A low-cost optical resonator cavity length locking device and method based on phase-locked loop

Abstract

The application relates to a low-cost optical resonant cavity length locking device and method based on a phase-locked loop, which mainly comprises a reference laser frequency stabilization structure and an optical resonant cavity length locking structure. The frequency of the reference laser is locked at an atomic resonance point through a saturated absorption frequency stabilization method, then laser containing a modulation component is coupled into the optical resonant cavity and transmission spectrum is collected, a modulation signal in a laser frequency stabilization loop is taken as a reference based on a phase-locked loop, a demodulation signal with the same frequency and adjustable phase is locked, which is used for demodulating an error signal of the locked optical resonant cavity length, and the cavity length of the optical resonant cavity can be locked through the error signal. The application does not need to use external optical modulation devices, which greatly reduces the cost and is not limited by the modulation bandwidth, can demodulate an error signal with low frequency discrimination slope, reduces loop gain and increases frequency discrimination range, and reduces the influence of a control system on the precision of the optical resonant cavity.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser frequency stabilization and optical resonator cavity length locking in quantum precision measurement technology, and particularly relates to a low-cost optical resonator cavity length locking device and method based on a phase-locked loop. BACKGROUND

[0002] In the field of atomic spin inertial and magnetic field measurement, the frequency stability of the pumping laser and the detection laser will affect the atomic polarization performance and the optical spin angle detection, thereby directly affecting the performance of the atomic spin inertial and magnetic field measurement system. In order to suppress the pumping effect introduced by the detection laser to the atom, the working frequency of the detection laser needs to deviate from the alkali metal atom absorption line, however, there is a lack of stable frequency reference at the frequency point deviating from the metal atom absorption line, so it is difficult to realize the frequency stabilization of the detection light. Because the optical resonator has longitudinal mode distribution in a large range, most of the existing technologies lock the frequency of the detection laser on the optical resonator, and the representative is the PDH (Pound-Drever-Hall) frequency stabilization technology. However, the ordinary optical resonator is easily affected by the environmental temperature and vibration, thereby causing the deterioration of the laser frequency stability. The super-temperature cavity placed in the vacuum chamber and the temperature control system can suppress the cavity length drift of the resonator to a certain extent, but this scheme is high in volume and cost, and is difficult to apply. Therefore, researchers choose to lock the optical resonator on the frequency-stable laser, and lock the detection light on the optical resonator to complete the transmission of the frequency stability.

[0003] The optical resonator fineness has the following relationship:

[0004] ;

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

[0006] To solve the above technical problems, the application provides a low-cost optical resonant cavity cavity length locking device and method based on a phase-locked loop, which can lock the cavity length without destroying the precision of the optical resonant cavity and without using an external modulator.

[0007] To achieve the above object, the technical scheme adopted by the application is as follows:

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

[0009] Further, the laser frequency locking part comprises a laser, an optical isolator, a first half-wave plate, a polarization beam splitter prism, a 50:50 transmissive-reflection mirror, an atomic cell, a first mirror, a first photodetector, a signal generator, a first frequency discriminator, a first low-pass filter and a first PI controller; the laser emitted by the laser passes through the optical isolator, and is then split into two parts, i.e., S-polarized reflected light and P-polarized transmitted light, through the first half-wave plate and the polarization beam splitter prism; the S-polarized reflected light is reflected into the 50:50 transmissive-reflection mirror, half of the power of the light is transmitted, and the other half of the power of the light is reflected into the atomic cell as pump light; the light passing through the atomic cell is reflected by the first mirror back to the atomic cell as detection light; the detection light and the pump light intersect to generate saturated absorption spectrum, and the detection light carrying spectral information is detected by the first photodetector after transmitting through the 50:50 transmissive-reflection mirror.

[0010] The signal generator applies a sweep signal to the laser, and the frequency of the signal is The modulated signal is transmitted to the phase-locked loop, the phase-locked loop generates a demodulation signal with the same frequency and adjustable phase, and then is sent to the second frequency discriminator phase detector to be mixed with the electrical signal detected by the second photodetector and demodulate an error signal; the error signal is transmitted to the second PI controller through the second low-pass filter and generates a feedback control signal; finally, the control signal is transmitted to the piezoelectric ceramic controller of the optical resonant cavity to realize the cavity length locking of the optical resonant cavity.

[0011] Further, the optical resonant cavity length locking part comprises a second mirror, a third mirror, a convex lens, an optical resonant cavity, a second photodetector, a phase-locked loop, a second frequency discriminator phase 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 mirror and the third mirror to the convex lens, is focused by the lens and is coupled into the optical resonant cavity, and is detected by the second photodetector after being transmitted through the optical resonant cavity. The modulated signal generated by the signal generator is transmitted to the phase-locked loop, the phase-locked loop generates a demodulation signal with the same frequency and adjustable phase, and then is sent to the second frequency discriminator phase detector to be mixed with the electrical signal detected by the second photodetector and demodulate an error signal; the error signal is transmitted to the second PI controller through the second low-pass filter and generates a feedback control signal; finally, the control signal is transmitted to the piezoelectric ceramic controller of the optical resonant cavity to realize the cavity length locking of the optical resonant cavity.

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

[0013] Step one, generating a saturated absorption spectrum by laser through a laser frequency locking optical path;

[0014] Step two, locking the laser frequency at a saturated absorption peak point, i.e., an atomic resonance point;

[0015] Step three, then coupling the laser containing a modulated component into an optical resonant cavity and collecting a transmission spectrum;

[0016] Step four, taking the modulated signal in the laser frequency stabilization loop as a reference signal based on the phase-locked loop, phase-locked to generate a demodulation signal with the same frequency and adjustable phase, which is used to demodulate an error signal for locking the cavity length of the optical resonant cavity, and the cavity length of the optical resonant cavity is locked through the error signal.

[0017] Further, the step one includes: the laser emitted by the laser passes through the optical isolator, and is divided into two parts by the first half-wave plate and the polarization beam splitter prism, which are S polarization state reflected light and P polarization state transmitted light respectively, the S polarization state reflected light is reflected into the 50:50 transmissive mirror, half of the power of the light is transmitted, and the other half of the power of the light is reflected into the atomic cell as pump light, the light passing through the atomic cell is reflected by the first mirror back to the atomic cell as detection light, the detection light and the pump light intersect to generate saturated absorption spectrum, and the detection light carrying spectral information is detected by the first photodetector after passing through the 50:50 transmissive mirror.

[0018] Further, the step two includes: the signal generator applies a sweep signal and a modulation signal with a frequency of to the laser 1, simultaneously generates a demodulation signal with the same frequency and inputs the demodulation signal into the first frequency discriminator, the laser detected by the first photodetector is converted into an electrical signal carrying modulation information and spectral information, and is output to the first frequency discriminator to be mixed with the demodulation signal and demodulate an error signal, the error signal is transmitted to the first PI controller through the first low-pass filter and generates a feedback control signal, and finally the control signal is transmitted to the frequency tuning port of the laser to realize laser frequency locking.

[0019] Further, the step three includes: the P polarization state transmitted light of the polarization beam splitter prism is reflected to the convex lens by the second mirror and the third mirror, is coupled into the optical resonant cavity after focusing by the lens, and is detected by the second photodetector after passing through the optical resonant cavity.

[0020] Further, the step four includes: the modulation signal with a frequency of generated by the signal generator is transmitted to the phase-locked loop, the phase-locked loop generates a demodulation signal with the same frequency and adjustable phase, and then sends the demodulation signal to the second frequency discriminator to be mixed with the electrical signal detected by the second photodetector and demodulate an error signal, the error signal is transmitted to the second PI controller through the second low-pass filter and generates a feedback control signal, and finally the control signal is transmitted to the piezoelectric ceramic controller of the optical resonant cavity to realize cavity length locking of the optical resonant cavity.

[0021] Further, in the step three, the 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 the step two, and after entering the optical resonant cavity, multi-beam interference occurs, and the transmitted light carries the optical resonant cavity spectral information.

[0022] Further, in the step four, the power divider is used to transmit the modulation signal with a frequency of The modulation signal is divided into two paths, one of which is transmitted to the laser for modulation, and the other is transmitted to the phase-locked loop; the phase-locked loop comprises a phase detector, a loop filter and a voltage-controlled oscillator; the pure digital phase-locked loop is built based on FPGA, and the frequency generated by the signal generator is The modulation signal is converted into a digital signal by the power divider and sent to the analog-to-digital converter as a reference signal, and the frequency generated by the voltage-controlled oscillator is set to be approximately The demodulation signal is transmitted to the phase detector, and the two signals 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 to generate a control signal 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 output by the digital-to-analog converter and output to the second frequency discriminator.

[0023] Compared with the prior art, the present application has the following beneficial effects:

[0024] The present application uses the modulation signal in the laser frequency control loop as a reference signal, uses the phase-locked loop to lock the demodulation signal with the same frequency and adjustable phase, mixes the demodulation signal in the frequency discriminator to demodulate the error signal, applies feedback control to the PZT of the optical resonant cavity through the PI controller, and thereby completes the cavity length locking of the resonant cavity. The present application does not need to use external optical modulation devices, which greatly reduces the cost, and secondly, the method is not limited by the modulation bandwidth, can demodulate the error signal with a low frequency discrimination slope, thereby reducing the loop gain and increasing the frequency discrimination range, and reducing the influence of the control system on the precision of the optical resonant cavity.

[0025] Specifically, the device of the present application adopts a double-modulation structure composed of a plano-convex lens, a Faraday rotator, a mirror and a polarization beam splitter prism. The structure sets the single-modulation first-order diffraction light exit point of the acousto-optic frequency shifter at the focal point position of the plano-convex lens, so that the exit light is parallel to the optical axis, and the reflected light is still incident on the plano-convex lens according to the original light path and passes through the focal point into the acousto-optic frequency shifter, so that the second-order modulation first-order diffraction light can coincide with the original laser light path, and therefore the propagation angle of the second-order modulation first-order diffraction light can not be affected by the frequency shift of the acousto-optic frequency shifter, avoiding the problem of fluctuation of the beat signal amplitude caused by the change of the modulation light angle. In addition, the structure can make the second-order modulation light always maintain an orthogonal polarization state with the original laser and the single-modulation light, avoiding the problem of fluctuation of the beat signal amplitude caused by the fluctuation of the polarization state. BRIEF DESCRIPTION OF DRAWINGS

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

[0027] Figure 2 It is Figure 1A schematic diagram of the phase-locked loop in the diagram.

[0028] The reference numerals in the attached figures are as follows: 1 is a laser, 2 is an optical isolator, 3 is a first half-wave plate, 4 is a polarizing beam splitter, 5 is a second reflecting mirror, 6 is a third reflecting mirror, 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 transmission mirror, 13 is an atomic gas cell, 14 is a first reflecting mirror, 15 is a first frequency and phase 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 and phase detector, 21 is a second low-pass filter, and 22 is a second PI controller. Detailed Implementation

[0029] The embodiments of the present invention are described in detail below: These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. The present invention will be further described below with reference to the accompanying drawings and specific implementation examples.

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

[0031] The laser frequency locking section includes a laser 1, an optical isolator 2, a first half-wave plate 3, a polarizing beam splitter 4, a 50:50 transmission mirror 12, an atomic gas cell 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 section 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 and phase detector 20, a second low-pass filter 21, and a second PI controller 22.

[0033] The laser emitted by the laser 1 passes through the optical isolator 2, and is divided into two parts by the first half-wave plate 3 and the polarization beam splitter prism 4, which are the reflected light of S polarization state and the transmitted light of P polarization state respectively, the reflected light of S polarization state is reflected into the 50:50 transmissive mirror 12, half of the power of the light is transmitted, and the other half of the power of the light is reflected into the atomic cell 13 to serve as the pumping light, the light passing through the atomic cell 13 is reflected by the first mirror 14 back to the atomic cell 13 to serve as the detection light, the detection light and the pumping light intersect to generate a saturated absorption spectrum, and the detection light carrying the spectral information is detected by the first photodetector 11 after passing through the 50:50 transmissive mirror 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, simultaneously generates a demodulation signal with a frequency of 30 kHz and inputs the demodulation signal into the first frequency discriminator 15, the laser detected by the first photodetector 11 is converted into an electric signal carrying modulation information and spectral information, and is output to the first frequency discriminator 15 to be mixed with the demodulation signal and demodulate an error signal, the error signal is transmitted to the first PI controller 17 after passing through the first low-pass filter 16 and generates a feedback control signal, and finally the control signal is transmitted to the frequency tuning port of the laser 1 to realize laser frequency locking;

[0035] The P polarization state transmitted light of the polarization beam splitter prism 4 is reflected by the second mirror 5 and the third mirror 6 to the convex lens 7, is focused and coupled into the optical resonant cavity 8 after passing through the lens, 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, the phase-locked loop 19 generates a demodulation signal with the same frequency and adjustable phase, and then sends the demodulation signal to the second frequency discriminator 20 to be mixed with the electric signal detected by the second photodetector 10 and demodulate an error signal, the error signal is transmitted to the second PI controller 22 after passing through the second low-pass filter 21 and generates a feedback control signal, and finally the control signal is transmitted to the piezoelectric ceramic controller 9 of the optical resonant cavity 8 to realize the cavity length locking of the optical resonant cavity.

[0037] Therefore, the cavity length locking method of the low-cost optical resonant cavity based on the phase-locked loop in the embodiment of the application comprises the following steps:

[0038] Step one, generating a saturated absorption spectrum by laser through a laser frequency locking optical path;

[0039] Step two, locking the laser frequency at the saturated absorption peak point, i.e. the atomic resonance point;

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

[0041] Step four, based on the phase-locked loop, the modulation signal in the laser frequency stabilization loop as the reference signal, phase-locked out a frequency same and phase adjustable demodulation signal, for demodulating the error signal of the length of the optical resonator, through the error signal can lock the length of the optical resonator.

[0042] Specifically, the step one includes: the laser emitted by the laser 1 passes through the optical isolator 2, and is divided into two parts through the first half-wave plate 3 and the polarization beam splitter prism 4, which are respectively the reflected light of S polarization state and the transmitted light of P polarization state, the reflected light of S polarization state is reflected into the 50:50 transmissive mirror 12, half of the power of the light is transmitted, and the other half of the power of the light is reflected into the atomic cell 13 as the pumping light, the light passing through the atomic cell 13 is reflected by the first mirror 14 back to the atomic cell 13 as the detection light, the detection light intersects with the pumping light to generate a saturated absorption spectrum, and the detection light carrying the spectrum information is detected by the first photodetector 11 after passing through the 50:50 transmissive mirror 12.

[0043] Specifically, the step two 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, simultaneously generates a demodulation signal with a frequency of 30 kHz and inputs the demodulation signal into the first frequency discriminator 15, the laser carrying the modulation information and the spectrum information detected by the first photodetector 11 is converted into an electrical signal and output to the first frequency discriminator 15, the electrical signal is mixed with the demodulation signal and demodulates an error signal, the error signal is transmitted to the first PI controller 17 after passing through the first low-pass filter 16 and generates a feedback control signal, and finally the control signal is transmitted to the frequency tuning port of the laser 1 to realize the frequency locking of the laser;

[0044] Specifically, the step three includes: the P polarization state transmitted light of the polarization beam splitter prism 4 is reflected to the convex lens 7 by the second mirror 5 and the third mirror 6, is coupled into the optical resonator 8 after focusing by the lens, and is detected by the second photodetector 10 after passing through the optical resonator 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 demodulation signal with the same frequency and adjustable phase, and then sends the demodulation signal to the second frequency discriminator 20 to mix with the electrical signal detected by the second photodetector 10 and demodulate an error signal, the error signal is transmitted to the second PI controller 22 after passing through the second low-pass filter 21 and generates a feedback control signal, and finally the control signal is transmitted to the piezoelectric ceramic controller 9 of the optical resonator 8 to realize the length locking of the optical resonator.

[0046] Further, in the third step, a piezoelectric ceramic controller 9 is attached to one of the mirrors of the optical resonant cavity 8, and the 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 modulated information as in the second step, and 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 the fourth step, a power divider is used to divide the modulated signal with a frequency of 30 kHz generated by the signal generator 18 into two paths, one of which is transmitted to the laser 1 for modulation, and the other of which is transmitted to the phase-locked loop 19.

[0048] It is worth noting that the power divider will evenly distribute the power of the modulated signal, so the amplitude of the modulated signal generated by the signal generator 18 should be 40 mV. As shown in Figure 2 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 based on FPGA. The modulated 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 and converted into a digital signal, which is sent to the phase detector as a reference signal. The voltage-controlled oscillator generates a demodulation signal with a frequency of approximately 30 kHz and transmits it to the phase detector. The two signals are mixed in the phase detector to produce an error signal proportional to the difference in frequency and phase between the two signals. The error signal is sent to the loop filter for amplification and integration to generate a control signal that is fed back to the voltage-controlled oscillator, thereby achieving the effect of tracking the frequency and phase of the modulated signal in the laser frequency stabilization loop. Finally, the demodulation signal generated by the phase-locked loop is converted into an analog signal by the digital-to-analog converter and output to the second frequency and phase detector 20.

[0050] In the fourth step, the method of optical phase-locked reference modulation signal phase-locked in the present application is:

[0051] Let the modulated signal in the laser frequency stabilization loop be:

[0052] ;

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

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

[0055] ;

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

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

[0058] ;

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

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

[0061] The contents not described in detail in the present invention belong to the prior art known to those skilled in the art.

Claims

1. A low cost optical resonator cavity length locking device based on a phase-locked loop, characterized in that, The laser frequency locking part is used for locking the laser frequency to the atomic resonance point, and the optical resonant cavity length locking part locks the cavity length of the optical resonant cavity by taking the frequency of the frequency-stabilized laser as a frequency reference. The laser frequency locking part comprises a laser, an optical isolator, a first half-wave plate, a polarization beam splitter prism, a 50:50 transmissive-reflection mirror, an atomic cell, a first mirror, a first photodetector, a signal generator, a first frequency discriminator, a first low-pass filter and a first PI controller. The laser emitted by the laser passes through the optical isolator, and is then split into two parts by the first half-wave plate and the polarization beam splitter prism, i.e. S-polarized reflected light and P-polarized transmitted light. The S-polarized reflected light is reflected into the 50:50 transmissive-reflection mirror, half of the power of the light is transmitted, and the other half of the power of the light is reflected into the atomic cell as pump light. The light passing through the atomic cell is reflected by the first mirror back to the atomic cell as detection light. The detection light and the pump light intersect to generate saturated absorption spectrum. The detection light carrying spectral information is transmitted through the 50:50 transmissive-reflection mirror and is detected by the first photodetector. The signal generator applies a frequency sweeping signal and a modulation signal with a frequency of w to the laser, and simultaneously generates a demodulation signal with the same frequency as the modulation signal and inputs the demodulation signal into the first frequency discriminator. The laser detected by the first photodetector carries modulation information and spectral information, and is converted into an electrical signal. The electrical signal is output to the first frequency discriminator, mixed with the demodulation signal and demodulated to obtain an error signal. The error signal is transmitted to the first PI controller through the first low-pass filter and a feedback control signal is generated. Finally, the control signal is transmitted to the laser frequency tuning port to realize laser frequency locking. The optical resonant cavity length locking part comprises a second mirror, a third mirror, a convex lens, an optical resonant cavity, a second photodetector, a phase-locked loop, a second frequency discriminator, a second low-pass filter and a second PI controller. The P-polarized transmitted light of the polarization beam splitter prism is reflected by the second mirror and the third mirror to the convex lens, is focused by the lens and is coupled into the optical resonant cavity. The light transmitted through the optical resonant cavity is detected by the second photodetector. The modulation signal with a frequency of w generated by the signal generator is transmitted to the phase-locked loop. The phase-locked loop generates a demodulation signal with the same frequency and adjustable phase, and then inputs the demodulation signal into the second frequency discriminator. The demodulation signal is mixed with the electrical signal detected by the second photodetector, and an error signal is demodulated. The error signal is transmitted to the second PI controller through the second low-pass filter and a feedback control signal is generated. Finally, the control signal is transmitted to the piezoelectric ceramic controller of the optical resonant cavity to realize the locking of the cavity length of the optical resonant cavity.

2. The cavity length locking method of the low-cost optical resonator cavity length locking device based on a phase-locked loop according to claim 1, characterized in that, The method comprises the following steps: Step one, generating saturated absorption spectrum by laser through the laser frequency locking optical path; Step two, locking the laser frequency at the saturated absorption peak point, i.e. the atomic resonance point; Step three, then coupling the laser containing modulation components into the optical resonant cavity and collecting the transmission spectrum; Step four, taking the modulation signal in the laser frequency stabilization loop as a reference signal based on the phase-locked loop, phase-locked to obtain 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 is locked through the error signal.

3. The cavity length locking method of claim 2, wherein, The step one includes: 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 prism, which are S polarization state reflected light and P polarization state transmitted light respectively, the S polarization state reflected light is reflected into the 50:50 transmission mirror, half of the power light is transmitted, and half of the power light is reflected into the atomic cell as the pump light, the light passing through the atomic cell is reflected by the first mirror back to the atomic cell as the detection light, the detection light intersects with the pump light to generate a saturated absorption spectrum, and the detection light carrying the spectrum information is detected by the first photodetector after passing through the 50:50 transmission mirror.

4. The cavity length locking method of claim 3, wherein, The step two includes: the signal generator applies a sweep signal and a modulation signal with a frequency of w to the laser 1, simultaneously generates a demodulation signal with the same frequency and inputs the demodulation signal into the first frequency discriminator, the laser detected by the first photodetector is converted into an electrical signal carrying modulation information and spectrum information, and the electrical signal is output to the first frequency discriminator and mixed with the demodulation signal to demodulate an error signal, the error signal is transmitted to the first PI controller through the first low-pass filter and generates a feedback control signal, and finally the control signal is transmitted to the frequency tuning port of the laser to realize laser frequency locking.

5. The cavity length locking method of claim 4, wherein, The step three includes: the P polarization state transmitted light of the polarization beam splitter prism is reflected to the convex lens by the second mirror and the third mirror, is coupled into the optical resonant cavity after focusing by the lens, and is detected by the second photodetector after passing through the optical resonant cavity.

6. The cavity length locking method of claim 5, wherein, The step four includes: transmitting the modulation signal with a frequency of w generated by the signal generator to the phase-locked loop, generating a demodulation signal with the same frequency and adjustable phase by the phase-locked loop, then sending the demodulation signal to the second frequency discriminator, mixing the electrical signal detected by the second photodetector with the demodulation signal to demodulate an error signal, transmitting the error signal to the second PI controller through the second low-pass filter to generate a feedback control signal, and finally transmitting the control signal to the piezoelectric ceramic controller of the optical resonant cavity to realize the cavity length locking of the optical resonant cavity.

7. The cavity length locking method of claim 2, wherein, In the step three, the 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 spectrum information of the optical resonant cavity.

8. The cavity length locking method of claim 2, wherein, In the fourth step, the modulated signal with frequency w generated by the signal generator is divided into two paths by a power divider, one path is transmitted to the laser for modulation, and the other path is transmitted to the phase-locked loop; the phase-locked loop comprises a phase detector, a loop filter and a voltage-controlled oscillator, and is a pure digital phase-locked loop based on FPGA; the modulated signal with frequency w generated by the signal generator is converted into a digital signal by an analog-to-digital converter after passing through the power divider and is sent to the phase detector as a reference signal; the voltage-controlled oscillator generates a demodulated signal with frequency approximately equal to w and transmits the demodulated signal to the phase detector; the two signals are mixed in the phase detector to generate an error signal proportional to the frequency difference and the phase difference of the two signals; the error signal is sent to the loop filter for amplification and integration to generate a control signal which is fed back to the voltage-controlled oscillator, thereby achieving the effect of frequency and phase tracking of the demodulated signal of the phase-locked loop relative to the modulated signal in the laser frequency stabilization loop; finally, the demodulated signal generated by the phase-locked loop is converted into an analog output by a digital-to-analog converter and is output to the second frequency and phase detector.

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