Electro-optical modulator bias control device and method based on sine amplitude modulation signal

Abstract

The invention discloses an electrooptical modulator bias control device and method based on sine amplitude modulation signals. The device comprises a photoelectric conversion amplification module, an analog-to-digital conversion module, a controller, a sine signal generator, a radio frequency signal output circuit and a direct current bias output circuit. The photoelectric conversion amplification module is used for collecting signal output of the electro-optical modulator as feedback and transmitting the signal output to the controller through the analog-to-digital conversion module; the controller is used for generating a reference sinusoidal signal, generating a radio frequency modulation signal through the radio frequency signal output circuit, and sending the radio frequency modulation signal into the electro-optical modulator; the control module is used for judging a working drift condition according to a feedback signal and a reference signal and generating a corresponding control signal; the DC bias output circuit generates a voltage signal according to the control signal and outputs the voltage signal to the electro-optical modulator; according to the invention, feedback control is carried out by using modulated light consistent with the frequency of a radio frequency signal, so that the limitation of a traditional structure is well eliminated, and high-precision bias control is realized on the premise of not influencing the output of an amplitude-modulated light time domain waveform.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and more particularly to a device and method for controlling bias of an electro-optical modulator based on a sinusoidal amplitude modulation signal. Background Art

[0002] Currently, electro-optic modulators (EOMs) are essential for generating AM light in AM NMOR magnetometers. However, due to material inhomogeneities and manufacturing process errors, EO modulators have an initial bias phase. Furthermore, variations in ambient temperature, mechanical distortion, mechanical vibration, and input electrical signals can cause this initial bias phase to shift, causing the operating point to drift. This results in waveform distortion and amplitude fluctuations in the AM light, which in turn affects the sensitivity of the magnetometer. Therefore, automatic tracking and control of the EO modulator's operating point is essential.

[0003] In addition, when automatically tracking and controlling the operating point of the EO modulator, traditional EO modulator operating point control schemes often use a low-frequency disturbance signal input at the DC bias end as the feedback signal. This method will introduce a single-frequency noise signal into the output energy spectrum of the EO modulator, affecting the time domain waveform of the output amplitude modulated light and, in turn, the sensitivity of the magnetometer.

[0004] Therefore, how to achieve high-precision bias control of an electro-optic modulator is an urgent problem that those skilled in the art need to solve. Summary of the Invention

[0005] In view of this, the present invention provides an electro-optical modulator bias control device and method based on a sinusoidal amplitude modulated signal, which utilizes the sinusoidal signal at the RF input end for feedback modulation, effectively eliminating the limitations of the traditional structure and achieving high-precision bias control without affecting the output amplitude modulated light time domain waveform.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A bias control device for an electro-optical modulator based on a sinusoidal amplitude modulation signal, comprising: a photoelectric conversion and amplification module, an analog-to-digital conversion module, a controller, a sinusoidal signal generator, a radio frequency signal output circuit, and a DC bias output circuit;

[0008] The photoelectric conversion and amplification module is used to collect the signal output of the electro-optical modulator as feedback and transmit it to the controller via the analog-to-digital conversion module;

[0009] The controller is used to generate a reference sinusoidal signal, generate a radio frequency modulated signal through the radio frequency signal output circuit and enter the electro-optical modulator; and is used to determine the working drift according to the feedback signal and the reference signal, and generate a corresponding control signal;

[0010] The DC bias output circuit generates a voltage signal according to the control signal and outputs the voltage signal to the electro-optical modulator.

[0011] Preferably, it also includes an optical coupler, the input end of the optical coupler is connected to the output end of the electro-optical modulator, and is used to divide the signal of the electro-optical modulator into a first optical path and a second optical path; the first optical path is used as modulated light output, and the second optical path is used as the feedback.

[0012] Preferably, the radio frequency modulation signal is a high frequency signal.

[0013] An amplitude modulated light generating device comprises the above-mentioned bias control device, a laser and an electro-optical modulator.

[0014] A method for controlling bias of an electro-optical modulator based on a sinusoidal amplitude modulated signal comprises the following steps:

[0015] S1: Use a sinusoidal signal of a specific frequency to modulate the light source;

[0016] S2: Receive the modulated optical signal and extract the fundamental wave amplitude R1 and the second-order harmonic amplitude R2 and phase θ2 as drift data using a phase-locked amplification method based on the specific frequency;

[0017] S3: Determine the current drift state based on the drift data of the previous moment and generate a corresponding bias voltage output value.

[0018] Preferably, the S3 includes:

[0019] If the ratio R2 / R1 is equal to 0, it means that the working point at this time is the optimal working point;

[0020] If the ratio R2 / R1 is not equal to 0, the output voltage is judged to have exceeded the optimal operating point based on whether θ2 changes by 180°; the θ2 obtained at the current moment is compared with the θ2 obtained at the previous moment (using θ 2_previous Is the difference between (indicates) ±180°?

[0021] If the ratio R2 / R1 is not equal to 0 and θ2 does not change by 180°, it indicates that the output voltage has not exceeded the optimal operating point. The sign of the ratio R2 / R1 continues to use the sign of the ratio R2 / R1 at the previous moment. At this time, the feedback value R2 / R1 is substituted into the PID controller to calculate the bias voltage output value at this moment.

[0022] If the ratio R2 / R1 is not equal to 0 and θ2 changes by 180°, it indicates that the output voltage has exceeded the optimal operating point; the sign of the ratio R2 / R1 is opposite to the sign of the ratio R2 / R1 at the previous moment. At this time, the feedback value R2 / R1 with the opposite sign is substituted into the PID controller to calculate the bias voltage output value at this moment.

[0023] Preferably, the S3 further includes:

[0024] After the electro-optic modulator bias control device is powered on, the controller controls the output voltage of the DC bias output circuit to 0V, calculates the R2 / R1 ratio at this moment, and assumes that the ratio read back at this moment is a positive value. The output voltage at this moment is calculated by PID. If the R2 / R1 ratio decreases at the next moment, it indicates that the sign of the initially assumed ratio is correct; if the R2 / R1 ratio increases at the next moment, it indicates that the sign of the initially assumed ratio is incorrect, and the initial ratio sign should be negative.

[0025] It can be seen from the above technical solutions that, compared with the prior art, the present invention provides a method based on a sinusoidal amplitude modulation signal, which has the following effects:

[0026] (1) By applying a sinusoidal signal to the RF input port, feedback control of the bias control can be performed while modulating the laser, avoiding single-frequency noise caused by applying a low-frequency signal to the DC input port;

[0027] (2) The present invention quickly identifies the operating point crossing through phase mutation detection, dynamically corrects the feedback signal sign, and improves the convergence speed and stability of PID control. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0029] Figure 1 It is a schematic diagram of an electro-optic modulator whose operating point voltage is slightly lower than the optimal operating point;

[0030] Figure 2 are the amplitude spectrum and phase spectrum of the electro-optic modulator output signal when the operating point voltage drifts below the optimal operating point voltage; (a) is the amplitude spectrum, (b) is the phase spectrum;

[0031] Figure 3 It is a schematic diagram of an electro-optic modulator with an operating point voltage higher than the optimal operating point;

[0032] Figure 4 are the amplitude spectrum and phase spectrum of the electro-optic modulator output signal when the operating point voltage drifts above the optimal operating point voltage; (a) is the amplitude spectrum, and (b) is the phase spectrum;

[0033] Figure 5 Schematic diagram of the bias voltage control system of the electro-optic modulator of the present invention;

[0034] Figure 6 This is a flow chart for confirming the sign of the ratio at the initial moment in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] like Figure 1 In a first aspect, an embodiment of the present invention discloses an electro-optical modulator bias control device based on a sinusoidal amplitude modulated signal, comprising a photoelectric conversion amplification module, an analog-to-digital conversion module, a controller, a sinusoidal signal generator, a radio frequency signal output circuit, and a DC bias output circuit.

[0037] The photoelectric conversion and amplification module is used to collect the signal output of the electro-optical modulator as feedback and send it to the controller through the analog-to-digital conversion module.

[0038] The controller is used to generate a reference sinusoidal signal, generate an RF modulated signal through the RF signal output circuit, and enter the electro-optical modulator; it is used to judge the working drift according to the feedback signal and the reference signal, and generate a corresponding control signal.

[0039] The DC bias output circuit generates a voltage signal according to the control signal and outputs the voltage signal to the electro-optic modulator.

[0040] In order to further implement the above technical solution, an optical coupler is also included, the input end of the optical coupler is connected to the output end of the electro-optical modulator, and is used to split the signal of the electro-optical modulator into a first optical path and a second optical path; the first optical path is used as modulated light output, and the second optical path is used as feedback.

[0041] To prevent bias control from affecting the amplitude-modulated light output by the electro-optic modulator, this device directly uses the modulated light output by the electro-optic modulator as a feedback signal to control the modulator. Laser light generated by a laser is transmitted to the modulator via optical fiber. Due to the electro-optic effect of the modulator, the RF signal and bias signal applied to the modulator simultaneously modulate the laser light, resulting in modulated light at the output of the modulator with sinusoidal intensity variation and a frequency consistent with the input RF signal.

[0042] Because the RF signal is high-frequency, this device uses the principle of a sampling lock-in amplifier to extract the fundamental and harmonic information of the modulated light. To improve control security, the lock-in amplifier adopts a bidirectional lock-in amplifier design. However, the output amplitude of a bidirectional lock-in amplifier is always positive, so the phase of the RF signal's second harmonic is introduced to correct the sign of the output amplitude.

[0043] When the operating point voltage is lower than the optimal operating point, such as Figure 2 The lock-in amplifier obtains the amplitude and phase of each order harmonic at the moment u1, as shown in Figure 3 Assume that the ratio R2 / R1 at time u1 is positive, and bring the ratio into the PID control to calculate the output control voltage. If the control output is appropriate, the working voltage gradually approaches the optimal working point; if the control output is too large, the working voltage exceeds the optimal working point, as shown in the following example. Figure 4 The lock-in amplifier obtains the amplitude and phase of each order harmonic at the moment u2, as shown in Figure 5 As shown. Figure 5 (b) and Figure 5 As shown in (a), the phase of the second harmonic at time u2 changes by 180°. By flipping the sign of the ratio R2 / R1 at time u2 to a negative value and then applying it to the PID control output voltage, the operating voltage gradually approaches the optimal operating point. By determining whether the phase of the RF signal's second harmonic has changed by 180°, the direction of the operating point's offset can be directly determined, allowing the controller to control the operating point. In actual implementation, the PID parameters and phase jump threshold (e.g., a tolerance of ±170° to ±190°) must be adjusted according to the specific modulator characteristics.

[0044] In this embodiment, the photoelectric conversion amplification module also includes a photodetector and a transimpedance amplifier circuit. When this embodiment is used to generate amplitude modulated light in an amplitude modulated NMOR magnetometer, since the modulation frequency required by the NMOR magnetometer is between 14kHz and 1.4MHz, in order to meet the detection requirements of the NMOR magnetometer, the bandwidth of the photodetector should be greater than twice the maximum required modulation frequency. In this embodiment, the maximum required modulation frequency is 1.4MHz. In addition, the dark current of the photodetector should be as small as possible. When the driving voltage is 3.3V, the dark current should be less than 1nA, so as to ensure a good signal-to-noise ratio and ensure that the signal received by the controller is correct. In this embodiment, the bandwidth of the photodetector is 500MHz, and the spectral sensitivity band is at 760nm. When the driving voltage is 3.3V, the dark current of the photodetector is a maximum of 100pA, which meets the above requirements. The photodetector converts the light signal into a current signal.

[0045] The transimpedance amplifier circuit is mainly composed of an operational amplifier. The function of the transimpedance amplifier circuit is to convert the current signal output by the photodetector into a voltage signal and amplify it. The current signal output by the photodetector is a high-frequency signal, so the bandwidth of the transimpedance amplifier circuit should also be greater than twice the maximum required modulation frequency. In addition, the input bias current of the transimpedance amplifier circuit should be less than 100nA to avoid affecting the current signal converted by the photodetector. In this embodiment, the bandwidth of the transimpedance amplifier circuit is 10MHz and the maximum input bias current is 2nA, which meets the above requirements. The transimpedance amplifier circuit amplifies the current signal into a voltage signal.

[0046] The analog-to-digital conversion module is mainly composed of an analog-to-digital converter (ADC). The analog-to-digital conversion module converts the output voltage signal of the transimpedance amplifier circuit into a digital signal and transmits it to the controller. The performance of the analog-to-digital conversion module will affect the control accuracy of the controller. Due to the limitations of the Nyquist sampling theorem and the output voltage signal of the transimpedance amplifier circuit, the bandwidth of the analog-to-digital conversion module should be greater than four times the maximum required modulation frequency. In addition, the resolution of the analog-to-digital converter should be greater than 8 bits to meet the requirements of control accuracy. In this embodiment, the bandwidth of the analog-to-digital converter is 60MHz and the resolution is 16 bits. The analog-to-digital converter converts the voltage signal into a digital signal.

[0047] The controller consists of a controller chip and its peripheral circuits. The controller processes the digital signal output by the analog-to-digital converter with a reference signal generated by itself. It simultaneously generates four sinusoidal signals, performs PID control calculations, controls the output of the DC bias output circuit, and controls the RF signal output circuit. The selected controller should complete these functions within a specific control cycle. In this embodiment, the control cycle is 1000 Hz, and the chip selected is a Xilinx Zynq-7000 series chip.

[0048] The DC bias output circuit primarily consists of a digital-to-analog converter (DAC). This DAC converts the bias control signal from the controller into a voltage signal, which is then output to the DC bias input port of the electro-optical modulator. Because the operating point of the electro-optical modulator drifts very slowly, the DAC does not require high-speed conversion. Instead, its resolution should be greater than 10 bits to meet control accuracy requirements. In this embodiment, the DAC in the DC bias output circuit has a 14-bit resolution.

[0049] The sine signal generator is generated by a controller using Direct Digital Frequency Synthesis (DDS). The DDS frequency control word should have as high a bit count as possible, and the number of waveform lookup tables should be as large as possible to generate a sine signal with high frequency resolution and low phase noise. In this embodiment, the frequency control word count is 128 bits, and the number of waveform lookup tables is 4096.

[0050] The RF signal output circuit primarily consists of a digital-to-analog converter (DAC). This DAC converts the sinusoidal signal output by the controller into a voltage signal, which is then output to the RF input port of the electro-optical modulator. The DAC's conversion rate and resolution should be as high as possible to generate a high-quality sinusoidal signal. In this embodiment, the DAC in the RF signal output circuit has a conversion rate of 60 MHz and a resolution of 14 bits.

[0051] In a second aspect, an embodiment of the present invention provides a banner light generating device, which is based on the control device provided in the first aspect and further includes a laser and an electro-optical modulator; the laser output is modulated by the electro-optical modulator and then output; in addition, the radio frequency signal output circuit and the DC bias output circuit of the control device are respectively connected to the DC input terminal and the RF input terminal of the electro-optical modulator; at the same time, the photoelectric conversion and amplification module of the control device collects the output of the electro-optical modulator to achieve closed-loop control.

[0052] It is known that in order to prevent the feedback process from affecting the normal output, an optical coupler can be connected to the output end of the electro-optical modulator to achieve beam splitting, which can be used for output and feedback respectively.

[0053] In a third aspect, based on the same inventive concept, an embodiment of the present invention provides a method for controlling a bias of an electro-optical modulator based on a sinusoidal amplitude modulated signal, comprising the following steps:

[0054] S1: Use a sinusoidal signal of a specific frequency to modulate the light source;

[0055] S2: Receives the modulated optical signal and uses a phase-locked amplification method based on a specific frequency to extract the fundamental wave amplitude R1 and the second-order harmonic amplitude R2 and phase θ2 as drift data;

[0056] S3: Determine the current drift state based on the drift data of the previous moment and generate a corresponding bias voltage output value.

[0057] Exemplarily, in the aforementioned electro-optical modulator bias voltage control process, the controller uses a DDS to generate a sinusoidal signal of the desired frequency and outputs the sinusoidal signal to the RF signal output circuit to generate an RF modulated signal. The optical signal generated by the electro-optical modulator is split into two paths via a 9:1 optical coupler, with 90% of the laser output as amplitude modulated light and 10% as a feedback optical signal input to the optoelectronic conversion and amplification module. The optoelectronic conversion and amplification module converts the optical signal into a voltage signal, which is then converted into a digital signal via an analog-to-digital conversion module and output to the controller. The controller processes the feedback signal and the reference signal to determine the drift of the electro-optical modulator's operating point. Based on the drift, the controller performs a PID operation to output a digital control variable. This is then passed through the DC bias output circuit to generate an analog control voltage, which brings the electro-optical modulator's operating point closer to the optimal operating point.

[0058] Control Method: The EO modulator bias voltage control system tracks the EO modulator's operating point in two stages. The first stage is the initialization stage, which determines the sign of the initial ratio. The second stage is the control stage, which gradually controls the EO modulator's operating point toward the optimal operating point.

[0059] Phase 1: After the electro-optic modulator bias control device is powered on, the controller sets the output voltage of the DC bias output circuit to 0V. The R2 / R1 ratio and phase at time T0 are obtained. Assuming the ratio is positive, the PID controller calculates the output voltage at time T0 and obtains the R2 / R1 ratio and phase at time T1. If the phase difference between T0 and T1 is 180°, the output voltage at T0 causes the electro-optic modulator's bias point to pass the optimal operating point. The sign at T0 is positive, while the sign at T1 is negative. If the phase difference between T0 and T1 is significantly less than 180°, and the ratio at T0 is greater than the ratio at T1, the output voltage at T0 causes the electro-optic modulator's bias point to gradually approach the optimal operating point. The sign at T0 is positive, while the sign at T1 is positive. If the phase difference between time T0 and time T1 is much less than 180°, and the ratio at time T0 is smaller than the ratio at time T1, it means that the output voltage at time T0 makes the bias point of the electro-optic modulator far away from the optimal operating point. The sign of the ratio initially assumed is wrong, with the sign at time T0 being negative and the sign at time T1 being negative. The flowchart for obtaining the sign of the ratio at the initial time is as follows: Figure 6 shown.

[0060] Phase 2: Determine the operating point drift based on the R2 / R1 ratio and θ2, and control the operating point of the electro-optic modulator to gradually approach the optimal operating point, as follows:

[0061] If the ratio R2 / R1 is equal to 0, it means that the working point at this time is the optimal working point;

[0062] If the ratio R2 / R1 is not equal to 0, the output voltage is judged to have exceeded the optimal operating point based on whether θ2 changes by 180°. 2_current ) and the θ2 obtained at the previous moment (expressed by θ 2_previous Is the difference between (indicates) ±180°?

[0063] If the ratio R2 / R1 is not equal to 0 and θ2 does not change by 180°, it indicates that the output voltage has not exceeded the optimal operating point. The sign of the ratio R2 / R1 continues to use the sign of the ratio R2 / R1 at the previous moment. At this time, the feedback value R2 / R1 is substituted into the PID controller to calculate the bias voltage output value at this moment, gradually approaching the optimal operating point.

[0064] If the ratio R2 / R1 is not equal to 0 and θ2 changes by 180°, it indicates that the output voltage has exceeded the optimal operating point. The sign of the ratio R2 / R1 is opposite to that of the previous ratio. At this time, the feedback value R2 / R1 with the opposite sign is input into the PID controller to calculate the bias voltage output value at this moment, gradually approaching the optimal operating point. The control process of stage 2 is repeated to gradually lock in the optimal operating point of the electro-optic modulator.

[0065] The PID controller outputs the bias voltage and gradually locks the optimal operating point of the electro-optic modulator in the following process. At discrete time k, the controller output u(k) is:

[0066]

[0067] Where u(k) is the output at the kth moment; e(k) is the error at the kth moment (the difference between the set value r(k) and the actual value y(k), e(k) = r(k) - y(k) = 0 - R2 / R1 = -R2 / R1 = -R2 / R1); K p , K i , K d is the proportional, integral and differential gain; T s is the sampling time.

[0068] Assuming that R2 / R1 is positive at time k, it indicates that the operating point at time k is above the optimal operating point, as shown at time u2 in Figure (3). Since the error is negative, the output voltage calculated by formula (1) at time k will decrease, so that the operating point gradually approaches the optimal operating point. Assuming that R2 / R1 is negative at time k, it indicates that the operating point at time k is below the optimal operating point, as shown at time u1 in Figure (1). Since the error is positive, the output voltage calculated by formula (1) at time k will increase, so that the operating point gradually approaches the optimal operating point.

[0069] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0070] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An electro-optic modulator bias control device based on a sinusoidal amplitude modulated signal, characterized in that: include: Photoelectric conversion amplifier module, analog-to-digital conversion module, controller, sinusoidal signal generator, radio frequency signal output circuit and DC bias output circuit; The photoelectric conversion and amplification module is used to collect the signal output of the electro-optical modulator as feedback and transmit it to the controller via the analog-to-digital conversion module; The controller is used to generate a reference sinusoidal signal, and generate a radio frequency modulated signal through the radio frequency signal output circuit and enter the electro-optical modulator; Used to judge the working drift according to the feedback signal and reference signal, and generate corresponding control signals; The DC bias output circuit generates a voltage signal according to the control signal and outputs the voltage signal to the electro-optical modulator.

2. The electro-optic modulator bias control device based on a sinusoidal amplitude modulation signal according to claim 1, characterized in that: It also includes an optical coupler, the input end of the optical coupler is connected to the output end of the electro-optical modulator, and is used to divide the signal of the electro-optical modulator into a first optical path and a second optical path; the first optical path is used as modulated light output, and the second optical path is used as the feedback.

3. The electro-optic modulator bias control device based on a sinusoidal amplitude modulation signal according to claim 1, characterized in that: The radio frequency modulation signal is a high frequency signal.

4. An amplitude modulated light generating device, characterized in that: The invention comprises the bias control device, laser and electro-optic modulator according to any one of claims 1 to 3.

5. A method for controlling bias of an electro-optical modulator based on a sinusoidal amplitude modulated signal, characterized in that: The following steps are involved: S1: Use a sinusoidal signal of a specific frequency to modulate the light source; S2: Receive the modulated optical signal and extract the fundamental wave amplitude R1 and the second-order harmonic amplitude R2 and phase θ2 as drift data using a phase-locked amplification method based on the specific frequency; S3: Determine the current drift state based on the drift data of the previous moment and generate a corresponding bias voltage output value.

6. The method for controlling bias of an electro-optical modulator based on a sinusoidal amplitude modulated signal according to claim 5, wherein: The S3 includes: If the amplitude ratio R2 / R1 of the second-order harmonic to the fundamental amplitude is equal to 0, it indicates that the operating point at this time is the optimal operating point; If the amplitude ratio R2 / R1 is not equal to 0, whether the output voltage has exceeded the optimal operating point is determined by determining whether the phase θ2 of the second-order harmonic has changed by 180°; If the amplitude ratio R2 / R1 is not equal to 0 and the phase θ2 of the second-order harmonic does not change by 180°, it indicates that the output voltage has not exceeded the optimal operating point; the sign of the amplitude ratio R2 / R1 continues to use the sign of the ratio at the previous moment, and the amplitude ratio R2 / R1 with the sign is used as the feedback value to be fed into the PID controller to calculate the bias voltage output value at this moment; If the ratio R2 / R1 is not equal to 0 and the amplitude θ2 of the second-order harmonic changes by 180°, it indicates that the output voltage has exceeded the optimal operating point; the sign of the ratio R2 / R1 is opposite to the sign of the ratio at the previous moment, and the amplitude ratio R2 / R1 with the sign is used as the feedback value to be brought into the PID controller to calculate the bias voltage output value at this moment.

7. The method for controlling bias of an electro-optical modulator based on a sinusoidal amplitude modulated signal according to claim 6, wherein: Said S3 further comprises: After the electro-optic modulator bias control device is powered on, the controller controls the output voltage of the DC bias output circuit to 0V, calculates the R2 / R1 ratio at this moment, sets the initial ratio sign to positive, and uses PID to calculate the output voltage at this moment. If the R2 / R1 ratio decreases at the next moment, it indicates that the initial ratio sign is set correctly; if the R2 / R1 ratio increases at the next moment, it indicates that the initial ratio sign is set incorrectly and the sign at the current moment is negative, and the current feedback value is brought into the PID controller to calculate the bias voltage output value.

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