A dual-modulation-depth large-dynamic-range high-sensitivity PDH frequency stabilization method

Abstract

The application discloses a kind of dual-modulation depth's large dynamic range, high sensitivity PDH frequency stabilization method.Laser frequency is far from the resonant frequency of optical resonator, interference signal is obtained after orthogonal downmixing and arctangent operation initial phase difference, realize PDH demodulation reference signal phase automatic matching;Then PDH error signal is divided by transmission power square to obtain a new error signal with larger linear dynamic range;Finally, respectively using the new error signal with large dynamic range under large modulation depth and the error signal with high sensitivity under small modulation depth as the feedback signal of PDH capture initial lock and frequency stabilization precision lock, realize the precise locking of laser frequency / resonator cavity length.The method of the application not only expands the linear dynamic range of PDH frequency stabilization, reduces the scanning and capture time of laser frequency / resonator cavity length, but also improves the anti-interference ability of the system after laser frequency stabilization / optical resonator cavity length locking, and can be widely applied in the fields of laser frequency stabilization and resonator cavity length locking.

Description

Technical Field

[0001] This invention belongs to the field of laser frequency stabilization and optical resonator length locking technology, specifically involving a high dynamic range and high sensitivity PDH frequency stabilization method with dual modulation depths. Background Technology

[0002] Pound-Drever-Hall (PDH) technology is one of the most commonly used methods for active laser frequency stabilization or optical resonator length locking. By employing radio frequency electro-optic phase modulation and optical resonator heterodyne spectroscopy, a frequency discrimination error signal is obtained indicating that the laser frequency deviates from the resonant frequency of the optical resonator. When the optical resonator is used as a reference, the error signal is used to feedback control and adjust the laser's cavity length / current, stabilizing the laser frequency at the resonant frequency. Conversely, when the laser frequency is used as a reference, the error signal is used to feedback control and lock the cavity length. The linear dynamic range and sensitivity of the error signal are the main indicators affecting the stability and accuracy of laser frequency stabilization / cavity length locking. The error signal is typically obtained at a modulation depth of approximately 1.08 rad, using the 0th and ±1st order sideband beat frequencies generated by electro-optic phase modulation of the laser. Its linear dynamic range is relatively small, making it difficult to achieve large-scale, rapid locking of the laser frequency / cavity length. Even when locked, large fluctuations in ambient temperature or environmental vibrations can easily lead to loss of laser frequency / cavity length lock. To address the aforementioned issues, current approaches primarily focus on expanding the system's control range through nonlinear control methods or by acquiring signals with a wider linear range. Nonlinear control methods include linear quadratic Gaussian control (LQG) and time-varying Kalman filtering, which extend the dynamic range of PDH technology. However, these algorithms are quite complex. Alternatively, methods such as near-Q phase demodulation, combining in-phase demodulated signals modulated by multiple odd frequencies, and normalizing the transmitted power signal can be used to obtain signals with a larger linear dynamic range, but these methods reduce the sensitivity of PDH technology. Summary of the Invention

[0003] To address the problems existing in the background technology, this invention discloses a high dynamic range and high sensitivity PDH frequency stabilization method with dual modulation depths. This invention not only expands the linear dynamic range of PDH frequency stabilization and reduces the scanning and acquisition time of laser frequency / resonant cavity length, but also improves the system's anti-interference capability after laser frequency stabilization / optical resonant cavity length locking. It can be widely applied in fields such as laser frequency stabilization and resonant cavity length locking.

[0004] The technical solution adopted by the present invention to achieve the above objectives includes the following steps:

[0005] I. A high dynamic range, high sensitivity PDH frequency stabilization device with dual modulation depths:

[0006] Includes a laser, an optical isolator, an electro-optic phase modulator, a first photodetector, a polarizing beam splitter, a quarter-wave plate, an optical resonator, and a second photodetector;

[0007] The laser emitted by the laser enters the optical isolator. The laser exiting the optical isolator enters the electro-optic phase modulator, is modulated, and then incident on the polarization beam splitter, where it is transmitted. The laser transmitted from the polarization beam splitter is p-polarized light and passes through a quarter-wave plate, where it is reflected and transmitted to the optical resonant cavity. The laser reflected from the optical resonant cavity returns along the same path, passes through the quarter-wave plate, and is converted from p-polarized to s-polarized. It then returns to the polarization beam splitter and is reflected. The laser reflected from the polarization beam splitter is received by the first photodetector, and the laser transmitted from the optical resonant cavity is received by the second photodetector.

[0008] It also includes a signal acquisition and processing module. The output terminals of the first photodetector and the second photodetector are connected to the input terminal of the signal acquisition and processing module. The output terminal of the signal acquisition and processing module is connected to an electro-optic phase modulator. At the same time, the output terminal of the signal acquisition and processing module is connected to a laser or an optical resonant cavity.

[0009] The signal acquisition and processing module specifically includes a digital frequency synthesizer, a phase shifter, a first multiplier, a first low-pass filter, an arctangent operation module, a gain module, a triangular wave scanning module, a second multiplier, a squaring operation module, a second low-pass filter, a first threshold judgment module, a division operation module, a first data selector, a PID control module, a second threshold judgment module, and a second data selector.

[0010] One output of the digital frequency synthesizer is connected to the input of the gain module. The output of the gain module is connected to the drive input of the electro-optic phase modulator. The other two quadrature outputs of the digital frequency synthesizer are connected to the two inputs of the phase shifter. The two outputs of the phase shifter are connected to the inputs of the first multiplier and the second multiplier, respectively. The first photodetector is connected to the inputs of the first multiplier and the second multiplier. The outputs of the first multiplier and the second multiplier are connected to the inputs of the first low-pass filter and the second low-pass filter, respectively. The output of the first low-pass filter is connected to one input of the arctangent operation module. The output of the second low-pass filter is divided into three paths and connected to the inputs of the arctangent operation module, the division operation module, and the first data selector, respectively.

[0011] The output of the second photodetector is divided into three paths and connected to the inputs of the squaring module, the first threshold judgment module, and the second threshold judgment module, respectively. The output of the squaring module is connected to another input of the division module. The output of the division module is connected to another input of the first data selector and the second threshold judgment module, respectively. The output of the first threshold judgment module is connected to the selection control terminal of the first data selector. The output of the first data selector is connected to the input of the PID control module. The outputs of the PID control module and the triangular wave scanning module are connected to the two data inputs of the second data selector, respectively. The output of the second threshold judgment module is connected to the selection control terminal of the second data selector. The output of the second data selector is connected to the optical resonant cavity after digital-to-analog conversion.

[0012] When the device is used to stabilize the laser frequency emitted by a laser, the laser is a laser with adjustable laser frequency, and the optical resonant cavity is an optical resonant cavity with fixed cavity length.

[0013] When the device is used to lock the cavity length of the optical resonator, the laser is a laser with a laser frequency, and the optical resonator is an optical resonator with an adjustable cavity length.

[0014] II. A high dynamic range and high sensitivity PDH frequency stabilization method with dual modulation depth, the method comprising the following steps:

[0015] Step 1) The laser emitted by the laser enters the electro-optic phase modulator after passing through the optical isolator. The electro-optic phase modulator generates a phase modulation signal to modulate the phase of the laser. The phase-modulated laser then passes sequentially through a polarizing beam splitter and a quarter-wave plate before entering the optical resonant cavity, where it undergoes multiple reflections and transmissions. The laser reflected from the optical resonant cavity passes back through the quarter-wave plate, where its polarization state changes from p-state to s-state. It is then reflected by the polarizing beam splitter to the first photodetector, where multi-beam interference occurs. The interference signal is detected, sampled, and converted from analog to digital to obtain the reflected multi-beam interference signal I. r1 (t); where the sampling frequency for sampling the interference signal is higher than the laser phase modulation frequency ω of the electro-optic phase modulator. m 2 times.

[0016] The reflected multibeam interference signal I r1 (t) represents the following:

[0017]

[0018]

[0019] Among them, I r1(t) represents the interference signal detected by the photodetector at the reflecting end of the optical resonant cavity at time t, and j represents the highest order of the laser frequency sideband generated after laser phase modulation, that is, the highest order of the laser frequency sideband (or sideband) generated by the laser phase modulation of the electro-optic phase modulator is j; DC terms and p·ω m terms represent the DC component and ω of the interference signal, respectively. m The p-th harmonic component, and the remaining terms are ω. m The first harmonic component; E0 represents the laser amplitude, ω c ω represents the laser fundamental frequency, l represents the cavity length of the optical resonant cavity, and ω represents the fundamental frequency of the laser. m J represents the phase modulation frequency, k represents the order of the modulation sideband, and k = 0 to j-1; k (β) represents the k-th order Bessel function of the first kind at a modulation depth of β; Re{} and Im{} represent the real and imaginary parts of the complex number, respectively; F[ω,l] represents the reflection coefficient of the optical resonator, which is a function of the incident laser frequency ω and the cavity length l; F*[ω,l] represents the conjugate of the reflection coefficient of the optical resonator, n is the refractive index of the air inside the cavity, and i is the imaginary unit. r represents the reflectivity of the two cavity mirrors of the optical resonator, and FSR represents the free spectral range of the optical resonator.

[0020] Step 2) The digital frequency synthesizer generates two quadrature reference signals with the same frequency as the phase modulation signal, including a cosine reference signal. and sinusoidal reference signal After the two orthogonal reference signals are shifted to the same phase by a phase shifter, they are respectively interfered with the reflected multi-beam interference signal I. r1 (t) The components are sequentially multiplied by two separate multipliers (903, 908) and filtered by low-pass filters (904, 910) to complete the quadrature downmixing operation, retaining the DC component, and obtaining a pair of quadrature harmonic amplitude signals, which include a sinusoidal component S. I S and cosine component Q ;

[0021] The orthogonal harmonic amplitude signals, simplified to single trigonometric function form, are expressed as follows:

[0022]

[0023] R(k)=F[ω c +kω m ,l]F*[ω c +(k+1)ω m ,l]-F*[ω c -kω m ,l]F[ω c -(k+1)ω m ,l]

[0024] Among them, S I S Q Let LPF[] represent the sine and cosine components in the quadrature harmonic amplitude signal, respectively. Let LPF[] represent the low-pass filtering operation, and R(k) represent the ω generated by the beat frequencies of the k-th and (k+1)-th sidebands and the (-k)-th and (-(k+1))-th sidebands, respectively. m The sum of the amplitudes of the first harmonic components is called the amplitude coefficient of the k-th order, and |R(k)| is the modulus of the amplitude coefficient R(k). Let R(k) be the phase angle of the k-th order amplitude coefficient. Let ω be the initial phase difference between the first harmonic component of the interference signal and the sinusoidal reference signal. m J represents the phase modulation frequency. k (β) is the k-th order Bessel function of the first kind when the modulation depth is β, k represents the order of the modulation sideband, k = 0 to j-1, j represents the highest order of the sideband after laser modulation, and t represents the time.

[0025] Step 3) Perform arctangent operation on a pair of orthogonal harmonic amplitude signals in the arctangent operation module to obtain the phase. The calculation is as follows:

[0026]

[0027] Adjusting the laser frequency or the cavity length of the optical resonator ensures that the frequency intervals between the laser's fundamental frequency and its sideband frequencies and the optical resonator's resonant frequency are both greater than twice the full width at half maximum (FWHM) of the optical resonator. The phase obtained under these conditions... As the initial phase difference

[0028] Specifically, when the frequency intervals between the laser's fundamental frequency and its sideband frequencies and the optical resonant cavity's resonant frequency are both greater than twice the cavity's full width at half maximum (FWHM), both the fundamental laser and its modulated sideband lasers are totally reflected, meaning the reflection coefficients at the corresponding frequencies are all pure real numbers. Therefore, the phase of the amplitude coefficient of any order... k = 0 to j-1, where j represents the highest order of the sideband after laser modulation. The initial phase difference of the sinusoidal reference signal is then calculated using the first harmonic component of the interference signal according to the following formula.

[0029]

[0030] Then use the obtained initial phase difference The feedback signal is sent to the phase shifter to shift the phase of the two orthogonal reference signals, thereby realizing the interference signal I. r1 Automatic matching of the first harmonic component of (t) with the phase of the sinusoidal reference signal ensures the linearity and high sensitivity of the error signal within the frequency discrimination range;

[0031] Step 4) Extract the sinusoidal component S from the quadrature harmonic amplitude signal. I After phase matching is completed, it serves as the error signal S after phase matching. PDH The expression is:

[0032]

[0033] Where k represents the order of the modulation sideband, k = 0 to j-1, j represents the highest order of the laser frequency sideband generated after laser phase modulation, that is, the highest order of the laser frequency sideband (referred to as sideband) generated by laser phase modulation by the electro-optic phase modulator is j; E0 represents the laser amplitude, J k (β) is the first-order Bessel function of the kind at modulation depth β; R(k) represents the ω generated by the beat frequencies of the k-th and (k+1)-th order sidebands, and the (-k)-th and (-(k+1))-th order sidebands, respectively. m The sum of the amplitudes of the first harmonic components is called the amplitude coefficient of the k-th order, and |R(k)| is the modulus of the amplitude coefficient R(k). Let R(k) be the phase angle of the k-th order amplitude coefficient, and t represent time. This represents the phase difference between the first harmonic component of the interference signal and the sinusoidal reference signal after matching. When the laser phase modulation frequency is lower than the full width at half maximum (FWHM) of the optical resonator, the phase difference after matching is 0°; when the laser phase modulation frequency is higher than the FWHM of the optical resonator, the phase difference after matching is 90°.

[0034] Step 5) The light transmitted from the optical resonant cavity is received and detected by the second photodetector to obtain the DC power signal. The DC power signal is sampled and converted from analog to digital to obtain the transmitted power signal P. tran The expression is:

[0035]

[0036]

[0037] Among them, P tran The transmitted power signal is represented by l, the cavity length of the optical resonant cavity is l, and ω is ω. m J represents the phase modulation frequency, |k| represents the absolute value of k, and J represents the phase modulation frequency. |k| (β) represents the k-th or -k-th order Bessel function of the first kind when the modulation depth is β, j represents the highest order of the sideband after laser modulation; T(ω,l) represents the transmission coefficient of the optical resonator, which is a function related to the incident laser frequency ω and the cavity length l; T*(ω,l) represents the conjugate of the transmission coefficient of the optical resonator, n is the refractive index of the air inside the cavity, i is the imaginary unit, r represents the reflectivity of the two cavity mirrors of the optical resonator, and FSR represents the free spectral range of the optical resonator;

[0038] Then, the error signal S after phase matching is... PDH With the square of the transmitted power signal P tran 2 A new error signal S is obtained by performing a division operation in the division module. new The calculation formula is as follows:

[0039]

[0040] Step 6) At this point, the error signal S after phase matching has been obtained in real time. PDH New error signal S new and transmitted power signal P tran Then, the cavity length of the optical resonator / laser frequency will be automatically locked using these three real-time signals. This mainly includes the scanning, acquisition and preliminary locking stages as well as the precise locking stage, in order to achieve frequency stabilization control.

[0041] Step 6) specifically refers to:

[0042] Step 6.1) Scanning, capturing, and initial locking:

[0043] First, the signal acquisition and processing module controls and adjusts the amplitude of the drive signal of the electro-optic phase modulator to achieve laser phase modulation with a large modulation depth.

[0044] Then, the output of the triangular wave scanning module is connected to the laser or optical resonant cavity via the control of the second data selector. The triangular wave scanning module emits a triangular wave scanning signal to perform a triangular wave pre-scan on the laser frequency emitted by the laser and the cavity length of the optical resonant cavity, thereby obtaining a new error signal S. new The maximum value S newmax and transmitted power signal P tran transmission peak P tranmax and with a new error signal S new The maximum value S newmax Three-quarters as the locking capture threshold S th ;

[0045] Next, the laser frequency and cavity length of the optical resonator emitted by the laser are locked within a specific range. Under the control of the triangular wave scanning signal, the output of the division module is connected to the PID control module via the control of the first data selector. The PID control module then inputs a new error signal S. new The following judgments and actions will be taken:

[0046] When the new error signal S new The value did not reach the maximum value S newmax Or not from the maximum value S newmaxDrops below the lock capture threshold S th If the laser frequency emitted by the laser is not processed, then the triangular wave pre-scan of the optical resonant cavity length is maintained without processing.

[0047] When the new error signal S new The value from the maximum value S newmax Drops below the lock capture threshold S th At that time, the output of the PID control module is connected to the laser or optical resonant cavity through the control of the second data selector, switching from the triangular wave scanning signal to an S-wave scanning signal. new The input signal, processed by the PID control module, is the laser frequency emitted by the laser and the cavity length of the optical resonator. The PID control module then uses this ratio to provide feedback control, resulting in a new error signal S. new Fluctuations around zero indicate an initial lock-in.

[0048] Step 6.2) Precise locking:

[0049] The output of the second low-pass filter is connected to the PID control module by the control of the first data selector. The PID control module receives the phase-matched error signal S as input. PDH The input to the PID control module will be changed from the error signal S. new Error signal S after switching to phase matching PDH Simultaneously, by adjusting the amplitude of the drive signal of the electro-optic phase modulator, the phase modulation depth is reduced to a small modulation depth, retaining only the 0th and ±1st order sideband components. Furthermore, through the control of the second data selector, the output of the PID control module remains connected to the laser or optical resonator. The PID control module uses feedback control of the laser frequency emitted by the laser and the cavity length of the optical resonator to ensure that the phase-matched error signal S... PDH Fluctuating around zero allows for precise locking.

[0050] In step 6.1), when using the PID control module for feedback control, the final transmitted power signal P tran The value will be in the transmission peak P tranmax Based on the surrounding fluctuations, the following judgments should be made:

[0051] When the transmitted power signal P tran The fluctuation over a fixed period of time is less than the transmission peak value P. tranmax When one-third of the laser frequency is reached, the ratio of the laser frequency emitted by the laser to the cavity length of the optical resonator is initially locked, allowing for the next step of precise locking.

[0052] When the transmitted power signal P tranThe fluctuation over a fixed period of time is not less than the peak transmission value P. tranmax When one-third of the value is reached, the PID parameters in the PID control module are adjusted.

[0053] In step 6.1), during the process of adjusting the amplitude of the drive signal of the electro-optic phase modulator to perform laser phase modulation with a large modulation depth, the first-order Bessel function J of the kth order (k is a positive integer) is used. k The properties of (β) are determined by the following criteria:

[0054] For a fixed modulation depth β, when k > β + 1, then J k (β)<0.1, when k<=β+1, then J k (β)>0.1, that is, J k The higher-order sideband components with orders greater than β+1 corresponding to (β) can be ignored, while the sideband components with orders greater than or equal to β+1 cannot be ignored. Under large modulation depth, the highest order of the laser frequency sideband is j≥2.

[0055] During steps 6.1) to 6.2), the following judgments and actions are performed in real time:

[0056] When the transmitted power signal P tran Greater than or equal to the transmission peak value P tranmax When the modulation depth of the electro-optic phase modulator is half of the original value, the modulation depth of the electro-optic phase modulator is small. The output of the second low-pass filter is connected to the PID control module through the control of the first data selector. The input of the PID control module is the error signal. The PID control module is used to perform feedback control on the laser frequency of the laser emitted by the laser and the cavity length of the optical resonator, so that the laser frequency of the laser emitted by the laser and the cavity length of the optical resonator are precisely locked.

[0057] When the transmitted power signal P tran Less than the transmission peak value P tranmax Half of the transmission peak value P tranmax When the value is one-tenth, the modulation depth of the electro-optic phase modulator is switched back to a large modulation depth, and the output of the division module is connected to the PID control module through the control of the first data selector. The input of the PID control module is then switched to the new error signal S. new The PID control module is used to perform feedback control on the laser frequency and cavity length of the optical resonant cavity emitted by the laser, so that the laser frequency and cavity length of the optical resonant cavity emitted by the laser are relocked.

[0058] If external interference is very strong, thus affecting the transmitted power signal P tran The value is less than P tranmaxWhen the output of the triangular wave scanning module is connected to the laser or optical resonant cavity through the control of the second data selector, the output of the triangular wave scanning module is connected to the laser or optical resonant cavity. The triangular wave scanning module sends a triangular wave scanning signal to control the laser frequency of the laser emitted by the laser and the cavity length of the optical resonant cavity. That is, the control signal of the laser frequency of the laser emitted by the laser and the cavity length of the optical resonant cavity is switched to the triangular wave scanning signal again, and the process returns to step 6.1) to scan, capture and lock again.

[0059] In this invention, the error signal S PDH For processing at small modulation depths, the error signal S at small modulation depths PDH It features high sensitivity and a narrow linear dynamic range, while the new error signal S new For processing at large modulation depths, the new error signal S at large modulation depths new It features a large linear dynamic range and low sensitivity. Increasing the modulation depth of the laser phase modulation in an electro-optic phase modulator increases the linear dynamic range and decreases the sensitivity for both; conversely, decreasing the modulation depth decreases the linear dynamic range and increases the sensitivity for both.

[0060] The reflected multi-beam interference signal described in this invention originates from the interference signal detected by the photodetector at the reflecting end of the optical resonator in PDH technology, and the transmitted power signal is the DC power signal detected by the photodetector at the transmitting end of the optical resonator.

[0061] In a specific implementation, the cavity length of the optical resonant cavity, i.e. its resonant frequency, is locked as an example.

[0062] The modulation frequency of the laser phase modulation in the electro-optic phase modulator is less than the full width at half maximum (FWHM) of the optical resonator. A modulation depth β greater than or equal to 1.2 rad is considered a large modulation depth, and a modulation depth β less than 1.2 rad is considered a small modulation depth. In a specific implementation, the large modulation depth is set to β1 = 1.80 rad, and the small modulation depth is set to β2 = 1.08 rad.

[0063] This invention first performs orthogonal downmixing and arctangent operations on the obtained interference signal and a pair of orthogonal reference signals to obtain an initial phase difference. This initial phase difference is then used to perform phase shifting using a phase shifter, achieving automatic phase matching and avoiding manual adjustment of the phase shifter. Next, a new error signal is generated using the phase-matched error signal and the squared resonant cavity transmission power signal, expanding the linear dynamic range and reducing the scanning and acquisition time for the laser frequency / resonant cavity length. Finally, the new error signal with a large dynamic range at a large modulation depth and the error signal with high sensitivity at a small modulation depth are switched as inputs to the PID feedback control module. This ensures locking accuracy while improving the system's anti-interference capability after laser frequency stabilization / optical resonant cavity length locking, making it less prone to laser frequency / cavity length loss of lock.

[0064] Compared with the prior art, the beneficial effects of the present invention are:

[0065] (1) The present invention calculates the initial phase difference by performing orthogonal downmixing and arctangent operations on the reflected multi-beam interference signal and the local oscillator signal at the same frequency obtained at different modulation depths, and uses a phase shifter to achieve automatic phase matching of the two signals, which ensures the linearity and high slope of the error signal in the frequency discrimination interval and avoids the process of manually adjusting the phase shifter.

[0066] (2) The present invention obtains a new error signal with a larger linear dynamic range by simultaneously utilizing the reflection error signal and the transmission power signal. At the same time, by increasing the phase modulation depth, the linear dynamic range is expanded, and the scanning and acquisition time of the laser frequency / optical resonant cavity length during the automatic locking process is reduced.

[0067] (3) This invention achieves a large dynamic range and high sensitivity locking of laser frequency / cavity length by switching the new error signal under large modulation depth and the error signal under small modulation depth as the input of PID control module. While ensuring locking accuracy, it improves the anti-interference capability of the system after laser frequency stabilization / optical resonant cavity length locking, making laser frequency / cavity length less likely to lose lock. Attached Figure Description

[0068] Figure 1 This is a schematic diagram of the device principle of the method of the present invention.

[0069] Figure 2 This is a block diagram illustrating the principle of the signal acquisition and processing module of the method of the present invention.

[0070] Figure 3 This is a schematic diagram of the error signal and the new error signal in the method of the present invention.

[0071] Figure 4 This is a simulation diagram showing the relationship between the linear dynamic range of the error signal and the new error signal and the modulation depth in the method of this invention.

[0072] In the diagram: 1. Laser, 2. Optical isolator, 3. Electro-optic phase modulator, 4. First photodetector, 5. Polarizing beam splitter, 6. Quarter-wave plate, 7. Optical resonant cavity, 8. Second photodetector, 9. Signal acquisition and processing module, 901. Digital frequency synthesizer, 902. Phase shifter, 903. First multiplier, 904. First low-pass filter, 905. Arctangent operation module, 906. Gain module, 907. Triangular wave scanning module, 908. Second multiplier, 909. Squaring operation module, 910. Second low-pass filter, 911. First threshold judgment module, 912. Division operation module, 913. First data selector, 914. PID control module, 915. Second threshold judgment module, 916. Second data selector. Detailed Implementation

[0073] The present invention will now be described in detail with reference to the accompanying drawings.

[0074] like Figure 1 As shown, the specific implementation method adopts the following optical path: including laser 1, optical isolator 2, electro-optic phase modulator 3, first photodetector 4, polarizing beam splitter 5, quarter-wave plate 6, optical resonator 7, second photodetector 8, and signal acquisition and processing module 9; laser 1, optical isolator 2, electro-optic phase modulator 3, polarizing beam splitter 5, quarter-wave plate 6, optical resonator 7, and second photodetector 8 can be arranged along the optical axis.

[0075] The laser emitted by laser 1 enters optical isolator 2. The laser exiting optical isolator 2 enters electro-optic phase modulator 3, is modulated, and then incident on polarizing beam splitter 5, where it is transmitted. The laser transmitted from polarizing beam splitter 5 is p-polarized light and passes through quarter-wave plate 6, then enters optical resonant cavity 7, where it is reflected and transmitted. The laser reflected from optical resonant cavity 7 returns along the original path, passes through quarter-wave plate 6, and is converted from p-polarized to s-polarized state, returning to polarizing beam splitter 5 where it is reflected again. The laser reflected from polarizing beam splitter 5 is incident on first photodetector 4 and received, obtaining the reflected multi-beam interference signal I. r1 (t), the laser transmitted from the optical resonant cavity 7 is incident on the second photodetector 8 and received to obtain the transmitted power signal P. tran The output terminals of the first photodetector 4 and the second photodetector 8 are connected to the input terminal of the signal acquisition and processing module 9. The output terminal of the signal acquisition and processing module 9 is connected to the electro-optic phase modulator 3. At the same time, the output terminal of the signal acquisition and processing module 9 is connected to the laser 1 or the optical resonant cavity 7.

[0076] Signal acquisition and processing module 9 receives detection signals from the first photodetector 4 and the second photodetector 8, outputs a sinusoidal drive signal to the electro-optic phase modulator 3, and outputs a PID control signal U. PIDThe controlled object can be a laser 1 or an optical resonant cavity 7.

[0077] like Figure 2 As shown, the signal acquisition and processing module 9 specifically includes a digital frequency synthesizer 901, a phase shifter 902, a first multiplier 903, a first low-pass filter 904, an arctangent operation module 905, a gain module 906, a triangular wave scanning module 907, a second multiplier 908, a squaring operation module 909, a second low-pass filter 910, a first threshold judgment module 911, a division operation module 912, a first data selector 913, a PID control module 914, a second threshold judgment module 915, and a second data selector 916.

[0078] One of the outputs of the digital frequency synthesizer 901 is connected to the input of the gain module 906, and the output of the gain module 906 is connected to... Figure 1 The drive input terminal of the electro-optic phase modulator 3 in the optical path and the other two quadrature output terminals of the digital frequency synthesizer 901 are connected to the two input terminals of the phase shifter 902. The two output terminals of the phase shifter 902 are respectively connected to the input terminals of the first multiplier 903 and the second multiplier 908. The reflected multi-beam interference signal I from the first photodetector 4... r1 (t) The input is connected to the input terminals of the first multiplier 903 and the second multiplier 908. The output terminals of the first multiplier 903 and the second multiplier 908 are respectively connected to the input terminals of the first low-pass filter 904 and the second low-pass filter 910. The output terminal of the first low-pass filter 904 is connected to one input terminal of the arctangent operation module 905. The output terminal of the second low-pass filter 910 is divided into three paths and connected to the input terminals of the arctangent operation module 905, the division operation module 912 and the first data selector 913 respectively.

[0079] The transmission power signal P at the output of the second photodetector 8 tranThe circuit is divided into three paths and connected to the input terminals of the squaring operation module 909, the first threshold judgment module 911, and the second threshold judgment module 915, respectively. The output terminal of the squaring operation module 909 is connected to another input terminal of the division operation module 912. The output terminal of the division operation module 912 is connected to another input terminal of the first data selector 913 and the second threshold judgment module 915, respectively. The output terminal of the first threshold judgment module 911 is connected to the selection control terminal of the first data selector 913. The output terminal of the first data selector 913 is connected to the input terminal of the PID control module 914. The output terminals of the PID control module 914 and the triangular wave scanning module 907 are connected to the two data input terminals of the second data selector 916, respectively. The output terminal of the second threshold judgment module 915 is connected to the selection control terminal of the second data selector 916. The output terminal of the second data selector 916 is connected to the optical resonant cavity 7 after digital-to-analog conversion, specifically to the input terminal of the piezoelectric ceramic mounted on a cavity mirror of the optical resonant cavity 7.

[0080] The specific implementation process of this invention is as follows:

[0081] like Figure 1 As shown, the laser emitted by laser 1 passes through isolator 2 and enters electro-optic phase modulator 3 for phase modulation. It is then transmitted through polarizing beam splitter 5 and incident on quarter-wave plate 6 into optical resonant cavity 7. The cavity length of optical resonant cavity 7 can be controlled by a piezoelectric ceramic PZT mounted on a cavity mirror. The light reflected from optical resonant cavity 7 passes through quarter-wave plate 6 again, changing its polarization state from p to s. After being reflected by polarizing beam splitter 5, it undergoes multi-beam interference at first photodetector 4, where it is detected and converted. The interference signal is called the reflected multi-beam interference signal I. r1 (t); After passing through the optical resonant cavity 7, the transmitted light is detected and converted by the second photodetector 8, and the detected DC signal is called the transmitted power signal P. tran Reflected multi-beam interference signal I r1 (t) and transmission power signal P tran The signal is sampled, converted from analog to digital, and then processed by the signal acquisition and processing module 9.

[0082] Reflected multibeam interference signal I r1 (t) represents the following:

[0083]

[0084]

[0085] Among them, I r1 (t) represents the interference signal detected by the first photodetector 4 at the reflective end of the optical resonator 7 at time t, j represents the highest order of the sideband, DC terms and p·ωm terms represent the DC component and ω of the interference signal, respectively. m The p-th harmonic component, and the remaining terms are ω. m The first harmonic component; E0 represents the laser amplitude, ω c ω represents the fundamental frequency of the laser. m The modulation frequency is represented by l, the cavity length of the optical resonator is represented by l, and the order of the modulation sideband is represented by k, where k = 0 to j-1; J k (β) represents the k-th order Bessel function of the first kind when the modulation depth is β; Re{} and Im{} represent the real and imaginary parts of the complex number, respectively; F[ω,l] represents the reflection coefficient of the optical resonator 7, which is a function of the incident laser frequency ω and the cavity length l; F*[ω,l] represents the conjugate of the reflection coefficient of the optical resonator 7, n is the refractive index of the air inside the cavity, and i is the imaginary unit. r represents the reflectivity of the two cavity mirrors of the optical resonator 7, and FSR represents the free spectral range of the optical resonator 7.

[0086] Transmitted power signal P tran The expression is as follows:

[0087]

[0088]

[0089] Among them, P tran The transmitted power signal is represented by l, which represents the cavity length of the optical resonant cavity (7), and ω represents the cavity length of the optical resonant cavity (7). m J represents the phase modulation frequency, |k| represents the absolute value of k, and J represents the phase modulation frequency. |k| (β) is the k-th or -k-th order Bessel function of the first kind when the modulation depth is β, T(ω,l) represents the transmission coefficient of the optical resonator (7), which is a function related to the incident laser frequency ω and cavity length l; T*(ω,l) represents the conjugate of the transmission coefficient of the optical resonator (7), n is the refractive index of the air inside the cavity, i is the imaginary unit, r represents the reflectivity of the two cavity mirrors of the optical resonator (7), and FSR represents the free spectral range of the optical resonator (7).

[0090] like Figure 2 As shown, in obtaining the reflected multibeam interference signal I r1 After (t), the first step is to perform quadrature downmixing, specifically as follows: the digital frequency synthesizer 901 generates a pair of quadrature signals, which are then shifted by the same phase by the phase shifter 902 to obtain a pair of quadrature reference signals. The reflected multibeam interference signal I is processed by the first multiplier 903 and the second multiplier 908 respectively. r1 (t) is multiplied by this pair of orthogonal reference signals, and then filtered by the first low-pass filter 904 and the second low-pass filter 910 to obtain the orthogonal harmonic amplitude signal (including the cosine component S).Q Sine component S I The sinusoidal component S is among them. I After phase matching is completed, the error signal S is obtained. PDH This completes the orthogonal under-mixing operation. Simplified to single trigonometric function form, it can be expressed as follows:

[0091]

[0092] R(k)=F[ω c +kω m ,l]F*[ω c +(k+1)ω m ,l]-F*[ω c -kω m ,l]F[ω c -(k+1)ω m ,l](6)

[0093] Among them, S I S Q Let LPF[] represent the sine and cosine components in the quadrature harmonic amplitude signal, respectively. Let LPF[] represent the low-pass filtering operation, and R(k) represent the ω generated by the beat frequencies of the k-th and (k+1)-th sidebands and the (-k)-th and (-(k+1))-th sidebands, respectively. m The sum of the amplitudes of the first harmonic components is called the amplitude coefficient of the k-th order, and |R(k)| is the modulus of the amplitude coefficient R(k). Let R(k) be the phase angle of the k-th order amplitude coefficient. The first harmonic component of the interference signal and the initial phase difference between the sinusoidal reference signal are given.

[0094] Then, the arctangent operation is performed on the quadrature harmonic amplitude signal to obtain the interference signal I. r1 The phase of the first harmonic component of (t) and the phase of the sinusoidal reference signal are determined as follows: The quadrature harmonic amplitude signals obtained by the first low-pass filter 904 and the second low-pass filter 910 are processed by the arctangent operation module 905 to obtain the phase. The formula is as follows:

[0095]

[0096] Adjust the laser frequency of laser 1 or the cavity length of optical resonator 7 so that the frequency interval between the fundamental frequency and its sideband frequencies of the laser and the resonant frequency of optical resonator 7 is greater than twice the full width at half maximum (FWHM) of optical resonator 7. Then, calculate the phase at this point. As the initial phase difference At this point, both the fundamental frequency laser and the laser from its modulation sidebands are totally reflected, meaning the reflection coefficients at the corresponding frequencies are all real numbers. Therefore, the phase angle of the amplitude coefficient of any order is... The phase obtained from the arctangent operation is equal to zero. That is, the reflected multi-beam interference signal I r1 The first harmonic component in (t) and the initial phase difference with the sinusoidal reference signal The specific calculation process is as follows:

[0097]

[0098] Then use the obtained initial phase difference The feedback signal is sent to phase shifter 902 to shift the phase of the two orthogonal reference signals, thereby realizing the reflection of the multi-beam interference signal I. r1 The first harmonic component in (t) is automatically matched with the phase of the sinusoidal reference signal to ensure that the error signal S PDH Linearity and high sensitivity within the frequency discrimination interval. The expression for the error signal after phase matching is:

[0099]

[0100] in, This represents the phase difference between the first harmonic component of the interference signal and the sinusoidal reference signal after matching. When the laser phase modulation frequency is lower than the full width at half maximum (FWHM) of the optical resonator, the phase difference after matching is 0°; when the laser phase modulation frequency is higher than the FWHM of the optical resonator, the phase difference after matching is 90°.

[0101] Then, the error signal S after phase matching is used. PDH and transmitted power signal P tran The square of the result yields a new error signal with a larger linear dynamic range. The specific process is as follows: Transmitted power signal P tran The square P of the transmitted power signal is obtained after squaring the signal through the squaring module 909. tran 2 .

[0102] Then, the error signal S after phase matching is processed by the division module 912. PDH With the square of the transmitted power signal P tran 2 A new error signal S is obtained by performing a division operation. new The calculation formula is as follows:

[0103]

[0104] Thus, the error signal S was obtained. PDH New error signal S new and the transmitted power signal P tran The schematic diagram of the two error signals is as follows: Figure 3 As shown, the two error signals are related to the phase.

[0105] The three signals will then be used to achieve automatic locking of the cavity length of the optical resonator 7. The process mainly includes scanning, acquisition and preliminary locking stages as well as precise locking stage.

[0106] First, by adjusting the gain of the gain module 906, the phase modulation depth β is made to 1.80 rad, thereby achieving laser phase modulation with a large modulation depth.

[0107] Then, under the control of the second data selector 916, the output of the triangular wave scanning module 907 is connected to the laser 1 or the optical resonant cavity 7. The triangular wave scanning module 907 emits a triangular wave scanning signal to perform a triangular wave pre-scan on the cavity length of the optical resonant cavity 7, thereby obtaining a new error signal S. new The maximum value S newmax and transmitted power signal P tran transmission peak P tranmax and define S newmax Three-quarters of it is the locked capture threshold S th .

[0108] During this stage, the output of the first data selector 913 is selected as the new error signal S. new The output of the second data selector 916 is selected as the output of the triangular wave scanning module 907.

[0109] Next, the laser frequency of the laser emitted by laser 1 and the cavity length of the optical resonant cavity 7 are locked within a specific range. Under the control of the triangular wave scanning signal, the output of the division module 912 is connected to the PID control module 914 through the control of the first data selector 913. The PID control module 914 then inputs a new error signal S. new The following judgments and actions will be taken:

[0110] When the new error signal S new The value did not reach the maximum value S newmax Or not from the maximum value S newmax Drops below the lock capture threshold S th When the laser frequency emitted by the laser (1) is equal to the cavity length of the optical resonant cavity (7), the triangular wave pre-scan is maintained and no processing is performed.

[0111] When the new error signal S new The value from the maximum value S newmax Drops below the lock capture threshold S th At that time, the output of the PID control module 914 is connected to the laser 1 or the optical resonant cavity 7 through the control of the second data selector 916, switching from the triangular wave scanning signal to an S-wave scanning signal. newThe output signal, processed by the PID control module 914, is used as the input signal to control the laser frequency of the laser emitted by the laser 1 and the cavity length of the optical resonant cavity 7. The PID control module 914 then performs feedback control on this laser frequency / cavity length, resulting in a new error signal S. new Fluctuations around zero indicate an initial lock-in.

[0112] When using the PID control module 914 for feedback control, the transmitted power signal P tran The value will be in the transmission peak P tranmax Based on the surrounding fluctuations, the following judgments should be made:

[0113] When the transmitted power signal P tran The fluctuation over a fixed period of time is less than the transmission peak value P. tranmax When one-third of the laser frequency emitted by laser 1 is equal to the cavity length of optical resonant cavity 7, the laser frequency is initially locked, and the next step of precise locking is carried out.

[0114] Thus, a new error signal S with a large linear dynamic range at a large modulation depth is utilized. new To achieve the capture of the resonant range and the initial locking of the laser frequency / optical resonant cavity length.

[0115] When the transmitted power signal P tran The fluctuation over a fixed period of time is not less than the peak transmission value P. tranmax When one-third of the volume is reached, the PID parameters in the PID control module (914) are adjusted.

[0116] During the process of adjusting the amplitude of the driving signal of the electro-optic phase modulator 3 to perform laser phase modulation with a large modulation depth, according to the k-th order Bessel function of the first kind J... k The properties of (β) are determined by the following criteria:

[0117] For a fixed modulation depth β, when k > β + 1, then J k (β)<0.1, when k<=β+1, then J k (β)>0.1, that is, J k (β) The higher-order sideband components with orders greater than β+1 can be ignored, while the sideband components with orders greater than or equal to β+1 cannot be ignored. Under large modulation depth, the highest order of the higher-order sideband is j≥2.

[0118] The output of the second low-pass filter 910 is connected to the PID control module 914 under the control of the first data selector 913. The PID control module 914 receives the phase-matched error signal S. PDH The input to the PID control module 914 will be changed from the error signal S.new Error signal S after switching to phase matching PDH Meanwhile, by adjusting the gain of the gain module 906, the phase modulation depth β is made to be 1.00 rad, and the highest order of the laser frequency sideband generated by the laser phase modulation is j2 = 1.

[0119] Furthermore, through the control of the second data selector 916, the output of the PID control module 914 remains connected to the laser 1 or the optical resonant cavity 7. The PID control module 914 uses feedback control to control the laser frequency emitted by the laser 1 and the cavity length of the optical resonant cavity 7, so that the phase-matched error signal S PDH Fluctuating around zero allows for precise locking.

[0120] Thus, the error signal S, which has high sensitivity at a small modulation depth, is utilized. PDH To achieve precise locking of the laser frequency / optical resonant cavity length.

[0121] External environmental vibrations and other disturbances can affect locking, causing the laser frequency / optical resonant cavity length to deviate from the resonant center momentarily, or even leading to loss of locking.

[0122] During steps 6.1) to 6.2), the following judgments and actions are performed in real time:

[0123] When the transmitted power signal P tran Greater than or equal to the transmission peak value P tranmax When the modulation depth of the electro-optic phase modulator (3) is half, the modulation depth is small, and the output of the second low-pass filter (910) is connected to the PID control module (914) by the control of the first data selector (913). The input of the PID control module (914) is the error signal. The PID control module (914) is used to perform feedback control on the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7) so that the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7) is precisely locked.

[0124] When the transmitted power signal P tran Less than the transmission peak value P tranmax Half of the transmission peak value P tranmax When the modulation depth of the electro-optic phase modulator 3 is reduced to one-tenth, the modulation depth is switched back to a large modulation depth. Furthermore, the output of the division module 912 is connected to the PID control module 914 under the control of the first data selector 913. The input of the PID control module 914 is then switched to 1.80 rad and the new error signal S. newThe PID control module 914 is used to perform feedback control on the laser frequency of the laser emitted by the laser 1 and the cavity length of the optical resonant cavity 7, so that the laser frequency of the laser emitted by the laser 1 and the cavity length of the optical resonant cavity 7 are relocked.

[0125] If external interference is very strong, thus affecting the transmitted power signal P tran The value is less than P tranmax When the output of the triangular wave scanning module 907 is connected to the laser 1 or the optical resonant cavity 7, the second data selector 916 controls the output of the triangular wave scanning module 907. The triangular wave scanning module 907 sends a triangular wave scanning signal to control the laser frequency of the laser emitted by the laser 1 and the cavity length of the optical resonant cavity 7. That is, the control signal for the laser frequency of the laser emitted by the laser 1 and the cavity length of the optical resonant cavity 7 is switched back to the triangular wave scanning signal, and the process returns to step 6.1) to scan, capture and lock again.

[0126] In summary, the method of this invention first performs orthogonal downmixing and arctangent operations on the obtained interference signal and a pair of orthogonal reference signals to obtain the initial phase difference. This initial phase difference is then used to perform phase shifting using a phase shifter, achieving automatic phase matching and ensuring the linearity and high sensitivity of the error signal within the frequency discrimination interval, thus avoiding the need for manual adjustment of the phase shifter. Next, a new error signal is generated by combining the phase-matched error signal with the square of the resonant cavity transmission power signal, expanding the linear dynamic range and reducing the scanning and acquisition time for the laser frequency / resonant cavity length. Finally, by switching between the new error signal with a large dynamic range at a large modulation depth and the error signal with high sensitivity at a small modulation depth as inputs to the PID feedback control module, the system maintains laser frequency stability and optical resonant cavity length locking accuracy while improving the anti-interference capability of the locked system, making it less prone to laser frequency / cavity length loss of lock.

[0127] The above specific embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A high dynamic range, high sensitivity PDH frequency stabilization method with dual modulation depths, characterized in that: The method includes the following steps: Step 1) The laser emitted by the laser (1) enters the electro-optic phase modulator (3) after passing through the optical isolator (2). The electro-optic phase modulator (3) generates a phase modulation signal to modulate the phase of the laser. The phase-modulated laser passes through the polarization beam splitter (5) and the quarter-wave plate (6) in sequence and is then incident on the optical resonant cavity (7). It is reflected and transmitted back and forth multiple times in the cavity. The laser reflected from the optical resonant cavity (7) passes through the quarter-wave plate (6) in reverse and its polarization state changes from p state to s state. Then it is reflected by the polarization beam splitter (5) to the first photodetector (4) to generate multi-beam interference. The interference signal is detected and obtained. The interference signal is sampled and converted from analog to digital to obtain the reflected multi-beam interference signal I. r1 (t); Step 2) The digital frequency synthesizer (901) generates two quadrature reference signals with the same frequency as the phase modulation signal, including a cosine reference signal. and sinusoidal reference signal After the two orthogonal reference signals are shifted to the same phase by the phase shifter (902), they are respectively interfered with the reflected multi-beam interference signal I. r1 (t) The components are sequentially multiplied by two separate multipliers (903, 908) and filtered by low-pass filters (904, 910) to complete the quadrature downmixing operation, retaining the DC component, and obtaining a pair of quadrature harmonic amplitude signals, which include sinusoidal components. Sum and cosine components ; Step 3) Perform arctangent operation on a pair of quadrature harmonic amplitude signals in the arctangent operation module (905) to obtain the phase. The calculation is as follows: ； Adjust the laser frequency of the laser (1) or the cavity length of the optical resonator (7) so that the frequency interval between the laser fundamental frequency and its sideband frequencies and the resonant frequency of the optical resonator (7) is greater than twice the full width at half maximum (FWHM) of the optical resonator (7). Then, calculate the phase at this time. As the initial phase difference ; Then use the obtained initial phase difference The feedback is sent to the phase shifter (902) to shift the two quadrature reference signals; Step 4) Extract the sinusoidal component from the quadrature harmonic amplitude signal. After phase matching is completed, it serves as the error signal after phase matching. The expression is: ； Where k represents the order of the modulation sideband, k = 0 ~ j-1, j represents the highest order of the laser frequency sideband generated after laser phase modulation, E0 represents the laser amplitude, and J k (β) is the first-order Bessel function of the kind at modulation depth β; R(k) represents the ω generated by the beat frequencies of the k-th and (k+1)-th order sidebands, and the (-k)-th and (-(k+1))-th order sidebands, respectively. m The sum of the amplitudes of the first harmonic components is called the amplitude coefficient of the k-th order. Let R(k) be the modulus of the k-th order amplitude coefficient. Let R(k) be the phase angle of the k-th order amplitude coefficient, and t represent time. This represents the phase difference between the first harmonic component of the interference signal and the sinusoidal reference signal after matching. When the laser phase modulation frequency is lower than the full width at half maximum (FWHM) of the optical resonator, the phase difference after matching is 0°; when the laser phase modulation frequency is higher than the FWHM of the optical resonator, the phase difference after matching is 90°. Step 5) The light transmitted from the optical resonant cavity (7) is received by the second photodetector (8) to obtain the DC power signal. The DC power signal is sampled and converted from analog to digital to obtain the transmitted power signal. The expression is: ； ； in, The transmitted power signal is represented by l, which represents the cavity length of the optical resonant cavity (7), and ω represents the cavity length of the optical resonant cavity (7). m J represents the phase modulation frequency, |k| represents the absolute value of k, and J represents the phase modulation frequency. |k| (β) is the k-th or -k-th order Bessel function of the first kind when the modulation depth is β, j represents the highest order of the sideband after laser modulation; T(ω,l) represents the transmission coefficient of the optical resonator (7), which is a function related to the incident laser frequency ω and cavity length l; T*(ω,l) represents the conjugate of the transmission coefficient of the optical resonator (7), n is the refractive index of the air inside the cavity, i is the imaginary unit, r represents the reflectivity of the two cavity mirrors of the optical resonator (7), and FSR represents the free spectral range of the optical resonator (7); Then the error signal after phase matching With the square of the transmitted power signal A new error signal is obtained by performing a division operation. The calculation formula is as follows: ； Step 6) At this point, the error signal after phase matching has been obtained in real time. New error signal and transmitted power signal Then, the cavity length of the optical resonator (7) and the laser frequency of the laser (1) will be automatically locked using these three real-time signals to achieve frequency stabilization control.

2. The high dynamic range and high sensitivity PDH frequency stabilization method with dual modulation depth according to claim 1, characterized in that: Step 6) specifically refers to: Step 6.1) Scanning, capturing, and initial locking: First, the amplitude of the driving signal of the electro-optic phase modulator (3) is adjusted to achieve laser phase modulation with a large modulation depth. Then, a triangular wave scanning signal is emitted to perform a triangular wave pre-scan on the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7) to obtain a new error signal. maximum value and transmitted power signal transmission peak and with a new error signal maximum value Three-quarters as the lock capture threshold ; Next, the laser frequency emitted by the laser (1) and the cavity length of the optical resonant cavity (7) are locked within a specific range. Under the control of the triangular wave scanning signal, the PID control module (914) inputs a new error signal. The following judgments and actions will be taken: When the new error signal The value did not reach the maximum value. Or not from the maximum value Drop to below the lock capture threshold When the laser frequency emitted by the laser (1) is equal to the cavity length of the optical resonant cavity (7), the triangular wave pre-scan is maintained and no processing is performed. When the new error signal The value from the maximum value Drop to below the lock capture threshold At that time, the signal is switched from a triangular wave scanning signal to a signal with a triangular wave scanning signal. As an input signal, the output signal processed by the PID control module (914) controls the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7). The PID control module (914) then performs feedback control on the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7), so that the new error signal Fluctuations around zero indicate an initial lock-in. Step 6.2) Precise locking: The PID control module (914) inputs the error signal after phase matching. Meanwhile, by adjusting the amplitude of the driving signal of the electro-optic phase modulator (3) to reduce the phase modulation depth to a small modulation depth, the PID control module (914) is used to perform feedback control on the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonator (7), so that the error signal after phase matching is reduced. Fluctuating around zero allows for precise locking.

3. The high dynamic range and high sensitivity PDH frequency stabilization method with dual modulation depth according to claim 2, characterized in that: In step 6.1), when using the PID control module (914) for feedback control, the following judgments are made: When the transmitted power signal The fluctuation over a fixed period of time is less than the transmission peak value. When one-third of the laser frequency emitted by the laser (1) is equal to the cavity length of the optical resonant cavity (7), the laser frequency is initially locked, and the next step of precise locking is carried out. When the transmitted power signal The fluctuation over a fixed period of time is not less than the peak transmission value. When one-third of the value is reached, the PID parameters in the PID control module (914) are adjusted.

4. The high dynamic range and high sensitivity PDH frequency stabilization method with dual modulation depth according to claim 2, characterized in that: During steps 6.1) to 6.2), the following judgments and actions are performed in real time: When the transmitted power signal Greater than or equal to the transmission peak value When the modulation depth of the electro-optic phase modulator (3) is half of the modulation depth, the input of the PID control module (914) is an error signal. The PID control module (914) is used to perform feedback control on the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7) so that the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7) is precisely locked. When the transmitted power signal Less than the transmission peak value Half of the transmission peak value When the modulation depth of the electro-optic phase modulator (3) is reduced to one-tenth, the modulation depth is switched back to a large modulation depth, and the input of the PID control module (914) is switched back to the new error signal. The PID control module (914) is used to perform feedback control on the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7), so that the laser frequency of the laser emitted by the laser (1) / the cavity length of the optical resonant cavity (7) is relocked. If the transmitted power signal The value is less than When the laser frequency is one-tenth of the laser, a triangular wave scanning signal is emitted to control the laser frequency emitted by the laser (1) / the cavity length of the optical resonant cavity (7), and the process returns to step 6.1) to re-scan, capture and lock.

5. A dual-modulation-depth, large dynamic range, high-sensitivity PDH frequency stabilization device for implementing the method described in any one of claims 1-4, characterized in that: Includes a laser (1), an optical isolator (2), an electro-optic phase modulator (3), a first photodetector (4), a polarizing beam splitter (5), a quarter-wave plate (6), an optical resonator (7), and a second photodetector (8); The laser emitted by the laser (1) enters the optical isolator (2). The laser coming out of the optical isolator (2) enters the electro-optic phase modulator (3), is modulated, and is incident on the polarizing beam splitter (5) for transmission. The laser transmitted from the polarizing beam splitter (5) is p-polarized light and passes through the quarter-wave plate (6) and is incident on the optical resonant cavity (7) for reflection and transmission. The laser reflected from the optical resonant cavity (7) returns along the original path and passes through the quarter-wave plate (6) to change from p-polarized to s-polarized, and returns to the polarizing beam splitter (5) for reflection. The laser reflected from the polarizing beam splitter (5) is incident on the first photodetector (4) and is received. The laser transmitted from the optical resonant cavity (7) is incident on the second photodetector (8) and is received.

6. The PDH frequency stabilization device with large dynamic range and high sensitivity and dual modulation depth according to claim 5, characterized in that: It also includes a signal acquisition and processing module (9), the output terminals of the first photodetector (4) and the second photodetector (8) are connected to the input terminal of the signal acquisition and processing module (9), the output terminal of the signal acquisition and processing module (9) is connected to the electro-optic phase modulator (3), and the output terminal of the signal acquisition and processing module (9) is connected to the laser (1) or the optical resonator (7).

7. The PDH frequency stabilization device with large dynamic range and high sensitivity and dual modulation depth according to claim 6, characterized in that: The signal acquisition and processing module (9) specifically includes a digital frequency synthesizer (901), a phase shifter (902), a first multiplier (903), a first low-pass filter (904), an arctangent operation module (905), a gain module (906), a triangular wave scanning module (907), a second multiplier (908), a square operation module (909), a second low-pass filter (910), a first threshold judgment module (911), a division operation module (912), a first data selector (913), a PID control module (914), a second threshold judgment module (915), and a second data selector (916). One output of the digital frequency synthesizer (901) is connected to the input of the gain module (906), and the output of the gain module (906) is connected to the drive input of the electro-optic phase modulator (3). The other two quadrature outputs of the digital frequency synthesizer (901) are connected to the two inputs of the phase shifter (902). The two outputs of the phase shifter (902) are connected to the inputs of the first multiplier (903) and the second multiplier (908), respectively. The first photodetector (4) is connected to the inputs of the first multiplier (903) and the second multiplier (908). The input terminal of the multiplier (908) is connected to the input terminals of the first multiplier (903) and the second multiplier (908), respectively. The output terminals of the first multiplier (903) and the second multiplier (908) are connected to the input terminals of the first low-pass filter (904) and the second low-pass filter (910), respectively. The output terminal of the first low-pass filter (904) is connected to one input terminal of the arctangent operation module (905). The output terminal of the second low-pass filter (910) is divided into three paths and connected to the input terminals of the arctangent operation module (905), the division operation module (912) and the first data selector (913), respectively. The output of the second photodetector (8) is divided into three paths and connected to the input of the square operation module (909), the first threshold judgment module (911), and the second threshold judgment module (915), respectively. The output of the square operation module (909) is connected to the other input of the division operation module (912). The output of the division operation module (912) is connected to the other input of the first data selector (913) and the second threshold judgment module (915), respectively. The output of the first threshold judgment module (911) is connected to the selection control terminal of the first data selector (913). The output of the first data selector (913) is connected to the input of the PID control module (914). The outputs of the PID control module (914) and the triangular wave scanning module (907) are connected to the two data inputs of the second data selector (916), respectively. The output of the second threshold judgment module (915) is connected to the selection control terminal of the second data selector (916). The output of the second data selector (916) is connected to the optical resonant cavity (7) after digital-to-analog conversion.

8. The PDH frequency stabilization device with large dynamic range and high sensitivity and dual modulation depth according to claim 5, characterized in that: When the device is used to stabilize the laser frequency emitted by the laser, the laser (1) is a laser with adjustable laser frequency, and the optical resonant cavity (7) is an optical resonant cavity with fixed cavity length. When the device is used to lock the cavity length of the optical resonator, the laser (1) is a laser with a laser frequency, and the optical resonator (7) is an optical resonator with an adjustable cavity length.

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