Control device and control method

Abstract

To reduce the size of a bias control device such as a Mach-Zehnder modulator.SOLUTION: A control device 2 holds an output of a Mach-Zehnder type modulator or the like before and after changing a bias in first and second holding sections, and controls the bias based on a difference of the held outputs.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a control device and a control method. [Background technology]

[0002] A device whose output changes depending on the bias (e.g., a Mach-Zehnder modulator) is used with the bias controlled. The bias is controlled based on the response of the device to a bias superimposed with a sine wave (so-called dither signal) whose frequency is lower than that of the input signal and whose amplitude is smaller than that of the input signal. In this technology, a signal that oscillates at the same frequency as the dither signal (hereinafter referred to as the dither component) is extracted from the output of the device, and the bias is controlled based on the extracted dither component. A band-pass filter is used to extract the dither component.

[0003] In order to miniaturize the circuit for bias control, a technology has been proposed that controls the bias based on the response to a bias on which a square wave is superimposed instead of a sine wave (see, for example, Patent Documents 1 and 2). In this technology, a signal that oscillates at the same frequency as the square wave is extracted from the output of the device, and the bias is controlled based on the extracted signal. Circuits such as analog-to-digital conversion circuits, digital-to-analog conversion circuits, and switch circuits are often used to extract the signal that oscillates at the same frequency as the square wave. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-111398 A [Patent Document 2] U.S. Pat. No. 9,686,017 Summary of the Invention [Problem to be solved by the invention]

[0005] The capacitors included in the bandpass filters are devices that are difficult to miniaturize. For this reason, it is also difficult to miniaturize the bandpass filters used to extract the dither components. On the other hand, devices with controlled bias (e.g., Mach-Zehnder modulators) are becoming smaller thanks to technological innovations (e.g., silicon photonics).

[0006] For this reason, bias control devices using dither signals are relatively large compared to the devices they control. Additionally, devices manufactured using silicon photonics have a slow response to bias changes, so the capacitors of the bandpass filters that extract the dither signals from these devices are large.

[0007] Patent Documents 1 and 2 propose techniques for miniaturizing a bias control circuit. However, the techniques of Patent Documents 1 and 2 make extensive use of analog-digital conversion circuits and the like, and therefore the bias control circuit cannot be sufficiently miniaturized.

[0008] Therefore, an object of the present invention is to solve such problems. [Means for solving the problem]

[0009] In one embodiment, the control device is a control device that controls a device in which an output from a region including a first portion changes depending on a state of the first portion, and includes a first holding unit that performs a first sample-and-hold processing to which a monitor signal that changes depending on the output is input, changes a first output depending on an intensity of the input monitor signal, and then holds the changed first output, a second holding unit that is different from the first holding unit, to which the monitor signal is input, changes a second output depending on an intensity of the input monitor signal, and then holds the changed second output, and a control unit that causes the first holding unit to perform the first sample-and-hold processing while keeping the state of the first portion constant, changes the state of the first portion, and causes the second holding unit to perform the second sample-and-hold processing while maintaining the changed state of the first portion, and further controls the state of the first portion based on a difference between the intensity of the first output held by the first holding unit and the intensity of the second output held by the second holding unit. Effect of the Invention

[0010] According to one aspect of the present invention, a control device for controlling the bias of a Mach-Zehnder type modulator or the like can be made compact. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is an example of a functional block diagram of the control device 2 according to the first embodiment. [Diagram 2] FIG. 2 is a diagram illustrating the operation of the control device 2. As shown in FIG. [Diagram 3] FIG. 3 is a diagram showing an example of control by the control unit 10. As shown in FIG. [Figure 4] FIG. 4 is a diagram showing an example of control by the control unit 10. As shown in FIG. [Diagram 5] FIG. 5 is a diagram illustrating control using a bias 38 on which a sine wave 36 is superimposed. [Figure 6] FIG. 6 is a diagram illustrating control using a bias 38 on which a sine wave 36 is superimposed. [Figure 7]FIG. 7 is a diagram illustrating an example of the control method according to the first embodiment. [Figure 8] FIG. 8 is an example of a functional block diagram of the control device 202 according to the second embodiment. [Figure 9] FIG. 9 is a diagram showing an example of the hardware configuration of the control device 202. As shown in FIG. [Figure 10] FIG. 10 is a diagram for explaining the operation of each piece of hardware in the control device 202. As shown in FIG. [Figure 11] FIG. 11 is a diagram showing an example of a device 212 controlled by the control device 202. As shown in FIG. [Figure 12] FIG. 12 is a diagram illustrating the operation of the control device 202. [Figure 13] FIG. 13 is a flowchart showing an example of the process executed by control unit 210. [Figure 14] FIG. 14 is a diagram showing the relationship between the first bias 226a and the second bias 226b and the output 216 of the Mach-Zehnder modulator 312. As shown in FIG. [Figure 15] FIG. 15 is a diagram illustrating an example of a hardware configuration of the control device 502 according to the third embodiment. [Figure 16] FIG. 16 is a diagram showing an example of the control circuit 508. [Figure 17] FIG. 17 is a diagram illustrating the operation of the control device 502. [Figure 18] FIG. 18 is a diagram illustrating the operation of the control unit 210. As shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, the embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and their equivalents. Parts having the same structure are given the same reference numerals even in different drawings, and the description thereof will be omitted.

[0013] (Embodiment 1) (1) Structure FIG. 1 is an example of a functional block diagram of the control device 2 according to the first embodiment.

[0014] 1, the control device 2 of the first embodiment has a first holding unit 6a, a second holding unit 6b, a differential unit 8, and a control unit 10. The first holding unit 6a and the second holding unit 6b are realized by, for example, a sample-and-hold circuit. The control unit 10 is realized by, for example, an integrated circuit or the like.

[0015] 1 also shows a device 12 controlled by the control device 2. The device 12 has, for example, a region 14 (hereinafter referred to as a control region) in which a Mach-Zehnder modulator is formed. The device 12 further has a monitor unit 4. The monitor unit 4 is realized, for example, by a photodiode and a resistor.

[0016] (2) Operation FIG. 2 is a diagram illustrating the operation of the control device 2. As shown in FIG.

[0017] The control region 14 of the device 12 includes a first portion 18a. The intensity of the output 16 from the control region 14 (hereinafter referred to as a controlled variable) changes according to the state of the first portion 18a. The first portion 18a is, for example, a heater (e.g., a thin-film heater) disposed near an optical waveguide (so-called an arm) of a Mach-Zehnder modulator. The state of the first portion 18a is, for example, a state that can be quantified by the physical quantity (e.g., temperature, carrier density, electric field, position, size, etc.) of the first portion 18a. The same applies to the state of the second portion described in the second embodiment.

[0018] For example, if the first portion 18a is a heater, the "state of the first portion" refers to the state of the heater heated to 100°C or the state of the heater heated to 120°C.

[0019] The device 12 may directly output the output 16 of the control area 14. Alternatively, the device 12 may process the output 16 of the control area 14 (for example, modulate, combine, split, etc.) before outputting it.

[0020] The monitor unit 4 generates a monitor signal 17 that changes depending on the intensity (i.e., the control amount) of the output 16 of the control region 14. The monitor unit 4 generates the monitor signal 17, for example, from a branched light 20 branched from the output 16 of the control region 14. The monitor signal 17 is input to a first holding unit 6a and a second holding unit 6b.

[0021] The monitor signal 17 may be generated by a circuit other than the monitor unit 4. For example, a circuit (e.g., a current-voltage conversion circuit) in the control device 2 may generate the monitor signal 17 from the output (i.e., photocurrent) of a photodetector to which the branched light 20 is input.

[0022] The first holding unit 6a changes its output 22a (hereinafter referred to as the first output) in response to the intensity of the input monitor signal 17, and then performs a process (hereinafter referred to as the first sample-and-hold process) of holding the intensity of the changed first output 22a. The second holding unit 6b changes its output 22b (hereinafter referred to as the second output) in response to the intensity of the monitor signal 17, and then performs a process (hereinafter referred to as the second sample-and-hold process) of holding the intensity of the changed second output 22b.

[0023] The differential section 8 changes its output 24 according to the difference between the intensity of the first output 22a held by the first holding section 6a and the intensity of the second output 22b held by the second holding section 6b. That is, the output 24 of the differential section 8 (hereinafter referred to as the differential output) changes according to the difference between the intensity of the first output 22a and the intensity of the second output 22b.

[0024] The control unit 10 causes the first holding unit 6a to perform the first sample-and-hold process while keeping the state of the first portion 18a constant (for example, a state of temperature T1), and then changes the state of the first portion 18a to another state (for example, a state of temperature T2 (≠ T1)). The control unit 10 then causes the second holding unit 6b to perform the second sample-and-hold process while keeping the state of the first portion 18a in the other state.

[0025] The control unit 10 then controls the state (e.g., temperature) of the first portion 18a based on the differential output 24 of the differential unit 8. For example, if the absolute value of the differential output 24 is greater than the allowable value, the control unit 10 increases or decreases the temperature of the first portion 18a depending on the sign of the differential output 24.

[0026] The control unit 10 repeats the above series of processes so that the average value of the intensity of the output 16 in the control region 14 approaches the target value of the output 16 .

[0027] When the first portion 18a is the heater, the arm of the Mach-Zehnder modulator is heated by the heater, and the refractive index of the arm changes according to the temperature of the heater. Therefore, the output of the Mach-Zehnder modulator (i.e., the output 16 of the control region 14) changes according to the temperature of the heater (i.e., the state of the first portion 18a). The temperature of the heater is adjusted, for example, by the power supplied to the heater.

[0028] The control unit 10 controls the state of the first portion 18a of the device 12 by an output 26 (hereinafter referred to as bias). For example, the control unit 10 adjusts the electrical quantity (e.g., power, current, voltage, etc.) of the bias 26 applied to the first portion 18a to control the physical quantity (e.g., temperature, carrier density, electric field, position, size, etc.) indicating the state of the first portion 18a. The same applies to the bias 226b applied to the second portion 18b described later.

[0029] The target value of the output 16 of the control region 14 is, for example, the intensity of the output 16 at a null point (i.e., the minimum value of the output 16). However, the target value is not limited to the intensity of the output 16 at a null point. The target point may be, for example, the intensity of the output 16 at a peak point or a quadrature point.

[0030] The monitor signal 17 is, for example, an analog signal (for example, a voltage or a current). The same is true for the first output 22a, the second output 22b, and the differential output 24.

[0031] 3-4 are diagrams showing an example of control by the control unit 10. The vertical axis is the intensity of the output 16 of the control region 14 (i.e., the amount of control). The horizontal axis is the intensity of the bias 26. The same is true for the vertical and horizontal axes in FIGS. 5-6. FIGS. 3-4 show a case where the output 16 is the output of a Mach-Zehnder type modulator. In this case, the output 16 changes periodically, but FIGS. 3-4 only show the vicinity of the null point 32.

[0032] The lower part of Fig. 3 shows the time change TC26 of the bias 26 (hereinafter referred to as bias time change). The vertical position indicates the time. The upper position indicates a later time than the lower position. The horizontal position indicates the strength of the bias 26. The same applies to the time progression shown in the lower parts of Figs. 4 to 6.

[0033] The right part of Fig. 3 shows the time change TC16 of the output 16 of the control region 14. The horizontal position indicates the time. The position on the right side indicates a later time than the position on the left side. The same is true for the time progression shown on the right parts of Figs. 4 to 6.

[0034] 3 shows a case where the center 34 of the rectangular wave portion 27 of the bias time change TC 26 approximately coincides with the bias value 132 of the null point 32. First, the control unit 10 sets the strength of the bias 26 to a constant value PI - Then, the intensity of the first output 22a of the first holding unit 6a changes to a constant value I1 and is held at that constant value I1.

[0035] The control unit 10 then adjusts the intensity of the bias 26 to PI - Different PI + and change the strength of bias 26 to PI + Then, the intensity of the second output 22b of the second holding unit 6b changes to a constant value I2 and is held at that constant value I2.

[0036] 3, when the center 34 of the rectangular wave portion 27 of the bias time change TC 26 and the bias value 132 of the null point 32 are approximately equal, the intensity Ia (here, I1) of the output 16 before the bias 26 is changed is approximately equal to the intensity Ib (here, I2) after the change. Therefore, the differential output 24 of the differential section 8 is approximately zero.

[0037] 4 shows a case where the center 34 of the rectangular wave portion 27 of the bias time change TC26 does not coincide with the bias value 132 at the null point 32. - Then, the intensity of the first output 22a of the first holding unit 6a changes to a constant value I3, and is held at that constant value I3.

[0038] The control unit 10 then adjusts the intensity of the bias 26 to PII - Different PII + Change it to PII with the bias strength of 26. + Then, the intensity of the second output 22b of the second holding unit 6b changes to a constant value I4 and is held at that constant value I4.

[0039] As shown in Fig. 4, if the center 34 of the square wave portion of the bias time change TC 26 does not match the bias value 132 at the null point 32, the intensity Ia of the output 16 of the control region 14 before the bias 26 is changed will not match the intensity Ib after the change. Therefore, the absolute value of the differential output 24 will be larger than that in Fig. 3.

[0040] In such a case, the control unit 10 changes the center 34 of the bias time change TC 26 (i.e., the average value of the bias 26) so as to reduce the absolute value of the differential output 24. For example, in the example shown in Fig. 4, the control unit 10 controls the bias 26 so as to reduce the center 34 of the bias time change TC 26.

[0041] By repeating the above process by the control unit 10, the differential output 24 eventually becomes approximately zero, and the center 34 of the bias time change TC 26 approximately coincides with the bias value 132 at the null point 32 (see FIG. 3).

[0042] In other words, the average value (=(Ia+Ib) / 2) of the output 16 of the control region 14 is the minimum value I min The process of the control unit 10 is repeated so as to approach the average value I AV Finally, the minimum value I min Approximately equal to.

[0043] (3) Control using a bandpass filter 5 and 6 are diagrams for explaining control using a bias 38 superimposed with a sine wave 36. Here, bias control of a Mach-Zehnder type modulator using a sine wave 36 (hereinafter referred to as a sine wave dither) will be considered.

[0044] Similar to the examples shown in FIGS. 3 and 4, the bias 38 is input to a heater provided in the vicinity of the optical waveguide of the Mach-Zehnder type modulator.

[0045] A response 40 of the Mach-Zehnder modulator to bias 38 (hereinafter referred to as the dither response) is input to a bandpass filter (not shown). The bandpass filter to which dither response 40 is input passes a signal (hereinafter referred to as the dither component) that oscillates at the same frequency f as sinusoidal dither 36, but blocks a signal that oscillates at a frequency 2f that is twice that of the sinusoidal dither. Bias 38 is controlled based on the dither component that has passed through the bandpass filter.

[0046] 5 shows a case where the center 44 of the sinusoidal dither 36 and the bias value 132 of the null point 32 are approximately aligned. In this case, the dither response 40 oscillates at a frequency 2f that is twice the frequency f of the sinusoidal dither 36. Therefore, the dither response 40 does not include a dither component (i.e., a signal that oscillates at the same frequency f as the sinusoidal dither 36).

[0047] 6 illustrates a case where the center 44 of the sinusoidal dither 36 and the null point bias value 132 do not coincide. In this case, the dither response 140 oscillates at the same frequency f as the sinusoidal dither 36. Thus, the dither response 140 includes a dither component that oscillates at frequency f.

[0048] The dither component is extracted by a bandpass filter, and the bias 38 is controlled to reduce the amplitude of the extracted dither component.

[0049] This control is repeated until the amplitude of the dither component becomes approximately zero. As a result, the center 44 of the sine wave dither 36 eventually coincides approximately with the bias value 132 of the null point 32. At this time, the average value I AV is the minimum value of output 16, I min Approximately equal to.

[0050] However, the capacitors included in bandpass filters are devices that are difficult to miniaturize. For this reason, it is also difficult to miniaturize the bandpass filters used to extract the dither components. On the other hand, optical devices such as Mach-Zehnder modulators have been miniaturized through technological innovation (e.g., silicon photonics).

[0051] For this reason, a control device using a sinusoidal dither signal is relatively large compared to the device to be controlled. On the other hand, the control device 2 of the first embodiment described with reference to Figs. 1 to 4 does not use a bandpass filter, and therefore can be easily miniaturized. That is, according to the first embodiment, a device for controlling the bias of a Mach-Zehnder modulator or the like (i.e., a bias control device) can be miniaturized.

[0052] Incidentally, the bandpass filter used to extract the dither component not only removes signals oscillating at a frequency 2f that is a multiple of the frequency f of the sinusoidal dither 36 (i.e., harmonics of the dither component), but also removes noise contained in the dither responses 40, 140 (see FIGS. 5 and 6).

[0053] In the control device 2 described with reference to Figs. 1 to 4, low-frequency noise lower in frequency than the frequency of the rectangular wave portion 27 of the bias 26 (hereinafter referred to as the bias frequency) is removed by differential processing in the differential unit 8. On the other hand, high-frequency noise higher in frequency than the bias frequency passes through the differential unit 8. Such high-frequency noise can be easily removed by digital signal processing in the control unit 10.

[0054] 1 and 2 includes a differential unit 8. However, the differential unit 8 may be omitted. In this case, the control device 2 can calculate the difference between the first output 22a and the second output 22b by digital signal processing in the control unit 10, for example, and control the bias 26 based on the calculated difference.

[0055] (4) Control method FIG. 7 is a diagram illustrating an example of the control method according to the first embodiment.

[0056] First, a first signal (ie, first output 22a) that changes according to the intensity of output 16 is generated while the state of first portion 18a (see FIG. 2) is kept constant, and the generated first signal is held (step S2).

[0057] After step S2, the state of the first portion 18a is changed (step S4).

[0058] After step S4, a second signal (i.e., second output 22b) that changes according to the intensity of output 16 is generated while keeping the changed state of first portion 18a constant, and the generated second signal is held (step S6).

[0059] After step S6, the state of the first portion 18a is controlled based on the difference between the intensity of the held first signal and the intensity of the held second signal (step S8).

[0060] Steps S2 to S8 are preferably repeated so that the average value of the intensity of output 16 before the change and the intensity of output 16 after the change approaches the target value of output 16.

[0061] 1 to 4, in the first embodiment, the bias 26 is controlled based on the difference between the first and second outputs 22a, 22b of the first and second holding units 6a, 6b, which changes according to the intensity (i.e., the controlled variable) of the output 16 of the control region 14. Therefore, bias control without using a bandpass filter is possible, and according to the first embodiment, the bias control device for a Mach-Zehnder type modulator or the like can be easily miniaturized.

[0062] (Embodiment 2) The second embodiment is similar to the first embodiment. Therefore, the description of the same configuration as the first embodiment will be omitted or simplified.

[0063] (1) Structure (1-1) Functional block 8 is an example of a functional block diagram of the control device 202 according to the second embodiment. As shown in FIG. 8, the control device 202 is similar to the control device 2 according to the first embodiment.

[0064] However, in the second embodiment, the control area 214 of the control device 202 includes, in addition to the first portion 18a, a second portion 18b different from the first portion 18a. The control area 214 is an example of the control area 14 (see FIG. 1) of the first embodiment. The output (i.e., the controlled variable) of the control area 214 changes depending on the state of the first portion 18a and the state of the second portion 18b. The control unit 210 of the control device 202 controls the state of the first portion 18a and the state of the second portion 18b based on the differential output of the differential unit 8. The control device 202 further includes a monitor signal generating unit 301.

[0065] (1-2) Hardware configuration Fig. 9 is a diagram showing an example of the hardware configuration of the control device 202 (see Fig. 8). Fig. 9 also shows the hardware configuration of the device 212 controlled by the control device 202.

[0066] The control device 202 includes a transimpedance amplifier 302, a first sample-and-hold circuit 304a, a second sample-and-hold circuit 304b, and a differential amplifier 306. The control device 202 further includes an integrated circuit 308 for digital coherent communication and a digital-to-analog converter 310.

[0067] 9, the integrated circuit 308 has a function of converting an analog signal into a digital signal. If the integrated circuit 308 does not have the above function, the control device 202 may have an analog-to-digital converter that converts the output of the differential amplifier 306 into a digital signal.

[0068] The monitor signal generating unit 301 (see FIG. 8) is realized by, for example, a transimpedance amplifier 302. The monitor signal generating unit 301 generates a monitor signal 217 (see FIG. 10) that changes according to an output 216 (see FIG. 10 described later) of the control area 214. The monitor signal generating unit 301 may be provided outside the control device 202 (see FIG. 8) (for example, inside the device 212 or between the device 212 and the control device 202). In other words, the transimpedance amplifier 302 may be provided outside the control device 202.

[0069] The first holding unit 6a (see FIG. 8) is realized by a first sample-and-hold circuit 304a. The second holding unit 6b is realized by a second sample-and-hold circuit 304b. The differential unit 8 is realized by a differential amplifier 306. The control unit 210 is realized by an integrated circuit 308 and a digital-to-analog converter 310.

[0070] The control unit 210 may be realized by an analog-to-digital converter connected to the differential amplifier 306, a memory, a CPU (Central Processing Unit) coupled to the memory, and a digital-to-analog converter 310 (see embodiment 3). Alternatively, the control unit 210 may be realized by an FPGA (Field-Programmable Gate Array) and the digital-to-analog converter 310.

[0071] The device 212 has a Mach-Zehnder modulator 312 and a photodetector 314 formed in the control region 214. The Mach-Zehnder modulator 312 is, for example, a modulator formed using silicon photonics. The photodetector 314 is, for example, a photodiode. The photodetector 314 is, for example, optically connected to a multiplexer 320 of the Mach-Zehnder modulator 312 via a Y-branch 319.

[0072] The Mach-Zehnder modulator 312 includes a splitter 316, a first optical waveguide 318a, a second optical waveguide 318b, and a multiplexer 320. The Mach-Zehnder modulator 312 further includes a first electrode 322a connected to the core of the first optical waveguide 318a, and a second electrode 322b connected to the core of the second optical waveguide 318b. The cores of the first optical waveguide 318a and the second optical waveguide 318b are, for example, silicon having a pn junction.

[0073] The Mach-Zehnder modulator 312 further includes a first heater 324a provided along the first optical waveguide 318a and a second heater 324b provided along the second optical waveguide 318b. The first heater 324a and the second heater 324b are, for example, thin-film heaters. The first heater 324a heats a portion of the first optical waveguide 318a adjacent to the first heater 324a. The second heater 324a heats a portion of the second optical waveguide 318b adjacent to the second heater 324b.

[0074] 9, the first portion 18a (see FIG. 8) is realized by a first heater 324a. The second portion 18b (see FIG. 8) is realized by a second heater 324b. The monitor unit 204 (see FIG. 8) is realized by a photodetector 314.

[0075] 10 is a diagram for explaining the operation of each piece of hardware in the control device 202. Light 326 (for example, continuous light) input to the control region 214 is split by the splitter 316 into a first split light 321a and a second split light 321b.

[0076] The first branched light 321a is input to the first optical waveguide 318a. The first optical waveguide 318a modulates the first branched light 321a with a first electrical signal 328a input to the first electrode 322a. The second branched light 321b is input to the second optical waveguide 318b. The second optical waveguide 318b modulates the second branched light 321b with a second electrical signal 328b input to the second electrode 322b.

[0077] The multiplexer 320 multiplexes the first branched light 321a modulated by the first optical waveguide 318a and the second branched light 321b modulated by the second optical waveguide 318b to generate an output 216, and outputs the generated output 216 from the control region 214.

[0078] When power is supplied to the first heater 324a and the temperature of the first heater 324a rises, the temperature of the portion of the first optical waveguide 318a close to the first heater 324a also rises. Then, the refractive index of this portion increases, and the phase of the first branched light 321a changes. That is, the phase of the first branched light 321a changes according to the state (here, the temperature) of the first heater 324a (i.e., the first portion 18a). Similarly, the phase of the second branched light 321b changes according to the state (here, the temperature) of the second heater 324b (i.e., the second portion 18b).

[0079] The modulation of the first branched light 321a by the first electric signal 328a (hereinafter referred to as the first modulation) is a process different from the power supply to the first heater 324a (i.e., the control of the state of the first portion). Similarly, the modulation of the second branched light 321b by the second electric signal 328b (hereinafter referred to as the second modulation) is a process different from the power supply to the second heater 324b (i.e., the control of the state of the second portion).

[0080] The Y-branch 319 splits the output 216 of the control region 214, and inputs one of the split outputs 216 (hereinafter referred to as monitor light) to the photodetector 314. The photodetector 314 photoelectrically converts the input monitor light to generate a photocurrent 315.

[0081] The transimpedance amplifier 302 is, for example, a current-voltage conversion circuit having an operational amplifier 330 and a resistor 332. The photocurrent 315 is input to the resistor 332 connected to the inverting input terminal of the operational amplifier 330. A reference potential Vr is input to the non-inverting input terminal of the operational amplifier 330. The transimpedance amplifier 302 converts the photocurrent 315 into a monitor signal 217. The monitor signal 217 changes according to the intensity of the photocurrent 315.

[0082] Each of the first and second sample-and-hold circuits 304 a and 304 b includes a switch circuit 336 , a capacitor 338 , and a voltage follower 340 .

[0083] The switch circuit 336 is closed by a first signal 342a (hereinafter referred to as a close signal). As a result, the transimpedance amplifier 302 and the voltage follower 340 are connected. The switch circuit 336 is opened by a second signal 342b (hereinafter referred to as an open signal) different from the close signal 342a. As a result, the transimpedance amplifier 302 is disconnected from the voltage follower 340.

[0084] When the transimpedance amplifier 302 and the voltage follower 340 are connected by the switch circuit 336, the voltage of the output 344 of the voltage follower 340 becomes substantially the same voltage as the monitor signal 217. After that, when the transimpedance amplifier 302 and the voltage follower 340 are disconnected by the switch circuit 336, the voltage of the output 344 of the voltage follower 340 is fixed to the voltage before the disconnection.

[0085] The first output 222a of the first sample and hold circuit 304a is the output 344a of the voltage follower 340a of the first sample and hold circuit 304a. The second output 222b of the second sample and hold circuit 304b is the output 344b of the voltage follower 340b of the second sample and hold circuit 304b.

[0086] Therefore, when the switch circuit 336a is closed, the first output 222a of the first sample and hold circuit 304a changes according to the intensity of the monitor signal 217. When the switch circuit 336a is opened, the intensity of the first output 222a is held at the intensity just before the switch circuit 336a was opened. That is, the first output 222a is held.

[0087] Similarly, the second output 222b of the second sample-and-hold circuit 304b (i.e., the output 344b of the voltage follower 340b) changes according to the intensity of the monitor signal 217 when the switch circuit 336b is closed. When the switch circuit 336a is opened, the intensity of the second output 222b is fixed to the intensity before the switch circuit 336b was opened. That is, the second output 222b is held.

[0088] The first output 222a and the second output 222b are input to the differential amplifier 306. The output 224 of the differential amplifier 306 changes according to the difference between the voltage of the first output 222a and the voltage of the second output 222b.

[0089] The integrated circuit 308 controls the opening and closing of the switch circuit 336. The integrated circuit 308 further calculates the value of the voltage (hereinafter referred to as the voltage value) to be applied to the first and second heaters 324a and 324b based on the differential output 224 of the differential amplifier 306. The integrated circuit 308 first calculates the power to be supplied to the first and second heaters 324a and 324b, and converts the calculated power into the voltage value.

[0090] The calculated voltage value is input to the digital-to-analog converter 310. The digital-to-analog converter 310 converts the input voltage value into a voltage, and supplies it to the first and second heaters 324a and 324b.

[0091] In the above example, the first portion 18a is the first heater 324a. However, the first portion 18a is not limited to a heater. The first portion 18a may be, for example, a core of the first optical waveguide 318a in which a pn junction is provided. In this case, the carrier concentration of the core changes according to the voltage output by the digital-to-analog converter 310. As a result, the phase of the first branched light 321a changes.

[0092] In the above example, the photocurrent 315 is converted into the monitor signal 217 by the transimpedance amplifier 302. However, the device that converts the photocurrent 315 into the monitor signal 217 is not limited to the transimpedance amplifier 302. For example, the photocurrent 315 may be converted into the monitor signal 217 by a resistor (see embodiment 3).

[0093] FIG. 11 is a diagram showing an example of the device 212 controlled by the control device 202 (see FIG. 8). The device 212 illustrated in FIG. 11 is a DQPSK (Differential Quadrature Phase-Shift Keying) modulator. DQPSK has two child Mach-Zehnder modulators 342 and a parent Mach-Zehnder modulator 345. The Mach-Zehnder modulator 312 shown in FIG. 9 is, for example, one of the child Mach-Zehnder modulators 342. The integrated circuit 308 (i.e., the control unit 210) may control both the child Mach-Zehnder modulators 342 and the other one. The integrated circuit 308 (i.e., the control unit 210) may further control the heater 324 of the parent Mach-Zehnder modulator 345.

[0094] (2) Operation 12 is a diagram for explaining the operation of the control device 202. The operations of the first holding unit 6a and the second holding unit 6b have been described in the first embodiment. The operations of the monitor unit 4 and the differential unit 8 have also been described in the first embodiment. The control unit 210 inputs the first bias 226a to the first portion 18a while inputting the second bias 226b to the second portion 18b. The control unit 210 controls the first bias 226a and the second bias 226b based on the differential output 224 of the differential unit 8.

[0095] Fig. 13 is a flowchart showing an example of processing executed by the control unit 210. In the following explanation, a case is considered in which the control device 202 has a resistor (see Fig. 15 described later) instead of the transimpedance amplifier 302. The other hardware configurations are the same as the hardware configurations of the control device 202 (see Fig. 12) explained with reference to Figs. 9 and 10.

[0096] -Step S102- First, the control unit 210 (see FIG. 12) calculates a variable P p The control unit 210 further assigns an initial value P0 to the variable P n Substitute the initial value P0 into

[0097] -Step S104- After step S102, the control unit 210 sets the variable P p The control unit 210 subtracts ΔP (>0) from the calculated power (=P p -ΔP) is supplied to the first heater 324a (see FIG. 10). At this time, the control unit 210 converts the calculated power into a voltage value, and the calculated voltage value is converted into a voltage by the digital-to-analog converter 310. The same applies to the subsequent steps. ΔP is, for example, a constant equal to or smaller than P0.

[0098] The control unit 210 further determines the variable P n The control unit 210 (see FIG. 12) calculates the value of the power (=P n +ΔP) is supplied to the second heater 324b (see FIG. 10).

[0099] The temperatures of the first heater 324a and the second heater 324b change in step S104, which results in a change in the output 216 of the Mach-Zehnder modulator 312.

[0100] -Step S106- After step S104, the control unit 210 (see FIG. 12) transmits a close signal 342a to the first switch circuit 336a (see FIG. 10). The first switch circuit 336a closes in response to the close signal 342a.

[0101] -Step S108- After step S106, the control unit 210 (see FIG. 12) transmits an open signal 342b to the first switch circuit 336a (see FIG. 10). The first switch circuit 336a opens in response to the open signal 342b.

[0102] During the interval between the end of step S106 and the start of step S108, the first holding unit 6a (see FIG. 12) changes the intensity of the first output 222a in accordance with the intensity of the monitor signal 217. When the first switch circuit 336a (see FIG. 10) is opened in step S108, the first holding unit 6a (see FIG. 12) holds the changed intensity of the first output 222a.

[0103] -Step S110- After step S108, the control unit 210 sets the variable P p The control unit 210 (see FIG. 12) calculates the value of the power (=P p +ΔP) is supplied to the first heater 324a.

[0104] The control unit 210 further determines the variable P n The control unit 210 subtracts ΔP from the calculated power (=P n -ΔP) is supplied to the second heater 324b.

[0105] In step S110, the temperatures of the first heater 324a and the second heater 324b are changed, and as a result, the output 216 of the Mach-Zehnder modulator 312 (see FIG. 10) is also changed.

[0106] -Step S112- After step S110, the control unit 210 (see FIG. 12) transmits a close signal 342a to the second switch circuit 336b (see FIG. 10). The second switch circuit 336b closes in response to the close signal 342a.

[0107] -Step S114- After step S112, the control unit 210 (see FIG. 12) transmits an open signal 342b to the second switch circuit 336b (see FIG. 10). The second switch circuit 336b opens in response to the open signal 342b.

[0108] During the interval between the end of step S112 and the start of step S114, the second holding unit 6b (see FIG. 12) changes the intensity of the second output 222b in accordance with the intensity of the monitor signal 217. When the second switch circuit 336b (see FIG. 10) is opened in step S114, the second holding unit 6b (see FIG. 12) holds the changed intensity of the second output 222b.

[0109] -Step S116- The control section 210 performs analog-to-digital conversion on the differential output 224 of the differential section 8, and calculates a value Δ of the differential output 224 (hereinafter referred to as a difference value).

[0110] -Step S118- After step S116, the control unit 210 determines whether or not the absolute value of the difference value Δ calculated in step S116 is greater than the allowable value ε. If the absolute value of the difference value Δ is equal to or smaller than the allowable value ε, the control unit 210 ends the control.

[0111] If the absolute value of the difference value Δ is greater than the allowable value ε, the control unit 210 proceeds to step S120.

[0112] -Step S120- The control unit 210 multiplies the difference value Δ by a gain G (>0) and sets the result as a variable P P Subtract from to get the new P p (=P p -G × ΔP) is calculated.

[0113] The control unit 210 further multiplies the difference value Δ by a gain G (>0) and sets the result as a variable P n Add to and get the new P n (=P n + G × ΔP).

[0114] After step S120, the control unit 210 resumes steps S104 to S120. The temperatures of the first heater 324a and the second heater 324b are updated by steps S104 and S110 after the resumption. By this update, the absolute value of the difference value Δ calculated in step S116 decreases from the absolute value before the update, and the average value of the output 216 of the Mach-Zehnder modulator 312 approaches the target value (the minimum value of the output 216 in the second embodiment). For example, when the difference value Δ is a negative value as shown in FIG. 14 described later, P p increases and P n decreases. As a result, the average value of the output 216 approaches a minimum value.

[0115] Fig. 14 is a diagram showing the relationship between the first bias 226a and the second bias 226b and the output 216 of the Mach-Zehnder modulator 312. The drawing shown at the top of Fig. 14 (hereinafter referred to as the upper drawing) shows a relationship 346 between the phase difference Δφ between the first branched light 321a (see Fig. 10) and the second branched light 321b and the output 216 of the Mach-Zehnder modulator 312. The vertical axis is the output 216 of the Mach-Zehnder modulator 312. The horizontal axis is the phase difference Δφ.

[0116] 14 (hereinafter referred to as the lower diagram) shows a relationship A between the intensity (i.e., power) of the first bias 226a and the phase difference Δφ. The lower diagram further shows a relationship B between the intensity (i.e., power) of the second bias 226b and the phase difference Δφ.

[0117] The vertical axis represents the intensity of the first bias 226a and the second bias 226b, and the horizontal axis represents the phase difference Δφ.

[0118] Between the upper and lower figures, the time change 348 of the phase difference Δφ is shown. The vertical position indicates time. The upper position indicates a later time than the lower position. The same is true for FIG. 18, which will be described later. The horizontal position indicates the phase difference Δφ.

[0119] The graph on the left side of the upper diagram shows the time change TC216 of the output 216 of the Mach-Zehnder modulator 312. The horizontal position indicates the time. The position on the right side indicates a later time than the position on the left side. The same is true for FIG. 18, which will be described later. The vertical position indicates the intensity of the output 216 of the Mach-Zehnder modulator 312.

[0120] As shown by the time change TC216 of the output 216, when the first and second biases 226a, 226b are changed in step 110, the output 216 changes in a rectangular shape like the output 16 of the device 12 in the first embodiment (see the time change TC16 in FIG. 4). However, the amount of change in the output 216 in the second embodiment is larger than the amount of change in the output 16 in the first embodiment.

[0121] In step S110, the intensity of the first bias 226a is P p -ΔP to P p +ΔP by 2×ΔP. On the other hand, the intensity of the second bias 226b is changed to P p +ΔP to P p It changes from -ΔP to -2×ΔP.

[0122] As a result, the phase difference Δφ between the first branched light 321a and the second branched light 321b becomes twice as large as that in the first embodiment in which only the first bias 226a is changed. Therefore, the amount of change in the output 216 of the Mach-Zehnder modulator 312 also becomes twice as large. As a result, the change in the monitor signal 217 also becomes twice as large.

[0123] That is, the control device 202 of embodiment 2 changes the temperature (state) of the second heater 324b (second portion 18b) so as to promote a change (difference) in the monitor signal 217 before and after the temperature (state) of the first heater 324a (first portion 18a) is changed.

[0124] This increases the signal-to-noise ratio of the monitor signal 217, improving the accuracy of bias control.

[0125] As described above, according to the second embodiment, the signal-to-noise ratio of the monitor signal 217 is increased, and therefore the accuracy of the bias control is improved.

[0126] (Embodiment 3) The third embodiment is similar to the first and second embodiments. Therefore, the description of the same configuration as the first or second embodiment will be omitted or simplified.

[0127] (1) Structure Fig. 15 is a diagram showing an example of a hardware configuration of the control device 502 according to the third embodiment. In Fig. 15, a device 512 controlled by the control device 502 is also shown.

[0128] The control device 502 of the third embodiment includes a resistor 601, a first sample-and-hold circuit 304a, a second sample-and-hold circuit 304b, and a differential amplifier 306. The control device 502 further includes an analog-to-digital converter 307 and a control circuit 508. The resistor 601 may be provided outside the control device 502. For example, the resistor 601 may be provided in a device 512 controlled by the control device 502.

[0129] The first sample-and-hold circuit 304a is the circuit described in the second embodiment. Therefore, the description of the first sample-and-hold circuit 304a will be omitted. The same applies to the second sample-and-hold circuit 304b and the differential amplifier 306.

[0130] One end of the resistor 601 is connected to the input ports of the first sample-and-hold circuit 304a and the second sample-and-hold circuit 304b, and the other end of the resistor 601 is connected to a reference potential (i.e., ground).

[0131] A functional block diagram of the control device 502 according to the third embodiment is substantially the same as the functional block diagram of the second embodiment (see FIG. 8).

[0132] The monitor signal generating unit 301 of the third embodiment is realized by a resistor 601 (see FIG. 15). The first holding unit 6a is realized by a first sample-and-hold circuit 304a. The second holding unit 6b is realized by a second sample-and-hold circuit 304b. The differential unit 8 is realized by a differential amplifier 306. The control unit 210 is realized by a control circuit 508 and an analog-to-digital converter 307.

[0133] The functional block diagram of device 512 controlled by control device 502 of embodiment 3 is the same as device 12 (see FIG. 1) of embodiment 1. Therefore, the portion controlled by control unit 210 of embodiment 3 is the first portion.

[0134] 16 is a diagram showing an example of a control circuit 508. The control circuit 508 has a CPU (Central Processing Unit) 602, a memory 604, and a non-volatile memory 606. The memory 604 is, for example, a RAM (Random Access Memory). The non-volatile memory 606 is, for example, a flash memory. The control circuit 508 further has a bus 610. The CPU 602, the memory 604, and the non-volatile memory 606 are connected to each other via the bus 610.

[0135] The control circuit 508 further has a first interface 608a that connects the first switch circuit 336a to the bus 610 and a second interface 608b that connects the second switch circuit 336b to the bus 610. The control circuit 508 further has a third interface 608c that connects the analog-to-digital converter 307 to the bus 610. The control circuit 508 further has a fourth interface 608d that connects the TOF driver control circuit 510, which will be described later, to the bus 610.

[0136] The CPU 602 is coupled to a memory 604 via a bus 610 and is configured to execute programs stored in, for example, a non-volatile memory 606 .

[0137] (2) Operation 17 is a diagram for explaining the operation of the control device 502. The device 512 (device controlled by the control device 502) has a tunable optical filter (hereinafter referred to as TOF) 612 and a TOF driver control circuit 510. The device 512 further has a photodetector 314.

[0138] The photodetector 314 photoelectrically converts the light branched off from the output 516 of the TOF 612 to generate a photocurrent 315. The photocurrent 315 is input to a resistor 601, which generates a monitor signal 217. The monitor signal 217 is a voltage generated in the resistor 601 by the photocurrent 315.

[0139] The TOF 612 has, for example, a diffraction grating that separates light 525 (hereinafter referred to as input light) input to the TOF 612, and a slit plate. The input light 525 separated by the diffraction grating is irradiated onto the slit plate, and a part of the separated input light 525 passes through a slit provided in the slit plate. The part of the input light 525 that passes through the slit is output from the TOF 612.

[0140] The position of the slit plate determines the center wavelength (e.g., center of the 3 dB band) of the light output from the TOF 612. The position of the slit plate changes according to the control voltage (i.e., bias) 526 output from the TOF driver control circuit 510.

[0141] For a constant wavelength of input light 525 to the TOF 612, the intensity of output 516 from region 514 including the TOF 612 varies depending on the position of the slit plate. The slit plate is part of the TOF 612, and is the first portion 18a described with reference to Figure 1. The state of the slit plate is a state that is quantified by the position of the slit plate (e.g., the distance from a reference position).

[0142] FIG. 18 is a diagram for explaining the operation of the control unit 210 (see FIG. 8).

[0143] 18 (hereinafter referred to as the upper diagram) shows a relationship 614 between the output 516 of the TOF 612 and the central wavelength. The vertical axis is the intensity of the output 516 of the TOF 612. The horizontal axis is the central wavelength of the output 516.

[0144] The diagram shown at the bottom of FIG. 18 (hereinafter referred to as the lower diagram) is a diagram showing a relationship C between the control voltage 526 output by the TOF driver control circuit 510 and the central wavelength of the output 516. The vertical axis is the control voltage 526. The horizontal axis is the central wavelength of the output 516. The lower diagram further shows the time change TC526 of the control voltage 526. Between the upper and lower diagrams, the time change TCλ of the central wavelength is shown. The upper diagram shows the time change TC516 of the output 516 of the TOC 612.

[0145] As shown in the relationship 614 in the figure, the output 516 of the TOF 612 increases with the center wavelength, reaches a maximum value, and then decreases. The control circuit 508 controls the position of the diffraction grating of the TOF 612 via the TOF driver control circuit 510 so that the output 516 of the TOF 612 reaches a maximum value 532. That is, the target value of the output 516 is the maximum value.

[0146] Specifically, the control circuit 508 first transmits a command 616 to the TOF driver control circuit 510. In response to the command 616, the TOF driver control circuit 510 generates a control voltage 526 (see time change TC 526) on which a square wave is superimposed, and applies it to the TOF 612.

[0147] The output 516 of the TOF 612 changes in response to the control voltage 526. The control circuit 508 controls the TOF driver control circuit 510 so that the absolute value of the output 524 of the differential amplifier 306 caused by the change in the output 516 (see time change TC 516) decreases. By this control, the average value (=(I7+I8) / 2) of the output 516 of the TOF 612 approaches the maximum value (i.e., the target value) of the output 516. By repeating the above control, the intensity of the output 516 of the TOF 612 eventually reaches approximately the maximum value 533.

[0148] According to the third embodiment, the output of the controlled area 514 can be maximized.

[0149] Although the embodiments of the present invention have been described above, the first to third embodiments are merely illustrative and not restrictive. For example, the cores of the Mach-Zehnder modulators of the first and second embodiments are made of silicon. However, the cores of the Mach-Zehnder modulators of the first and second embodiments may be made of a material other than silicon. For example, the cores of the Mach-Zehnder modulators of the first and second embodiments may be made of InP or a ferroelectric.

[0150] The following supplementary notes are further disclosed regarding the above first to third embodiments.

[0151] (Appendix 1) A control device for controlling a device in which an output from a region including a first portion changes in response to a state of the first portion, a first holding unit that performs a first sample-and-hold process to which a monitor signal that changes according to the output is input, changes a first output according to the intensity of the input monitor signal, and then holds the changed first output; a second holding unit which is different from the first holding unit and receives the monitor signal, changes a second output according to the intensity of the input monitor signal, and performs a second sample-and-hold process to hold the changed second output; a control unit that changes the state of the first portion after causing the first holding unit to perform the first sample-and-hold process while keeping the state of the first portion constant, and causes the second holding unit to perform the second sample-and-hold process while keeping the changed state of the first portion, and further controls the state of the first portion based on a difference between an intensity of the first output held by the first holding unit and an intensity of the second output held by the second holding unit. Control device.

[0152] (Appendix 2) The process of the control unit is repeated so that an average value of the intensity of the output before the change and the intensity of the output after the change approaches a target value of the intensity of the output. The control device according to claim 1.

[0153] (Appendix 3) the region includes a second portion different from the first portion; The output of the region further varies in response to a state of the second portion; The change is a process of changing the state of the second part together with the state of the first part so as to promote a change in the output before and after the state of the first part is changed; The control unit controls a state of the first portion and a state of the second portion based on the difference. 3. The control device according to claim 1 or 2.

[0154] (Appendix 4) the region of the device includes a splitter that splits input light into a first branched light and a second branched light, a first optical waveguide that modulates the phase of the first branched light, a second optical waveguide that modulates the phase of the second branched light, and a multiplexer that multiplexes the modulated first branched light and the modulated second branched light and outputs the multiplexed light from the region; the first portion has a heater that heats the first optical waveguide; The second portion has a heater for heating the second optical waveguide. The control device according to claim 3.

[0155] (Appendix 5) 1. A method of controlling an apparatus in which an output from a region including a first portion varies in response to a state of the first portion, comprising: generating a first signal that changes according to the intensity of the output while keeping the state of the first portion constant, holding the generated first signal, and then changing the state of the first portion; generating a second signal that changes according to the intensity of the output while keeping the changed state of the first portion constant, and holding the generated second signal; and further controlling the state of the first portion based on a difference between the intensity of the held first signal and the intensity of the held second signal. Control methods. [Explanation of symbols]

[0156] 2: Control device 6a: 1st holding part 6b:Second holding part 10: Control section 14 :Area 16: Output 17: Monitor signal 18a: 1st part 18b :Second part 22a: 1st output 22b: 2nd output 24: Differential output

Claims

1. A control device (excluding a device for controlling a modulation current of a laser diode) for controlling a device in which an output from a region including a first portion and a second portion different from the first portion changes in response to a state of the first portion and a state of the second portion, a first holding unit that performs a first sample-and-hold process to which a monitor signal that changes in response to the output is input, to change a first output in response to the intensity of the input monitor signal, and then to hold the changed first output; a second holding unit, which is a holding unit different from the first holding unit, receives the monitor signal, changes a second output in response to the intensity of the input monitor signal, and performs a second sample-and-hold process to hold the changed second output; a control unit that changes the state of the first portion after causing the first holding unit to perform the first sample-and-hold process while keeping the state of the first portion constant, and causes the second holding unit to perform the second sample-and-hold process while keeping the changed state of the first portion, and further controls the state of the first portion and the state of the second portion (however, this is a process performed in the region that outputs modulated light, and does not include the modulation of the light) based on a difference between the intensity of the first output held by the first holding unit and the intensity of the second output held by the second holding unit, The change is a process of changing the state of the second part together with the state of the first part so as to promote a change in the output before and after the state of the first part is changed. Control device.

2. A control device for controlling a device in which an output from a region including a first portion changes in response to a state of the first portion, a first holding unit that performs a first sample-and-hold process to which a monitor signal that changes in response to the output is input, to change a first output in response to the intensity of the input monitor signal, and then to hold the changed first output; a second holding unit, which is a holding unit different from the first holding unit, receives the monitor signal, changes a second output in response to the intensity of the input monitor signal, and performs a second sample-and-hold process to hold the changed second output; a control unit that changes the state of the first portion after causing the first holding unit to perform the first sample-and-hold process while keeping the state of the first portion constant, and causes the second holding unit to perform the second sample-and-hold process while keeping the changed state of the first portion, and further controls the state of the first portion so that a difference between an intensity of the first output held by the first holding unit and an intensity of the second output held by the second holding unit becomes approximately zero. Control device.

3. The process of the control unit is repeated so that an average value of the intensity of the output before the change and the intensity of the output after the change approaches a target value of the intensity of the output.

3. A control device according to claim 1 or 2.

4. A method for controlling an apparatus in which an output from a region including a first portion and a second portion different from the first portion varies depending on a state of the first portion and a state of the second portion (excluding a method for controlling a modulation current of a laser diode), generating a first signal that changes according to the intensity of the output while keeping the state of the first portion constant, holding the generated first signal, and then changing the state of the first portion; generating a second signal that changes according to the intensity of the output while keeping the changed state of the first portion constant, and holding the generated second signal; and further controlling the state of the first portion and the state of the second portion (however, this is a process performed in the region that outputs modulated light, and does not include modulation of the light) based on a difference between the intensity of the held first signal and the intensity of the held second signal; The change is a process of changing the state of the second part together with the state of the first part so as to promote a change in the output before and after the state of the first part is changed. Control methods.

5. 1. A method of controlling an apparatus in which an output from a region including a first portion varies in response to a state of the first portion, comprising the steps of: generating a first signal that changes according to the intensity of the output while keeping the state of the first portion constant, holding the generated first signal, and then changing the state of the first portion; while keeping the changed state of the first portion constant, generating a second signal that changes according to the intensity of the output, holding the generated second signal; and further controlling the state of the first portion so that a difference between the intensity of the held first signal and the intensity of the held second signal becomes approximately zero. Control methods.

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