Optical transmitter and method for controlling optical transmitter

Abstract

To provide an optical transmitter and a method for controlling the optical transmitter with which it is possible to suitably perform bias control of a modulator and wavelength control of a light source.SOLUTION: The optical transmitter comprises: a mode synchronous laser light source; a plurality of semiconductor modulators; an optical filter for passing light of a passband through from modulated light having been modulated by a first semiconductor modulator; an optical monitor for monitoring the light having passed through; wavelength control means for controlling the wavelength of light-source light of the mode synchronous laser light source on the basis of results of monitoring; and bias control means for controlling the bias voltage of the first semiconductor modulator on the basis of results of monitoring. The mode synchronous laser light source is provided with a reflection type semiconductor optical amplifier, an external resonator including a multiplication filter for multiplying the vertical mode interval of the mode synchronous laser light source, and a heater for multiplication filter adjustment capable of heating the multiplication filter. The wavelength control means controls a heater current injected into the heater for multiplication filter adjustment so as to control the wavelength of the mode synchronous laser light source.SELECTED DRAWING: Figure 4

Description

Technical Field

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

Background Art

[0002] In recent years, with the popularization of the Internet, 5G, etc., research on high-speed and large-capacity optical communication, which is used as a basic technology, has been underway. For example, by using a digital coherent method that combines an optical phase modulation method and a polarization multiplexing separation technique, a large capacity exceeding 100 Gbps (Giga bit per second) has been realized. As such a modulator for high-speed transmission, an MZ modulator (MZM: Mach-Zehnder Modulator) is used.

[0003] As a related technology, Patent Document 1 is known. Patent Document 1 describes an LN (LiNbO3: lithium niobate) modulator that constitutes an MZ modulator and a control unit that controls the bias voltage of the LN modulator.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] On the other hand, the development of a semiconductor modulator in which an MZ modulator is formed on a semiconductor substrate has been progressing, and in the future, miniaturization and power saving are expected by integrating optical devices including the semiconductor modulator. However, in related technologies, integration of optical devices such as semiconductor modulators is not considered. For this reason, in related technologies, when integrating a modulator and a light source in an optical transmitter, it may be difficult to suitably perform bias control of the modulator and wavelength control of the light source.

[0006] In view of such problems, one object of the present disclosure is to provide an optical transmitter and a method for controlling the optical transmitter that can suitably perform bias control of a modulator and wavelength control of a light source.

Means for Solving the Problems

[0007] An optical transmitter according to an aspect of the present disclosure includes a mode-locked laser light source that generates multi-wavelength light source light, a plurality of semiconductor modulators that modulate the generated multi-wavelength light source light into modulated light for each wavelength, an optical filter that transmits light in a transmission band from the modulated light modulated by a first semiconductor modulator among the plurality of semiconductor modulators, an optical monitor that monitors the transmitted light, wavelength control means for controlling the wavelength of the light source light based on the monitored result, and bias control means for controlling the bias voltage of the first semiconductor modulator based on the monitored result. The mode-locked laser light source includes a reflective semiconductor optical amplifier that emits light, and an external resonator that resonates the light from the reflective semiconductor optical amplifier, the external resonator including a frequency-doubling filter that doubles the longitudinal mode interval of the mode-locked laser light source, and a frequency-doubling filter adjustment heater that can heat the frequency-doubling filter. The wavelength control means controls the wavelength of the light source light by controlling the heater current injected into the frequency-doubling filter adjustment heater.

[0008] A method for controlling an optical transmitter according to an aspect of the present disclosure includes generating light source light of multiple wavelengths by a mode-locked laser light source, modulating the generated light source light of multiple wavelengths into modulated light for each wavelength by a plurality of semiconductor modulators, transmitting light within a transmission band from the modulated light modulated by a first semiconductor modulator among the plurality of semiconductor modulators, monitoring the transmitted light, controlling the wavelength of the light source light based on the monitored result, and controlling the bias voltage of the first semiconductor modulator based on the monitored result. In the mode-locked laser light source, light is emitted by a reflective semiconductor optical amplifier, and in an external resonator that resonates light from the reflective semiconductor optical amplifier, a multiplication filter multiplies the longitudinal mode interval of the mode-locked laser light source. Controlling the wavelength includes controlling the wavelength of the light source light by controlling a heater current injected into a heater for adjusting the multiplication filter, which can be heated.

Advantages of the Invention

[0009] According to the present disclosure, bias control of the modulator and wavelength control of the light source can be suitably performed.

Brief Description of the Drawings

[0010]

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Embodiments for Carrying Out the Invention

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

[0012] (Consideration of Related Technologies) First, the related technologies will be examined. FIG. 1 shows the configuration of an optical transmitter 9 in the related technologies based on, for example, the description in Patent Document 1. FIG. 1 is an example configuration of an optical transmitter using an LN modulator. As shown in FIG. 1, the related optical transmitter 9 includes a light source 901, an LN modulator 902, a monitor PD 903, and a bias control unit 904.

[0013] The light source 901 is an external light source provided outside the LN modulator 902 and is connected to the LN modulator 902 via an optical fiber or the like. The light source 901 is, for example, a DFB (Distributed Feedback) laser. The LN modulator 902 modulates the light output from the light source 901. The monitor PD 903 is a light receiving unit provided outside the LN modulator 902. The light modulated by the LN modulator 902 is received and monitored by the monitor PD 903. The bias control unit 904 performs feedback control on the bias voltage of the LN modulator 902 so that the output deviation due to the bias point (operating point) vibration of the LN modulator 902 becomes small according to the monitor result of the monitor PD 903.

[0014] In addition, since the wavelength of the light output from the light source fluctuates due to aging deterioration or the like, it is necessary to perform wavelength control so that light of a desired wavelength is output from the light source. As a related technology, a wavelength control method using a temperature adjustment function can be considered. FIG. 2 is a schematic side view showing an example arrangement of the temperature adjustment function of the light source in the related technologies. As shown in FIG. 2, in the related technology, a TEC (Thermoelectric Cooler) 905 is arranged under the external light source 901 as the temperature adjustment function. By adjusting the temperature of the entire lower surface of the light source 901 with the TEC 905, the wavelength of the light source 901 is controlled.

[0015] However, in related optical transmitters, since the light source is external to the LN modulator and the temperature adjustment function of the light source is significant, it becomes an obstacle to miniaturization. Also, when attempting to integrate the semiconductor modulator and the light source with the LN modulator as a semiconductor modulator, it is difficult to locally control the wavelength of the light source with a large temperature adjustment function such as a TEC. Therefore, in the embodiments, it is made possible to perform wavelength control of the light source and bias control of the semiconductor modulator in close proximity with a reduced area.

[0016] (Overview of Embodiments) First, the overview of the embodiments will be described. Here, it will be described as an overview of the embodiments, but it may also be implemented as one embodiment.

[0017] FIG. 3 shows a configuration example of an optical transmitter 10 according to some embodiments. The optical transmitter 10 is, for example, an optical transmitter for digital coherent communication. In the example of FIG. 3, the optical transmitter 10 includes a light source 11, a semiconductor modulator 12, an optical filter 13, an optical monitor 14, a bias control unit 15, and a wavelength control unit 16. For example, the light source 11 and the semiconductor modulator 12 may be formed on the same semiconductor substrate. Also, the light source 11, the semiconductor modulator 12, the optical filter 13, and the optical monitor 14 may be formed on the same semiconductor substrate.

[0018] The light source 11 generates and outputs source light. The light source 11 is, for example, a semiconductor laser such as a DFB laser. The light source 11 may be a single-wavelength light source that generates single-wavelength source light or a multi-wavelength light source that generates multi-wavelength source light.

[0019] The semiconductor modulator 12 modulates the source light generated by the light source 11 and outputs the modulated light. The semiconductor modulator 12 is, for example, an IQ (In-phase and Quadrature) modulator.

[0020] The optical filter 13 transmits the light within the transmission band from the modulated light modulated by the semiconductor modulator 12. The optical filter 13 is, for example, a semiconductor optical BPF (Band Pass Filter). The modulated light modulated by the semiconductor modulator 12 may be branched by a demultiplexer (brancher) into an output optical signal for output and a monitor optical signal for monitoring, and the optical filter 13 may transmit the branched monitor optical signal.

[0021] The optical monitor 14 monitors the light transmitted through the optical filter 13 and outputs the monitored result. The optical monitor 14 is, for example, a semiconductor PD (Photo Detector).

[0022] The wavelength control unit 16 controls the wavelength of the light source light generated by the light source 11 based on the result monitored by the optical monitor 14 after passing through the optical filter 13. For example, the light source 11 is provided with a heater that can be heated, and the wavelength control unit 16 may control the calorific value of the heater according to the monitoring result of the optical monitor 14. The wavelength control unit 16 may control the wavelength of the light source light by controlling the heater current injected (supplied) to the heater.

[0023] For example, when the power of the light monitored by the optical monitor 14 has decreased by a predetermined specified value or more from the initial value, the wavelength control unit 16 may control the heater current. The initial value is the power of the light initially set when the wavelength of the light source light of the light source 11 is not deviated. For example, when the power of the light monitored by the optical monitor 14 has decreased by a predetermined specified value or more from the initial value, the wavelength control unit 16 increases and decreases the heater current by a certain amount, and controls the heater current according to the amount of change in the power of the light monitored when the heater current is increased and decreased. In this case, the wavelength control unit 16 controls the heater current according to the slope of the change in the power of the light with respect to the increase and decrease of the heater current. That is, the wavelength control unit 16 may control the heater current according to whether the power of the light monitored when the heater current is increased increases or decreases, and whether the power of the light monitored when the heater current is decreased increases or decreases.

[0024] The bias control unit 15 controls the bias voltage of the semiconductor modulator 12 based on the result monitored by the optical monitor 14 after passing through the optical filter 13. The bias control unit 15 may control the bias voltage based on the power of the light monitored by the optical monitor 14, or may control the bias voltage based on the function value obtained by converting the power of the light monitored by the optical monitor 14 using a predetermined function. The predetermined function is an objective function for bias control.

[0025] For example, the bias control unit 15 may control the bias voltage when the function value based on the power of the light monitored by the optical monitor 14 increases by a predetermined specified value or more from the initial value. The initial value is the bias voltage initially set when the bias point (operating point) of the semiconductor modulator 12 is not deviated. For example, when the function value based on the power of the light monitored by the optical monitor 14 increases by a predetermined specified value or more from the initial value, the bias control unit 15 increases and decreases the bias voltage, and controls the bias voltage according to the amount of change in the function value based on the power of the light monitored when the bias voltage is increased and decreased. In this case, the bias control unit 15 controls the bias voltage according to the slope of the function value with respect to the increase and decrease of the bias voltage. That is, the bias control unit 15 may control the bias voltage according to whether the function value based on the power of the light monitored when the bias voltage is increased increases or decreases, and whether the function value based on the power of the light monitored when the bias voltage is decreased increases or decreases.

[0026] FIG. 4 is a configuration example of the optical transmitter 10 according to some embodiments, and shows a configuration example when the light source 11 is a multi-wavelength light source.

[0027] In the example of FIG. 4, the light source 11 is a mode-locked laser light source 20. Not limited to the mode-locked laser light source 20, other multi-wavelength light sources may also be used. When the light source 11 is a multi-wavelength light source, the optical transmitter 10 includes a plurality of semiconductor modulators 12. The plurality of semiconductor modulators 12 modulate the multi-wavelength source light generated by the multi-wavelength light source into modulated light for each wavelength. The optical filter 13 transmits the light in the transmission band from the modulated light modulated by the first semiconductor modulator among the plurality of semiconductor modulators 12. The wavelength control unit 16 controls the wavelength of the source light of the multi-wavelength light source based on the result monitored by the optical monitor 14 from the first semiconductor modulator via the optical filter 13. The bias control unit 15 controls the bias voltage of the first semiconductor modulator based on the result monitored by the optical monitor 14 from the first semiconductor modulator via the optical filter 13.

[0028] In the example of FIG. 4, the mode-locked laser light source 20 includes a reflective semiconductor optical amplifier 21 that emits light, and an external resonator 22 that resonates the light from the reflective semiconductor optical amplifier 21. The external resonator 22 includes a frequency doubling filter 23 that doubles the longitudinal mode interval of the mode-locked laser light source 20. Further, a frequency doubling filter adjustment heater 24 that can heat the frequency doubling filter 23 may be disposed in the external resonator 22. In this case, the wavelength control unit 16 controls the wavelength of the source light of the mode-locked laser light source 20 by controlling the heater current injected into the frequency doubling filter adjustment heater 24.

[0029] Further, the external resonator 22 may include an optical waveguide for adjusting the resonator length that includes the reflective semiconductor optical amplifier 21 and the external resonator 22. A heater for adjusting the resonator length that can heat the optical waveguide for adjusting the resonator length may be disposed in the external resonator 22. In this case, the wavelength control unit 16 may control the wavelength of the light source light of the mode-locked laser light source 20 by controlling the heater current injected into the heater for adjusting the doubling filter and the heater for adjusting the resonator length. For example, the doubling filter 23 is a ring resonator filter, and the wavelength control unit 16 controls the heater current injected into the heater for adjusting the doubling filter and the heater for adjusting the resonator length so that the round-trip length of the doubling filter 23 is an integer fraction of the resonator round-trip length including the reflective semiconductor optical amplifier 21 and the external resonator 22. The resonator round-trip length is the round-trip length (optical length) when light circulates in the resonator (when going and coming back, that is, when making a round trip).

[0030] As described above, in the embodiment, in the optical transmitter, in front of the optical monitor, an optical filter that transmits the light monitored by the optical monitor for wavelength control and bias control is disposed. Thereby, wavelength control and bias control can be suitably performed with one optical monitor. Further, by forming a light source, a semiconductor modulator, etc. on the same semiconductor substrate, miniaturization and power reduction of the optical transmitter are possible, and wavelength control of the light source and bias control of the semiconductor modulator can be performed in a space-saving manner in the vicinity. Furthermore, when the light source is a multi-wavelength light source such as a mode-locked laser, by controlling the heater current injected into the heater disposed in the multi-wavelength light source, the wavelength shift of the multi-wavelength light can be adjusted all at once.

[0031] (Embodiment 1) Next, Embodiment 1 will be described. In this embodiment, an example of performing wavelength control and bias control in an optical transmitter including a single-wavelength light source and a semiconductor modulator will be described.

[0032] FIG. 5 shows a configuration example of the optical transmitter 1 according to some embodiments. FIGS. 6 and 7 are schematic side views showing arrangement examples of respective parts of the semiconductor substrate 200 according to some embodiments. FIG. 8 shows a specific configuration example of the semiconductor modulator 120 according to some embodiments.

[0033] The optical transmitter 1 according to the present embodiment is, for example, an optical transmitter for digital coherent communication. In the example of FIG. 5, the optical transmitter 1 includes a single-wavelength light source 110, a semiconductor modulator 120, a demultiplexer 130, a band-pass optical filter 140, a monitor PD 150, a bias control unit 160, and a wavelength control unit 170.

[0034] For example, the single-wavelength light source 110, the semiconductor modulator 120, the demultiplexer 130, the band-pass optical filter 140, and the monitor PD 150 are formed on one semiconductor substrate 200. At least the single-wavelength light source 110 and the semiconductor modulator 120 may be formed on the semiconductor substrate 200. The semiconductor substrate 200 is, for example, a semiconductor substrate such as indium phosphide (InP), gallium arsenide (GaAs), or silicon (Si), but may also be other semiconductor substrates such as germanium (Ge).

[0035] The bias control unit 160 and the wavelength control unit 170 are formed as semiconductor chips separate from the semiconductor substrate 200. The bias control unit 160 and the wavelength control unit 170 may be constituted by separate semiconductor chips or may be constituted by one semiconductor chip. The bias control unit 160 and the wavelength control unit 170 may be constituted by hardware or software, or both. The functions of the bias control unit 160 and the wavelength control unit 170 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 160 and the wavelength control unit 170 may be mounted on the semiconductor substrate 200 by flip chip or the like. A semiconductor device including all the configurations of FIG. 5 may be used as one semiconductor package.

[0036] The single-wavelength light source 110 generates a light source light SO1 that is light of a preset single wavelength (set wavelength). The single-wavelength light source 110 is a semiconductor laser formed on a semiconductor substrate 200 such as indium phosphide or silicon. The single-wavelength light source 110 may be, for example, a DFB laser, a DBR (Distributed Bragg Reflector) laser, or an external cavity laser (ECL). The single-wavelength light source 110 is a wavelength-fixed light source in which the set wavelength of the light source light SO1 is fixed, but it may also be a wavelength-variable light source in which the set wavelength of the light source light SO1 is variable.

[0037] FIG. 6 shows an example of mounting a single-wavelength light source 110 on a semiconductor substrate 200. In the example of FIG. 6, the single-wavelength light source 110 is configured by laminating a lower cladding layer 201, an active layer 202, and an upper cladding layer 203 on the semiconductor substrate 200, for example. Further, a heater 204 is formed on the uppermost layer of the single-wavelength light source 110 (semiconductor substrate 200), that is, on the upper cladding layer 203. The heater 204 is a heating element capable of heating the single-wavelength light source 110. The heater 204 is formed of a temperature-controllable resistor such as a TiN heater. The heater 204 is formed corresponding to the formation region of the single-wavelength light source 110 in the semiconductor substrate 200. For example, the heater 204 may be formed within the range of the formation region of the single-wavelength light source 110. The heater 204 may have the same shape and size as the formation region (active layer 202) of the single-wavelength light source 110 in plan view, or may have a shape and size smaller than the formation region of the single-wavelength light source 110. The heater 204 may be larger than the formation region of the single-wavelength light source 110, but preferably does not overlap with the formation regions of other optical devices such as the semiconductor modulator 120. The heater 204 generates heat according to the heater current HT supplied from the wavelength control unit 170, and the energy gap of the active layer 202 decreases due to the heat of the heater 204 and the center wavelength shifts to the longer wavelength side, thereby controlling the wavelength of the light source light SO1 generated in the active layer 202. Since it is only necessary to adjust the temperature of the active layer 202, the heater 204 is not limited to being on the upper cladding layer 203, and may be within the upper cladding layer 203, or may be formed under the lower cladding layer 201 or under the semiconductor substrate 200. Further, when the single-wavelength light source 110 is an external resonance type laser, a heater may be formed on the silicon waveguide constituting the external resonance type laser, and the wavelength may be controlled by adjusting the refractive index of the silicon waveguide with the heat of the formed heater.

[0038] FIG. 7 shows another example of mounting the single-wavelength light source 110 on the semiconductor substrate 200. In the example of FIG. 7, the semiconductor chip of the single-wavelength light source 110 is flip-chip mounted on the semiconductor substrate 200. That is, the single-wavelength light source 110 may be monolithically mounted on the semiconductor substrate 200 as shown in FIG. 6, or may be hybridly mounted on the semiconductor substrate 200 as shown in FIG. 7. In the example of FIG. 7, the single-wavelength light source 110 is mounted on the semiconductor substrate 200 by joining the heater 204 (the lowermost in FIG. 7) formed on the single-wavelength light source 110 and the semiconductor substrate 200 via the solder bump 205.

[0039] The semiconductor modulator 120 is an IQ modulator for digital coherent communication. The semiconductor modulator 120 is a semiconductor modulator formed on a semiconductor substrate 200 such as indium phosphide or silicon. The semiconductor modulator 120 modulates the light source light SO1 output from the single-wavelength light source 110 into a modulated optical signal SO2. The semiconductor modulator 120 performs IQ modulation (coherent modulation) on the light source light SO1 output from the single-wavelength light source 110 and outputs the modulated modulated optical signal SO2.

[0040] In the example of FIG. 8, the semiconductor modulator 120 is constituted by an MZ interferometer including two MZ modulators. The semiconductor modulator 120 includes a demultiplexer 121, MZ modulators 122-1 and 122-2, a phase shifter 123, and a multiplexer 124. The demultiplexer 121 branches (demultiplexes) the light source light SO1 from the single-wavelength light source 110 into light for Ich (in-phase component) and light for Qch (quadrature component).

[0041] The MZ modulator 122-1 is, for example, a modulator for Ich. The data signal DT1 for Ich generated from the transmission data signal is applied to the MZ modulator 122-1 as a drive signal. The MZ modulator 122-1 modulates the light for Ich branched by the demultiplexer 121 according to the data signal DT1. A bias voltage BS1 for adjusting the bias point is applied to the MZ modulator 122-1 from the bias control unit 160.

[0042] The MZ modulator 122-2 is, for example, a modulator for Qch. The data signal DT2 for Qch generated from the transmission data signal is applied to the MZ modulator 122-2 as a drive signal. The MZ modulator 122-2 modulates the light for Qch branched by the optical demultiplexer 121 according to the data signal DT2. Also, a bias voltage BS2 for adjusting the bias point is applied to the MZ modulator 122-2 from the bias control unit 160.

[0043] The phase shifter 123 shifts the phase of the optical signal of Ich or the phase of the optical signal of Qch so that the phases of the optical signal of Ich and the optical signal of Qch are orthogonal. In this example, the phase shifter 123 shifts the phase of the modulated optical signal of Qch modulated by the MZ modulator 122-2 by π / 2. The phase shifter 123 may be configured by an MZ interferometer in the same manner as the MZ modulator, or may be configured to shift the phase by changing the refractive index of the waveguide. Also, a bias voltage BS3 for adjusting the phase shift amount is applied to the phase shifter 123 from the bias control unit 160. When the phase shifter 123 is configured by an MZ interferometer, the phase shift amount may be controlled by the bias voltage BS3 applied to the MZ interferometer. A heater is formed in the waveguide constituting the phase shifter 123, and the phase shift amount may be controlled by changing the refractive index of the waveguide due to the heat generated by applying the bias voltage BS3 to the formed heater.

[0044] The multiplexer 124 multiplexes the optical signal modulated by the MZ modulator 122-1 and the optical signal modulated by the MZ modulator 122-2 and phase-shifted by the phase shifter 123, and outputs the multiplexed modulated optical signal SO2.

[0045] The optical demultiplexer 130 branches the modulated optical signal SO2 generated by the semiconductor modulator 120 (IQ modulator) into an output optical signal SO3 for output and a monitor optical signal SO4 for monitoring. The branched output optical signal SO3 is output to the outside (such as an optical transmission line) as transmission light. Note that the multiplexer 124 and the optical demultiplexer 130 of the semiconductor modulator 120 may be a single optical coupler (2×2 optical coupler).

[0046] The band-pass optical filter 140 is an optical BPF (Band Pass Filter) that transmits an optical signal within a predetermined transmission band. The band-pass optical filter 140 receives the monitored optical signal SO4 branched by the optical demultiplexer 130 and transmits the light within the transmission band of the monitored optical signal SO4. The band-pass optical filter 140 is a semiconductor filter formed on a semiconductor substrate 200 such as indium phosphide or silicon. The band-pass optical filter 140 is constituted by, for example, an asymmetric MZ interferometer. The band-pass optical filter 140 may be constituted by a ring resonator or a resonator using opposing mirrors.

[0047] The transmission band of the band-pass optical filter 140 corresponds to the set wavelength (desired target wavelength) of the light source light SO1 output from the single-wavelength light source 110. When the single-wavelength light source 110 is a wavelength-variable light source, the band-pass optical filter 140 may be a band-variable filter, and the transmission band of the band-pass optical filter 140 may be switched according to a change in the set wavelength of the light source light SO1. When the band-pass optical filter 140 is constituted by a resonator, a heater may be formed in the waveguide of the resonator, and the transmission band of the band-pass optical filter may be controlled by adjusting the heat of the formed heater. Further, a plurality of band-pass optical filters having different transmission bands may be provided, and the band-pass optical filter to be used may be switched according to the set wavelength of the light source light SO1 of the single-wavelength light source 110.

[0048] The monitor PD 150 is a PD (Photo Detector) that monitors (detects) the optical signal transmitted through the band-pass optical filter 140. The monitor PD 150 is a semiconductor PD formed on a semiconductor substrate 200 such as indium phosphide or silicon. The monitor PD 150 photoelectrically converts the optical signal transmitted through the band-pass optical filter 140 and outputs a monitored signal MO that has been photoelectrically changed.

[0049] The bias control unit 160 controls the bias voltage BS of the semiconductor modulator 120 based on the monitor signal MO which is the monitoring result of the monitor PD 150. The modulated light output from the semiconductor modulator 120 is branched, and the bias voltage of the semiconductor modulator 120 is feedback-controlled according to the monitoring result of the branched light, thereby automatically optimizing the bias voltage. In this example, the bias control unit 160 controls the bias voltage BS1 applied to the MZ modulator 122-1, the bias voltage BS2 applied to the MZ modulator 122-2, and the bias voltage BS3 applied to the phase shifter 123 based on the monitor signal MO, respectively.

[0050] The wavelength control unit 170 controls the wavelength of the light source light SO1 of the single-wavelength light source 110 based on the monitor signal MO which is the monitoring result of the monitor PD 150. The wavelength control unit 170 controls the calorific value of the heater 204 according to the monitoring result of the monitor PD 150. The wavelength control unit 170 generates a heater current HT according to the monitor signal MO, and supplies the generated heater current HT to the single-wavelength light source 110. For example, the heater current HT is passed through a resistor, and a voltage corresponding to the heater current HT is applied to the heater 204 through the resistor to generate heat in the heater 204, thereby controlling the wavelength of the light source light SO1 of the single-wavelength light source 110. Note that the wavelength control unit 170 may adjust the wavelength of the light source light SO1 by controlling the drive current for driving the single-wavelength light source 110. That is, the wavelength control unit 170 may control the drive current for driving the single-wavelength light source 110 according to the monitoring result of the monitor PD 150. In that case, since the power of the light source light SO1 output by the single-wavelength light source 110 changes, an optical amplifier for adjusting the power of the light source light SO1 may be provided.

[0051] FIG. 9 shows an operation example of the optical transmitter 1 according to some embodiments. In the example of FIG. 9, first, the optical transmitter 1 sets initial values of the heater current HT of the single-wavelength light source 110 and the bias voltage BS of the semiconductor modulator 120 (S101). The wavelength control unit 170 generates a heater current HT with a predetermined initial value, and supplies the generated heater current HT to the single-wavelength light source 110. Specifically, the wavelength control unit 170 applies, via a resistor, a voltage corresponding to the heater current HT with the initial value to the heater 204. The single-wavelength light source 110 generates the source light SO1 with a wavelength set by the heater current HT with the initial value.

[0052] Also, the bias control unit 160 generates a bias voltage BS with a predetermined initial value, and supplies the generated bias voltage BS to the semiconductor modulator 120. Specifically, the bias control unit 160 applies the bias voltage BS1 with the initial value to the MZ modulator 122-1, applies the bias voltage BS2 with the initial value to the MZ modulator 122-2, and applies the bias voltage BS3 with the initial value to the phase shifter 123. The semiconductor modulator 120 performs a modulation operation at a bias point set by the bias voltage BS with the initial value.

[0053] Subsequently, the optical transmitter 1 performs wavelength control processing on the single-wavelength light source 110 (S102 to S104). In the wavelength control processing, the monitor PD 150 monitors the light passing through the band-pass optical filter 140 (S102). Specifically, when the source light SO1 is input from the single-wavelength light source 110 and the data signal DT is input to the semiconductor modulator 120, the semiconductor modulator 120 modulates the source light SO1 according to the data signal DT1, and outputs the modulated optical signal SO2. The demultiplexer 130 branches the generated modulated optical signal SO2 into an output optical signal SO3 and a monitor optical signal SO4. The band-pass optical filter 140 transmits the light in the transmission band from the branched monitor optical signal SO4. The monitor PD 150 detects the optical signal passing through the band-pass optical filter 140, and converts the detected optical signal into a monitor current (monitor signal MO).

[0054] Subsequently, the wavelength control unit 170 determines whether wavelength control (adjustment) of the single-wavelength light source 110 is necessary based on the monitoring result of the monitor PD 150 (S103). For example, the wavelength control unit 170 determines whether the monitor current (monitor signal MO) output from the monitor PD 150 is equal to or less than a predetermined threshold value, and determines that wavelength control is necessary when the monitor current is equal to or less than the predetermined threshold value.

[0055] Subsequently, when it is determined that wavelength control is necessary, the wavelength control unit 170 controls (adjusts) the wavelength of the single-wavelength light source 110 (S104). The wavelength control unit 170 adjusts the heater current HT according to the detected monitor current, and supplies the adjusted heater current HT to the single-wavelength light source 110. For example, the wavelength control unit 170 repeats the adjustment of the heater current HT until the monitor current exceeds a predetermined threshold value.

[0056] FIGs. 10A and 10B to FIGS. 12A and 12B show specific examples of wavelength control of the single-wavelength light source 110. FIGS. 10A and 10B show examples of the spectrum and monitor power when the wavelength of the light source light SO1 output from the single-wavelength light source 110 does not deviate from the set wavelength (λ0). In this case, as shown in FIG. 10A, the spectra of the modulated optical signal SO2 generated by the semiconductor modulator 120 and the monitor optical signal SO4 branched from the demultiplexer 130 are included in the transmission band spectrum of the band-pass optical filter 140. The bandwidth of the transmission band spectrum of the band-pass optical filter 140 corresponds to the bandwidth of the modulated optical signal SO2 output from the semiconductor modulator 120. For example, the bandwidth of the transmission band of the band-pass optical filter 140 is wider than the bandwidth of the modulated optical signal SO2, or is approximately the same width as the bandwidth of the modulated optical signal SO2, and is at least a bandwidth capable of transmitting the modulated optical signal SO2. Then, as shown in FIG. 10B, when the wavelength of the light source light SO1 does not deviate from the set wavelength (λ0), the monitor power (monitor current) of the monitor optical signal SO4 transmitted through the band-pass optical filter 140 becomes the maximum value. For example, since the monitor power is greater than a predetermined threshold value, the wavelength control unit 170 determines that wavelength control is unnecessary (S103).

[0057] Figures 11A and 11B show examples of the spectrum and monitor power when the wavelength of the light source light SO1 output from the single-wavelength light source 110 is shifted to the shorter wavelength side than the set wavelength (λ0). In this case, as shown in FIG. 11A, the spectra of the modulated optical signal SO2 generated by the semiconductor modulator 120 and the monitor optical signal SO4 branched from the optical demultiplexer 130 are shifted to the shorter wavelength side than the transmission band spectrum of the band-pass optical filter 140. Therefore, the portion on the shorter wavelength side of the monitor optical signal SO4 that is not included in the transmission band is cut by the band-pass optical filter 140. Then, as shown in FIG. 11B, when the wavelength of the light source light SO1 is shifted to the shorter wavelength side than the set wavelength (λ0), the monitor power (monitor current) of the monitor optical signal SO4 that has passed through the band-pass optical filter 140 becomes smaller than the maximum value. For example, since the monitor power is smaller than a predetermined threshold, the wavelength control unit 170 determines that wavelength control is necessary (S103) and controls the wavelength of the single-wavelength light source 110 (S104). In this example, the wavelength of the single-wavelength light source 110 is adjusted to the longer wavelength side by increasing the heater current according to the monitor power.

[0058] Figs. 12A and 12B show examples of the spectrum and monitor power when the wavelength of the light source light SO1 output from the single-wavelength light source 110 is shifted to the longer wavelength side than the set wavelength (λ0). In this case, as shown in Fig. 12A, the spectra of the modulated optical signal SO2 generated by the semiconductor modulator 120 and the monitor optical signal SO4 branched from the optical demultiplexer 130 are shifted to the longer wavelength side than the transmission band spectrum of the band-pass optical filter 140. Therefore, the portion on the longer wavelength side of the monitor optical signal SO4 that is not included in the transmission band is cut by the band-pass optical filter 140. Then, as shown in Fig. 12B, when the wavelength of the light source light SO1 is shifted to the longer wavelength side than the set wavelength (λ0), the monitor power (monitor current) of the monitor optical signal SO4 that has passed through the band-pass optical filter 140 becomes smaller than the maximum value. For example, since the monitor power is smaller than a predetermined threshold value, the wavelength control unit 170 determines that wavelength control is necessary (S103), and controls the wavelength of the single-wavelength light source 110 (S104). In this example, the wavelength of the single-wavelength light source 110 is adjusted to the shorter wavelength side by decreasing the heater current according to the monitor power.

[0059] Fig. 13 shows a detailed operation example of the wavelength control process of the single-wavelength light source 110 according to some embodiments. For example, S201 to S206 in Fig. 13 correspond to S103 to S104 in Fig. 9. Fig. 14 shows an example of the monitor power for explaining the operation example of Fig. 13.

[0060] In the example of Fig. 13, the wavelength control unit 170 determines whether or not the monitor power of the monitor signal MO output from the monitor PD150 has decreased by a specified value (ΔP0) or more (S201). For example, the wavelength control unit 170 compares the initial value of the monitor power (monitor current) of the monitor signal MO from the monitor PD150 when the initial value of the heater current HT is set in S101 of Fig. 9 with the current monitor power of the monitor signal MO from the monitor PD150. The initial value of the monitor power is the monitor power of the monitor signal MO monitored by the monitor PD150 in a state where the wavelength of the light source light SO1 is not shifted. The wavelength control unit 170 determines whether or not the current monitor power has decreased by a specified value or more with respect to the initial value of the monitor power.

[0061] When the monitor power of the monitor signal MO has not decreased by a specified value (ΔP0) or more, the wavelength control unit 170 determines that wavelength control is unnecessary and ends the wavelength control process. Also, when the monitor power of the monitor signal MO has decreased by a specified value (ΔP0) or more, the wavelength control unit 170 determines that wavelength control is necessary and increases or decreases the heater current HT by a certain amount (±ΔI) (S202). For example, the wavelength control unit 170 decreases the heater current HT from the current set value (I n ) by (-ΔI) and acquires the monitor signal MO from the monitor PD150. Also, the wavelength control unit 170 increases the heater current HT from the current set value (I n ) by (+ΔI) and acquires the monitor signal MO from the monitor PD150.

[0062] Next, the wavelength control unit 170 determines whether the amount of change in the monitor power of the monitor signal MO is within the specified value (ΔP0) (S203). Note that the specified value for the determination criterion in S201 and the specified value for the determination criterion in S203 may be the same value or different values. For example, the wavelength control unit 170 compares the monitor power (MP n ) of the monitor signal MO when the heater current HT is at the current set value (I n ) with the monitor power (MP1) of the monitor signal MO when the heater current HT is decreased by (-ΔI) from the set value (I n ), and determines whether the difference (ΔP1) between the monitor power (MP n ) and the monitor power (MP1) is less than or equal to the specified value (ΔP0). Also, the wavelength control unit 170 compares the monitor power (MP n ) of the monitor signal MO when the heater current HT is at the current set value (I n ) with the monitor power (MP2) of the monitor signal MO when the heater current HT is increased by (+ΔI) from the set value (I n ), and determines whether the difference (ΔP2) between the monitor power (MP n ) and the monitor power (MP2) is less than or equal to the specified value (ΔP0).

[0063] When the fluctuation amount of the monitor power of the monitor signal MO is within the specified value (ΔP0), the wavelength control unit 170 determines that wavelength control is unnecessary and ends the wavelength control process. FIG. 14(b) shows an example in which the heater current dependence (1401) of the monitor power of the monitor signal MO has not shifted, that is, the wavelength of the monitor signal MO has not shifted. For example, as shown by 1401 in FIG. 14(b), when the heater current HT is at the set value (I n ), the monitor power (MP n ), and the difference (ΔP1) between the monitor power (MP1) when the heater current HT is decreased by (-ΔI) is equal to or less than the specified value (ΔP0), and when the heater current HT is at the set value (I n ), the monitor power (MP n ), and the difference (ΔP2) between the monitor power (MP2) when the heater current HT is increased by (+ΔI) is equal to or less than the specified value (ΔP0), the wavelength control unit 170 determines that there is no shift in the wavelength of the light source light SO1 because the heater current dependence of the monitor power has not shifted and that wavelength control is unnecessary.

[0064] Also, when the fluctuation amount of the monitor power of the monitor signal MO is greater than the specified value (ΔP0), the wavelength control unit 170 determines that wavelength control is necessary and proceeds to the next S204. For example, when the difference (ΔP1) between the monitor power (MP n ) when the heater current HT is at the set value (I n ) and the monitor power (MP1) when the heater current HT is decreased by (-ΔI) is greater than the specified value (ΔP0), or when the difference (ΔP2) between the monitor power (MP n ) when the heater current HT is at the set value (I n ) and the monitor power (MP2) when the heater current HT is increased by (+ΔI) is greater than the specified value (ΔP0), the wavelength control unit 170 determines that wavelength control is necessary.

[0065] In this case, the wavelength control unit 170 determines that the fluctuation amount (ΔP1) of the monitor power of the monitor signal MO when the heater current HT is (I n -ΔI) is less than 0, and when the heater current HT is (I nDetermine whether the amount of change (ΔP2) in the monitor power of the monitor signal MO when (I + ΔI) is greater than 0 (S204). That is, the wavelength control unit 170 determines whether the slope of the change in the monitor power when the heater current is increased or decreased is positive.

[0066] For example, the wavelength control unit 170 determines when the heater current HT is at the set value (I n ) of the monitor power (MP n ), and the difference (ΔP1) between the monitor power (MP1) when the heater current HT is decreased by (-ΔI) is a negative value, and when the heater current HT is at the set value (I n ) of the monitor power (MP n ), and determines whether the difference (ΔP2) between the monitor power (MP2) when the heater current HT is increased by (+ΔI) is a positive value. Thereby, it is determined whether the wavelength of the light source light SO1 is shifted to the longer wavelength side (is the slope positive) or the shorter wavelength side (is the slope negative) than the set wavelength (λ0).

[0067] When the amount of change (ΔP1) in the monitor power of the monitor signal MO when the heater current HT is (I n -ΔI) is less than 0, and the amount of change (ΔP2) in the monitor power of the monitor signal MO when the heater current HT is (I n +ΔI) is greater than 0, the wavelength control unit 170 sets the heater current HT to (I n +ΔI) (S205). FIG. 14(c) shows an example in which the heater current dependence (1402) of the monitor power of the monitor signal MO is shifted to the high current side, that is, the wavelength of the monitor signal MO is shifted to the longer wavelength side. For example, as shown in 1402 of FIG. 14(c), when the heater current HT is at the set value (I n ) of the monitor power (MP n ), and the difference (ΔP1) between the monitor power (MP1) when the heater current HT is decreased by (-ΔI) is a negative value, and when the heater current HT is at the set value (I n ) of the monitor power (MP n) When the difference (ΔP2) between the monitor power (MP2) when the heater current HT is increased by (+ΔI) is a positive value, the wavelength control unit 170 determines that the slope of the change in the monitor power is positive and the heater current dependence of the monitor power has shifted to the high current side, so the wavelength of the light source light SO1 has shifted to the shorter wavelength side than the set wavelength (λ0). In this case, the wavelength control unit 170 adjusts the wavelength of the light source light SO1 to the longer wavelength side by increasing the set value of the heater current HT by (+ΔI). The amount by which the heater current HT is increased may be changed according to the slope of the monitor power.

[0068] Conversely, when the amount of change (ΔP1) in the monitor power of the monitor signal MO when the heater current HT is (I n -ΔI) is >0, and the amount of change (ΔP2) in the monitor power of the monitor signal MO when the heater current HT is (I n +ΔI) is <0, the wavelength control unit 170 sets the heater current HT to (I n -ΔI) (S206). FIG. 14(a) shows an example in which the heater current dependence (1403) of the monitor power of the monitor signal MO has shifted to the low current side, that is, the wavelength of the monitor signal MO has shifted to the shorter wavelength side. For example, as shown in 1403 of FIG. 14(a), when the heater current HT is the set value (I n ), the difference (ΔP1) between the monitor power (MP n ) and the monitor power (MP1) when the heater current HT is decreased by (-ΔI) is a positive value, and when the heater current HT is the set value (I n ), the difference (ΔP2) between the monitor power (MP n ) and the monitor power (MP2) when the heater current HT is increased by (+ΔI) is a negative value, the wavelength control unit 170 determines that the slope of the change in the monitor power is negative and the heater current dependence of the monitor power has shifted to the low current side, so the wavelength of the light source light SO1 has shifted to the longer wavelength side than the set wavelength (λ0). In this case, the wavelength control unit 170 adjusts the wavelength of the light source light SO1 to the shorter wavelength side by decreasing the set value of the heater current HT by (-ΔI). The amount by which the heater current HT is decreased may be changed according to the slope of the monitor power.

[0069] Returning to FIG. 9, when it is determined in S103 that wavelength control is unnecessary, or when the wavelength control in S104 (including the wavelength control in FIG. 13) is completed, the optical transmitter 1 performs bias control processing on the semiconductor modulator 120 (S105 to S107).

[0070] In the bias control processing, the monitor PD150 monitors the light passing through the band-pass optical filter 140 (S105). Similar to S102, the semiconductor modulator 120 modulates the wavelength-controlled light source light SO1, the monitor PD150 detects the optical signal passing through the band-pass optical filter 140, and converts the detected optical signal into a monitor current (monitor signal MO).

[0071] Subsequently, the bias control unit 160 determines whether bias control (adjustment) of the semiconductor modulator 120 is necessary based on the monitoring result of the monitor PD150 (S106). For example, the bias control unit 160 determines whether the monitor current (monitor signal MO) output from the monitor PD150 is within a predetermined range, and determines that bias control is necessary when the monitor current exceeds the predetermined range.

[0072] Subsequently, when it is determined that bias control is necessary, the bias control unit 160 controls (adjusts) the bias voltage BS of the semiconductor modulator 120 (S107). The bias control unit 160 controls the bias voltage BS1 applied to the MZ modulator 122-1, the bias voltage BS2 applied to the MZ modulator 122-2, and the bias voltage BS3 applied to the phase shifter 123 based on the detected monitor current. For example, the bias control unit 160 repeats the adjustment of the bias voltages BS1 to BS3 until the monitor current is within a predetermined range.

[0073] FIG. 15 shows a detailed operation example of the bias control process of the semiconductor modulator 120 according to some embodiments. For example, S301 to S306 in FIG. 15 correspond to S106 to S107 in FIG. 9. FIG. 16 shows an example of the objective function value for explaining the operation example of FIG. 15. In this example, the objective function value obtained by converting the monitor power (monitor current) of the monitor signal MO output from the monitor PD150 by a predetermined objective function is used. The objective function is a function used for bias voltage control of the MZ modulator. In the case of an MZ modulator, when deviating from the optimal bias point, the non-modulated component passes through the optical modulator (semiconductor modulator 120) as a DC component. For this reason, the monitor signal MO mainly includes this DC component, and as it approaches the optimal bias point, the DC component is reduced. Therefore, a function for reducing this component is set as the objective function. For example, as shown in FIG. 16, when the bias voltage supplied to the modulator is optimal, that is, when the bias point of the modulator is not deviated, the objective function value obtained from the power of the light modulated by the modulator becomes the minimum value.

[0074] In the example of FIG. 15, the bias control unit 160 determines whether or not the objective function value based on the monitor signal MO output from the monitor PD150 has increased by a specified value (Δf0) or more (S301). For example, the bias control unit 160 compares the initial value of the objective function value obtained by converting the monitor power of the monitor signal MO from the monitor PD150 when the initial value of the bias voltage BS is set in S101 of FIG. 9 with the objective function value, and the objective function value obtained by converting the monitor power of the current monitor signal MO from the monitor PD150 by the objective function. The initial value of the objective function value is the objective function value obtained by converting the monitor power of the monitor signal MO monitored by the monitor PD150 with the bias point of the semiconductor modulator 120 not deviated by the objective function. The bias control unit 160 determines whether or not the current objective function value is equal to or greater than the specified value with respect to the initial value of the objective function value.

[0075] When the objective function value based on the monitor signal MO has not increased by a specified value (Δf0) or more, the bias control unit 160 determines that bias control is unnecessary and ends the bias control process. Also, when the objective function value based on the monitor signal MO has increased by a specified value (Δf0) or more, the bias control unit 160 determines that bias control is necessary and increases or decreases the bias voltage BS by a certain amount (±ΔV) (S302). For example, the bias control unit 160 decreases the bias voltage BS from the current set value (V n ) by (-ΔV) and acquires the monitor signal MO from the monitor PD150. Also, the bias control unit 160 increases the bias voltage BS from the current set value (V n ) by (+ΔV) and acquires the monitor signal MO from the monitor PD150.

[0076] Next, the bias control unit 160 determines whether the variation amount of the objective function value based on the monitor signal MO is within the specified value (Δf0) (S303). Note that the specified value for the determination criterion in S301 and the specified value for the determination criterion in S303 may be the same value or different values. For example, the bias control unit 160 compares the objective function value (Mf n ) based on the monitor signal MO when the bias voltage BS is the current set value (V n ) with the objective function value (Mf1) based on the monitor signal MO when the bias voltage BS is decreased by (-ΔV) from the set value (V n ), and determines whether the difference (Δf1) between the objective function value (Mf n ) and the objective function value (Mf1) is less than or equal to the specified value (Δf0). Also, the bias control unit 160 compares the objective function value (Mf n ) based on the monitor signal MO when the bias voltage BS is the current set value (V n ) with the objective function value (Mf2) based on the monitor signal MO when the bias voltage BS is increased by (+ΔV) from the set value (V n ), and determines whether the difference (Δf1) between the objective function value (Mf n ) and the objective function value (Mf2) is less than or equal to the specified value (Δf0).

[0077] When the amount of change in the objective function value based on the monitor signal MO is within the specified value (Δf0), the bias control unit 160 determines that bias control is unnecessary and ends the bias control process. FIG. 16(b) shows an example where the bias value dependency (1601) of the objective function has not shifted. For example, as shown by 1601 in FIG. 16(b), when the bias voltage BS is the set value (V n ), the objective function value (Mf n ), and the difference (Δf1) between the objective function value (Mf1) when the bias voltage BS is decreased by (-ΔV) is less than or equal to the specified value (Δf0), and when the bias voltage BS is the set value (V n ), the objective function value (Mf n ), and the difference (Δf2) between the objective function value (Mf2) when the bias voltage BS is increased by (+ΔV) is less than or equal to the specified value (Δf0), the bias control unit 160 determines that bias control is unnecessary because the bias value dependency of the objective function has not shifted.

[0078] Also, when the amount of change in the objective function value based on the monitor signal MO is greater than the specified value (Δf0), the bias control unit 160 determines that bias control is necessary and proceeds to the next S304. For example, when the bias voltage BS is the set value (V n ), the objective function value (Mf n ), and the difference (Δf1) between the objective function value (Mf1) when the bias voltage BS is decreased by (-ΔV) is greater than the specified value (Δf0), or when the bias voltage BS is the set value (V n ), the objective function value (Mf n ), and the difference (Δf2) between the objective function value (Mf2) when the bias voltage BS is increased by (+ΔV) is greater than the specified value (Δf0), the bias control unit 160 determines that bias control is necessary.

[0079] In this case, the bias control unit 160 has the amount of change (Δf1) in the objective function value based on the monitor signal MO when the bias voltage BS is (V n -ΔV) less than 0, and the bias voltage (V nDetermine whether the amount of change (Δf2) in the objective function value based on the monitor signal MO when (V + ΔV) is greater than 0 (S304). That is, the bias control unit 160 determines whether the slope of the change in the objective function value when the bias voltage is increased or decreased is positive.

[0080] For example, the bias control unit 160 determines the objective function value (Mf n ) when the bias voltage BS is the set value (V n ) and the difference (Δf1) between the objective function value (Mf1) when the bias voltage BS is decreased by (-ΔV) is a negative value, and the objective function value (Mf n ) when the bias voltage BS is the set value (V n ) and the difference (Δf2) between the objective function value (Mf2) when the bias voltage BS is increased by (+ΔV) is a positive value. Thus, it is determined whether the bias point of the semiconductor modulator 120 has shifted to the negative side (whether the slope is positive) or to the positive side (whether the slope is negative) from the initial setting.

[0081] When the amount of change (Δf1) in the objective function value based on the monitor signal MO when the bias voltage BS is (V n -ΔV) is less than 0, and the amount of change (Δf2) in the objective function value based on the monitor signal MO when the bias voltage (V n +ΔV) is greater than 0, the bias control unit 160 sets the bias voltage BS to (V n -ΔV) (S305). FIG. 16(a) shows an example when the bias value dependency (1602) of the objective function has shifted to the low bias side. For example, as shown in 1602 of FIG. 16(a), the difference (Δf1) between the objective function value (Mf n ) when the bias voltage BS is the set value (V n ) and the objective function value (Mf1) when the bias voltage BS is decreased by (-ΔV) is a negative value, and the objective function value (Mf n ) when the bias voltage BS is the set value (V n) When the difference (Δf2) between the objective function value (Mf2) when the bias voltage BS is increased by (+ΔV) is a positive value, the bias control unit 160 determines that the slope of the change in the objective function value is positive and the bias value dependency of the objective function has shifted to the low-bias side. In this case, the bias control unit 160 adjusts the objective function value to decrease by decreasing the set value of the bias voltage BS by (-ΔV). The amount by which the bias voltage BS is decreased may be changed according to the slope of the objective function value.

[0082] Conversely, when the amount of change (Δf1) in the objective function value based on the monitor signal MO when the bias voltage BS is (V n -ΔV) is >0 and the amount of change (Δf2) in the objective function value based on the monitor signal MO when the bias voltage is (V n +ΔV) is <0, the bias control unit 160 sets the bias voltage BS to (V n +ΔV) (S306). FIG. 16(c) shows an example when the bias value dependency (1603) of the objective function has shifted to the high-bias side. For example, as shown in 1603 of FIG. 16(c), when the difference (Δf1) between the objective function value (Mf n ) when the bias voltage BS is the set value (V n ) and the objective function value (Mf1) when the bias voltage BS is decreased by (-ΔV) is a positive value, and when the difference (Δf2) between the objective function value (Mf n ) when the bias voltage BS is the set value (V n ) and the objective function value (Mf2) when the bias voltage BS is increased by (+ΔV) is a negative value, the bias control unit 160 determines that the slope of the change in the objective function value is negative and the bias value dependency of the objective function has shifted to the high-bias side. In this case, the bias control unit 160 adjusts the objective function value to decrease by increasing the set value of the bias voltage BS by (+ΔV). The amount by which the bias voltage BS is increased may be changed according to the slope of the objective function value.

[0083] Returning to FIG. 9, thereafter, the optical transmitter 1 repeatedly performs wavelength control processing (S102 to S104) and bias control processing (S105 to S107) in order. The wavelength control processing and the bias control processing may be repeatedly performed always, or may be repeatedly performed periodically. Once the wavelength of the single-wavelength light source 110 is adjusted by the wavelength control processing, the wavelength control processing and the bias control processing may be executed independently. That is, the wavelength control processing and the bias control processing may be performed in parallel, or may be performed at different periods. Since the period from when the wavelength of the light source is adjusted until the wavelength drifts is longer than the period from when the bias voltage is adjusted until the bias point of the semiconductor modulator drifts, the bias control processing may be performed in a first period, and the wavelength control processing may be performed in a second period longer than the first period.

[0084] As described above, in the present embodiment, in an optical transmitter in which a single-wavelength light source such as a DFB laser or a wavelength-variable light source and a semiconductor modulator are integrated on the same substrate, an optical band-pass filter having a set wavelength as a center transmission wavelength is arranged in front of the monitor PD. Further, the light from the semiconductor modulator is branched by a coupler, and the branched light is input to the band-pass filter. In wavelength control and bias control, wavelength control is performed by identifying with a wavelength control unit using the electrical signal output from the monitor PD and controlling the heater current of the light source, and then, identification processing is performed by a bias control unit using the electrical signal output from the monitor PD to control the bias voltage of the optical modulator.

[0085] As a result, it is possible to integrate a semiconductor modulator, a light source, etc., and to suitably perform bias control and wavelength control. Specifically, by integrating a semiconductor modulator, a light source, etc. on the same semiconductor substrate, miniaturization and power saving can be achieved. By monitoring the light passing through the band-pass filter and performing bias control and wavelength control based on the monitoring result, it is possible to perform bias control and wavelength control with one monitor PD. By controlling the heat of the heater formed in the light source with a heater current according to the monitoring result, the wavelength of the light source can be controlled without using a large-scale temperature adjustment function such as a TEC. Therefore, the wavelength control of the light source and the bias control of the semiconductor modulator can be performed in close proximity with a reduced area. Also, by branching the light from the semiconductor modulator and monitoring the branched light, it is possible to perform wavelength control and bias control while suppressing the influence on the light output to the outside such as an optical transmission path.

[0086] (Embodiment 2) Next, Embodiment 2 will be described. In this embodiment, an example will be described in which the transmission bandwidth of the band-pass optical filter is narrowed and the presence or absence of the band-pass optical filter can be switched. Note that this embodiment can be implemented in combination with Embodiment 1, and each configuration shown in Embodiment 1 may be used as appropriate.

[0087] FIG. 17 shows a configuration example of the optical transmitter 1 according to some embodiments. In the example of FIG. 17, the optical transmitter 1 includes an optical switch 180 in addition to the configuration of FIG. 5. Other configurations are the same as those in the example of FIG. 5.

[0088] The optical switch 180 is disposed between the demultiplexer 130 and the band-pass optical filter 140, and is a switching unit that can switch to bypass the band-pass optical filter 140. It can also be said that the optical switch 180 switches the presence or absence (use or non-use) of the band-pass optical filter 140. The optical switch 180 is formed on the semiconductor substrate 200, similar to the demultiplexer 130, the band-pass optical filter 140, etc. The optical switch 180 switches the output destination of the monitor optical signal SO4 from the demultiplexer 130, and outputs the monitor optical signal SO4 to either the band-pass optical filter 140 or the monitor PD150. Thereby, it is switched whether to bypass the band-pass optical filter 140, that is, whether to let the monitor optical signal SO4 be filtered by the band-pass optical filter 140.

[0089] For example, when the wavelength control unit 170 performs wavelength control processing, the output of the demultiplexer 130 is connected to the input of the band-pass optical filter 140, and the output destination of the monitor optical signal SO4 is switched to the band-pass optical filter 140. Thereby, the monitor optical signal SO4 is filtered by the band-pass optical filter 140, and the monitor optical signal SO4 that has passed through the band-pass optical filter 140 is detected by the monitor PD150. Also, when the bias control unit 160 performs bias control processing, the output of the demultiplexer 130 is connected to the input of the monitor PD150, and the output destination of the monitor optical signal SO4 is switched to the monitor PD150. Thereby, the band-pass optical filter 140 is bypassed, and the monitor optical signal SO4 branched by the demultiplexer 130 is directly detected by the monitor PD150.

[0090] Also, in this embodiment, the bandwidth of the transmission band of the band-pass optical filter 140 is made narrower than that in the first embodiment. FIG. 18 shows an example of the transmission band spectrum of the band-pass optical filter 140. As shown in FIG. 18, for example, the bandwidth of the transmission band spectrum is narrower than the bandwidth of the modulated optical signal SO2 output from the semiconductor modulator 120. The bandwidth of the transmission band of the band-pass optical filter 140 may correspond to the bandwidth of the light source light SO1 output from the single-wavelength light source 110. The bandwidth of the transmission band of the band-pass optical filter 140 may be wider than the bandwidth of the light source light SO1, or may have substantially the same width as the bandwidth of the light source light SO1, and may be at least a bandwidth that can transmit the light source light SO1.

[0091] FIG. 19 shows an operation example of the optical transmitter 1 according to some embodiments. In the example of FIG. 19, S111 and S112 are added to the operation example of FIG. 9, and S105 is changed to S115.

[0092] In the example of FIG. 19, after setting the initial values of the heater current HT of the single-wavelength light source 110 and the bias voltage BS of the semiconductor modulator 120 in the same manner as in FIG. 9 (S101), the optical transmitter 1 switches the connection of the optical switch 180 to use the band-pass optical filter 140 in order to perform wavelength control processing (S111). The optical switch 180 sets the output destination of the monitor optical signal SO4 from the demultiplexer 130 to the band-pass optical filter 140.

[0093] Subsequently, the optical transmitter 1 performs wavelength control processing (S102 to S104) in the same manner as in FIG. 9. The band-pass optical filter 140 transmits the monitor optical signal SO4 from the optical switch 180 with a narrow transmission band as shown in FIG. 18. The monitor PD150 monitors the optical signal transmitted through the band-pass optical filter 140, and the wavelength control unit 170 controls the wavelength of the single-wavelength light source 110 according to the monitoring result.

[0094] Subsequently, the optical transmitter 1 switches the connection of the optical switch 180 to bypass the band-pass optical filter 140 in order to perform bias control processing (S112). The optical switch 180 sets the output destination of the monitor optical signal SO4 from the demultiplexer 130 to the monitor PD150.

[0095] Subsequently, in the same manner as in FIG. 9, the optical transmitter 1 performs bias control processing (S115, S106 to S107). The monitor PD150 monitors the monitor optical signal SO4 from the optical switch 180 without passing through the band-pass optical filter 140 (S115). The bias control unit 160 controls the bias voltage BS of the semiconductor modulator 120 according to the monitoring result (S106 to S107).

[0096] As described above, in the present embodiment, the transmission band of the band-pass filter is narrowed, and the presence or absence of the band-pass filter can be selected. When performing wavelength control, by monitoring the light that has passed through the band-pass filter with a narrow transmission band, the accuracy of wavelength control can be improved. Further, when performing bias control, by bypassing the band-pass filter, the influence of the band-pass filter with a narrow transmission band can be suppressed, and bias control can be performed accurately.

[0097] (Embodiment 3) Next, Embodiment 3 will be described. In the present embodiment, an example in which the optical transmitter includes a DP-IQ modulator as a semiconductor modulator will be described. Note that the present embodiment can be implemented in combination with Embodiment 1 or 2, and each configuration shown in Embodiment 1 or 2 may be used as appropriate.

[0098] FIG. 20 shows a configuration example of the optical transmitter 1 according to some embodiments. FIG. 21 shows a specific configuration example of the DP-IQ modulator 220 in FIG. 20. In the example of FIG. 20, in the optical transmitter 1, a DP-IQ modulator 220 is provided as the semiconductor modulator 120 in FIG. 5. Other configurations are the same as those in the example of FIG. 5.

[0099] The DP-IQ modulator 220 is a DP (dual polarization) type IQ modulator and is a semiconductor modulator formed on the semiconductor substrate 200. As shown in FIG. 21, the DP-IQ modulator 220 includes two IQ modulators that constitute the semiconductor modulator 120 of FIG. 8. One of the two IQ modulators is an IQ modulator for the X polarization, and the other is an IQ modulator for the Y polarization.

[0100] Specifically, the DP-IQ modulator 220 includes optical splitters 121-1 to 121-2, MZ modulators 122-1 to 122-4, phase shifters 123-1 to 123-2, optical combiners 124-1 to 124-2, an optical splitter 125, a polarization rotator 126, and a polarization coupler 127. The optical splitter 125 branches (splits) the light source light SO1 from the single-wavelength light source 110 into light for the X polarization and light for the Y polarization.

[0101] The optical splitter 121-1, the MZ modulators 122-1 to 122-2, the phase shifter 123-1, and the optical combiner 124-1 constitute, for example, an IQ modulator for the X polarization. The optical splitter 121-2, the MZ modulators 122-3 to 122-4, the phase shifter 123-2, and the optical combiner 124-2 constitute, for example, an IQ modulator for the Y polarization. The configurations of the IQ modulator for the X polarization and the IQ modulator for the Y polarization are the same as those of the semiconductor modulator 120 of FIG. 8.

[0102] A bias voltage BS1 is applied to the MZ modulator 122-1, and the light for Ich of the X polarization branched by the optical splitter 121-1 is modulated according to the data signal DT1. A bias voltage BS2 is applied to the MZ modulator 122-2, and the light for Qch of the X polarization branched by the optical splitter 121-1 is modulated according to the data signal DT2. A bias voltage BS3 is applied to the phase shifter 123-1, and the phase of the modulated optical signal for Qch of the X polarization modulated by the MZ modulator 122-2 is shifted.

[0103] The MZ modulator 122-3 has a bias voltage BS4 applied thereto, and modulates the light for Ich of the Y polarization wave branched by the demultiplexer 121-2 in accordance with the data signal DT3. The MZ modulator 122-4 has a bias voltage BS5 applied thereto, and modulates the light for Qch of the Y polarization wave branched by the demultiplexer 121-2 in accordance with the data signal DT4. The phase shifter 123-2 has a bias voltage BS6 applied thereto, and shifts the phase of the modulated optical signal for Qch of the Y polarization wave modulated by the MZ modulator 122-4.

[0104] The polarization rotator 126 rotates the polarization direction of the modulated optical signal output from the IQ modulator for the X polarization wave or the modulated optical signal output from the IQ modulator for the Y polarization wave so that the polarization directions of the modulated optical signals output from the IQ modulator for the X polarization wave and the IQ modulator for the Y polarization wave are orthogonal to each other. In this example, the polarization rotator 126 rotates the polarization direction of the modulated optical signal output from the IQ modulator for the X polarization wave (the multiplexer 124-1) by 90°.

[0105] The polarization combiner 127 polarization-combines the modulated optical signal of the X polarization wave output from the IQ modulator for the X polarization wave (the multiplexer 124-1) with its polarization direction rotated and the modulated optical signal of the Y polarization wave output from the IQ modulator for the Y polarization wave (the multiplexer 124-2), and outputs the polarization-combined optical signal as the modulated optical signal SO2.

[0106] In the example of FIG. 20, the optical demultiplexer 130 branches the modulated optical signal SO2 including the X polarization wave and the Y polarization wave generated by the DP-IQ modulator 220 into an output optical signal SO3 for output and a monitor optical signal SO4 for monitoring. The bias control unit 160 controls the bias voltage BS of the DP-IQ modulator 220 based on the monitor signal MO that has passed through the band-pass optical filter 140 and is monitored by the monitor PD 150. Specifically, the bias control unit 160 controls the bias voltage BS1 of the MZ modulator 122-1, the bias voltage BS2 of the MZ modulator 122-2, the bias voltage BS3 of the phase shifter 123-1, the bias voltage BS4 of the MZ modulator 122-3, the bias voltage BS5 of the MZ modulator 122-4, and the bias voltage BS6 of the phase shifter 123-2 based on the monitor signal MO. Similar to Embodiment 1, the wavelength control unit 170 controls the wavelength of the single-wavelength light source 110 based on the monitor signal MO.

[0107] As described above, the optical transmitter may include a DP-IQ modulator as a semiconductor modulator. Even in this case, similar to Embodiment 1, the DP-IQ modulator and the like can be integrated, and bias control and wavelength control can be suitably performed.

[0108] (Embodiment 4) Next, Embodiment 4 will be described. In this embodiment, an example in which the optical transmitter includes a plurality of DP-IQ modulators will be described. Note that this embodiment can be implemented in combination with any of Embodiments 1 to 3, and each configuration shown in any of Embodiments 1 to 3 may be appropriately used.

[0109] FIG. 22 shows a configuration example of the optical transmitter 1 according to some embodiments. In the example of FIG. 22, the optical transmitter 1 includes a plurality of DP-IQ modulators 220-1 to 220-N (modulator array). The DP-IQ modulators 220-1 to 220-N are the DP-IQ modulators shown in FIG. 20.

[0110] In the example of FIG. 22, a single-wavelength light source 110, a demultiplexer 111, DP-IQ modulators 220-1 to 220-N, demultiplexers 130-1 to 130-N, a band-pass optical filter 140, and monitor PDs 150-1 to 150-N are provided on a semiconductor substrate 200.

[0111] The demultiplexer 111 branches (demultiplexes) the source light SO1 from the single-wavelength light source 110 to the DP-IQ modulators 220-1 to 220-N. The DP-IQ modulators 220-1 to 220-N modulate the light branched from the demultiplexer 111 respectively. The demultiplexers 130-1 to 130-N branch the modulated optical signals generated by the DP-IQ modulators 220-1 to 220-N into output optical signals and monitor optical signals respectively.

[0112] The band-pass optical filter 140 transmits the monitor optical signal branched by any one of the demultiplexers 130-1 to 130-N through the transmission band. In this example, the band-pass optical filter 140 transmits the monitor optical signal generated by the DP-IQ modulator 220-N (for example, the first semiconductor modulator) and branched by the demultiplexer 130-N (for example, the first demultiplexer). The band-pass optical filter 140 is not limited to the demultiplexer 130-N and may transmit the monitor optical signal branched by other demultiplexers 130.

[0113] The monitor PD 150-N monitors the optical signal transmitted through the band-pass optical filter 140. The other monitor PDs 150-1 to 150-N-1 monitor the monitor optical signals generated by the DP-IQ modulators 220-1 to 220-N-1 and branched by the demultiplexers 130-1 to 130-N-1 respectively.

[0114] The bias control unit 160 controls the bias voltages BS of the DP-IQ modulators 220-1 to 220-N based on the monitor signals MO monitored by the monitor PDs 150-1 to 150-N. That is, the bias control unit 160 controls the bias voltages BS of the DP-IQ modulators 220-1 to 220-N based on the optical signals modulated by the DP-IQ modulators 220-1 to 220-N and branched by the optical splitters 130-1 to 130-N. The bias control unit 160 controls the bias voltage BS of the DP-IQ modulator 220-1 according to the monitoring result of the monitor PD 150-1, controls the bias voltage BS of the DP-IQ modulator 220-2 according to the monitoring result of the monitor PD 150-2, ···, and controls the bias voltage BS of the DP-IQ modulator 220-N according to the monitoring result of the monitor PD 150-N. The wavelength control unit 170 controls the wavelength of the single-wavelength light source 110 based on the monitor signal MO monitored by the monitor PD 150-N, as in the first embodiment. That is, the wavelength control unit 170 controls the wavelength of the single-wavelength light source 110 based on the optical signal modulated by any one of the DP-IQ modulators 220-1 to 220-N and branched by the optical splitter 130.

[0115] As described above, the optical transmitter may include a plurality of DP-IQ modulators as semiconductor modulators. Even in this case, as in the first embodiment, a plurality of DP-IQ modulators and the like can be integrated, and bias control and wavelength control can be suitably performed.

[0116] (Embodiment 5) Next, Embodiment 5 will be described. In this embodiment, an example in which the optical transmitter includes a plurality of DP-IQ modulators and a multi-wavelength light source will be described. Note that this embodiment can be implemented in combination with any one of Embodiments 1 to 4, and each configuration shown in any one of Embodiments 1 to 4 may be appropriately used.

[0117] FIG. 23 shows a configuration example of the optical transmitter 1 according to some embodiments. In the example of FIG. 23, the optical transmitter 1 includes a multi-wavelength light source 112 instead of the single-wavelength light source 110 and a wavelength demultiplexer 113 instead of the demultiplexer 111 with respect to the configuration of FIG. 22. Other configurations are the same as those in the example of FIG. 22.

[0118] The multi-wavelength light source 112 is a light source that outputs source light SO1 of multiple wavelengths (a plurality of wavelengths), and outputs light of wavelengths λ1 to λN. For example, the multi-wavelength light source 112 outputs light of a plurality of wavelengths with equally spaced wavelength intervals. The multi-wavelength light source 112 is, for example, a comb light source such as a mode-locked laser.

[0119] The wavelength demultiplexer 113 demultiplexes (branches) the source light SO1 of multiple wavelengths output from the multi-wavelength light source 112 for each wavelength. The wavelength demultiplexer 113 is, for example, an AWG (Arrayed waveguide gratings). The wavelength demultiplexer 113 outputs the optical signals of wavelengths λ1 to λN from the multi-wavelength light source 112 to the DP-IQ modulators 220-1 to 220-N for each wavelength. Specifically, the wavelength demultiplexer 113 outputs the optical signal of wavelength λ1 to the DP-IQ modulator 220-1, the optical signal of wavelength λ2 to the DP-IQ modulator 220-2, ···, and the optical signal of wavelength λN to the DP-IQ modulator 220-N.

[0120] The DP-IQ modulators 220-1 to 220-N modulate the light of each wavelength branched from the wavelength demultiplexer 113 in the same manner as in FIG. 22. The demultiplexers 130-1 to 130-N, the band-pass optical filters 140, the monitor PDs 150-1 to 150-N, and the bias control unit 160 are also the same as those in FIG. 22. When the band-pass optical filter 140 transmits the monitor optical signal generated by the DP-IQ modulator 220-N and branched by the demultiplexer 130-N, the transmission band of the band-pass optical filter 140 corresponds to the wavelength (λN) of the optical signal modulated by the DP-IQ modulator 220-N.

[0121] The wavelength control unit 170 controls all of the plurality of wavelengths of the multi-wavelength light source 112 based on the monitor signal MO monitored by the monitor PD 150-N. Similar to the first embodiment, the wavelength control unit 170 controls the plurality of wavelengths by supplying a heater current to the heater formed in the multi-wavelength light source 112. For example, in the multi-wavelength light source 112, since the intervals between the plurality of wavelengths are locked, the plurality of wavelengths can be collectively controlled by controlling the heater current according to the monitor result of the optical signal modulated by any one of the modulators.

[0122] FIG. 24 is a schematic top view showing a configuration example of the multi-wavelength light source 112 according to some embodiments. FIG. 25 is a schematic side view showing an example of the arrangement of each part in the semiconductor substrate 200 according to some embodiments, and shows a side configuration example of the multi-wavelength light source 112.

[0123] In the examples of FIGS. 24 and 25, the multi-wavelength light source 112 is an external resonator type semiconductor laser light source including an optical amplifier element 300 and an external resonator 400, and constitutes a mode-locked laser light source that generates light with a predetermined longitudinal mode interval.

[0124] The optical amplifier element 300 is a semiconductor chip including a reflective semiconductor optical amplifier (RSOA). The reflective semiconductor optical amplifier 310 is an optical amplifier that emits the amplified light to the external resonator 400 and reflects the light from the external resonator 400. An AR (Anti Reflection) coat 321 is formed on the end face on the emission side (external resonator side) of the reflective semiconductor optical amplifier 310. The AR coat 321 suppresses unnecessary reflection of the light reciprocating between the reflective semiconductor optical amplifier 310 and the external resonator 400. The end face of the reflective semiconductor optical amplifier 310 opposite to the AR coat 321 becomes a reflective end face 322 that reflects light. For example, the reflective end face 322 has a reflectivity of about 30% in cleavage based on the crystal structure, but an HR (High Reflection) coat may be formed.

[0125] As shown in FIG. 25, an optical amplifier element 300 including a reflective semiconductor optical amplifier 310 is configured by laminating a lower cladding layer 331, an active layer 332, and an upper cladding layer 333. In this example, the optical amplifier element 300 is flip-chip mounted on a semiconductor substrate 200 in the same manner as the single-wavelength light source of FIG. 7. Note that the optical amplifier element 300 may be monolithically mounted on the semiconductor substrate 200 in the same manner as the single-wavelength light source of FIG. 6.

[0126] The reflective semiconductor optical amplifier 310 is a reflective semiconductor optical amplifier with a saturable absorption region, and includes a saturable absorption region 311 and a gain region 312. A saturable absorption region electrode (first electrode) 334 is formed on the surface of the upper cladding layer 333 that becomes the saturable absorption region (the lowermost surface in FIG. 25), and a gain region electrode (second electrode) 335 is formed on the surface of the upper cladding layer 333 that becomes the gain region 312. The reflective semiconductor optical amplifier 310 is based on a two-electrode semiconductor laser including the saturable absorption region electrode 334 and the gain region electrode 335. By bonding the saturable absorption region electrode 334 and the gain region electrode 335 to the semiconductor substrate 200 with solder bumps or the like, the optical amplifier element 300 including the reflective semiconductor optical amplifier 310 is mounted on the semiconductor substrate 200.

[0127] For example, a back surface electrode is formed on the back surface of the lower cladding layer 331 (the uppermost surface in FIG. 25), and a ground potential is supplied to the back surface electrode. By connecting the negative electrode of a constant voltage source (not shown) to the saturable absorption region electrode 334 and applying a reverse bias voltage to the saturable absorption region electrode 334, the saturable absorption region 311 is formed. By connecting the positive electrode of a constant current source (not shown) to the gain region electrode 335 and injecting a constant current into the gain region electrode 335, the gain region 312 is formed. For example, the wavelength control unit 170 controls the application of the reverse bias voltage to the saturable absorption region electrode 334 and the injection of the constant current into the gain region electrode 335.

[0128] The external resonator 400 is a resonator that resonates the light from the reflective semiconductor optical amplifier 310. As shown in FIG. 24, the external resonator 400 includes a broadband transmission filter 410, a frequency doubling filter 420, an optical waveguide 430 for resonator length adjustment, and a partial transmission mirror 440. The external resonator 400 includes an optical waveguide 401 that propagates and reciprocates the light from the reflective semiconductor optical amplifier 310. In the optical waveguide 401, the broadband transmission filter 410, the frequency doubling filter 420, the optical waveguide 430 for resonator length adjustment, and the partial transmission mirror 440 are formed in series in the order from the side of the reflective semiconductor optical amplifier 310 to the output side of the light source light SO1.

[0129] The broadband transmission filter 410 transmits the light in the band required by the multi-wavelength light source from the light generated by the mode-locked laser light source including the reflective semiconductor optical amplifier 310 and the external resonator 400. The gain band of the reflective semiconductor optical amplifier 310 is appropriately set wide, and the broadband transmission filter 410 transmits a range narrower than the gain band of the reflective semiconductor optical amplifier 310. The generation of unnecessary light is suppressed by the broadband transmission filter 410. For example, the broadband transmission filter 410 may be constituted by a lattice filter in which asymmetric MZ interferometers are cascade-connected.

[0130] The frequency doubling filter 420 extracts the light with the longitudinal mode interval multiplied (n times) from the light with the longitudinal mode interval generated by the mode-locked laser light source including the reflective semiconductor optical amplifier 310 and the external resonator 400. That is, the frequency doubling filter 420 is a filter that multiplies the longitudinal mode interval of the mode-locked laser light source by n. For example, the frequency doubling filter 420 is a ring resonator filter that circulates an optical waveguide. The circulation length (optical length) of the frequency doubling filter 420 is an integer fraction of the resonator circulation length including the reflective semiconductor optical amplifier 310 and the external resonator 400.

[0131] A heater 421 for adjusting the multiplication filter is formed above (or on the upper side of) the multiplication filter 420. The heater 421 for adjusting the multiplication filter may be formed below (or on the lower side of) the multiplication filter 420. For example, the heater 421 for adjusting the multiplication filter is disposed on the semiconductor substrate 200 including the multiplication filter 420, but may be disposed below the semiconductor substrate 200 or formed within the semiconductor substrate 200.

[0132] The heater 421 for adjusting the multiplication filter is a heating element capable of heating the multiplication filter 420. The heater 421 for adjusting the multiplication filter is formed of a resistor capable of temperature control, such as a TiN heater. The heater 421 for adjusting the multiplication filter is formed corresponding to the region of the optical waveguide of the multiplication filter 420. The heater 421 for adjusting the multiplication filter may be formed in a region overlapping the entire optical waveguide of the multiplication filter 420, or may be formed in a region overlapping a part of the optical waveguide of the multiplication filter 420.

[0133] For example, the optical waveguide of the multiplication filter 420 circulates in a rounded rectangular shape in a top view. The optical waveguides forming the rounded rectangular shape include optical waveguides 420a - 420d. The optical waveguide 420a is a directional coupler adjacent to an optical waveguide linearly extending from the output side of the light source light SO1 from the broadband transmission filter 410 (on the reflection type semiconductor optical amplifier 310 side). The optical waveguide 420b is opposite to the optical waveguide 420a and is a directional coupler adjacent to an optical waveguide linearly extending from the output side of the light source light SO1 from the resonator length adjustment optical waveguide 430 (on the reflection type semiconductor optical amplifier 310 side). The optical waveguides 420c and 420d are between the optical waveguides 420a and 420b and linearly extend oppositely from both ends of the optical waveguides 420a and 420b. For example, the heater 421 for adjusting the multiplication filter is formed in a region overlapping the optical waveguides 420c and 420d among the optical waveguides constituting the multiplication filter 420. In this example, the heater 421 for adjusting the multiplication filter is formed in a U - shaped (C - shaped) manner so as to overlap the optical waveguides 420c and 420d. Note that the optical waveguide of the multiplication filter 420 is not limited to a rounded rectangular shape and may circulate in a circular shape or an elliptical shape.

[0134] The heater 421 for adjusting the multiplier filter has one end connected to the ground potential and the other end connected to the wavelength control unit 170. The heater current HT1 for the multiplier filter is injected from the wavelength control unit 170 to the other end of the heater 421 for adjusting the multiplier filter. The heater 421 for adjusting the multiplier filter generates heat according to the injected heater current HT1 for the multiplier filter, and the refractive index of the optical waveguide of the multiplier filter 420 changes due to the heat of the heater 421 for adjusting the multiplier filter, so that the round-trip length (optical length) of the multiplier filter 420 can be adjusted.

[0135] The optical waveguide 430 for adjusting the resonator length is an optical waveguide for adjusting the overall resonator length (optical length) of a mode-locked laser light source (multi-wavelength light source 112) including the reflective semiconductor optical amplifier 310 and the external resonator 400. Note that adjusting the resonator length is also to adjust the resonator round-trip length. For example, the optical waveguide 430 for adjusting the resonator length is an optical waveguide formed between the multiplier filter 420 and the partially transmissive mirror 440. The optical waveguide 430 for adjusting the resonator length adjusts the resonator length so that the round-trip length (optical length) of the multiplier filter 420 is an integer fraction of the resonator round-trip length including the reflective semiconductor optical amplifier 310 and the external resonator 400.

[0136] The heater 431 for adjusting the resonator length is formed above (or on top of) the optical waveguide 430 for adjusting the resonator length. The heater 431 for adjusting the resonator length may be formed below (or beneath) the optical waveguide 430 for adjusting the resonator length. For example, the heater 431 for adjusting the resonator length is disposed on the semiconductor substrate 200 including the optical waveguide 430 for adjusting the resonator length, but it may also be disposed below the semiconductor substrate 200 or formed within the semiconductor substrate 200.

[0137] The resonator length adjustment heater 431 is a heating element capable of heating the resonator length adjustment optical waveguide 430. The resonator length adjustment heater 431 is formed of a temperature controllable resistor such as a TiN heater. The resonator length adjustment heater 431 is formed corresponding to the region of the resonator length adjustment optical waveguide 430. The resonator length adjustment heater 431 may be formed in a region overlapping with the optical waveguide between the multiplier filter 420 and the partial transmission mirror 440.

[0138] For example, the resonator length adjustment optical waveguide 430 is formed by being bent in a U-shape (U-shaped) in a top view between the multiplier filter 420 and the partial transmission mirror 440. The U-shaped resonator length adjustment optical waveguide 430 includes optical waveguides 430a - 430c. The optical waveguide 430a extends linearly from the multiplier filter 420 (output side of the light source light SO1) toward the reflective semiconductor optical amplifier 310. The optical waveguide 430b extends linearly from the side of the reflective semiconductor optical amplifier 310 toward the partial transmission mirror 440 (output side of the light source light SO1) facing the optical waveguide 430a. The optical waveguide 430c extends linearly between the optical waveguide 430a and the optical waveguide 430b from the ends of the optical waveguide 430a and the optical waveguide 430b on the side of the reflective semiconductor optical amplifier 310. For example, the resonator length adjustment heater 431 is formed in a U-shape so as to overlap with the optical waveguide 430a (for example, a part from the optical waveguide 430c to the multiplier filter 420), the optical waveguide 430b (for example, a part from the optical waveguide 430c to the partial transmission mirror 440), and the optical waveguide 430c. The resonator length adjustment heater 431 may be formed in a region overlapping with any one of the optical waveguides 430a - 430c.

[0139] One end of the resonator length adjustment heater 431 is connected to the ground potential, and the other end is connected to the wavelength control unit 170. A resonator length adjustment heater current HT2 is injected from the wavelength control unit 170 to the other end of the resonator length adjustment heater 431. The resonator length adjustment heater 431 generates heat according to the injected resonator length adjustment heater current HT2, and the refractive index of the resonator length adjustment optical waveguide 430 changes due to the heat of the resonator length adjustment heater 431, so that the optical length (i.e., the resonator length) of the resonator length adjustment optical waveguide 430 can be adjusted.

[0140] The partial transmission mirror 440 reflects a part of the light from the optical waveguide 430 for resonator length adjustment, and transmits (outputs) the remaining part of the light as the light source light SO1. For example, the partial transmission mirror 440 is a Sagnac waveguide mirror. The partial transmission mirror 440 is formed on the end face on the output side of the external resonator 400.

[0141] The mode-locked laser light source (multi-wavelength light source 112) including the reflective semiconductor optical amplifier 310 and the external resonator 400 resonates by reciprocating light between the reflective end face 322 of the reflective semiconductor optical amplifier 310 and the partial transmission mirror 440. The optical length between the reflective end face 322 of the reflective semiconductor optical amplifier 310 and the partial transmission mirror 440 (the center of the mirror) becomes the overall resonator length of the mode-locked laser light source. In order for the mode-locked laser light source to output the light source light SO1 with a wavelength interval multiplied by the multiplication filter 420, the resonator length (optical length) L of the mode-locked laser light source t and the round-trip length (optical length) L of the multiplication filter 420 ring need to satisfy the following relationship. L t =L ring ·n / 2 (n: positive integer) ···(1) When transformed with Equation (1), the round-trip length L of the multiplication filter 420 ring is expressed as follows. That is, as described above, the round-trip length L of the multiplication filter 420 ring is 1 / n of the resonator round-trip length 2L t . L ring =2L t / n (n: positive integer) ···(2) In the present embodiment, the multiplication filter heater current HT1 and the resonator length adjustment heater current HT2 are adjusted so as to satisfy the relationship of Equation (1) or Equation (2), and the round-trip length of the multiplication filter 420 and the resonator length (resonator round-trip length) are adjusted.

[0142] FIG. 26 shows an operation example of the optical transmitter 1 according to some embodiments. In the example of FIG. 26, first, the optical transmitter 1 sets initial values of the heater current HT of the multi-wavelength light source 112 and the bias voltage BS of the plurality of DP-IQ modulators 220 (S401).

[0143] FIG. 27 shows an example of each optical signal at the time of initial setting, and shows an example of the spectrum of the multi-wavelength source light SO1 output from the multi-wavelength light source 112, one wavelength of the source light SO1 input to the DP-IQ modulator 220, and the monitor optical signal SO4 input to the band-pass optical filter 140.

[0144] The wavelength control unit 170 applies a reverse bias voltage to the saturable absorption region electrode 334 and injects a constant current into the gain region electrode 335. Then, as shown in 2701 of FIG. 27(a), the multi-wavelength light source 112 generates light having a longitudinal mode interval (Δf) of the external resonator length by an external resonator mode synchronization operation. Next, as shown in 2702 of FIG. 27(a), the broadband transmission filter 410 transmits light included in the band of the band-pass filter characteristics of the broadband transmission filter 410 from the generated light having the longitudinal mode interval (Δf). Next, as shown in 2703 of FIG. 27(a), the multiplication filter 420 extracts light having an interval n times that of the light having the longitudinal mode interval (Δf) that has passed through the broadband transmission filter 410. The partial transmission mirror 440 outputs multi-wavelength light having an interval of nΔf as the source light SO1.

[0145] The wavelength demultiplexer 113 demultiplexes the multi-wavelength source light SO1 output from the multi-wavelength light source 112 for each wavelength, and inputs, for example, one wavelength of the demultiplexed source light SO1 to the DP-IQ modulator 220-N as shown in 2704 of FIG. 27(b). The DP-IQ modulator 220-N modulates one wavelength of the source light SO1. The demultiplexer 130-N branches the modulated modulated optical signal SO2 and inputs the branched monitor optical signal SO4 to the band-pass optical filter 140. When there is no wavelength shift in the initial state as shown in 2705 of FIG. 27(c), the center of the spectrum of the monitor optical signal SO4 coincides with the center of the transmission band spectrum of the band-pass optical filter 140. The monitor PD 150-N monitors the light that has passed through the band-pass optical filter 140.

[0146] The wavelength control unit 170 controls the resonator length L of the multi-wavelength light source 112 t (or the resonator circumference 2L t ) and the circumference L of the frequency doubling filter 420 ring so that they satisfy the relationship of the above formula (1) or formula (2), and adjusts the heater current HT1 for the frequency doubling filter and the heater current HT2 for adjusting the resonator length. The resonator length L of the multi-wavelength light source 112 t (or the resonator circumference 2L t ) and the circumference L of the frequency doubling filter 420 ring satisfy the relationship of the above formula (1) or formula (2), multi-wavelength light with an interval of nΔf is normally output from the multi-wavelength light source 112, and thus the monitor power of the monitor signal MO becomes maximum. Therefore, the wavelength control unit 170 adjusts the heater current HT1 for the frequency doubling filter and the heater current HT2 for adjusting the resonator length so that the monitor power of the monitor signal MO monitored by the monitor PD150 becomes maximum, thereby making the relationship between the resonator length L of the multi-wavelength light source 112 t (or the resonator circumference 2L t ) and the circumference L of the frequency doubling filter 420 ring the same as the relationship of the above formula (1) or formula (2). The wavelength control unit 170 sets the heater current HT1 for the frequency doubling filter and the heater current HT2 for adjusting the resonator length when the monitor power of the monitor signal MO is maximum as the initial values.

[0147] Furthermore, the bias control unit 160 modulates each wavelength of the multi-wavelength source light SO1 by the DP-IQ modulators 220-1 to 220-N, and adjusts the bias voltage BS of the DP-IQ modulators 220-1 to 220-N based on the monitor signals MO monitored by the monitor PDs 150-1 to 150-N, and sets the adjusted bias voltage BS as the initial value.

[0148] Subsequently, the optical transmitter 1 performs wavelength control processing on the multi-wavelength light source 112 (S402 to S407). The wavelength control processing of the multi-wavelength light source 112 includes heater current control processing for the frequency doubling filter (S402 to S404) and heater current control processing for adjusting the resonator length (S405 to S407).

[0149] In the heater current control process for the multiplier filter, the monitor PD150-N monitors the light passing through the band-pass optical filter 140 (S402). Specifically, the wavelength demultiplexer 113 demultiplexes the multi-wavelength source light SO1 from the multi-wavelength light source 112, and the DP-IQ modulator 220-N modulates the optical signal demultiplexed from the multi-wavelength source light SO1. The demultiplexer 130-N branches the modulated optical signal SO2 modulated by the DP-IQ modulator 220-N into an output optical signal SO3 and a monitor optical signal SO4. The band-pass optical filter 140 transmits the light in the transmission band from the branched monitor optical signal SO4. The monitor PD150-N detects the optical signal passing through the band-pass optical filter 140 and converts the detected optical signal into a monitor current (monitor signal MO).

[0150] Subsequently, based on the monitoring result of the monitor PD150, the wavelength control unit 170 determines whether control (adjustment) of the heater current HT1 for the multiplier filter is necessary (S403). For example, the wavelength control unit 170 determines whether the monitor current (monitor signal MO) output from the monitor PD150 is below a predetermined threshold (threshold for determining adjustment of the heater current for the multiplier filter). If the monitor current is below the predetermined threshold, it is determined that control of the heater current HT1 for the multiplier filter is necessary.

[0151] Subsequently, when it is determined that control of the heater current HT1 for the multiplier filter is necessary, the wavelength control unit 170 controls (adjusts) the heater current HT1 for the multiplier filter (S404). The wavelength control unit 170 adjusts the heater current HT1 for the multiplier filter according to the detected monitor current and injects the adjusted heater current HT1 for the multiplier filter into the heater 421 for multiplier filter adjustment. For example, the wavelength control unit 170 repeats the adjustment of the heater current HT1 for the multiplier filter until the monitor current exceeds the predetermined threshold.

[0152] Following the heater current control process for the multiplier filter, a heater current control process for resonator length adjustment is performed. In the heater current control process for resonator length adjustment, similarly to S402, the monitor PD150-N monitors the light passing through the band-pass optical filter 140 (S405).

[0153] Subsequently, the wavelength control unit 170 determines whether control (adjustment) of the resonator length adjustment heater current HT2 is necessary based on the monitoring result of the monitor PD150 (S406). For example, the wavelength control unit 170 determines whether the monitor current (monitor signal MO) output from the monitor PD150 is less than or equal to a predetermined threshold value (threshold value for determining adjustment of the resonator length adjustment heater current). If the monitor current is less than or equal to the predetermined threshold value, it is determined that control of the resonator length adjustment heater current HT2 is necessary.

[0154] Subsequently, when it is determined that control of the resonator length adjustment heater current HT2 is necessary, the wavelength control unit 170 controls (adjusts) the resonator length adjustment heater current HT2 (S407). The wavelength control unit 170 adjusts the resonator length adjustment heater current HT2 according to the detected monitor current, and injects the adjusted resonator length adjustment heater current HT2 into the resonator length adjustment heater 431. For example, the wavelength control unit 170 repeats the adjustment of the resonator length adjustment heater current HT2 until the monitor current exceeds a predetermined threshold value. By the multiplier filter heater current control process and the resonator length adjustment heater current control process, the multiplier filter heater current HT1 and the resonator length adjustment heater current HT2 can be adjusted so that the resonator length of the multi-wavelength light source and the round-trip length of the multiplier filter maintain a predetermined relationship. After adjusting the round-trip length of the multiplier filter 420 by adjusting the multiplier filter heater current HT1, the wavelength of the multi-wavelength light source can be accurately adjusted by further adjusting the resonator length by adjusting the resonator length adjustment heater current HT2.

[0155] In the multiplier filter heater current control process and the resonator length adjustment heater current control process, each heater current may be controlled in the same manner as in the examples of FIGS. 13 and 14. That is, when the monitor power of the monitor signal MO decreases by a specified value or more, the multiplier filter heater current HT1 may be adjusted according to the amount of change in the monitor power when the multiplier filter heater current HT1 is increased or decreased. When the monitor power of the monitor signal MO decreases by a specified value or more, the resonator length adjustment heater current HT2 may be adjusted according to the amount of change in the monitor power when the resonator length adjustment heater current HT2 is increased or decreased.

[0156] Subsequent to the heater current control process for the frequency doubling filter and the heater current control process for resonator length adjustment, bias control processing for a plurality of DP-IQ modulators 220 is performed (S408 to S410). The bias control processing is a process in which the processing of S109 to S107 in FIG. 9 is applied to the bias control of a plurality of modulators.

[0157] That is, in the bias control processing, the monitor PDs 150-1 to PD150-N monitor the monitor signal MO modulated and branched by the DP-IQ modulators 220-1 to 220-N (S408). The bias control unit 160 determines whether bias control (adjustment) of the DP-IQ modulators 220-1 to 220-N is necessary based on the monitoring results of the monitor PDs 150-1 to PD150-N (S409). When it is determined that bias control is necessary, the bias control unit 160 controls (adjusts) the bias voltage BS of the corresponding DP-IQ modulator 220 (S410).

[0158] In the bias control processing, the bias voltage may be controlled in the same manner as in the examples of FIGS. 15 and 16. That is, for each DP-IQ modulator 220, when the objective function value of the monitor signal MO increases by a specified value or more, the bias voltage BS may be adjusted according to the amount of change in the objective function value when the bias voltage BS is increased or decreased.

[0159] As described above, the optical transmitter may include a plurality of DP-IQ modulators and a multi-wavelength light source. Even in this case, as in the first embodiment, a plurality of DP-IQ modulators, a multi-wavelength light source, etc. can be integrated, and bias control and wavelength control can be suitably performed. By adjusting the heater current of the multi-wavelength light source, the wavelength shift of the multi-wavelength light can be adjusted all at once. By adjusting the heater current for the frequency doubling filter and the heater current for resonator length adjustment of the mode-locked laser, the wavelength shift of the multi-wavelength light can be accurately adjusted.

[0160] Note that the present disclosure is not limited to the above-described embodiments, and can be appropriately changed without departing from the spirit.

[0161] Each configuration in the above-described embodiment is constituted by hardware or software, or both, and may be constituted by one piece of hardware or software, or may be constituted by a plurality of pieces of hardware or software. Each function (processing) such as the bias control unit and the wavelength control unit may be realized by a computer 30 having a processor 31 such as a CPU (Central Processing Unit) and a memory 32 which is a storage device, as shown in FIG. 28. 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.

[0162] These programs, when loaded into a computer, include a set of instructions (or software code) for causing the computer to perform one or more functions described in the embodiment. The program may be stored in a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, a computer-readable medium or a tangible storage medium includes random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD), or other memory technologies, CD-ROM, digital versatile disc (DVD), Blu-ray (registered trademark) disc, or other optical disc storage, magnetic cassette, magnetic tape, magnetic disk storage, or other magnetic storage devices. The program may also be transmitted on a transient computer-readable medium or a communication medium. By way of example and not limitation, a transient computer-readable medium or a communication medium includes electrical, optical, acoustic, or other forms of propagated signals.

[0163] Although the present disclosure has been described with reference to the embodiments, the present disclosure is not limited to the above-described embodiments. Various changes 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. And each embodiment can be combined with other embodiments as appropriate.

[0164] Each drawing is merely an illustration for explaining one or more embodiments. Each drawing may be associated with not only one specific embodiment but also one or more other embodiments. As can 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, for example, to create embodiments that are not explicitly illustrated or described. Not all of the features or steps shown in any one drawing for explaining exemplary embodiments are necessarily essential, and some features or steps may be omitted. The order of the steps described in any drawing may be changed as appropriate.

[0165] Some or all of the above embodiments may be described as follows, but are not limited thereto.

[0166] (Appendix 1) A mode-locked laser light source that generates multi-wavelength light source light, A plurality of semiconductor modulators that modulate the generated multi-wavelength light source light into modulated light for each wavelength, An optical filter that transmits light in a transmission band from the modulated light modulated by a first semiconductor modulator among the plurality of semiconductor modulators, An optical monitor that monitors the transmitted light, Wavelength control means for controlling the wavelength of the light source light based on the monitored result, Bias control means for controlling the bias voltage of the first semiconductor modulator based on the monitored result, Comprising The mode-locked laser light source A reflective semiconductor optical amplifier that emits light, An external resonator that resonates the light from the reflective semiconductor optical amplifier and includes a frequency-doubling filter that doubles the longitudinal mode interval of the mode-locked laser light source, A frequency-doubling filter adjustment heater capable of heating the frequency-doubling filter, Comprising The wavelength control means controls the wavelength of the light source light by controlling the heater current injected into the heater for adjusting the multiplier filter. Optical transmitter. (Appendix 2) The external resonator includes a resonator length adjustment optical waveguide that adjusts the resonator length including the reflective semiconductor optical amplifier and the external resonator. The mode-locked laser light source includes a heater for adjusting the resonator length that can heat the resonator length adjustment optical waveguide. The wavelength control means controls the wavelength of the light source light by controlling the heater current injected into the heater for adjusting the multiplier filter and the heater for adjusting the resonator length. The optical transmitter according to Appendix 1. (Appendix 3) The multiplier filter is a ring resonator filter. The wavelength control means controls the heater current injected into the heater for adjusting the multiplier filter and the heater for adjusting the resonator length so that the round-trip length of the multiplier filter is an integer fraction of the resonator round-trip length including the reflective semiconductor optical amplifier and the external resonator. The optical transmitter according to Appendix 2. (Appendix 4) The wavelength control means controls the heater current according to whether the power of the monitored light has decreased by a predetermined specified value or more from the initial value. The optical transmitter according to any one of Appendices 1 to 3. (Appendix 5) When the power of the monitored light has decreased by the predetermined specified value or more from the initial value, the wavelength control means controls the heater current according to the amount of change in the power of the monitored light when the heater current is increased and decreased. The optical transmitter according to Appendix 4. (Appendix 6) The wavelength control means controls the heater current according to whether the power of the monitored light increases or decreases when the heater current is increased, and whether the power of the monitored light increases or decreases when the heater current is decreased. The optical transmitter according to Appendix 5. (Appendix 7) The bias control means controls the bias voltage according to whether the function value obtained by converting the monitored optical power by a predetermined objective function has increased by a predetermined specified value or more from the initial value. The optical transmitter according to Appendix 1. (Appendix 8) When the function value based on the monitored optical power has increased by the predetermined specified value or more from the initial value, the bias control means controls the bias voltage according to the amount of change in the function value based on the monitored optical power when the bias voltage is increased and decreased. The optical transmitter according to Appendix 7. (Appendix 9) The bias control means controls the bias voltage according to whether the function value based on the monitored optical power increases or decreases when the bias voltage is increased, and whether the function value based on the monitored optical power increases or decreases when the bias voltage is decreased. The optical transmitter according to Appendix 8. (Appendix 10) Generating light source light of multiple wavelengths by a mode-locked laser light source, Modulating the generated multi-wavelength light source light into modulated light for each wavelength by a plurality of semiconductor modulators, Transmitting light in a transmission band from the modulated light modulated by a first semiconductor modulator among the plurality of semiconductor modulators, Monitoring the transmitted light, Controlling the wavelength of the light source light based on the monitored result, Controlling the bias voltage of the first semiconductor modulator based on the monitored result, including In the mode-locked laser light source, Emitting light by a reflective semiconductor optical amplifier, Doubling the longitudinal mode interval of the mode-locked laser light source by a frequency doubling filter in an external resonator that resonates the light from the reflective semiconductor optical amplifier, including Controlling the wavelength includes controlling the wavelength of the light from the light source by controlling the heater current injected into the heater for adjusting the multiplier filter, which is a heatable multiplier filter. Method for controlling an optical transmitter. (Appendix 11) A light source for generating light from the light source, A heater capable of heating the light source, A semiconductor modulator that modulates the generated light from the light source into modulated light, An optical filter that transmits light in a transmission band from the modulated modulated light, An optical monitor for monitoring the transmitted light, Wavelength control means for controlling the wavelength of the light from the light source by controlling the heater current injected into the heater based on the monitored result, Bias control means for controlling the bias voltage of the semiconductor modulator based on the monitored result, Comprising The wavelength control means controls the heater current according to whether the power of the monitored light has decreased from an initial value by a predetermined specified value or more. Optical transmitter. (Appendix 12) When the power of the monitored light has decreased from the initial value by the predetermined specified value or more, the wavelength control means controls the heater current according to the amount of change in the power of the monitored light when the heater current is increased and decreased. The optical transmitter according to Appendix 11. (Appendix 13) The wavelength control means controls the heater current according to whether the power of the monitored light increases or decreases when the heater current is increased, and whether the power of the monitored light increases or decreases when the heater current is decreased. The optical transmitter according to Appendix 12. (Appendix 14) A light source for generating light from the light source, A heater capable of heating the light source, A semiconductor modulator that modulates the generated light from the light source into modulated light, An optical filter that transmits light in a transmission band from the modulated modulated light; An optical monitor that monitors the transmitted light; Wavelength control means for controlling the wavelength of the light source light by controlling the heater current injected into the heater based on the monitored result; Bias control means for controlling the bias voltage of the semiconductor modulator based on the monitored result; Comprising; The bias control means controls the bias voltage according to whether or not a function value obtained by converting the power of the monitored light by a predetermined objective function has increased by a predetermined specified value or more from an initial value. Optical transmitter. (Appendix 15) When the function value based on the power of the monitored light has increased by the predetermined specified value or more from the initial value, the bias control means controls the bias voltage according to the amount of change in the function value based on the power of the monitored light when the bias voltage is increased and decreased. The optical transmitter according to Appendix 14. (Appendix 16) The bias voltage control means controls the bias voltage according to whether the function value based on the power of the monitored light increases or decreases when the bias voltage is increased, and whether the function value based on the power of the monitored light increases or decreases when the bias voltage is decreased. The optical transmitter according to Appendix 15. (Appendix 17) Generating light source light with a light source; Modulating the generated light source light into modulated light by a semiconductor modulator; Transmitting light in a transmission band from the modulated modulated light; Monitoring the transmitted light; Controlling the wavelength of the light source light by controlling the heater current injected into a heater capable of heating the light source based on the monitored result; Controlling the bias voltage of the semiconductor modulator based on the monitored result; including controlling the wavelength includes controlling the heater current according to whether the power of the monitored light has decreased from an initial value by a predetermined specified value or more. A method for controlling an optical transmitter. (Appendix 18) generating light source light by a light source; modulating the generated light source light into modulated light by a semiconductor modulator; transmitting light in a transmission band from the modulated modulated light; monitoring the transmitted light; controlling the wavelength of the light source light by controlling the heater current injected into a heater capable of heating the light source based on the monitored result; controlling the bias voltage of the semiconductor modulator based on the monitored result; including controlling the bias voltage includes controlling the bias voltage according to whether a function value obtained by converting the power of the monitored light by a predetermined objective function has increased from an initial value by a predetermined specified value or more. A method for controlling an optical transmitter.

[0167] Some or all of the elements (e.g., configuration and function) described in Appendices 2 to 9 that are subordinate to the optical transmitter of Appendix 1 may be subordinate to the method for controlling the optical transmitter of Appendix 10 in the same subordinate relationship as Appendices 2 to 9. Some or all of the elements (e.g., configuration and function) described in Appendices 12 to 13 that are subordinate to the optical transmitter of Appendix 11 may be subordinate to the method for controlling the optical transmitter of Appendix 17 in the same subordinate relationship as Appendices 12 to 13. Some or all of the elements (e.g., configuration and function) described in Appendices 15 to 16 that are subordinate to the optical transmitter of Appendix 14 may be subordinate to the method for controlling the optical transmitter of Appendix 18 in the same subordinate relationship as Appendices 15 to 16. Some or all of the elements described in any appendix may be applied to various hardware, software, recording means for recording software, systems, and methods.

Explanation of Reference Numerals

[0168] 1. 10 Optical transmitter 11 Light source 12 Semiconductor modulator 13 Optical filter 14 Optical monitor 15 Bias control unit 16 Wavelength control unit 20 Mode-locked laser light source 21 Reflective semiconductor optical amplifier 22 External resonator 23 Multiplier filter 24 Multiplier filter adjustment heater 30 Computer 31 Processor 32 Memory 110 Single-wavelength light source 111 Demultiplexer 112 Multi-wavelength light source 113 Wavelength demultiplexer 120 Semiconductor modulator 121 Demultiplexer 122, 122-1 to 122-4 MZ modulators 123, 123-1 to 123-2 Phase shifters 124, 124-1 to 124-2 Multiplexers 125 Demultiplexer 126 Polarization rotator 127 Polarization coupler 130, 130-1 to 130-N Demultiplexers 140 Bandpass optical filter 150, 150-1 to 150-N Monitor PDs 160 Bias control unit 170 Wavelength control unit 180 Optical switch 200 Semiconductor substrate 201 Lower cladding layer 202 Active layer 203 Upper cladding layer 204 Heater 205 Solder bump 220, 220-1 to 220-N DP-IQ modulators 300 Optical amplifier element 310 Reflective semiconductor optical amplifier 311 Saturable absorption region 312 Gain region 321 AR coating 322 Reflective end face 331 Lower cladding layer 332 Active layer 333 Upper cladding layer 334 Saturable absorption region electrode 335 Gain region electrode 400 External resonator 401 Optical waveguide 410 Broadband transmission filter 420 Doubling filter 420a~420d Optical waveguide 421 Heater for adjusting doubling filter 430 Optical waveguide for adjusting resonator length 430a~430c Optical waveguide 431 Heater for adjusting resonator length 440 Partially transmissive mirror

Claims

1. A mode-locked laser light source that generates multi-wavelength source light, a plurality of semiconductor modulators that modulate the generated multi-wavelength source light into modulated light for each wavelength, an optical filter that transmits light in a transmission band from the modulated light modulated by a first semiconductor modulator among the plurality of semiconductor modulators, an optical monitor that monitors the transmitted light, wavelength control means for controlling the wavelength of the source light based on the monitored result, bias control means for controlling the bias voltage of the first semiconductor modulator based on the monitored result, comprising: The mode-locked laser light source a reflective semiconductor optical amplifier that emits light, an external resonator that resonates the light from the reflective semiconductor optical amplifier, the external resonator including a frequency doubling filter that doubles the longitudinal mode interval of the mode-locked laser light source, a frequency doubling filter adjustment heater capable of heating the frequency doubling filter, comprising: The wavelength control means controls the wavelength of the source light by controlling the heater current injected into the frequency doubling filter adjustment heater. An optical transmitter.

2. The external resonator includes an optical waveguide for resonator length adjustment that adjusts the resonator length including the reflective semiconductor optical amplifier and the external resonator, The mode-locked laser light source includes a resonator length adjustment heater capable of heating the optical waveguide for resonator length adjustment, The wavelength control means controls the wavelength of the source light by controlling the heater current injected into the frequency doubling filter adjustment heater and the resonator length adjustment heater. The optical transmitter according to claim 1.

3. The frequency doubling filter is a ring resonator filter, The wavelength control means controls the heater current injected into the frequency doubling filter adjustment heater and the resonator length adjustment heater so that the round-trip length of the frequency doubling filter is an integer fraction of the round-trip length of the resonator including the reflective semiconductor optical amplifier and the external resonator. The optical transmitter according to claim 2.

4. The wavelength control means controls the heater current according to whether the power of the monitored light has decreased by a predetermined specified value or more from an initial value. The optical transmitter according to any one of claims 1 to 3.

5. When the power of the monitored light has decreased by the predetermined specified value or more from the initial value, the wavelength control means controls the heater current according to the amount of change in the power of the monitored light when the heater current is increased and decreased. The optical transmitter according to claim 4.

6. The wavelength control means controls the heater current according to whether the power of the monitored light increases or decreases when the heater current is increased, and whether the power of the monitored light increases or decreases when the heater current is decreased. The optical transmitter according to claim 5.

7. The bias control means controls the bias voltage according to whether the function value obtained by converting the power of the monitored light by a predetermined objective function has increased by a predetermined specified value or more from the initial value. The optical transmitter according to claim 1.

8. When the function value based on the monitored light power has increased by the predetermined specified value or more from the initial value, the bias control means controls the bias voltage according to the amount of change in the function value based on the monitored light power when the bias voltage is increased and decreased. The optical transmitter according to claim 7.

9. The bias control means controls the bias voltage according to whether the function value based on the monitored light power increases or decreases when the bias voltage is increased, and whether the function value based on the monitored light power increases or decreases when the bias voltage is decreased. The optical transmitter according to claim 8.

10. Generating light source light of multiple wavelengths by a mode-locked laser light source; Modulating the generated multi-wavelength light source light into modulated light for each wavelength by a plurality of semiconductor modulators; Transmitting light in a transmission band from the modulated light modulated by a first semiconductor modulator among the plurality of semiconductor modulators; Monitoring the transmitted light; Controlling the wavelength of the light source light based on the monitored result; Controlling the bias voltage of the first semiconductor modulator based on the monitored result; including In the mode-locked laser light source, Emitting light by a reflective semiconductor optical amplifier; Doubling the longitudinal mode interval of the mode-locked laser light source by a doubling filter in an external resonator that resonates the light from the reflective semiconductor optical amplifier; including Controlling the wavelength includes controlling the wavelength of the light source light by controlling the heater current injected into a heater for adjusting the doubling filter, which can heat the doubling filter. A control method for an optical transmitter.

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