An external cavity semiconductor laser frequency stabilization system and a frequency stabilization method

Abstract

This application belongs to the field of quantum precision measurement technology, and specifically discloses an external cavity semiconductor laser frequency stabilization system and a frequency stabilization method. The system includes a saturated absorption spectrum module, a wavelet peak seeking module, a modulation and demodulation module, a bias amplification module, and a switch module. The saturated absorption spectrum module outputs an electrical signal of the saturated absorption spectrum. Under the control of a frequency locking instruction, the wavelet peak seeking module outputs a target bias voltage signal corresponding to the locked frequency based on the electrical signal. The modulation and demodulation module performs signal modulation and demodulation based on the electrical signal and a reference signal corresponding to the locked frequency, and outputs a modulation signal and a demodulation signal. The bias amplification module superimposes and amplifies the modulation signal, the demodulation signal, and the target bias voltage signal, and outputs a target superimposed signal to the PZT modulation port of the laser, so that the output laser operates freely at the locked frequency. Through this application, the laser is not prone to mode hopping during the frequency locking process, making the entire frequency stabilization system more stable, and having a simple structure and low cost.

Description

Technical Field

[0001] This application belongs to the field of quantum precision measurement technology. More specifically, it relates to an external cavity semiconductor laser frequency stabilization system and a frequency stabilization method. Background Art

[0002] With the successful trapping of cold atoms, the field of quantum precision measurement has developed rapidly. The laser for trapping cold atoms needs to be stabilized within the required frequency range to meet the experimental conditions. Since the linewidth of the external cavity semiconductor laser is narrow enough to introduce less noise to the entire optical path system, in a cold atom gravimeter, an external cavity semiconductor laser is usually used as a light source to generate lasers of various frequencies required for experiments.

[0003] In the prior art, during the process of stabilizing the frequency of an external cavity semiconductor laser, the system usually uses a current feedback loop for laser frequency scanning and locking. However, since the current feedback loop is a fast feedback loop, when scanning the current, the laser is prone to mode hopping, making it difficult to lock the laser frequency. In addition, in the prior art, the modulation transfer spectrum of an electro-optic modulator (EOM) external modulation is usually used to stabilize the frequency of the external cavity semiconductor laser. Although it can achieve laser frequency locking, the price of the EOM is expensive.

[0004] Therefore, how to better achieve the frequency stabilization of an external cavity semiconductor laser has become an urgent technical problem in the industry. Summary of the Invention

[0005] Aiming at the defects of the prior art, the purpose of this application is to better achieve the frequency stabilization of an external cavity semiconductor laser, aiming to solve the problems of easy mode hopping and high cost existing in the existing frequency stabilization methods.

[0006] To achieve the above purpose, in the first aspect, this application provides an external cavity semiconductor laser frequency stabilization system, including:

[0007] A saturated absorption spectrum module, a wavelet peak searching module, a modulation and demodulation module, a bias amplification module, and a switch module;

[0008] The input end of the saturated absorption spectrum module is used to receive the laser output by an external laser. The saturated absorption spectrum module is used to measure the atomic saturated absorption spectrum of the laser and output the electrical signal of the saturated absorption spectrum;

[0009] The first input terminal of the switch module is connected to the output terminal of the saturable absorption spectrum module. The output terminal of the switch module is respectively connected to the input terminal of the wavelet peak-seeking module and the input terminal of the modulation and demodulation module. The second input terminal of the switch module is used to receive an external frequency locking instruction. The switch module is used to close under the control of the frequency locking instruction to transmit the electrical signal to the wavelet peak-seeking module and the modulation and demodulation module respectively.

[0010] The wavelet peak-seeking module is used to output a target bias voltage signal corresponding to the locked frequency according to the electrical signal.

[0011] The modulation and demodulation module includes a first output terminal and a second output terminal. The modulation and demodulation module is used to perform signal modulation and demodulation according to the electrical signal and a reference signal corresponding to the locked frequency, and output a modulation signal from the first output terminal and a demodulation signal from the second output terminal.

[0012] The first output terminal and the second output terminal of the modulation and demodulation module, and the output terminal of the wavelet peak-seeking module are respectively connected to the input terminal of the bias amplification module. The bias amplification module is used to perform superposition and amplification processing on the modulation signal, the demodulation signal, a preset bias voltage, and the target bias voltage signal, and output a target superposition signal to the PZT modulation port of the laser to adjust the cavity length of the laser so that the output laser operates freely at the locked frequency.

[0013] Optionally, the wavelet peak-seeking module includes a differential module, a wavelet denoising module, a peak-seeking module, and a locking module connected in sequence.

[0014] The input terminal of the differential module serves as the input terminal of the wavelet peak-seeking module and is used to output a frequency discrimination signal corresponding to the electrical signal.

[0015] The wavelet denoising module is used to perform wavelet decomposition and signal reconstruction on the frequency discrimination signal to obtain a denoised frequency discrimination signal.

[0016] The peak-seeking module is used to identify the peak-to-peak information corresponding to the transition peak of the locked frequency in the denoised frequency discrimination signal.

[0017] The output terminal of the locking module serves as the output terminal of the wavelet peak-seeking module and is used to perform zero-crossing detection on the peak-to-peak information and output a target bias voltage signal corresponding to the locked frequency.

[0018] Optionally, the modulation and demodulation module includes a first signal generator, and a DC blocker, a lock-in amplifier, a low-pass filter, and a feedback controller connected in sequence.

[0019] The reference signal output terminal of the first signal generator is connected to the input terminal of the lock-in amplifier; the reference signal output terminal is used to output the reference signal corresponding to the locked frequency;

[0020] The modulation signal output terminal of the first signal generator serves as the first output terminal of the modulation and demodulation module and is used to output the modulation signal;

[0021] The input terminal of the modulation and demodulation module includes a first input terminal; the input terminal of the DC blocker serves as the first input terminal of the modulation and demodulation module and is used to filter out the DC signal of the electrical signal to obtain the AC signal of the electrical signal;

[0022] The lock-in amplifier is used to subtract the AC signal from the reference signal and output an error signal;

[0023] The low-pass filter is used to filter out the high-frequency signal in the error signal to obtain a low-frequency error signal;

[0024] The output terminal of the feedback controller serves as the second output terminal of the modulation and demodulation module and is used to output the demodulation signal based on the low-frequency error signal.

[0025] Optionally, the bias amplification module includes a first adder, a second adder, and a power amplifier connected in sequence;

[0026] The input terminals of the bias amplification module include a first input terminal, a second input terminal, a third input terminal, and a fourth input terminal. The first input terminal of the first adder serves as the first input terminal of the bias amplification module and is connected to the first output terminal of the modulation and demodulation module. The second input terminal of the first adder serves as the second input terminal of the bias amplification module and is connected to the second output terminal of the modulation and demodulation module. The third input terminal of the first adder serves as the third input terminal of the bias amplification module and is used to receive the preset bias voltage; the input terminal of the second adder serves as the fourth input terminal of the bias amplification module and is connected to the output terminal of the wavelet peak search module;

[0027] The first adder is used to perform a superposition process on the modulation signal, the demodulation signal, and the preset bias voltage and output a first superposition signal;

[0028] The second adder is used to perform a superposition process on the target bias voltage signal and the first superposition signal and output a second superposition signal;

[0029] The power amplifier serves as the output terminal of the bias amplification module and is used to amplify the second superposition signal and output the target superposition signal.

[0030] Optionally, the modulation and demodulation module further includes a second signal generator; the modulation and demodulation module further includes a second input terminal and a third output terminal;

[0031] The output terminal of the second signal generator serves as the second input terminal of the modulation and demodulation module, and is used to receive an external unlocking instruction and generate a scanning signal under the control of the unlocking instruction;

[0032] The second input terminal of the switch module is further used to receive the unlocking instruction to control the switch module to disconnect, blocking the electrical signal from being transmitted to the wavelet peak seeking module and the modulation and demodulation module;

[0033] The output terminal of the second signal generator serves as the third output terminal of the modulation and demodulation module and is connected to the input terminal of the bias amplification module, and is used to output the scanning signal to the bias amplification module;

[0034] The bias amplification module is further used to perform superposition and amplification processing on the modulation signal and the scanning signal, and transmit the output superposition signal to the PZT modulation port to continuously adjust the cavity length of the laser, so that the output laser operates freely within a preset frequency scanning range.

[0035] Optionally, the saturated absorption spectrum module includes an optical isolator, a transmission optical fiber, a half-wave plate, a polarization beam splitter, a coupling head, a wavelength meter, a high-transmission and low-reflection mirror, a reflector, a rubidium cell, a semi-transmissive and semi-reflective mirror, and a photodetector arranged in sequence along the main optical path;

[0036] The coupling head and the wavelength meter are arranged in sequence on the transmission optical path of the polarization beam splitter, the high-transmission and low-reflection mirror and the reflector are arranged in sequence on the reflection optical path of the polarization beam splitter, and the rubidium cell, the semi-transmissive and semi-reflective mirror and the photodetector are arranged in sequence on the reflection optical path of the high-transmission and low-reflection mirror;

[0037] The optical isolator is used to prevent the light reflected by the optical fiber echo on the main optical path from returning to the laser and affecting the operation of the laser;

[0038] The transmission optical fiber is used to transmit the laser passing through the optical isolator to the half-wave plate;

[0039] The half-wave plate is used to adjust the polarization state of the input laser to linear polarization;

[0040] The polarization beam splitter is used to divide the linearly polarized laser into a first reflected light and a first transmitted light. The first transmitted light is converted into spatial light by the coupling head and then transmitted to the wavelength meter to monitor the frequency change of the laser; the first reflected light is transmitted to the high-transmission and low-reflection mirror;

[0041] The high-transmission and low-reflection mirror is used to divide the first reflected light into a second reflected light and a second transmitted light. The second reflected light is transmitted to the semi-transparent and semi-reflective mirror after generating a saturated absorption spectrum signal of rubidium atoms through the rubidium cell; the second transmitted light is reflected by the reflector to the semi-transparent and semi-reflective mirror;

[0042] The semi-transparent and semi-reflective mirror is used to divide the saturated absorption spectrum signal into a third reflected light and a third transmitted light. The third transmitted light is transmitted to the photodetector as the probe light; and is used to convert the second transmitted light reflected by the reflector into the repump light;

[0043] The photodetector is used to convert the probe light into an electrical signal.

[0044] In a second aspect, the present application provides a frequency stabilization method applied to the frequency stabilization system of the external cavity semiconductor laser described in any one of the foregoing. The method includes:

[0045] Performing atomic saturated absorption spectrum measurement on the laser output by the laser, and outputting an electrical signal of the saturated absorption spectrum;

[0046] Under the control of a frequency locking instruction, outputting a target bias voltage signal corresponding to the locked frequency according to the electrical signal;

[0047] Performing signal modulation and demodulation according to the electrical signal and a reference signal corresponding to the locked frequency, and outputting a modulation signal and a demodulation signal;

[0048] Performing superposition and amplification processing on the modulation signal, the demodulation signal, a preset bias voltage, and the target bias voltage signal, and outputting a target superposition signal to the PZT modulation port to adjust the cavity length of the laser, so that the output laser operates freely at the locked frequency.

[0049] Optionally, the outputting a target bias voltage signal corresponding to the locked frequency according to the electrical signal under the control of a frequency locking instruction includes:

[0050] Outputting a frequency discrimination signal corresponding to the electrical signal;

[0051] Performing wavelet decomposition and signal reconstruction on the frequency discrimination signal to obtain a denoised frequency discrimination signal;

[0052] Identifying the peak-to-peak information of the transition peak of the locked frequency corresponding to the denoised frequency discrimination signal;

[0053] Performing zero-crossing detection on the peak-to-peak information, and outputting a target bias voltage signal corresponding to the locked frequency.

[0054] Optionally, the performing signal modulation and demodulation according to the electrical signal and a reference signal corresponding to the locked frequency, and outputting a modulation signal and a demodulation signal includes:

[0055] Filter out the DC signal of the electrical signal to obtain the AC signal of the electrical signal;

[0056] Generate two signals, namely the reference signal corresponding to the locked frequency and the modulation signal, under the control of the frequency locking instruction;

[0057] Subtract the AC signal from the reference signal and output an error signal;

[0058] Filter out the high-frequency signal in the error signal to obtain a low-frequency error signal;

[0059] Output the demodulation signal based on the low-frequency error signal.

[0060] Optionally, the method further includes:

[0061] Output a scanning signal and a modulation signal under the control of an unlocking instruction;

[0062] Perform superposition and amplification processing on the modulation signal and the scanning signal, and transmit the output superposed signal to the PZT modulation port to continuously adjust the cavity length of the laser, so that the output laser operates freely within a preset frequency scanning range.

[0063] Generally speaking, compared with the prior art through the above technical solutions provided by this application, the following beneficial effects are obtained:

[0064] (1) This application provides an external cavity semiconductor laser frequency stabilization system. By introducing the method of PZT internal modulation to achieve frequency stabilization, the laser is less likely to experience mode hopping during the phase locking process, making the entire frequency stabilization system more stable and overcoming the defect that the scanning current of the external cavity semiconductor laser is prone to mode hopping;

[0065] (2) On the premise of meeting the narrow linewidth of the external cavity semiconductor laser, by adopting the method of processing the saturated absorption spectrum to achieve frequency stabilization, the overall structure is simple and easy to integrate, and it can better adapt to the dynamic measurement and handling process of the cold atom gravimeter. Compared with the traditional modulation transfer spectrum frequency stabilization method using EOM external modulation, the cost is greatly reduced;

[0066] (3) By introducing the wavelet peak seeking method to identify the transition peak to be locked, the problems that the frequency discrimination signal will decay in amplitude over time and the peak signal will be submerged by noise are overcome, making the locked laser frequency more accurate and less susceptible to the influence of saturated absorption spectrum line drift and line noise. Description of the Drawings

[0067] Figure 1 is a schematic structural diagram of the external cavity semiconductor laser frequency stabilization system provided by the embodiment of this application;

[0068] Figure 2It is a schematic structural diagram of the saturated absorption spectrum module provided by an embodiment of the present application;

[0069] Figure 3 It is a schematic diagram of the saturated absorption spectrum measured by the saturated absorption spectrum module provided by an embodiment of the present application;

[0070] Figure 4 It is a schematic structural diagram of the wavelet peak searching module provided by an embodiment of the present application;

[0071] Figure 5 It is a schematic diagram of the first derivative signal of the saturated absorption spectral line provided by an embodiment of the present application;

[0072] Figure 6 It is a schematic diagram of the first derivative signal of the saturated absorption spectral line after wavelet transform provided by an embodiment of the present application;

[0073] Figure 7 It is a schematic structural diagram of the modulation and demodulation module provided by an embodiment of the present application;

[0074] Figure 8 It is a schematic structural diagram of the bias amplification module provided by an embodiment of the present application;

[0075] Figure 9 It is a schematic diagram of the frequency change before and after frequency locking of the frequency stabilization system of the external cavity semiconductor laser provided by an embodiment of the present application;

[0076] Figure 10 It is a schematic diagram of the error signal change before and after frequency locking of the frequency stabilization system of the external cavity semiconductor laser provided by an embodiment of the present application;

[0077] Figure 11 It is a schematic flowchart of the frequency stabilization method of the frequency stabilization system of the external cavity semiconductor laser provided by an embodiment of the present application.

[0078] In all the drawings, the same reference numerals are used to represent the same elements or structures, where:

[0079] 1 is the saturated absorption spectrum module; 101 is an optical isolator; 102 is a transmission optical fiber; 103 is a half-wave plate; 104 is a polarization beam splitter; 105 is a coupling head; 106 is a wavemeter; 107 is a high-transmission and low-reflection mirror; 108 is a reflector; 109 is a rubidium cell; 110 is a semi-transmissive and semi-reflective mirror; 111 is a photodetector; 2 is the small wave peak-finding module; 21 is a differential module; 22 is a small wave denoising module; 23 is a peak-finding module; 24 is a locking module; 3 is the modulation and demodulation module; 31 is a first signal generator; 32 is a DC blocker; 33 is a lock-in amplifier; 34 is a low-pass filter; 35 is a feedback controller; 36 is a second signal generator; 4 is the bias amplification module; 41 is a first adder; 42 is a second adder; 43 is a power amplifier; 5 is the switch module; 6 is a laser (external cavity semiconductor laser); 61 is a PZT modulation port. Detailed implementation manners

[0080] In order to make the objectives, technical solutions and advantages of this application clearer, the following further elaborates on this application in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not used to limit this application.

[0081] The terms "first" and "second" in the description and claims of this application are used to distinguish different objects, rather than to describe a specific order of the objects. For example, the first signal generator and the second signal generator are used to distinguish signal generators with different functions, rather than to describe the specific order of the signal generators.

[0082] In the embodiments of this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or explanations. Any embodiment or design solution described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or having more advantages than other embodiments or design solutions. Rather, the use of words such as "exemplary" or "for example" is intended to present relevant concepts in a specific manner.

[0083] The following describes the embodiments of this application in conjunction with the accompanying drawings in the embodiments of this application.

[0084] Figure 1 is a schematic structural diagram of an external cavity semiconductor laser frequency stabilization system provided by an embodiment of this application. As Figure 1 shown, this external cavity semiconductor laser frequency stabilization system (the part marked by the dashed box) includes:

[0085] a saturated absorption spectrum module 1, a small wave peak-finding module 2, a modulation and demodulation module 3, a bias amplification module 4 and a switch module 5;

[0086] The input end of the saturated absorption spectrum module 1 is used to receive the laser output by the external laser 6. The saturated absorption spectrum module 1 is used to measure the atomic saturated absorption spectrum of the laser and output the electrical signal of the saturated absorption spectrum.

[0087] The first input end of the switch module 5 is connected to the output end of the saturated absorption spectrum module 1. The output end of the switch module 5 is respectively connected to the input end of the wavelet peak seeking module 2 and the input end of the modulation and demodulation module 3. The second input end of the switch module 5 is used to receive the external frequency locking instruction. The switch module 5 is used to close under the control of the frequency locking instruction to transmit the electrical signal to the wavelet peak seeking module and the modulation and demodulation module respectively.

[0088] The wavelet peak seeking module 2 is used to output the target bias voltage signal corresponding to the locked frequency according to the electrical signal.

[0089] The modulation and demodulation module 3 includes a first output end and a second output end. The modulation and demodulation module 3 is used to perform signal modulation and demodulation according to the electrical signal and the reference signal corresponding to the locked frequency, output the modulation signal from the first output end, and output the demodulation signal from the second output end.

[0090] The first output end and the second output end of the modulation and demodulation module 3, and the output end of the wavelet peak seeking module 2 are respectively connected to the input end of the bias amplification module 4. The bias amplification module 4 is used to perform superposition and amplification processing on the modulation signal, the demodulation signal, the preset bias voltage, and the target bias voltage signal, and output the target superposition signal to the PZT modulation port 61 of the laser 6 to adjust the cavity length of the laser 6 so that the output laser operates freely at the locked frequency.

[0091] Specifically, the target bias voltage signal described in the embodiment of the present application refers to the zero-crossing voltage signal corresponding to the peak-to-peak value of the locked frequency of the laser. Here, the laser is an external cavity semiconductor laser (ECDL); the frequency locking instruction can be set by the upper computer and sent to the switch module. Among them, the switch module can specifically adopt a relay switch module; the locked frequency can be obtained from the frequency locking information carried by the frequency locking instruction.

[0092] The modulation signal described in the embodiment of the present application is used to modulate the phase or frequency of the laser signal, and can be adjusted accordingly according to the actual experiment requirements. For example, in a specific embodiment of the present application, a modulation signal with a modulation frequency of 0.01716 MHz, a modulation amplitude of 0.084 V, and a modulation phase of 98.38 degrees is adopted.

[0093] The demodulation signal described in the embodiments of this application refers to the control signal generated by performing control calculations on the frequency error signal, which is used to reduce the deviation between the laser frequency and the desired locked frequency. Among them, the frequency error signal can be obtained by comparing the modulated saturated absorption spectrum signal with a preset reference signal and extracting it through filtering and other means.

[0094] The target superposition signal described in the embodiments of this application refers to the voltage signal obtained by superimposing the foregoing three types of signals, namely the modulation signal, the demodulation signal, and the target bias voltage signal, and performing signal amplification processing on the obtained superimposed signal. It is used to be applied to the PZT modulation port of the laser to achieve frequency locking of the laser. Among them, the PZT modulation port is generally set on the chassis of the external cavity semiconductor laser.

[0095] In the embodiments of this application, by controlling the on / off of the switch module 5 through the frequency locking instruction issued by the host computer, the output end of the saturated absorption spectrum module 1 can be connected to the input ends of the wavelet peak seeking module 2 and the modulation and demodulation module 3 respectively. That is to say, under the control of the frequency locking instruction, the output signal of the saturated absorption spectrum module 1 is transmitted in two paths. One path is transmitted to the bias amplification module 4 after being processed by the wavelet peak seeking module 2, and the other path is transmitted to the bias amplification module 4 after being processed by the modulation and demodulation module 3.

[0096] Among them, the saturated absorption spectrum module 1 is used to measure the atomic saturated absorption spectrum of the laser output by the laser 6. By adjusting the temperature, current of the laser 6 and the bias voltage of the PZT modulation port, it scans out the saturated absorption spectrum of cold atoms and converts it into an electrical signal as a detection signal for output, facilitating subsequent laser frequency locking.

[0097] Here, in the application of the cold atom gravimeter, the cold atoms can specifically use rubidium (Rb) atoms. Due to the stable energy level structure of Rb atoms, its saturated absorption peak is usually used as the frequency reference. It should be noted that in practical applications, alkali metal atoms are applicable to cold atoms.

[0098] It should be noted that the laser 6 can be set outside the saturated absorption spectrum module 1 or can be integrated inside the saturated absorption spectrum module 1. This application does not make specific limitations on this.

[0099] In the embodiments of this application, the wavelet peak seeking module 2 can respond to the frequency locking instruction issued by the host computer. When the host computer issues a frequency locking instruction, the switch module 5 closes. At this time, the wavelet peak seeking module 2 can receive the electrical signal of the saturated absorption spectrum and perform wavelet analysis and peak search on the electrical signal of the saturated absorption spectrum. In the specific implementation process, the wavelet peak seeking module 2 can first perform data denoising on the electrical signal of the saturated absorption spectrum to obtain a frequency discrimination signal.

[0100] Here, in the actual implementation process, there are problems that the frequency discrimination signal will attenuate in amplitude over time and the peak signal will be submerged by noise. Therefore, in the embodiment of the present application, the wavelet peak seeking module 2 can further filter and denoise the frequency discrimination signal to obtain the frequency discrimination signal after filtering and denoising, and then can search for peaks by setting a threshold, identify the position of the transition peak corresponding to the locked frequency in the frequency discrimination signal after filtering and denoising, find the corresponding zero-crossing bias voltage through the peak-to-peak value, and finally output the target bias voltage signal corresponding to the locked frequency.

[0101] Furthermore, in the embodiment of the present application, the modulation and demodulation module 3 can also respond to the frequency locking instruction issued by the host computer. When the host computer issues a frequency locking instruction, the modulation and demodulation module 3 can receive the electrical signal of the saturated absorption spectrum and modulate the electrical signal of the saturated absorption spectrum; at the same time, the modulation signal and the reference signal corresponding to the locked frequency can be output inside the module, and then the modulated signal and the reference signal corresponding to the locked frequency can be compared and signal demodulated to output a demodulated signal. Thus, the modulation and demodulation module 3 outputs a modulation signal and a demodulated signal under the frequency locking instruction issued by the host computer, and outputs the demodulated signal and the modulation signal to the bias amplification module 4 together.

[0102] In the frequency locking process of the embodiment of the present application, the bias amplification module 4 further performs bias superposition and amplification on the modulation solution signal, the demodulated signal, the preset bias voltage, and the target bias voltage signal, and applies the voltage corresponding to the amplified target superposition signal to the PZT modulation port 61 of the laser 6 to form a feedback loop, and adjusts the cavity length of the laser 6 by controlling the PZT modulation port 61, so that the output laser operates freely at the locked frequency, realizing the frequency locking of the external cavity semiconductor laser.

[0103] Among them, the preset bias voltage is usually a direct current (DC) signal, which is used to prevent the laser from scanning to a negative voltage and damaging the PZT. In the actual operation process, the above preset bias voltage and amplification factor can be adjusted through the knobs on the bias amplification module 4.

[0104] In the specific implementation process, the process of achieving frequency stabilization of the external cavity semiconductor laser is as follows: First, adjust the temperature and current of the laser 6 to make the laser operate freely near the required frequency. Before frequency locking, the saturated absorption spectrum module 1 is in the laser scanning mode, and the saturated absorption spectrum of Rb atoms can be scanned. During the process, an oscilloscope can be used to observe the saturated absorption spectrum line signal; when the upper computer issues a frequency locking instruction, the electrical signal of the saturated absorption spectrum used as the detection signal is input into the modulation and demodulation module 3; the modulation and demodulation module 3 outputs a modulation signal and a demodulation signal, and after passing through the bias amplification module 4, it is connected to the PZT modulation port 61 of the laser 6 to form a feedback loop, and at the same time, the scanning is automatically turned off. At the same time, the wavelet peak search module 2 performs peak search on the saturated absorption spectrum, and combines the modulation and demodulation signals output by the modulation and demodulation module 3 to achieve the locking of the laser frequency.

[0105] It should be noted that locking the laser to different frequencies can meet different stages of the atomic gravimeter, and different peak locking points can be selected through the upper computer.

[0106] It should also be noted that the change in the external cavity length of the external cavity semiconductor laser is proportional to the applied bias voltage, which causes a corresponding change in the laser frequency; applying a bias voltage to the PZT modulation port of the laser can increase the fine frequency adjustment range of the laser. Among them, the corresponding relationship between the PZT control voltage and frequency adjustment is 0.794 GHz / V. When a bias voltage is applied, the frequency scanning range can be increased. At the same time, the response frequency of the PZT is relatively low, usually in the kHz order of magnitude, which is a slow feedback loop. Compared with the traditional method of stabilizing the frequency by scanning current, the PZT feedback loop has a high adjustment accuracy, which can make the laser less likely to jump modes during the phase-locking process, and it is easier to scan out the saturated absorption spectrum signal required for frequency locking, making the entire frequency stabilization system more stable.

[0107] The external cavity semiconductor laser frequency stabilization system of the embodiment of the present application realizes frequency stabilization by introducing the method of PZT internal modulation, which can make the laser less likely to jump modes during the phase-locking process and make the entire frequency stabilization system more stable, overcoming the defect that the scanning current of the external cavity semiconductor laser is prone to jump modes; on the premise of meeting the narrow linewidth of the external cavity semiconductor laser, frequency stabilization is achieved by adopting the method of processing the saturated absorption spectrum, making the overall structure simple and easy to integrate, and being able to better adapt to the dynamic measurement and handling process of the cold atom gravimeter. Compared with the traditional modulation transfer spectrum frequency stabilization method using EOM external modulation, the cost is greatly reduced; by introducing the wavelet peak search method to identify the transition peak to be locked, the problems that the amplitude of the frequency discrimination signal decays with time and the peak signal is submerged by noise are overcome, making the locked laser frequency more accurate and less affected by the drift of the saturated absorption spectrum line and the spectral line noise.

[0108] Figure 2It is a schematic structural diagram of the saturated absorption spectrum module provided by an embodiment of the present application. As shown in Figure 2 Based on the content of the above embodiment, as an alternative embodiment, the saturated absorption spectrum module 1 includes a laser 6, an optic isolator (OI) 101, a transmission optical fiber 102, a half wave plate (HWP) 103, a polarizing beam splitter (PBS) 104, a coupling head 105, a wavemeter 106, a high-transmission and low-reflection mirror 107, a reflector 108, a rubidium cell (Rb cell) 109, a semi-transmissive and semi-reflective mirror 110, and a photodetector (PD) 111 arranged in sequence along the main optical path;

[0109] The coupling head 105 and the wavemeter 106 are arranged in sequence on the transmission optical path of the polarizing beam splitter 104, the high-transmission and low-reflection mirror 107 and the reflector 108 are arranged in sequence on the reflection optical path of the polarizing beam splitter 104, and the rubidium cell 109, the semi-transmissive and semi-reflective mirror 110 and the photodetector 111 are arranged in sequence on the reflection optical path of the high-transmission and low-reflection mirror 107;

[0110] The laser 6 and the optic isolator 101 are used to prevent the light reflected by the optical fiber echo on the main optical path from returning to the laser 6 and affecting the operation of the laser 6;

[0111] The transmission optical fiber 102 is used to transmit the laser passing through the optic isolator 101 to the half wave plate 103;

[0112] The half wave plate 103 is used to adjust the polarization state of the input laser to linear polarization;

[0113] The polarizing beam splitter 104 is used to divide the linearly polarized laser into a first reflected light and a first transmitted light. The first transmitted light is converted into spatial light by the coupling head 105 and then transmitted to the wavemeter 106 to monitor the frequency change of the laser; the first reflected light is transmitted to the high-transmission and low-reflection mirror 107;

[0114] The high-transmission and low-reflection mirror 107 is used to divide the first reflected light into a second reflected light and a second transmitted light. The second reflected light generates a saturated absorption spectrum signal of Rb atoms after passing through the rubidium cell 109 and then is transmitted to the semi-transmissive and semi-reflective mirror 110; the second transmitted light is reflected by the reflector 108 to the semi-transmissive and semi-reflective mirror 110;

[0115] The semi-transmissive and semi-reflective mirror 110 is used to divide the saturated absorption spectrum signal into a third reflected light and a third transmitted light. The third transmitted light is used as the detection light and transmitted to the photodetector 111; and it is used to convert the second transmitted light reflected by the reflector 108 into the pump-back light;

[0116] The photodetector 111 is used to convert the detection light into an electrical signal.

[0117] Specifically, in the embodiments of the present application, the transmission optical fiber 102 may specifically adopt a single-mode polarization-maintaining fiber (PMF), which has characteristics such as high transmission quality, stable polarization characteristics, and strong anti-interference ability, and can ensure the stable transmission of laser; the transmittance-to-reflectance ratio (T:R) of the high-transmission and low-reflection mirror 107 can be set to 90:10; the transmittance-to-reflectance ratio (T:R) of the beam splitter 110 is 50:50.

[0118] In the embodiments of the present application, the laser 6 is an external cavity semiconductor laser, and it can be selected to be integrated inside the saturated absorption spectrum module 1. The external cavity semiconductor laser 6 is used to output laser, and the laser is transmitted into the transmission optical fiber 102 through the optical isolator 101. Here, the optical isolator 101 can prevent the light reflected back through the fiber echo on the main optical path from returning to the external cavity semiconductor laser 6 and affecting the operation and performance of the external cavity semiconductor laser 6. The laser passing through the optical isolator 101 is transmitted to the half-wave plate 103 through the transmission optical fiber 102 intact. The polarization state of the input laser can be adjusted to linear polarization through the half-wave plate 103, and then the linearly polarized laser is transmitted to the polarization beam splitter 104.

[0119] In the embodiments of the present application, the polarization beam splitter 104 further divides the linearly polarized laser into two paths of signals, namely reflected light and transmitted light, that is, the first reflected light and the first transmitted light are generated. The first transmitted light is converted into spatial light through the coupling head 105 and then transmitted to the wavelength meter 106. Here, by rotating the half-wave plate 103, the ratio of the first reflected light and the first transmitted light can be controlled, so that the first transmitted light can be better connected to the wavelength meter 106, thereby observing the saturated absorption spectrum line signal in real time and monitoring the laser frequency change; at the same time, the first reflected light is transmitted to the high-transmission and low-reflection mirror 107, and the high-transmission and low-reflection mirror 107 further divides the first reflected light into reflected light and transmitted light, that is, the second reflected light and the second transmitted light are generated. Then, the second reflected light acts on the stable Rb atoms in the rubidium cell 109 to generate a saturated absorption spectrum signal of the Rb atoms and is transmitted to the beam splitter 110, while the second transmitted light is also reflected to the beam splitter 110 through the mirror 108, and the beam splitter 110 converts the second transmitted light into the repump light.

[0120] At the same time, the saturated absorption spectrum signal of the Rb atoms is divided into reflected light and transmitted light after passing through the beam splitter 110, that is, the third reflected light and the third transmitted light are generated. The third transmitted light is used as the probe light and is transmitted to the photodetector 111, and then converted into an electrical signal, that is, the electrical signal of the saturated absorption spectrum is generated.

[0121] The frequency stabilization system of the external cavity semiconductor laser in the embodiment of the present application has a compact saturated absorption spectrum optical path structure built, which can be integrated in a square box, making the whole system highly mobile and facilitating adaptation to the movement process of the cold atom gravimeter.

[0122] Figure 3 It is a schematic diagram of the saturated absorption spectrum measured by the saturated absorption spectrum module provided in the embodiment of the present application. As Figure 3 shown, it is the saturated absorption spectrum of Rb atoms obtained by the external cavity semiconductor laser frequency stabilization system provided in the embodiment of the present application through scanning by regulating the PZT modulation port.

[0123] Figure 4 It is a schematic structural diagram of the wavelet peak seeking module provided in the embodiment of the present application. As Figure 4 shown, based on the content of the above embodiment, as an optional embodiment, the wavelet peak seeking module 2 includes a differential module 21, a wavelet denoising module 22, a peak seeking module 23, and a locking module 24 connected in sequence;

[0124] The input end of the differential module 21 serves as the input end of the wavelet peak seeking module 2 and is used to output the frequency discrimination signal corresponding to the electrical signal;

[0125] The wavelet denoising module 22 is used to perform wavelet decomposition and signal reconstruction on the frequency discrimination signal to obtain the denoised frequency discrimination signal;

[0126] The peak seeking module 23 is used to identify the peak-to-peak information corresponding to the transition peak of the locking frequency in the denoised frequency discrimination signal;

[0127] The output end of the locking module 24 serves as the output end of the wavelet peak seeking module 2 and is used to perform zero-crossing detection on the peak-to-peak information and output the target bias voltage signal corresponding to the locking frequency.

[0128] Specifically, the peak-to-peak information described in the embodiment of the present application refers to the difference information between the maximum value (positive peak) and the minimum value (negative peak) of the denoised frequency discrimination signal within one period.

[0129] In the embodiment of the present application, the wavelet peak seeking module 2 may mainly consist of four modules: a differential module 21, a wavelet denoising module 22, a peak seeking module 23, and a locking module 24 connected in sequence.

[0130] Among them, the differential module 21 is used to receive the electrical signal of the Rb atom saturated absorption spectrum and perform a first-order differential calculation on the electrical signal to perform data denoising and obtain the frequency discrimination signal.

[0131] The wavelet denoising module 22 is used to receive the frequency discrimination signal. According to the characteristics of the saturated absorption spectrum electrical signal, an appropriate wavelet basis function and decomposition level are selected to perform wavelet transform on the frequency discrimination signal, calculate the wavelet coefficients of the signal at different scales, and the signal can be decomposed into components of different frequency bands through wavelet transform. Furthermore, the wavelet coefficients at a specific scale are selected to reconstruct an approximate representation of the signal, and the frequency discrimination signal after denoising is obtained.

[0132] The peak searching module 23 is used to detect peaks in the constructed signal approximation using a peak searching algorithm, that is, to further identify and lock the peak-to-peak information corresponding to the transition peaks of the frequency in the denoised frequency discrimination signal according to a preset peak threshold, and verify the detected peaks to ensure that they conform to the characteristics of the saturated absorption spectrum.

[0133] Finally, the locking module 24 performs zero-crossing detection on the above-mentioned identified peak-to-peak information. During the process, the zero-crossing points of the signal are detected by comparing the sign changes of adjacent sampling points. When one of two adjacent sampling points is positive and the other is negative, it is considered that the signal has a zero-crossing, that is, a zero-crossing point. According to the number of detected zero-crossing points, the frequency of the signal can be calculated, and then, according to the comparison result between the calculated frequency and the locking frequency, the target bias voltage signal corresponding to the locking frequency is output.

[0134] Exemplarily, as Figure 5 shown, before processing, the frequency discrimination signal obtained by differentiating the electrical signal of the Rb atom saturated absorption spectrum once will have problems such as amplitude attenuation over time and peak signals being obscured by noise. As Figure 6 shown, after performing wavelet transform and signal reconstruction processing on the frequency discrimination signal, the denoised frequency discrimination signal obtained is not affected by the drift of the saturated absorption spectrum line and spectral noise.

[0135] In the frequency stabilization system of the embodiment of the present application, the frequency discrimination signal is obtained by performing a first-order differential calculation on the detection signal received by the photodetector. Since the amplitude of the first-order differential signal will decay over time, it is not easy to lock the frequency. Therefore, the wavelet transform method is used to reduce the noise of the saturated absorption spectrum line, and then its first-order differential signal is decomposed and reconstructed to eliminate the noise signal while retaining the useful peak signal. Finally, by setting a threshold to identify the peak positions of each frequency point to be locked, the frequency locked by the system can be made more accurate.

[0136] Figure 7 is a schematic structural diagram of the modulation and demodulation module provided by the embodiment of the present application. As Figure 7 shown, based on the content of the above embodiment, as an optional embodiment, the modulation and demodulation module 3 includes a first signal generator 31, and a direct current blocker 32, a lock-in amplifier 33, a low-pass filter 34, and a feedback controller 35 connected in sequence;

[0137] The reference signal output terminal of the first signal generator 31 is connected to the input terminal of the lock-in amplifier 33; the reference signal output terminal is used to output a reference signal corresponding to the locked frequency;

[0138] The modulation signal output terminal of the first signal generator 31 serves as the first output terminal of the modulation and demodulation module 3, and is used to output a modulation signal;

[0139] The input terminal of the modulation and demodulation module 3 includes a first input terminal; the input terminal of the DC blocker 32 serves as the first input terminal of the modulation and demodulation module 3, and is used to filter out the DC signal of the electrical signal to obtain the AC signal of the electrical signal;

[0140] The lock-in amplifier 33 is used to subtract the AC signal from the reference signal and output an error signal;

[0141] The low-pass filter 34 is used to filter out the high-frequency signal in the error signal to obtain a low-frequency error signal;

[0142] The output terminal of the feedback controller 35 serves as the second output terminal of the modulation and demodulation module 3, and is used to output a demodulation signal based on the low-frequency error signal.

[0143] Specifically, in the embodiment of the present application, the modulation and demodulation module 3 may be composed of a first signal generator 31, and a DC blocker 32, a lock-in amplifier 33, a low-pass filter 34, and a feedback controller 35 connected in sequence. The output terminal of the photodetector 111 is connected to the input terminal of the DC blocker 32. Among them, the feedback controller 35 may specifically adopt a PID controller.

[0144] In this embodiment, after the host computer issues a locking command, the electrical signal of the saturation absorption spectrum output by the photodetector 111 will filter out the DC signal through the DC blocker 32 to obtain an AC modulation signal f 1, which is subtracted from the reference signal f 2 corresponding to the locked frequency output by the first signal generator 31 through the lock-in amplifier 33, and an error signal can be obtained. This error signal contains two frequencies, which are respectively . After this signal is connected to the low-pass filter 34 and the high-frequency signal is filtered out, the size of the filtered signal that can be obtained is , that is, a low-frequency error signal is output. Further, the filtered and amplified low-frequency error signal is input to the PID module, and the PID control algorithm is used to calculate the control signal for the low-frequency error signal, and a demodulation signal is output.

[0145] The frequency stabilization system of the external cavity semiconductor laser in the embodiment of the present application filters out the DC signal in the detection signal through a DC blocker, and can obtain a modulation signal with lower noise, avoiding the introduction of more circuit noise due to back-and-forth wiring. At the same time, the designed modulation and demodulation module uses the method of filtering to reduce noise and then amplifying during signal processing to improve the signal-to-noise ratio of the modulation signal, avoiding the introduction of more spectral line noise, and can further improve the accuracy of the system's locked frequency.

[0146] Continuing to refer to Figure 7 , based on the content of the above embodiment, as an alternative embodiment, the modulation and demodulation module 3 further includes a second signal generator 36; the modulation and demodulation module 3 further includes a second input terminal and a third output terminal;

[0147] The output terminal of the second signal generator 36 serves as the second input terminal of the modulation and demodulation module 3, and is used to receive an external unlocking instruction, and generate a scanning signal under the control of the unlocking instruction;

[0148] The second input terminal of the switch module 5 is also used to receive the unlocking instruction to control the switch module 5 to disconnect, blocking the electrical signal from being transmitted to the wavelet peak seeking module 2 and the modulation and demodulation module 3;

[0149] The output terminal of the second signal generator 36 serves as the third output terminal of the modulation and demodulation module 3 and is connected to the input terminal of the bias amplification module 4, and is used to output the scanning signal to the bias amplification module 4;

[0150] The bias amplification module 4 is further used to perform superposition and amplification processing on the modulation signal and the scanning signal, and transmit the output superposition signal to the PZT modulation port 61 to continuously adjust the cavity length of the laser 6, so that the output laser operates freely within the preset frequency scanning range.

[0151] Specifically, in the embodiment of the present application, the modulation and demodulation module 3 further includes a second signal generator 36. When the host computer issues an unlocking instruction, it will control the second signal generator 36 in the modulation and demodulation module 3 to output a scanning signal, and the first signal generator 31 outputs a modulation signal, and the scanning signal and the modulation signal are jointly connected to the bias amplification module 4. Among them, the scanning signal can specifically adopt a triangular wave signal.

[0152] In the embodiment of the present application, specifically, the ±1V voltage of the superposition signal obtained by processing the scanning signal and the modulation signal output by the modulation and demodulation module 3 is used to apply a bias voltage to the PZT modulation port 61 through the bias amplification module 4 and amplify it to meet the preset frequency scanning range corresponding to 0-10V of the laser PZT, so as to be able to scan out the complete saturated absorption spectrum of Rb atoms.

[0153] In the frequency stabilization system according to the embodiment of the present application, by superimposing a modulation signal and a scanning signal, the composite signal can be continuously changed within the working voltage range of the PZT, realizing continuous scanning of the output parameters of the laser, so that the complete saturated absorption spectrum of Rb atoms can be scanned out.

[0154] An external cavity semiconductor laser frequency stabilization system based on PZT internal modulation provided by the embodiment of the present application adjusts the temperature and current of the external cavity semiconductor laser 6 to freely operate near the required frequency; before frequency locking, the modulation and demodulation module 3 provides a scanning signal to superimpose the modulation signal, which is connected to the PZT modulation port 61 of the laser 6 through the bias amplification module 4, and the saturated absorption spectrum of Rb atoms can be observed on the oscilloscope and the upper computer; the applied bias voltage and scanning range are adjusted by the bias amplification module 4; the wavelet peak seeking module 2 performs differential operation on the electrical signal of the saturated absorption spectrum, obtains the frequency discrimination signal after the first derivative, and can be displayed on the upper computer; the upper computer issues a frequency locking command according to the transition peak corresponding to the frequency to be locked. After responding to the frequency locking command, the scanning is turned off, the signal generator in the modulation and demodulation module 3 outputs a modulation signal, the PID module outputs a demodulation signal, and the modulation and demodulation module 3 identifies the zero-crossing voltage of the frequency discrimination signal corresponding to the transition peak to be locked through the wavelet peak seeking module 2 and superimposes the modulation signal and the demodulation signal, and inputs them into the PZT modulation port 61 of the laser 6 through the bias amplification module 4. In this way, the laser 6 can stabilize the frequency to the selected transition peak and maintain high stability, and it is not easy to occur mode hopping.

[0155] It should be noted that in the embodiment of the present application, the specific parameters of signals such as the modulation signal, the scanning signal, the reference signal, and the preset bias voltage can be freely set and adjusted according to the actual frequency stabilization experiment requirements of the system, and the present application does not make specific limitations on this.

[0156] In the embodiment of the present application, when the upper computer issues a frequency locking command, the modulation and demodulation module 3 outputs a modulation and demodulation signal to complete the frequency locking; when the upper computer issues an unlocking command, the modulation and demodulation module 3 outputs a scanning signal to superimpose the modulation signal to scan out the saturated absorption spectrum of Rb atoms.

[0157] Specifically, in the embodiment of the present application, the first signal generator 31 in the modulation and demodulation module 3 can output two signals. One is the reference signal corresponding to the locked frequency, which is used to act on the lock-in amplifier 33, and the other is the modulation signal. After the upper computer issues a frequency locking command, the modulation and demodulation module 3 turns off the scanning and outputs the superposition of the modulation signal and the demodulation signal; after the upper computer issues an unlocking command, the modulation and demodulation module 3 outputs the superposition of the scanning signal and the modulation signal; the output of the modulation and demodulation module 3 is input into the PZT modulation port 61 of the laser 6 through the bias amplification module 4 to form a frequency stabilization feedback loop.

[0158] When it is necessary to lock to other frequencies, only need to issue an unlocking instruction through the host computer, and then issue a frequency locking instruction again. The wavelet peak seeking module 2 is used to find the target bias voltage value corresponding to the newly required locked peak and the modulation and demodulation signals, and apply them to the PZT modulation port 61 of the laser 6 through the bias amplification module 4 to achieve the locking of the laser frequency.

[0159] More specifically, after unlocking, restart the scanning, and re-determine the bias voltage value corresponding to the transition peak of the frequency to be locked, and then re-feed it to the PZT modulation port 61 of the laser 6 and then close the scanning. Reforming the PZT feedback loop can obtain a more accurate frequency locking result by fine-tuning the parameters of the PID controller.

[0160] Figure 8 It is a schematic structural diagram of the bias amplification module provided by the embodiment of the present application. As Figure 8 shown, based on the content of the above embodiment, as an optional embodiment, the bias amplification module 4 includes a first adder 41, a second adder 42 and a power amplifier 43 connected in sequence;

[0161] The input end of the bias amplification module 4 includes a first input end, a second input end, a third input end and a fourth input end. The first input end of the first adder 41 is used as the first input end of the bias amplification module 4 and is connected to the first output end of the modulation and demodulation module 3. The second input end of the first adder 41 is used as the second input end of the bias amplification module 4 and is connected to the second output end of the modulation and demodulation module 3. The third input end of the first adder 41 is used as the third input end of the bias amplification module 4 for receiving a preset bias voltage;; The input end of the second adder 42 is used as the fourth input end of the bias amplification module 4 and is connected to the output end of the wavelet peak seeking module 2;

[0162] The first adder 41 is used to perform superposition processing on the modulation signal, the demodulation signal and the preset bias voltage, and output a first superposition signal;

[0163] The second adder 42 is used to perform superposition processing on the target bias voltage signal and the first superposition signal, and output a second superposition signal;

[0164] The power amplifier 43 is used as the output end of the bias amplification module 4 to amplify the second superposition signal and output a target superposition signal.

[0165] Specifically, in the embodiment of the present application, the bias amplification module 4 may be composed of a first adder 41, a second adder 42 and a power amplifier 43 connected in sequence.

[0166] In an embodiment of the present application, in a frequency locking scenario, the main function of the first adder 41 is to superimpose an input modulation signal, a demodulation signal, and a preset bias voltage to obtain a superimposed signal, that is, to output a first superimposed signal.

[0167] Further, the second adder 42 is used to superimpose the target bias voltage corresponding to the locked frequency output by the wavelet peak seeking module 2 and the above first superimposed signal, so as to output a second superimposed signal. The power amplifier 43 can amplify the second superimposed signal, and apply the voltage of the amplified target superimposed signal to the PZT modulation port 61 of the laser 6 to achieve frequency locking of the laser 6.

[0168] In the frequency stabilization system of the embodiment of the present application, the bias amplification module can effectively achieve precise control and efficient amplification of the superimposed composite signal through a combined adder and a power amplifier, providing important support for subsequent frequency locking control of the PZT modulation port of the laser.

[0169] In an embodiment of the present application, in a scenario controlled by an unlocking instruction, in combination with Figure 7 and Figure 8 , the third output end of the modulation and demodulation module 3 is connected to the fourth input end of the first adder 41 in the bias amplification module 4, and is used to input a scanning signal into the first adder 41. The main function of the first adder 41 is to superimpose the input modulation signal, scanning signal, and preset bias voltage. The second adder 42 then transmits the superimposed signal output by the first adder 41 to the power amplifier 43.

[0170] In this process, after adjusting the voltage bias and amplification factor applied by the bias amplification module 4 to the PZT modulation port of the laser, the modulation signal output by the modulation and demodulation module 3 is superimposed with a scanning signal and input into the power amplifier 43. The power amplifier 43 amplifies the signal to increase the scanning range of the PZT modulation port of the laser, so that it covers the transition peak of the saturated absorption spectrum that needs to be locked, thereby meeting the requirement of the 0-10V scanning range of the PZT modulation port.

[0171] Based on the content of the above embodiments, as an alternative embodiment, the system can be used to generate repumping light, probe light, and imaging light with different frequencies required in the measurement process of a cold atom gravimeter, and can also be used to generate light with various frequencies required by devices such as atomic clocks and magnetometers in the field of quantum precision measurement.

[0172] Specifically, in an embodiment of the present application, taking Figure 9 as an example, the frequency of the frequency-stabilized laser output by the external cavity semiconductor laser is stabilized to 87 Rb atoms The transition frequency is 384.228115 GHz. By locking to different frequencies, it can be used as the repumping light during the atomic cooling stage, the probing light and imaging light during the probing stage, and can also be used in quantum measurement devices such as atomic clocks and magnetometers.

[0173] Figure 10 FIG. 4 is a schematic diagram showing the change of the error signal before and after frequency locking of the external cavity semiconductor laser frequency stabilization system provided by the embodiment of the present application. As Figure 10 shown, in the embodiment of the present application, the fluctuation of the error signal before and after frequency locking is on the order of 10 -4 magnitude, and the amplitude of the error signal after frequency locking becomes smaller relative to the amplitude of the error signal before locking.

[0174] Next, the frequency stabilization method of the external cavity semiconductor laser frequency stabilization system provided by the present application will be described. The frequency stabilization method of the external cavity semiconductor laser frequency stabilization system described below can be correspondingly referred to the external cavity semiconductor laser frequency stabilization system described above.

[0175] Figure 11 FIG. 5 is a schematic flowchart of the frequency stabilization method of the external cavity semiconductor laser frequency stabilization system provided by the embodiment of the present application. It can be understood that it can be applied to the frequency stabilization method of any of the foregoing external cavity semiconductor laser frequency stabilization systems. As Figure 11 shown, the method includes:

[0176] Step S1, measuring the cold atomic saturated absorption spectrum of the laser output by the laser and outputting the electrical signal of the saturated absorption spectrum;

[0177] Step S2, under the control of the frequency locking instruction, outputting the target bias voltage signal corresponding to the locked frequency according to the electrical signal;

[0178] Step S3, performing signal modulation and demodulation according to the electrical signal and the reference signal corresponding to the locked frequency, and outputting the modulation signal and the demodulation signal;

[0179] Step S4, performing superposition and amplification processing on the modulation signal, the demodulation signal, the preset bias voltage and the target bias voltage signal, and outputting the target superposition signal to the PZT modulation port to adjust the cavity length of the laser so that the output laser operates freely at the locked frequency.

[0180] It should be understood that the method in the above embodiment is applied to the foregoing external cavity semiconductor laser frequency stabilization system, and its implementation principle and technical effect are similar to the description in the above system. The detailed process of this method can refer to the corresponding description process in the above system, which will not be elaborated here.

[0181] The frequency stabilization method of the embodiment of the present application realizes frequency stabilization by introducing the method of internal modulation of PZT, which can prevent the laser from jumping modes during the phase-locking process, making the entire frequency stabilization system more stable and overcoming the defect that the scanning current of the external cavity semiconductor laser is prone to jumping modes. On the premise of meeting the narrow linewidth of the external cavity semiconductor laser, frequency stabilization is achieved by processing the saturated absorption spectrum, making the overall structure simple and easy to integrate, and being able to better adapt to the dynamic measurement and handling process of the cold atom gravimeter. Compared with the traditional modulation transfer spectrum frequency stabilization method using EOM external modulation, the cost is greatly reduced. By introducing the wavelet peak search method to identify the transition peak to be locked, the problems that the frequency discrimination signal will decay in amplitude over time and the peak signal will be submerged by noise are overcome, making the locked laser frequency more accurate and not easily affected by the drift and noise of the saturated absorption spectrum lines.

[0182] Based on the content of the above embodiment, as an alternative embodiment, step S2, under the control of the frequency locking instruction, output the target bias voltage signal corresponding to the locked frequency according to the electrical signal, including:

[0183] Output the frequency discrimination signal corresponding to the electrical signal;

[0184] Perform wavelet decomposition and signal reconstruction on the frequency discrimination signal to obtain the denoised frequency discrimination signal;

[0185] Identify the peak-to-peak information of the transition peak of the locked frequency corresponding to the denoised frequency discrimination signal;

[0186] Perform zero-crossing detection on the peak-to-peak information and output the target bias voltage signal corresponding to the locked frequency.

[0187] Based on the content of the above embodiment, as an alternative embodiment, step S3, perform signal modulation and demodulation according to the electrical signal and the reference signal corresponding to the locked frequency, and output the modulation signal and the demodulation signal, including:

[0188] Filter out the DC signal of the electrical signal to obtain the AC signal of the electrical signal;

[0189] Generate two signals, namely the reference signal and the modulation signal corresponding to the locked frequency, under the control of the frequency locking instruction;

[0190] Subtract the AC signal from the reference signal and output the error signal;

[0191] Filter out the high-frequency signal in the error signal to obtain the low-frequency error signal;

[0192] Output the demodulation signal based on the low-frequency error signal.

[0193] Based on the content of the above embodiment, as an alternative embodiment, the method further includes:

[0194] Output a scanning signal and a modulation signal under the control of an unlocking instruction;

[0195] Superimpose and amplify the modulation signal and the scanning signal, and transmit the output superimposed signal to the PZT modulation port to continuously adjust the cavity length of the laser, so that the output laser operates freely within a preset frequency scanning range.

[0196] The method steps in the embodiments of the present application can be implemented in a hardware manner or by a processor executing software instructions. The software instructions can be composed of corresponding software modules, and the software modules can be stored in a random access memory (Random Access Memory, RAM), flash memory, read-only memory (Read-only Memory, ROM), programmable read-only memory (Programmable ROM, PROM), erasable programmable read-only memory (Erasable PROM, EPROM), electrically erasable programmable read-only memory (Electrically EPROM, EEPROM), registers, hard disks, removable hard disks, CD-ROMs, or any other form of storage medium well known in the art. An exemplary storage medium is coupled to the processor so that the processor can read information from the storage medium and write information to the storage medium. Of course, the storage medium can also be a component of the processor. The processor and the storage medium can be located in an ASIC.

[0197] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware, or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the processes or functions described in the embodiments of the present application are generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices. The computer instructions can be stored in a computer-readable storage medium or transmitted through the computer-readable storage medium. The computer instructions can be transmitted from a website, computer, server, or data center to another website, computer, server, or data center in a wired manner (such as coaxial cable, optical fiber, digital subscriber line (DSL)) or wirelessly (such as infrared, wireless, microwave, etc.). The computer-readable storage medium can be any available medium that the computer can access or a data storage device such as a server or data center that includes one or more integrated available media. The available medium can be a magnetic medium (such as a floppy disk, hard disk, magnetic tape), an optical medium (such as a DVD), or a semiconductor medium (such as a solid state disk (Solid State Disk, SSD)), etc.

[0198] It should be understood that the various numerical numbers involved in the embodiments of the present application are only for the convenience of description and are not used to limit the scope of the embodiments of the present application.

[0199] It should be understood that expressions such as "including" and "may include" that can be used in the present application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In the present application, terms such as "including" and / or "having" can be interpreted as indicating specific characteristics, numbers, operations, constituent elements, components, or combinations thereof, but cannot be interpreted as excluding the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0200] In the description of the embodiments of the present application, it should be noted that unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium. Among them, "fixed connection" means that the two are connected and the relative positional relationship after connection remains unchanged. "Rotational connection" means that the two are connected and can rotate relative to each other after connection. "Sliding connection" means that the two are connected and can slide relative to each other after connection. The orientation terms mentioned in the embodiments of the present application, such as "inside" and "outside", etc., are only with reference to the direction of the drawings. Therefore, the orientation terms used are for better and clearer description and understanding of the embodiments of the present application, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the embodiments of the present application.

[0201] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of changes or substitutions, which should all be covered by the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. An external cavity semiconductor laser frequency stabilization system, characterized in that, Comprising: A saturated absorption spectrum module, a wavelet peak searching module, a modulation and demodulation module, a bias amplification module and a switch module; The input end of the saturated absorption spectrum module is used to receive the laser output by an external laser, and the saturated absorption spectrum module is used to measure the atomic saturated absorption spectrum of the laser and output the electrical signal of the saturated absorption spectrum; The first input end of the switch module is connected to the output end of the saturated absorption spectrum module, and the output end of the switch module is respectively connected to the input end of the wavelet peak searching module and the input end of the modulation and demodulation module; The second input end of the switch module is used to receive an external frequency locking instruction, and the switch module is used to close under the control of the frequency locking instruction to transmit the electrical signal to the wavelet peak searching module and the modulation and demodulation module respectively; The wavelet peak searching module is used to output a target bias voltage signal corresponding to the locked frequency according to the electrical signal; The modulation and demodulation module includes a first output end and a second output end, and the modulation and demodulation module is used to perform signal modulation and demodulation according to the electrical signal and the reference signal corresponding to the locked frequency, and output a modulation signal from the first output end and output a demodulation signal from the second output end; The first output end and the second output end of the modulation and demodulation module, and the output end of the wavelet peak searching module are respectively connected to the input end of the bias amplification module; The bias amplification module is used to perform superposition and amplification processing on the modulation signal, the demodulation signal, a preset bias voltage and the target bias voltage signal, and output a target superposition signal to the PZT modulation port of the laser to adjust the cavity length of the laser so that the output laser operates freely at the locked frequency.

2. The external cavity semiconductor laser frequency stabilization system according to claim 1, wherein The wavelet peak searching module includes a differential module, a wavelet denoising module, a peak searching module and a locking module connected in sequence; The input end of the differential module serves as the input end of the wavelet peak searching module and is used to output a frequency discrimination signal corresponding to the electrical signal; The wavelet denoising module is used to perform wavelet decomposition and signal reconstruction on the frequency discrimination signal to obtain a denoised frequency discrimination signal; The peak searching module is used to identify the peak-to-peak information of the transition peak of the locked frequency corresponding to the denoised frequency discrimination signal; The output end of the locking module serves as the output end of the wavelet peak searching module and is used to perform zero-crossing detection on the peak-to-peak information and output a target bias voltage signal corresponding to the locked frequency.

3. The external cavity semiconductor laser frequency stabilization system according to claim 1, characterized in that, The modulation and demodulation module includes a first signal generator, and a direct current blocking capacitor, a lock-in amplifier, a low-pass filter and a feedback controller connected in sequence; The reference signal output end of the first signal generator is connected to the input end of the lock-in amplifier; The reference signal output end is used to output a reference signal corresponding to the locked frequency; The modulation signal output end of the first signal generator serves as the first output end of the modulation and demodulation module and is used to output the modulation signal; The input end of the modulation and demodulation module includes a first input end; The input end of the direct current blocking capacitor serves as the first input end of the modulation and demodulation module and is used to filter out the direct current signal of the electrical signal to obtain the alternating current signal of the electrical signal; The lock-in amplifier is used to subtract the AC signal from the reference signal and output an error signal; The low-pass filter is used to filter out the high-frequency signals in the error signal to obtain a low-frequency error signal; The output end of the feedback controller serves as the second output end of the modulation and demodulation module, and is used to output the demodulation signal based on the low-frequency error signal.

4. The external cavity semiconductor laser frequency stabilization system according to claim 1, characterized in that, The bias amplification module includes a first adder, a second adder, and a power amplifier connected in sequence; The input end of the bias amplification module includes a first input end, a second input end, a third input end, and a fourth input end. The first input end of the first adder serves as the first input end of the bias amplification module and is connected to the first output end of the modulation and demodulation module. The second input end of the first adder serves as the second input end of the bias amplification module and is connected to the second output end of the modulation and demodulation module. The third input end of the first adder serves as the third input end of the bias amplification module and is used to receive the preset bias voltage. The input end of the second adder serves as the fourth input end of the bias amplification module and is connected to the output end of the wavelet peak search module; The first adder is used to perform a superposition process on the modulation signal, the demodulation signal, and the preset bias voltage, and output a first superposition signal; The second adder is used to perform a superposition process on the target bias voltage signal and the first superposition signal, and output a second superposition signal; The power amplifier serves as the output end of the bias amplification module, and is used to amplify the second superposition signal and output the target superposition signal.

5. The external cavity semiconductor laser frequency stabilization system according to claim 3, characterized in that, The modulation and demodulation module further includes a second signal generator; the modulation and demodulation module further includes a second input end and a third output end; The output end of the second signal generator serves as the second input end of the modulation and demodulation module, and is used to receive an external unlocking instruction and generate a scanning signal under the control of the unlocking instruction; The second input end of the switch module is further used to receive the unlocking instruction to control the switch module to disconnect and block the electrical signal from being transmitted to the wavelet peak search module and the modulation and demodulation module; The output end of the second signal generator serves as the third output end of the modulation and demodulation module and is connected to the input end of the bias amplification module, and is used to output the scanning signal to the bias amplification module; The bias amplification module is further used to perform a superposition and amplification process on the modulation signal and the scanning signal, and transmit the output superposition signal to the PZT modulation port to continuously adjust the cavity length of the laser, so that the output laser operates freely within a preset frequency scanning range.

6. The external cavity semiconductor laser frequency stabilization system according to any one of claims 1-5, characterized in that, The saturated absorption spectrum module includes an optical isolator, a transmission optical fiber, a half-wave plate, a polarization beam splitter, a coupling head, a wavelength meter, a high-transmission and low-reflection mirror, a reflector, a rubidium cell, a semi-transmissive and semi-reflective mirror, and a photodetector arranged in sequence along the main optical path; The coupling head and the wavelength meter are arranged in sequence on the transmission optical path of the polarization beam splitter. The high-transmission and low-reflection mirror and the reflector are arranged in sequence on the reflection optical path of the polarization beam splitter. The rubidium cell, the semi-transmissive and semi-reflective mirror, and the photodetector are arranged in sequence on the reflection optical path of the high-transmission and low-reflection mirror; The optical isolator is used to prevent the light reflected by the optical fiber echo on the main optical path from returning to the laser and affecting the operation of the laser; The transmission optical fiber is used to transmit the laser passing through the optical isolator to the half-wave plate; The half-wave plate is used to adjust the polarization state of the input laser to linear polarization; The polarization beam splitter is used to divide the linearly polarized laser into a first reflected light and a first transmitted light. The first transmitted light is converted into spatial light by the coupling head and then transmitted to the wavelength meter to monitor the frequency change of the laser. The first reflected light is transmitted to the high-transmission and low-reflection mirror; The high-transmission and low-reflection mirror is used to divide the first reflected light into a second reflected light and a second transmitted light. The second reflected light generates a saturated absorption spectrum signal of rubidium atoms after passing through the rubidium cell and then is transmitted to the semi-transmissive and semi-reflective mirror. The second transmitted light is reflected by the reflector to the semi-transmissive and semi-reflective mirror; The semi-transmissive and semi-reflective mirror is used to divide the saturated absorption spectrum signal into a third reflected light and a third transmitted light. The third transmitted light is used as the detection light and transmitted to the photodetector. And it is used to convert the second transmitted light reflected by the reflector into the repump light; The photodetector is used to convert the detection light into an electrical signal.

7. A frequency stabilization method applied to the frequency stabilization system of an external cavity semiconductor laser as described in any one of claims 1-6, characterized in that, It includes: Performing atomic saturated absorption spectrum measurement on the laser output by the laser and outputting the electrical signal of the saturated absorption spectrum; Under the control of the frequency locking instruction, outputting the target bias voltage signal corresponding to the locked frequency according to the electrical signal; Performing signal modulation and demodulation according to the electrical signal and the reference signal corresponding to the locked frequency, and outputting the modulation signal and the demodulation signal; Performing superposition and amplification processing on the modulation signal, the demodulation signal, the preset bias voltage, and the target bias voltage signal, and outputting the target superposition signal to the PZT modulation port to adjust the cavity length of the laser so that the output laser operates freely at the locked frequency.

8. The frequency stabilization method according to claim 7, wherein The under the control of the frequency locking instruction, outputting the target bias voltage signal corresponding to the locked frequency according to the electrical signal includes: Outputting the frequency discrimination signal corresponding to the electrical signal; Performing wavelet decomposition and signal reconstruction on the frequency discrimination signal to obtain the denoised frequency discrimination signal; Identifying the peak-to-peak information of the transition peak of the locked frequency corresponding to the denoised frequency discrimination signal; Performing zero-crossing detection on the peak-to-peak information and outputting the target bias voltage signal corresponding to the locked frequency.

9. The frequency stabilization method according to claim 7, wherein The performing signal modulation and demodulation according to the electrical signal and the reference signal corresponding to the locked frequency, and outputting the modulation signal and the demodulation signal includes: Filtering the DC signal of the electrical signal to obtain the AC signal of the electrical signal; Generating two signals, namely the reference signal corresponding to the locked frequency and the modulation signal, under the control of the frequency locking instruction; Subtracting the AC signal from the reference signal and outputting the error signal; Filtering the high-frequency signal in the error signal to obtain the low-frequency error signal; Output the demodulation signal based on the low-frequency error signal.

10. The frequency stabilization method according to claim 7, characterized in that The method further includes: Output a scanning signal and a modulation signal under the control of an unlocking instruction; Perform superposition and amplification processing on the modulation signal and the scanning signal, and transmit the output superposed signal to the PZT modulation port to continuously adjust the cavity length of the laser so that the output laser operates freely within a preset frequency scanning range.

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