JP7308771B2 - Optical transmitter and method for controlling optical transmitter - Google Patents

Description

The present disclosure relates to an optical transmitter and a control method for the optical transmitter.

An optical transmitter using QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) comprises an Ich optical modulator, which is a Mach-Zehnder (MZ) type optical modulator for Ich (in-phase channel), and a Qch (quadrature- phase channel), and the phase difference between the light output from the Ich optical modulator and the light output from the Qch optical modulator ("phase difference between Ich and Qch") and an optical phase control unit for controlling the In order to generate a QPSK signal or a QAM signal, a bias voltage (that is, Ich bias and It is necessary to apply a bias voltage (that is, a PS bias) to the optical phase control unit so that the phase difference between Ich and Qch is π/2.

In Patent Document 1, in an optical transmitter, an Ich optical modulator and a Qch optical modulator are operated in a state in which no modulation signal is input to the Ich optical modulator and the Qch optical modulator (that is, in a state in which the modulation signal is turned off). , and the optical phase control section, a control method for determining the desired bias voltage applied to each of them in a short time.

JP 2015-114499 A

However, the optical transmitter described in Patent Document 1 is a device of a push-pull drive system in which both arms of the Ich optical modulator and the Qch optical modulator are subjected to phase modulation of the same magnitude and opposite sign. Therefore, in principle, when the control method described in Patent Document 1 is applied to a single-end driving device in which phase modulation is performed on only one arm in each of the Ich optical modulator and the Qch optical modulator, it should be avoided. There is a problem that the bias voltage applied to the Ich optical modulator, the Qch optical modulator, and the optical phase control section cannot be set to an optimum value due to the phase modulation of the output light.

The present disclosure applies to the first optical modulator, the second optical modulator, and the optical phase adjustment section, respectively, in an optical transmitter having single-ended drive type MZ-type first and second optical modulators. The purpose is to control the applied bias voltage to an optimum value.

An optical transmitter according to the present disclosure includes an optical modulation section that modulates input light and outputs output light, a modulation signal driving section that inputs a modulation signal to the optical modulation section, the optical modulation section and the modulation signal driving section. and a control unit for controlling the optical modulation unit, wherein the optical modulation unit is a first optical modulator and a second optical modulator, both of which are single-ended driving Mach-Zehnder optical modulators, and the second optical modulator an optical phase adjustment unit connected to the modulator via an optical waveguide; an optical intensity detection unit for detecting the intensity of the output light; to the first optical modulator and the second optical modulator to the second optical modulator; and the first optical modulator guided through the first optical modulator. The output light is generated by combining the third light wave, which is a light wave, and the fourth light wave, which is the second light wave guided through the second optical modulator and the optical phase adjustment unit. and a multiplexing waveguide, wherein the controller controls the first optical modulator and the second optical modulator in a state where the modulated signal is not input to the first optical modulator and the second optical modulator. bias control for determining a set value of the first bias voltage, a set value of the second bias voltage of the second optical modulator, and a set value of the third bias voltage of the optical phase adjustment unit; The bias control includes a first search for searching for a first value of the first bias voltage that maximizes the intensity of the output light or a second search for the second bias voltage that maximizes the intensity of the output light. By alternately performing the second search for searching for the value of and the third search for searching for the third value of the third bias voltage that maximizes the intensity of the output light, the first and the second value; a fourth search for searching for a fourth value of the first bias voltage that minimizes the intensity of the output light; setting the first bias voltage to the converged first value; and a sixth search for searching for a sixth value of the third bias voltage that maximizes the intensity of the output light while the second bias voltage is set to the converged second value. and determining the set value of the first bias voltage based on the fourth value, and determining the set value of the second bias voltage based on the fifth value. and a fourth process of determining the set value of the third bias voltage based on the sixth value.

A control method for an optical transmitter according to the present disclosure includes an optical modulator that modulates input light and outputs output light, a modulated signal driver that inputs a modulated signal to the optical modulator, the optical modulator and the modulator. a controller for controlling a signal driver, wherein the optical modulators are both single-ended drive type Mach-Zehnder optical modulators; a first optical modulator and a second optical modulator; an optical phase adjusting section connected to the optical modulator of No. 2 by an optical waveguide; an optical intensity detecting section for detecting the intensity of the output light; and splitting the input light into a first light wave and a second light wave, a branch waveguide for inputting the first light wave to the first optical modulator and inputting the second light wave to the second optical modulator; By combining the third light wave, which is the first light wave, and the fourth light wave, which is the second light wave guided through the second optical modulator and the optical phase adjustment unit, the output and a multiplexing waveguide for generating light, wherein the control unit generates the first light in a state where the modulated signal is not input to the first optical modulator and the second optical modulator. Bias control for determining a first bias voltage setting value for the modulator, a second bias voltage setting value for the second optical modulator, and a third bias voltage setting value for the optical phase adjustment section. wherein the first search searches for a first value of the first bias voltage that maximizes the intensity of the output light or the method for maximizing the intensity of the output light. Alternating between a second search for a second value of the second bias voltage and a third search for a third value of the third bias voltage that maximizes the intensity of the output light. a first process of converging the first value and the second value, and a fourth process of searching for a fourth value of the first bias voltage that minimizes the intensity of the output light. and a fifth search for searching for a fifth value of the second bias voltage that minimizes the intensity of the output light; and the second bias voltage is set to the converged second value, the sixth bias voltage of the third bias voltage maximizing the intensity of the output light. a third process of performing a sixth search for searching for a value; determining the set value of the first bias voltage based on the fourth value; determining the set value of the first bias voltage based on the fifth value; and a fourth process of determining the set value of the bias voltage and determining the set value of the third bias voltage based on the sixth value.

According to the optical transmitter and the control method thereof of the present disclosure, in the optical transmitter having the single-ended drive system MZ type first and second optical modulators, the first optical modulator and the second optical modulator It is possible to control the bias voltages applied to the detector and the optical phase adjuster to optimum values.

1 is a diagram showing the configuration of an optical transmitter according to Embodiment 1; FIG. 2A and 2B are diagrams showing configurations of an Ich optical modulator and a Qch optical modulator shown in FIG. 1; FIG. 4 is a flow chart showing bias control of an Ich optical modulator, a Qch optical modulator, and an optical phase adjuster in the optical transmitter according to Embodiment 1. FIG. FIG. 4 is a flowchart showing processing in step S11 of FIG. 3; FIG. FIG. 4 is a flowchart showing processing in step S16 of FIG. 3; FIG. (A) and (B) are diagrams showing constellations in an optical modulation unit of a push-pull drive system as a comparative example, and (C) and (D) are diagrams of a single-ended drive system according to the first embodiment. It is a figure which shows the constellation in an optical modulation part. 5A to 5F are diagrams showing constellations in each step of the bias control in FIG. 4; FIG. 4 is a diagram showing calculation results of the power of output light from the optical transmitter according to the first embodiment; FIG. FIG. 10 is a diagram showing the configuration of an optical transmitter according to Embodiment 2; 9 is a flow chart showing PS bias control in the optical transmitter according to the second embodiment.

An optical transmitter and a method for controlling the optical transmitter according to the embodiments will be described below with reference to the drawings. The following embodiments are merely examples, and various modifications are possible within the scope of the present disclosure.

Embodiment 1.
FIG. 1 is a diagram showing the configuration of an optical transmitter according to Embodiment 1. FIG. As shown in FIG. 1, the optical transmitter according to the first embodiment includes a light source 31, and an optical modulator 10 that modulates input light L10, which is a light wave output from the light source 31, and outputs output light L30. , a modulation signal driving section 32 which is a circuit for giving modulation signals M1 and M2 to the optical modulation section 10, and a control section 20 which is a circuit for controlling the optical modulation section 10 and the modulation signal driving section 32. FIG. The optical transmitter according to the first embodiment is a circuit that converts a monitor current D1, which is a feedback signal corresponding to the intensity (that is, power) of the output light L30 output from the optical modulator 10, into a monitor voltage D2. A current-voltage conversion unit 33 , a storage unit 34 such as a semiconductor memory, and a calculation function unit 35 are provided. The calculation function unit 35 has, for example, a memory that stores a program as software and a processor as an information processing unit that executes this program. The calculation function unit 35 may be a computer. Also, part of the control unit 20 may be implemented by a processor that executes a program.

The optical modulator 10 includes an Ich optical modulator 11 which is a first MZ optical modulator for Ich (in-phase channel) and a second MZ optical modulator for Qch (quadrature-phase channel). Qch optical modulator 12, optical waveguides 15a, 15b, 15c, 15d, 15e, 15f, and 15g, and an optical phase adjustment unit (PS (Phase Shift) unit) that controls the phase difference between Ich and Qch. 13 and a light intensity detector 14 for outputting a monitor current D1 corresponding to the power of the output light L30. The current-voltage converter 33 converts the monitor current D1 output from the light intensity detector 14 into a monitor voltage D2 and inputs the monitor voltage D2 to the controller 20 .

The optical waveguides 15a, 15b, and 15c split the input light L10 into a light wave L21 as a first light wave and a light wave L22 as a second light wave, input the light wave L21 to the Ich optical modulator 11, and transmit the light wave L22. A branch waveguide for input to the Qch optical modulator 12 is constructed. An input light L10 output from the light source 31 and input to the optical waveguide 15a of the optical modulation section 10 is branched into light waves L21 and L22. The light waves L21 and L22 travel through the optical waveguides 15b and 15c, respectively, and enter the Ich optical modulator 11 and the Qch optical modulator 12, respectively.

The optical waveguides 15d, 15f, and 15g are composed of a light wave L21a as a third light wave, which is the light wave L21 guided through the Ich optical modulator 11, and a light wave L22 guided through the Qch optical modulator 12 and the optical phase adjustment unit 13. A multiplexing waveguide for multiplexing the light wave L22b as a certain fourth light wave is configured. The light wave L21a output from the Ich optical modulator 11 travels through the optical waveguide 15d to the optical waveguide 15g. The optical wave L22 output from the Qch optical modulator 12 is input to the PS section 13 via the optical waveguide 15e and phase-adjusted to become the optical wave L22b. The light wave L22b travels through the optical waveguide 15f to the optical waveguide 15g. That is, the optical wave L21a output from the Ich optical modulator 11 and the optical wave L22b output from the PS section 13 are combined in the optical waveguide 15g, and the combined optical wave is output from the optical modulation section 10 as the output light L30. is output as The PS section 13 can also be connected to the Ich optical modulator 11 instead of the Qch optical modulator 12 .

When transmitting output light as an optical signal from the optical transmitter according to Embodiment 1, the Ich optical modulator 11 and the Qch optical modulator 12 receive the modulated signals M1 and M2 from the modulated signal driving unit 32, respectively, Based on the modulation signals M1 and M2, intensity modulation of the input light waves L21 and L22 is performed. However, the bias control of the transmitter according to Embodiment 1 (processing shown in FIG. 3 to be described later) is performed in a state in which no modulation signal is applied from the modulation signal driving section 32 to the Ich optical modulator 11 and the Qch optical modulator 12; That is, it is performed with the modulation signal turned off.

The control unit 20 receives input from the synchronous detection unit 21, bias application units 22a, 22b, and 22c, low-frequency signal generation units 23a, 23b, and 23c, addition units 25a, 25b, and 25c, and a current-voltage conversion unit 33. and a lock determination unit 26. The monitor unit 24 receives the monitor voltage D2.

The bias voltage B1 output from the bias applying unit 22a and the dither signal, which is a low-frequency signal generated by the low-frequency signal generating unit 23a, are superimposed by the adding unit 25a to obtain a first bias voltage on which the dither signal is superimposed. C 1 , and the first bias voltage C 1 is applied to the Ich optical modulator 11 . The bias voltage B2 output from the bias applying unit 22b and the dither signal, which is a low-frequency signal generated by the low-frequency signal generating unit 23b, are superimposed by the adding unit 25b to obtain a second bias voltage on which the dither signal is superimposed. C 2 , and the second bias voltage C 2 is applied to the Qch optical modulator 12 . The bias voltage B3 output from the bias application unit 22c and the dither signal, which is a low-frequency signal generated by the low-frequency signal generation unit 23c, are superimposed by the addition unit 25c to obtain a third bias voltage on which the dither signal is superimposed. C3 and the third bias voltage C3 is applied to the PS section 13 .

The bias voltages B1, B2, B3 output from the bias applying units 22a, 22b, 22c and the dither signal are superimposed by digital signal processing in the adding units 25a, 25b, 25c. Also, the bias voltages B1, B2, and B3 are output from a DAC (digital-to-analog converter) provided in the control section 20. FIG. An ADC (analog-to-digital converter) provided in the monitor section 24 converts the monitor voltage D2 input from the current-voltage conversion section 33 into a digital value D3. A lock determination unit 26 performs lock determination of bias control based on a predetermined threshold value TH. Lock determination is the determination in step S114 in FIG. 4, which will be described later.

The synchronous detection unit 21 synchronously detects the dither signals generated by the low-frequency signal generation units 23a, 23b, and 23c and the digital value D3, which is the monitor signal acquired by the monitor unit 24, to detect an error signal. is controlled to be zero. The synchronous detection section 21 detects first to third bias voltages C1, C2, and C3 applied to the Ich optical modulator 11, the Qch optical modulator 12, and the PS section 13 by controlling the bias voltages B1, B2, and B3. Control. The first to third bias voltages C1, C2 and C3 are also called Ich bias, Qch bias and PS bias, respectively.

2A and 2B are diagrams showing configurations of the Ich optical modulator 11 and the Qch optical modulator 12 shown in FIG. As shown in FIG. 2A, the Ich optical modulator 11 has an Ich arm #1 and an Ich arm #2. Ich arm # 2 has Ich phase adjustment section 111 . The Ich optical modulator 11 does not phase-modulate the lightwave input to the Ich arm #1, but phase-modulates the lightwave input to the Ich arm #2 by the Ich phase adjustment unit 111, thereby converting the lightwave L21 into an intensity-modulated lightwave. This is a single-ended drive type MZ optical modulator that outputs L21a. As shown in FIG. 2B, the Qch optical modulator 12 has a Qch arm #1 and a Qch arm #2. Qch arm # 2 has Qch phase adjustment section 121 . The Qch optical modulator 12 does not phase-modulate the light wave input to the Qch arm #1, but phase-modulates the light wave input to the Qch arm #2 by the Qch phase adjustment unit 121, thereby converting the light wave L22 into an intensity-modulated light wave. This is a single-ended drive type MZ optical modulator that outputs L22a.

Next, the bias control in the optical transmitter, that is, the method of controlling the optical transmitter according to the first embodiment will be described. FIG. 3 is a flow chart showing bias control in the optical transmitter according to the first embodiment. FIG. 4 is a flow chart showing the processing in step S11 of FIG. FIG. 5 is a flow chart showing the processing in step S16 of FIG.

First, the outline of bias control will be described. In the bias control, first to third bias voltages C1, C2, and C3 applied to the Ich optical modulator 11, Qch optical modulator 12, and PS section 13, respectively, are output from bias application sections 22a, 22b, and 22c. This is the process of setting the bias voltages B1, B2 and B3 to optimum values by adjusting the applied bias voltages B1, B2 and B3.

The control unit 20 controls the setting value of the Ich bias of the Ich optical modulator 11 and Bias control for determining the set value of the Qch bias of the Qch optical modulator 12 and the set value of the PS bias of the PS section 13 is performed. In Embodiment 1, bias control has the following first to fourth processes.

The first process is shown in steps S11 and S12 of FIG. 3 and FIG. The first process is a first search ( Step S112) or a second search for searching for a second value of the Qch bias that maximizes the power of the output light L30 (that is, maximizes the power of the light wave output from the Qch optical modulator 12) (step S112). ), a third search for a third value of the PS bias that maximizes the power of the output light L30 (that is, maximizes the power of the light wave output from the PS section 13) (step S113); are alternately performed to converge the first value (Vi_peak) and the second value (Vq_peak).

The second process is shown in steps S13 and S14 of FIG. The second process searches for a fourth value (Vi_null) of the Ich bias that minimizes the power of the output light L30 (that is, minimizes the power of the light wave output from the Ich optical modulator 11). (step S13), and the fifth value (Vq_null) of the Qch bias that minimizes the power of the output light L30 (that is, minimizes the power of the light wave output from the Qch optical modulator 12). This is a process of performing a fifth search (step S14).

The third process is shown in steps S15 and S16 in FIG. 3 and steps S161 and S162 in FIG. In the third process, the Ich bias is set to Vi_peak, which is the first converged value, and the Qch bias is set to Vq_peak, which is the second converged value (step S15). A sixth search (steps S16 and S161) is performed to search for a sixth value (Vp_peak) of the PS bias that maximizes the power (that is, maximizes the power of the light wave output from the PS unit 13), and outputs A seventh search (steps S16, S162) for searching for a seventh value (Vp_null) of the PS bias that minimizes the power of the light L30 (that is, minimizes the power of the light wave output from the PS unit 13) is performed. This is the process to be performed.

The fourth process is shown in step S17 of FIG. 3 and step S163 of FIG. In the fourth process, a value determined based on Vp_peak and Vp_null (for example, an intermediate value between Vp_peak and Vp_null) is set as the PS bias setting value Vps (step S161), and the Ich bias is set to the fourth value. This is a process of setting a value (eg, Vi_null) based on a certain Vi_null and setting the Qch bias to a value (eg, Vq_null) based on a fifth value Vq_null (step S17).

Next, bias control in the optical transmitter will be described in detail with reference to FIGS. 3 to 5. FIG. As shown in FIG. 3, in the bias control, the controller 20 adjusts the power of the output light L30 of the optical modulator 10 while turning off the output of the modulated signals M1 and M2 from the modulated signal driver 32. The Ich bias, Qch bias, and PS bias are controlled to maximize (step S11).

Step S11 will be described in detail with reference to FIG. In step S11, the control unit 20 controls the PS bias to search for Vp_old, which is the PS bias value that maximizes the power of the output light L30, and stores it in the storage unit 34 (step S111).

Next, the controller 20 controls the Ich bias to search for Vi_peak, which is the value of the Ich bias that maximizes the power of the output light L30 (step S112).

Next, the control unit 20 controls the PS bias to search for Vp_new, which is the PS bias value that maximizes the power of the output light L30, and stores it in the storage unit 34 (step S113).

Next, the lock determination unit 26 calculates the difference between Vp_old and Vp_new (that is, the amount of change in the PS bias value), and compares the absolute value |Vp_new−Vp_old| of the difference with a predetermined threshold TH ( step S114). As a result of the comparison, when the absolute value of the difference |Vp_new−Vp_old| is equal to or greater than the threshold TH, the control unit 20 overwrites Vp_old with the value of Vp_new (step S115), and returns the process to step S112. If the absolute value of the difference |Vp_new−Vp_old| is smaller than the threshold TH, it is considered that Vi_peak has converged, and the Ich bias control in step S11 is completed.

Next, for Qch as well, the Qch bias is controlled by performing the processing of steps S111 to S115 in the same manner as for Ich. Also, the value of the Qch bias when the Qch bias is controlled to maximize the power of the output light L30 (step S112) is denoted as Vq_peak.

After step S11 in FIG. 3, the control unit 20 stores the converged Vi_peak and the converged Vq_peak, which are the values obtained in step S11, in the storage unit 34 (step S12).

Next, the control unit 20 controls the Ich bias to search for Vi_null, which is the value of the Ich bias that minimizes the power of the output light L30, and stores it in the storage unit 34 (step S13). Next, the control unit 20 controls the Qch bias to search for Vq_null, which is the Qch bias value that minimizes the power of the output light L30, and stores it in the storage unit 34 (step S14).

Next, the controller 20 returns the Ich bias value to Vi_peak and the Qch bias value to Vq_peak (step S15).

Next, the controller 20 controls the PS bias so that the phase difference between the light waves L21a and L22b (that is, the phase difference between Ich and Qch) becomes π/2 (step S16).

The PS bias control in step S16 will be described in detail with reference to FIG. The control unit 20 controls the PS bias to search for Vp_peak, which is the PS bias value that maximizes the power of the output light L30, and stores it in the storage unit 34 (step S161).

Next, the control unit 20 controls the PS bias to search for Vp_null, which is the PS bias value that minimizes the power of the output light L30, and stores it in the storage unit 34 (step S162).

Next, the calculation function unit 35 calculates the set value Vps, which is the PS bias value at which the phase difference between Ich and Qch becomes π/2. The bias application section 22c of the control section 20 outputs the calculated set value Vps as the PS bias. For example, when the light modulation unit 10 is driven by voltage control, the calculation function unit 35 calculates an intermediate value between Vp_null and Vp_peak as the set value Vps (step S163). Further, when the optical modulation unit 10 is driven by power control, the calculation function unit 35 calculates an intermediate value between the square of Vp_null and the square of Vp_peak as the set value Vps.

After step S16 in FIG. 3, the control unit 20 sets the Ich bias and the Qch bias to Vi_null and Vq_null, respectively, stored in the storage unit 34 (step S17).

By the above, the setting of the Ich bias, the Qch bias, and the PS bias in the optical transmitter according to the first embodiment, that is, the bias control is completed. After completing the setting, the control unit 20 causes the modulation signal driving unit 32 to output the modulation signals M1 and M2. That is, the optical modulator 10 modulates the input light L10 based on the modulated signals M1 and M2, and outputs the output light L30 as signal light.

Next, the control shown in step S11 of FIG. 3 will be described in detail. In principle, a single-ended drive type MZ optical modulator causes a phase change (that is, a frequency chirp) accompanying intensity modulation. FIGS. 6A and 6B are diagrams showing constellations in a push-pull drive type optical modulation unit as a comparative example. FIGS. 6C and 6D are diagrams showing constellations in the single-end drive type optical modulator 10 according to the first embodiment. In FIGS. 6A to 6D, the horizontal axis is the in-phase axis (ie I axis) and the vertical axis is the quadrature-phase axis (ie Q axis).

As shown in FIG. 6(A), the push-pull drive type MZ optical modulator gives the light wave traveling in arm #1 and the light wave traveling in arm #2 phase changes of the same magnitude and in opposite directions. , and as shown in FIG. 6B, the trajectory of the light wave obtained by combining the light wave traveling in arm #1 and the light wave traveling in arm #2 overlaps the in-phase axis. In other words, in principle, no frequency chirp occurs in the push-pull drive type MZ optical modulator. The trajectory of the light wave is the trajectory of the position of the constellation of the light wave, and is the trajectory of the position of the tip of the arrow in the figure.

On the other hand, as shown in FIG. 6C, in the single-ended drive type MZ optical modulator, the phase of the light wave traveling in arm #1 does not change, so the trajectory of the light wave is fixed on the in-phase axis. , only light waves traveling in arm #2 undergo a phase change. Therefore, as shown in FIG. 6(D), the trajectory of the light wave obtained by combining the light wave traveling in arm #1 and the light wave traveling in arm #2 is indicated by the arrow indicating the light wave in arm #1. A circle is formed with the position of the tip as the center. That is, in principle, a frequency chirp occurs in the single-ended drive type MZ optical modulator. Therefore, if the Ich bias is controlled or the Qch bias is controlled after controlling the PS bias, the PS bias is relatively changed, and the phase of the light wave guided through the PS section 13 is shifted.

However, in the single-ended drive type MZ optical modulator, the trajectory of the light wave is in- It exists on the phase axis and has no phase component. Therefore, by controlling the Ich buffer and the Qch buffer to adjust the power of the output light L30 to the peak point of the modulation curve, it is possible to control the PS bias without being affected by the frequency chirp. From the above description using FIGS. 6C and 6D, in the optical modulation section 10 using two MZ optical modulators of the single-ended drive system, the Ich bias and Qch bias are adjusted without being affected by frequency chirp. It can be seen that it is possible to control the bias and the PS bias.

FIGS. 7A to 7F are diagrams showing constellations in each step of the bias control in FIG. 4. FIG. In FIGS. 7A to 7F, the phase of the light wave guided through Ich arm #1 is used as the reference phase. As shown in FIG. 7A, before bias control is started, light waves traveling through Ich arm #1, Ich arm #2, Qch arm #1, and Qch arm #2 are combined in different phase states. being waved.

Next, as shown in FIG. 7B and step S111 in FIG. 4, the control unit 20 controls the PS bias, and modulates the light wave L21a modulated by the Ich optical modulator 11 and the Qch optical modulator 12. and the phase of the light wave L22b modulated by the PS unit 13 and phase-adjusted by the PS unit 13. At this time, the power of the output light L30 obtained by combining the light waves L21a and L22b is maximum.

Next, as shown in FIG. 7C and step S112 in FIG. 4, the controller 20 controls the Ich bias to maximize the power of the output light L30 of the optical modulator 10. FIG. However, the value of the Ich bias that maximizes the power of the output light L30 of the optical modulation section 10 at this time due to the frequency chirp is finally adopted for maximizing the power of the output light L30 of the optical modulation section 10. does not necessarily match the normal Ich bias value.

Next, as shown in FIG. 7(D) and step S113 in FIG. 4, the control unit 20 controls the PS bias so that the light wave L21a output from the Ich optical modulator 11 and the Qch optical modulator 12 and The phase of the light wave L22b guided through the PS section 13 is again made the same. That is, the control unit 20 controls the PS bias to cancel the phase change of the optical wave output from the Ich optical modulator 11 caused by the frequency chirp. When the power of the light wave L21a output from the Ich optical modulator 11 reaches the peak point of the modulation curve, there is no phase change due to Ich bias control. As a result, the change in the PS bias value obtained as a result of searching for the peak point of the modulation curve in PS bias control is reduced.

Next, as shown in step S114 in FIG. 4, the lock determination unit 26 of the control unit 20 determines whether or not the change in the PS bias value is sufficiently small. That is, when the absolute value of the PS bias difference |Vp_new−Vp_old| Bias control is performed to maximize the power of the output light L30 of the optical modulation section 10. FIG.

After that, in step S113 of FIG. 4, the control unit 20 again controls the PS bias, and as shown in FIG. The phase of the light wave L22b guided through the PS section 13 is again made the same. If the absolute value |Vp_new−Vp_old| of the PS bias difference is smaller than the threshold TH in step S114 of FIG. judge that it did.

By performing the processing of FIG. 4 for the Qch as well as for the Ich, the power of the light wave L22b guided through the Qch optical modulator 12 and the PS section 13 converges to the peak point.

Next, the correlation between the power of the light wave L21a and the power of the output light L30 of the optical modulation section 10 in controlling the Ich bias will be described. Due to the frequency chirp described above, the phase relationship between the light wave L21a guided through the Ich optical modulator 11 and the light wave L22b guided through the Qch optical modulator 12 and the PS section 13 changes depending on the Ich bias. The power of the output light L30 of the modulation unit 10 is determined by the phase relationship between the light wave L21a guided through the Ich optical modulator 11 and the light wave L22b guided through the Qch optical modulator 12 and the PS unit 13. and the phase difference between Qch.

FIG. 8 is a diagram showing calculation results of the power of the output light L30 of the optical transmitter according to the first embodiment. FIG. 8 shows the calculation results of the power of the output light L30 of the optical modulation section 10 with respect to the reference phase for each Qch phase, using the Ich phase, which is the phase of the light wave guided through the Ich arm #1, as the reference phase. For example, under the condition that the Qch phase is +0.25π, if the Ich bias is controlled to search for the maximum point of the power of the output light L30 of the optical modulator 10, then the Ich phase is +0.085π (and Ich arm #2 When the phase of the light wave guided through is +0.17π), the power of the output light L30 of the optical modulation section 10 becomes maximum. That is, the maximum point of the power of the output light L30 of the optical modulation section 10 does not necessarily coincide with the peak point on the Ich modulation curve shown in FIG.

FIG. 8 shows the Ich phase at which the power of the output light L30 of the optical modulation section 10 is maximized for arbitrary Qch phases (for example, Q=0, ±0.25π, ±0.5π, ±0.75π). also decreases as the Qch phase increases, and its absolute value approaches zero. Therefore, as shown in step S11 in FIG. 3, after the Ich bias is controlled in an arbitrary Qch phase, the light wave L21a guided through the Ich optical modulator 11 by PS bias control and the Qch optical modulator 12 and The phase of the light wave L22b guided through the PS section 13 is made the same again, and then the control of the Ich bias is repeated. Both the phase of the light wave L21a and the phase of the light wave L22b that has traveled through the Qch arm #1 and the Qch arm #2 and have been combined can be converged to the phase of the Ich arm #1, which is the reference phase.

By controlling the Qch arm #1 and the Qch arm #2 in the same manner as the Ich arm #1 and the Ich arm #2, the total number of the Ich arm #1 and the Ich arm #2 and the Qch arm #1 and the Qch arm #2 can be adjusted. The phases of the four light waves guided through the four arms can all be converged to the same phase.

In the first embodiment, in steps S11, S13, and S14 of FIG. 3, the Qch bias control is performed after the Ich bias control, but the Ich bias control may be performed after the Qch bias control.

As described above, in the optical transmitter and its control method according to the first embodiment, the control unit 20 is configured to use the lock determination unit 26, and the Ich bias control and the PS bias control are alternately repeated. PS bias threshold determination (step S114 in FIG. 4) is performed, and the Qch bias control and PS bias control are alternately repeated to perform PS bias threshold determination (step S114 in FIG. 4). As a result, it is possible to eliminate the effect of frequency chirp in the optical modulation section 10 using two single-ended drive type MZ optical modulators, and to set the optimum Ich bias, Qch bias, and PS bias. .

Embodiment 2.
In the optical transmitter according to Embodiment 1, as shown in FIG. A PS bias value that makes the phase difference of .pi./2 is calculated, and the controller 20 sets the calculated value as the PS bias set value. On the other hand, in the optical transmitter according to the second embodiment, the controller 20a sets the PS bias to a value corresponding to −3 dB with respect to the voltage value PS_peak that maximizes the power of the output light L30. By adjusting the bias set value, the phase difference between Ich and Qch is set to π/2.

FIG. 9 is a diagram showing the configuration of an optical transmitter according to the second embodiment. In FIG. 9, the same reference numerals as those shown in FIG. 1 are attached to the same or corresponding constituent elements as those shown in FIG. The optical transmitter according to the second embodiment differs from the optical transmitter according to the first embodiment in that it does not include the calculation function unit 35 . In the second embodiment, the control unit 20a stores in the storage unit 34a in advance a value PS_-3dB corresponding to -3dB with respect to the digital value D3 output from the ADC of the monitor unit 24. FIG. The bias control of the optical transmitter according to the second embodiment is as shown in FIG. 3, but differs from the bias control of the optical transmitter according to the first embodiment in the content of step S16 in FIG. In other words, the bias control of the optical transmitter according to the second embodiment has the first to fourth processes in the same manner as the bias control of the optical transmitter according to the first embodiment. It differs from the bias control of the optical transmitter according to the first embodiment in one point.

FIG. 10 is a flow chart showing PS bias control in the optical transmitter according to the second embodiment. In step S16, as shown in FIG. 10, the control unit 20a controls the PS bias to obtain the voltage value PS_peak after AD conversion in the monitor unit 24 when the power of the output light L30 becomes maximum, It saves in the storage unit 34a (step S261). Next, the control unit 20a adjusts the PS bias to a value obtained by subtracting a value PS_-3dB corresponding to -3dB from the AD-converted voltage value PS_peak in the monitor unit 24 (step S262). As described above, the control unit 20a can control the PS bias to a value that makes the phase difference between Ich and Qch π/2.

As described above, in the transmitter and its control method according to the second embodiment, the value PS_-3dB corresponding to -3dB is subtracted from the AD-converted voltage value PS_peak of the monitor unit 24 in the storage unit 34a. By storing the value, the PS bias can be controlled to a value that makes the phase difference between Ich and Qch π/2 without providing the calculation function unit 35 . Moreover, since the optical transmitter according to the second embodiment does not include the calculation function unit 35, the configuration can be simplified.

10 optical modulator 11 Ich optical modulator 111 Ich phase adjuster 12 Qch optical modulator 121 Qch phase adjuster 13 PS section 14 optical intensity detector 15a, 15b, 15c, 15d, 15e, 15f , 15g optical waveguide 20, 20a control unit 21 synchronous detection unit 22a, 22b, 22c bias application unit 23a, 23b, 23c low frequency signal generation unit 24 monitor unit 25a, 25b, 25c addition unit 26 lock Determination unit 31 Light source 32 Modulation signal driver 33 Current voltage converter 34, 34a Storage unit 35 Calculation function unit L10 Input light L21, L21a, L22, L22a, L22b Light wave L30 Output light.

Claims (8)

an optical modulator that modulates input light and outputs output light;
a modulation signal driving unit that inputs a modulation signal to the optical modulation unit;
a control unit that controls the optical modulation unit and the modulation signal driving unit;
with
The optical modulation unit
a first optical modulator and a second optical modulator, both of which are single-ended drive type Mach-Zehnder optical modulators;
an optical phase adjuster connected to the second optical modulator via an optical waveguide;
a light intensity detector that detects the intensity of the output light;
splitting the input light into a first light wave and a second light wave, inputting the first light wave to the first optical modulator and inputting the second light wave to the second optical modulator a branch waveguide that
A third light wave that is the first light wave guided through the first optical modulator, and a fourth light wave that is the second light wave guided through the second optical modulator and the optical phase adjustment unit a combining waveguide that generates the output light by combining the light waves of
has
The control unit controls the set value of the first bias voltage of the first optical modulator and the performing bias control for determining a set value of a second bias voltage of the second optical modulator and a set value of a third bias voltage of the optical phase adjustment unit;
The bias control is
a first search for a first value of the first bias voltage that maximizes the intensity of the output light or a second value of the second bias voltage that maximizes the intensity of the output light; and a third search for searching for a third value of the third bias voltage that maximizes the intensity of the output light, thereby obtaining the first value and the a first process of converging the second value;
a fourth search for searching for a fourth value of the first bias voltage that minimizes the intensity of the output light; and a fifth value for the second bias voltage that minimizes the intensity of the output light. a fifth search for searching; a second process for performing;
setting the first bias voltage to the converged first value and setting the second bias voltage to the converged second value, and maximizing the intensity of the output light; a third process of performing a sixth search for searching for a sixth value of the third bias voltage;
determining the set value of the first bias voltage based on the fourth value; determining the set value of the second bias voltage based on the fifth value; and determining the set value of the second bias voltage based on the sixth value. a fourth process of determining the set value of the third bias voltage based on
An optical transmitter characterized by comprising: In the first process, when the absolute value of the amount of change in the third value generated by the first search becomes smaller than a predetermined threshold value, the control unit changes the first value to It is determined that convergence has occurred, and when the absolute value of the amount of change in the third value generated by the second search is smaller than the threshold, it is determined that the second value has converged. 2. The optical transmitter of claim 1. The control unit
In the third processing, performing a seventh search for searching for a seventh value of the third bias voltage that minimizes the intensity of the output light;
3. The optical transmission according to claim 1, wherein in said fourth processing, said set value of said third bias voltage is determined based on said sixth value and said seventh value. vessel. 3. The control unit sets the set value of the third bias voltage to an intermediate value between the sixth value and the seventh value in the fourth process. The optical transmitter according to . In the fourth process, the controller sets the set value of the third bias voltage to an intermediate value between the square of the sixth value and the square of the seventh value. 4. The optical transmitter according to claim 3, wherein The controller adjusts the set value of the third bias voltage to a value obtained by subtracting a value corresponding to −3 dB from the sixth value in the fourth process. 3. The optical transmitter according to claim 1 or 2, wherein In the fourth process, the controller sets the set value of the first bias voltage to the fourth value and sets the set value of the second bias voltage to the fifth value. 7. An optical transmitter according to any one of claims 1 to 6, characterized in that: an optical modulation unit that modulates input light and outputs output light, a modulation signal driving unit that inputs a modulation signal to the optical modulation unit, and a control unit that controls the optical modulation unit and the modulation signal driving unit. prepared,
The optical modulator includes a first optical modulator and a second optical modulator, both of which are Mach-Zehnder optical modulators of a single-end drive system, and an optical waveguide connected to the second optical modulator. a phase adjusting section, a light intensity detecting section for detecting the intensity of the output light, and a first light wave and a second light wave from the input light, and the first light wave is transmitted to the first optical modulator. a branching waveguide for inputting the second light wave to the second optical modulator; a third light wave that is the first light wave guided through the first optical modulator; a combining waveguide that generates the output light by combining a fourth light wave that is the second light wave guided through the second optical modulator and the optical phase adjustment unit;
The control unit controls the set value of the first bias voltage of the first optical modulator and the A control method for an optical transmitter, which performs bias control for determining a set value of a second bias voltage of a second optical modulator and a set value of a third bias voltage of the optical phase adjustment section, comprising:
a first search for a first value of the first bias voltage that maximizes the intensity of the output light or a second value of the second bias voltage that maximizes the intensity of the output light; and a third search for searching for a third value of the third bias voltage that maximizes the intensity of the output light, thereby obtaining the first value and the a first process of converging the second value;
a fourth search for searching for a fourth value of the first bias voltage that minimizes the intensity of the output light; and a fifth value for the second bias voltage that minimizes the intensity of the output light. a fifth search for searching; a second process for performing;
setting the first bias voltage to the converged first value and setting the second bias voltage to the converged second value, and maximizing the intensity of the output light; a third process of performing a sixth search for searching for a sixth value of the third bias voltage;
determining the set value of the first bias voltage based on the fourth value; determining the set value of the second bias voltage based on the fifth value; and determining the set value of the second bias voltage based on the sixth value. a fourth process of determining the set value of the third bias voltage based on
A method of controlling an optical transmitter, comprising:

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