / 2
When the bias voltage is changed as shown in (v) and (c) Vb = −Vπ (v), the DC extinction characteristic P0 of the light intensity modulator changes as shown in FIGS. 3 (a) to 3 (c). The light output waveform of the light intensity modulator 5 changes as shown in FIGS. 4 (a) to 4 (c).
When Vb is further increased, the light output waveform of the light intensity modulator 5 changes as shown in FIGS. The conversion of the sign of the α parameter of the optical transmitter is Vb = 0, −Vπ in FIG.
4A corresponds to (v), and the optical transmission waveform at that time is (a) in FIG.
The light emission logic is inverted as shown in (c). That is, the α parameter positive / negative sign and the light emission logic of the optical transmitter are determined based on the α parameter positive / negative code conversion signal.
And the bias voltage value applied to the light intensity modulator via the bias voltage applying circuit 3.

Bias voltage Vb approximated by equation 2 in FIG.
And the average value of the output light of the light intensity modulator, that is, the average light output detection circuit output voltage V0.

Now, assuming that the target value of V0 is C, it is assumed that a deviation occurs between the target value C and the output value of the average light output detection circuit due to the characteristic change of the extinction ratio or the like. In the bias voltage control circuit 6,
A bias control voltage is output so that the deviation is amplified so as to correct this deviation and negative feedback is applied. Therefore, when the bias control voltage value is larger than ± 1/2 of the half frequency Vπ,
The slope of the increase or decrease of the average light output value of the light intensity modulator 5 with respect to the bias voltage Vb becomes a region in which the average light output cannot be controlled to the target value. Further, the larger the range in which the bias control voltage can be output becomes, V0 = C (V). Since there are a plurality of Vb, Vb = 0, ± Vπ, ± 2Vπ, ... (V), there are a plurality of control solutions for performing bias control. As a result, stable output control of the light intensity modulator cannot be performed.

Further, the light emission logic of the signal light whose light intensity is modulated by the light intensity modulator is determined by the bias voltage Vb applied to the light intensity modulator, and the light emission logic determination voltage selection circuit output values 0 and -Vπ (v ) Is centered around each range of ± Vπ / 2 (v). Therefore, when the value of Vb becomes larger than ± Vπ / 2 (v) around the light emission logic determination voltage selection circuit output value, that is, the logic selection circuit output value is ± Vπ / 2.
When it becomes larger than (v), the light emission logic of the light intensity modulated light output from the light intensity modulator may be inverted with respect to the light emission logic determined by the α parameter positive / negative sign conversion signal input.

[0013]

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

In order to solve the above-mentioned problems, the present invention provides a light intensity modulator 5 and a light intensity modulator 5 each including a light intensity modulator 11 and a light receiver 10 for branching the optical output and detecting the output light. DC light source module 8 for emitting DC light to be input to
A light intensity modulator 5, a Mach-Zehnder drive circuit 7 for driving the light intensity modulator 5 to perform light intensity modulation, and a light receiver 10.
Thus, the average light output detection circuit 9 for detecting the average value of the output light of the light intensity modulator, and the light intensity modulator 5 so as to correct the deviation by comparing the output value of the average light output detection circuit 9 with the target value. Bias voltage control circuit 6 that determines the bias voltage value applied to the optical signal, bias voltage application circuit 3 that applies the bias voltage to the optical intensity modulator 5, and the light emission logic of the transmission signal is inverted to set the α parameter of the optical transmitter. A light emission logic determination voltage selecting circuit 4 for applying a light emission logic determination voltage for converting a positive / negative sign to the light intensity modulator 5 via a bias voltage applying circuit 3,
An optical output intensity modulation optical transmitter comprising a logic selection circuit 2 for automatically inverting the positive / negative control direction of a bias voltage control output value while inverting the emission logic of a transmission signal,
A limit circuit 1 in which the output range of the bias voltage value Vb applied to the light intensity modulator 5 is set to ± 1/2 or less of the half cycle voltage Vπ centering on the output value of the light emission logic determination voltage selection circuit.
Is provided between the logic selection circuit 2 and the bias voltage application circuit 3 to provide a light intensity modulator bias voltage control circuit.

[0015]

The operation principle of the present invention will be described below with reference to FIGS.

The DC light emitted from the DC light source module 8 is incident on the light intensity modulator 5 and is subjected to light intensity modulation by the modulation signal applied from the Mach-Zehnder drive circuit 7.

The branched light of the light intensity modulated light of the light intensity modulator 5 is voltage-converted by the light receiver 10, and the average light output detection circuit 9
Is input to the bias voltage control circuit 6 as the average light output detection circuit output voltage. The bias voltage control circuit 6 compares the input average optical output detection voltage with the target value and outputs a bias voltage for correcting the difference.

In order to perform the sign conversion of the α parameter of the transmitter, when a sign conversion command signal is input from the sign conversion terminal of the α parameter, the light emission logic determination voltage selection circuit 4 causes the bias voltage application circuit 3 to operate. The bias voltage Vb is changed from 0 (v) to -Vπ (v). Along with this, the slope of the change in the optical output with respect to the bias voltage Vb of the light intensity modulator 5 is inverted, so the positive / negative control direction of the bias voltage Vb for keeping the average optical output detection circuit output voltage constant is determined by the logic selection circuit 2 to emit light. Invert according to.
By providing the limit circuit 1 in which the bias control voltage range input from the logic selection circuit 2 to the bias voltage application circuit 3 is ± Vπ / 2 or less between the bias voltage application circuit 3 and the logic selection circuit 2, The average light output detection circuit output voltage Vb is Vb = 0 (v) before the light emission logic inversion and Vb = −V after the light emission logic inversion so that the average light output detection circuit output voltage Vb stabilizes at the target value C.
π (v) and Vb of one control solution are always controlled, and even if the extinction characteristic of the light intensity modulator 5 changes or the like, the light emission logic is controlled to match the positive / negative sign conversion signal of the input α parameter. To be done.

[0019]

EXAMPLE As an example of the present invention, a Mach-Zehnder half-period voltage Vπ = 4 (v), and the modulation signal amplitude voltage Vm applied to the light intensity modulator 5 at this time is 4 (vp) which is the same as Vπ.
p), the target value C of the average optical output detection circuit output voltage V0 is V
When b = 0 (v) and Vb = −Vπ (v), C = 0.5
(V), the output voltage amplitude B of the average optical output detection circuit output voltage V0 of FIG. V
When b = 0 (v), the α parameter is a positive real number, Vb = −
In the light intensity modulator bias voltage control circuit in the configuration of FIG. 1 using the light intensity modulator 5 in which the α parameter at Vπ (v) is a negative real number, the sign of the α parameter is changed to 10 Gb / s. , NRZ signal pseudo random pattern behavior of the optical output waveform will be described.

The α parameter is a positive sign, that is, Vb =
When the light emission logic inversion is not performed with 0 (v),
Light is emitted when the modulation input signal is High, as shown in FIG.
When w, a good light output waveform with no light emission is obtained.

Further, the α parameter has a negative sign, that is, V
When the light emission logic inversion is performed with b = −Vπ (v), light emission occurs when the modulation input signal is low, as shown in FIG.
When it is high, a good light output waveform with no light emission is obtained.

Now, the Mach-Zehnder half-period voltage Vπ is 4v.
If it changes from 4.5 v to 4.5 v, the average optical output V0 increases from 0.5 v to 0.55 v before the light emission logic sign is inverted. Due to this variation, the bias voltage Vb is set to V so that the average light output detection circuit output value V0 becomes the target value C according to Equation
It is controlled to increase in the positive direction from b = 0 (v) to Vb = + 0.25 (v). Although the average light output detection circuit output value V0 becomes large after the light emission logic sign inversion, since the bias voltage control direction is reversed by the logic selection circuit 2, Vb changes from Vb = -4 (v) to Vb = -4. 25
As in (v), the bias voltage Vb is controlled so as to increase in the negative direction so that V0 becomes the target value C as before the light emission logic inversion.

This is because the output range of the bias control voltage input to the bias voltage applying circuit is ±± by the limit circuit 1.
Since it is limited to Vπ / 2 (v), the bias voltage Vb applied to the light intensity modulator is stabilized at the target value C so that Vb = + 0.25 (v) before the light emission logic inversion, and the light emission logic. This is because after the inversion, the control is performed only to Vb = −4.25 (v) and one control solution Vb. Therefore, even if the extinction characteristic of the light intensity modulator 5 changes or the like, the light emission logic corresponding to the α parameter positive / negative sign conversion input is always selected from the plurality of control solutions for the light output which is the control target. Since the control is performed so as to converge to only one control solution, a stable light intensity modulator output light can be obtained.

FIG. 7 is a diagram showing a concrete configuration example of the light intensity modulator bias voltage control circuit of the present invention. In this configuration example, regardless of the presence or absence of the logic inversion, the change in the Mach-Zehnder extinction characteristic is always limited to the emission logic corresponding to the α parameter positive / negative sign conversion signal, and a stable light intensity modulator output light is obtained. By performing resistance division using resistors, the output value of the bias voltage control circuit is ± Vπ / 2.
(V) A limit circuit that suppresses the following is created. this is,
Since the variable resistor is used, it can be freely adjusted for any Mach-Zehnder half-period voltage Vπ.

Although FIG. 7 shows one example of the configuration,
Alternatively, a limit circuit can be created by using a Zener diode or the like.

[0026]

According to the present invention, as compared with the conventional bias voltage control circuit, by using the limit circuit 1 that suppresses the output value of the bias voltage control circuit to ± 1/2 or less of the Mach-Zehnder half-cycle voltage Vπ, the α It becomes possible to limit the light emission logic corresponding to the parameter positive / negative sign conversion signal and to stably control the output light of the light intensity modulator.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration example of a Mach-Zehnder type optical intensity modulator bias voltage control circuit of the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional Mach-Zehnder type optical intensity modulator bias voltage control circuit.

FIG. 3 is an explanatory diagram showing extinction characteristics of a Mach-Zehnder optical intensity modulator and conditions for signal input.

FIG. 4 is an explanatory diagram showing output light of a Mach-Zehnder type optical intensity modulator having the extinction characteristic shown in FIG.

5 is an explanatory diagram showing the relationship between the output and the bias voltage when the output light of the Mach-Zehnder type optical intensity modulator having the extinction characteristic shown in FIG. 3 is received by the average optical output detection circuit.

FIG. 6 is a light output waveform diagram of a Mach-Zehnder type optical intensity modulator using the present invention.

FIG. 7 is an explanatory diagram showing a specific configuration example of a Mach-Zehnder type optical intensity modulator bias voltage control circuit of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Limit circuit, 2 ... Logic selection circuit, 3 ... Bias voltage application circuit, 4 ... Emission logic determination voltage selection circuit, 5 ... Mach-Zehnder type light intensity modulator, 6 ... Bias voltage control circuit, 7 ... Mach-Zehnder drive circuit, 8 ... DC light source module, 9 ... Average light output detection circuit, 10 ... Photoreceiver, 11 ... Mach-Zehnder type light intensity modulator.

Claims (1)

[Claims] 1. A light intensity modulator having a light intensity modulator and a light receiver for detecting branched light of its light output, a DC light source module for emitting DC light input to the light intensity modulator, and A Mach-Zehnder drive circuit that drives a light intensity modulator to provide light intensity modulation; an average light output detection circuit that detects an average value of the output light of the light intensity modulator by the light receiver; and an average light output detection circuit A bias voltage control circuit that determines the bias voltage value applied to the light intensity modulator so as to correct the deviation by comparing the output value and the target value, and the bias voltage that applies the bias voltage to the Mach-Zehnder type light intensity modulator. A bias voltage for inverting the light emission logic of the transmission signal and converting the sign of the α parameter of the optical transmitter is applied to the optical intensity modulator via the bias voltage application circuit. In an optical output intensity modulation optical transmitter comprising a light emission logic determination voltage selection circuit and a logic selection circuit for inverting the light emission logic of a transmission signal and automatically inverting the positive / negative control direction of a bias voltage control output value, The output range of the bias voltage value applied to the intensity modulator is set to ± 1/2 or less of the Mach-Zehnder half-cycle voltage Vπ centering on the output value of the light emission logic determination voltage selection circuit. A Mach-Zehnder type optical intensity modulator bias voltage control circuit provided between application circuits.