Optical transmitter and optical transceiver

Abstract

To provide an optical transmitter capable of stabilizing optical output.SOLUTION: An optical transmitter includes a driver amplifier that amplifies a high-frequency signal, an adding unit that adds a dither signal to the high-frequency signal amplified by the driver amplifier, and an optical modulating unit that modulates an optical signal in accordance with the high-frequency signal to which the dither signal has been added. The optical transmitter also includes a detecting unit that detects a fluctuation level of the dither signal from the modulated optical signal, and a control unit that controls the gain of the driver amplifier that amplifies the high-frequency signal on the basis of the detected fluctuation level of the dither signal such that the output amplitude of the driver amplifier is constant.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical transmitter and an optical transceiver. [Background technology]

[0002] Currently, network traffic demands are increasing both domestically and internationally, and the advancement of 5G is expected to further increase the speed and capacity of edge networks. In light of this situation, further improvements in transmission capacity will be required both domestically and internationally.

[0003] Therefore, optical transmission systems are increasing their transmission capacity, and it is expected that transmission speeds exceeding 1 terabit per second (TBP) per wavelength will become practical in the future. Known methods for improving transmission capacity include increasing the number of bits per symbol (code) to increase the information length, and increasing the symbol rate to increase the number of symbols per unit time.

[0004] To achieve these higher multi-level and higher symbol rates, optical devices such as E / O converters that convert electrical signals to optical signals at high speed, and O / E converters that convert optical signals to electrical signals, are important. While improvements in transmission capacity depend heavily on increasing the speed of these devices, there is a stronger demand for increased device transmission capacity than for device speed, and in reality, devices with slightly insufficient bandwidth are being used by optimizing them with bandwidth compensation and other adjustments. Furthermore, in recent years, such optimization adjustments have become more difficult due to the trend toward higher multi-level and higher symbol rates, and the number of adjustment points is also on the rise. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 180537 [Patent Document 2] U.S. Patent No. 09124364 Summary of the Invention

Problems to be Solved by the Invention

[0006] As one of the adjustment points, there is gain adjustment of the driver amplifier in the CDM (Coherent Driver Modulator). However, in the driver amplifier used in the E / O conversion section, suppression of amplitude fluctuation due to temperature change is required.

[0007] FIG. 15 is an explanatory diagram showing an example of the characteristics of the PI (Peak Indicator) value and gain of a driver amplifier at an environmental temperature of 25°C. The PI value is the sensitivity of the output amplitude monitor of the driver amplifier. As shown in FIG. 15, the PI value varies greatly depending on the setting range of the gain of the driver amplifier. Incidentally, the XI channel is the channel of the I component of the X polarization, the XQ channel is the channel of the Q component of the X polarization, the YI channel is the channel of the I component of the Y polarization, and the YQ channel is the channel of the Q component of the Y polarization. The target PI values of each channel are as shown in FIG. 15.

[0008] For example, when the gain of the driver amplifier is in the range of 0≦X≦64, the PI value of each channel is low, and the sensitivity of the output amplitude monitor is low. Also, when the gain is in the range of 64<X≦128, the PI value of each channel is high, and the sensitivity of the output amplitude monitor is high. Focusing on the characteristics shown in FIG. 15, the gain corresponding to the target PI values of the XI, XQ, YI, and YQ channels is around 80.

[0009] On the other hand, when the environmental temperature of the driver amplifier increases, the sensitivity of the output amplitude monitor fluctuates greatly. FIG. 16 is an explanatory diagram showing an example of the characteristics of the PI value and gain of a driver amplifier at an environmental temperature of 50°C. At a high environmental temperature of 50°C, since the PI value becomes high even when the gain is small, it exceeds the target PI value of each channel. Therefore, it becomes impossible to adjust the PI value by adjusting the gain. As shown in FIG. 16, for example, the PI values of the XI, XQ, and YI channels can be adjusted, but the PI value of the YQ channel cannot be adjusted.

[0010] Therefore, because gain adjustment of the driver amplifier in the E / O conversion unit is controlled using an electrical signal in the electrical stage, the problem of fluctuations due to the temperature characteristics of the PI value itself arises, making it difficult to stabilize the optical output.Therefore, there is a need for a method that can stabilize the optical output of an optical transmitter even when the environmental temperature fluctuates by using, for example, the optical output of the optical stage in the optical transmitter instead of the electrical stage to adjust the gain of the driver amplifier.

[0011] In one aspect, an object is to provide an optical transmitter or the like that can stabilize optical output. [Means for solving the problem]

[0012] An optical transmitter of one embodiment includes a driver amplifier that amplifies a high-frequency signal, an adding unit that adds a dither signal to the high-frequency signal amplified by the driver amplifier, and an optical modulating unit that modulates an optical signal in accordance with the high-frequency signal to which the dither signal has been added. The optical transmitter also includes a detecting unit that detects a fluctuation level of the dither signal from the modulated optical signal, and a control unit that controls a gain of the driver amplifier that amplifies the high-frequency signal based on the detected fluctuation level of the dither signal so that the output amplitude of the driver amplifier is constant. [Effects of the Invention]

[0013] According to one aspect, the light output can be stabilized. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is an explanatory diagram showing an example of an optical transceiver according to this embodiment. [Figure 2] FIG. 2 is an explanatory diagram illustrating an example of an optical transmitter according to the first embodiment. [Figure 3] FIG. 3 is an explanatory diagram illustrating an example of an output of the driver amplifier according to the first embodiment. [Figure 4] FIG. 4 is an explanatory diagram illustrating an example of the optical output level due to the second dither signal of the optical modulation unit according to the first embodiment and the fluctuation level of the second dither signal. [Figure 5] FIG. 5 is a flowchart showing an example of the processing operation of the control unit related to the control processing of the optical modulation unit. [Figure 6A] FIG. 6A is a flowchart showing an example of a processing operation of the ABC control unit related to the ABC control processing of the optical modulation unit. [Figure 6B] FIG. 6B is a flowchart showing an example of the processing operation of the ABC control unit related to the ABC control processing of the optical modulation unit. [Figure 7A] FIG. 7A is a flowchart showing an example of a processing operation of the DRV control unit related to the first DRV control processing of the optical modulation unit. [Figure 7B] FIG. 7B is a flowchart showing an example of the processing operation of the DRV control unit related to the first DRV control processing of the optical modulation unit. [Figure 8] FIG. 8 is an explanatory diagram illustrating an example of an optical transmitter according to the second embodiment. [Figure 9] FIG. 9 is an explanatory diagram showing an example of an optical output level and a first fluctuation level due to a first dither signal of the optical modulation unit according to the second embodiment, and an example of an optical output level and a second fluctuation level due to a second dither signal. [Figure 10A] FIG. 10A is a flowchart showing an example of a processing operation of the DRV control unit related to the second DRV control processing of the optical modulation unit. [Figure 10B] FIG. 10B is a flowchart showing an example of the processing operation of the DRV control unit related to the second DRV control processing of the optical modulation unit. [Figure 11] FIG. 11 is an explanatory diagram illustrating an example of an optical transmitter according to the third embodiment. [Figure 12] FIG. 12 is an explanatory diagram illustrating an example of the optical output level and the first fluctuation level due to the first dither signal of the optical modulation unit according to the third embodiment. [Figure 13] FIG. 13 is an explanatory diagram illustrating an example of the optical output level and the second fluctuation level due to the second dither signal of the optical modulation unit according to the third embodiment. [Figure 14A] FIG. 14A is a flowchart showing an example of the processing operation of the DRV control unit related to the third DRV control processing of the optical modulation unit. [Figure 14B]FIG. 14B is a flowchart showing an example of the processing operation of the DRV control unit related to the third DRV control processing of the optical modulation unit. [Figure 15] FIG. 15 is an explanatory diagram showing an example of the characteristics of the PI value and gain of the driver amplifier when the environmental temperature is 25°C. [Figure 16] FIG. 16 is an explanatory diagram showing an example of the characteristics of the PI value and gain of the driver amplifier when the environmental temperature is 50°C. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the optical transmitter and the like disclosed in the present application will be described in detail with reference to the drawings. Note that the disclosed technology is not limited to these embodiments. Furthermore, the embodiments described below may be combined as appropriate within the scope of not causing any contradiction. [Example]

[0016] FIG. 1 is an explanatory diagram illustrating an example of an optical transceiver 1 according to the present embodiment. The optical transceiver 1 shown in FIG. 1 is, for example, a coherent optical transmitter / receiver using a DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) system. The optical transceiver 1 includes an optical transmitter 2, an optical receiver 3, a laser diode (LD) 4, and a digital signal processor (DSP) 5. The optical transmitter 2 includes an optical modulator 2A that modulates an optical signal from the LD 4 in response to an electrical signal from the DSP 5, and outputs the modulated transmission light from the optical modulator 2A through an optical fiber FC. The optical receiver 3 includes an optical receiver 3B that uses the optical signal from the LD 4 to obtain reception light from the signal light received through the optical fiber, converts the obtained reception light into an electrical reception signal, and outputs the electrical reception signal to the DSP 5. The LD 4 is a light source that emits an optical signal. The DSP 5 is a signal processing unit that generates an electrical signal to be sent to the optical transmitter 2 based on the data, and also acquires data from the signal received from the optical receiver 3 .

[0017] 2 is an explanatory diagram illustrating an example of an optical transmitter 2 according to a first embodiment. The optical transmitter 2 includes a CDM (Coherent Driver Modulator) 11, a detector 12, a DAC (Digital-Analog Converter) 13, and a microcomputer 15. The CDM 11 includes a driver amplifier 21 for each channel and an optical modulator 2A. The driver amplifier 21 amplifies an RF signal, which is a high-frequency signal output to a phase modulator in the optical modulator 2A. The driver amplifier 21 includes a driver amplifier 21A for an Xi channel, which is an I component of the X polarization, and a driver amplifier 21B for an Xq channel, which is a Q component of the X polarization. The driver amplifier 21 also includes a driver amplifier 21C for a Yi channel, which is an I component of the Y polarization, and a driver amplifier 21D for a Yq channel, which is a Q component of the Y polarization.

[0018] The optical modulation unit 2A has a first branching unit 22, a second branching unit 23, an X polarization modulation unit 31, a Y polarization modulation unit 32, a PR (Polarization Rotator) 34, and a PBC (Polarization Beam Combiner) 35. The first branching unit 22 branches and outputs an optical signal from the LD 4 to the second branching unit 23. The second branching unit 23 branches and outputs the optical signal from the first branching unit 22 to the X polarization modulation unit 31 and the Y polarization modulation unit 32.

[0019] The X-polarization modulation unit 31 has two RF-side MZMs 31A, two DC-side daughter MZMs 31B, and one DC-side parent MZM 33A. The RF-side MZM 31A is, for example, a phase modulation unit that phase-modulates an optical signal in response to an RF signal from the X-channel driver amplifier 21. The DC-side daughter MZM 31B and DC-side parent MZM 33A are, for example, phase adjustment units that adjust the phase of the X-channel optical signal after phase modulation.

[0020] The Xi channel RF side MZM31A1 in the X polarization modulation unit 31 is, for example, a Xi channel phase modulation unit that phase-modulates the X polarization I component of the optical signal in response to the RF signal from the Xi channel driver amplifier 21A. The Xi channel DC side element MZM31B1 in the X polarization modulation unit 31 is, for example, a Xi channel phase adjustment unit that adjusts the phase of the X polarization I component optical signal after phase modulation in response to the bias signal from the DAC 13. The Xi channel DC side element MZM31B1 outputs the phase-adjusted X polarization I component optical signal to the DC side parent MZM 33A.

[0021] The Xq-channel RF-side MZM31A2 in the X-polarization modulation unit 31 is, for example, an Xq-channel phase modulation unit that phase-modulates the X-polarized Q-component of the optical signal in response to the RF signal from the Xq-channel driver amplifier 21B. Furthermore, the Xq-channel DC-side element MZM31B2 in the X-polarization modulation unit 33 is, for example, an Xq-channel phase adjustment unit that adjusts the phase of the X-polarized Q-component optical signal after phase modulation in response to the bias signal from the DAC 13. The Xq-channel DC-side element MZM31B2 outputs the phase-adjusted X-polarized Q-component optical signal to the DC-side parent MZM 33A.

[0022] The DC-side parent MZM 33A in the X polarization modulation unit 31 is, for example, an Xphi channel phase adjustment unit that quadrature-modulates the phase-adjusted X polarization I-component optical signal and the phase-adjusted X polarization Q-component optical signal in accordance with the bias signal from the DAC 13. The DC-side parent MZM 33A multiplexes the quadrature-modulated X polarization I-component optical signal and the quadrature-modulated X polarization Q-component optical signal, and outputs the multiplexed X polarization optical signal to the PBC 35.

[0023] The Y polarization modulation unit 32 has two RF side MZMs 32A, two DC side daughter MZMs 32B, and one DC side parent MZM 33B. The RF side MZM 32A is, for example, a Y channel phase modulation unit that phase-modulates the optical signal in response to the RF signal from the Y channel driver amplifier 21. The DC side daughter MZM 32B and DC side parent MZM 33B are, for example, Y channel phase adjustment units that adjust the phase of the optical signal after phase modulation.

[0024] The Yi-channel RF-side MZM32A1 in the Y-polarization modulation unit 32 is, for example, a Yi-channel phase modulation unit that phase-modulates the Y-polarized I-component of the optical signal in response to the RF signal from the Yi-channel driver amplifier 21C. The Yi-channel DC-side element MZM32B1 in the Y-polarization modulation unit 32 is, for example, a Yi-channel phase adjustment unit that adjusts the phase of the Y-polarized I-component optical signal after phase modulation in response to the RF signal from the Yi-channel driver amplifier 21C. The Yi-channel DC-side element MZM32B1 outputs the phase-adjusted Y-polarized I-component optical signal to the DC-side parent MZM33B.

[0025] The Yq-channel RF-side MZM32A2 in the Y-polarization modulation unit 32 is, for example, a Yq-channel phase modulation unit that phase-modulates the Q component of the Y polarization of the optical signal in response to the RF signal from the Yq-channel driver amplifier 21D. Furthermore, the Yq-channel DC-side element MZM32B2 in the Y-polarization modulation unit 32 is, for example, a Yq-channel phase adjustment unit that adjusts the phase of the Q component of the Y polarization after phase modulation in response to the bias signal from the DAC 13. The Yq-channel DC-side element MZM32B2 outputs the phase-adjusted Q component optical signal to the DC-side parent MZM33B in the Y-polarization modulation unit 32.

[0026] The DC-side parent MZM 33B in the Y polarization modulation unit 32 is, for example, a phase adjustment unit for the Yphi channel, that quadrature-modulates the phase-adjusted I-component optical signal of the Y polarization and the phase-adjusted Q-component optical signal of the Y polarization in accordance with the bias signal from the DAC 13. The DC-side parent MZM 33B multiplexes the quadrature-modulated I-component optical signal of the Y polarization and the quadrature-modulated Q-component optical signal of the Y polarization, and outputs the multiplexed Y-polarized optical signal to the PR 34.

[0027] The PR 34 rotates the Y-polarized optical signal by 90 degrees, and outputs the optical signal of the Y-polarized component after the polarization rotation to the PBC 35. The PBC 35 polarization-multiplexes the optical signal of the X-polarized component from the X polarization modulation unit 31 and the optical signal of the Y-polarized component after the 90-degree polarization rotation from the PR 34, and outputs the polarization-multiplexed optical signal to an optical fiber as transmission light.

[0028] The detection unit 12 includes a branching coupler 41, a photodetector (PD) 42, a transimpedance amplifier (TIA) 43, a band pass filter (BPF) 44, and an amplifier 45. The branching coupler 41 branches a portion of the polarization-multiplexed optical signal output from the optical modulation unit 2A. The PD 42 converts the optical signal branched by the branching coupler 41 into an electrical signal. The TIA 43 amplifies the electrical signal after electrical conversion and outputs the amplified electrical signal to the BPF 44. The BPF 44 extracts electrical signal components of a specific frequency from the amplified electrical signal. For example, when the filter frequency of the BPF 44 is f1 Hz, this corresponds to the electrical signal component of a dither signal, which is a low-frequency component described below. The amplifier 45 amplifies the dither signal component extracted by the BPF 44 and outputs the amplified dither signal component to the microcomputer 15.

[0029] The microcomputer 15 has an ADC (Analog Digital Converter) 51, a generator 52, a setting unit 54, and a controller 55. The ADC 51 digitally converts the dither signal components amplified by the amplifier 45. The generator 52 generates a predetermined dither signal. The setting unit 54 changes the setting of the wavelength of the optical signal output from the LD 4.

[0030] The control unit 55 controls the entire microcomputer 15. The control unit 55 has an ABC (Auto Bias Control) control unit 55A and a DRV (Driver) control unit 55B. The ABC control unit 55A executes ABC control processing for each channel. The ABC control processing is processing for adjusting the bias signals of the DC side daughter MZMs 31B and 32B and DC side parent MZMs 33A and 33B for each channel to optimal bias points. The ABC control unit 55A has a first adding unit 55A1 that adds a first dither signal to the bias signal. The first dither signal is, for example, an electrical signal of f1 Hz.

[0031] The DRV control unit 55B executes DRV control processing for the driver amplifier 21 for each channel. The DRV control processing is processing for adjusting the optimum gain of the driver amplifier 21 for each channel. The DRV control unit 55B has a second adding unit 55B1 that adds a second dither signal to the RF signal. The second dither signal is an electrical signal with the same frequency as the first dither signal, for example, f1 Hz.

[0032] When the optical output of the optical transmitter 2 is on, the ABC control unit 55A outputs to the DAC 13 a bias value obtained by adding a first dither signal to the bias signal applied to each of the DC side electrodes MZM31B and 32B. The DAC 13 converts the bias value to an analog bias signal by adding the first dither signal, and outputs the analog-converted bias signal to the DC side electrodes MZM31B and 32B. The ABC control unit 55A detects the first dither signal component from the optical output power for the first dither signal via the detection unit 12. The ABC control unit 55A performs ABC control to adjust the bias signals of the DC side electrodes MZM31B and 32B while searching for the optimal bias point for the DC side electrodes MZM31B and 32B so that the detected first dither signal component is minimized. That is, the ABC control unit 55A sequentially adjusts the bias signals of the DC side daughters MZM31B, 32B for each channel, and after adjusting the bias signals of all the DC side daughters MZM31B, 32B, performs ABC control to sequentially adjust the bias signal of each DC side parent MZM33A, 33B. By performing ABC control of the DC side daughters MZM31B, 32B and the DC side parent MZM33A, 33B, respectively, the ABC control unit 55A can adjust the bias signals of the DC side daughters MZM31B, 32B and the DC side parent MZM33A, 33B to their optimum bias points.

[0033] After performing ABC control of the DC-side slave MZMs 31B and 32B and the DC-side master MZMs 33A and 33B, the DRV control unit 55B starts DRV control of the driver amplifier 21 for each channel. The DRV control unit 55B sets the RF signal gain and a second dither signal of the low-frequency component for the driver amplifier 21 for each channel. The gain is the gain adjustment amount of the driver amplifier 21 that optimizes signal quality. The gain of the driver amplifier 21 fluctuates, for example, in response to changes in ambient temperature. Therefore, when a second dither signal is added to the RF signal, the fluctuation level, which is a component of the second dither signal, changes in response to changes in ambient temperature, just like the gain. In other words, when the gain of the driver amplifier 21 decreases, the fluctuation level also decreases, and when the gain of the driver amplifier 21 increases, the fluctuation level also increases.

[0034] FIG. 3 is an explanatory diagram showing an example of the output of the driver amplifier 21 in the first embodiment. The driver amplifier 21 of each channel adjusts the gain of the RF signal from the DSP 5 based on the gain for each channel set by the DRV control unit 55B. The driver amplifier 21 uses the I and Q control axes (based on the extinction point) after ABC control as reference axes, and amplitude-modulates the gain-adjusted RF signal with a second dither signal of a constant amplitude, as shown in FIG. 3. The RF-side MZM 31A phase-modulates the optical signal from the LD 4 in accordance with the RF signal amplitude-modulated with the second dither signal. In other words, the second dither signal of amplitude P is converted into level fluctuations in the optical output power by the RF-side MZM 31A. The DRV control unit 55B stores the fluctuation level P' of the second dither signal of each channel after the optimization adjustment as a predetermined fluctuation level that serves as a reference for each channel. Note that the optimization adjustment is the adjustment of the bias signal for the ABC control described above.

[0035] Then, the DRV control unit 55B detects the fluctuation level of the second dither signal, which is the amplitude of the electrical signal according to the optical output power for the second dither signal, through the detection unit 12. The DRV control unit 55B executes DRV control to adjust the gain of the driver amplifier 21 so that the detected fluctuation level becomes a predetermined fluctuation level.

[0036] 4 is an explanatory diagram showing an example of the optical output level and fluctuation level of the second dither signal due to the second dither signal of the optical modulation unit 2A of the first embodiment. In FIG. 4, the optical output level due to the second dither signal is the optical output of the optical modulation unit 2A, with the vertical axis representing the light amount and the horizontal axis representing the amplitude of the RF signal. The DRV control unit 55B can obtain the fluctuation level of the second dither signal from the optical output level through the detection unit 12. By adjusting the gain of the driver amplifier 21 in an increasing or decreasing direction, the fluctuation level of the second dither signal can be adjusted to a predetermined fluctuation level P'.

[0037] That is, the DRV control unit 55B performs FB control on the gain of the driver amplifier 21 for each channel so that the fluctuation level of the second dither signal for each channel detected by the detection unit 12 becomes a predetermined fluctuation level. As a result, the output amplitude of the driver amplifier 21 can be controlled to be constant.

[0038] Then, the control unit 55 sequentially adjusts the gain of the driver amplifier 21 for each channel, and executes the ABC control process again after adjusting the gain of all the driver amplifiers 21. In other words, the control unit 55 can ensure stable and highly accurate signal quality by repeatedly executing the ABC control process and the DRV control process.

[0039] 5 is a flow chart showing an example of the processing operation of the control unit 55 related to the control processing of the optical modulation unit 2A. The ABC control unit 55A in the control unit 55 executes the ABC control processing, for example, at regular intervals (step S11). The ABC control processing is the processing shown in FIGS. 6A and 6B, which will be described later. After executing the ABC control processing, the control unit 55 determines whether the ABC control processing for all the DC side daughter MZMs 31B and 32B and DC side parent MZMs 33A and 33B has been completed (step S12).

[0040] When the ABC control process for all the DC side slave MZMs 31B, 32B and the DC side master MZMs 33A, 33B is completed (step S12: Yes), the DRV control unit 55B executes the first DRV control process for all the driver amplifiers 21 (step S13). The first DRV control process is the process shown in Figs. 7A and 7B, which will be described later. The control unit 55 determines whether the first DRV control process for all the driver amplifiers 21 is completed (step S14).

[0041] When the first DRV control process for all the driver amplifiers 21 has been completed (step S14: Yes), the control unit 55 proceeds to the process of step S11 to execute the ABC control process.

[0042] Furthermore, if the ABC control processing for all DC side slave MZMs 31B, 32B and DC side master MZMs 33A, 33B has not been completed (step S12: No), the control unit 55 returns to the processing of step S12 to determine whether the ABC control processing has been completed.

[0043] If the first DRV control process has not been completed for all of the driver amplifiers 21 (step S14: No), the control unit 55 returns to the process of step S14 to determine whether the first DRV control process has been completed.

[0044] 5, after executing the ABC control process for all the DC side daughter MZMs 31B, 32B and DC side parent MZMs 33A, 33B, the first DRV control process is executed for all the driver amplifiers 21. As a result, the optical transmitter 2 can ensure stable and highly accurate signal quality.

[0045] 6A and 6B are flow charts showing an example of the processing operation of the ABC control unit 55A related to the ABC control processing of the optical modulation unit 2A. In FIG. 6A, the ABC control unit 55A starts ABC control for the Xi channel DC side electronic MZM31B1 (step S21). The first adding unit 55A1 in the ABC control unit 55A turns on the first dither signal to be added to the bias signal for the Xi channel DC side electronic MZM31B1 (step S22). As a result, the DAC 13 converts the bias value to which the first dither signal has been added into an analog signal and outputs the analog-converted bias signal to which the first dither signal has been added to the Xi channel DC side electronic MZM31B1. The Xi channel DC side electronic MZM31B1 adjusts the phase of the X polarized I component optical signal from the Xi channel RF side MZM31A1 in accordance with the bias signal and outputs the phase-adjusted X polarized I component optical signal to the DC side parent MZM33A. The detector 12 then detects the component of the first dither signal of the Xi channel from the optical output level of the optical signal including the first dither signal of the Xi channel from the output stage of the optical modulator 2A.

[0046] The ABC control unit 55A performs feedback control of the bias signal for the DC element MZM31B1 of the Xi channel so as to reach an optimum bias point at which the component of the first dither signal of the Xi channel is minimized (step S23).The ABC control unit 55A determines whether the feedback control of the bias signal for the DC element MZM31B1 of the Xi channel is completed (step S24).

[0047] If the feedback control of the bias signal for the DC element MZM31B1 of the Xi channel is completed (step S24: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC element MZM31B1 (step S25). Then, the ABC control unit 55A stops the ABC control for the DC element MZM31B1 of the Xi channel (step S26). If the feedback control of the bias signal for the DC element MZM31B1 of the Xi channel is not completed (step S24: No), the ABC control unit 55A returns to the processing of step S23 in which the feedback control of the bias signal for the DC element MZM31B1 of the Xi channel is performed.

[0048] The ABC control unit 55A stops the ABC control for the Xi-channel DC-side electronics MZM31B1, and then starts the ABC control for the Xq-channel DC-side electronics MZM31B2 (step S21A). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Xq-channel DC-side electronics MZM31B2 (step S22A). As a result, the DAC 13 converts the bias signal to which the first dither signal has been added to an analog signal, and outputs the analog-converted bias signal to which the first dither signal has been added to the Xq-channel DC-side electronics MZM31B2. The Xq-channel DC-side electronics MZM31B2 adjusts the phase of the X-polarized Q-component optical signal from the Xq-channel RF-side electronics MZM31A2 in accordance with the bias signal, and outputs the phase-adjusted X-polarized Q-component optical signal to the DC-side parent MZM33A. The detector 12 then detects the component of the first dither signal of the Xq channel from the optical output level of the optical signal including the first dither signal of the Xq channel from the output stage of the optical modulator 2A.

[0049] The ABC control unit 55A performs feedback control of the bias signal for the DC element MZM31B2 of the Xq channel to reach the optimum bias point at which the component of the first dither signal for the Xq channel is minimized (step S23A).The ABC control unit 55A determines whether the feedback control of the bias signal for the DC element MZM31B2 of the Xq channel is completed (step S24A).

[0050] If the feedback control of the bias signal for the DC element MZM31B2 of the Xq channel is completed (step S24A: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC element MZM31B (step S25A). Then, the ABC control unit 55A stops the ABC control for the DC element MZM31B2 of the Xq channel (step S26A). If the feedback control of the bias signal for the DC element MZM31B2 of the Xq channel is not completed (step S24A: No), the ABC control unit 55A returns to the processing of step S23A in which the feedback control of the bias signal for the DC element MZM31B2 of the Xq channel is performed.

[0051] Furthermore, the ABC control unit 55A stops the ABC control for the Yi-channel DC-side element MZM32B1, and then starts the ABC control for the Yi-channel DC-side element MZM32B1 (step S21B). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Yi-channel DC-side element MZM32B1 (step S22B). As a result, the DAC 13 converts the bias signal to which the first dither signal has been added into an analog signal, and outputs the analog-converted bias signal to which the first dither signal has been added to the Yi-channel DC-side element MZM32B1. The Yi-channel DC-side element MZM32B1 adjusts the phase of the Y-polarized I-component optical signal from the Yi-channel RF-side element MZM32A1 in accordance with the bias signal, and outputs the phase-adjusted Y-polarized I-component optical signal to the DC-side parent MZM33B. The detector 12 then detects the component of the first dither signal of the Yi channel from the optical output level of the optical signal including the first dither signal of the Yi channel from the output stage of the optical modulator 2A.

[0052] The ABC control unit 55A performs feedback control of the bias signal for the DC element MZM32B1 of the Yi channel to reach the optimum bias point at which the component of the first dither signal for the Yi channel is minimized (step S23B).The ABC control unit 55A determines whether the feedback control of the bias signal for the DC element MZM32B1 of the Yi channel is completed (step S24B).

[0053] If the feedback control of the bias signal for the Yi-channel DC element MZM32B1 is completed (step S24B: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC element MZM32B1 (step S25B). Then, the ABC control unit 55A stops the ABC control for the Yi-channel DC element MZM32B1 (step S26B) and proceeds to M1 shown in FIG. 6B. If the feedback control of the bias signal for the Yi-channel DC element MZM32B1 is not completed (step S24B: No), the ABC control unit 55A returns to the processing of step S23B, in which the feedback control of the bias signal for the Yi-channel DC element MZM32B1 is performed.

[0054] In M1 shown in FIG. 6B, the ABC control unit 55A stops the ABC control for the Yq-channel DC-side electronic device MZM32B2 and then starts the ABC control for the Yq-channel DC-side electronic device MZM32B2 (step S21C). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Yq-channel DC-side electronic device MZM32B2 (step S22C). As a result, the DAC 13 converts the bias signal to which the first dither signal has been added to an analog signal and outputs the analog-converted bias signal to which the first dither signal has been added to the Yq-channel DC-side electronic device MZM32B2. The Yq-channel DC-side electronic device MZM32B2 adjusts the phase of the Y-polarized Q-component optical signal from the Yq-channel RF-side electronic device MZM32A2 in accordance with the bias signal and outputs the phase-adjusted Y-polarized Q-component optical signal to the DC-side parent MZM33B. The detector 12 then detects the component of the first dither signal of the Yq channel from the optical output level of the optical signal including the first dither signal of the Yq channel from the output stage of the optical modulator 2A.

[0055] The ABC control unit 55A performs feedback control of the bias signal for the DC element MZM32B2 of the Yq channel to reach the optimum bias point at which the component of the first dither signal for the Yq channel is minimized (step S23C).The ABC control unit 55A determines whether the feedback control of the bias signal for the DC element MZM32B2 of the Yq channel is completed (step S24C).

[0056] If the feedback control of the bias signal for the DC element MZM32B2 of the Yq channel is completed (step S24C: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC element MZM32B2 (step S25C). Then, the ABC control unit 55A stops the ABC control for the DC element MZM32B2 of the Yq channel (step S26C). If the feedback control of the bias signal for the DC element MZM32B2 of the Yq channel is not completed (step S24C: No), the ABC control unit 55A returns to the processing of step S23C in which the feedback control of the bias signal for the DC element MZM32B2 of the Yq channel is performed.

[0057] The ABC control unit 55A stops ABC control for the DC-side daughter MZM32B2 of the Yq channel and then starts ABC control for the DC-side parent MZM33A of the Xphi channel (step S21D). The Xphi channel is an X-polarized channel. The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the DC-side parent MZM33A of the Xphi channel (step S22D). As a result, the DAC 13 converts the bias signal to which the first dither signal has been added to an analog signal and outputs the analog-converted bias signal to which the first dither signal has been added to the DC-side parent MZM33A of the Xphi channel. The DC-side parent MZM33A of the Xphi channel multiplexes the optical signal from the DC-side daughter MZM31B1 of the Xi channel and the optical signal from the DC-side daughter MZM31B2 of the Xq channel, and adjusts the phase of the multiplexed signal light according to the bias signal. The DC-side parent MZM 33A of the Xphi channel outputs the phase-adjusted optical signal to the PBC 35. Then, the detection unit 12 detects the component of the first dither signal of the Xphi channel from the optical output level of the optical signal including the first dither signal of the Xphi channel from the output stage of the optical modulation unit 2A.

[0058] The ABC control unit 55A performs feedback control of the bias signal for the DC-side parent MZM 33A of the Xphi channel to reach the optimal bias point at which the component of the first dither signal for the Xphi channel is minimized (step S23D).The ABC control unit 55A determines whether the feedback control of the bias signal for the DC-side parent MZM 33A of the Xphi channel is complete (step S24D).

[0059] If the feedback control of the bias signal for the DC side parent MZM33A of the Xphi channel is completed (step S24D: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC side parent MZM33A of the Xphi channel (step S25D). Then, the ABC control unit 55A stops the ABC control of the DC side parent MZM33A of the Xphi channel (step S26D). On the other hand, if the feedback control of the bias signal for the DC side parent MZM33A is not completed (step S24D: No), the ABC control unit 55A returns to the processing of step S23D, in which the feedback control of the bias signal for the DC side parent MZM33A of the Xphi channel is performed.

[0060] The ABC control unit 55A then stops ABC control for the DC-side parent MZM33A of the Xphi channel and then starts ABC control for the DC-side parent MZM33B of the Yphi channel (step S21E). The Yphi channel is a Y-polarized channel. The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the DC-side parent MZM33B of the Yphi channel (step S22E). As a result, the DAC 13 converts the bias signal to which the first dither signal has been added to an analog signal and outputs the analog-converted bias signal to which the first dither signal has been added to the DC-side parent MZM33B of the Yphi channel. The DC-side parent MZM33B of the Yphi channel multiplexes the optical signal from the DC-side child MZM32B1 of the Yi channel with the optical signal from the DC-side child MZM32B2 of the Yq channel and adjusts the phase of the multiplexed optical signal in accordance with the bias signal. The DC-side parent MZM 33B of the Yphi channel outputs the phase-adjusted optical signal to the PR 34. The detector 12 then detects the component of the Yphi channel first dither signal from the optical output level of the optical signal including the Yphi channel first dither signal from the output stage of the optical modulator 2A.

[0061] The ABC control unit 55A performs feedback control of the bias signal for the DC side parent MZM 33B of the Yphi channel to reach the optimal bias point at which the component of the first dither signal for the Yphi channel is minimized (step S23E).The ABC control unit 55A determines whether the feedback control of the bias signal for the DC side parent MZM 33B of the Yphi channel is complete (step S24E).

[0062] If the feedback control of the bias signal for the DC side parent MZM33B of the Yphi channel is completed (step S24E: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC side parent MZM33B (step S25E). Then, the ABC control unit 55A stops the ABC control of the DC side parent MZM33B of the Yphi channel (step S26E) and ends the processing operation shown in Figure 6B. On the other hand, if the feedback control of the bias signal for the DC side parent MZM33B is not completed (step S24E: No), the ABC control unit 55A returns to the processing of step S23E, in which the feedback control of the bias signal for the DC side parent MZM33B of the Yphi channel is performed.

[0063] In the ABC control process, the ABC control process is executed for all the DC side daughters MZM31B, 32B and DC side parent MZM33A, 33B. As a result, the optical modulation unit 2A can adjust the bias signals of the DC side daughters MZM31B, 32B and DC side parent MZM33A, 33B to the optimum bias points.

[0064] 7A and 7B are flow charts showing an example of the processing operation of the DRV control unit 55B related to the first DRV control processing of the optical modulation unit 2A. In FIG. 7A, the DRV control unit 55B in the microcomputer 15 starts DRV control for the driver amplifier 21A of the Xi channel (step S31). The second adding unit 55B1 in the DRV control unit 55B turns on the second dither signal to be added to the RF signal for the driver amplifier 21A of the Xi channel (step S32). As a result, the driver amplifier 21A amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the RF-side MZM 31A1 of the Xi channel. Then, the RF-side MZM 31A1 of the Xi channel phase-modulates the optical signal in accordance with the RF signal of the Xi channel. The detector 12 detects the fluctuation level, which is a component of the second dither signal of the Xi channel, from the optical output level, which is an optical signal phase-modulated with an RF signal including the second dither signal of the Xi channel from the output stage of the optical modulator 2A.

[0065] The DRV control unit 55B executes feedback control to adjust the gain of the driver amplifier 21A of the Xi channel so that the fluctuation level of the component of the second dither signal of the Xi channel becomes the reference fluctuation level (step S33). The DRV control unit 55B determines whether the feedback control of the gain adjustment for the driver amplifier 21A of the Xi channel is completed (step S34).

[0066] If the feedback control of the gain adjustment for the Xi channel driver amplifier 21A is completed (step S34: Yes), the second adding unit 55B1 turns off the second dither signal to be added to the RF signal for the Xi channel driver amplifier 21A (step S35). Then, the DRV control unit 55B stops the DRV control for the Xi channel driver amplifier 21A (step S36). On the other hand, if the feedback control of the gain adjustment for the Xi channel driver amplifier 21A is not completed (step S34: No), the DRV control unit 55B returns to the processing of step S33, in which the feedback control for adjusting the gain of the Xi channel driver amplifier 21A is executed.

[0067] The DRV control unit 55B stops DRV control for the Xi channel driver amplifier 21A and then starts DRV control for the Xq channel driver amplifier 21B (step S31A). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Xq channel driver amplifier 21B (step S32A). As a result, the driver amplifier 21B amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the Xq channel RF side MZM 31A2. The Xq channel RF side MZM 31A2 then phase-modulates the optical signal in accordance with the Xq channel RF signal. The detection unit 12 detects the fluctuation level, which is a component of the Xq channel second dither signal, from the optical output level of the optical signal phase-modulated with the RF signal including the Xq channel second dither signal from the output stage of the optical modulation unit 2A.

[0068] The DRV control unit 55B executes feedback control to adjust the gain of the driver amplifier 21B of the Xq channel so that the fluctuation level of the component of the second dither signal of the Xq channel becomes the reference fluctuation level (step S33A). The DRV control unit 55B determines whether the feedback control of the gain adjustment for the driver amplifier 21B of the Xq channel is completed (step S34A).

[0069] If the feedback control of the gain adjustment for the driver amplifier 21B of the channel Xq is completed (step S34A: Yes), the second adding unit 55B1 turns off the second dither signal to be added to the RF signal for the driver amplifier 21B (step S35A). Then, the DRV control unit 55B stops the DRV control for the driver amplifier 21B of the channel Xq (step S36A) and proceeds to the processing of M2 shown in Fig. 7B. On the other hand, if the feedback control of the gain adjustment for the driver amplifier 21B of the channel Xq is not completed (step S34A: No), the DRV control unit 55B returns to the processing of step S33A, in which the feedback control for adjusting the gain of the driver amplifier 21B of the channel Xq is executed.

[0070] In M2 shown in FIG. 7B, the DRV control unit 55B stops DRV control for the Xq-channel driver amplifier 21B and then starts DRV control for the Yi-channel driver amplifier 21C (step S31B). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Yi-channel driver amplifier 21C (step S32B). As a result, the driver amplifier 21C amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the Yi-channel RF-side MZM 32A1. The Yi-channel RF-side MZM 32A1 then phase-modulates the optical signal in accordance with the Yi-channel RF signal. The detecting unit 12 detects the fluctuation level, which is a component of the Yi-channel second dither signal, from the optical output level of the optical signal phase-modulated with the RF signal including the Yi-channel second dither signal from the output stage of the optical modulating unit 2A.

[0071] The DRV control unit 55B executes feedback control to adjust the gain of the driver amplifier 21C for the Yi channel so that the fluctuation level of the component of the second dither signal for the Yi channel becomes the reference fluctuation level (step S33B). The DRV control unit 55B determines whether the feedback control of the gain adjustment for the driver amplifier 21C for the Yi channel is completed (step S34B).

[0072] If the feedback control of the gain adjustment for the Yi channel driver amplifier 21C is completed (step S34B: Yes), the DRV control unit 55B turns off the second dither signal for the Yi channel driver amplifier 21C (step S35B). Then, the DRV control unit 55B stops the DRV control for the Yi channel driver amplifier 21C (step S36B). If the feedback control of the gain adjustment for the Yi channel driver amplifier 21C is not completed (step S34B: No), the DRV control unit 55B returns to the processing of step S33B, where the feedback control for adjusting the gain of the Yi channel driver amplifier 21C is executed.

[0073] The DRV control unit 55B stops DRV control for the Yi-channel driver amplifier 21C and then starts DRV control for the Yq-channel driver amplifier 21D (step S31C). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Yq-channel driver amplifier 21D (step S32C). As a result, the driver amplifier 21D amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the Yq-channel RF-side MZM 32A2. The Yq-channel RF-side MZM 32A2 then phase-modulates the optical signal in accordance with the Yq-channel RF signal. The detection unit 12 detects the fluctuation level, which is a component of the second dither signal for the Yq channel, from the optical output level of the optical signal phase-modulated with the RF signal including the second dither signal for the Yq channel from the output stage of the optical modulation unit 2A.

[0074] The DRV control unit 55B executes feedback control to adjust the gain of the driver amplifier 21D of the Yq channel so that the fluctuation level of the component of the second dither signal of the Yq channel becomes the reference fluctuation level (step S33C). The DRV control unit 55B determines whether the feedback control of the gain adjustment for the driver amplifier 21D of the Yq channel is completed (step S34C).

[0075] If the feedback control of the gain adjustment for the Yq channel driver amplifier 21D is completed (step S34C: Yes), the second adding unit 55B1 turns off the second dither signal to be added to the RF signal for the driver amplifier 21D (step S35C). Then, the DRV control unit 55B stops the DRV control for the Yq channel driver amplifier 21D (step S36C) and ends the processing operation shown in Fig. 7B. If the feedback control of the gain adjustment for the Yq channel driver amplifier 21D is not completed (step S34C: No), the DRV control unit 55B returns to the processing of step S33C, where the feedback control for adjusting the gain of the Yq channel driver amplifier 21D is executed.

[0076] In the first DRV control process, after the ABC control process is executed, the gain of the driver amplifier 21 of each channel is adjusted so that the fluctuation level of the second dither signal for each channel becomes the reference fluctuation level. As a result, the output amplitude of the driver amplifier 21 can be controlled to be constant, and the optical output of the optical modulation unit 2A can be stabilized.

[0077] In the optical transmitter 2 of the first embodiment, the gain of the driver amplifier 21 that amplifies the RF signal is controlled based on the fluctuation level of the second dither signal detected at the output stage of the optical modulation unit 2 A. As a result, the output amplitude of the driver amplifier 21 becomes constant, and the optical output of the optical modulation unit 2 A can be stabilized.

[0078] In the optical transmitter 2, the gain of the driver amplifier 21 is controlled so that the fluctuation level of the second dither signal detected at the output stage of the optical modulation unit 2A matches a predetermined fluctuation level that has been set in advance. As a result, the output amplitude of the driver amplifier 21 becomes constant, and the optical output of the optical modulation unit 2A can be stabilized.

[0079] The optical transmitter 2 performs ABC control on the DC-side daughter MZMs 31B and 32B and the DC-side parent MZMs 33A and 33B based on the fluctuation level of the first dither signal. Furthermore, after performing ABC control, the optical transmitter 2 controls the gain of the driver amplifier 21 based on the fluctuation level of the second dither signal. As a result, the optical output of the optical modulation unit 2A can be stabilized while adjusting the bias signal of each channel to the optimal bias point. [Example]

[0080] Fig. 8 is an explanatory diagram showing an example of an optical transmitter 2 according to a second embodiment. Note that the same components as those of the optical transmitter 2 according to the first embodiment are denoted by the same reference numerals, and explanations of the overlapping components and operations will be omitted. The detecting unit 12 shown in Fig. 8 has a BPF 44A with a filter frequency of f1 x 2 instead of the BPF 44 with a filter frequency of f1. The BPF 44A detects the components of the first dither signal and also detects the components of the second dither signal that are in an anti-phase relationship with the first dither signal.

[0081] FIG. 9 is an explanatory diagram showing an example of the optical output level and first fluctuation level due to the first dither signal of the optical modulation unit 2A of the second embodiment, and the optical output level and second fluctuation level due to the second dither signal. In FIG. 9, the optical output level due to the first dither signal is the optical output of the optical modulation unit 2A, with the vertical axis representing the light intensity and the horizontal axis representing the amplitude of the bias signal. The DRV control unit 55B can obtain the first fluctuation level, which is the fluctuation level of the first dither signal, from the optical output level through the detection unit 12. The optical output level due to the second dither signal is the optical output of the optical modulation unit 2A, with the vertical axis representing the light intensity and the horizontal axis representing the amplitude of the RF signal. The DRV control unit 55B can obtain the second fluctuation level, which is the fluctuation level of the second dither signal, from the optical output level through the detection unit 12.

[0082] The second dither signal and the first dither signal in the same channel are in opposite phase, so when the amplitudes of the first and second fluctuation levels are the same, the second fluctuation level disappears.

[0083] However, if the first fluctuation level has a larger amplitude than the second fluctuation level, they are not completely canceled out, and the amplitude component of the first fluctuation level remains.Also, if the second fluctuation level has a larger amplitude than the first fluctuation level, they are not completely canceled out, and the amplitude component of the second fluctuation level remains.

[0084] Next, the operation of the optical transmitter 2 of the second embodiment will be described. First, the ABC control unit 55A adjusts the bias values ​​of the DC side daughter MZMs 31B, 32B and DC side parent MZMs 33A, 33B to optimal bias values ​​by performing ABC control on the DC side daughter MZMs 31B, 32B and DC side parent MZMs 33A, 33B, respectively. In other words, after the ABC control is performed, the ABC control is stopped at the convergence point, and therefore the fluctuation level (f) of the first dither signal at the convergence point of the ABC control becomes a fixed value.

[0085] The DRV control unit 55B starts DRV control for each channel after the ABC control unit 55A executes ABC control of the DC-side daughter MZMs 31B, 32B and the DC-side parent MZMs 33A, 33B. The control unit 55 adjusts the first adding unit 55A1 and the second adding unit 55B1 so that the timing of adding the second dither signal to the RF signal and the timing of adding the first dither signal to the bias signal coincide with each other for each channel. In other words, the control unit 55 adjusts the timing of adding the dither signals of the first adding unit 55A1 and the second adding unit 55B1 so that the phase timing is such that the second fluctuation level and the first fluctuation level cancel each other out at the output stage of the optical modulation unit 2A.

[0086] The DRV control unit 55B adjusts the gain of the driver amplifier 21 for each channel during an adjustment process that optimizes the communication quality of the output of the optical modulation unit 2A. The DRV control unit 55B then adjusts the gain of the driver amplifier 21 so that the second fluctuation level is completely canceled out by the first fluctuation level, and sets the completely canceled gain as the adjustment amount. The set adjustment amount is a fixed value. In other words, if the gain does not fluctuate, the second fluctuation level does not change either, and the first fluctuation level and the second fluctuation level are completely canceled out.

[0087] Furthermore, for example, if the gain of the driver amplifier 21 decreases in response to a change in the ambient temperature, the second fluctuation level also decreases in response to the decrease in gain. In this case, the first fluctuation level has a larger amplitude than the second fluctuation level, so they are not completely canceled out, and the amplitude component of the first dither signal remains.

[0088] Furthermore, for example, when the gain of the driver amplifier 21 increases in response to a change in the ambient temperature, the second fluctuation level also increases in response to the increase in the gain. In this case, the second fluctuation level has a larger amplitude than the first fluctuation level, and therefore the two are not completely canceled out, and the amplitude component of the second dither signal remains.

[0089] Therefore, even if the gain of the driver amplifier 21 fluctuates, the DRV control unit 55B adjusts the gain of the driver amplifier 21 so that the first fluctuation level is completely offset by the second fluctuation level. As a result, the output amplitude of the driver amplifier 21 can be controlled to be constant.

[0090] 10A and 10B are flow charts showing an example of the processing operation of the DRV control unit 55B related to the second DRV control processing of the optical modulation unit 2A. In FIG. 10A, the DRV control unit 55B starts DRV control for the Xi channel driver amplifier 21A (step S41). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Xi channel DC side element MZM31B1 (step S42). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Xi channel driver amplifier 21A (step S43).

[0091] The control unit 55 adjusts the phase so that the first dither signal added to the bias signal for the DC-side element MZM31B1 of the Xi channel and the second dither signal added to the RF signal for the driver amplifier 21A of the Xi channel are in opposite phase (step S44). As a result, the driver amplifier 21A amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the RF-side MZM31A1 of the Xi channel. The RF-side MZM31A1 of the Xi channel then phase-modulates the optical signal in accordance with the RF signal of the Xi channel. The detection unit 12 detects the first fluctuation level and the second fluctuation level of the Xi channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0092] The DRV control unit 55B executes feedback control of the gain of the driver amplifier 21A of the Xi channel so that the first fluctuation level of the Xi channel and the second fluctuation level of the Xi channel cancel each other out (step S45). The DRV control unit 55B determines whether the feedback control of the gain of the driver amplifier 21A of the Xi channel is completed (step S46).

[0093] If the feedback control of the gain of the Xi channel driver amplifier 21A is completed (step S46: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the Xi channel DC side amplifier MZM31B1 (step S47). Furthermore, the second adding unit 55B1 turns off the second dither signal added to the RF signal for the Xi channel driver amplifier 21A (step S48). Then, the DRV control unit 55B stops the DRV control for the Xi channel driver amplifier 21A (step S49). If the feedback control of the gain adjustment for the Xi channel driver amplifier 21A is not completed (step S46: No), the DRV control unit 55B returns to the processing of step S45, where the feedback control for adjusting the gain of the Xi channel driver amplifier 21A is executed.

[0094] The DRV control unit 55B stops the DRV control for the Xi channel driver amplifier 21A, and then starts the DRV control for the Xq channel driver amplifier 21B (step S41A). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Xq channel DC side element MZM31B2 (step S42A). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Xq channel driver amplifier 21B (step S43A).

[0095] The control unit 55 adjusts the phase so that the first dither signal added to the bias signal for the DC side MZM 31B2 of the Xq channel and the second dither signal added to the RF signal for the driver amplifier 21B of the Xq channel are in opposite phase (step S44A). As a result, the driver amplifier 21B amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the RF side MZM 31A2 of the Xq channel. The RF side MZM 31A2 of the Xq channel then phase-modulates the optical signal in accordance with the RF signal of the Xq channel. The detection unit 12 detects the first fluctuation level and the second fluctuation level of the Xq channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0096] The DRV control unit 55B executes feedback control of the gain of the driver amplifier 21B of the Xq channel so that the first fluctuation level of the Xq channel and the second fluctuation level of the Xq channel cancel each other out (step S45A). The DRV control unit 55B determines whether the feedback control of the gain of the driver amplifier 21B of the Xq channel is completed (step S46A).

[0097] If the feedback control of the gain of the Xq-channel driver amplifier 21B is completed (step S46A: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the Xq-channel DC side terminal MZM31B2 (step S47A). Furthermore, the second adding unit 55B1 turns off the second dither signal added to the RF signal for the Xq-channel driver amplifier 21B (step S48A). Then, the DRV control unit 55B stops the DRV control for the Xq-channel driver amplifier 21B (step S49A) and proceeds to M3 shown in FIG. 10B. If the feedback control of the gain adjustment for the Xq-channel driver amplifier 21B is not completed (step S46A: No), the DRV control unit 55B returns to the processing of step S45A, where the feedback control for adjusting the gain of the Xq-channel driver amplifier 21B is executed.

[0098] 10B, the DRV control unit 55B stops the DRV control for the Xq channel driver amplifier 21B and then starts the DRV control for the Yi channel driver amplifier 21C (step S41B). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Yi channel DC side element MZM32B1 (step S42B). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Yi channel driver amplifier 21C (step S43B).

[0099] The control unit 55 adjusts the phase so that the first dither signal added to the bias signal for the Yi-channel DC-side element MZM31B and the second dither signal added to the RF signal for the Yi-channel driver amplifier 21C are in opposite phase (step S44B). As a result, the driver amplifier 21C amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the Yi-channel RF-side MZM32A1. The Yi-channel RF-side MZM32A1 then phase-modulates the optical signal in accordance with the Yi-channel RF signal. The detection unit 12 detects the first and second fluctuation levels of the Yi channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0100] The DRV control unit 55B executes feedback control of the gain of the Yi channel driver amplifier 21C so that the first fluctuation level of the Yi channel and the second fluctuation level of the Yi channel cancel each other out (step S45B). The DRV control unit 55B determines whether the feedback control of the gain of the Yi channel driver amplifier 21C is completed (step S46B).

[0101] If the feedback control of the gain of the Yi-channel driver amplifier 21C is completed (step S46B: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the Yi-channel DC side terminal MZM32B1 (step S47B). Furthermore, the second adding unit 55B1 turns off the second dither signal added to the RF signal for the Yi-channel driver amplifier 21C (step S48B). Then, the DRV control unit 55B stops the DRV control for the Yi-channel driver amplifier 21C (step S49B). If the feedback control of the gain adjustment for the Yi-channel driver amplifier 21C is not completed (step S46B: No), the DRV control unit 55B returns to the processing of step S45B, where the feedback control for adjusting the gain of the Yi-channel driver amplifier 21C is executed.

[0102] The DRV control unit 55B stops the DRV control for the Yi channel driver amplifier 21C, and then starts the DRV control for the Yq channel driver amplifier 21D (step S41C). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the DC side element MZM32B2 of the Yq channel (step S42C). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Yq channel driver amplifier 21D (step S43C).

[0103] The control unit 55 adjusts the phase so that the first dither signal added to the bias signal for the DC side MZM 32B2 of the Yq channel and the second dither signal added to the RF signal for the driver amplifier 21D of the Yq channel are in opposite phase (step S44C). As a result, the driver amplifier 21D amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the RF side MZM 32A2 of the Yq channel. The RF side MZM 32A2 of the Yq channel then phase-modulates the optical signal in accordance with the RF signal of the Yq channel. The detection unit 12 detects the first fluctuation level and the second fluctuation level of the Yq channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0104] The DRV control unit 55B executes feedback control of the gain of the driver amplifier 21D of the Yq channel so that the first fluctuation level of the Yq channel and the second fluctuation level of the Yq channel cancel each other out (step S45C). The DRV control unit 55B determines whether the feedback control of the gain of the driver amplifier 21D of the Yq channel is completed (step S46C).

[0105] If the feedback control of the gain of the Yq-channel driver amplifier 21D is completed (step S46C: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC side terminal MZM32B2 for the Yq-channel (step S47C). Furthermore, the second adding unit 55B1 turns off the second dither signal for the Yq-channel driver amplifier 21D (step S48C). Then, the DRV control unit 55B stops the DRV control for the Yq-channel driver amplifier 21D (step S49C) and ends the processing operation shown in FIG. 10B. If the feedback control of the gain adjustment for the Yq-channel driver amplifier 21D is not completed (step S46C: No), the DRV control unit 55B returns to the processing of step S45C, where the feedback control for adjusting the gain of the Yq-channel driver amplifier 21D is executed.

[0106] In the second DRV control process, after the ABC control process is executed, the gain of the driver amplifier 21 of each channel is adjusted so that the second fluctuation level and the first fluctuation level cancel each other out for each channel. As a result, the output amplitude of the driver amplifier 21 can be controlled to be constant, and the optical output of the optical modulation unit 2A can be stabilized.

[0107] In the optical transmitter 2 of the second embodiment, the first dither signal and the second dither signal are in an anti-phase relationship, and the gain of the driver amplifier 21 of each channel is adjusted so that the second fluctuation level and the first fluctuation level cancel each other out for each channel. As a result, the output amplitude of the driver amplifier 21 becomes constant, and the optical output of the optical modulation unit 2A can be stabilized.

[0108] In the optical transmitter 2 of Example 2, the first fluctuation level of f at the ABC convergence point is used as an example of the reference for the feedback information, but this is not limited to this, and an embodiment thereof will be described below as Example 3. [Example]

[0109] 11 is an explanatory diagram showing an example of an optical transmitter 2 according to a third embodiment. The same components as those in the optical transmitter 2 according to the second embodiment are denoted by the same reference numerals, and explanations of the overlapping components and operations will be omitted. The optical transmitter 2 according to the second embodiment differs from the optical transmitter 2 according to the third embodiment in that the first fluctuation level of 2f at the ABC convergence point is used as the reference for feedback information. The first fluctuation level of 2f is twice the fluctuation level of the first fluctuation level of f in the second embodiment.

[0110] FIG. 12 is an explanatory diagram showing an example of the optical output level and first fluctuation level due to the first dither signal of the optical modulation unit 2A of the third embodiment. In FIG. 12, the optical output level due to the first dither signal is the optical output of the optical modulation unit 2A, with the vertical axis representing the light intensity and the horizontal axis representing the amplitude of the bias signal. The DRV control unit 55C can obtain the first fluctuation level of 2f, which is the fluctuation level of the first dither signal, from the optical output level via the detection unit 12. It cannot be denied that the first fluctuation level of 2f may change due to, for example, aging or temperature fluctuation. Therefore, the initial first fluctuation level of 2f is stored in advance in the first memory 53A. Note that the amplitude value of the first fluctuation level of 2f is set to the initial amplitude value A.

[0111] The control unit 55 has a DRV control unit 55C including a second adding unit 55B1 instead of the DRV control unit 55B. When starting the second DRV control process, the DRV control unit 55C measures the current first fluctuation level of 2f and stores the current first fluctuation level of 2f in the second memory 53B. At this time, the amplitude value of the current first fluctuation level of 2f is set to the current amplitude value B.

[0112] The DRV control unit 55C periodically measures and stores the current amplitude value B of the first fluctuation level at the current time, and calculates the fluctuation ratio C of the first fluctuation level from the initial value based on (current amplitude value B ÷ initial amplitude value A).

[0113] FIG. 13 is an explanatory diagram showing an example of the optical output level and second fluctuation level due to the second dither signal of the optical modulation unit 2A of the third embodiment. In FIG. 13, the optical output level due to the second dither signal is the optical output of the optical modulation unit 2A, with the vertical axis representing the light intensity and the horizontal axis representing the amplitude of the RF signal. The DRV control unit 55B can obtain the second fluctuation level, which is the fluctuation level of the second dither signal, from the optical output level via the detection unit 12. The DRV control unit 55C detects the amplitude P of the second fluctuation level of the second dither signal for each channel and corrects the second fluctuation level of 2f based on (amplitude P of the second fluctuation level × fluctuation ratio C). As a result, the secular fluctuation of the first fluctuation level of 2f, which serves as the feedback reference, can be reflected in the second fluctuation level, thereby absorbing errors due to secular fluctuation.

[0114] 14A and 14B are flow charts showing an example of the processing operation of the DRV control unit 55C related to the third DRV control processing of the optical modulation unit 2A. In FIG. 14A, the DRV control unit 55C starts DRV control for the driver amplifier 21A of the Xi channel (step S51). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the DC side element MZM31B1 of the Xi channel (step S52). The second adding unit 55B1 turns on the second dither signal for the driver amplifier 21A of the Xi channel (step S53).

[0115] The control unit 55 adjusts the phase so that the first dither signal added to the bias signal for the DC-side element MZM31B1 of the Xi channel and the second dither signal added to the RF signal for the driver amplifier 21A of the Xi channel are in opposite phase (step S54). As a result, the driver amplifier 21A amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the RF-side MZM31A1 of the Xi channel. The RF-side MZM31A1 of the Xi channel then phase-modulates the optical signal in accordance with the RF signal of the Xi channel. The detection unit 12 detects the first fluctuation level and the second fluctuation level of 2f of the Xi channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0116] The DRV control unit 55C calculates the fluctuation ratio of the first fluctuation level of the Xi channel based on (A1 / A2) using the first fluctuation level A1 of 2f of the Xi channel and the first fluctuation level A2 of 2f of the Xi channel, which is the initial value stored in advance (step S55).

[0117] The DRV control unit 55C uses the second fluctuation level B1 of the Xi channel and the fluctuation ratio of the first fluctuation level of the Xi channel to calculate the second fluctuation level of the Xi channel that reflects the fluctuation ratio based on (B1 x fluctuation ratio) (step S56).

[0118] The DRV control unit 55C executes feedback control of the gain of the driver amplifier 21A of the Xi channel so that the first fluctuation level of the current 2f of the Xi channel and the second fluctuation level reflecting the fluctuation ratio of the Xi channel cancel each other out (step S57). The DRV control unit 55C determines whether the feedback control of the gain of the driver amplifier 21A of the Xi channel is completed (step S58).

[0119] If the feedback control of the gain of the Xi channel driver amplifier 21A is completed (step S58: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the Xi channel DC side amplifier MZM31B1 (step S59). Furthermore, the second adding unit 55B1 turns off the second dither signal added to the RF signal for the Xi channel driver amplifier 21A (step S60). Then, the DRV control unit 55C stops the DRV control for the Xi channel driver amplifier 21A (step S61). If the feedback control of the gain adjustment for the Xi channel driver amplifier 21A is not completed (step S58: No), the DRV control unit 55C returns to the processing of step S57, where the feedback control for adjusting the gain of the Xi channel driver amplifier 21A is executed.

[0120] The DRV control unit 55C stops DRV control for the Xi channel driver amplifier 21A and then starts DRV control for the Xq channel driver amplifier 21B (step S51A). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Xq channel DC side element MZM31B2 (step S52A). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Xq channel driver amplifier 21B (step S53A).

[0121] The control unit 55 adjusts the phase of the first dither signal for the DC-side MZM 31B2 of the Xq channel and the second dither signal for the driver amplifier 21B of the Xq channel so that they are in opposite phases (step S54A). As a result, the driver amplifier 21B amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the RF-side MZM 31A2 of the Xq channel. The RF-side MZM 31A2 of the Xq channel then phase-modulates the optical signal in accordance with the RF signal of the Xq channel. The detection unit 12 detects the first and second fluctuation levels of 2f of the Xq channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0122] The DRV control unit 55C calculates the fluctuation ratio of the first fluctuation level of 2f of the Xq channel based on (A1 / A2) using the first fluctuation level A1 of 2f of the Xq channel and the first fluctuation level A2 of 2f of the Xq channel, which is the initial value stored in advance (step S55A).

[0123] The DRV control unit 55C uses the second fluctuation level B1 of the Xq channel and the fluctuation ratio of the first fluctuation level of the Xq channel to calculate the second fluctuation level of the Xq channel that reflects the fluctuation ratio based on (B1 x fluctuation ratio) (step S56A).

[0124] The DRV control unit 55C executes feedback control of the gain of the driver amplifier 21B of the Xq channel so that the current first fluctuation level of the Xq channel and the second fluctuation level reflecting the fluctuation ratio of the Xq channel cancel each other out (step S57A). The DRV control unit 55C determines whether the feedback control of the gain of the driver amplifier 21B of the Xq channel is completed (step S58A).

[0125] If the feedback control of the gain of the Xq-channel driver amplifier 21B is completed (step S58A: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC side terminal MZM31B2 of the Xq-channel (step S59A). Furthermore, the second adding unit 55B1 turns off the second dither signal added to the RF signal for the Xq-channel driver amplifier 21B (step S60A). Then, the DRV control unit 55C stops the DRV control for the Xq-channel driver amplifier 21B (step S61A) and proceeds to the processing of M4 shown in FIG. 14B. If the feedback control of the gain adjustment for the Yi-channel driver amplifier 21B is not completed (step S58A: No), the DRV control unit 55C returns to the processing of step S57A, where the feedback control for adjusting the gain of the Yi-channel driver amplifier 21B is executed.

[0126] 14B, the DRV control unit 55C stops DRV control for the Xq-channel driver amplifier 21B and then starts DRV control for the Yi-channel driver amplifier 21C (step S51B). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the Yi-channel DC side terminal MZM32B1 (step S52B). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Yi-channel driver amplifier 21C (step S53B).

[0127] The control unit 55 adjusts the phase so that the first dither signal added to the bias signal for the Yi-channel DC-side element MZM32B1 and the second dither signal added to the RF signal for the Yi-channel driver amplifier 21C are in opposite phase (step S54B). As a result, the driver amplifier 21C amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the Yi-channel RF-side MZM32A1. The Yi-channel RF-side MZM32A1 then phase-modulates the optical signal in accordance with the Yi-channel RF signal. The detection unit 12 detects the first and second fluctuation levels of 2f for the Yi channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0128] The DRV control unit 55C calculates the fluctuation ratio of the first fluctuation level of 2f of the Yi channel based on (A1 / A2) using the first fluctuation level A1 of 2f of the Yi channel and the first fluctuation level A2 of 2f of the Yi channel, which is the initial value stored in advance (step S55B).

[0129] The DRV control unit 55C uses the second fluctuation level B1 of the Yi channel and the fluctuation ratio of the first fluctuation level of the Yi channel to calculate the second fluctuation level of the Yi channel reflecting the fluctuation ratio based on (B1 x fluctuation ratio) (step S56B).

[0130] The DRV control unit 55C executes feedback control of the gain of the driver amplifier 21C of the Yi channel so that the first fluctuation level of the current 2f of the Yi channel and the second fluctuation level reflecting the fluctuation ratio of the Yi channel cancel each other out (step S57B). The DRV control unit 55C determines whether the feedback control of the gain of the driver amplifier 21C of the Yi channel is completed (step S58B).

[0131] If the feedback control of the gain of the Yi-channel driver amplifier 21C is completed (step S58B: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the Yi-channel DC side amplifier MZM32B1 (step S59B). Furthermore, the second adding unit 55B1 turns off the second dither signal for the Yi-channel driver amplifier 21C (step S60B). Then, the DRV control unit 55C stops the DRV control for the Yi-channel driver amplifier 21C (step S61B). If the feedback control of the gain adjustment for the Yi-channel driver amplifier 21C is not completed (step S58B: No), the DRV control unit 55C returns to the processing of step S57B, where it performs the feedback control to adjust the gain of the Yi-channel driver amplifier 21C.

[0132] The DRV control unit 55C stops the DRV control for the Yi channel driver amplifier 21C, and then starts the DRV control for the Yq channel driver amplifier 21D (step S51C). The first adding unit 55A1 turns on the first dither signal to be added to the bias signal for the DC side element MZM32B2 of the Yq channel (step S52C). The second adding unit 55B1 turns on the second dither signal to be added to the RF signal for the Yq channel driver amplifier 21D (step S53C).

[0133] The control unit 55 adjusts the phase so that the first dither signal added to the bias signal for the DC side MZM32B2 of the Yq channel and the second dither signal added to the RF signal for the driver amplifier 21D of the Yq channel are in opposite phase (step S54C). As a result, the driver amplifier 21D amplitude-modulates the gain-adjusted RF signal with the second dither signal and outputs the amplitude-modulated RF signal to the RF side MZM32A2 of the Yq channel. The RF side MZM32A2 of the Yq channel then phase-modulates the optical signal in accordance with the RF signal of the Yq channel. The detection unit 12 detects the first fluctuation level and the second fluctuation level of 2f of the Yq channel from the optical output level, which is the optical signal from the output stage of the optical modulation unit 2A.

[0134] The DRV control unit 55C calculates the fluctuation ratio of the first fluctuation level of the Yq channel based on (A1 / A2) using the first fluctuation level A1 of 2f of the Yq channel and the first fluctuation level A2 of 2f of the Yq channel, which is the initial value stored in advance (step S55C).

[0135] The DRV control unit 55C uses the second fluctuation level B1 of the Yq channel and the fluctuation ratio of the first fluctuation level of 2f of the Yq channel to calculate the second fluctuation level of the Yq channel reflecting the fluctuation ratio based on (B1 x fluctuation ratio) (step S56C).

[0136] The DRV control unit 55C executes feedback control of the gain of the driver amplifier 21D of the Yq channel so that the first fluctuation level of the current 2f of the Yq channel and the second fluctuation level reflecting the fluctuation ratio of the Yq channel cancel each other out (step S57C). The DRV control unit 55C determines whether the feedback control of the gain of the driver amplifier 21D of the Yq channel is completed (step S58C).

[0137] If the feedback control of the gain of the Yq-channel driver amplifier 21D is completed (step S58C: Yes), the first adding unit 55A1 turns off the first dither signal added to the bias signal for the DC side terminal MZM32B2 for the Yq-channel (step S59C). Furthermore, the second adding unit 55B1 turns off the second dither signal added to the RF signal for the Yq-channel driver amplifier 21D (step S60C). Then, the DRV control unit 55C stops the DRV control for the Yq-channel driver amplifier 21D (step S61C) and ends the processing operation shown in FIG. 14B. If the feedback control of the gain adjustment for the Yq-channel driver amplifier 21D is not completed (step S58C: No), the DRV control unit 55C returns to the processing of step S57C, where the feedback control for adjusting the gain of the Yq-channel driver amplifier 21D is executed.

[0138] In the third DRV control process, after executing the ABC control process, the gain of the driver amplifier 21 of each channel is adjusted so that the second fluctuation level reflecting the fluctuation ratio of the first fluctuation level and the current 2f first fluctuation level cancel each other out for each channel. As a result, the output amplitude of the driver amplifier 21 can be controlled to be constant, thereby stabilizing the optical output of the optical modulation unit 2A. The aging fluctuation of the 2f first fluctuation level, which serves as the feedback reference, can be reflected in the second fluctuation level to absorb errors due to the aging fluctuation.

[0139] In the optical transmitter 2 of the third embodiment, a fluctuation ratio between the detected first fluctuation level and a preset initial first fluctuation level is calculated. Then, the optical transmitter 2 adjusts the detected second fluctuation level according to the calculated fluctuation ratio, and controls the gain of the driver amplifier 21 so that the adjusted second fluctuation level and the detected first fluctuation level cancel each other out. As a result, the output amplitude of the driver amplifier 21 can be controlled to be constant, and the optical output of the optical modulation unit 2A can be stabilized. The secular fluctuation of the first fluctuation level of 2f, which serves as a feedback reference, can be reflected in the second fluctuation level, thereby absorbing errors due to the secular fluctuation.

[0140] For ease of explanation, the optical transceiver 1 has been illustrated as incorporating the optical transmitter 2 and the optical receiver 3, but the optical transceiver 1 may incorporate either the optical transmitter 2 or the optical receiver 3. For example, the optical transceiver 1 may incorporate the optical transmitter 2, and modifications can be made as appropriate.

[0141] Furthermore, the components of each unit shown in the figure do not necessarily have to be physically configured as shown in the figure. In other words, the specific form of distribution and integration of each unit is not limited to that shown in the figure, and all or part of them can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc.

[0142] Furthermore, the various processing functions performed by each device may be executed in whole or in part on a CPU (Central Processing Unit) (or a microcomputer such as an MPU (Micro Processing Unit) or MCU (Micro Controller Unit)). Needless to say, the various processing functions may be executed in whole or in part on a program analyzed and executed by a CPU (or a microcomputer such as an MPU or MCU), or on hardware using wired logic. [Explanation of symbols]

[0143] 2 Optical transmitter 12 Detector 21 Driver amplifier 31A RF side MZM 31B DC side child MZM 32A RF side MZM 32B DC side child MZM 33A DC side parent MZM 33B DC side parent MZM 55A ABC control unit 55A1 First additional part 55B DRV control unit 55B1 Second additional part

Claims

1. a driver amplifier that amplifies high-frequency signals; an adding unit that adds a dither signal to the high-frequency signal amplified by the driver amplifier; an optical modulation unit that modulates an optical signal in accordance with the high-frequency signal to which the dither signal has been added; a detector for detecting a fluctuation level of the dither signal from a modulated optical signal; a control unit that controls a gain of the driver amplifier that amplifies the high frequency signal based on the detected fluctuation level of the dither signal so that the output amplitude of the driver amplifier becomes constant; An optical transmitter comprising:

2. The control unit 2. The optical transmitter according to claim 1, wherein the gain of the driver amplifier is controlled so that the fluctuation level of the dither signal detected by the detection unit matches a predetermined fluctuation level that is set in advance.

3. The optical modulation unit a phase modulation unit that phase-modulates an optical signal in response to the high-frequency signal; a phase adjusting unit that adjusts the phase of the optical signal phase-modulated by the phase modulating unit in accordance with a bias signal, The adding unit a second adding unit that adds a second dither signal, which is the dither signal, to the high-frequency signal amplified by the driver amplifier; a first adding unit that adds a first dither signal having the same frequency as the second dither signal to the bias signal, The detection unit detecting fluctuation levels of the first dither signal and the second dither signal from the optical signal modulated by the optical modulation unit; The control unit a first control unit that controls the bias signal based on a fluctuation level of the first dither signal; a second control unit that controls a gain of the driver amplifier based on a fluctuation level of the second dither signal after the first control unit controls the bias signal; 2. The optical transmitter according to claim 1, further comprising:

4. the first dither signal and the second dither signal are in an anti-phase relationship, The control unit 4. The optical transmitter according to claim 3, wherein the gain of the driver amplifier is controlled so that the first dither signal and the second dither signal cancel each other out.

5. The control unit 5. The optical transmitter according to claim 4, wherein the fluctuation level of the second dither signal detected by the detection unit is adjusted in accordance with a fluctuation ratio between the fluctuation level of the first dither signal detected by the detection unit and a preset initial fluctuation level, and the gain of the driver amplifier is controlled so that the adjusted fluctuation level of the second dither signal and the fluctuation level of the first dither signal detected by the detection unit cancel each other out.

6. An optical transceiver having an optical transmitter that outputs a transmission light using an electrical signal and an optical signal corresponding to transmission data, an optical receiver that receives a reception light using the optical signal and obtains an electrical signal corresponding to the reception data from the received reception light, and a processor that performs signal processing on the electrical signal, The optical transmitter comprises: a driver amplifier that amplifies high-frequency signals; an adding unit that adds a dither signal to the high-frequency signal amplified by the driver amplifier; an optical modulation unit that modulates an optical signal in accordance with the high-frequency signal to which the dither signal has been added; a detector for detecting a fluctuation level of the dither signal from a modulated optical signal; a control unit that controls a gain of the driver amplifier that amplifies the high frequency signal based on the detected fluctuation level of the dither signal so that the output amplitude of the driver amplifier becomes constant; An optical transceiver comprising:

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