Optical transmitter and control method for optical transmitter

Abstract

The present invention provides an optical transmitter capable of precisely controlling the bias voltage and a method for controlling the optical transmitter. [Solution] The optical transmitter comprises an optical modulator that modulates the input light and outputs the modulated light, a calculation unit that calculates an objective function based on the modulated light output from the optical modulator, a suppression unit that suppresses amplitude fluctuations of the objective function calculated by the calculation unit, and a setting unit that sets the bias voltage of the optical modulator according to the value of the objective function whose amplitude fluctuations have been suppressed by the suppression unit.

Description

[Technical Field]

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

[0002] In recent years, with the spread of the internet and 5G, research has been progressing on increasing the speed and capacity of optical communications, which are used as foundational technologies. For example, by using a digital coherent method that combines optical phase modulation and polarization multiplexing / decoupling technology, a capacity exceeding 100 Gbps (Gigabits per second) has been achieved. The MZ modulator (MZM: Mach-Zehnder Modulator) is used as a modulator for such high-speed transmission.

[0003] Related technologies are known in Patent Document 1 and Non-Patent Document 1. 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. Non-Patent Document 1 describes an algorithm for controlling the bias of an MZ modulator using an objective function. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 5924349 [Non-patent literature]

[0005] [Non-Patent Document 1] Takanori Shimizu, Mingqi Wu, and Kohei Hosokawa, “Demonstration of ditherless auto bias control for optical IQ modulator using genetic algorithm and greedy method,” 29th Opto-Electronics and Communication Conference (OECC2024), Melbourne, Australia, July, 2024, paper no. WeD1 / Track4 #379 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In related technologies such as Patent Document 1 and Non-Patent Document 1, an optical modulator modulates light from a light source, and the bias voltage of the optical modulator is controlled based on the modulated light. However, because the light modulated by the optical modulator contains frequency components that are unnecessary for controlling the bias voltage, it can be difficult to control the bias voltage accurately.

[0007] In view of these challenges, one of the objectives of this disclosure is to provide an optical transmitter and a method for controlling an optical transmitter that can control the bias voltage with high precision. [Means for solving the problem]

[0008] An optical transmitter according to one aspect of the present disclosure includes: an optical modulator that modulates input light and outputs the modulated light; a calculation unit that calculates an objective function based on the output modulated light; a suppression unit that suppresses amplitude fluctuations of the calculated objective function; and a setting unit that sets the bias voltage of the optical modulator according to the value of the objective function in which the amplitude fluctuations have been suppressed.

[0009] A method for controlling an optical transmitter according to an aspect of the present disclosure modulates input light by an optical modulator, outputs the modulated modulated light, calculates an objective function based on the output modulated light, suppresses amplitude fluctuations of the calculated objective function, and sets a bias voltage of the optical modulator according to the value of the objective function with suppressed amplitude fluctuations.

Advantages of the Invention

[0010] According to the present disclosure, the bias voltage can be accurately controlled.

Brief Description of the Drawings

[0011] [Figure 1] It is a configuration diagram showing the configuration of an optical transmitter in a related technology. [Figure 2] It is a graph showing the bias dependence of an objective function in a related technology. [Figure 3] It is a graph showing the bias dependence of an objective function when a low-pass filter is used in a related technology. [Figure 4] It is a configuration diagram showing a configuration example of an optical transmitter according to some embodiments. [Figure 5] It is a configuration diagram showing a configuration example of an optical transmitter according to some embodiments. [Figure 6] It is a flowchart showing an operation example of an optical transmitter according to some embodiments. [Figure 7] It is a graph showing the bias dependence of an objective function according to some embodiments. [Figure 8] It is a graph showing the bias dependence of an objective function according to some embodiments. [Figure 9] It is a flowchart showing a detailed operation example of the bias control process of an optical transmitter according to some embodiments. [Figure 10] It is a flowchart showing an operation example of an optical transmitter according to some embodiments. [Figure 11] It is a configuration diagram showing a configuration example of the hardware of a computer according to some embodiments. [Modes for carrying out the invention]

[0012] The embodiments will be described below with reference to the drawings. In each drawing, the same or corresponding elements are denoted by the same reference numerals, and redundant explanations are omitted where necessary for clarity. The arrows shown in each drawing are illustrative examples and do not limit the type or direction of the signal.

[0013] (Consideration of related technologies) First, we will examine the related technologies. Figure 1 shows the configuration of an optical transmitter 9 in a related technology, based on the description in Patent Document 1, for example. Figure 1 is an example of the configuration of an optical transmitter using an LN modulator. As shown in Figure 1, the related optical transmitter 9 comprises a light source 901, an LN modulator 902, a monitor PD 903, and a bias control unit 904.

[0014] In the associated optical transmitter 9, the wavelength-controlled light source (light source module) 901 is connected to the LN modulator 902 via a fiber or the like. The LN modulator 902 modulates the light output from the light source 901. The monitor PD (Photo Detector) 903 receives and monitors the light modulated by the LN modulator 902. The bias control unit 904 feedback-controls the bias voltage of the LN modulator 902 according to the monitoring results of the monitor PD 903 so as to reduce the output deviation due to bias point (operating point) oscillation of the LN modulator 902.

[0015] The bias control unit 904 calculates an objective function from the power of the monitor signal converted into an electrical signal by the monitor PD903, and adjusts the bias voltage according to the calculated value of the objective function (objective function value). The objective function is an objective function for controlling the bias voltage of the optical modulator, and the optimal bias point can be set by adjusting the bias voltage so that the objective function value is minimized. For example, the objective function calculates the average value and variance of the power of the monitor signal. Although Patent Document 1 employs a dithering method in which a dithering signal is superimposed on the input signal, the example in Figure 1 (and the embodiment thereafter) employs a ditherless method that does not use a dithering signal, similar to Non-Patent Document 1. By controlling the bias voltage according to the objective function calculated from modulated light that does not contain a dithering signal, the influence of the dithering signal on the main signal is suppressed, and the bias voltage can be controlled with high precision with a simple configuration.

[0016] Figure 2 shows the characteristics of the objective function with respect to the bias voltage (bias dependence of the objective function). Figure 2 is an example of calculating the average value of the monitor signal using the objective function, similar to Non-Patent Document 1.

[0017] When controlling the bias voltage using an objective function, the DC (Direct Current) component and its surrounding low-frequency components are used from the monitor signal converted into an electrical signal by the monitor PD. Typically, a monitor PD designed for acquiring low-frequency components is used, and although the frequency characteristics are suppressed, the monitor signal also contains the main signal transmitted by the optical transmitter, so medium to high-frequency components are superimposed. Therefore, as shown in Figure 2, in the bias dependence of the objective function, such as the mean and variance, amplitude fluctuations (fluctuations in the direction of increase or decrease of the objective function value) occur due to the influence of the medium to high-frequency components added to the objective function. When searching for the optimal bias voltage that minimizes the objective function value, if the objective function contains amplitude fluctuations, it becomes difficult to converge to the accurate bias point.

[0018] To solve this problem, one possible method is to use a low-pass filter. Figure 3 shows the bias dependence of the objective function when the cutoff frequency of the low-pass filter is changed. Figure 3 is an example in which the variance of the monitor signal is calculated using the objective function and the calculation result is applied to a low-pass filter. As shown in Figure 3, lowering the cutoff frequency of the low-pass filter can suppress amplitude fluctuations of the objective function. However, a low-pass filter cuts out components above the cutoff frequency. Therefore, lowering the cutoff frequency of the low-pass filter increases the frequency band that is cut off, and the value of the objective function decreases. For this reason, as shown in Figure 3, if the value of the objective function decreases significantly by lowering the cutoff frequency, the amount of change in the value of the objective function in response to the bias voltage also becomes small, making it difficult to converge to the accurate bias point.

[0019] Therefore, in this embodiment, the amplitude fluctuations of the objective function are suppressed while preventing a significant decrease in the objective function value.

[0020] (Embodiment 1) Next, Embodiment 1 will be described. This embodiment will outline several embodiments.

[0021] Figure 4 shows some configuration examples of an optical transmitter 10 according to several embodiments. The optical transmitter 10 is, for example, an optical transmitter for digital coherent communication. In the example in Figure 4, the optical transmitter 10 includes an optical modulator 11, a calculation unit 12, a suppression unit 13, and a setting unit 14.

[0022] For example, the optical modulator 11 receives light from a light source as input. The optical modulator 11 modulates the input light and outputs the modulated light. For example, the optical modulator 11 may be an IQ (In-phase and Quadrature) modulator.

[0023] The calculation unit 12 calculates an objective function based on the modulated light output from the optical modulator 11. For example, the modulated light output from the optical modulator 11 is monitored by the monitor PD, and the calculation unit 12 calculates an objective function from the monitored signal. For example, the calculation unit 12 calculates the average value and variance of the power of the monitor signal using the objective function.

[0024] The suppression unit 13 suppresses amplitude fluctuations in the objective function calculated by the calculation unit 12. The suppression unit 13 may also perform an averaging process of the objective function. In the averaging process of the objective function, the average of the objective function values ​​calculated from multiple monitor signals (measurement signals) obtained by monitoring (measuring) the modulated light multiple times may be obtained. Alternatively, in the averaging process of the objective function, a moving average of the objective function values ​​calculated from multiple sampled values ​​obtained by sampling the monitor signals (measurement signals) obtained by monitoring (measuring) the modulated light may be obtained. The suppression unit 13 is not limited to averaging, but may also perform a bandpass filter process to remove components in a predetermined frequency band from the objective function value calculated by the calculation unit 12. In this case, the predetermined frequency band to be cut off by the filter process includes a frequency band where the amplitude fluctuation is greater than the predetermined value.

[0025] The setting unit 14 sets the bias voltage of the optical modulator 11 according to the objective function value whose amplitude fluctuations have been suppressed by the suppression unit 13. For example, the setting unit 14 adjusts the bias voltage so that the objective function value is minimized.

[0026] Thus, in this embodiment, the bias voltage can be controlled with high precision by suppressing amplitude fluctuations in the objective function. For example, by suppressing amplitude fluctuations in the objective function through averaging or bandpass filtering, it is possible to suppress amplitude fluctuations in the objective function while preventing a significant decrease in the objective function value.

[0027] (Embodiment 2) Next, Embodiment 2 will be described. In this embodiment, a specific example of Embodiment 1 will be described.

[0028] Figure 5 shows an example configuration of an optical transmitter 1 according to several embodiments. The optical transmitter 1 is, for example, an optical transmitter for digital coherent communication. In the example in Figure 5, the optical transmitter 1 includes a light source 110, an optical modulator 120, a demultiplexer 130, a monitor PD 140, and a bias control unit 150.

[0029] For example, the light source 110, optical modulator 120, demultiplexer 130, and monitor PD 140 may be mounted or integrated on a single semiconductor substrate. The bias control unit 150 may be included in a semiconductor chip separate from the semiconductor substrate containing the optical modulator 120. The bias control unit 150 may be composed of hardware, software, or both. The function of the bias control unit 150 may be implemented by executing a program stored in memory in a processor such as a CPU (Central Processing Unit). For example, a semiconductor chip including the bias control unit 150 may be mounted on the semiconductor substrate containing the optical modulator 120 using a flip-chip or the like. A semiconductor device including all the configurations shown in Figure 5 may be a single semiconductor package.

[0030] The light source 110 generates light SO1 of a predetermined wavelength. The light source 110 may be a semiconductor laser formed on the same semiconductor substrate as the semiconductor modulator. The light source 110 may also be an external light source located outside the LN modulator. For example, the light source 110 may be a DFB (Distributed Feedback) laser, a DBR (Distributed Bragg Reflector) laser, or an External Cavity Laser (ECL).

[0031] The optical modulator 120 is an IQ modulator for digital coherent communication. The optical modulator 120 may be a semiconductor modulator or an LN modulator. The optical modulator 120 modulates the optical light SO1 output from the light source 110 into a modulated optical signal SO2. The optical modulator 120 applies IQ modulation (coherent modulation) to the optical light SO1 output from the light source 110 and outputs the modulated optical signal SO2.

[0032] The optical modulator 120 is composed of an MZ interferometer including two MZ modulators. The optical modulator 120 comprises a demultiplexer 121, MZ modulators 122-1 and 122-2, a phase shifter 123, and a multiplexer 124. The demultiplexer 121 splits (demultiplexes) the light SO1 from the light source 110 into light for Ich (in-phase component) and light for Qch (orthogonal component).

[0033] The MZ modulator 122-1 is, for example, a modulator for channel I. The MZ modulator 122-1 receives the data signal DT1 for channel I, which is generated from the transmitted data signal, as a drive signal. The MZ modulator 122-1 modulates the light for channel I, which has been split by the demultiplexer 121, according to the data signal DT1. In addition, the MZ modulator 122-1 receives a bias voltage BS1 from the bias control unit 150 to adjust the bias point.

[0034] The MZ modulator 122-2 is, for example, a modulator for the Q channel. The MZ modulator 122-2 receives the data signal DT2 for the Q channel, which is generated from the transmitted data signal, as a drive signal. The MZ modulator 122-2 modulates the light for the Q channel, which is branched by the demultiplexer 121, according to the data signal DT2. In addition, the MZ modulator 122-2 receives a bias voltage BS2 from the bias control unit 150 to adjust the bias point.

[0035] The phase shifter 123 shifts the phase of the optical signal of Ich or the optical signal of Qch so that the phases of the optical signals of Ich and Qch are orthogonal. In this example, the phase shifter 123 shifts the phase of the modulated optical signal of Qch, which is modulated by the MZ modulator 122-2, by π / 2. The phase shifter 123 may be configured as an MZ interferometer, similar to the MZ modulator, or it may be configured to shift the phase by changing the refractive index of the waveguide. A bias voltage BS3 is applied to the phase shifter 123 from the bias control unit 150 to adjust the amount of phase shift. When the phase shifter 123 is configured as an MZ interferometer, the amount of phase shift may be controlled by the bias voltage BS3 applied to the MZ interferometer. Alternatively, a heater may be formed in the waveguide constituting the phase shifter 123, and the amount of phase shift may be controlled by changing the refractive index of the waveguide with the heat generated by applying the bias voltage BS3 to the formed heater.

[0036] The multiplexer 124 combines the optical signal modulated by the MZ modulator 122-1 with the optical signal modulated by the MZ modulator 122-2 and phase-shifted by the phase shifter 123, and outputs the combined modulated optical signal SO2.

[0037] The demultiplexer 130, located downstream of the optical modulator 120, splits the modulated optical signal SO2 generated by the optical modulator 120 (IQ modulator) into an output optical signal SO3 and a monitor optical signal SO4. The split output optical signal SO3 is output externally (to an optical transmission line, etc.) as transmitted light. Alternatively, the multiplexer 124 and demultiplexer 130 of the optical modulator 120 may be combined into a single optical coupler (2x2 optical coupler).

[0038] The monitor PD140 is a photodetector that monitors (detects) the monitor optical signal SO4 (a part of the modulated optical signal) that has been split by the demultiplexer 130. For example, the monitor PD140 may be a semiconductor PD formed on a semiconductor substrate. The monitor PD140 performs photoelectric conversion of the monitor optical signal SO4 and outputs the photoelectrically converted monitor signal MO.

[0039] The bias control unit 150 controls the bias voltage BS of the optical modulator 120 based on the monitor signal MO, which is the monitoring result of the monitor PD 140. The modulated light output from the optical modulator 120 is branched, and the bias voltage of the optical modulator 120 is automatically optimized by feedback control of the bias voltage of the optical modulator 120 according to the monitoring result of the branched light. In this example, the bias control unit 150 uses the electrical signal output from the monitor PD 140 to perform a predetermined identification process and controls the bias voltages BS1 to BS3. That is, the bias control unit 150 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, respectively, based on the monitor signal MO. The bias control unit 150 may control all of the bias voltages BS1 to BS3, or it may control only the bias voltage BS that needs adjustment.

[0040] Furthermore, the bias control unit 150 includes an AD converter 151, a calculation unit 152, a suppression unit 153, a setting unit 154, and a DA converter 155.

[0041] The AD converter 151 performs analog-to-digital (AD) conversion of the monitor signal MO, an analog electrical signal output from the monitor PD140, into a digital electrical signal. The AD converter 151 performs AD conversion by sampling the monitor signal MO at a predetermined sampling interval.

[0042] The calculation unit 152 calculates an objective function based on the digital signal of the monitor signal MO converted by the AD converter 151. The objective function is a function used for controlling the bias voltage of the MZ modulator or phase shifter. In the case of an MZ modulator, if the bias point deviates from the optimal bias point, the unmodulated component passes through the optical modulator 120 as a DC component. Therefore, the monitor signal MO mainly contains this DC component, and as it approaches the optimal bias point, the DC component decreases, so a function that reduces this component is set as the objective function. For example, when the bias voltage supplied to the optical modulator is optimal, that is, when the bias point of the optical modulator is not deviated, the objective function value obtained from the power of the light modulated by the optical modulator will be the minimum value. The objective function may be used to calculate the average value of the power (digital signal value) of the monitor signal MO, or to calculate the variance value of the power of the monitor signal MO. For example, when controlling the bias voltage of the MZ modulator, the average value of the monitor signal MO may be calculated, and when controlling the bias voltage of the phase shifter, the variance value of the monitor signal MO may be calculated.

[0043] The suppression unit 153 performs a suppression process to suppress amplitude fluctuations in the objective function calculated by the calculation unit 152. For example, the suppression process may be a process of averaging objective function values ​​obtained by measuring (monitoring) the modulated optical signal of the optical modulator 120 multiple times. That is, after n measurements, the average of the objective function values ​​obtained from the n measurements may be calculated. The suppression process may also be a process of measuring the modulated optical signal of the optical modulator 120 once and calculating a moving average of multiple objective function values ​​obtained by AD conversion. For example, if the monitor signal MO is AD converted and 5000 digital signal values ​​are obtained, a moving average may be calculated every 10 points. Alternatively, the suppression process may be a bandpass filter process that cuts only the frequency band in which the amplitude fluctuation is greater than a predetermined value. That is, instead of cutting all frequency components above the cutoff frequency like a lowpass filter, only the frequency band in a predetermined range that includes the frequency of the amplitude fluctuation is cut. For example, the frequency band to be cut off is set in advance. The frequency band of amplitude fluctuations may be identified from the frequency components of the monitor signal MO, and this identified frequency band may be used as the cutoff frequency band. The suppression unit 153 may perform these suppression processes in combination. For example, a moving average may be calculated for each measurement, and the average of n measurements may be calculated after n measurements. Alternatively, the average of the values ​​passed through a bandpass filter may be calculated.

[0044] The setting unit 154 sets 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, respectively, based on the objective function value from which amplitude fluctuations have been suppressed by the suppression unit 153.

[0045] The DA converter 155 performs a digital-to-analog (DA) conversion of the bias voltage BS of the digital electrical signal set by the setting unit 154 into an analog electrical signal. The DA converter 155 applies the DA-converted analog electrical signals of bias voltages BS1 to BS3 to the MZ modulators 122-1 and 122-2 and the phase shifter 123.

[0046] Figure 6 shows an example of the operation of the optical transmitter 1 according to several embodiments. In the example in Figure 6, first, the bias control unit 150 supplies initial values ​​of bias voltages BS1 to BS3 to the MZ modulators 122-1 and 122-2 and the phase shifter 123 (S101). The setting unit 154 generates bias voltages BS1 to BS3, which are predetermined initial values. The DA converter 155 performs DA conversion on the generated bias voltages BS1 to BS3 and applies the analog electrical signals of the DA-converted bias voltages BS1 to BS3 to the MZ modulators 122-1 and 122-2 and the phase shifter 123. The optical modulator 120 performs IQ modulation operation with a bias point and phase shift amount corresponding to the initial bias voltages BS1 to BS3.

[0047] Next, bias control processing (S102~S105) is performed for MZ modulators 122-1 and 122-2, followed by bias control processing (S106~S109) for phase shifter 123. Note that in the bias control processing for MZ modulators 122-1 and 122-2, the bias control processing for MZ modulator 122-1 and the bias control processing for MZ modulator 122-2 may be performed simultaneously, or one may be performed first.

[0048] First, in the bias control process of MZ modulators 122-1 and 122-2, the monitor PD 140 monitors the modulated optical signal modulated by the optical modulator 120 and converts it into a monitor signal (S102). Specifically, when optical modulator 120 receives optical SO1 from light source 110 via demultiplexer 121 and data signals DT1 and DT2, it IQ modulates optical SO1 according to the data signals DT1 and DT2. Demultiplexer 130 splits the IQ modulated modulated optical signal SO2 into an output optical signal SO3 for output and a monitor optical signal SO4 for monitoring. The monitor PD 140 detects the split monitor optical signal SO4 and converts the detected optical signal into a monitor signal MO (monitor current). Furthermore, the AD converter 151 AD converts the monitor signal MO, an analog electrical signal output from monitor PD 140, into a digital electrical signal.

[0049] Next, the bias control unit 150 performs a process to suppress amplitude fluctuations in the objective function F1 derived from the monitor signal (S103). The calculation unit 152 calculates the objective function F1 (e.g., the average value) for bias control of the MZ modulators 122-1 and 122-2 from the digital signal value of the AD-converted monitor signal MO. The suppression unit 153 performs a suppression process to suppress amplitude fluctuations in the calculated objective function F1. As described above, the suppression unit 153 performs one of the following as amplitude fluctuation suppression processes, or a combination thereof: a process to calculate the average of n objective function values ​​after n measurements; a process to calculate a moving average of multiple objective function values ​​obtained from a single measurement; or a process to cut out only the frequency band with large amplitude fluctuations.

[0050] Figure 7 shows the bias dependence of the objective function F1 before amplitude fluctuation suppression, and Figure 8 shows the bias dependence of the objective function F1 after amplitude fluctuation suppression processing. Figure 8 is an example of calculating the average value after 5 measurements. As shown in Figure 7, the objective function F1 calculated from the monitor signal MO contains amplitude fluctuations. As shown in Figure 8, by performing amplitude fluctuation suppression processing, the power level can be maintained while suppressing the error amplitude. Note that Figure 8 shows the result of applying amplitude fluctuation processing to all bias voltages as an example of the bias dependence of the objective function for explanatory purposes, but in the flow chart of Figure 6, amplitude fluctuation processing is performed only on the objective function measured and calculated at the bias voltage set for adjustment.

[0051] Next, the bias control unit 150 determines whether or not adjustment (control) of the bias voltages BS1 and BS2 is necessary based on the objective function F1 with suppressed amplitude fluctuations (S104). For example, the setting unit 154 determines whether or not the value of the objective function F1 after fluctuation suppression is within a predetermined range, as shown in Figure 8. If the value of the objective function F1 exceeds the predetermined range, it determines that bias adjustment of the MZ modulators 122-1 and 122-2 is necessary.

[0052] Next, if it is determined that bias adjustment is necessary, the bias control unit 150 adjusts (controls) the bias voltages BS1 and BS2 so that the objective function F1 with suppressed amplitude fluctuations is minimized (S105). The setting unit 154 controls the bias voltage BS1 applied to the MZ modulator 122-1 and the bias voltage BS2 applied to the MZ modulator 122-2, respectively, based on the detected monitor current.

[0053] In the case of an MZ modulator, if the bias voltage deviates from the optimal value, the unmodulated component passes through the optical modulator as a DC component. Based on the monitor signal MO, the setting unit 154 adjusts the bias voltages BS1 and BS2 so that the DC component of the modulated optical signal of the optical modulator 120 is less than or equal to the set value. As shown in Figure 8, as the bias voltage moves away from the optimal value, the DC component increases, and the objective function value also increases. As the bias voltage approaches the optimal value, the DC component decreases, and the objective function value also decreases. Therefore, as shown in Figure 8, the setting unit 154 repeatedly adjusts the bias voltages BS1 and BS2 so that the value of the objective function F1 after amplitude fluctuation suppression is minimized. The adjustment may also be made so that the value of the objective function F1 is less than or equal to the set value. The DA converter 155 performs DA conversion on the bias voltages BS1 and BS2 set by the setting unit 154 for adjustment, and applies the DA-converted analog electrical signals to the MZ modulators 122-1 and 122-2.

[0054] Next, in the bias control process of the phase shifter 123, similar to S102, the monitor PD140 monitors the modulated optical signal modulated by the optical modulator 120 and converts it into a monitor signal (S106).

[0055] Next, the bias control unit 150 performs a process to suppress amplitude fluctuations in the objective function F2 derived from the monitor signal (S107). The calculation unit 152 calculates the objective function F2 (e.g., variance value) for bias control of the phase shifter 123 from the digital signal value of the AD-converted monitor signal MO. The suppression unit 153 performs a suppression process to suppress amplitude fluctuations in the calculated objective function F2, similar to S103.

[0056] Next, the bias control unit 150 determines whether or not adjustment (control) of the bias voltage BS3 is necessary based on the objective function F2 in which amplitude fluctuations have been suppressed (S108). For example, the setting unit 154, similar to S104, determines whether or not the objective function F2 after fluctuation suppression, according to the monitor signal MO (monitor current), is within a predetermined range (which may be a different range than that in S104), and if the objective function F2 exceeds the predetermined range, it determines that bias adjustment of the phase shifter 123 is necessary.

[0057] Next, if it is determined that bias adjustment is necessary, the bias control unit 150 adjusts (controls) the bias voltage BS3 so that the objective function F2 with suppressed amplitude fluctuations is minimized (S109). The setting unit 154 repeats the adjustment of the bias voltage BS3, similar to S105, so that the objective function F2 after amplitude fluctuation suppression is minimized. The DA converter 155 performs a DA conversion on the bias voltage BS3 set by the setting unit 154 for adjustment, and applies the DA converted analog electrical signal to the phase shifter 123. After that, for example, steps S102 to S105 and S106 to S109 are repeated periodically.

[0058] Figure 9 shows detailed operation examples of the bias control process of the optical modulator 120 according to several embodiments. For example, S201 to S206 in Figure 9 correspond to S104 to S105 and S108 to S109 in Figure 6. Here, an example of adjusting the bias voltage BS (one of BS1 to BS3) based on the objective function (F1 or F2) is described. Note that in the following description, the specified value (Δf0) and the amount of variation (±ΔV) differ depending on whether the objective function is F1 or F2.

[0059] In the example shown in Figure 9, the bias control unit 150 determines whether the objective function value based on the monitor signal MO output from the monitor PD 140 has increased by a specified value (Δf0) or more (S201). For example, the bias control unit 150 compares the initial value of the objective function value based on the monitor signal MO from the monitor PD 140 when the initial value of the bias voltage BS was set in S101 of Figure 6 with the current objective function value based on the monitor signal MO from the monitor PD 140. The initial value of the objective function value is the objective function value based on the monitor signal MO monitored by the monitor PD 140 when the bias point of the optical modulator 120 is not shifted. The bias control unit 150 determines whether the current objective function value is greater than or equal to the specified value relative to the initial value of the objective function value.

[0060] If the objective function value based on the monitor signal MO does not increase by more than the specified value (Δf0), the bias control unit 150 determines that bias control is unnecessary and terminates the bias control process.

[0061] Furthermore, if the objective function value based on the monitor signal MO increases by a specified value (Δf0) or more, the bias control unit 150 determines that bias control is necessary and increases or decreases the bias voltage BS by a certain amount (±ΔV) (S202). For example, the bias control unit 150 sets the bias voltage BS to the current set value (V n The monitor signal MO is obtained from the monitor PD140 by reducing the bias voltage BS from (-ΔV). The bias control unit 150 also sets the bias voltage BS to the current set value (V n The monitor signal MO is obtained from monitor PD140 by increasing the voltage from (+ΔV).

[0062] Next, the bias control unit 150 determines whether the amount of variation in the objective function value based on the monitor signal MO is within a specified value (Δf0) (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 or different. For example, the bias control unit 150 determines whether the bias voltage BS is within the current set value (V n The objective function value (Mf) based on the monitor signal MO in the case of ) n ) and the bias voltage BS set to the value (V nCompare it with the objective function value (Mf1) based on the monitor signal MO when it is decreased from (-ΔV), and determine 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 150 determines whether the bias voltage BS is the current set value (V n ) and compares the objective function value (Mf n ) based on the monitor signal MO 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 (Δf2) between the objective function value (Mf n ) and the objective function value (Mf2) is less than or equal to the specified value (Δf0).

[0063] When the variation amount of the objective function value based on the monitor signal MO is within the specified value (Δf0), the bias control unit 150 determines that bias control is unnecessary and ends the bias control process.

[0064] Also, when the variation amount of the objective function value based on the monitor signal MO is greater than the specified value (Δf0), the bias control unit 150 determines that bias control is necessary and proceeds to the next S204 for processing.

[0065] In this case, the bias control unit 150 compares the variation amount (Δf1) of the objective function value based on the monitor signal MO when the bias voltage BS is (V n -ΔV) with the variation amount (Δf2) of the objective function value based on the monitor signal MO when the bias voltage is (V n +ΔV) (S204). That is, the bias control unit 150 determines whether the slope of the variation of the objective function value when the bias voltage is increased or decreased is positive.

[0066] When the variation amount (Δf1) of the objective function value based on the monitor signal MO when the bias voltage BS is (V n -ΔV) is smaller than the variation amount (Δf2) of the objective function value based on the monitor signal MO when the bias voltage is (V n +ΔV), the bias control unit 150 sets the bias voltage BS to (V nThe bias voltage is set to -ΔV (S205). In this case, the bias control unit 150 determines that the slope of the fluctuation of the objective function value is positive and that the bias value dependence of the objective function has shifted to the low bias side. In this case, the bias control unit 150 adjusts the bias voltage BS by decreasing the set value by (-ΔV) so that the objective function value decreases. The amount by which the bias voltage BS is decreased may be changed according to the slope of the objective function value.

[0067] Conversely, if the bias voltage BS is (V n The variation in the objective function value (Δf1) based on the monitor signal MO in the case of -ΔV) is equal to the bias voltage (V n If the bias control unit 150 is greater than the variation in the objective function value (Δf2) based on the monitor signal MO in the case of +ΔV, the bias voltage BS is set to (V n The bias voltage is set to (+ΔV) (S206). In this case, the bias control unit 150 determines that the slope of the fluctuation of the objective function value is negative, and that the bias value dependence of the objective function has shifted to the high bias side. In this case, the bias control unit 150 adjusts the bias voltage BS by increasing its set value by (+ΔV) so that the objective function value decreases. The amount by which the bias voltage BS is increased may be changed according to the slope of the objective function value. Furthermore, by repeating steps S202 onward, the bias voltage BS can be adjusted so that the amount of fluctuation of the objective function value is within a specified value (minimum objective function value).

[0068] (Modified version of Embodiment 2) As shown in Figures 6 and 9, when the bias control process is performed repeatedly, the degree of fluctuation suppression (number of processing cycles) and the amount of change in bias voltage adjustment (amount of fluctuation) may be changed. With each repetition of the bias control process, the degree of fluctuation suppression (number of processing cycles) may be increased, or the amount of change in bias voltage adjustment may be decreased. This makes it possible to improve the accuracy of bias voltage adjustment with each repetition. Note that both the degree of fluctuation suppression and the amount of change in bias voltage adjustment may be changed, or only one of them may be changed.

[0069] Figure 10 shows examples of operation of the optical transmitter 1 according to several embodiments. Here, for the sake of simplicity, the control processing of the bias voltages BS1 and BS2 of the MZ modulators 122-1 and 122-2 (corresponding to S102 to S105 in Figure 6) and the control processing of the bias voltage BS3 of the phase shifter 123 (corresponding to S106 to S109 in Figure 6) will be explained together. In Figure 10, only the first bias control processing (S302 to S305) and the second bias control processing (S306 to S309) are shown, but the bias control processing is repeated further.

[0070] In the example in Figure 10, first, similar to S101 in Figure 6, the bias control unit 150 supplies initial values ​​of bias voltages BS1 to BS3 to the MZ modulators 122-1 and 122-2 and the phase shifter 123 (S301). Subsequently, similar to S102 and S106 in Figure 6, the monitor PD 140 monitors the modulated optical signal modulated by the optical modulator 120 and converts it into a monitor signal (S302).

[0071] Next, the bias control unit 150 performs amplitude fluctuation suppression processing on the objective functions F1 and F2 derived from the monitor signal with a suppression degree of 1 (first suppression degree) (S303). Similar to S103 and S107 in Figure 6, the calculation unit 152 calculates the objective function F1 for bias control of the MZ modulators 122-1 and 122 and the objective function F2 for bias control of the phase shifter 123 from the digital signal value of the AD-converted monitor signal MO. The suppression unit 153 performs suppression processing to suppress the amplitude fluctuations of the calculated objective functions F1 and F2 with a suppression degree of 1. For example, the amplitude fluctuations are suppressed with a suppression degree of 1 by performing suppression processing for a processing number N1 (first processing number). As the processing for processing number N1, for example, processing to calculate the average of N1 objective function values ​​after N1 measurements, or processing to calculate a moving average of N1 sizes from multiple objective function values ​​obtained by one measurement, etc., may be performed. Note that the number of iterations N1 may differ between the case of objective function F1 and the case of objective function F2.

[0072] Next, similar to S104 and S108 in Figure 6, the bias control unit 150 determines whether or not adjustment (control) of the bias voltages BS1 to BS3 is necessary based on the objective functions F1 and F2, whose amplitude fluctuations have been suppressed (S304).

[0073] Next, if it is determined that bias adjustment is necessary, the bias control unit 150 applies bias voltages BS1 to BS3 with a constant change amount ΔV1 (first change amount) and adjusts (controls) the bias voltages BS1 to BS3 so that the objective functions F1 and F2 with suppressed amplitude fluctuations are minimized (S305). The setting unit 154, similar to S105 and S109 in Figure 6, sets the bias voltages BS1 to BS3, which have been changed by a change amount ΔV1, when adjusting the bias voltages BS1 to BS3 so that the objective functions F1 and F2 with suppressed amplitude fluctuations are minimized. Specifically, as shown in Figure 9, in S202 the bias voltages BS1 to BS3 are increased or decreased by ±ΔV1, and in S205 or S206 the bias voltages BS1 to BS3 are set to V n -ΔV1 or V n Set the value to +ΔV1 and repeat until the objective functions F1 and F2 are minimized. Note that the change in ΔV1 may differ between the cases of objective function F1 and objective function F2.

[0074] Next, similar to S302, the monitor PD140 monitors the modulated optical signal modulated by the optical modulator 120 and converts it into a monitor signal (S306).

[0075] Next, the bias control unit 150 performs amplitude fluctuation suppression processing on the objective functions F1 and F2 derived from the monitor signal with a suppression degree of 2 (second suppression degree) (S307). For example, by performing suppression processing for a number of processing steps N2 (second number of processing steps), amplitude fluctuations are suppressed with a suppression degree of 2. Suppression degree 2 is greater than suppression degree 1 in S303. That is, amplitude fluctuations are suppressed more strongly than in S303. As the processing for the number of processing steps N2, for example, processing to calculate the average of N2 objective function values ​​after N2 measurements greater than N1, or processing to calculate a moving average of N2 sizes from multiple objective function values ​​obtained from one measurement, etc. Note that the number of processing steps N2 (the amount increased from N1) may differ between the case of objective function F1 and the case of objective function F2.

[0076] Next, similar to S304, the bias control unit 150 determines whether or not adjustment (control) of the bias voltages BS1 to BS3 is necessary based on the objective functions F1 and F2, whose amplitude fluctuations have been suppressed (S308).

[0077] Next, if it is determined that bias adjustment is necessary, the bias control unit 150 applies bias voltages BS1 to BS3 with a constant change amount ΔV2 (second change amount) and controls (adjusts) the bias voltages BS1 to BS3 so that the objective functions F1 and F2 with suppressed amplitude fluctuations are minimized (S309). The setting unit 154, similar to S305, sets the bias voltages BS1 to BS3, which have been changed by a change amount ΔV2, when adjusting to minimize the objective functions F1 and F2 with suppressed amplitude fluctuations. The change amount ΔV2 is smaller than the change amount ΔV1 in S305. Specifically, as shown in Figure 9, in S202 the bias voltages BS1 to BS3 are increased or decreased by ±ΔV2, and in S205 or S206 the bias voltages BS1 to BS3 are set to V n -ΔV2 or V n Set the value to +ΔV2 and repeat until the objective functions F1 and F2 are minimized. Note that the change in ΔV2 (the amount to reduce from ΔV1) may differ between the case of objective function F1 and the case of objective function F2.

[0078] As described above, in this embodiment, the optical modulator monitor PD converts the optical signal to an electrical signal, performs processing to suppress amplitude fluctuations of the objective function, and then the bias control unit performs identification processing to control the bias voltage of the optical modulator. For example, as processing to suppress amplitude fluctuations of the objective function, processing is performed to average n times after n measurements, processing to calculate a moving average, and processing to cut only the frequency band with large amplitude fluctuations. This makes it possible to suppress amplitude fluctuations of the objective function and suppress a decrease in signal power, and to control the bias voltage with high precision.

[0079] This disclosure is not limited to the embodiments described above, and may be modified as appropriate without departing from its spirit.

[0080] Each configuration in the above-described embodiment may consist of hardware, software, or both, and may consist of one piece of hardware or software, or multiple pieces of hardware or software. Each function (process), such as the bias control unit, may be implemented by a computer 20 having a processor 21 such as a CPU and a memory 22 which is a storage device, as shown in Figure 11. For example, a program for performing the method (control method) in the embodiment may be stored in the memory 22, and each function may be implemented by executing the program stored in the memory 22 with the processor 21.

[0081] These programs, when loaded into a computer, include a set of instructions (or software code) for causing the computer to perform one or more of the functions described in the embodiments. The programs may be stored on non-temporary computer-readable media or tangible storage media. Examples, but not limited to, include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drives (SSDs), or other memory technologies, CD-ROMs, digital versatile discs (DVDs), Blu-ray® discs, or other optical disc storage, magnetic cassettes, magnetic tapes, magnetic disk storage, or other magnetic storage devices. The programs may be transmitted over temporary computer-readable media or communication media. Examples, but not limited to, include electrical, optical, acoustic, or other forms of propagating signals.

[0082] Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0083] Each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated with one or more other embodiments rather than with only one specific embodiment. As those skilled in the art will understand, 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 not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps shown in any of the drawings may be changed as appropriate.

[0084] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0085] (Note 1) An optical modulator that modulates incoming light and outputs the modulated light, A calculation unit that calculates an objective function based on the output modulated light, A suppression unit that suppresses amplitude fluctuations of the calculated objective function, A setting unit sets the bias voltage of the optical modulator according to the value of the objective function in which the amplitude fluctuations are suppressed. An optical transmitter equipped with the following features. (Note 2) The suppression unit performs the averaging process of the objective function. The optical transmitter described in Appendix 1. (Note 3) In the averaging process of the objective function, the average of the values ​​of the objective function calculated from multiple measurement signals obtained by measuring the modulated light multiple times is obtained. The optical transmitter described in Appendix 2. (Note 4) In the averaging process of the objective function, a moving average of the values ​​of the objective function calculated from a plurality of sampled values ​​obtained by sampling the measurement signal of the modulated light is obtained. The optical transmitter described in Appendix 2. (Note 5) The suppression unit performs a bandpass filter process to remove components in a predetermined frequency band from the calculated objective function value. An optical transmitter as described in any one of the items 1 to 4 of the appendix. (Note 6) The predetermined frequency band includes a frequency band in which the amplitude fluctuation is greater than the predetermined value. The optical transmitter described in Appendix 5. (Note 7) The suppression unit suppresses the amplitude fluctuations with a first degree of suppression, and after the bias voltage is set, it suppresses the amplitude fluctuations with a second degree of suppression that is greater than the first degree of suppression. An optical transmitter as described in any one of the items 1 to 4 of the appendix. (Note 8) The suppression unit suppresses the amplitude fluctuations by performing amplitude fluctuation processing for a first number of processing cycles, and suppresses the amplitude fluctuations by performing amplitude fluctuation processing for a second number of processing cycles greater than the first number of processing cycles, The optical transmitter described in Appendix 7. (Note 9) The setting unit adjusts the bias voltage by changing the bias voltage by a first amount of change, and then adjusts the bias voltage by changing the bias voltage by a second amount of change that is smaller than the first amount of change. An optical transmitter as described in any one of the items 1 to 4 of the appendix. (Note 10) The optical modulator modulates the input light and outputs the modulated light. Based on the output modulated light, the objective function is calculated. The amplitude fluctuations of the calculated objective function are suppressed, The bias voltage of the optical modulator is set according to the value of the objective function in which the amplitude fluctuations are suppressed. A method for controlling an optical transmitter. (Note 11) The objective function is calculated based on the modulated light output from the optical modulator. The amplitude fluctuations of the calculated objective function are suppressed, The bias voltage of the optical modulator is set according to the value of the objective function in which the amplitude fluctuations are suppressed. A control program that instructs a computer to perform a process.

[0086] Some or all of the elements (e.g., configuration and function) described in Appendices 2 to 9 that are dependent on Appendice 1 (Optical Transmitter) may also be dependent on Appendices 10 (Control Method for Optical Transmitter) and 11 (Control Program) in the same way as in Appendices 2 to 9. Some or all of the elements described in any appendice may be applied to various hardware, software, recording means, systems, and methods for recording software. [Explanation of Symbols]

[0087] 1. 10 Optical Transmitter 11. Optical modulator 12 Calculation Section 13 Suppression part 14. Settings section 20 Computers 21 processors 22 memory 110 Light source 120 Optical modulators 121 Duplexer 122, 122-1~122-2 MZ Modulator 123 Phase Shifter 124 Multiplexer 130 Duplexer 140 Monitor PD 150 Bias Control Unit 151 AD Converter 152 Calculation Section 153 Suppression part 154 Settings Section 155 DA Converter

Claims

1. An optical modulator that modulates incoming light and outputs the modulated light, A calculation unit that calculates an objective function based on the output modulated light, A suppression unit that suppresses amplitude fluctuations of the calculated objective function, A setting unit sets the bias voltage of the optical modulator according to the value of the objective function in which the amplitude fluctuations are suppressed. An optical transmitter equipped with the following features.

2. The suppression unit performs the averaging process of the objective function. The optical transmitter according to claim 1.

3. In the averaging process of the objective function, the average of the values ​​of the objective function calculated from multiple measurement signals obtained by measuring the modulated light multiple times is obtained. The optical transmitter according to claim 2.

4. In the averaging process of the objective function, a moving average of the values ​​of the objective function calculated from a plurality of sampled values ​​obtained by sampling the measurement signal of the modulated light is obtained. The optical transmitter according to claim 2.

5. The suppression unit performs a bandpass filter process to remove components in a predetermined frequency band from the calculated objective function value. The optical transmitter according to any one of claims 1 to 4.

6. The predetermined frequency band includes a frequency band in which the amplitude fluctuation is greater than the predetermined value. The optical transmitter according to claim 5.

7. The suppression unit suppresses the amplitude fluctuations with a first degree of suppression, and after the bias voltage is set, it suppresses the amplitude fluctuations with a second degree of suppression greater than the first degree of suppression. The optical transmitter according to any one of claims 1 to 4.

8. The suppression unit suppresses the amplitude fluctuations by performing amplitude fluctuation processing for a first number of processing cycles, and suppresses the amplitude fluctuations by performing amplitude fluctuation processing for a second number of processing cycles greater than the first number of processing cycles, The optical transmitter according to claim 7.

9. The setting unit adjusts the bias voltage by changing the bias voltage by a first amount of change, and then adjusts the bias voltage by changing the bias voltage by a second amount of change that is smaller than the first amount of change. The optical transmitter according to any one of claims 1 to 4.

10. The optical modulator modulates the input light and outputs the modulated light. Based on the output modulated light, the objective function is calculated. The amplitude fluctuations of the calculated objective function are suppressed, The bias voltage of the optical modulator is set according to the value of the objective function in which the amplitude fluctuations are suppressed. A method for controlling an optical transmitter.

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