Laser saturation absorption frequency stabilizing device and method

Abstract

The application provides a laser saturation absorption frequency stabilization device and method, and relates to the technical field of laser frequency stabilization. The laser saturation absorption frequency stabilization device comprises four modules, namely, an outgoing light path module, a probe light path module, a pump light path module and a signal processing and feedback module. The outgoing light path module comprises a half wave plate HWP0, a polarization beam splitter PBS0 and a half wave plate HWP1. The probe light path module comprises a polarization beam splitter PBS1, a lens LENS0, a hollow cathode lamp, a lens LENS1 and a polarization beam splitter PBS2. The pump light path module comprises a mirror MR0, a lens LENS2, an acousto-optic modulator and a mirror MR1. The signal processing and feedback module comprises a photodetector, an FPGA, a radio frequency switch, a voltage-controlled oscillator and an amplifier. The application simplifies the device of the original saturation absorption frequency stabilization method, is easy to adjust, and improves the stability of the laser frequency stabilization.

Description

Technical Field

[0001] This invention relates to the field of laser frequency stabilization technology, and in particular to a laser saturated absorption frequency stabilization device and method. Background Technology

[0002] Currently, semiconductor lasers are widely used in fields such as atomic cooling, atomic and molecular spectroscopy, quantum precision measurement, and quantum time and frequency analysis due to their advantages such as small size, high diffraction efficiency, and ease of operation in single-mode. However, because semiconductor lasers have a large spectral range, are susceptible to external environmental interference, and have poor frequency stability, frequency stabilization is necessary before conducting research in fields such as atomic cooling and atomic and molecular spectroscopy. Saturated absorption stabilization is a conventional method for stabilizing the frequency of semiconductor lasers. Because this method uses an external reference frequency for stabilization and does not use the transition center frequencies of the laser working material atoms themselves as a reference, it is widely used.

[0003] Saturated absorption frequency stabilization devices are divided into those with a reference beam that eliminates Doppler broadening and those without a reference beam that exhibits Doppler broadening. Since a saturated absorption system with a reference beam can produce a saturated absorption peak without Doppler broadening, it greatly improves the stability of the laser frequency. Therefore, the saturated absorption frequency stabilization method with a reference beam is often used in precision spectral research.

[0004] like Figure 1 As shown, the basic principle of the saturated absorption frequency stabilization method with a reference beam is that the laser beam is split into a stronger pump beam, a weaker probe beam, and a reference beam by a polarizing beam splitter 2. The pump beam and probe beam cross each other in opposite directions and pass through the hollow cathode lamp 6. The reference beam and probe beam pass through the hollow cathode lamp 6 parallel to each other and at a certain distance. When the laser frequency is not equal to the center frequency of atomic absorption, the probe beam is absorbed by the atom with the opposite direction of its motion due to the Doppler effect. When the laser frequency equals the center frequency of atomic absorption, the atoms are excited to the point of saturation by the pump light. The probe light passes through the hollow cathode lamp 6 with almost no atomic absorption. Therefore, when the laser frequency equals the center frequency of atomic absorption, a Lamb Dip occurs. The probe light passing through the hollow cathode lamp is then subtracted from the reference light signal to obtain the absorption peak signal that eliminates Doppler broadening. After modulation and demodulation, this signal is used as a frequency-stabilized discrimination signal.

[0005] Figure 1 The saturated absorption frequency stabilization method shown involves three optical paths and is quite complex. In practical applications, it presents at least the following technical problems:

[0006] 1. Existing saturated absorption frequency stabilization methods require that the power density of the probe light and the reference light passing through the hollow cathode lamp 6 be equal, which greatly increases the difficulty of adjustment;

[0007] 2. There is a certain distance between the probe light and the reference light of the hollow cathode lamp 6. Due to the uneven distribution of atomic density inside the hollow cathode lamp 6, there are errors in the signals of the probe light and the reference light, which will affect the strength of the absorption peak signal.

[0008] 3. Existing saturated absorption frequency stabilization methods have complex signal processing system topologies and frequency locking processes, which increases the impact of noise during signal processing;

[0009] 4. Changes in ambient air pressure and temperature can affect the optical path, causing changes in the frequency discrimination signal and degrading the frequency stabilization performance of the laser. Summary of the Invention

[0010] To overcome the shortcomings of existing saturated absorption frequency stabilization methods, such as high adjustment difficulty and poor stability, this application provides a laser saturated absorption frequency stabilization device and method. This solution greatly reduces the adjustment difficulty by sharing a single laser beam between the probe light and the reference light. The all-digital design of signal processing greatly simplifies the topology of the connection line, reduces the impact of noise during signal processing, and improves robustness.

[0011] The technical solution adopted by the embodiments of this application to solve its technical problem is:

[0012] The laser saturable absorption frequency stabilization device consists of four modules: an output optical path module, a probe optical path module, a pump optical path module, and a signal processing and feedback module.

[0013] The outgoing optical path module includes a half-wave plate HWP0, a polarizing beam splitter PBS0, and a half-wave plate HWP1;

[0014] The detection optical path module includes a polarizing beam splitter PBS1, a lens LENS0, a hollow cathode lamp, a lens LENS1, and a polarizing beam splitter PBS2.

[0015] The pump optical path module includes a reflector MR0, a lens LENS2, an acousto-optic modulator, and a reflector MR1.

[0016] The signal processing and feedback module includes a photodetector, an FPGA, a radio frequency switch, a voltage-controlled oscillator, and an amplifier.

[0017] Preferably, the lens LENS0 and the lens LENS1 are located on both sides of the hollow cathode lamp, with their focal points coinciding, and the focal points of the lens LENS0 and the lens LENS1 are located at the center of the hollow cathode lamp.

[0018] Preferably, the signal of the acousto-optic modulator includes a 100MHz radio frequency signal provided by the voltage-controlled oscillator and a 30kHz square wave modulated signal generated by the FPGA programming.

[0019] Preferably, the photodetector is connected to the input terminal of the FPGA, the voltage-controlled oscillator, the radio frequency switch, and the laser controller in the laser are respectively connected to the three output terminals of the FPGA, and the radio frequency switch is electrically connected to the acousto-optic modulator through the voltage-controlled oscillator and the amplifier.

[0020] Preferably, the functions implemented by the FPGA include a digital signal time-domain digital subtractor, a digital lock-in amplifier, a digital low-pass filter, a digital PID controller, a digital sine wave generator, and a scan / lock switch.

[0021] Preferably, the digital signal time-domain digital subtractor performs time-domain digital subtraction on the probe optical signal and the reference optical signal at the previous and next time points to obtain an absorption peak signal without Doppler broadening.

[0022] Preferably, the scan / lock switch represents two different operating modes: scan mode and lock mode.

[0023] Preferably, in the scanning mode, the digital sine wave generator generates a scanning signal and sends it to the piezoelectric driver of the laser, while simultaneously adding a modulation signal to the pump light to scan the voltage of the laser and locate the locking point of the hollow cathode lamp.

[0024] Preferably, in the locked mode, the output signal of the photodetector is subjected to time-domain subtraction, phase-locked amplification, and low-pass filtering. The processed signal is then fed back through the digital PID controller, and the feedback signal is output to the piezoelectric driver of the laser. At the same time, a square wave signal is connected to the radio frequency switch for rapid switching of the pump light.

[0025] The laser saturated absorption frequency stabilization method based on amplitude modulation, and based on the aforementioned laser saturated absorption frequency stabilization device, specifically includes the following steps:

[0026] S1. Is this the first time using this software? If it is the first time using this software, proceed to S2; otherwise, proceed to S7.

[0027] S2. The acousto-optic modulator starts working and receives square wave signals;

[0028] S3. Set the FPGA to scanning mode and turn off the voltage-controlled oscillator and amplifier. At this time, no square wave signal will be sent to the acousto-optic modulator. At the same time, the piezoelectric ceramic of the laser receives the scanning voltage of the FPGA, and the laser wavelength changes accordingly. At this time, the FPGA transmits the data of the photodetector to the computer, and the Doppler absorption peak with a frequency range of 3.3 GHz can be observed.

[0029] S4. Keep the FPGA in scanning mode and turn on the voltage-controlled oscillator and amplifier. At this time, the acousto-optic modulator intermittently generates a first-order spot at a frequency of 30KHz and coincides with the optical paths of the reference light and the probe light through the polarization beam splitter PBS2. When the square wave signal is low, the acousto-optic modulator does not generate a first-order spot. At this time, the photodetector receives the reference light signal. When the square wave signal is high, the acousto-optic modulator generates a first-order spot as the pump light. The photodetector receives the reference light signal. At this time, the Doppler free saturation absorption spectrum can be observed through the time-domain subtraction program in the FPGA. In addition, it is necessary to adjust the reflector MR1 and the polarization beam splitter PBS2 to maximize the signal of the saturation absorption spectrum.

[0030] S5. Keep the FPGA in scanning mode, enable digital demodulation, fine-tune the signal applied by the acousto-optic modulator, reduce the residual amplitude modulation of the signal, and eliminate signal bias.

[0031] S6. Determine the digital PID parameters, gradually reduce the value of the scanning voltage, determine the lock point, and after selection, enter the lock mode, adjust the digital PID parameters to achieve laser frequency lock, and save the digital PID parameters.

[0032] S7. Using the saved digital PID parameters, enable automatic lock point search and complete automatic locking.

[0033] The advantages of the embodiments of this application are:

[0034] This invention simplifies the apparatus of the original saturated absorption frequency stabilization method, makes it easy to adjust, facilitates the modularization of the frequency stabilization device, and improves the stability of laser frequency stabilization. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of a saturated absorption frequency stabilization device with reference light in the prior art.

[0036] Figure 2 This is a schematic diagram of the laser saturated absorption frequency stabilization device of the present invention;

[0037] Figure 3 This is a schematic diagram of the internal structure of the FPGA of this invention;

[0038] Figure 4 This is a schematic diagram of the TTL signal of the present invention.

[0039] Explanation of key figure labels:

[0040] 1. Half-wave plate HWP0; 2. Polarizing beam splitter PBS0; 3. Half-wave plate HWP1; 4. Polarizing beam splitter PBS1; 5. Lens LENS0; 6. Hollow cathode lamp; 7. Lens LENS1; 8. Polarizing beam splitter PBS2; 9. Mirror MR0; 10. Lens LENS2; 11. Acousto-optic modulator; 12. Mirror MR1; 13. Photodetector; 14. FPGA; 15. RF switch; 16. Voltage-controlled oscillator; 17. Amplifier; 18. Quartz beam splitter; 19. Digital signal time-domain digital subtractor; 20. Digital low-pass filter; 21. Digital PID controller; 22. Digital sine wave generator; 23. Scan / lock switch. Detailed Implementation

[0041] This application provides a laser saturated absorption frequency stabilization device and method, which solves the problems of high adjustment difficulty and poor stability in the existing saturated absorption frequency stabilization method. In the solution, by sharing a single laser beam for the probe light and the reference light, the adjustment difficulty is greatly reduced. The all-digital design of signal processing greatly simplifies the topology of the connection line, reduces the impact of noise during signal processing, and improves robustness.

[0042] For ion trap quantum computing experiments, a laser is required to provide frequency-stable cooling light. The laboratory constructed a saturated absorption frequency stabilization system to stabilize the DLC DL PRO HP 399 laser to the frequency of ytterbium atoms. 1 S0- 1 The P1 absorption peak is used to stabilize the laser output frequency.

[0043] The technical solution in this application is to solve the above-mentioned technical problems, and the general idea is as follows:

[0044] Example 1:

[0045] This invention provides a laser saturable absorption frequency stabilization device, such as... Figure 2 As shown, the laser saturated absorption frequency stabilization device includes four modules: an output optical path module, a probe optical path module, a pump optical path module, and a signal processing and feedback module.

[0046] The outgoing optical path module includes a half-wave plate HWP0 (1), a polarizing beam splitter PBS0 (2), and a half-wave plate HWP1 (3).

[0047] The detection optical path module includes a polarizing beam splitter PBS1 (4), a lens LENS0 (5), a hollow cathode lamp (6), a lens LENS1 (7), and a polarizing beam splitter PBS2 (8), which are arranged in a straight line along the optical axis. The focal points of the lenses LENS0 (5) and LENS1 (7) located on both sides of the hollow cathode lamp (6) coincide, and the focal points of the lenses LENS0 (5) and LENS1 (7) are located at the center of the hollow cathode lamp (6).

[0048] The pump optical path module includes a mirror MR0 (9), a lens LENS2 (10), an acousto-optic modulator (11), and a mirror MR1 (12) arranged in a straight line along the optical axis. The signal of the acousto-optic modulator (11) includes a 100MHz radio frequency signal provided by a voltage-controlled oscillator (16) and a 30kHz square wave modulated signal generated by programming the FPGA (14). The 30kHz square wave modulated signal is controlled by a radio frequency switch (15).

[0049] The signal processing and feedback module includes a photodetector (13), an FPGA (14), a radio frequency switch (15), a voltage-controlled oscillator (16), and an amplifier (17). The photodetector (13) is connected to the input terminal of the FPGA (14). The voltage-controlled oscillator (16), the radio frequency switch (15), and the laser controller in the laser are respectively connected to the three output terminals of the FPGA (14). The radio frequency switch (15) is electrically connected to the acousto-optic modulator (11) through the voltage-controlled oscillator (16) and the amplifier (17). Figure 3As shown, the functions implemented by FPGA (14) include a digital signal time-domain digital subtractor (19), a digital lock-in amplifier (not shown in the figure), a digital low-pass filter (20), a digital PID (21), a digital sine wave generator (22), and a scan / lock switch (23). The digital signal time-domain digital subtractor (19) performs time-domain digital subtraction on the probe signal T- and the reference signal T+ at the previous and next time points to obtain an absorption peak signal without Doppler broadening. The scan / lock switch (23) represents two different operating modes: scan mode and lock mode. In scan mode, the digital sine wave generator (22) generates a scan signal and sends it to the piezoelectric driver of the laser. At the same time, the output signal of OUT0 is connected to the RF switch (15), which enters the acousto-optic modulator (11) after passing through the voltage-controlled oscillator (16). This adds a modulation signal to the pump light and scans the voltage of the laser to find the lock point of the hollow cathode lamp (6). In locked mode, the output signal of the photodetector (13) is digitally processed, and the processed signal is output to the piezoelectric driver of the laser. Specifically, the output signal of the photodetector (13) is processed by signal time-domain subtraction, lock-in amplification, low-pass filtering, etc., and then the corresponding signal is processed by the digital PID (21) to perform the corresponding feedback, and the feedback signal is output to the piezoelectric driver of the laser. At the same time, the TTL signal output by OUT1 is connected to the RF switch (15) for fast switching of the pump light. The TTL signal (30KHz square wave signal) issued by the FPGA (14) controls the switching state of the RF switch (15), and the RF switch (15) switches the 30KHz RF modulation signal, such as Figure 4 As shown, when the TTL signal input to the RF switch (15) is high, the pump light is turned on, and the light from both directions passes through the hollow cathode lamp (6) simultaneously. Therefore, the photodetector (13) obtains the probe light signal; when the TTL signal input to the RF switch (15) is low, the reverse pump light is turned off, and only a single beam of light passes through the hollow cathode lamp (6). Therefore, the photodetector (13) obtains the reference light signal. By modulating the pump light, the probe light and the reference light share the same laser beam. This process is equivalent to the acousto-optic modulator (11) performing rapid switching and frequency modulation on the pump light. The scanning / locking mode and the parameters of the digital PID are switched on the programmed control interface. By writing the FPGA program, the lock point can be automatically found. Only the parameters need to be adjusted on the computer, and the lock point position needs to be confirmed to achieve the frequency stabilization target. The signals required for the experiment are uniformly time-controlled by the FPGA (14).

[0050] The half-wave plate HWP0 (1), polarizing beam splitter PBS0 (2), half-wave plate HWP1 (3), polarizing beam splitter PBS1 (4), lens LENS0 (5), hollow cathode lamp (6), lens LENS1 (7), polarizing beam splitter PBS2 (8), mirror MR0 (9), lens LENS2 (10), acousto-optic modulator (11), mirror MR1 (12), and photodetector (13) are sequentially mounted on the same optical breadboard along the same optical axis.

[0051] The light transmission path in the laser saturated absorption frequency stabilization device is as follows:

[0052] The p-polarized light generated by the polarizing beam splitter PBS0(2) enters the main optical path experimental system. The s-polarized light generated by the polarizing beam splitter PBS0(2) passes through the half-wave plate HWP1(3) and then enters the polarizing beam splitter PBS1(4) as incident light. The p-polarized light generated by the polarizing beam splitter PBS1(4) exits and becomes the incident light of the pump optical path, illuminating the reflecting mirror MR0(9). After being focused by the lens LENS(10), it enters the acousto-optic modulator (11). The first-order diffracted light from the acousto-optic modulator (11) After being reflected by the mirror MR1 (12), the light enters the polarizing beam splitter PBS2 (8) and the resulting s-polarized light passes through the lens LENS1 (7) and enters the hollow cathode lamp (6). The s-polarized light generated by the polarizing beam splitter PBS1 (4) serves as the incident light in the detection light path and illuminates the lens LENS0 (5). After passing through the hollow cathode lamp (6) and the lens LENS1 (7), the light enters the polarizing beam splitter PBS2 (8). The p-polarized light generated by the polarizing beam splitter PBS2 (8) is incident on the photodetector (13).

[0053] Example 2:

[0054] This invention proposes a laser saturated absorption frequency stabilization method based on amplitude modulation. The laser saturated absorption frequency stabilization method specifically includes the following steps:

[0055] S1. Is this the first time using this software? If it is the first time using this software, proceed to S2; otherwise, proceed to S7.

[0056] S2, the acoustic-optic modulator (11) starts working and receives square wave signals.

[0057] S3. Set the FPGA (14) to scanning mode and turn off the voltage-controlled oscillator (16) and amplifier (17). At this time, no square wave signal will be sent to the acousto-optic modulator (11). At the same time, the piezoelectric ceramic of the laser receives the scanning voltage of the FPGA (14), and the laser wavelength changes accordingly. At this time, the FPGA (14) transmits the data of the photodetector (13) to the computer, and the Doppler absorption peak with a frequency range of 3.3 GHz can be observed.

[0058] S4. Keep FPGA (14) in scanning mode and turn on voltage-controlled oscillator (16) and amplifier (17). At this time, acousto-optic modulator (11) intermittently generates a first-order spot at a frequency of 30KHz and overlaps with the reference and probe optical paths through polarization beam splitter PBS2 (8). When the square wave signal (TTL) is low, acousto-optic modulator (11) does not generate a first-order spot. At this time, photodetector (13) receives the reference signal. When the square wave signal (TTL) is high, acousto-optic modulator (11) generates a first-order spot as pump light. Photodetector (13) receives the reference signal. At this time, the Doppler free saturated absorption spectrum can be observed through the time-domain subtraction program in FPGA (14). In addition, it is necessary to adjust the reflector MR1 (12) and polarization beam splitter PBS2 (8) to maximize the signal of the saturated absorption spectrum.

[0059] S5. Keep FPGA (14) in scanning mode, enable digital demodulation function, fine-tune the signal added by the acousto-optic modulator (11), reduce the residual amplitude modulation of the signal, and eliminate the bias of the signal.

[0060] S6. Determine the digital PID parameters, gradually reduce the value of the scanning voltage, and determine the lock point. The lock point is generally selected as the point with the strongest demodulation signal and the largest slope. After selection, enter the lock mode, adjust the digital PID parameters to achieve laser frequency lock, and save the digital PID parameters.

[0061] S7. Using the saved digital PID parameters, enable automatic lock point search and complete automatic locking.

[0062] Compared with existing technologies, the present invention has the following advantages:

[0063] 1) By sharing a single laser beam for both the probe and reference beams, the power required for the saturated absorption frequency stabilization device is reduced, and the utilization rate of laser power in the main optical path is improved;

[0064] 2) By sharing a single laser beam between the probe and reference beams, there is no need to build a reference beam optical path, which simplifies the optical system for saturated absorption frequency stabilization and greatly reduces the difficulty of adjustment;

[0065] 3) The all-digital design of signal processing greatly simplifies the topology of the connection lines, reduces the impact of noise during signal processing, and improves robustness;

[0066] 4) The programmed control chart can automatically find the laser frequency lock point. By adjusting the parameters on the computer and confirming the lock point position, the frequency stabilization target can be achieved, saving time and facilitating actual experimental operation.

[0067] Finally, it should be noted that the above embodiments are merely examples for clearly illustrating the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A laser saturated-absorption frequency stabilisation apparatus characterised in that The laser saturation absorption frequency stabilizing device comprises four modules, namely, an outgoing light path module, a probe light path module, a pump light path module and a signal processing and feedback module. The outgoing light path module comprises a half wave plate HWP0, a polarization beam splitter PBS0 and a half wave plate HWP1. The probe light path module comprises a polarization beam splitter PBS1, a lens LENS0, a hollow cathode lamp, a lens LENS1 and a polarization beam splitter PBS2. The pump light path module comprises a mirror MR0, a lens LENS2, an acousto-optic modulator and a mirror MR1. The signal processing and feedback module comprises a photodetector, an FPGA, a radio frequency switch, a voltage-controlled oscillator and an amplifier.

2. The laser saturated-absorption frequency stabilisation device of claim 1, wherein, The lens LENS0 and the lens LENS1 are located on the two sides of the hollow cathode lamp, and the focal points are coincident, and the focal points of the lens LENS0 and the lens LENS1 are located at the central position of the hollow cathode lamp.

3. The laser saturated-absorption frequency stabilisation device of claim 1, wherein, The signal of the acousto-optic modulator comprises a 100MHz radio frequency signal provided by the voltage-controlled oscillator and a 30KHz square wave modulation signal generated by the FPGA.

4. The laser saturated-absorption frequency stabilisation device of claim 1, wherein, The photodetector is connected to the input end of the FPGA, the voltage-controlled oscillator, the radio frequency switch and a laser controller in the laser are connected to three output ends of the FPGA, and the radio frequency switch is electrically connected to the acousto-optic modulator through the voltage-controlled oscillator and the amplifier.

5. The laser saturated-absorption frequency stabilisation device of claim 1, wherein, The FPGA realizes functions including a digital signal time domain digital subtractor, a digital phase-locked amplifier, a digital low-pass filter, a digital PID, a digital sine wave generator and a scanning / locking switch.

6. The laser saturated-absorption frequency stabilisation device of claim 5, wherein, The digital signal time domain digital subtractor performs time domain digital subtraction on the probe light signal and the reference light signal at different time points to obtain an absorption peak signal without Doppler broadening.

7. The laser saturated-absorption frequency stabilisation device of claim 5, wherein, The scanning / locking switch represents two different operation modes, namely, a scanning mode and a locking mode.

8. The laser saturated-absorption frequency stabilisation device of claim 7, wherein, In the scanning mode, the digital sine wave generator generates a scanning signal and sends the scanning signal to the piezoelectric driver of the laser, and at the same time, a modulation signal is added to the pump light, so that the voltage of the laser is scanned to find the locking point of the hollow cathode lamp.

9. The laser saturated-absorption frequency stabilisation device of claim 7, wherein, In the locking mode, the output signal of the photodetector is subjected to signal time domain subtraction, phase-locked amplification and low-pass filtering, and then the processed signal is subjected to feedback through the digital PID, and the feedback signal is output to the piezoelectric driver of the laser, and at the same time, the square wave signal is connected to the radio frequency switch to quickly switch the pump light.

10. An amplitude modulation based laser saturated absorption frequency stabilization method based on the laser saturated absorption frequency stabilization device of any one of claims 1-9, characterized in that, The laser saturation absorption frequency stabilizing method comprises the following steps: S1, whether it is the first use, if it is the first use, entering S2, otherwise, entering S7; S2, the acousto-optic modulator starts to work and receives a square wave signal; S3, the FPGA is set to the scanning mode, and at the same time, the voltage-controlled oscillator and the amplifier are turned off, so that no square wave signal is sent to the acousto-optic modulator, and at the same time, the piezoelectric ceramic of the laser receives the scanning voltage of the FPGA, and the wavelength of the laser changes accordingly, and at this time, the data of the photodetector is transmitted to the computer by the FPGA, and the Doppler absorption peak with a frequency range of 3.3GHz can be observed; S4, keep FPGA in scan mode, open voltage-controlled oscillator and amplifier, at this time, acousto-optic modulator intermittently generates first order spot at 30KHz frequency and coincides with reference light and probe light path through polarizing beam splitter PBS2, when square wave signal is low, acousto-optic modulator does not generate first order spot, at this time, photoelectric detector receives reference light signal, when square wave signal is high, acousto-optic modulator generates first order spot as pump light, photoelectric detector receives reference light signal, at this time, Doppler free saturated absorption spectrum can be observed through time domain subtraction program in FPGA, in addition, polarizing beam splitter PBS2 and mirror MR1 need to be adjusted to make saturated absorption spectrum signal maximum; S5, keep FPGA in scan mode, open digital demodulation function, fine tune signal added by acousto-optic modulator, reduce residual amplitude modulation of signal, eliminate signal bias; S6, determine digital PID parameter, gradually reduce value of scan voltage, determine lock point, after selection, enter lock mode, adjust digital PID parameter, realize laser frequency locking, and save digital PID parameter; S7, use saved digital PID parameter, open automatic lock point searching, complete automatic locking.

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