JP6710105B2 - Drive control device, optical transmitter, optical transmission system, and drive control method - Google Patents

Description

The present invention relates to a drive control device, an optical transmitter, an optical transmission system, and a drive control method, and more particularly to a drive control device, an optical transmitter, an optical transmission system, and a drive that include an optical modulator that modulates light based on a drive signal. Control method

The most common optical modulator used in recent optical communication systems is a Mach-Zehnder type optical modulator (hereinafter referred to as MZ optical modulator). However, the MZ optical modulator has a problem that the optical output characteristic with respect to the drive signal temporally drifts due to changes in temperature, changes with time, and the like. In order to solve such a problem and stably control the operating point of the MZ optical modulator, it is necessary to control the operating point by changing the bias voltage according to the drift of the optical output characteristic.

The stabilization control of the bias voltage in the intensity modulation method using the conventional NRZ (Non Return to Zero) code or the like is described in Patent Document 1, for example. Patent Document 1 proposes a compensation technique that superimposes a low-frequency signal on a drive signal to detect a variation amount and a variation direction of an operating point and controls a bias voltage by feedback to keep the operating point normal. In this control, the optimum bias voltage is approximately the midpoint between the voltage at which the optical output is turned on and the voltage at which the optical output is turned off.

Further, for example, Patent Document 2 also describes stabilization control of a bias voltage in a modulation method such as BPSK (Binary Phase Shift Keying) and QPSK (Quadrature Phase Shift Keying). Patent Document 2 proposes a compensation technique in which a low-frequency signal is superimposed on a bias voltage to detect a variation amount and a variation direction of an operating point, and the bias voltage is controlled by feedback to keep the operating point normal. In the case of an ideal MZ optical modulator, in a modulation method such as BPSK and QPSK, the voltage at which the optical output is turned off (hereinafter referred to as NULL point) is the optimum bias voltage. Therefore, in the bias control described in Patent Document 2, control is performed such that the bias voltage is at the NULL point.

In order to perform such bias control, it is necessary to detect the output light from the MZ optical modulator, but in the case of a general MZ optical modulator, a photodiode for detecting the output light (hereinafter, Built-in PD). However, in the MZ optical modulator having such a built-in PD, the built-in PD is installed in the multiplexing part of the MZ optical modulator and installed so as to monitor the diffused light. Bias shift occurs.

In the case of an ideal MZ optical modulator, when the output light of the MZ optical modulator is maximum, the light intensity incident on the built-in PD is minimum, and when the output light is minimum, the light intensity incident on the built-in PD. Is the maximum. However, the positional deviation of the built-in PD or the stray light in the waveguide causes a deviation in this relationship. This is called bias shift. When the bias control shown in Patent Document 2 or the like is performed on the optical modulator having the bias shift characteristic, the optical signal cannot be controlled to the optimum bias voltage, and thus the characteristic of the optical signal is deteriorated.

In order to solve this problem, for example, Patent Documents 3 and 4 propose a compensation technique for controlling an optimum bias voltage even in an optical modulator having a bias shift characteristic.

JP-A-3-251815 JP, 2003-283432, A JP, 2013-174761, A WO2015/129192

By using the technique disclosed in Patent Document 3 or Patent Document 4, it is possible to perform optimum bias control even with an MZ optical modulator having a bias shift.
However, in the bias control in Patent Document 3 and Patent Document 4, it is necessary to know the bias shift amount in advance. Therefore, there is a problem that a shipping inspection for a huge amount of time is required to grasp all the bias shift amounts having wavelength dependence, and the circuit becomes complicated to store such data.

Therefore, it is an object of the present invention to realize highly accurate bias voltage correction without the need to know the bias shift amount in advance.

A drive control device according to an aspect of the present invention includes a first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input, and the first drive signal and the second drive generates an optical signal having four values of phase by modulating the light by the signal, and outputs the optical signal, correcting a light modulation unit that detects the intensity of the optical signal, the operating point of the optical signal In order to achieve this, a bias controller that applies a bias voltage to each of the first waveguide and the second waveguide based on the intensity detected by the light modulator, and an output from the light modulator. A bias shift detection unit that detects a bias shift amount that is an offset between the intensity of the optical signal that is generated and the intensity of the optical signal that is detected by the optical modulation unit, wherein the bias shift detection unit is the The first optical signal is applied to either one of the first waveguide and the second waveguide, and the optical signal is turned off when the first drive signal and the second drive signal are not input to the optical modulator. A difference between a bias voltage and a second bias voltage that is applied to the one side and that turns off the optical signal when the first drive signal and the second drive signal are input to the optical modulator, It is characterized in that it is detected as a bias shift amount, and the bias control unit corrects the bias voltage based on the bias shift amount.

An optical transmitter according to an aspect of the present invention includes a light source, a first waveguide to which a first drive signal is input, and a second waveguide to which a second drive signal is input, and the first drive signal and the second drive signal. An optical modulator that generates an optical signal having a quaternary phase by modulating the light from the light source with a second drive signal , outputs the optical signal, and detects the intensity of the optical signal; A bias control unit that applies a bias voltage to each of the first waveguide and the second waveguide based on the intensity detected by the optical modulator in order to correct the operating point in the optical signal. , and the intensity of the optical signal output from the light modulation unit, and a bias shift detector for detecting a bias shift amount is an offset between the intensity of the optical signal detected by the light modulation unit, the The bias shift detector is applied to either one of the first waveguide and the second waveguide, and the optical signal is applied in a state where the first drive signal and the second drive signal are not input to the optical modulator. A first bias voltage that turns off the signal and a second bias that is applied to the one side and turns off the optical signal while the first drive signal and the second drive signal are input to the optical modulator. A difference from a voltage is detected as the bias shift amount, and the bias control unit corrects the bias voltage based on the bias shift amount.

An optical transmission system according to an aspect of the present invention is an optical transmission system including an optical transmitter and an optical receiver that receives an optical signal transmitted from the optical transmitter, wherein the optical transmitter is a light source. And a first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input, and the light from the light source is modulated by the first drive signal and the second drive signal. This generates an optical signal having a four-valued phase, outputs the optical signal, detects the intensity of the optical signal, and an optical modulator for correcting the operating point in the optical signal. A bias controller that applies a bias voltage to each of the first waveguide and the second waveguide based on the intensity detected by the modulator, and the intensity of an optical signal output from the optical modulator. And a bias shift detection unit that detects a bias shift amount that is an offset between the intensity of the optical signal detected by the light modulation unit and the bias shift detection unit, wherein the bias shift detection unit includes the first waveguide and the A first bias voltage that is applied to one of the second waveguides and that turns off the optical signal when the first drive signal and the second drive signal are not input to the optical modulator; And a second bias voltage that is applied to the optical modulator and turns off the optical signal while the first drive signal and the second drive signal are being input to the optical modulator, and detects the difference as the bias shift amount. The bias control unit corrects the bias voltage based on the bias shift amount.

A drive control method according to one aspect of the present invention has a four-valued phase by modulating light with a first drive signal input to a first waveguide and a second drive signal input to a second waveguide. The optical signal is generated and output, the intensity of the optical signal is detected, and the bias shift amount, which is an offset between the intensity of the optical signal and the detected intensity, is detected. A drive control method for correcting a bias voltage specified based on the detected intensity based on the bias shift amount in order to correct an operating point, the method including: the first waveguide and the second waveguide. A first bias voltage applied to either one of the first drive signal and the second drive signal and turning off the optical signal in a state where the first drive signal and the second drive signal are not input; And a difference from the second bias voltage at which the optical signal is turned off while the second drive signal is being input, is detected as the bias shift amount .

According to one embodiment of the present invention, since the bias shift amount is included in the algorithm, it is not necessary to know the bias shift amount in advance, and highly accurate bias control can be realized.

FIG. 3 is a block diagram showing a drive control device according to the first embodiment. It is a schematic diagram showing an example of the optical output characteristic to the drive signal of the MZ optical modulator. It is a schematic diagram showing an example of the optical output characteristic when the drive amplitude is 2Vπ. It is the schematic which shows the bias shift in a MZ light modulator. 6 is a flowchart showing a process of calculating a bias shift amount and compensating for a bias voltage in the first embodiment. (A)-(D) is a schematic diagram which shows the intensity|strength and phase of light in the 1st case in which light is not detected by PD in Embodiment 1. (A)-(D) is a schematic diagram which shows the intensity|strength and phase of light in the 2nd case in which light is not detected by PD in Embodiment 1. FIG. (A)-(D) is a schematic diagram which shows the intensity|strength and phase of light when changing the bias voltage of Pch in the 2nd case in which light is not detected by PD in Embodiment 1. FIG. FIG. 7 is a block diagram showing a drive control device according to a second embodiment. 9 is a flowchart showing a process of calculating a bias shift amount and compensating for a bias voltage in the second embodiment. FIG. 9 is a schematic diagram showing an intensity relationship between an output of an optical modulator and input light to a PD in the second embodiment. (A)-(D) is the schematic which shows the intensity|strength and phase of light in Embodiment 2. FIG. FIG. 7 is a block diagram showing a drive control device according to a third embodiment. (A) And (B) is a schematic diagram for explaining the average light intensity when the bias voltage is deviated from the NULL point in the third embodiment. (A) And (B) is a schematic diagram for explaining the average light intensity when the bias voltage is controlled to the NULL point in the third embodiment. FIG. 9 is a block diagram showing an optical transmitter according to a fourth embodiment. FIG. 16 is a block diagram showing an optical transmission system according to a fifth embodiment. (A) And (B) is the schematic which shows the hardware structural example of the control part in Embodiments 1-5.

Embodiment 1.
FIG. 1 is a block diagram showing a drive control device 100 according to the first embodiment.
The drive control device 100 includes a light modulation unit 110, a control unit 120, a storage unit 130, and amplifiers 101A and 101B.

The optical modulator generates an optical signal by modulating the light based on the drive signal, outputs the optical signal, and detects the intensity of the optical signal. Here, the optical modulator 110 is a QPSK-type MZ optical modulator that generates an optical signal having a quaternary phase by two drive signals (electrical signals). Therefore, in the optical modulator 110, the first Mach-Zehnder waveguide 111A (first waveguide) and the second Mach-Zehnder waveguide 111B (second waveguide) are arranged in one Mach-Zehnder waveguide 112. In the following description, the first Mach-Zehnder waveguide 111A is called Ich, the second Mach-Zehnder waveguide 111B is called Qch, and the Mach-Zehnder waveguide 112 is called Pch.

Each of Ich and Qch includes modulation electrodes 111Aa and 111Ba and bias electrodes 111Ab and 111Bb. There is also a QPSK type optical modulator in which the modulation electrode and the bias electrode are the same electrode. In this case, the same configuration can be achieved by adding and applying the drive signal and the bias voltage outside the light modulator. The drive signal input to Ich is also referred to as a first drive signal, and the drive signal input to Qch is also referred to as a second drive signal. The optical signal generated by Ich is also called a first optical signal, and the optical signal generated by Qch is also called a second optical signal.

Pch has a bias electrode 112a. By adjusting the bias voltage, Pch adjusts the phase difference between the Ich optical signal and the Qch optical signal to π/2, and combines them. Here, the voltage applied to the Pch bias electrode 112a is also referred to as a phase adjustment bias voltage. The phase adjustment bias voltage is a bias voltage for adjusting the phase difference between the Ich and Qch optical signals.
The light modulator 110 also includes a PD 113 that is a built-in PD. The PD 113 detects the intensity of the optical signal output from the optical modulator 110.

Here, the operating point control and bias shift of the MZ optical modulator will be described with reference to the drawings.
FIG. 2 is a schematic diagram showing an example of optical output characteristics with respect to a drive signal of the MZ optical modulator.
In the conventional NRZ modulation code, the high level and the low level of the drive signal S1 whose level changes in the cycle T are adjusted to the emission peak A and the extinction peak B of the optical output O1, respectively, so that the optical output is turned on and off. It is carried out.

In the following description, the difference between the bias voltages corresponding to the respective vertices A and B of light emission and light extinction of the light output O1 that changes periodically is represented by Vπ. According to this, the optimum drive signal amplitude in the NRZ modulation method is Vπ.

However, as described above, the MZ optical modulator has a problem that the optical output characteristic with respect to the drive signal drifts with time due to changes in temperature, changes with time, and the like. To deal with such a problem, in the conventional technology, a low frequency signal is superimposed on a drive signal to detect a variation amount and a variation direction of an operating point, and a bias voltage is controlled by feedback to keep the operating point normal. Compensation techniques have been proposed. In these controls, the optimum bias voltage is approximately the midpoint between the voltage at which the optical output O1 is turned on and the voltage at which the optical output is turned off.

FIG. 3 is a schematic diagram showing an example of the optical output characteristics when the drive amplitude is 2Vπ as in BPSK and QPSK.
In this case, in the ideal MZ optical modulator, the NULL point where the optical output O2 is turned off is the optimum bias voltage.

In order to perform the bias control as described above, the intensity of the output light from the MZ optical modulator is usually detected by the built-in PD. The built-in PD is installed in the multiplexing section of the MZ optical modulator. Therefore, a bias shift occurs due to the arrangement of the built-in PD.

FIG. 4 is a schematic diagram showing the bias shift in the MZ optical modulator.
In the case of an ideal optical modulator having no bias shift, the light intensity incident on the built-in PD becomes minimum when the output light O3 becomes maximum, as in the output light O3 shown in FIG. The intensity of light incident on the built-in PD is maximized when is minimized. However, such a shift in the relationship as in the output light O4 shown in FIG. 4 due to the positional displacement of the built-in PD or stray light in the waveguide is called bias shift. In other words, there is an offset between the intensity of the output light output from the optical modulator and the intensity of the output light detected by the built-in PD. When the conventional bias control is performed on the MZ optical modulator having the bias shift characteristic, the characteristic cannot be controlled to the optimum bias voltage, so that the characteristic deterioration of the optical signal is caused.

Returning to FIG. 1, the control unit 120 controls the light modulation unit 110.
The control unit 120 includes a bias shift detection unit 121 and a bias control unit 122.

The bias shift detection unit 121 detects a bias shift amount that is an offset between the intensity of the optical signal output from the optical modulator 110 and the intensity of the optical signal detected by the PD 113.
In the first embodiment, the bias shift detector 121 is configured such that the optical signal detected by the PD 113 is turned off in the state where the drive signal is not input to both Ich and Qch, and either one of the Ich and Qch is turned off. Of the first bias voltage and the second bias voltage on one side where the optical signal detected by the PD 113 is turned off in the state where the drive signals are input to both Ich and Qch, the bias shift is calculated. Detect as quantity.

The bias controller 122 controls the bias voltage applied to the optical modulator 110 based on the intensity of the optical signal detected by the PD 113.
Here, the bias controller 122 corrects the bias voltage applied to the optical modulator 110 based on the bias shift amount detected by the bias shift detector 121.

The bias control unit 122 includes an ABC (Automatic Bias Control) circuit similar to the configuration disclosed in Patent Document 2, for example. Specifically, the ABC circuit superimposes a low frequency signal on the bias voltage applied to each of the bias electrodes 111Ab and 111Bb, and causes the PD 113 to detect the low frequency signal component. The amplitude of the detected low-frequency signal component gradually decreases as it approaches the optimum bias voltage, and becomes “0” at the point of reaching the optimum bias voltage. Therefore, the ABC circuit detects the low-frequency signal component detected by the PD 113. The bias voltage is adjusted so that the amplitude of is 0.

However, as described above, the light modulator 110 causes a bias shift. Therefore, the bias voltage controlled by the ABC circuit is not the true optimum bias voltage, but deviates from the true optimum bias voltage by the bias shift amount. Therefore, in order to compensate the bias shift included in the monitor signal from the PD 113, the bias control unit 122 adds the offset to the output from the ABC circuit by the amount of compensation from the bias shift detection unit 121 to reduce the bias voltage. to correct.

Next, a process of calculating the bias shift amount and compensating for the bias voltage in the first embodiment will be described.
FIG. 5 is a flowchart showing processing for calculating the bias shift amount and compensating for the bias voltage in the first embodiment.
First, after light is incident on the light modulator 110, the outputs from the amplifiers 101A and 101B are turned off (S10). Here, the signals themselves input to the amplifiers 101A and 101B may be turned off, or the power sources of the amplifiers 101A and 101B may be turned off. The means for turning off the outputs from the amplifiers 101A and 101B may be any means.

Next, the bias shift detection unit 121 searches for NULL points in each of Ich and Qch (S11). Specifically, the bias shift detection unit 121 searches for a combination of Ich and Qch bias voltages in which light is not detected by the PD 113 even if the Pch bias voltage is changed. The PD 113 of the light modulator 110 monitors the diffused light after being multiplexed by Pch. Therefore, the following two cases can be considered that the light is not detected by the PD 113.

The first case where the PD 113 does not detect light is when each of Ich and Qch is controlled to the NULL point. In this case, since the light is extinguished on the Ich and Qch, the light is not input to the Pch, and the PD 113 does not detect the light either. 6A to 6D show the light intensity and phase in this case in polar coordinates. FIG. 6A is a polar coordinate showing the intensity and phase of the optical output of Ich. FIG. 6B is polar coordinates showing the intensity and phase of the optical output of Qch. FIG. 6C is a polar coordinate showing the intensity and phase of the Pch optical output. FIG. 6D is polar coordinates showing the intensity and phase of light detected by the PD 113.
As shown in FIGS. 6A and 6B, in this case, since Ich and Qch are exactly at the NULL point, they are in the extinction state, and light is input to Pch. Not done. Therefore, as shown in FIGS. 6C and 6D, both the output light of the optical modulator 110 and the diffused light detected by the PD 113 are in the extinction state.

The second case where the PD 113 does not detect light is when the phase difference between the light from the Ich and the light from the Qch is “0”. 7A to 7D show polarities of light intensity and phase in this case. FIG. 7A is a polar coordinate showing the intensity and phase of the optical output of Ich. FIG. 7B is a polar coordinate showing the intensity and phase of the optical output of Qch. FIG. 7C is polar coordinates showing the intensity and phase of the Pch optical output. FIG. 7D is polar coordinates showing the intensity and phase of light detected by the PD 113. 7A to 7D are diagrams when the phase difference between each of the Ich and Qch and the input light is π/4.

The light from Ich and the light from Qch are multiplexed on Pch. At this time, as shown in FIGS. 7A and 7B, the phase difference between the Ich and Qch with respect to the input light is π/4, so the phase difference between Ich and Qch is “0”. Become. Therefore, as shown in FIG. 7C, in Pch, the lights are added together, and all the lights become the output lights of the light modulator 110. This output light also has a phase difference of π/4 from the input light. On the other hand, no diffused light is generated. Therefore, as shown in FIG. 7D, light is not detected by the PD 113.

However, in the second case, Ich and Qch cannot actually be controlled to the NULL point. In order to exclude such a case, the bias shift detection unit 121 changes the bias voltage of Pch by instructing the bias control unit 122. Varying the bias voltage of Pch corresponds to changing the phase difference between the light from Ich and the light from Qch. Therefore, if the phase difference between Ich and Qch is other than “0”, the light is diffused as a result of being multiplexed by Pch.

In the second case, an example in which the Pch bias is changed is shown in FIGS. FIG. 8A is a polar coordinate showing the intensity and phase of the optical output of Ich. FIG. 8B is a polar coordinate showing the intensity and phase of the Qch optical output. FIG. 8C is polar coordinates showing the intensity and phase of the Pch optical output. FIG. 8D is polar coordinates showing the intensity and phase of light detected by the PD 113.
As shown in FIG. 8B, here, it is assumed that the phase of Qch is changed by π by changing the bias voltage of Pch. Further, as shown in FIGS. 8A and 8C, it is assumed here that the light intensity of Ich is higher than that of Qch. In such a case, as shown in FIG. 8(C), lights of opposite phases are multiplexed in Pch. Therefore, as the output light, the Ich light is weakened, and the diffused light is increased accordingly (FIG. 8D).

In FIGS. 8A to 8D, there is a difference in the light intensity between Ich and Qch. For example, the phase difference between the light from Ich and the light from Qch is π, and the light intensity of these is If they are equal, all the light becomes diffused light and is incident on the PD 113.

Conversely, if Ich and Qch are NULL points, the PD 113 does not detect light even if the bias voltage of Pch is changed, and these can be controlled to an accurate NULL point. .. As a specific method, a combination of varying the Pch bias voltage with respect to various combinations of the Ich bias voltage and the Qch bias voltage so that the light intensity detected by the PD 113 is equal to or lower than a certain intensity (threshold value). The method of detecting is considered. Alternatively, a method of finely adjusting the Ich bias voltage and the Qch bias voltage by varying the Pch bias voltage after searching for the NULL point including the bias shift by the bias control by the conventional ABC circuit may be used. The method according to the first embodiment is not limited to the above method.

Returning to FIG. 5, the bias shift detection unit 121 stores the Ich bias voltage and the Qch bias voltage controlled to the NULL point as described above, for example, in the storage unit 130 as the first bias voltage (S12). ). Here, the first bias voltage of Ich is BiasIA and the first bias voltage of Qch is BiasQA.

Next, the outputs of the amplifiers 101A and 101B are turned on, and the drive signal is input to the optical modulator 110 (S13). Here, the signals themselves input to the amplifiers 101A and 101B may be turned on, or the power sources of the amplifiers 101A and 101B may be turned on. The means for turning on the outputs from the amplifiers 101A and 101B may be any means.

Next, the bias control unit 122 carries out bias control in the ABC circuit (S14). The control here is the same as the conventional control method disclosed in Patent Document 2 or the like. When such bias control is performed, Ich and Qch are controlled to the NULL point with the drive signal turned on. However, the light modulator 110 has a bias shift. Therefore, the NULL point controlled by the bias control is not the true NULL point but the NULL point including the bias shift of the optical modulator 110.

The bias control unit 122 stores, as the second bias voltage, the Ich bias voltage and the Qch bias voltage controlled to the NULL point as described above, for example, in the storage unit 130 (S15). Here, the second bias voltage of Ich is BiasIB and the second bias voltage of Qch is BiasQB.

Then, the bias shift detection unit 121 calculates the bias shift amount from the difference between the first bias voltage and the second bias voltage stored in the storage unit 130 (S16). For example, the bias shift detection unit 121 calculates the difference between the first bias voltage BiasIA and the second bias voltage BiasIB in Ich or the difference between the first bias voltage BiasQA and the second bias voltage BiasQB in Qch, and the bias shift is detected. The amount. Then, the bias shift detection unit 121 gives this bias shift amount to the bias control unit 122, and the bias control unit 122 adds this bias shift amount to the above-mentioned ABC control results (BiasIB, BiasQB) (S17). As described above, the bias controller 122 can control the Ich and Qch to the accurate NULL point.

It is known that the bias shift amount of the optical modulator 110 has wavelength dependence. Therefore, when the wavelength of the light input to the light modulator 110 changes, the bias shift amount can be detected again by the control method according to the first embodiment. Therefore, it is not necessary to measure the amount of bias shift at all wavelengths, and the bias shift of the optical modulator 110 can be compensated.

In addition, even when the intensity of the light input to the optical modulator 110 is changed, such as when the light input to the optical modulator 110 is turned off, the bias shift amount is again adjusted by the control method according to the first embodiment. Is desirable to detect.

Embodiment 2.
The drive control device 100 according to the first embodiment makes it possible to detect the bias shift amount of the optical modulator 110. However, in the drive control device 100 according to the first embodiment, the drive signal is turned off to search for the true NULL point, and then the drive signal is turned on to search for the NULL point including the bias shift amount.

A general QPSK type optical modulator includes both a modulation electrode and a bias electrode, and the modulation electrode has a single-phase input. For example, the modulation electrode and the bias electrode are composed of one electrode. If so, DC drift occurs by turning on the drive signal. The DC drift is a phenomenon in which charges are accumulated in the dielectric layer of the optical modulator, and the extinction characteristic is shifted by the charges.
In other words, the drive signal is turned on to find the NULL point, but the NULL point is shifted by the sum of the bias shift amount of the optical modulator and the DC drift amount, and the bias shift amount is correctly detected. Can not do it. Further, when the modulation electrodes have a differential input, if there is a difference in the drive amplitude applied to the positive phase and the negative phase, this difference becomes a phase difference, and even in this case, the bias shift amount can be detected correctly. I can't.

Therefore, in the second embodiment, the bias shift amount can be accurately detected even when the drive signal input to the optical modulator is turned on and DC drift occurs.

FIG. 9 is a block diagram showing the drive control device 200 according to the second embodiment.
The drive control device 200 includes an optical modulation unit 210, a control unit 220, a storage unit 230, and amplifiers 101A and 101B.

The optical modulator 210 is a QPSK-type MZ optical modulator that generates an optical signal having a quaternary phase by two drive signals. Therefore, in the optical modulator 210, the first Mach-Zehnder waveguide 211A (first waveguide) and the second Mach-Zehnder waveguide 211B (second waveguide) are arranged in one Mach-Zehnder waveguide 212. In the following description, the first Mach-Zehnder waveguide 211A is called Ich, the second Mach-Zehnder waveguide 211B is called Qch, and the Mach-Zehnder waveguide 212 is called Pch.

The Ich and Qch are provided with modulation bias electrodes 211Aa and 211Ba, respectively. The drive signal and the bias voltage are added and applied to the modulation bias electrodes 211Aa and 211Ba. The drive signal input to Ich is also referred to as a first drive signal, and the drive signal input to Qch is also referred to as a second drive signal. The optical signal generated by Ich is also called a first optical signal, and the optical signal generated by Qch is also called a second optical signal.

Pch has a bias electrode 212a. By adjusting the bias voltage, Pch adjusts the phase difference between the Ich optical signal and the Qch optical signal to π/2, and combines them.
The light modulator 210 also includes a PD 113 that is a built-in PD. The PD 113 has the same configuration as that of the first embodiment.

The control unit 220 controls the light modulation unit 210.
The control unit 220 includes a bias shift detection unit 221 and a bias control unit 222.

The bias shift detector 221 detects the amount of bias shift of the light modulator 210.
Here, the bias shift detection unit 221 according to the second embodiment accurately detects the bias shift amount even if DC drift occurs.
For example, in the second embodiment, the bias shift detection unit 221 biases the Ich and Qch so that the optical signal detected by the PD 113 is turned off in the state where the drive signal is not input to both Ich and Qch. After the voltage is adjusted, the optical signal detected by the PD 113 is turned off in the state where the drive signal is input to only one of Ich and Qch. In the state where the drive signal is input to both of the two, the optical signal detected by the PD 113 is turned off, and the difference from the second bias voltage on one side is detected as the bias shift amount.

The bias controller 222 controls the bias voltage applied to the light modulator 210. For example, the bias control unit 222 corrects each bias voltage of the Ich bias and the Qch bias of the optical modulation unit 210 based on the bias shift amount detected by the bias shift detection unit 221.

FIG. 10 is a flowchart showing a process of calculating the bias shift amount and compensating the bias voltage in the second embodiment.
First, after light is incident on the light modulator 210, the outputs from the amplifiers 101A and 101B are turned off (S20). Here, the signals themselves input to the amplifiers 101A and 101B may be turned off, or the power sources of the amplifiers 101A and 101B may be turned off.

Next, the bias shift detection unit 221 searches for NULL points in each of Ich and Qch (S21). The method for searching the NULL point is the same as that in the first embodiment.

Next, the output of the amplifier 101A is turned on, and the Ich drive signal is input to the optical modulator 210 (S22). Here, the signal itself input to the amplifier 101A may be turned on, or the power source of the amplifier 101A may be turned on. Although only the Ich signal is turned on here for the sake of explanation, only the Qch signal may be turned on. Note that the following description will be made on the case where only the Ich signal is turned on, and the description when only the Qch is turned on will be omitted.

When the signal of Ich is turned on, DC drift occurs in Ich. However, since the Qch signal remains off, the Qch maintains the true NULL point. Therefore, the light from the Qch is not incident on the Pch, and the light on the Ich alone is incident. Ideally, all the light is output light, and the light is not incident on the PD 113.

However, in the actual light modulator 210, the isolation between the output light and the PD 113 is finite, and for example, when the output light is 0 dBm, light of about −30 dBm is incident on the PD 113. However, since this light is not generated by multiplexing with Qch, bias shift does not occur. Further, when the output light becomes maximum, the incident light on the PD 113 also becomes maximum, which is opposite to the phase relationship when the lights from Ich and Qch are both combined. FIG. 11 shows the intensity relationship between the output of the light modulator 210 and the input light to the PD 113.

12A to 12D show polarities of the light intensity and phase in this case. FIG. 12A is a polar coordinate showing the intensity and phase of the optical output of Ich. FIG. 12B is a polar coordinate showing the intensity and phase of the Qch optical output. FIG. 12C is a polar coordinate showing the intensity and phase of the optical output of Pch. FIG. 12D is polar coordinates showing the intensity and phase of light detected by the PD 113.
Since the drive signal is input to Ich, there are two phase states in which the phases are deviated by π due to the drive signals “0” and “1”. In FIG. 12A, only the phase when the drive signal is “0” is shown, and the case of π/4 is shown as an example. However, when the drive signal is “1”, the phase is “0” and the phase is “0”. There is a π-shifted light output. As shown in FIG. 12C, only the output light of Ich is input to Pch and becomes the output light of the optical modulator 210. However, as shown in FIG. Light of the same phase with a small value becomes the input light of the PD 113.

That is, this case can be regarded as an optical modulator having an ideal built-in PD in which bias shift does not occur, if it is limited to Ich.

Next, the bias controller 222 performs bias control only for Ich by the bias control in the ABC circuit described above (S23). Here, the bias control unit 222 does not change the bias of the Qch, and the Qch remains at the true NULL point.
When such control is performed, the signal from Qch is not input to Pch, and thus Ich can be controlled to the true NULL point even when the drive signal is turned on. The bias control unit 222 stores the Ich bias voltage at this time as the first bias voltage in, for example, the storage unit 230.

Next, the Qch signal is turned on (S25), and the bias control unit 222 performs bias control in the ABC circuit and searches for a NULL point (S26). At this time, since light is input to Pch for both Ich and Qch, the NULL point controlled by the bias control is not the true NULL point but the NULL point including the bias shift of the optical modulator 210.
The bias control unit 222 stores the Ich bias voltage controlled to the NULL point as described above, for example, in the storage unit 230 as the second bias voltage (S27).

Then, the bias shift detection unit 221 calculates the bias shift amount from the difference between the first bias voltage and the second bias voltage stored in the storage unit 230 (S28).
Then, the bias shift detection unit 221 gives this bias shift amount to the bias control unit 222, and the bias control unit 222 adds this bias shift amount to the ABC control result in step S26 (S29). As described above, the bias controller 222 can control the Ich and the Qch to the accurate NULL point.

Embodiment 3.
With the drive control device 200 described in the second embodiment, the bias shift amount can be accurately detected even if DC drift occurs when the drive signal is turned on. However, the drive control device 200 according to the second embodiment requires two circuits, an ABC circuit that turns off the drive signal to perform the bias control and an ABC circuit that turns on the drive signal only for the Ich to perform the bias control. Since the phase relationship between the modulator and the optical output of the modulator is opposite, the two ABC circuits cannot be shared and there is a problem that the circuit becomes complicated.

Therefore, the third embodiment realizes simplification of the circuit by using only one ABC circuit.

FIG. 13 is a block diagram showing a drive control device 300 according to the third embodiment.
The drive control device 300 includes an optical modulator 210, a controller 320, a storage 230, amplifiers 101A and 101B, and a signal generator 340.
In the drive control device 300 according to the third embodiment, the light modulation unit 210 and the storage unit 230 are configured similarly to the drive control device 200 according to the second embodiment, and the amplifiers 101A and 101B are the same as those in the first embodiment. The drive control device 100 according to the first embodiment has the same configuration.

The signal generation unit 340 has a function of turning on and off the signals of Ich and Qch, respectively, and a function of varying the mark ratio of the signal sequence to be generated. The mark ratio indicates the probability of occurrence of "0" or "1" in the drive signal. The normal mark rate is 50%, and in this case, "0" and "1" occur with the same probability.

The control unit 320 controls the light modulation unit 210.
The controller 320 includes a bias shift detector 221 and a bias controller 322.
The bias shift detector 221 according to the third embodiment has the same configuration as that of the second embodiment.

The bias controller 322 controls the bias voltage applied to the light modulator 210. For example, the bias control unit 322 corrects each bias voltage of the Ich bias and the Qch bias of the optical modulation unit 210 based on the bias shift amount detected by the bias shift detection unit 221.
In the second embodiment, the bias control unit 222 searches for a NULL point using the ABC circuit, but in the third embodiment, the bias control unit 322 marks the signal sequence generated by the signal generation unit 340. The true NULL point is searched by varying the rate. The determination of the true NULL point by the bias control unit 322 will be described below.

First, the bias controller 322 causes the signal generator 340 to generate a signal with a changed mark ratio.
The signal generator 340 generates a signal with a changed mark ratio and uses it as a drive signal. For example, here, the signal generation unit 340 generates a signal with a mark ratio of 50+α% (α is greater than 0 and 50 or less). At this time, the average value of the light intensity detected by the PD 113 is defined as the average light intensity P+.
Next, the signal generation unit 340 generates a signal that sets the mark rate to 50-α% (α is greater than 0 and 50 or less). At this time, the average value of the light intensity detected by the built-in PD is defined as the average light intensity P-.

The average light intensity P+ and the average light intensity P- when the bias voltage is deviated from the NULL point will be described with reference to FIGS. Here, it is assumed that the mark rate indicates the occurrence probability of "1".
As shown in FIG. 14A, when the bias voltage is slightly higher than the NULL point, a difference occurs between the light intensity when the drive signal is “0” and the light intensity when the drive signal is “1”, and the drive is performed. The light intensity when the signal is “1” is higher. In such a case, if the mark rate of the signal is 50%+α, the probability of occurrence of “1” is higher, so that the average light intensity P+ becomes larger. On the other hand, as shown in FIG. 14B, when the signal mark rate is 50%-α, the probability of occurrence of “1” is lower, and the average light intensity P− is smaller.

Next, with reference to FIGS. 15A and 15B, the average light intensity P+ and the average light intensity P− when the bias voltage is controlled to the NULL point will be described.
As shown in FIG. 15A, when the bias voltage is at the NULL point, there is no difference between the light intensity when the drive signal is “0” and the light intensity when the drive signal is “1”. In such a case, if the mark ratio of the signal is 50%+α, the probability of occurrence of the drive signal “1” is higher. Does not change, the average light intensity P+ also does not change. Similarly, even if the mark rate is 50%-α, the probability that the drive signal is "1" is lower, but the average light intensity P- remains unchanged.

That is, the bias control unit 322 can control the Ich and Qch to the true NULL point by changing the bias voltage of Ich and Qch so that the average light intensity P+ and the average light intensity P− are equal. It will be possible. For example, in step S23 shown in FIG. 10, the bias control unit 322 can control the Ich to the true NULL point by changing the mark ratio of the Ich drive signal in the signal generation unit 340.

In the above example, the method of controlling to the NULL point by using two kinds of mark ratios and making the two average light intensities equal to each other has been described. Not limited to. For example, the bias control unit 322 can control the light intensity information to the NULL point by approximating the light intensity information with a quadratic function or the like and obtaining the pole value. Further, the bias control unit 322 can realize more accurate approximation by using two or more types of mark ratios.

Fourth Embodiment
FIG. 16 is a block diagram showing the optical transmitter 400 according to the fourth embodiment.
The optical transmitter 400 according to the fourth embodiment is configured by adding a light source 450 to the drive control device 100 according to the first embodiment.

The drive control device 100 according to the first embodiment makes it possible to detect the bias shift amount of the optical modulator. However, it is known that the bias shift amount has wavelength dependence. Further, it is necessary to re-execute the control by the drive control device 100 even when the light input is turned off once and the light is input again to the optical modulator.
Therefore, the drive control device 100 needs to detect that the wavelength has changed and that the optical input has been turned off.

Therefore, the optical transmitter 400 according to the fourth embodiment is provided with the light source 450, so that it is very easy to detect that the wavelength is changed or that the optical input is turned off. Become.
Although the light source 450 is added to the drive control device 100 according to the first embodiment in the fourth embodiment, the present invention is not limited to such an example.
For example, the optical transmitter 400 according to the fourth embodiment may be configured by adding the light source 450 to the drive control devices 200 and 300 according to the second or third embodiment.

Embodiment 5.
FIG. 17 is a block diagram showing an optical transmission system 500 according to the fifth embodiment.
The optical transmission system 500 according to the fifth embodiment is configured by adding optical fibers 560A and 560B, an optical repeater 561, and an optical receiver 562 to the optical transmitter 400 according to the fourth embodiment. There is.

The optical fiber 560A transmits the output light from the optical modulator 110.
The optical repeater 561 amplifies the light transmitted by the optical fiber 560A.
The optical fiber 560B transmits the optical signal amplified by the optical repeater 561.
The optical receiver 562 receives the optical signal from the optical fiber 560B.
Here, the optical repeater 561 may be omitted or plural depending on the distance of the optical fiber 560A.

The optical transmission system 500 according to the fifth embodiment includes an optical transmitter 400 configured by adding a light source 450 to the drive control devices 200 and 300 according to the second or third embodiment, and optical fibers 560A and 560B. The optical repeater 561 and the optical receiver 562 may be added.

Part or all of the control units 120, 220, and 320 described above are, for example, as illustrated in FIG. 18A, a memory 10 and a CPU that executes a program stored in the memory 10. (Processing Unit) 11 or the like. Such a program may be provided via a network, or may be provided by being recorded in a recording medium.

In addition, part or all of the control units 120, 220, and 320 are, for example, as illustrated in FIG. 18B, a single circuit, a decoding circuit, a programmed processor, a parallel programmed processor, an ASIC. It can also be configured by a processing circuit 12 such as (Application Specific Integrated Circuits) or FPGA (Field Programmable Gate Array).

100, 200, 300 Drive control device, 400 Optical transmitter, 500 Optical transmission system, 101A, 101B Amplifier, 110, 210 Optical modulator, 111A, 211A 1st Mach-Zehnder waveguide, 111B, 211B 2nd Mach-Zehnder waveguide, 111Aa , 111Ba modulation electrode, 111Ab, 111Bb bias electrode, 112,212 Mach-Zehnder waveguide, 211Aa, 211Ba modulation bias electrode, 112a, 212a bias electrode, 120, 220, 320 control unit, 121, 221 bias shift detection unit, 122, 222 , 322 bias control unit, 130, 230 storage unit, 340 signal generation unit, 450 light source, 560A, 560B optical fiber, 561 optical repeater, 562 optical receiver.

Claims (13)

It has a first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input, and a four-value phase by modulating light with the first drive signal and the second drive signal. And an optical modulator that detects the intensity of the optical signal and that outputs the optical signal.
A bias controller that applies a bias voltage to each of the first waveguide and the second waveguide based on the intensity detected by the optical modulator in order to correct the operating point in the optical signal. When,
A bias shift detector that detects a bias shift amount that is an offset between the intensity of the optical signal output from the optical modulator and the intensity of the optical signal detected by the optical modulator,
The bias shift detector is applied to one of the first waveguide and the second waveguide, and the first drive signal and the second drive signal are not input to the optical modulator. A first bias voltage that turns off the optical signal and a second bias voltage that is applied to the one side and turns off the optical signal while the first drive signal and the second drive signal are input to the optical modulator. The difference from the bias voltage is detected as the bias shift amount,
The drive control device, wherein the bias control unit corrects the bias voltage based on the bias shift amount. The bias control unit applies a phase adjustment bias voltage, which is a voltage for adjusting the phase difference, to the optical modulation unit,
The optical modulator includes a first optical signal generated in the first waveguide based on the first drive signal, and a second optical signal generated in the second waveguide based on the second drive signal. A phase difference between signals is adjusted by the phase adjustment bias voltage, and the optical signal is generated by multiplexing the first optical signal and the second optical signal,
The bias shift detector detects the optical modulation even if the bias controller changes the phase adjustment bias voltage in a state where the first drive signal and the second drive signal are not input to the optical modulator. the voltage detected intensity is below a predetermined threshold value which in parts, the drive control apparatus according to claim 1, characterized in that specified as the first bias voltage. It has a first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input, and a four-value phase by modulating light with the first drive signal and the second drive signal. And an optical modulator that detects the intensity of the optical signal and that outputs the optical signal.
A bias control unit that controls a bias voltage applied to the optical modulation unit based on the intensity detected by the optical modulation unit in order to correct the operating point in the optical signal;
A bias shift detector that detects a bias shift amount that is an offset between the intensity of the optical signal output from the optical modulator and the intensity of the optical signal detected by the optical modulator,
The bias shift detection unit is configured such that the optical signal is turned off when the first drive signal and the second drive signal are not input to the optical modulation unit, and the first waveguide and the second guide are provided. After adjusting the bias voltage applied to each of the waveguides, the optical signal is turned off in a state where only one of the first drive signal and the second drive signal is input to the optical modulator. When the first bias voltage applied to the one waveguide to which the one is input and both the first drive signal and the second drive signal are input to the optical modulator, the optical signal is A difference with the second bias voltage applied to the one waveguide, which is turned off, is detected as the bias shift amount,
The drive control device, wherein the bias control unit corrects the bias voltage based on the bias shift amount. Signal generation for generating the first drive signal and the second drive signal, and changing the mark ratio that is the probability of occurrence of "0" or "1" in the first drive signal and the second drive signal, respectively. More parts,
The bias control unit applies a voltage that does not change the average value of the intensities detected by the light modulation unit even when the mark ratio of the one is changed in a state where only the one is input to the light modulation unit. The drive control device according to claim 3 , wherein the drive control device is specified as the first bias voltage. The bias control unit, a voltage obtained by adding the bias shift amount to said second bias voltage, as corrected bias voltage, any one of 4 claims 1, characterized in that applied to the light modulation unit The drive control device according to item. When the intensity of the light input to the light modulator changes, the bias shift detector detects the bias shift amount, and the bias controller corrects the bias voltage. The drive control device according to any one of claims 1 to 5 . When the wavelength of the light input to the optical modulator changes, the bias shift detector detects the bias shift amount, and the bias controller corrects the bias voltage. The drive control device according to any one of claims 1 to 5 . A light source,
A first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input are provided, and the light from the light source is modulated by the first drive signal and the second drive signal. An optical modulator that generates an optical signal having a quaternary phase, outputs the optical signal, and detects the intensity of the optical signal;
A bias controller that applies a bias voltage to each of the first waveguide and the second waveguide based on the intensity detected by the optical modulator in order to correct the operating point in the optical signal. When,
A bias shift detector that detects a bias shift amount that is an offset between the intensity of the optical signal output from the optical modulator and the intensity of the optical signal detected by the optical modulator,
The bias shift detector is applied to one of the first waveguide and the second waveguide, and the first drive signal and the second drive signal are not input to the optical modulator. A first bias voltage that turns off the optical signal and a second bias voltage that is applied to the one side and turns off the optical signal while the first drive signal and the second drive signal are input to the optical modulator. The difference from the bias voltage is detected as the bias shift amount,
The optical transmitter according to claim 1, wherein the bias control unit corrects the bias voltage based on the bias shift amount. A light source,
A first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input are provided, and the light from the light source is modulated by the first drive signal and the second drive signal. An optical modulator that generates an optical signal having a quaternary phase, outputs the optical signal, and detects the intensity of the optical signal;
A bias control unit that controls a bias voltage applied to the optical modulation unit based on the intensity detected by the optical modulation unit in order to correct the operating point in the optical signal;
A bias shift detector that detects a bias shift amount that is an offset between the intensity of the optical signal output from the optical modulator and the intensity of the optical signal detected by the optical modulator,
The bias shift detection unit is configured such that the optical signal is turned off when the first drive signal and the second drive signal are not input to the optical modulation unit, and the first waveguide and the second guide are provided. After adjusting the bias voltage applied to each of the waveguides, the optical signal is turned off in a state where only one of the first drive signal and the second drive signal is input to the optical modulator. When the first bias voltage applied to the one waveguide to which the one is input and both the first drive signal and the second drive signal are input to the optical modulator, the optical signal is A difference with the second bias voltage applied to the one waveguide, which is turned off, is detected as the bias shift amount,
The optical transmitter according to claim 1, wherein the bias control unit corrects the bias voltage based on the bias shift amount. An optical transmission system comprising an optical transmitter and an optical receiver for receiving an optical signal transmitted from the optical transmitter,
The optical transmitter is
A light source,
A first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input are provided, and the light from the light source is modulated by the first drive signal and the second drive signal. An optical modulator that generates an optical signal having a quaternary phase, outputs the optical signal, and detects the intensity of the optical signal;
A bias controller that applies a bias voltage to each of the first waveguide and the second waveguide based on the intensity detected by the optical modulator in order to correct the operating point in the optical signal. When,
A bias shift detector that detects a bias shift amount that is an offset between the intensity of the optical signal output from the optical modulator and the intensity of the optical signal detected by the optical modulator,
The bias shift detector is applied to one of the first waveguide and the second waveguide, and the first drive signal and the second drive signal are not input to the optical modulator. A first bias voltage that turns off the optical signal and a second bias voltage that is applied to the one side and turns off the optical signal while the first drive signal and the second drive signal are input to the optical modulator. The difference from the bias voltage is detected as the bias shift amount,
The bias control unit corrects the bias voltage based on the bias shift amount. An optical transmission system comprising an optical transmitter and an optical receiver for receiving an optical signal transmitted from the optical transmitter,
The optical transmitter is
A light source,
A first waveguide to which a first drive signal is input and a second waveguide to which a second drive signal is input are provided, and the light from the light source is modulated by the first drive signal and the second drive signal. An optical modulator that generates an optical signal having a quaternary phase, outputs the optical signal, and detects the intensity of the optical signal;
A bias control unit that controls a bias voltage applied to the optical modulation unit based on the intensity detected by the optical modulation unit in order to correct the operating point in the optical signal;
A bias shift detector that detects a bias shift amount that is an offset between the intensity of the optical signal output from the optical modulator and the intensity of the optical signal detected by the optical modulator,
The bias shift detection unit is configured such that the optical signal is turned off when the first drive signal and the second drive signal are not input to the optical modulation unit, and the first waveguide and the second guide are provided. After adjusting the bias voltage applied to each of the waveguides, the optical signal is turned off in a state where only one of the first drive signal and the second drive signal is input to the optical modulator. When the first bias voltage applied to the one waveguide to which the one is input and both the first drive signal and the second drive signal are input to the optical modulator, the optical signal is A difference with the second bias voltage applied to the one waveguide, which is turned off, is detected as the bias shift amount,
The bias control unit corrects the bias voltage based on the bias shift amount. By modulating the light with the first drive signal input to the first waveguide and the second drive signal input to the second waveguide, an optical signal having a quaternary phase is generated and output.
Detecting the intensity of the optical signal,
Detecting a bias shift amount that is an offset between the intensity of the optical signal and the detected intensity,
A drive control method for correcting, based on the bias shift amount, a bias voltage specified based on the detected intensity in order to correct the operating point in the optical signal ,
A first bias voltage that is applied to one of the first waveguide and the second waveguide and that turns off the optical signal in a state where the first drive signal and the second drive signal are not input; A difference from a second bias voltage applied to the one side and turning off the optical signal in a state where the first drive signal and the second drive signal are input is detected as the bias shift amount. And drive control method. By modulating the light with the first drive signal input to the first waveguide and the second drive signal input to the second waveguide, an optical signal having a quaternary phase is generated and output.
Detecting the intensity of the optical signal,
Detecting a bias shift amount that is an offset between the intensity of the optical signal and the detected intensity,
A drive control method for correcting, based on the bias shift amount, a bias voltage specified based on the detected intensity in order to correct the operating point in the optical signal ,
The bias voltage applied to each of the first waveguide and the second waveguide was adjusted so that the optical signal was turned off when the first drive signal and the second drive signal were not input. After that, the optical signal is turned off in the state where only one of the first drive signal and the second drive signal is input, and the first is applied to the one waveguide to which the one is input. A difference between a bias voltage and a second bias voltage applied to the one waveguide, in which the optical signal is turned off when both the first drive signal and the second drive signal are input, A drive control method comprising detecting as the bias shift amount .

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