JPH1152310A - Bias voltage control circuit corresponding to mark rate fluctuation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably control a bias voltage by providing the circuit with a control target value selection circuit for matching the inclination of a change in the object value to be controlled changing according to α parameter positive and negative codes and the inclination of a change in the control target value at all times and outputting the control target value even at the time of the α parameter positive and negative code conversion. SOLUTION: A mark rate detecting circuit 3 detects the mark rate of an input electric signal from the opposite phase output of a Mach-Zehnder driving circuit 4. This mark rate detecting circuit 3, a standardization circuit 2 which matches the change width of the control target value and the fluctuation width of the object to be controlled at the time of a mark rate fluctuation and the control target value selection selection circuit 1 for matching the inclination of the change in the object value to be controlled changing according to the αparameter positive and negative codes and the inclination of the change in the control target value at all times are disposed between the Mach-Zehnder driving circuit 4 and a bias voltage control circuit 11. Even if the positive and negative codes of the α parameter are changed by an αparameter positive and negative code conversion input signal, the control target value matching the output value of an average light output detecting circuit 10 of the object to be controlled corresponding to the mark rate is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention can be used in an optical output intensity modulated optical transmitter using a Mach-Zehnder type optical intensity modulator as an external modulator. Set the desired control target value
The present invention relates to a bias voltage control circuit suitable for a mark ratio variation suitable for stably controlling output light of a Mach-Zehnder type light intensity modulator.

[0002]

2. Description of the Related Art In an optical output intensity modulated optical transmitter using a Mach-Zehnder type optical intensity modulator as an external converter, the Mach-Zehnder type optical intensity modulator changes over time or changes in temperature.
The extinction characteristic fluctuates, and the light intensity modulation output value changes. FIGS. 1 and 2 show control configuration examples of a conventional solution to this phenomenon. The circuit shown in FIG. 1 converts the branched light of the intensity-modulated light output from the Mach-Zehnder type light intensity modulator 7 into a voltage by the photodetector 9 and outputs the average light output detection circuit 1.
0 is detected as an average voltage value to be controlled. Further, a mark ratio detection circuit 3 for outputting a rated signal corresponding to the mark ratio according to the mark ratio of the input electric signal is provided, and the bias voltage control circuit 11 is provided with a bias voltage control circuit 11 so that the difference between the output value and the average voltage value is minimized. To output a bias voltage control voltage. As an example adopting a similar control format, there is a control circuit described in Japanese Patent Application Laid-Open No. 4-116618.

The circuit shown in FIG. 2 includes a light emitting logic selection circuit 1 for suppressing a dispersion penalty caused by chromatic dispersion.
5, the bias voltage value is changed to the Mach-Zehnder half cycle voltage Vπ.
Even when the positive / negative signal of the α parameter is converted by shifting by (v), by inverting the control direction in the logic selecting circuit 12, stable bias voltage control can be performed only when the mark ratio is 50%. As an example employing a similar control format, there is a control circuit described in Japanese Patent Application Laid-Open No. 4-140712.

However, when the sign conversion of the α parameter is performed and the bias voltage is to be controlled even when the mark ratio fluctuates, stable bias voltage control cannot be performed only by combining the above-mentioned conventional circuits. This is because the bias voltage is reduced to the Mach-Zehnder half-period voltage Vπ in order to suppress the dispersion penalty caused by chromatic dispersion.
When the α parameter sign conversion is performed by shifting by (V), not only the control direction is inverted because the light emission logic is inverted, but also the control target value set based on the mark ratio of the input electric signal is the mark ratio. This is because the opposite change is made to the fluctuation of the above, and stable light output control cannot be performed.

[0005]

Generally, in a Mach-Zehnder type optical intensity modulator in which a modulation signal and a bias voltage are applied to different waveguides, when α parameter> 0, (Equation 1), (Equation 2), α parameter If <0 (Equation 3),
Po has DC quenching characteristics approximated by (Equation 4). Here, Vm is a DC voltage applied to the modulation signal input side, Vb
Represents a bias voltage applied to a waveguide different from that to which the modulation signal is applied, and A represents a level difference between the maximum and minimum optical outputs of the Mach-Zehnder optical intensity modulator when Vm and Vb are applied.

[0006]

(Equation 1)

[0007]

(Equation 2)

Here, Po: Mach-Zehnder type light intensity modulator output light, Vb: Bias voltage, A: Mach-Zehnder type light intensity modulator light output amplitude, Vm: Signal input side DC voltage.

Further, if the branch light monitor value of the intensity-modulated light, that is, the average light output detection circuit output value Vmo, which is the control object value, does not fluctuate from the bias voltage value at which the light output waveform cross point becomes 50%, the mark rate fluctuation When α parameter> 0 (Equation 3), when α parameter <0 (Equation 4), it changes. Here, VmoMAX is the value of the average light output detection circuit output value Vmo when the mark ratio = 1, and B is the Vmo value when the mark ratio is 1 and Vm when the mark ratio is 0.
It shows the voltage difference of the o value.

[0010]

(Equation 3)

[0011]

(Equation 4)

Here, Vmo: average light output detection circuit output, VmoMAX: Vmo output when mark ratio = 1,
B: Voltage difference of Vmo output when mark ratio = 1,0.

FIG. 3 shows the DC of the output light of the Mach-Zehnder type optical intensity modulator which is approximated by (Equation 1, 2) and (Equation 3, 4).
The extinction characteristic is shown in FIG. 4 as an output characteristic diagram of the average light output detection circuit output Vmo approximated by (Equation 5) and (Equation 6).

When the conventional technique is used, Vref is set to (Equation 7) and (Equation 8) in order to perform stable light output control of the control target value Vref of the bias voltage in accordance with the mark rate.

[0015]

(Equation 5)

Here, X is a Vref adjustment constant.

However, if the bias voltage is shifted by the Mach-Zehnder half-period voltage Vπ (V) to suppress the dispersion penalty due to chromatic dispersion, and the sign of the α parameter of the optical transmitter is converted into a sign, that is, α parameter <0,
The control object value average output detection circuit output Vmo value at which the optical output waveform cross point becomes 50% has an inverted slope of change with respect to the mark ratio as shown in FIG. For this reason, the control target value is different from the value at which the optical output waveform cross point becomes 50% when the mark ratio changes, and the bias voltage is not correctly controlled, and the stable Mach-Zehnder type optical intensity modulator output light as shown in FIG. Control becomes impossible.

[0018]

A Mach-Zehnder type light intensity modulator 7 including a Mach-Zehnder type light intensity modulator 8 and a photodetector 9 for branching the light output and detecting an output light;
A DC light source module 6 that emits DC light input to the Mach-Zehnder type light intensity modulator 7, a Mach-Zehnder drive circuit 4 that drives the Mach-Zehnder type light intensity modulator 7 and performs light intensity modulation, and the output amplitude of the Mach-Zehnder drive circuit is always constant. The amplitude control circuit 5 controls the average value of the output light of the Mach-Zehnder type light intensity modulator by the photodetector 9, the output value of the average light output detection circuit to be controlled, A bias voltage control circuit 11 for determining a bias piezoelectric value to be applied to the Mach-Zehnder type optical intensity modulator 7 so as to compare with the control target value and correct the deviation, and apply the bias voltage to the Mach-Zehnder type optical intensity modulator 7 And a bias for inverting the light emission logic of the transmission signal and converting the sign of the α parameter of the optical transmitter. Logic voltage selection circuit 15 for applying a voltage to the Mach-Zehnder type light intensity modulator 7 via a bias voltage application circuit 14, the light emission logic of the transmission signal is inverted, and the positive / negative control direction of the bias voltage control output value is changed. The output range of the bias voltage applied to the logic selection circuit 12 for automatically inverting and the Mach-Zehnder type light intensity modulator 7 is set to ± 1/2 or less of the Mach-Zehnder anti-periodic voltage Vπ around the output value of the light emission logic determination voltage selection circuit. In the optical output intensity modulation optical transmitter bias control circuit comprising the set limit circuit 13, the reverse phase of the Mach-Zehnder drive circuit 4 is set in order to set a control target value corresponding to the mark rate even when the α parameter sign is converted. A mark ratio detection circuit 3 for detecting a mark ratio of an input electric signal from an output; A Mach-Zehnder drive circuit that includes a normalizing circuit 2 for matching the variation range of the control target value and a control target value selecting circuit 1 for always matching the gradient of the change of the control target value and the gradient of the change of the control target value that change according to the sign of the α parameter. This problem can be solved by providing a bias voltage control circuit corresponding to a mark ratio variation, which is provided between the bias voltage control circuit 7 and the bias voltage control circuit 11.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation principle of the present invention will be described below with reference to FIGS.

The DC light emitted by the DC light source module 6 is incident on a Mach-Zehnder type light intensity modulator 7, and the light intensity is modulated by a modulation signal applied from a Mach-Zehnder driving circuit 4 whose amplitude is controlled to be constant by an amplitude control circuit 5. Has been given.

The branched light of the light intensity modulated light of the Mach-Zehnder type light intensity modulator 7 is converted into a voltage by a light receiver 9,
The average light detection circuit 10 inputs the average light detection circuit output Vmo, which is a control target value, to the bias voltage control circuit 11. The bias voltage control circuit 11 compares the input control target value Vmo with the control target value Vref, and outputs a bias voltage for correcting the difference.

In order to convert the sign of the α parameter of the optical transmitter, when a code conversion command signal is input from the α parameter sign conversion terminal, the light emission logic determination voltage selection circuit 15 controls the bias voltage application circuit 14. The center value of the bias voltage Vb through the Mach-Zehnder half cycle voltage Vπ
(V). Accordingly, the light emission logic is inverted, and the inclination of the change in the optical output with respect to the bias voltage Vb of the Mach-Zehnder type light intensity modulator 7 is inverted. Therefore, the positive / negative control direction of the bias voltage Vb for keeping the output of the average optical output circuit constant is logical. The signal is inverted by the selection circuit 12 in accordance with the light emission logic. The bias voltage output range input from the logic selection circuit 12 to the bias voltage application circuit 14 is ± Vπ
The average light output detection circuit output Vmo is controlled by the limit circuit 13 set to be equal to or less than / 2 regardless of before and after the light emission logic inversion so as to be stabilized at the control target value Vref.

When the mark rate changes, the amplitude control circuit 5
The mark ratio of the input electric signal is detected by using the mark ratio detection circuit 3 using the inverted phase output of the Mach-Zehnder drive circuit 4 which always keeps a constant amplitude, and the normalization circuit 2 is determined based on the output value. Is standardized to B when the output Vmo of the average light output detection circuit becomes 50% of the light output waveform cross point when the mark ratio changes. (Equation 9) shows the normalized circuit output V1.

[0024]

(Equation 6)

Here, V1 is the output of the standardization circuit.

The control target value selection circuit 1 converts the normalization circuit output V1 into + V1 if α parameter> 0 and −V1 if α parameter <0 according to the α parameter sign conversion signal input, and detects the average light output. The addition is performed with the value Vmo-VmoMAX-B / 2 when the mark ratio of the Vmo value, which is the circuit output, is 1/2. Therefore, the control target value Vref is

[0027]

(Equation 7)

[0028]

(Equation 8)

Therefore, when α> 0

[0030]

(Equation 9)

When α <0

[0032]

(Equation 10)

When the sign of the α parameter is changed by the α parameter sign conversion input signal,
A control target value that matches the Vmo value (Equation 5, 6) of the control target corresponding to the mark rate is output. As a result, stable control corresponding to the mark rate fluctuation can be performed even during the α parameter sign conversion.

As an embodiment of the present invention, in a bias voltage control circuit using the Mach-Zehnder type light intensity modulator 7 and having the configuration shown in FIG.
The output value Vmoma of the average light output detection circuit output 10 at the time of
x is 2.5 V, the mark ratio = 0, the voltage difference B of the output of the average light output detection circuit 10 at 1 o'clock is 1.0 V, and the mark ratio is 1 /
An average optical output detection circuit output Vmo value which changes to 4/3 and becomes 50% of the optical output waveform cross point when the α parameter sign conversion is performed, and is created by the normalization circuit 2 and the target control value selection circuit 1. The control target value Vref and the behavior of the optical output waveform based on the 10 Gbit / s NRZ pseudo random pattern at that time will be described.

When the α parameter has a positive sign, that is, when the light emission logic is not inverted, if the mark ratio is varied from / to /, the Vmo value at which the optical output waveform cross point becomes 50% is 1 according to (Equation 5). It becomes .75V-2.25V. The control target value Vref at this time is from 1.75 V to 2.
The voltage is set to 25 V, and Vmo is controlled to a value at which the optical output waveform cross point becomes 50%, and stable bias voltage control is performed. FIG. 7 shows the behavior of Vmo, Vref, and the optical waveform at this time.

Now, in order to suppress the dispersion penalty caused by the chromatic dispersion, the bias voltage is shifted by the Mach-Zehnder half-period voltage Vπ (V) by the light emission logic decision voltage selection circuit 15, and the α parameter of the Mach-Zehnder type optical intensity modulator is shifted. When the sign conversion is performed, that is, when the α parameter <0, the Vmo value at which the optical output waveform cross point becomes 50% when the mark ratio is varied from / to / is calculated from (Equation 6) from 2.25 V to 1.V. 75V, and the slope of the change of the Vmo value is inverted as compared with the case where the α parameter> 0.

The Vref value at this time is given by (Equation 10).
25V to 1.75V. Even when the α parameter has a negative sign, the Vmo value is controlled to be 50% of the optical output waveform cross point, and stable bias voltage control is performed.

This is because the standardizing circuit 2 sets the fluctuation range of the control target value Vref to the fluctuation range B (1) of the average light output detection circuit output value Vmo value at which the light output waveform cross point becomes 50% when the mark rate changes. .0 V), and the control target value selection circuit always makes the gradient of the change of the control target value Vref coincide with the gradient of the change of the control target value Vmo. Therefore, the control target value Vref
When the mark ratio fluctuates from / to /, when α> 0, 1.75 to 2.25 V, and when α <0, 2.25 V to
The control target values are set to 1.75 V and 50%, respectively, of the optical output waveform cross point, and stable control corresponding to the mark ratio can be performed even when the α parameter fluctuates.

FIG. 8 is a diagram showing a specific configuration example of the bias voltage control circuit according to the present invention. In this configuration example, in order to always obtain a control target value corresponding to the mark rate variation regardless of the sign of the α parameter, the mark rate detection circuit is combined with a variable resistor and an operational amplifier by using a buffer circuit using an operational amplifier. Vmo fluctuation range B
(V) A standardization circuit for normalization, and an analog switch and an operational amplifier are used to determine the slope of the change in Vref by Vmo.
The control target value selection circuits that match the slopes of were created.

[0040]

As described above, according to the present invention, the α parameter sign conversion can be performed by using the mark ratio detection circuit 3, the normalization circuit 2 and the control target value selection circuit 1 as compared with the conventional bias voltage control circuit. In this case, the control target value can be always output, and stable bias voltage control can be performed.

FIG. 8 shows an example of the configuration, but the mark ratio detection circuit can be applied to a configuration using a clamp circuit.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration example of a conventional bias voltage control circuit corresponding to a mark ratio variation.

FIG. 2 is a block diagram showing an example of a configuration of a conventional bias voltage control circuit for α parameter code conversion.

FIG. 3 is a diagram showing a DC extinction characteristic of a Mach-Zehnder type optical intensity modulator.

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

FIG. 5 is a diagram showing the behavior of an optimum average optical output detection circuit output value, a control target value, and an optical output waveform when a mark ratio fluctuates when a conventional circuit is used.

FIG. 6 is a block diagram showing a configuration example of a bias voltage control circuit corresponding to a mark ratio variation according to the present invention.

FIG. 7 is a diagram showing the behavior of an optimum average optical output detection circuit output value, a control target value, and an optical output waveform when the mark ratio changes when the present invention is used.

FIG. 8 is a block diagram showing a specific configuration example of a bias voltage control circuit according to the present invention.

[Explanation of symbols]

1 ... control target value selection circuit 2 ... standardization circuit 3
... Mark ratio detection circuit, 4 ... Mach-Zehnder drive circuit, 5
... Amplitude control circuit, 6 ... DC light source module, 7 ... Mach-Zehnder light intensity modulator, 8 ... Mach-Zehnder light intensity modulation element, 9 ... Light receiver, 10 ... Average light output detection circuit, 11 ... Bias voltage control circuit, 12 ...
Logic selection circuit, 13: limit circuit, 14: bias voltage application circuit, 15: light emission logic determination voltage selection circuit.

Claims (1)

[Claims] 1. A Mach-Zehnder type light intensity modulator including a Mach-Zehnder type light intensity modulator and a photodetector for branching the optical output and detecting an output light, and emits DC light input to the Mach-Zehnder type light intensity modulator. A DC power supply module, a Mach-Zehnder type light intensity modulator, a Mach-Zehnder drive circuit for providing light intensity modulation, an amplitude control circuit for constantly controlling the output amplitude of the Mach-Zehnder drive circuit to a constant value, An average light output detection circuit that detects the average value of the output light from the intensity modulator, and a Mach-Zehnder type light intensity modulator that compares the output value of the average light output detection circuit to be controlled with the control target value and corrects the deviation. Voltage control circuit that determines a bias voltage value to be applied to the Mach-Zehnder type light intensity modulator A bias applying circuit, and a light emitting logic determining voltage for inverting the light emitting logic of the transmission signal and applying a bias voltage for converting the positive / negative signal of the α parameter of the optical transmitter to the Mach-Zehnder light intensity modulator via the bias voltage applying circuit. A selection circuit, a logic selection circuit that inverts the emission logic of the transmission signal and automatically reverses the positive / negative control direction of the bias voltage control output value, and emits the output range of the bias voltage applied to the Mach-Zehnder light intensity modulator In the optical output intensity modulation optical transmitter bias control circuit comprising a limit circuit which is set to ± 1/2 or less of the Mach-Zehnder anti-periodic voltage Vπ around the output value of the logic decision voltage selection circuit, even when the sign of the α parameter is converted. In order to set a control target value corresponding to the mark rate, the input electric signal is mapped from the negative phase output of the Mach-Zehnder drive circuit. A mark rate detecting circuit for detecting a click rate, and the normalized circuit to match the variation range of the control target value at the time of mark rate variation and fluctuation range of the control target, alpha
A control target value selection circuit that always matches the gradient of the change of the control target value and the gradient of the change of the control target value depending on the sign of the parameter is provided between the Mach-Zehnder drive circuit and the bias voltage control circuit. Bias voltage control circuit for mark rate fluctuation.