Phase-locked loop-based cavity length locking device and method for low-cost optical resonant cavity

Abstract

The invention relates to a cavity length locking device and method of a low-cost optical resonant cavity based on a phase-locked loop. The device mainly comprises a reference laser frequency stabilization structure and a cavity length locking structure of the optical resonant cavity. The method comprises the following steps: locking the frequency of a reference laser at an atomic resonance point through a saturated absorption frequency stabilization method, coupling laser containing a modulation component into an optical resonant cavity, collecting a transmission spectrum, taking a phase-locked loop as a basis, and taking a modulation signal in a frequency stabilization loop of the laser as a reference; a demodulation signal with the same frequency and adjustable phase is obtained through phase locking and used for demodulating an error signal for locking the cavity length of the optical resonant cavity, and the cavity length of the optical resonant cavity can be locked through the error signal. According to the invention, an external optical modulation device is not needed, so that the cost is greatly reduced, the method is not limited by the modulation bandwidth, and an error signal with a relatively low frequency discrimination slope can be demodulated, thereby reducing the loop gain, enlarging the frequency discrimination range, and reducing the influence of a control system on the fineness of the optical resonant cavity.

Description

Technical Field

[0001] The present invention relates to the technical field of laser frequency stabilization and optical resonant cavity length locking in quantum precision measurement technology, and in particular to a low-cost optical resonant cavity length locking device and method based on a phase-locked loop. Background Art

[0002] In the field of atomic spin inertia and magnetic field measurement, the frequency stability of the pump laser and the detection laser will affect the atomic polarization performance and optical rotation angle detection respectively, thus directly affecting the performance of the atomic spin inertia and magnetic field measurement system. Among them, in order to suppress the pumping effect introduced by the detection laser on the atom, the operating frequency of the detection laser needs to deviate from the alkali metal atomic absorption line. However, there is a lack of a stable frequency reference at the frequency point that deviates from the metal atomic absorption line, so it is difficult to achieve detuned frequency stabilization of the detection light. Because the optical resonant cavity has a longitudinal mode distribution in a large range, most of the existing technologies lock the detection laser frequency to the optical resonant cavity, and the representative one is the PDH (Pound-Drever-Hall) frequency stabilization technology. However, ordinary optical resonants are easily affected by ambient temperature and vibration, which causes the laser frequency stability to deteriorate. The super-temperature cavity placed in the vacuum chamber and the addition of a temperature control system can suppress the drift of the resonant cavity length to a certain extent, but this solution is very large in size and costly and difficult to apply. Therefore, the researchers chose to lock the optical resonant cavity to a frequency-stable laser, and then lock the detection light to the optical resonant cavity to complete the transmission of frequency stability.

[0003] The finesse of an optical resonator is related by:

[0004] ;

[0005] in, represents the finesse of the optical resonant cavity, c represents the speed of light, n represents the refractive index of the air in the cavity, L is the cavity length of the optical resonant cavity, R is the reflectivity of the optical resonant cavity mirror, and FSR represents the free spectral range of the optical resonant cavity. It can be seen that the modulation and regulation of the cavity length of the resonant cavity will affect the finesse of the resonant cavity, and thus affect the frequency stability of the laser locked in the cavity. In order to suppress this problem as much as possible, researchers mostly use external modulation devices such as electro-optical modulators to apply external modulation to the laser, but due to the bandwidth limitation of the external modulator, the demodulated error signal has a large frequency discrimination slope, which leads to a high gain in the cavity length locking loop, thereby destroying the finesse of the resonant cavity. In addition, the cost of the external optical modulator is also high. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a low-cost optical resonant cavity length locking device and method based on a phase-locked loop, which performs low-cost cavity length locking without destroying the fineness of the optical resonant cavity and without using an external modulator. The device of the present invention mainly includes a laser frequency locking part and an optical resonant cavity length locking part. The frequency of the reference laser is locked at the atomic resonance point by a saturated absorption frequency stabilization method, and then a laser containing a modulated component is coupled into the optical resonant cavity and the transmission spectrum is collected. Based on the phase-locked loop, the modulated signal in the laser frequency stabilization loop is used as a reference signal to phase-lock a demodulation signal with the same frequency and adjustable phase, which is used to demodulate the error signal for locking the cavity length of the optical resonant cavity. The cavity length of the optical resonant cavity can be locked by the error signal. The long-term frequency stability of the locked optical resonant cavity can be traced back to the atomic resonance frequency, which significantly improves the long-term stability of the optical resonant cavity. The present invention does not require the use of external optical modulation devices, which greatly reduces costs. Secondly, the present invention is not limited by the modulation bandwidth and can demodulate an error signal with a lower discrimination slope, thereby reducing the loop gain and increasing the discrimination range, and reducing the influence of the control system on the fineness of the optical resonant cavity.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A low-cost optical resonant cavity length locking device based on a phase-locked loop comprises a laser frequency locking part and an optical resonant cavity length locking part; the laser frequency locking part is used to lock the laser frequency to an atomic resonance point, and the optical resonant cavity length locking part uses the stabilized laser as a frequency reference to lock the cavity length of the optical resonant cavity.

[0009] Further, the laser frequency locking part includes a laser, an optical isolator, a first half-wave plate, a polarization beam splitter, a 50:50 transflector, an atomic gas chamber, a first reflector, a first photodetector, a signal generator, a first frequency detector, a first low-pass filter, and a first PI controller; after the laser emitted by the laser passes through the optical isolator, it is divided into two parts through the first half-wave plate and the polarization beam splitter, which are respectively the reflected light of the S polarization state and the transmitted light of the P polarization state; the reflected light of the S polarization state is reflected into the 50:50 transflector, half of the power of the light is transmitted, and half of the power of the light is reflected into the atomic gas chamber to act as pump light; the light passing through the atomic gas chamber is reflected back to the atomic gas chamber by the first reflector to act as detection light; the detection light intersects with the pump light to generate a saturated absorption spectrum; the detection light carrying spectral information is detected by the first photodetector after passing through the 50:50 transflector;

[0010] The signal generator applies a sweep signal and a frequency of The modulation signal is transmitted to the first PI controller through the first low-pass filter to generate a feedback control signal, and the control signal is finally transmitted to the laser frequency tuning port to realize laser frequency locking.

[0011] Furthermore, the optical resonant cavity length locking part includes a second reflector, a third reflector, a convex lens, an optical resonant cavity, a second photodetector, a phase-locked loop, a second frequency detector, a second low-pass filter, and a second PI controller; the P polarization state transmission light of the polarization beam splitter is reflected by the second reflector and the third reflector to the convex lens, is coupled into the optical resonant cavity after being focused by the lens, and is detected by the second photodetector after passing through the optical resonant cavity; the frequency generated by the signal generator is The modulated signal is transmitted to the phase-locked loop, and the phase-locked loop generates a demodulated signal with the same frequency and adjustable phase, which is then sent to the second frequency detector and phase detector to mix with the electrical signal detected by the second photodetector and demodulate the error signal. After passing through the second low-pass filter, it is transmitted to the second PI controller and generates a feedback control signal. The control signal is finally transmitted to the piezoelectric ceramic controller of the optical resonant cavity to achieve cavity length locking of the optical resonant cavity.

[0012] The present invention also provides a cavity length locking method of a cavity length locking device of a low-cost optical resonant cavity based on a phase-locked loop, comprising the following steps:

[0013] Step 1: The laser generates a saturated absorption spectrum through a laser frequency locking optical path;

[0014] Step 2: Lock the laser frequency at the saturation absorption peak point, that is, the atomic resonance point;

[0015] Step 3, then coupling the laser containing the modulation component into the optical resonant cavity and collecting the transmission spectrum;

[0016] Step 4: Based on the phase-locked loop, the modulation signal in the laser frequency stabilization loop is used as a reference signal to phase-lock a demodulation signal with the same frequency and adjustable phase, which is used to demodulate the error signal for locking the cavity length of the optical resonant cavity, and the cavity length of the optical resonant cavity is locked by the error signal.

[0017] Furthermore, the step one includes: after the laser emitted by the laser passes through the optical isolator, it is divided into two parts by the first half-wave plate and the polarization splitter prism, namely the reflected light of the S polarization state and the transmitted light of the P polarization state, the reflected light of the S polarization state is reflected into the 50:50 transflector, half of the power of the light is transmitted, and half of the power of the light is reflected into the atomic gas chamber to act as pump light, the light passing through the atomic gas chamber is reflected back to the atomic gas chamber by the first reflector to act as detection light, the detection light intersects with the pump light to generate a saturated absorption spectrum, and the detection light carrying the spectral information is detected by the first photodetector after passing through the 50:50 transflector.

[0018] Furthermore, the step 2 includes: the signal generator applies a sweep frequency signal and a frequency of The modulation signal is generated, and the demodulation signal of the same frequency is input to the first frequency detector. The laser carrying the modulation information and spectral information detected by the first photodetector is converted into an electrical signal, which is output to the first frequency detector, mixed with the demodulation signal and demodulated to obtain an error signal. After passing through the first low-pass filter, it is transmitted to the first PI controller and generates a feedback control signal. The control signal is finally transmitted to the frequency tuning port of the laser to achieve laser frequency locking.

[0019] Furthermore, the step three includes: the P-polarized state transmitted light of the polarization splitter prism is reflected to the convex lens by the second reflector and the third reflector, coupled into the optical resonant cavity after being focused by the lens, and detected by the second photodetector after passing through the optical resonant cavity.

[0020] Furthermore, the step 4 includes: setting the frequency generated by the signal generator to The modulated signal is transmitted to the phase-locked loop, and the phase-locked loop generates a demodulated signal with the same frequency and adjustable phase, which is then sent to the second frequency detector and phase detector to mix with the electrical signal detected by the second photodetector and demodulate the error signal. After passing through the second low-pass filter, it is transmitted to the second PI controller and generates a feedback control signal. The control signal is finally transmitted to the piezoelectric ceramic controller of the optical resonant cavity to achieve cavity length locking of the optical resonant cavity.

[0021] Furthermore, in step three, a piezoelectric ceramic controller is attached to one of the cavity mirrors of the optical resonant cavity, and the cavity length of the optical resonant cavity is adjusted by adjusting the piezoelectric ceramic controller; the laser coupled into the optical resonant cavity contains the same modulation information as in step two, and after entering the optical resonant cavity, multi-beam interference occurs, and the transmitted light carries the spectral information of the optical resonant cavity.

[0022] Furthermore, in step 4, a power divider is used to divide the frequency generated by the signal generator into The modulation signal is divided into two paths, one is transmitted to the laser for modulation, and the other is transmitted to the phase-locked loop; the phase-locked loop includes a phase detector, a loop filter, and a voltage-controlled oscillator. It is a pure digital phase-locked loop built on FPGA. The frequency generated by the signal generator is The modulated signal is sent to the analog-to-digital converter after the power divider to be converted into a digital signal, and sent to the phase detector as a reference signal. The frequency generated by the voltage-controlled oscillator is set to approximately The demodulated signal is transmitted to the phase detector, where the two are mixed to generate an error signal proportional to the frequency difference and phase difference of the two signals. The error signal is sent to the loop filter for amplification and integration, and a control signal is generated to be fed back to the voltage-controlled oscillator, thereby completing the frequency and phase tracking effect of the phase-locked loop demodulation signal relative to the modulation signal in the laser frequency stabilization loop; finally, the demodulated signal generated by the phase-locked loop is converted into an analog quantity by the digital-to-analog converter and output to the second frequency detector.

[0023] The beneficial effects of the present invention compared with the prior art are:

[0024] The present invention uses the modulation signal in the laser frequency stabilization control loop as a reference signal, uses a phase-locked loop to phase-lock a demodulation signal with the same frequency and adjustable phase, mixes and demodulates an error signal in a frequency detector and phase detector, and applies feedback control to the PZT of the optical resonant cavity through a PI controller, thereby completing the cavity length locking of the resonant cavity. The present invention does not require the use of an external optical modulation device, which greatly reduces the cost. Secondly, the method is not limited by the modulation bandwidth and can demodulate an error signal with a low frequency detection slope, thereby reducing the loop gain and increasing the frequency detection range, and reducing the influence of the control system on the fineness of the optical resonant cavity.

[0025] Specifically, the device of the present invention uses a plano-convex lens, a Faraday rotator, a reflector and a polarization beam splitter to form a double modulation structure. The structure sets the exit point of the single-modulated first-order diffraction light of the acousto-optic frequency shifter at the focal position of the plano-convex lens, so that the exit light is parallel to the optical axis, and the reflected light is parallel to the optical axis and enters the acousto-optic frequency shifter through the focal point according to the original light path after entering the plano-convex lens, so that the second-modulated first-order diffraction light can coincide with the original laser light path, so that the propagation angle of the second-modulated first-order diffraction light can be unaffected by the frequency shift of the acousto-optic frequency shifter, avoiding the problem of fluctuation of the beat frequency signal amplitude caused by the change of the modulated light angle. In addition, the structure can keep the second-modulated light in an orthogonal polarization state with the original laser and the single-modulated light, avoiding the problem of fluctuation of the beat frequency signal amplitude caused by fluctuation of the polarization state. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural diagram of a cavity length locking device of a low-cost optical resonant cavity based on a phase-locked loop of the present invention;

[0027] Figure 2 yes Figure 1Schematic diagram of the phase-locked loop in .

[0028] Among them, the meanings of the figure marks are: 1 is a laser, 2 is an optical isolator, 3 is a first half-wave plate, 4 is a polarization splitter prism, 5 is a second reflector, 6 is a third reflector, 7 is a convex lens, 8 is an optical resonant cavity, 9 is a piezoelectric ceramic controller, 10 is a second photodetector, 11 is a first photodetector, 12 is a 50:50 transflective mirror, 13 is an atomic gas chamber, 14 is a first reflector, 15 is a first frequency detector, 16 is a first low-pass filter, 17 is a first PI controller, 18 is a signal generator, 19 is a phase-locked loop, 20 is a second frequency detector, 21 is a second low-pass filter, and 22 is a second PI controller. DETAILED DESCRIPTION

[0029] The embodiments of the present invention are described in detail below: This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. The present invention is further described below in conjunction with the accompanying drawings and specific implementation examples.

[0030] like Figure 1 As shown, a low-cost optical resonant cavity length locking device based on a phase-locked loop of the present invention can be divided into a laser frequency locking part and an optical resonant cavity length locking part. The laser frequency locking part is used to lock the laser frequency to the atomic resonance point, and the optical resonant cavity length locking part uses the stabilized laser as the frequency reference to lock the cavity length of the optical resonant cavity.

[0031] The laser frequency locking part includes a laser 1, an optical isolator 2, a first half-wave plate 3, a polarization beam splitter prism 4, a 50:50 transflector 12, an atomic gas chamber 13, a first reflector 14, a first photodetector 11, a signal generator 18, a first frequency and phase detector 15, a first low-pass filter 16, and a first PI controller 17;

[0032] The optical resonant cavity length locking part includes a second reflector 5, a third reflector 6, a convex lens 7, an optical resonant cavity 8, a piezoelectric ceramic controller 9, a second photodetector 10, a phase-locked loop 19, a second frequency detector 20, a second low-pass filter 21, and a second PI controller 22.

[0033] The laser light emitted by the laser 1 passes through the optical isolator 2, and is then divided into two parts by the first half-wave plate 3 and the polarization beam splitter prism 4, namely, the reflected light of the S polarization state and the transmitted light of the P polarization state. The reflected light of the S polarization state is reflected to the 50:50 transflector 12, and half of the power of the light is transmitted, and the half of the power of the light is reflected into the atomic gas chamber 13 to act as pump light. The light passing through the atomic gas chamber 13 is reflected back to the atomic gas chamber 13 by the first reflector 14 to act as detection light. The detection light intersects with the pump light to generate a saturated absorption spectrum. The detection light carrying the spectrum information is detected by the first photodetector 11 after passing through the 50:50 transflector 12.

[0034] The signal generator 18 applies a sweep signal with a frequency of 10 Hz and an amplitude of 5V and a modulation signal with a frequency of 30 kHz and an amplitude of 20 mV to the laser 1, and generates a demodulation signal with a frequency of 30 kHz and inputs it into the first frequency detector 15. The laser light carrying modulation information and spectral information detected by the first photodetector 11 is converted into an electrical signal, which is output to the first frequency detector 15 to mix with the demodulation signal and demodulate an error signal. After passing through the first low-pass filter 16, it is transmitted to the first PI controller 17 and generates a feedback control signal. The control signal is finally transmitted to the frequency tuning port of the laser 1 to achieve laser frequency locking.

[0035] The P polarized state transmitted light of the polarization beam splitter 4 is reflected by the second reflector 5 and the third reflector 6 to the convex lens 7, is focused by the lens and coupled into the optical resonant cavity 8, and is detected by the second photodetector 10 after passing through the optical resonant cavity 8;

[0036] The modulation signal with a frequency of 30 kHz and an amplitude of 20 mV generated by the signal generator 18 is transmitted to the phase-locked loop 19, and the phase-locked loop 19 generates a demodulated signal with the same frequency and adjustable phase, which is then sent to the second frequency detector 20 to mix with the electrical signal detected by the second photodetector 10 and demodulate the error signal. After passing through the second low-pass filter 21, it is transmitted to the second PI controller 22 and generates a feedback control signal. The control signal is finally transmitted to the piezoelectric ceramic controller 9 of the optical resonant cavity 8 to achieve cavity length locking of the optical resonant cavity.

[0037] Therefore, a method for locking the cavity length of a low-cost optical resonant cavity based on a phase-locked loop according to an embodiment of the present invention comprises the following steps:

[0038] Step 1: The laser generates a saturated absorption spectrum through a laser frequency locking optical path;

[0039] Step 2: Lock the laser frequency at the saturation absorption peak point, that is, the atomic resonance point;

[0040] Step 3, then coupling the laser containing the modulation component into the optical resonant cavity and collecting the transmission spectrum;

[0041] Step 4: Based on the phase-locked loop, the modulation signal in the laser frequency stabilization loop is used as the reference signal to phase-lock a demodulation signal with the same frequency and adjustable phase, which is used to demodulate the error signal for locking the cavity length of the optical resonant cavity. The cavity length of the optical resonant cavity can be locked through the error signal.

[0042] Specifically, the step one comprises: the laser light emitted by the laser 1 passes through the optical isolator 2, and is then divided into two parts by the first half-wave plate 3 and the polarization splitter prism 4, namely, the reflected light in the S polarization state and the transmitted light in the P polarization state; the reflected light in the S polarization state is reflected into the 50:50 transflector 12, half of the power of the light is transmitted, and half of the power of the light is reflected into the atomic gas chamber 13 to act as pump light; the light passing through the atomic gas chamber 13 is reflected back to the atomic gas chamber 13 by the first reflector 14 to act as detection light; the detection light intersects with the pump light to generate a saturated absorption spectrum; the detection light carrying spectral information passes through the 50:50 transflector 12 and is detected by the first photodetector 11.

[0043] Specifically, the step 2 includes: the signal generator 18 applies a sweep signal with a frequency of 10 Hz and an amplitude of 5V and a modulation signal with a frequency of 30 kHz and an amplitude of 20 mV to the laser 1, and at the same time generates a demodulation signal with a frequency of 30 kHz and inputs it to the first frequency detector 15; the laser carrying modulation information and spectral information detected by the first photodetector 11 is converted into an electrical signal, which is output to the first frequency detector 15 to mix with the demodulation signal and demodulate an error signal, which is transmitted to the first PI controller 17 after passing through the first low-pass filter 16 and generates a feedback control signal, and the control signal is finally transmitted to the frequency tuning port of the laser 1 to achieve laser frequency locking;

[0044] Specifically, the step three includes: the P-polarized state transmitted light of the polarization beam splitter prism 4 is reflected to the convex lens 7 by the second reflector 5 and the third reflector 6, coupled into the optical resonant cavity 8 after being focused by the lens, and detected by the second photodetector 10 after passing through the optical resonant cavity 8.

[0045] Specifically, the step four includes: transmitting the modulation signal with a frequency of 30 kHz and an amplitude of 20 mV generated by the signal generator 18 to the phase-locked loop 19, the phase-locked loop 19 generates a demodulated signal with the same frequency and adjustable phase, and then sends it to the second frequency detector 20 to mix with the electrical signal detected by the second photodetector 10 and demodulate the error signal, and then transmits it to the second PI controller 22 after passing through the second low-pass filter 21 and generates a feedback control signal, and the control signal is finally transmitted to the piezoelectric ceramic controller 9 of the optical resonant cavity 8 to achieve cavity length locking of the optical resonant cavity.

[0046] Furthermore, in step 3, a piezoelectric ceramic controller 9 is attached to one of the cavity mirrors of the optical resonant cavity, and the cavity length of the optical resonant cavity 8 can be adjusted by adjusting the piezoelectric ceramic controller 9. The laser coupled into the optical resonant cavity 8 contains the same modulation information as in step 2. After entering the optical resonant cavity 8, multi-beam interference occurs, and the transmitted light carries the optical resonant cavity spectrum information.

[0047] Further, in step 4, a power divider is used to divide the frequency generated by the signal generator 18 into The modulation signal is divided into two paths, one path is transmitted to the laser 1 for modulation, and the other path is transmitted to the phase-locked loop 19; the phase-locked loop 19 includes a phase detector, a loop filter, and a voltage-controlled oscillator.

[0048] It is worth noting that the power divider will evenly distribute the modulated signal power, so the amplitude of the modulated signal generated by the signal generator 18 should be 40 mV. Figure 2 As shown, the phase-locked loop 19 includes an analog-to-digital converter (ADC), a phase detector (PFD), a loop filter, a voltage-controlled oscillator (VCO), and a digital-to-analog converter (DAC).

[0049] The phase-locked loop 19 in this embodiment is a pure digital phase-locked loop built on FPGA. The modulation signal with a frequency of 30 kHz generated by the signal generator 18 is sent to the analog-to-digital converter after the power divider to be converted into a digital signal, and sent to the phase detector as a reference signal. The demodulation signal with a frequency of approximately 30 kHz generated by the voltage-controlled oscillator is set and transmitted to the phase detector. The two are mixed in the phase detector to generate an error signal proportional to the frequency difference and phase difference of the two signals. The error signal is sent to the loop filter for amplification and integration and generates a control signal to be fed back to the voltage-controlled oscillator, thereby completing the frequency and phase tracking effect of the phase-locked loop demodulation signal relative to the modulation signal in the laser frequency stabilization loop. Finally, the demodulation signal generated by the phase-locked loop is converted into an analog quantity by the digital-to-analog converter and output to the second frequency detector 20.

[0050] In step 4, the method for phase locking of the optical phase-locked reference modulation signal in the present invention is:

[0051] Assume that the modulation signal in the laser frequency stabilization loop is:

[0052] ;

[0053] Where M(t) is the modulation signal in the laser frequency stabilization loop, A1 is the modulation signal amplitude, and are the initial frequency and phase of the modulation signal, and are the frequency and phase noise of the modulation signal respectively, and t represents time.

[0054] The demodulated signal generated by the voltage-controlled oscillator in the phase-locked loop is:

[0055] ;

[0056] Where, D(t) is the demodulated signal generated by the voltage-controlled oscillator in the phase-locked loop, A2 is the amplitude of the demodulated signal generated by the VCO, and They are respectively the initial frequency and phase of the demodulated signal generated by the voltage-controlled oscillator.

[0057] The signal after M(t) and D(t) are mixed and low-pass filtered by the loop filter is:

[0058] ;

[0059] Where E(t) is the error signal after low-pass filtering, K mix is the phase detector gain.

[0060] It can be seen from the above formula that the error signal contains the frequency and phase difference between the voltage-controlled oscillator output signal and the modulation reference signal, which is fed back to the voltage-controlled oscillator for control, thereby achieving the effect of frequency and phase tracking of the phase-locked loop demodulation signal relative to the modulation signal in the laser frequency stabilization loop. Phase adjustment can be achieved by superimposing a DC bias before the error signal reaches the loop filter.

[0061] The contents not described in detail in this specification belong to the prior art known to the professionals in this field.

Claims

1. A low-cost optical resonant cavity length locking device based on a phase-locked loop, characterized in that: It includes a laser frequency locking part and an optical resonant cavity length locking part; the laser frequency locking part is used to lock the laser frequency to the atomic resonance point, and the optical resonant cavity length locking part uses the stabilized laser as the frequency reference to lock the cavity length of the optical resonant cavity.

2. According to claim 1, a low-cost optical resonant cavity length locking device based on a phase-locked loop is characterized in that: The laser frequency locking part comprises a laser, an optical isolator, a first half-wave plate, a polarization beam splitter, a 50:50 transflector, an atomic gas chamber, a first reflector, a first photodetector, a signal generator, a first frequency detector, a first low-pass filter, and a first PI controller; the laser emitted by the laser passes through the optical isolator and is divided into two parts through the first half-wave plate and the polarization beam splitter, which are respectively a reflected light in an S polarization state and a transmitted light in a P polarization state; the reflected light in the S polarization state is reflected into the 50:50 transflector, half of the power of the light is transmitted, and half of the power of the light is reflected into the atomic gas chamber to serve as pump light; the light passing through the atomic gas chamber is reflected back to the atomic gas chamber by the first reflector to serve as detection light; the detection light intersects with the pump light to generate a saturated absorption spectrum; the detection light carrying spectrum information is detected by the first photodetector after passing through the 50:50 transflector; The signal generator applies a sweep signal and a frequency of The modulation signal is transmitted to the first PI controller through the first low-pass filter to generate a feedback control signal, and the control signal is finally transmitted to the laser frequency tuning port to realize laser frequency locking.

3. According to claim 1, a low-cost optical resonant cavity length locking device based on a phase-locked loop is characterized in that: The optical resonant cavity length locking part includes a second reflector, a third reflector, a convex lens, an optical resonant cavity, a second photodetector, a phase-locked loop, a second frequency detector, a second low-pass filter, and a second PI controller; the P polarization state transmission light of the polarization beam splitter is reflected to the convex lens by the second reflector and the third reflector, and is coupled into the optical resonant cavity after being focused by the lens, and is detected by the second photodetector after passing through the optical resonant cavity; the frequency generated by the signal generator is The modulated signal is transmitted to the phase-locked loop, and the phase-locked loop generates a demodulated signal with the same frequency and adjustable phase, which is then sent to the second frequency detector and phase detector to mix with the electrical signal detected by the second photodetector and demodulate the error signal. After passing through the second low-pass filter, it is transmitted to the second PI controller and generates a feedback control signal. The control signal is finally transmitted to the piezoelectric ceramic controller of the optical resonant cavity to achieve cavity length locking of the optical resonant cavity.

4. The cavity length locking method of a cavity length locking device of a low-cost optical resonant cavity based on a phase-locked loop according to any one of claims 1 to 3, characterized in that: The steps include: Step 1: The laser generates a saturated absorption spectrum through a laser frequency locking optical path; Step 2: Lock the laser frequency at the saturation absorption peak point, that is, the atomic resonance point; Step 3, then coupling the laser containing the modulation component into the optical resonant cavity and collecting the transmission spectrum; Step 4: Based on the phase-locked loop, the modulation signal in the laser frequency stabilization loop is used as a reference signal to phase-lock a demodulation signal with the same frequency and adjustable phase, which is used to demodulate the error signal for locking the cavity length of the optical resonant cavity, and the cavity length of the optical resonant cavity is locked by the error signal.

5. The cavity length locking method according to claim 4, characterized in that: The step 1 comprises: the laser light emitted by the laser passes through the optical isolator, and is then divided into two parts through the first half-wave plate and the polarization splitter prism, namely, the reflected light in the S polarization state and the transmitted light in the P polarization state; the reflected light in the S polarization state is reflected into the 50:50 transflector, half of the power of the light is transmitted, and half of the power of the light is reflected into the atomic gas chamber to act as pump light; the light passing through the atomic gas chamber is reflected back to the atomic gas chamber by the first reflector to act as detection light; the detection light intersects with the pump light to generate a saturated absorption spectrum; the detection light carrying spectral information passes through the 50:50 transflector and is detected by the first photodetector.

6. The cavity length locking method according to claim 5, characterized in that: The second step includes: the signal generator applies a sweep frequency signal and a frequency of The modulation signal is generated, and the demodulation signal of the same frequency is simultaneously generated and input into the first frequency detector. The laser carrying the modulation information and the spectrum information detected by the first photodetector is converted into an electrical signal, which is output to the first frequency detector, mixed with the demodulation signal and demodulated to obtain an error signal. After passing through the first low-pass filter, it is transmitted to the first PI controller and generates a feedback control signal. The control signal is finally transmitted to the frequency tuning port of the laser to realize laser frequency locking.

7. The cavity length locking method according to claim 6, characterized in that: The step three includes: the P polarized state transmitted light of the polarization beam splitter prism is reflected to the convex lens by the second reflector and the third reflector, coupled into the optical resonant cavity after being focused by the lens, and detected by the second photodetector after passing through the optical resonant cavity.

8. The cavity length locking method according to claim 7, characterized in that: The fourth step comprises: setting the frequency generated by the signal generator to The modulated signal is transmitted to the phase-locked loop, and the phase-locked loop generates a demodulated signal with the same frequency and adjustable phase, which is then sent to the second frequency detector and phase detector to mix with the electrical signal detected by the second photodetector and demodulate the error signal. After passing through the second low-pass filter, it is transmitted to the second PI controller and generates a feedback control signal. The control signal is finally transmitted to the piezoelectric ceramic controller of the optical resonant cavity to achieve cavity length locking of the optical resonant cavity.

9. The cavity length locking method according to claim 4, characterized in that: In the step three, a piezoelectric ceramic controller is attached to one of the cavity mirrors of the optical resonant cavity, and the cavity length of the optical resonant cavity is adjusted by adjusting the piezoelectric ceramic controller; the laser coupled into the optical resonant cavity contains the same modulation information as in step two, and after entering the optical resonant cavity, multi-beam interference occurs, and the transmitted light carries the spectral information of the optical resonant cavity.

10. The cavity length locking method according to claim 4, characterized in that: In the step 4, a power divider is used to divide the frequency generated by the signal generator into The modulation signal is divided into two paths, one is transmitted to the laser for modulation, and the other is transmitted to the phase-locked loop; the phase-locked loop includes a phase detector, a loop filter, and a voltage-controlled oscillator. It is a pure digital phase-locked loop built on FPGA. The frequency generated by the signal generator is The modulated signal is sent to the analog-to-digital converter after the power divider to be converted into a digital signal, and sent to the phase detector as a reference signal. The frequency generated by the voltage-controlled oscillator is set to approximately The demodulated signal is transmitted to the phase detector, where the two are mixed to generate an error signal proportional to the frequency difference and phase difference of the two signals. The error signal is sent to the loop filter for amplification and integration, and a control signal is generated to be fed back to the voltage-controlled oscillator, thereby completing the frequency and phase tracking effect of the phase-locked loop demodulation signal relative to the modulation signal in the laser frequency stabilization loop; finally, the demodulated signal generated by the phase-locked loop is converted into an analog quantity by the digital-to-analog converter and output to the second frequency detector.

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