Optical transmitter and method for controlling optical transmitter

Abstract

To provide an optical transmitter capable of appropriately performing bias control, and a control method of the optical transmitter.SOLUTION: The optical transmitter includes a first IQ modulator configured to generate first modulated light, a second IQ modulator connected in parallel with the first IQ modulator and configured to generate second modulated light, a phase shifter configured to shift a phase of the second modulated light, an optical coupler configured to couple the first modulated light and the phase-shifted second modulated light to generate third modulated light, and a bias controller configured to control first to third bias signals to be provided to the first IQ modulator, the second IQ modulator, and the phase shifter, respectively, based on the first to third modulated light.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an optical transmitter and a method for controlling an optical transmitter. [Background technology]

[0002] In recent years, with the spread of the Internet and 5G, research is being conducted to increase the speed and capacity of optical communications, which are used as fundamental technologies. For example, optical modulators that perform multi-level modulation have attracted attention in order to enable improved spectral efficiency. As related technology, Non-Patent Document 1 describes optical modulators that perform quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16QAM), and 256 quadrature amplitude modulation (256QAM). The optical modulator in Non-Patent Document 1 achieves multi-level modulation by connecting multiple Mach-Zehnder modulators (MZM) in parallel. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Tetsuya KAWANISHI, Takahide SAKAMOTO, Akito CHIBA, "Integrated Lithium Niobate Mach-Zehnder Interferometers for Advanced Modulation Formats", IEICE Trans. Electron, vol.E92-C, no.7, pp.915-921, July 2009 Summary of the Invention [Problem to be solved by the invention]

[0004] When a modulator is configured by connecting a plurality of MZ modulators in parallel as in Non-Patent Document 1, the bias control method for the modulator becomes complicated, making it difficult to perform appropriate bias control.

[0005] In view of the above problems, one object of the present disclosure is to provide an optical transmitter and a control method for an optical transmitter that are capable of performing appropriate bias control. [Means for solving the problem]

[0006] An optical transmitter according to one embodiment of the present disclosure includes a first IQ modulator that generates first modulated light, a second IQ modulator connected in parallel to the first IQ modulator that generates second modulated light, a phase shifter that shifts the phase of the second modulated light, an optical combiner that combines the first modulated light and the phase-shifted second modulated light to generate third modulated light, and bias control means that controls first to third bias signals that are applied to the first IQ modulator, the second IQ modulator, and the phase shifter, respectively, based on the first to third modulated lights.

[0007] An optical transmitter according to one embodiment of the present disclosure comprises a plurality of IQ modulators connected in parallel, each generating a plurality of modulated light beams, an optical coupler combining the plurality of modulated light beams to generate an output modulated light beam, a phase shifter located upstream of the optical coupler and shifting the phase of the modulated light beams contained in the plurality of modulated light beams, and bias control means for controlling a plurality of bias signals to be applied to the plurality of IQ modulators and the phase shifter, respectively, based on the plurality of modulated light beams and the output modulated light beam.

[0008] A method for controlling an optical transmitter according to one embodiment of the present disclosure generates first modulated light using a first IQ modulator, generates second modulated light using a second IQ modulator connected in parallel to the first IQ modulator, shifts the phase of the second modulated light using a phase shifter, combines the first modulated light and the phase-shifted second modulated light using an optical coupler to generate third modulated light, and controls first to third bias signals to be applied to the first IQ modulator, the second IQ modulator, and the phase shifter, respectively, based on the first to third modulated lights. [Effects of the Invention]

[0009] According to the present disclosure, bias control can be performed appropriately. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating a configuration of a multi-parallel modulator according to a related art technique. [Figure 2] 1 is a diagram illustrating an example of a configuration of an optical transmitter according to some embodiments. [Figure 3] 1 is a diagram illustrating an example of a configuration of an optical transmitter according to some embodiments. [Figure 4] 1 is a flowchart illustrating an example of the operation of an optical transmitter according to some embodiments. [Figure 5] FIG. 10 is a diagram for explaining a specific example of bias control of an IQ modulator according to some embodiments. [Figure 6] 10A and 10B are diagrams for explaining specific examples of bias control of a phase shifter according to some embodiments. [Figure 7] 1 is a diagram illustrating an example of a configuration of an optical transmitter according to some embodiments. [Figure 8] 1 is a diagram illustrating an example of a configuration of an optical transmitter according to some embodiments. [Figure 9] FIG. 1 is a diagram illustrating an example of the hardware configuration of a computer according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments will be described with reference to the drawings. In each drawing, the same or corresponding elements are designated by the same reference numerals, and for clarity of explanation, duplicate explanations will be omitted as necessary. Note that arrows shown in each drawing are for illustrative purposes only and do not limit the type or direction of signals.

[0012] (Review of related technologies) First, the related art will be considered. Fig. 1 shows the configuration of a multi-parallel modulator 9 in the related art described in Non-Patent Document 1.

[0013] As shown in Fig. 1, the related multi-parallel modulator 9 includes multiple IQ (In-phase and Quadrature) modulators 11 connected in parallel. The multi-parallel modulator 9 enables multi-level modulation by combining modulated light from the multiple IQ modulators 11.

[0014] In the example of Figure 1, the multi-parallel modulator 9 has two IQ modulators 11-1 and 11-2 arranged optically in parallel. Each IQ modulator 11 is composed of an MZ interference system including two MZ modulators and generates a QPSK optical signal. The multi-parallel modulator 9 is also a quad-parallel modulator including four MZ modulators. The multi-parallel modulator 9 adjusts the symbol amplitude by changing the loss of the optical waveguide and the coupling / branching ratio to generate a 16QAM signal. The multi-parallel modulator 9 generates a multi-level optical signal by electrically using a binary signal, which allows for reduced variation in signal level compared to when using an electrically multi-level signal.

[0015] On the other hand, in optical modulators, the bias point (operating point) fluctuates due to environmental changes, etc., so the bias voltage needs to be adjusted. However, a multi-parallel modulator has many phase adjustment parts. In the example of Figure 1, the bias voltage needs to be adjusted within the MZ modulator, between MZ modulators, and between the IQ modulators, so there are seven phase adjustment parts. This makes it difficult to properly adjust each bias voltage.

[0016] For example, one possible method is to place a monitor PD (Photo Detector) at the final stage of the optical modulator and control each bias voltage based on the optical power monitored by the monitor PD. However, while the phase is orthogonal between MZ modulators within an IQ modulator, the IQ modulators must be coupled in phase, which means that the phase adjustment methods are different. For this reason, it is difficult to optimize each bias voltage by simply monitoring the light at the final stage of the optical modulator.

[0017] (Embodiment 1) Next, a first embodiment will be described. In this embodiment, an outline of several embodiments will be described.

[0018] 2 shows an example of the configuration of an optical transmitter 20 according to some embodiments. The optical transmitter 20 is, for example, an optical transmitter for digital coherent communication.

[0019] 2, the optical transmitter 20 includes IQ modulators 11-1 and 11-2, a phase shifter 12, an optical coupler 13, and a bias control unit 21. For example, the IQ modulator 11-1 is a first IQ modulator, and the IQ modulator 11-2 is a second IQ modulator. The IQ modulators 11-1 and 11-2, the phase shifter 12, and the optical coupler 13 configure a multi-parallel modulator 10. The optical transmitter 20 may include a light source that generates light source light.

[0020] The IQ modulators 11-1 and 11-2 are the same as those in Fig. 1. The IQ modulator 11-1 modulates, for example, the input light source light (or a part of the light source light) to generate a first modulated light that is a QPSK optical signal. The IQ modulator 11-2 is connected in parallel to the IQ modulator 11-1 and modulates, for example, the input light source light (or a part of the light source light) to generate a second modulated light that is a QPSK optical signal.

[0021] The phase shifter 12 shifts the phase of the second modulated light generated by the IQ modulator 11-2. The phase shifter 12 adjusts the phase difference between the first modulated light generated by the IQ modulator 11-1 and the second modulated light generated by the IQ modulator 11-2.

[0022] The optical coupler 13 combines the first modulated light generated by the IQ modulator 11-1 with the second modulated light whose phase has been shifted by the phase shifter 12. The optical coupler 13 outputs the combined third modulated light. Note that an attenuator may be provided to attenuate the power of the second modulated light, or the coupling ratio of the optical coupler 13 may be changed.

[0023] The bias control unit 21 controls the bias signals provided to the IQ modulator 11-1, the IQ modulator 11-2, and the phase shifter 12. To control the bias signals, the bias control unit 21 may adjust the applied bias voltage to an optimum value, or may adjust the supplied bias current (e.g., heater current) to an optimum value. The bias control unit 21 controls the bias signals of the IQ modulator 11-1, the IQ modulator 11-2, and the phase shifter 12, based on the first modulated light generated by the IQ modulator 11-1, the second modulated light generated by the IQ modulator 11-2, and the third modulated light combined by the optical combiner 13.

[0024] The bias control unit 21 may control the first bias signal of the IQ modulator 11-1 and the second bias signal of the IQ modulator 11-2 based on the first modulated light generated by the IQ modulator 11-1 and the second modulated light generated by the IQ modulator 11-2, respectively, and then control the third bias signal of the phase shifter 12 based on the third modulated light coupled by the optical coupler 13. For example, the bias control unit 21 may control the first and second bias signals, respectively, so that the DC components (direct current components) of the first modulated light and the second modulated light are equal to or less than a set value. The bias control unit 21 may also control the third bias signal so that the power of the third modulated light is equal to or greater than a set value.

[0025] The multi-parallel modulator 10 may include a monitor for monitoring each modulated light. For example, a first optical monitor for monitoring the first modulated light may be arranged downstream of the IQ modulator 11-1, a second optical monitor for monitoring the second modulated light may be arranged downstream of the IQ modulator 11-2, and a third optical monitor for monitoring the third modulated light may be arranged downstream of the optical coupler 13. In this case, the bias control unit 21 may control the first bias signal of the IQ modulator 11-1 based on the monitoring result of the first optical monitor, control the second bias signal of the IQ modulator 11-2 based on the monitoring result of the second optical monitor, and control the bias signal of the phase shifter 12 based on the monitoring result of the third optical monitor.

[0026] The IQ modulators 11-1 and 11-2 may be LN (LiNbO3: lithium niobate) modulators or semiconductor modulators. In the case of semiconductor modulators, the IQ modulators 11-1 and 11-2, phase shifter 12, and optical coupler 13 may be formed on a single semiconductor substrate. In the case of including first to third optical monitors, the IQ modulators 11-1 and 11-2, phase shifter 12, optical coupler 13, and first to third optical monitors may be formed on a single semiconductor substrate.

[0027] In the example of FIG. 2 , the multi-parallel modulator 10 includes two IQ modulators, but may include more IQ modulators. That is, the multi-parallel modulator 10 may be configured to perform not only 16QAM modulation but also 256QAM modulation or other multi-level modulation with other symbol numbers. For example, the multi-parallel modulator 10 may include multiple IQ modulators 11 connected in parallel to generate multiple modulated light beams, an optical combiner 13 that combines the multiple modulated light beams to generate output modulated light, and a phase shifter 12 upstream of the optical combiner 13 that shifts the phase of the modulated light beams included in the multiple modulated light beams. In this case, the bias control unit 21 may control multiple bias signals to be applied to the multiple IQ modulators 11 and the phase shifter 12, respectively, based on the multiple modulated light beams and the output modulated light. For example, if the multi-parallel modulator includes n IQ modulators, n optical monitors may be arranged at the output stages of the n IQ modulators, and an optical monitor may be arranged at the final stage of the multi-parallel modulator. The final stage is an output stage that combines and finally outputs n modulated lights when n IQ modulators are connected in parallel.

[0028] As described above, in this embodiment, in a multi-parallel modulator including multiple IQ modulators, the bias signals of each IQ modulator and phase shifter are controlled based on the output light of each of the multiple IQ modulators and the output light of the multi-parallel modulator. For example, an optical monitor may be disposed at the output stage of each IQ modulator and the output stage of the multi-parallel modulator, and the bias signals of each IQ modulator and phase shifter may be controlled based on the monitoring results of each optical monitor. This allows appropriate bias control of each IQ modulator and phase shifter. In other words, the bias point of each IQ modulator can be appropriately adjusted, and the phase difference in the modulated light output from the multi-parallel modulator can be appropriately adjusted.

[0029] In the following embodiment, a specific example of the first embodiment will be described.

[0030] (Embodiment 2) Next, a description will be given of a second embodiment. In this embodiment, an example of bias control in a multi-parallel modulator that performs 16QAM modulation will be described.

[0031] 3 shows an example of the configuration of an optical transmitter 1 according to some embodiments. The optical transmitter 1 is, for example, an optical transmitter for digital coherent communication. In the example of FIG. 3, the optical transmitter 1 includes a light source 200, a multi-parallel modulator 100, and a bias control unit 300.

[0032] For example, the multi-parallel modulator 100 is a semiconductor modulator, but may be an LN modulator. Each component of the multi-parallel modulator 100 may be integrated on a semiconductor substrate. For example, the light source 200 and the multi-parallel modulator 100 may be formed on a single semiconductor substrate.

[0033] When the multi-parallel modulator 100 is a semiconductor modulator, the bias control unit 300 may be formed as a semiconductor chip separate from a semiconductor substrate including the multi-parallel modulator 100. The bias control unit 300 may be configured by hardware or software, or both. The functions of the bias control unit 300 may be implemented by a processor such as a CPU (Central Processing Unit) executing a program stored in a memory. For example, a semiconductor chip including the bias control unit 300 may be mounted on a semiconductor substrate including the multi-parallel modulator 100 by flip-chip mounting or the like. Furthermore, a semiconductor device including all of the configurations shown in FIG. 3 may be integrated into a single semiconductor package.

[0034] The light source 200 generates a light source light SO1, which is light of a predetermined wavelength. The light source 200 may be a semiconductor laser formed on the same semiconductor substrate as the semiconductor modulator. The light source 200 may also be an external light source disposed outside the LN modulator. For example, the light source 200 may be a DFB (Distributed Feedback) laser or a DBR (Distributed Bragg Reflector) laser, or an external cavity laser (ECL).

[0035] The multi-parallel modulator 100 is a multi-level modulator for digital coherent communication. In the example of Fig. 3, the multi-parallel modulator 100 is a 16QAM modulator that performs 16QAM modulation. The multi-parallel modulator 100 modulates the light source light SO1 output from the light source 200 into a modulated optical signal SO4. The multi-parallel modulator 100 applies 16QAM modulation to the light source light SO1 and outputs a 16QAM modulated optical signal SO4.

[0036] The multi-parallel modulator 100 includes two IQ modulators 110, which are QPSK modulators. 16QAM modulation is performed by arranging the two IQ modulators 110-1 and 110-2 in parallel. The multi-parallel modulator 100 includes an optical coupler 150, IQ modulators 110-1 and 110-2, monitor PDs 120-1 and 120-2, an attenuator 130, a phase shifter 140, an optical coupler 160, and a monitor PD 170. For example, the IQ modulators 110-1 and 110-2 may be semiconductor modulators, and the optical coupler 150, IQ modulators 110-1 and 110-2, monitor PDs 120-1 and 120-2, attenuator 130, phase shifter 140, optical coupler 160, and monitor PD 170 may all be integrated on a single semiconductor substrate.

[0037] The optical coupler 150 is a demultiplexer that demultiplexes (branches) the light source light SO1 from the light source 200 into light source light SO1-1 for the IQ modulator 110-1 and light source light SO1-2 for the IQ modulator 110-2. For example, the IQ modulator 110-1 is a modulator that does not change the amplitude and phase of the modulated optical signal, and the IQ modulator 110-2 is a modulator that changes the amplitude and phase of the modulated optical signal.

[0038] The IQ modulator 110-1 modulates the source light SO1-1 branched from the optical coupler 150 into a modulated optical signal SO2-1. The IQ modulator 110-1 applies IQ modulation (QPSK modulation) to the source light SO1-1 and outputs a QPSK modulated optical signal SO2-1.

[0039] The IQ modulator 110-1 includes an optical coupler 111-1, MZ modulators 112a-1 and 112b-1, a phase shifter 113-1, and an optical coupler 114-1. The optical coupler 111-1 branches (demultiplexes) the light source light SO1-1 branched from the optical coupler 150 into light for Ich (in-phase component) and light for Qch (quadrature component).

[0040] The MZ modulator 112a-1 is, for example, a modulator for Ich. An Ich data signal DT1 generated from a transmission data signal is applied to the MZ modulator 112a-1 as a drive signal. The MZ modulator 112a-1 modulates the Ich light branched by the optical coupler 111-1 in accordance with the data signal DT1. A bias voltage BS1 for adjusting a bias point is also applied to the MZ modulator 112a-1 from the bias control unit 300.

[0041] The MZ modulator 112b-1 is, for example, a modulator for Qch. A data signal DT2 for Qch generated from a transmission data signal is applied to the MZ modulator 112b-1 as a drive signal. The MZ modulator 112b-1 modulates the Qch light branched by the optical coupler 111-1 in accordance with the data signal DT2. A bias voltage BS2 for adjusting a bias point is also applied to the MZ modulator 112b-1 from the bias control unit 300.

[0042] The phase shifter 113-1 shifts the phase of the I-channel optical signal or the Q-channel optical signal so that the phases of the I-channel optical signal and the Q-channel optical signal are orthogonal to each other. In this example, the phase shifter 113-1 shifts the phase of the Q-channel modulated optical signal modulated by the MZ modulator 112b-1 by π / 2. The phase shifter 113-1 may be configured with an MZ interferometer like an MZ modulator, or may be configured to shift the phase by changing the refractive index of the waveguide. In addition, a bias voltage BS3 that adjusts the amount of phase shift is applied to the phase shifter 113-1 from the bias control unit 300. When the phase shifter 113-1 is configured with an MZ interferometer, the amount of phase shift may be controlled by the bias voltage BS3 applied to the MZ interferometer. The amount of phase shift may be controlled by forming a heater in the waveguide that constitutes the phase shifter 113-1 and applying a bias voltage BS3 corresponding to a heater current to the formed heater to change the refractive index of the waveguide using the generated heat.

[0043] The optical coupler 114-1 is a multiplexer that multiplexes an optical signal modulated by the MZ modulator 112a-1 and an optical signal modulated by the MZ modulator 112b-1 and phase-shifted by the phase shifter 113-1, and is also a demultiplexer that demultiplexes (branches) the multiplexed QPSK optical signal into a modulated optical signal SO2-1 for output from the IQ modulator 110-1 and a monitor optical signal SO3-1 for monitoring. For example, the optical coupler 114-1 is a 2×2 optical coupler that realizes the functions of a multiplexer and demultiplexer, but it may also be provided with a multiplexer and a demultiplexer. That is, a demultiplexer may be arranged downstream of a multiplexer that multiplexes an optical signal modulated by the MZ modulator 112a-1 with an optical signal modulated by the MZ modulator 112b-1 and phase-shifted by the phase shifter 113-1, and that demultiplexes the optical signal multiplexed by the multiplexer into a modulated optical signal SO2-1 for output from the IQ modulator 110-1 and a monitor optical signal SO3-1 for monitoring.

[0044] The monitor PD 120-1 is a PD that monitors (detects) the monitor optical signal SO3-1 branched by the optical coupler 114-1. It can also be said that the monitor PD 120-1 monitors a portion of the QPSK modulated optical signal SO2-1 modulated by the IQ modulator 110-1. For example, the monitor PD 120-1 may be a semiconductor PD formed on a semiconductor substrate. The monitor PD 120-1 photoelectrically converts the monitor optical signal SO3-1 and outputs the photoelectrically converted monitor signal MO-1.

[0045] The IQ modulator 110-2 is the same IQ modulator as the IQ modulator 110-1 and has the same configuration. That is, the IQ modulator 110-2 performs IQ modulation on the source light SO1-2 branched from the optical coupler 150, and outputs a QPSK modulated optical signal SO2-2.

[0046] Similar to the IQ modulator 110-1, the IQ modulator 110-2 includes an optical coupler 111-2, MZ modulators 112a-2 and 112b-2, a phase shifter 113-2, and an optical coupler 114-2. The optical coupler 111-2 branches (demultiplexes) the light source light SO1-2 branched from the optical coupler 150 into light for Ich and light for Qch.

[0047] The MZ modulator 112a-2 is applied with a bias voltage BS4 and modulates the I-channel light branched by the optical coupler 111-2 in accordance with the data signal DT3. The MZ modulator 112b-2 is applied with a bias voltage BS5 and modulates the Q-channel light branched by the optical coupler 111-2 in accordance with the data signal DT4. The phase shifter 113-2 is applied with a bias voltage BS6 and shifts the phase of the Q-channel modulated optical signal modulated by the MZ modulator 112b-2.

[0048] The optical coupler 114-2 combines the optical signal modulated by the MZ modulator 112a-2 with the optical signal modulated by the MZ modulator 112b-2 and phase-shifted by the phase shifter 113-2, and branches the combined QPSK optical signal into a modulated optical signal SO2-2 for output from the IQ modulator 110-2 and a monitor optical signal SO3-2 for monitoring.

[0049] The monitor PD 120-2 monitors the monitor optical signal SO3-2 branched by the optical coupler 114-2. In other words, the monitor PD 120-2 monitors a portion of the QPSK modulated optical signal SO2-2 modulated by the IQ modulator 110-2. The monitor PD 120-2 performs opto-electrical conversion on the monitor optical signal SO3-2 and outputs the opto-electrically converted monitor signal MO-2.

[0050] The attenuator 130 changes the amplitude of either the modulated optical signal SO2-1 modulated by the IQ modulator 110-1 or the modulated optical signal SO2-2 modulated by the IQ modulator 110-2. In this example, the attenuator 130 attenuates the power of the modulated optical signal SO2-2 modulated by the IQ modulator 110-2. For example, the attenuator 130 may attenuate the power of the modulated optical signal SO2-2 by 6 dB. Note that the function of the attenuator 130 may also be achieved by changing the coupling ratio of the optical coupler 160.

[0051] The phase shifter 140 shifts the phase of the modulated optical signal SO2-1 or the modulated optical signal SO2-2 so that the phase of the modulated optical signal SO2-1 modulated by the IQ modulator 110-1 and the phase of the modulated optical signal SO2-2 modulated by the IQ modulator 110-2 match (become in phase). In this example, the phase shifter 140 shifts the phase of the modulated optical signal SO2-2 modulated by the IQ modulator 110-2 and attenuated by the attenuator 130. The phase shifter 140 may be configured using an MZ interferometer, similar to the phase shifter 113 of the IQ modulator 110, or may be configured to shift the phase by changing the refractive index of the waveguide. In addition, a bias voltage BS7 that adjusts the amount of phase shift is applied to the phase shifter 140 from the bias control unit 300. Like the phase shifter 113, if the phase shifter 140 is configured using an MZ interferometer, the amount of phase shift may be controlled by the bias voltage BS7 applied to the MZ interferometer. The amount of phase shift may be controlled by forming a heater in the waveguide that constitutes the phase shifter 140, and applying a bias voltage BS7 corresponding to the heater current to the formed heater to generate heat that changes the refractive index of the waveguide.

[0052] The optical coupler 160 is a multiplexer that multiplexes a modulated optical signal SO2-1 modulated by the IQ modulator 110-1 and a modulated optical signal SO2-2 modulated by the IQ modulator 110-2 and attenuated and phase-shifted, and is also a demultiplexer that demultiplexes (branches) the multiplexed 16QAM optical signal into a modulated optical signal SO4 for output from the multi-parallel modulator 100 and a monitor optical signal SO5 for monitoring. For example, the optical coupler 160 is a 2×2 optical coupler that realizes the functions of a multiplexer and demultiplexer, but it may also be provided with a multiplexer and a demultiplexer. That is, a demultiplexer may be disposed downstream of a multiplexer that multiplexes modulated optical signal SO2-1 modulated by IQ modulator 110-1 and modulated optical signal SO2-2 modulated by IQ modulator 110-2 and attenuated and phase-shifted, to demultiplex the optical signal multiplexed by the multiplexer into modulated optical signal SO4 for output from multi-parallel modulator 100 and monitor optical signal SO5 for monitoring.

[0053] The monitor PD 170 monitors (detects) the monitor optical signal SO5 branched by the optical coupler 160. In other words, the monitor PD 170 monitors a portion of the 16QAM modulated optical signal SO4 modulated by the multi-parallel modulator 100. For example, the monitor PD 170 may be a semiconductor PD formed on a semiconductor substrate. The monitor PD 170 photoelectrically converts the monitor optical signal SO5 and outputs the photoelectrically converted monitor signal MO-3.

[0054] The bias control unit 300 controls each bias voltage based on the monitoring results of each monitor PD. The bias control unit 300 performs a predetermined discrimination process (feedback control) using the electrical signal output from the monitor PD 120-1 for the IQ modulator 110-1 to control the bias voltage of the IQ modulator 110-1, and also performs a predetermined discrimination process using the electrical signal output from the monitor PD 120-2 for the IQ modulator 110-2 to control the bias voltage of the IQ modulator 110-2. After controlling the bias voltages of the IQ modulators 110-1 and 110-2, the bias control unit 300 performs a predetermined discrimination process using the electrical signal output from the monitor PD 170 of the multi-parallel modulator 100 to control the bias voltage of the phase shifter 140.

[0055] The bias control unit 300 automatically optimizes the bias voltage and adjusts the bias point of the IQ modulator 110-1 by feedback-controlling the bias voltage of the IQ modulator 110-1 according to the monitoring results of a portion of the modulated optical signal modulated by the IQ modulator 110-1. In addition to adjusting the bias point, it can also be said that the bias control unit 300 controls the phases of the modulated optical signals in the output light of the IQ modulator 110-1 so that they are orthogonal. That is, the bias control unit 300 controls each bias voltage of the IQ modulator 110-1 based on the monitor signal MO-1 from the monitor PD 120-1. In this example, the bias voltage BS1 applied to the MZ modulator 112a-1, the bias voltage BS2 applied to the MZ modulator 112b-1, and the bias voltage BS3 applied to the phase shifter 113-1 are each controlled based on the monitor signal MO-1.

[0056] The bias control unit 300 automatically optimizes the bias voltage and adjusts the bias point of the IQ modulator 110-2 by feedback-controlling the bias voltage of the IQ modulator 110-2 according to the monitoring results of a portion of the modulated optical signal modulated by the IQ modulator 110-2. In addition to adjusting the bias point, it can also be said that the bias control unit 300 controls the phases of the modulated optical signals in the output light of the IQ modulator 110-2 so that they are orthogonal. That is, the bias control unit 300 controls the bias voltage of the IQ modulator 110-2 based on the monitor signal MO-2 from the monitor PD 120-2. In this example, the bias voltage BS4 applied to the MZ modulator 112a-2, the bias voltage BS5 applied to the MZ modulator 112b-2, and the bias voltage BS6 applied to the phase shifter 113-2 are each controlled based on the monitor signal MO-2.

[0057] The bias control unit 300 automatically optimizes the bias voltage by feedback-controlling the bias voltage of the phase shifter 140 in accordance with the monitoring results of a portion of the modulated optical signal modulated by the multi-parallel modulator 100, and adjusts the phase difference so that the modulated optical signals in the output light of the multi-parallel modulator 100 are in phase. In this example, the bias control unit 300 controls the bias voltage BS7 applied to the phase shifter 140 based on the monitor signal MO-3 of the monitor PD 170.

[0058] Fig. 4 shows an example of operation of the optical transmitter 1 according to some embodiments. In the example of Fig. 4, first, the optical transmitter 1 sets initial values ​​of bias voltages BS1 to BS7 (S101). The bias control unit 300 generates bias voltages BS1 to BS7 of predetermined initial values ​​and supplies the generated bias voltages BS1 to BS7. Specifically, the bias control unit 300 applies initial bias voltages BS1 to BS3 to the MZ modulator 112a-1, the MZ modulator 112b-1, and the phase shifter 113-1, respectively, as bias voltages for the IQ modulator 110-1. The IQ modulator 110-1 performs IQ modulation operation at a bias point and a phase shift amount according to the initial bias voltages BS1 to BS3.

[0059] Moreover, the bias control unit 300 applies initial bias voltages BS4 to BS6 to the MZ modulator 112a-2, MZ modulator 112b-2, and phase shifter 113-2 as bias voltages for the IQ modulator 110-2. The IQ modulator 110-2 performs IQ modulation operation at a bias point and a phase shift amount according to the initial bias voltages BS4 to BS6.

[0060] Furthermore, the bias control section 300 applies the bias voltage BS7 of the initial value to the phase shifter 140. The phase shifter 140 sets the phase difference between the IQ modulator 110-1 and the IQ modulator 110-2 according to the bias voltage BS7 of the initial value.

[0061] Next, the optical transmitter 1 performs bias control processing for the IQ modulator 110-1 (S102 to S104) and bias control processing for the IQ modulator 110-2 (S105 to S107). The bias control processing for the IQ modulator 110-1 and the bias control processing for the IQ modulator 110-2 may be performed in parallel, or one of them may be performed first.

[0062] In the bias control process of the IQ modulator 110-1, the monitor PD 120-1 monitors the modulated optical signal modulated by the IQ modulator 110-1 (S102). Specifically, when the light source light SO1-1 and data signals DT1 and DT2 are input from the light source 200 via the optical coupler 150 to the IQ modulator 110-1, the IQ modulator 110-1 IQ-modulates the light source light SO1-1 in accordance with the data signals DT1 and DT2. The optical coupler 114-1 branches the IQ-modulated optical signal into a modulated optical signal SO2-1 for output from the IQ modulator 110-1 and a monitor optical signal SO3-1 for monitoring. The monitor PD 120-1 detects the branched monitor optical signal SO3-1 and converts the detected optical signal into a monitor signal MO-1 (monitor current).

[0063] Next, the bias control unit 300 determines whether or not control (adjustment) of the bias voltages BS1 to BS3 of the IQ modulator 110-1 is necessary based on the monitoring result of the monitor PD 120-1 (S103). For example, the bias control unit 300 determines whether or not the monitor signal MO-1 (monitor current) output from the monitor PD 120-1 is within a predetermined range, and if the monitor signal MO-1 (monitor current) exceeds the predetermined range, determines that bias control is necessary.

[0064] Next, if it is determined that bias control is necessary, the bias control unit 300 controls (adjusts) the bias voltages BS1 to BS3 of the IQ modulator 110-1 (S104). The bias control unit 300 controls the bias voltage BS1 applied to the MZ modulator 112a-1, the bias voltage BS2 applied to the MZ modulator 112b-1, and the bias voltage BS3 applied to the phase shifter 113-1 based on the detected monitor signal MO-1 (monitor current). The bias control unit 300 adjusts the bias voltages BS1 to BS3 based on the monitor signal MO-1 so that the DC component of the modulated optical signal of the IQ modulator 110-1 is equal to or less than a set value. Specifically, the bias control unit 300 adjusts the bias voltages BS1 to BS3 so that the value of the objective function corresponding to the monitor signal MO-1 is equal to or less than a set value. For example, the bias control unit 300 repeats the adjustment of the bias voltages BS1 to BS3 until the monitor signal MO-1 (monitor current) falls within a predetermined range.

[0065] 5 shows an example of objective function values ​​depending on bias voltages as a specific example of bias control of an IQ modulator according to some embodiments. The bias voltages in Fig. 5 are, for example, bias voltages BS1 to BS3 of IQ modulator 110-1, but the same can be said for bias voltages BS4 to BS6 of IQ modulator 110-2.

[0066] In this example, an objective function value is used, which is obtained by converting the monitor power (monitor current) of the monitor signal MO-1 (or MO-2) output from the monitor PD 120-1 (or 120-2) using a predetermined objective function. The objective function value may be the average or variance of values ​​obtained from multiple monitored monitor signals MO-1. The objective function is a function used to control the bias voltage of the MZ modulator. In the case of an MZ modulator, when the bias voltage deviates from the optimal value, unmodulated components pass through the optical modulator as DC components. The monitor signal MO-1 mainly contains this DC component. As shown in FIG. 5, as the bias voltage deviates from the optimal value, the DC component increases, and the objective function value also increases. As the bias voltage approaches the optimal value, the DC component decreases, and the objective function value also decreases. For example, the bias control unit 300 adjusts the bias voltages BS1 to BS3 so that the objective function value calculated from the power of the light modulated by the IQ modulator 110-1 approaches a minimum value (for example, so that it is equal to or less than a set value).

[0067] The bias control process (S105 to S107) of the IQ modulator 110-2 is the same as the bias control process (S102 to S104) of the IQ modulator 110-1. That is, in the bias control process of the IQ modulator 110-2, the monitor PD 120-2 monitors the modulated optical signal modulated by the IQ modulator 110-2 (S105). Specifically, when the light source light SO1-2 and the data signals DT3 and DT4 are input from the light source 200 via the optical coupler 150, the IQ modulator 110-2 IQ-modulates the light source light SO1-2 in accordance with the data signals DT3 and DT4. The optical coupler 114-2 branches the IQ-modulated optical signal into a modulated optical signal SO2-2 to be output from the IQ modulator 110-2 and a monitor optical signal SO3-2 to be monitored. The monitor PD 120-2 detects the branched monitor optical signal SO3-2 and converts the detected optical signal into a monitor signal MO-2 (monitor current).

[0068] Next, the bias control unit 300 determines whether or not control (adjustment) of the bias voltages BS4 to BS6 of the IQ modulator 110-2 is necessary based on the monitoring result of the monitor PD 120-2 (S106). As in the case of the IQ modulator 110-1, for example, the bias control unit 300 determines whether or not the monitor signal MO-2 (monitor current) output from the monitor PD 120-2 is within a predetermined range, and if the monitor signal MO-2 (monitor current) exceeds the predetermined range, determines that bias control is necessary.

[0069] Next, if it is determined that bias control is necessary, the bias control unit 300 controls (adjusts) the bias voltages BS4 to BS6 of the IQ modulator 110-2 (S107). As in the case of the IQ modulator 110-1, the bias control unit 300 controls the bias voltage BS4 applied to the MZ modulator 112a-2, the bias voltage BS5 applied to the MZ modulator 112b-2, and the bias voltage BS6 applied to the phase shifter 113-2 based on the detected monitor signal MO-2 (monitor current). Specifically, as shown in FIG. 5, the bias control unit 300 adjusts the bias voltages BS4 to BS6 so that the objective function value corresponding to the monitor signal MO-2 is equal to or less than a set value. For example, the bias control unit 300 repeats the adjustment of the bias voltages BS4 to BS6 until the monitor signal MO-2 (monitor current) falls within a predetermined range.

[0070] Next, when the bias control process of the IQ modulator 110-1 and the bias control process of the IQ modulator 110-2 are completed, the optical transmitter 1 performs bias control process (S108 to S110) of the phase shifter 140. By adjusting the bias voltage of the phase shifter 140 after the bias points of the IQ modulators 110-1 and 110-2 are optimized, it is possible to reliably suppress phase shifts in the modulated optical signal of the multi-parallel modulator 100.

[0071] In the bias control process of the phase shifter 140, the monitor PD 170 monitors the modulated optical signal modulated by the multi-parallel modulator 100 (S108). Specifically, the optical coupler 160 combines an optical signal IQ-modulated by the IQ modulator 110-1 with an optical signal IQ-modulated by the IQ modulator 110-2 and passed through the attenuator 130 and the phase shifter 140, and branches the combined optical signal into a modulated optical signal SO4 to be output from the multi-parallel modulator 100 and a monitor optical signal SO5 for monitoring. The monitor PD 170 detects the branched monitor optical signal SO5 and converts the detected optical signal into a monitor signal MO-3 (monitor current).

[0072] Next, the bias control unit 300 determines whether or not it is necessary to control (adjust) the bias voltage BS7 of the phase shifter 140 based on the monitoring result of the monitor PD 170 (S109). For example, the bias control unit 300 determines whether or not the monitor signal MO-3 (monitor current) output from the monitor PD 170 is within a predetermined range, and if the monitor signal MO-3 (monitor current) exceeds the predetermined range, it determines that bias control is necessary.

[0073] Next, if it is determined that bias control is necessary, the bias control unit 300 controls (adjusts) the bias voltage BS7 of the phase shifter 140 (S110). The bias control unit 300 controls the bias voltage BS7 applied to the phase shifter 140 based on the detected monitor signal MO-3 (monitor current). Specifically, the bias control unit 300 adjusts the bias voltage BS7 so that the optical power of the monitor signal MO-3 is equal to or greater than a set value. For example, the bias control unit 300 repeats the adjustment of the bias voltage BS7 until the monitor signal MO-3 (monitor current) falls within a predetermined range.

[0074] 6 shows an example of optical power depending on a phase shift as a specific example of bias control of a phase shifter according to some embodiments. The phase shift in FIG. 6 is the phase difference between the modulated optical signal of IQ modulator 110-1 and the modulated optical signal of IQ modulator 110-2, and corresponds to, for example, bias voltage BS7.

[0075] 6, when the phase shift (bias voltage BS7) deviates from the optimal value (0), light leaks from the waveguide in optical coupler 160, causing the optical power of monitor signal MO-3 to decrease, and when the phase shift (bias voltage BS7) approaches the optimal value (0), the light leaking from the waveguide in optical coupler 160 decreases, causing the optical power of monitor signal MO-3 to increase. For example, bias control unit 300 adjusts the phase shift (bias voltage BS7) so that the power of the light modulated by multi-parallel modulator 100 increases (for example, to a set value or more).

[0076] Thereafter, the optical transmitter 1 repeats the bias control process of the IQ modulator 110-1 (S102 to S104), the bias control process of the IQ modulator 110-2 (S105 to S107), and the bias control process of the phase shifter 140 (S108 to S110).

[0077] As described above, in this embodiment, in a multi-parallel modulator that performs 16QAM modulation by connecting two IQ modulators in parallel, a monitor PD is placed in the output stage of the two IQ modulators, and the bias voltages of the two IQ modulators are adjusted according to the monitoring results, thereby optimizing the bias voltage of each IQ modulator.Furthermore, a monitor PD is placed in the final stage of the multi-parallel modulator, and the bias voltage of the phase shifter immediately preceding the final stage is adjusted according to the monitoring results, thereby suppressing phase shifts in modulated light from the multi-parallel modulator.

[0078] (Embodiment 3) Next, a description will be given of a third embodiment. In this embodiment, an example of bias control in a multi-parallel modulator that performs 256QAM modulation will be described.

[0079] 7 shows an example of the configuration of a multi-parallel modulator 100 according to some embodiments. In the example of FIG. 7, the multi-parallel modulator 100 is a 256QAM modulator that performs 256QAM modulation.

[0080] The multi-parallel modulator 100 includes four IQ modulators 110, which are QPSK modulators. The four IQ modulators 110-1 to 110-4 are arranged in parallel to perform 256QAM modulation. For example, the IQ modulators 110-1 to 110-2 perform 16QAM modulation, as in FIG. 3. The IQ modulators 110-3 to 110-4 also perform 16QAM modulation, and two 16QAM-modulated lights are combined to generate 256QAM-modulated light.

[0081] 7, the multi-parallel modulator 100 includes an optical coupler 151, an optical coupler 150, IQ modulators 110-1 to 110-4, monitor PDs 120-1 to 120-4, attenuators 130-1 and 130-2, phase shifters 140-1 and 140-2, optical couplers 160-1 and 160-2, monitor PDs 170-1 and 170-2, an attenuator 131, a phase shifter 141, an optical coupler 161, and a monitor PD 171. The basic configuration of the multi-parallel modulator 100 is the same as that in FIG.

[0082] The IQ modulators 110-1 and 110-2 IQ-modulate the light source light branched via the optical couplers 151 and 150-1 to generate a QPSK modulated optical signal. The optical coupler 160-1 combines the modulated optical signal modulated by the IQ modulator 110-1 with the modulated optical signal modulated by the IQ modulator 110-2 and passed through the attenuator 130-1 and phase shifter 140-1.

[0083] The IQ modulators 110-3 and 110-4 IQ-modulate the light source light branched via the optical couplers 151 and 150-2 to generate a QPSK modulated optical signal. The optical coupler 160-2 combines the modulated optical signal modulated by the IQ modulator 110-3 with the modulated optical signal modulated by the IQ modulator 110-4 and passed through the attenuator 130-2 and phase shifter 140-2.

[0084] The optical coupler 161 combines the modulated optical signal combined by the optical coupler 160-1 with the modulated optical signal combined by the optical coupler 160-2 and passed through the attenuator 131 and phase shifter 141, and outputs the combined 256QAM modulated optical signal.

[0085] 7, the multi-parallel modulator 100 includes monitor PDs 120-1 to 120-4, 170-1 to 170-2, and 171. Monitor PD 120-1 monitors the modulated optical signal modulated by IQ modulator 110-1 and branched by optical coupler 114-1, and outputs a monitor signal MO-1. Monitor PD 120-2 monitors the modulated optical signal modulated by IQ modulator 110-2 and branched by optical coupler 114-2, and outputs a monitor signal MO-2. Monitor PD 170-1 monitors the modulated optical signal modulated by IQ modulators 110-1 and 110-2 and branched by optical coupler 160-1, and outputs a monitor signal MO-3.

[0086] Monitor PD 120-3 monitors the modulated optical signal modulated by IQ modulator 110-3 and branched by optical coupler 114-3, and outputs monitor signal MO-4. Monitor PD 120-4 monitors the modulated optical signal modulated by IQ modulator 110-4 and branched by optical coupler 114-4, and outputs monitor signal MO-5. Monitor PD 170-2 monitors the modulated optical signal modulated by IQ modulator 110-3 and IQ modulator 110-4, and branched by optical coupler 160-2, and outputs monitor signal MO-6.

[0087] The monitor PD 171 monitors the modulated optical signals modulated by the IQ modulators 110-1 to 110-4 and branched by the optical coupler 161, and outputs a monitor signal MO-7.

[0088] 7, the bias control unit 300 performs bias control in the same manner as in the second embodiment. First, based on the monitor signal MO-1 from the monitor PD 120-1, the bias control unit 300 controls the bias voltages BS1-1 to BS3-1 applied to the MZ modulators 112a-1, 112b-1, and phase shifter 113-1 included in the IQ modulator 110-1, respectively, so that the value of the objective function becomes equal to or less than a set value. Also, based on the monitor signal MO-2 from the monitor PD 120-2, the bias control unit 300 controls the bias voltages BS4-1 to BS6-1 applied to the MZ modulators 112a-2, 112b-2, and phase shifter 113-2 included in the IQ modulator 110-2, respectively, so that the value of the objective function becomes equal to or less than a set value. Next, the bias control unit 300 controls the bias voltage BS7-1 applied to the phase shifter 140-1 based on the monitor signal MO-3 of the monitor PD 170-1 so that the optical power becomes equal to or greater than the set value.

[0089] The bias control of the IQ modulators 110-3 and 110-4 is the same as that of the IQ modulators 110-1 and 110-2. That is, based on the monitor signal MO-4 of the monitor PD 120-3, the bias control unit 300 controls the bias voltages BS1-2 to BS3-2 applied to the MZ modulator 112a-3, MZ modulator 112b-3, and phase shifter 113-3 included in the IQ modulator 110-3, respectively, so that the value of the objective function becomes equal to or less than a set value. Also, based on the monitor signal MO-5 of the monitor PD 120-4, the bias voltages BS4-2 to BS6-2 applied to the MZ modulator 112a-4, MZ modulator 112b-4, and phase shifter 113-4 included in the IQ modulator 110-4, respectively, so that the value of the objective function becomes equal to or less than a set value. Next, the bias control unit 300 controls the bias voltage BS7-2 applied to the phase shifter 140-2 based on the monitor signal MO-6 of the monitor PD 170-2 so that the optical power becomes equal to or greater than the set value.

[0090] After controlling the bias voltage BS7-1 of the phase shifter 140-1 and the bias voltage BS7-2 of the phase shifter 140-2, the bias control unit 300 controls the bias voltage BS8 applied to the phase shifter 141 based on the monitor signal MO-7 of the monitor PD 171 so that the optical power is equal to or greater than the set value.

[0091] In Fig. 7, monitor PDs 170-1 and 170-2 may be omitted. Fig. 8 shows an example configuration of multi-parallel modulator 100 when monitor PDs 170-1 and 170-2 are omitted. In this case, bias control unit 300 controls bias voltage BS7-1 of phase shifter 140-1, bias voltage BS7-2 of phase shifter 140-2, and bias voltage BS8 of phase shifter 141 based on monitor signal MO-7 of monitor PD 171 so that the optical power is equal to or greater than a set value.

[0092] As described above, in a multi-parallel modulator that performs 256-QAM modulation by connecting four IQ modulators in parallel, similar to embodiment 2, a monitor PD can be placed in the output stage of the four IQ modulators, and the bias voltage of each IQ modulator can be optimized. Furthermore, by placing a monitor PD in the final stage of the multi-parallel modulator and adjusting the bias voltage of the phase shifter, the phase shift of the modulated light of the multi-parallel modulator can be suppressed.

[0093] The present disclosure is not limited to the above-described embodiment, and can be modified as appropriate within the scope of the present disclosure.

[0094] Each component in the above-described embodiments may be configured by hardware or software, or both, and may be configured by a single piece of hardware or software, or may be configured by multiple pieces of hardware or software. Each function (processing) of the bias control unit and the like may be realized by a computer 30 having a processor 31 such as a CPU and a memory 32 serving as a storage device, as shown in FIG. 9. For example, a program for performing the method (control method) in the embodiment may be stored in the memory 32, and each function may be realized by the processor 31 executing the program stored in the memory 32.

[0095] These programs include instructions (or software code) that, when loaded into a computer, cause the computer to perform one or more functions described in the embodiments. The programs may be stored on a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, computer-readable media or tangible storage media include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD) or other memory technology, CD-ROM, digital versatile disc (DVD), Blu-ray® disc or other optical disk storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage device. The programs may also be transmitted on a transitory computer-readable medium or communication medium. By way of example and not limitation, transitory computer-readable media or communication media include electrical, optical, acoustic, or other forms of propagated signals.

[0096] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0097] Each drawing is merely an example for describing one or more embodiments. Each drawing may relate not only to one particular embodiment, but also to one or more other embodiments. As will be understood by those skilled in the art, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings to create, for example, an embodiment not explicitly shown or described. Not all features or steps shown in any one drawing are necessary to describe an exemplary embodiment, and some features or steps may be omitted. The order of steps described in any drawing may be changed as appropriate.

[0098] A part or all of the above-described embodiments can be described as, but not limited to, the following supplementary notes. (Appendix 1) a first IQ modulator that generates a first modulated light; a second IQ modulator connected in parallel with the first IQ modulator to generate second modulated light; a phase shifter that shifts the phase of the second modulated light; an optical combiner that combines the first modulated light and the phase-shifted second modulated light to generate a third modulated light; a bias control means for controlling first to third bias signals to be applied to the first IQ modulator, the second IQ modulator, and the phase shifter, respectively, based on the first to third modulated lights; An optical transmitter comprising: (Appendix 2) a first optical monitor to a third optical monitor for monitoring the first modulated light, the second modulated light, and the third modulated light; the bias control means controls the first to third bias signals based on the monitoring results of the first to third optical monitors, respectively; 10. The optical transmitter of claim 1. (Appendix 3) the bias control means controls the first and second bias signals to be provided to the first IQ modulator and the second IQ modulator, and then controls the third bias signal to be provided to the phase shifter. 10. The optical transmitter of claim 1. (Appendix 4) the bias control means controls the first and second bias signals so that DC components of the first modulated light and the second modulated light are equal to or less than a predetermined value, respectively. 4. An optical transmitter according to any one of claims 1 to 3. (Appendix 5) the bias control means controls the third bias signal so that the power of the third modulated light becomes equal to or greater than a predetermined value. 4. An optical transmitter according to any one of claims 1 to 3. (Appendix 6) the first and second IQ modulators are semiconductor modulators; the first and second IQ modulators, the phase shifter, and the optical coupler are formed on a single semiconductor substrate. 10. The optical transmitter of claim 1. (Appendix 7) the first and second IQ modulators are semiconductor modulators; the first and second IQ modulators, the phase shifter, the optical coupler, and the first to third optical monitors are formed on a single semiconductor substrate; 10. The optical transmitter of claim 2. (Appendix 8) a plurality of IQ modulators that generate a plurality of modulated lights and are connected in parallel; an optical combiner that combines the plurality of modulated lights to generate an output modulated light; a phase shifter that shifts the phase of modulated light included in the plurality of modulated light beams, the phase shifter being located upstream of the optical coupler; a bias control means for controlling a plurality of bias signals to be applied to the plurality of IQ modulators and the phase shifter, respectively, based on the plurality of modulated light beams and the output modulated light beam; An optical transmitter comprising: (Appendix 9) generating first modulated light by a first IQ modulator; generating second modulated light by a second IQ modulator connected in parallel with the first IQ modulator; shifting the phase of the second modulated light by a phase shifter; combining the first modulated light and the phase-shifted second modulated light with an optical combiner to generate a third modulated light; controlling first to third bias signals to be applied to the first IQ modulator, the second IQ modulator, and the phase shifter, respectively, based on the first to third modulated lights; A method for controlling an optical transmitter. (Appendix 10) monitoring the first to third modulated lights by first to third optical monitors, respectively; controlling the first to third bias signals based on the monitoring results of the first to third optical monitors, respectively; 10. A method for controlling an optical transmitter according to claim 9.

[0099] Some or all of the elements (e.g., configurations and functions) described in Supplementary Notes 2 to 7 that are dependent on Supplementary Note 1 (optical transmitter) may also be dependent on Supplementary Note 8 (optical transmitter) and Supplementary Note 9 (control method) in the same dependent relationship as Supplementary Notes 2 to 7. Some or all of the elements described in any Supplementary Note may be applied to various hardware, software, recording means for recording software, systems, and methods. [Explanation of symbols]

[0100] 1 Optical transmitter 10 Multi-parallel modulator 11, 11-1 to 11-2 IQ modulator 12 Phase Shifter 13 Optical coupler 20 Optical transmitter 21 Bias control section 30 Computer 31 processors 32 memory 100 Multi-parallel modulator 110, 110-1 to 110-4 IQ modulator 111, 111-1 to 111-4 Optical coupler 112a, 112a-1 to 112a-4 MZ modulator 112b, 112b-1 to 112b-4 MZ modulator 113, 113-1 to 113-4 Phase shifter 114, 114-1 to 114-4 Optical coupler 120, 120-1 to 120-4 Monitor PD 130, 130-1 to 130-2 attenuator 131 Attenuator 140, 140-1 to 140-2 Phase shifter 141 Phase Shifter 150, 150-1 to 150-2 optical coupler 151 Optical Coupler 160, 160-1 to 160-2 optical coupler 161 Optical Coupler 170, 170-1~170-2 Monitor PD 171 Monitor PD 200 light source 300 Bias control section

Claims

1. a first IQ modulator that generates a first modulated light; a second IQ modulator connected in parallel with the first IQ modulator to generate second modulated light; a phase shifter that shifts the phase of the second modulated light; an optical combiner that combines the first modulated light and the phase-shifted second modulated light to generate a third modulated light; a bias control means for controlling first to third bias signals to be applied to the first IQ modulator, the second IQ modulator, and the phase shifter, respectively, based on the first to third modulated lights; An optical transmitter comprising:

2. a first optical monitor to a third optical monitor for monitoring the first modulated light, the second modulated light, and the third modulated light; the bias control means controls the first to third bias signals based on the monitoring results of the first to third optical monitors, respectively; 2. The optical transmitter according to claim 1.

3. the bias control means controls the first and second bias signals to be applied to the first IQ modulator and the second IQ modulator, and then controls the third bias signal to be applied to the phase shifter.

2. The optical transmitter according to claim 1.

4. the bias control means controls the first and second bias signals so that DC components of the first modulated light and the second modulated light are equal to or less than a predetermined value, respectively; 4. The optical transmitter according to claim 1.

5. the bias control means controls the third bias signal so that the power of the third modulated light becomes equal to or greater than a predetermined value.

4. The optical transmitter according to claim 1.

6. the first and second IQ modulators are semiconductor modulators; the first and second IQ modulators, the phase shifter, and the optical coupler are formed on a single semiconductor substrate; 2. The optical transmitter according to claim 1.

7. the first and second IQ modulators are semiconductor modulators; the first and second IQ modulators, the phase shifter, the optical coupler, and the first to third optical monitors are formed on a single semiconductor substrate; 3. The optical transmitter according to claim 2.

8. a plurality of IQ modulators that generate a plurality of modulated lights and are connected in parallel; an optical combiner that combines the plurality of modulated lights to generate an output modulated light; a phase shifter that shifts the phase of modulated light included in the plurality of modulated light beams, the phase shifter being located upstream of the optical coupler; a bias control unit that controls a plurality of bias signals to be applied to the plurality of IQ modulators and the phase shifter, respectively, based on the plurality of modulated light beams and the output modulated light beam; An optical transmitter comprising:

9. generating first modulated light by a first IQ modulator; generating second modulated light by a second IQ modulator connected in parallel with the first IQ modulator; shifting the phase of the second modulated light by a phase shifter; combining the first modulated light and the phase-shifted second modulated light with an optical combiner to generate a third modulated light; controlling first to third bias signals to be applied to the first IQ modulator, the second IQ modulator, and the phase shifter, respectively, based on the first to third modulated lights; A method for controlling an optical transmitter.

10. monitoring the first to third modulated lights by first to third optical monitors, respectively; controlling the first to third bias signals based on the monitoring results of the first to third optical monitors, respectively; The method for controlling an optical transmitter according to claim 9.

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