Apparatus and method for performing bias control on mach-zehnder modulator on basis of pulse phase stability

Abstract

Disclosed in the present invention are an apparatus and method for performing bias control on an adaptive Mach-Zehnder modulator (MZM) on the basis of pulse phase stability. The apparatus comprises a mode-locking femtosecond laser, an MZM, an optical fiber coupler, a photoelectric detector, an amplification filter, an analog-to-digital converter, an FPGA bias control module, a digital-to-analog converter, a bias amplifier and an electric pulse generator, wherein the FPGA bias control module demodulates a high-frequency signal amplitude and outputs an initial bias value, sets an MZM working point at a Null point, and also demodulates a low-frequency signal phase at this moment as a target value, compares same with a low-frequency signal demodulation phase which is updated in real time, so as to generate an error signal, and adjusts a bias voltage in real time by means of PID, so as to realize the stability at the Null point. In the present invention, a method for maintaining pulse phase stability is used to stabilize an MZM at a Null point, without introducing additional noise, thereby improving the bias control precision, ensuring the long-term stability of a working point, and realizing a high-extinction-ratio output of laser pulses at any frequency.

Description

Bias control device and method for Mach-Zehnder modulator based on pulse phase stabilization Technical Field

[0001] The present invention relates to a control method of a Mach-Zehnder modulator in the field of ultrafast interferometric imaging, and more specifically to an adaptive Mach-Zehnder modulator bias control device and method based on pulse phase stabilization. Background Art

[0002] Ultrafast optical imaging is a method of imaging transient events using ultrashort pulses. By matching the repetition rate of femtosecond pulses with the exposure time of the camera, single-pulse imaging is achieved, allowing the observation of transient physical phenomena and chemical reactions on the femtosecond timescale. It has important applications in the dynamic analysis of ultrafast phenomena in the fields of biomedicine, precision manufacturing, and scientific research.

[0003] Laser pulses in ultrafast optical imaging systems are mostly generated by frequency downconversion using a modulator. Among these, the Mach-Zehnder Modulator (MZM) offers advantages such as low drive voltage, high modulation bandwidth, wide wavelength performance, ultrashort rise time, and zero chirp, making it a suitable choice for fast pulse extraction. By controlling the MZM's operating point at the null point and applying a square wave RF signal, the MZM acts as a fast optical switch, blocking or transmitting pulsed light at low or high levels of the square wave signal, thereby performing pulse selection. However, when the ambient temperature changes, the MZM's operating point tends to drift, which can seriously affect the performance of pulse extraction. Therefore, automatic bias control technology is required to adjust the bias voltage in real time to lock the operating point.

[0004] Existing modulator bias control technologies are mainly divided into two categories:

[0005] Methods based on direct optical power monitoring and methods based on low-frequency jitter signals. The method based on optical power monitoring uses the absolute value of the output optical power or the ratio of the input and output optical powers as the monitoring feedback signal. This method is greatly affected by the fluctuation of the input optical power and has low accuracy.

[0006] The dither signal-based method applies a kilohertz low-frequency dither signal to the DC bias end of the MZM, and uses the fundamental frequency or harmonics of the demodulated original dither signal as the feedback signal to control the bias point. However, this method introduces additional modulation into the output signal, which will reduce the extinction ratio of the output pulse.

[0007] Summary of the Invention

[0008] In view of the deficiencies in the prior art, the present invention aims to provide an adaptive Mach-Zehnder modulator bias control device and method based on pulse phase stabilization to solve the problem of a decrease in the output pulse extinction ratio caused by bias operating point drift, thereby improving the bias control accuracy of the modulator.

[0009] The present invention adopts a method of maintaining pulse phase stability to stabilize the Mach-Zehnder modulator (MZM) at the Null point without introducing additional noise, thereby improving the bias control accuracy, ensuring the long-term stability of the operating point, and achieving high extinction ratio output of laser pulses of any frequency.

[0010] The specific technical solution adopted by the present invention to achieve the above-mentioned purpose is:

[0011] 1. A bias control device for a Mach-Zehnder modulator based on pulse phase stabilization:

[0012] The device comprises a mode-locked femtosecond laser, a Mach-Zehnder modulator, a fiber coupler, a photodetector, a first amplifying filter, a first analog-to-digital converter, a second amplifying filter, a second analog-to-digital converter, an FPGA bias control module, a digital-to-analog converter, a bias amplifier, and an electric pulse generator;

[0013] The output end of the mode-locked femtosecond laser is connected to the input end of the Mach-Zehnder modulator, the output end of the Mach-Zehnder modulator is connected to the input end of the fiber coupler, the other output end of the fiber coupler is connected to the input end of the photodetector, the electrical output end of the photodetector is connected to the input end of the first amplifying filter and the second amplifying filter respectively, the output ends of the first amplifying filter and the second amplifying filter are connected to the input end of the first analog-to-digital converter and the second analog-to-digital converter respectively, the output ends of the first analog-to-digital converter and the second analog-to-digital converter are both connected to the FPGA bias control module, the FPGA bias control module is connected to the bias control end of the Mach-Zehnder modulator in turn through the digital-to-analog converter and the bias amplifier, and the electrical pulse generator is connected to the RF radio frequency input end of the Mach-Zehnder modulator.

[0014] The first amplifying filter is a low-pass filter, and the second amplifying filter is a band-pass filter.

[0015] The FPGA bias control module includes a first lock-in amplifier, a second lock-in amplifier and a bias generator. The input ends of the first lock-in amplifier and the second lock-in amplifier are respectively connected to the output ends of the first analog-to-digital converter and the second analog-to-digital converter. The output ends of the first lock-in amplifier and the second lock-in amplifier are both connected to the input end of the bias generator. The output end of the bias generator is connected to the input end of the digital-to-analog converter.

[0016] The two output ends of the optical fiber coupler perform light splitting processing, and a small portion of light is sent to the photoelectric detector for bias control.

[0017] 2. A bias control method based on a bias control device of a Mach-Zehnder modulator:

[0018] The output wavelength of the mode-locked femtosecond laser is λ and the repetition frequency is f rep The optical pulse is coupled into the Mach-Zehnder modulator, and the electrical pulse generator outputs the frequency f pick The electrical pulse signal is input to the RF input of the Mach-Zehnder modulator for pulse modulation, so that the pulse output frequency is reduced to the frequency f pick The Mach-Zehnder modulator outputs an optical pulse signal to the fiber coupler, and a portion of the light is separated by the fiber coupler and detected by the photodetector. The optical pulse signal is converted into an electrical signal by the photodetector and divided into two paths. One path is respectively passed through the first amplification filter and the first analog-to-digital converter to obtain a signal with the same frequency f as the electrical pulse signal. pick The low-frequency digital signal S1(t) is obtained by the second amplifying filter and the second analog-to-digital converter, and the other one is obtained by the second amplifying filter and the second analog-to-digital converter, which has the same frequency f as the output of the mode-locked femtosecond laser. rep Finally, the low-frequency digital signal S1(t) and the high-frequency digital signal S2(t) are sent to the FPGA bias control module for data processing. After processing, the FPGA bias control module outputs the bias control signal V through the digital-to-analog converter. b , bias control signal V b After being amplified by the bias amplifier, the bias of the Mach-Zehnder modulator is controlled.

[0019] Bias control specifically controls the voltage phase of a Mach-Zehnder modulator (MZM).

[0020] In the FPGA bias control module, the amplitude of the high-frequency digital signal S2(t) is demodulated by the second lock-in amplifier 10 and sent to the bias generator 11. After processing, the bias generator 11 outputs the initial Null point bias value V through the digital-to-analog converter 12. null After being amplified by the bias amplifier 13, it is input to the DC bias terminal of the MZM2 to set the operating point; the phase of the low-frequency digital signal S1(t) is demodulated by the first phase-locked amplifier 7 and sent to the bias generator 11. After processing, the bias generator 11 outputs the bias control signal V through the digital-to-analog converter 12. b After being amplified by the bias amplifier 13, it is input to the DC bias terminal of the MZM2 to stabilize the operating point.

[0021] In the initial case, the bias generator of the FPGA bias control module is set to the bias control signal V b Scan a triangle wave voltage cycle and set the output frequency of the electric pulse generator to f pick The electric pulse signal is zero, that is, Ep = 0, and then the second lock-in amplifier demodulates the amplitude A2 of the high-frequency digital signal S2(t) in real time, and scans to obtain the bias control signal V corresponding to the minimum amplitude A2. b As the initial bias voltage V null Input to the Mach-Zehnder modulator.

[0022] At the same time, the low-frequency digital signal phase β0 obtained according to the low-frequency signal phase demodulation formula is set as the target value, and compared with the real-time demodulated low-frequency signal phase β in the bias generator to generate an error feedback signal. The bias generator outputs a real-time adjusted bias voltage V b Closed-loop control locks the Null operating point to achieve pulse extraction with high extinction ratio.

[0023] The technical principle of the present invention is as follows:

[0024] The electric pulse generator 14 outputs a radio frequency pulse signal E p The repetition frequency of the Mach-Zehnder modulator MZM is f rep The light pulse is extracted, and according to the modulation transfer function of the Mach-Zehnder modulator MZM, the output light intensity P of the Mach-Zehnder modulator MZM out for:

[0025] Among them, E in is the output light field of the mode-locked femtosecond laser, k is the insertion loss of the Mach-Zehnder modulator MZM, K1 is the amplification factor of the bias amplifier, V π is the half-wave voltage of the Mach-Zehnder modulator MZM, E p Represents the RF pulse signal, V b Represents the bias control signal, V σ represents the equivalent bias voltage corresponding to random drift.

[0026] From the above formula, we can see that when E p =0,(K1V b +V σ )=V π When the Mach-Zehnder modulator MZM works in the OFF state, the optical pulse is blocked, and the bias voltage value V b It is called the Null point voltage V null .

[0027] Further expanding the above output light intensity formula of the Mach-Zehnder modulator MZM, the signal detected by the photodetector is expressed as:

[0028] In the above formula, the output light field E of the mode-locked femtosecond laser 1 is in Expressed as:

[0029] Where, f ceo is the carrier envelope offset frequency, n is the number of comb teeth, A n and are the amplitude and phase of the nth comb tooth, respectively, and t represents the beam propagation time.

[0030] In the above formula, the frequency is f pick The period of the electrical pulse signal E p Its Fourier series expansion is expressed as:

[0031] Where B0 is the DC component, B m and φ m Represent the amplitude and phase of the mth harmonic component respectively, and m represents the frequency f pick harmonic order.

[0032] The output light field E in and electrical pulse signal E p Substitute the expression into the output intensity expansion formula of the Mach-Zehnder modulator MZM, and the cutoff frequency of the first amplification filter is about 1.5f pick After low-pass filtering, the low-frequency digital signal S1(t) is obtained, which is expressed as:

[0033] Among them, S 10 is the DC component, S 11 、S 12 are the coefficients of the second and third terms, Φ1 and Φ2 are the phase constants of the second and third terms, respectively. 11 、S 12 , Φ1 and Φ2 are all related to E in and E p A constant related to the amplitude and phase of .

[0034] The above formula is further simplified to:

[0035] Where C represents the amplitude and θ represents the phase of the MZM operating point.

[0036] It can be seen from this formula that the phase β of the low-frequency digital signal S1(t) corresponds to the operating point θ of the Mach-Zehnder modulator MZM. null The phase of the corresponding low-frequency digital signal S1(t) is β0, so by adjusting the bias voltage V b To compensate V σ The drift of the low-frequency digital signal S1(t) is suppressed, so that the phase β0 of the low-frequency digital signal S1(t) remains stable, thereby locking the operating point at the Null point (zero point).

[0037] The FPGA bias control module of the present invention demodulates the amplitude of the high-frequency signal to determine and output an initial bias value, sets the Mach-Zehnder modulator (MZM) operating point at the Null point, and simultaneously, uses the demodulated low-frequency signal phase at this moment as a target value to compare with the real-time updated low-frequency signal demodulation phase to generate an error signal, and adjusts the bias voltage in real time through PID to achieve stability of the Null point.

[0038] The present invention has the following beneficial effects:

[0039] (1) The present invention adopts a method of maintaining pulse phase stability to realize the Mach-Zehnder modulator (MZM) operating point, locks the optimal operating point of the modulator, and realizes pulse extraction with a high extinction ratio without additional noise introduction.

[0040] (2) The present invention uses an orthogonal demodulation algorithm to accurately set and stabilize the operating point of the Mach-Zehnder modulator (MZM), and has high bias control accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG1 is a schematic diagram of the apparatus and method of the present invention.

[0042] FIG2 is a flow chart of the MZM operating point bias control according to the present invention.

[0043] FIG3 is a comparison diagram of the optical power output of the MZM of the present invention with bias control and without bias control for one hour.

[0044] In the figure: 1. Mode-locked femtosecond laser, 2. Mach-Zehnder modulator (MZM), 3. Fiber coupler, 4. Photodetector, 5. First amplifying filter, 6. First analog-to-digital converter, 7. First lock-in amplifier, 8. Second amplifying filter, 9. Second analog-to-digital converter, 10. Second lock-in amplifier, 11. Bias generator, 12. Digital-to-analog converter, 13. Bias amplifier, 14. Electric pulse generator. DETAILED DESCRIPTION

[0045] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0046] As shown in FIG1 , the present invention specifically includes a mode-locked femtosecond laser 1, a Mach-Zehnder modulator 2, a fiber coupler 3, a photodetector 4, a first amplifying filter 5, a first analog-to-digital converter 6, a second amplifying filter 8, a second analog-to-digital converter 9, an FPGA bias control module, a digital-to-analog converter 12, a bias amplifier 13, and an electric pulse generator 14; the output end of the mode-locked femtosecond laser 1 is connected to the input end of the Mach-Zehnder modulator 2, the output end of the Mach-Zehnder modulator 2 is connected to the input end of the fiber coupler 3, one output end of the fiber coupler 3 is used to connect to an external device, and the other output end of the fiber coupler 3 is used to connect to an external device. It is connected to the input end of the photodetector 4, the electrical output end of the photodetector 4 is connected to the input ends of the first amplifying filter 5 and the second amplifying filter 8 respectively, the output ends of the first amplifying filter 5 and the second amplifying filter 8 are connected to the input ends of the first analog-to-digital converter 6 and the second analog-to-digital converter 9 respectively, the output ends of the first analog-to-digital converter 6 and the second analog-to-digital converter 9 are both connected to the FPGA bias control module, the FPGA bias control module is connected to the bias control end of the Mach-Zehnder modulator 2 after passing through the digital-to-analog converter 12 and the bias amplifier 13 in sequence, and the electric pulse generator 14 is connected to the RF radio frequency input end of the Mach-Zehnder modulator 2.

[0047] The first amplifying filter 5 is a low-pass filter, and the second amplifying filter 8 is a band-pass filter.

[0048] The FPGA bias control module includes a first lock-in amplifier 7, a second lock-in amplifier 10, and a bias generator 11. The inputs of the first and second lock-in amplifiers 7 and 10 are connected to the outputs of the first analog-to-digital converter 6 and the second analog-to-digital converter 9, respectively. The outputs of the first and second lock-in amplifiers 7 and 10 are both connected to the input of the bias generator 11, and the output of the bias generator 11 is connected to the input of the digital-to-analog converter 12. The first and second lock-in amplifiers 7 and 10 demodulate the phase of the low-frequency digital signal and the amplitude of the high-frequency digital signal, respectively, which are then sent to the bias generator 11 for processing and output as bias control digital signals.

[0049] The fiber coupler 3 has only one input end and two output ends. The two output ends of the fiber coupler 3 perform light splitting processing, and a small portion of the light is sent to the photodetector 4 for bias control.

[0050] In a specific implementation, light can be split 99:1, with 99% being output from the first output port and 1% being input from the second output port to the photodetector 4. The light output from the first output port is typically used for applications such as ultrafast imaging, processing of micro- and nanostructures, and femtosecond laser direct writing.

[0051] The verification process specifically implemented by the present invention is as follows:

[0052] The femtosecond laser pulse output by the mode-locked femtosecond laser 1 is coupled into the optical fiber input end of the MZM2, and the electrical pulse generator 14 outputs an electrical pulse signal E p The RF input to MZM2 has a repetition frequency of f rep The femtosecond laser pulse is extracted. According to the modulation transfer function of MZM2, the output light intensity of MZM2 is expressed as

[0053] Among them, E in is the output light field of the mode-locked femtosecond laser 1, k is the insertion loss of MZM2, K1 is the amplification factor of the bias amplifier 13, V π is the half-wave voltage of MZM2, V σ represents the equivalent bias voltage corresponding to random drift.

[0054] From formula (1), we can see that when E p =0,(K1V b +V σ )=V π When MZM2 is in OFF state, the light pulse is blocked, and the bias voltage value V b It is called the Null point voltage V null .

[0055] By expanding formula (1), the electrical signal detected by the photodetector 4 is expressed as:

[0056] Output light field E of mode-locked femtosecond laser 1 in Expressed as:

[0057] Where, f ceo is the carrier envelope offset frequency, n is the number of comb teeth, A n and are the amplitude and phase of the nth comb tooth respectively.

[0058] For a frequency of f pick The periodic pulse signal E p , its Fourier series expansion is expressed as:

[0059] Where B0 is the DC component, B m and φ m represent the amplitude and phase of the mth harmonic component respectively.

[0060] Substitute (3) and (4) into (2) and calculate the cutoff frequency to be about 1.5f. pick After low-pass filtering, the low-frequency digital signal S1(t) is expressed as:

[0061] Among them, S 10 is the DC component, S 11 、S 12 , Φ1 and Φ2 are respectively in and E p A constant related to the amplitude and phase of .

[0062] Formula (5) is further simplified as follows:

[0063] From equation (6), we can see that the phase β of S1(t) corresponds to the operating point θ of MZM2. null The phase of the corresponding low-frequency digital signal S1(t) is β0, so by adjusting the bias voltage V b To compensate V σ The drift of β0 is kept stable, thus locking the operating point at the Null point.

[0064] The MZM2 operating point bias control flow chart is shown in Figure 2. First, set E p =0, K1=1.2V π , bias control signal V b Scanning a triangle wave cycle, the second lock-in amplifier 10 demodulates the amplitude of the high-frequency digital signal S2(t) in real time. The formula is as follows:

[0065] Get the high frequency signal amplitude A2 and V b Relationship curve, initial bias voltage V null The bias voltage V corresponding to the minimum amplitude A2 b value, the bias value V null After being amplified by the bias amplifier 13, it is input to the DC bias terminal of the MZM2 to set the working point. Then the electric pulse generator 14 loads the electric pulse signal E p To the RF input terminal of MZM2, according to formula (6), the first phase-locked amplifier 7 demodulates to obtain the initial bias voltage V null The phase of the low-frequency digital signal S1(t) at the time is β0. In the bias generator 11, β0 is set as the target value and compared with the phase β of the low-frequency signal demodulated in real time to generate an error feedback signal. The bias generator 11 adjusts the bias voltage V in real time through PID closed-loop control. b To compensate for random drift V σ , thereby achieving the stability of the MZM2 working point.

[0066] In the embodiment of the present invention, the mode-locked femtosecond laser 1 adopts the C-Fiber 780 mode-locked fiber femtosecond laser produced by Menlo Systems of Germany, and the center wavelength of the output pulse light is λ = 1560 nm; the Mach-Zehnder modulator MZM2 adopts the MXER-LN-10 Mach-Zehnder intensity modulator produced by iXblue of France; and the photodetector 4 adopts the 1611FC-AC fiber detector produced by Newport of the United States.

[0067] Setting E p =0, K1=1.2V π , bias control signal V b Scan a triangle wave voltage cycle with an amplitude of 1V, and demodulate the amplitude of the high-frequency digital signal S2(t) in real time by the second lock-in amplifier 10 to obtain the high-frequency signal amplitude A2 and V b and determine the initial Null point bias voltage V when the amplitude A2 is the smallest. null , the bias value V null After being amplified by the bias amplifier 13, it is input to the DC bias terminal of MZM2 to set the working point. Then the electric pulse generator 14 loads the amplitude of V π The electrical pulse signal E p To the RF input terminal of MZM2, according to formula (7), the initial bias voltage V null The phase of the low-frequency digital signal S1(t) is β0, and β0 is set as the target value and compared with the real-time demodulated low-frequency signal phase β to obtain the error feedback signal. The bias generator 11 adjusts the bias voltage V in real time. b Compensation for random drift V σ , completing the closed-loop control of the MZM2 operating point. To verify the effectiveness of the proposed bias control method, a 1-hour MZM2 output optical power comparison experiment was conducted with and without bias control. The experimental results are shown in Figure 3.

[0068] As can be seen from the above embodiments, the present invention adopts a method of maintaining pulse phase stability to achieve automatic bias control of the MZM without introducing additional noise. It uses an orthogonal demodulation algorithm to accurately set and stabilize the operating point of the MZM, thereby improving the bias control accuracy of the system, ensuring long-term stability of the operating point, and realizing pulse extraction with a high extinction ratio, thereby having outstanding and significant technical effects.

[0069] The above specific embodiments are used to illustrate the present invention rather than to limit the present invention. Any modifications and changes made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A bias control device for a Mach-Zehnder modulator based on pulse phase stabilization, characterized in that: It comprises a mode-locked femtosecond laser (1), a Mach-Zehnder modulator (2), an optical fiber coupler (3), a photodetector (4), a first amplifying filter (5), a first analog-to-digital converter (6), a second amplifying filter (8), a second analog-to-digital converter (9), an FPGA bias control module, a digital-to-analog converter (12), a bias amplifier (13) and an electric pulse generator (14); The output end of the mode-locked femtosecond laser (1) is connected to the input end of the Mach-Zehnder modulator (2), the output end of the Mach-Zehnder modulator (2) is connected to the input end of the optical fiber coupler (3), the other output end of the optical fiber coupler (3) is connected to the input end of the photodetector (4), the electrical output end of the photodetector (4) is respectively connected to the input ends of the first amplifying filter (5) and the second amplifying filter (8), the output ends of the first amplifying filter (5) and the second amplifying filter (8) are respectively connected to the input ends of the first analog-to-digital converter (6) and the second analog-to-digital converter (9), the output ends of the first analog-to-digital converter (6) and the second analog-to-digital converter (9) are both connected to an FPGA bias control module, the FPGA bias control module is connected to the bias control end of the Mach-Zehnder modulator (2) after being sequentially connected to the digital-to-analog converter (12) and the bias amplifier (13), and the electrical pulse generator (14) is connected to the RF radio frequency input end of the Mach-Zehnder modulator (2).

2. The bias control device for a Mach-Zehnder modulator based on pulse phase stabilization according to claim 1, characterized in that: The first amplifying filter (5) is a low-pass filter, and the second amplifying filter (8) is a band-pass filter.

3. The bias control device for a Mach-Zehnder modulator based on pulse phase stabilization according to claim 1, characterized in that: The FPGA bias control module comprises a first lock-in amplifier (7), a second lock-in amplifier (10) and a bias generator (11); the input ends of the first lock-in amplifier (7) and the second lock-in amplifier (10) are respectively connected to the output ends of the first analog-to-digital converter (6) and the second analog-to-digital converter (9); the output ends of the first lock-in amplifier (7) and the second lock-in amplifier (10) are both connected to the input end of the bias generator (11); and the output end of the bias generator (11) is connected to the input end of the digital-to-analog converter (12).

4. The bias control device for a Mach-Zehnder modulator based on pulse phase stabilization according to claim 1, characterized in that: The two output ends of the optical fiber coupler (3) perform light splitting processing, and a small portion of the light is split to the photoelectric detector (4) for bias control.

5. The bias control method applied to the bias control device of the Mach-Zehnder modulator according to any one of claims 1 to 4, characterized in that: The method uses a mode-locked femtosecond laser (1) to output a wavelength of λ and a repetition frequency of f. rep The optical pulse is coupled into the Mach-Zehnder modulator (2), and the electrical pulse generator (14) outputs a frequency f pick The electrical pulse signal is input to the RF radio frequency input end of the Mach-Zehnder modulator (2) for pulse modulation. The Mach-Zehnder modulator (2) outputs an optical pulse signal to the optical fiber coupler (3). A portion of light is separated by the optical fiber coupler (3) and detected by the photodetector (4). The optical pulse signal is converted into an electrical signal by the photodetector (4) and is divided into two paths. One path is respectively passed through the first amplifying filter (5) and the first analog-to-digital converter (6) to obtain a signal having the same frequency f as the electrical pulse signal. pick The other path is respectively passed through a second amplifying filter (8) and a second analog-to-digital converter (9) to obtain a low-frequency digital signal S1(t) having the same frequency f as the output of the mode-locked femtosecond laser (1). rep Finally, the low-frequency digital signal S1(t) and the high-frequency digital signal S2(t) are sent to the FPGA bias control module for data processing. After processing, the FPGA bias control module outputs the bias control signal V through the digital-to-analog converter. b , bias control signal V b After being amplified by the bias amplifier (13), the bias voltage of the Mach-Zehnder modulator (2) is controlled.

6. The bias control method of the bias control device of the Mach-Zehnder modulator according to claim 5, characterized in that: In the initial case, the bias generator (11) of the FPGA bias control module generates a bias control signal V b Scan a triangle wave voltage cycle, set the electric pulse signal output by the electric pulse generator (14) to zero, then demodulate the amplitude A2 of the high-frequency digital signal S2(t) in real time by the second lock-in amplifier, and scan to obtain the bias control signal V corresponding to the minimum amplitude A2. b As the initial bias voltage V null Input to the Mach-Zehnder modulator (2).

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