A cavity length locking system and method for a pulsed optical parametric oscillator

Abstract

The present application belongs to but is not limited to the technical field of oscillator, and particularly relates to a cavity length locking system and method of a pulse type optical parametric oscillator, comprising an OPO pulse light source, a pulse width feedback locking loop and a power-based auxiliary locking loop; the OPO pulse light source comprises a seed pulse light source and an OPO resonant cavity; the pulse width feedback locking loop is composed of a pulse width measuring device, a signal processing circuit and a cavity length control element; the power-based auxiliary locking loop is composed of a power signal measuring device, a signal processing circuit and a cavity length control element. The locking system works simultaneously using the pulse width feedback locking loop and the power-based auxiliary locking loop. The pulse width feedback locking loop has a Hz level bandwidth and a high gain feature, and the power-based auxiliary locking loop has a kHz-MHz level larger bandwidth. The two are matched for application, and can realize more stable OPO cavity length locking.

Description

TECHNICAL FIELD

[0001] The present application belongs to but is not limited to the field of oscillator technology, and particularly relates to a cavity length locking system and method of a pulse type optical parametric oscillator. BACKGROUND

[0002] The pulse type optical parametric oscillator (OPO) is based on synchronous pumping and resonant cavity technology. It can generate high-power and wide-spectrum-tunable high-quality pulse light source with only one pump light source, and has wide and important applications in the fields of precision spectroscopy, biological imaging, atmospheric environment science, optical frequency comb, etc. However, due to historical reasons of the development of ultra-short laser technology, the pulse type OPO with a spatial structure has not been friendly to people in the past. The main disadvantage is poor structural stability. This is because the matching error of the pulse type OPO cavity length and the pump pulse cavity length needs to be controlled at the sub-micron level due to the requirements of synchronous pumping and stable output. Typical OPO cavity length locking schemes include the "position sensing detection" feedback technology [Optics Communications 123:577-582 (1996)] and the jitter-locking technology [Optics Express 25(4):4190-4200 (2017)]. The former uses the OPO output wavelength as the feedback quantity and uses "position sensing detection" to extract the error signal, but it is very sensitive to the beam pointing of the optical path structure and is easily affected by the external environment, so it generally cannot achieve long-term locking at the top of the OPO resonance peak. The latter obtains an error signal related to the OPO power by applying a cavity length modulation signal, although this scheme can lock to the top of the OPO resonance peak, but due to the application of the external modulation signal, the output pulse power and spectrum frequency after locking will still have weak jitter, which limits its promotion in high-precision precision measurement and other application fields. Recently, researchers have found a new method to achieve OPO resonance peak locking in experiments. They successfully proved that without relying on external modulation means, the OPO resonance peak can also be effectively locked based on the OPO parasitic light pulse spectrum power as the feedback quantity [Optics Letters 45(3):768-771 (2020)]. This discovery is of great significance because parasitic light is ubiquitous in the leakage light of OPO, which can greatly simplify the optical path design of the system. Therefore, due to its simplicity and effectiveness, this scheme has shown broad application prospects and is considered as a very promising solution.

[0003] However, when using the spectral power of parasitic optical pulses as the feedback quantity, the signal-to-noise ratio of the generated error signal is limited by the spectral shape, while the signal strength is limited by the detection sensitivity of converting the spectral power into a voltage signal, making the current locking level still insufficient. Therefore, in the parasitic optical pulse scheme, how to effectively improve the signal-to-noise ratio and signal strength of the error signal has become the core issue for achieving high-level locking of the OPO cavity length.

[0004] To improve the locking performance, this invention proposes an innovative solution: utilizing the inverse Fourier transform term of the parasitic light pulse spectrum, i.e., using the pulse width in the time-domain waveform as a new feedback quantity. This method benefits from the significant advancements in ultrashort pulse measurement technology in recent years, enabling real-time, high-sensitivity (microwatt-level power), and high-resolution (femtosecond-level) measurement of ultrashort pulses. This high-sensitivity, high-resolution pulse width signal provides a strong guarantee for obtaining high signal-to-noise ratio and high-intensity error signals, thus promising the realization of OPO pulse light sources with higher locking levels.

[0005] Based on the above analysis, the urgent technical problem to be solved in the existing technology is that the modulation-free locking OPO cavity length scheme based on parasitic optical spectral power currently has a limiting factor, namely, the signal-to-noise ratio of the generated error signal is limited by the spectral shape and the signal strength is limited by the detection sensitivity, which results in the OPO cavity length locking level still not being high. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a cavity length locking system and method for pulsed optical parametric oscillators (OPOs). By using a commercial autocorrelation instrument, the parasitic optical pulse width variable caused by OPO cavity length jitter is measured in real time. This variable exhibits microwatt-level detection sensitivity and femtosecond-level detection resolution, thus significantly improving the signal-to-noise ratio and signal strength of the error signal. This invention is based on a dual-loop locking scheme, suppressing Hz-level bandwidth noise and kHz-MHz-level bandwidth noise respectively, achieving a high level of OPO cavity length locking.

[0007] The present invention is implemented as follows: a cavity length locking system for a pulsed optical parametric oscillator includes an OPO pulse light source, a pulse width feedback locking loop, and a power-based auxiliary locking loop.

[0008] The OPO pulse light source includes a seed pulse light source and an OPO resonant cavity;

[0009] The pulse width feedback lock loop is composed of a pulse width measuring device, a signal processing circuit, and a cavity length control element. The pulse width measuring device is used to measure the pulse width of the parasitic light pulse output by the OPO in real time and convert it into a digital voltage signal. The signal processing circuit compares the pulse width voltage signal with a reference voltage signal to obtain an error signal, and obtains a servo signal through a digital proportional-integral-derivative (PID) circuit, which is fed back to the control element of the cavity length. The cavity length control element adjusts the cavity length according to the servo voltage signal output by the signal processing circuit.

[0010] The power-based auxiliary lock loop is composed of a power signal measuring device, a signal processing circuit, and a cavity length control element. The power signal measuring device includes a filtering optical path and a low-noise power detector. The filtering optical path is based on a wave plate, a filter, and a mirror to extract the parasitic light pulse in the OPO leakage spectrum. The low-noise power detector has a kHz bandwidth and a low-noise detection level.

[0011] Further, the seed pulse light source can be a solid-state or fiber pulse laser, which outputs pulse laser coupled into the OPO resonant cavity, and the parametric conversion is carried out in the nonlinear crystal inside the cavity to generate the required harmonic pulse, and the parasitic light pulse is generated by the non-phase matching method.

[0012] Further, the pulse width measuring device includes a filtering optical path and an autocorrelator. The filtering optical path is based on a wave plate, a filter, and a mirror to extract the parasitic light pulse in the OPO leakage spectrum. The autocorrelator is based on a Michelson interferometer structure or a Mach-Zehnder interferometer structure to measure the pulse width in real time, with a power sensitivity of micro-watt level and a numerical resolution of femto-second level.

[0013] Further, the signal processing circuit is mainly composed of a digital PID circuit and a digital-to-analog conversion circuit. The digital PID circuit has a high-gain, low-bandwidth feedback loop characteristic, mainly for low-frequency noise suppression within the Hz-level bandwidth of the signal. The digital-to-analog conversion circuit converts the servo digital signal into an analog signal.

[0014] Further, the cavity length control element can be a piezoelectric ceramic or a stepper motor, which actively controls the OPO cavity length according to the output voltage of the signal processing circuit.

[0015] Further, the signal processing circuit is mainly composed of a comparison circuit and an analog PID circuit.

[0016] The comparison circuit compares the power detection voltage with the reference voltage to obtain an error signal. The comparison circuit has a wide bandwidth. The analog PID circuit has a wide-band feedback loop characteristic, mainly for effective suppression of kHz-MHz band noise.

[0017] The cavity length control element is a piezoelectric ceramic or a stepper motor, which is driven by the servo voltage to actively control the OPO cavity length.

[0018] Another object of the present application is to provide a cavity length locking method for the pulse optical parametric oscillator, comprising:

[0019] S1, after OPO resonance, two leakage lights are used to construct a power-based auxiliary locking loop and a pulse width feedback locking loop.

[0020] S2, first operate the power-based auxiliary locking loop, that is, measure the power variable of the parasitic light pulse related to the cavity length jitter through the signal measurement device, and obtain the error voltage signal through the comparison circuit, the error voltage signal passes through the analog PID circuit to obtain the servo voltage signal, which is used to drive the cavity length control element to realize the locking of one of the cavity length closed loops. The feedback loop has a kHz-MHz level bandwidth, which actively suppresses the kHz-MHz level noise of the OPO pulse.

[0021] S3, then operate the pulse width feedback locking loop, obtain the pulse width parameter of the parasitic pulse through the autocorrelator, and obtain the high signal-to-noise ratio, high intensity error signal and high gain low bandwidth servo signal through the signal processing circuit, which is used to drive another cavity length control element to realize the locking of the other cavity length closed loop. The feedback loop has a Hz level bandwidth, which actively suppresses the Hz level noise of the OPO pulse.

[0022] S4, the double loop will work at the same time, and finally realize the stable locking of the OPO cavity length.

[0023] Another object of the present application is to provide a cavity length locking method for the pulse optical parametric oscillator, comprising the following steps:

[0024] (1) extracting the parasitic light pulse of the OPO output, measuring the pulse width in real time through the pulse width measurement device, and converting the measurement result into a digital voltage signal;

[0025] (2) comparing the pulse width voltage signal with the reference voltage signal, and generating a servo signal by using a digital proportional-integral-differential (PID) circuit;

[0026] (3) adjusting the cavity length of the OPO resonant cavity according to the servo signal through the cavity length control element, so as to realize the real-time locking of the pulse width.

[0027] Further, in step (1), the method for extracting the parasitic light pulse of the OPO includes:

[0028] The parasitic light pulse in the OPO leakage light spectrum is extracted by using a filtering light path composed of a wave plate, a filter and a mirror.

[0029] The autocorrelator is used to measure the pulse width based on the Michelson interference structure or the Mach-Zehnder interference structure, and the measurement sensitivity reaches the micro-watt level and the resolution reaches the femtosecond level.

[0030] Further, comprising the following steps:

[0031] (1) Extracting OPO parasitic light pulses, measuring their power signals in real time through a low-noise power detector;

[0032] (2) Comparing the power signal with the reference power signal, and generating a servo signal using an analog PID circuit;

[0033] (3) Using a piezoelectric ceramic or a stepper motor to dynamically adjust the cavity length of the OPO resonant cavity according to the servo signal, realizing power-assisted cavity length locking.

[0034] Further, in step (1), the low-noise power detector has a kHz bandwidth and a low-noise detection level, which can realize high-sensitivity detection when the parasitic light pulse power is weak, and convert the power signal into a voltage signal for subsequent processing.

[0035] The basic method of cavity length locking using pulse width measurement includes measurement, signal processing and feedback control steps.

[0036] The pulse width measurement method emphasizes the specific structure and performance of the filtering optical path and the autocorrelator.

[0037] The method of using power signal for auxiliary cavity length locking proposes the basic steps of power detection and feedback control.

[0038] Further, the characteristics and role of the low-noise power detector are further clarified to ensure high sensitivity and accuracy of signal measurement.

[0039] These method claims clearly define the key technical steps and equipment characteristics of the locking process, providing a comprehensive protection range for the patented technology. In combination with the above technical solutions and solved technical problems, the technical solutions to be protected by the present application have the following advantages and positive effects:

[0040] First, the present application proposes an OPO cavity length locking scheme based on parasitic light pulse width as feedback. By using a commercial autocorrelator to obtain a pulse width variable with micro-watt level detection sensitivity and femtosecond level detection resolution, the signal-to-noise ratio and signal strength of the loop error signal can be greatly improved.

[0041] The locking system uses a pulse width feedback locking loop and a power-based auxiliary locking loop to work simultaneously. The pulse width feedback locking loop has a Hz-level bandwidth and high gain characteristics, while the power-based auxiliary locking loop has a kHz-MHz level of greater bandwidth. The matching application of the two can realize more stable OPO cavity length locking.

[0042] The application provides a cavity length locking method and system of a pulse type OPO. The locking system comprises a pulse width feedback locking loop and a power-based auxiliary locking loop. The former obtains a pulse width variable with micro-watt level detection sensitivity and femtosecond level detection resolution by using a commercial autocorrelator, can obtain a high signal-to-noise ratio and a high intensity error signal at a Hz level bandwidth, and thus realizes efficient suppression of low frequency noise; the latter uses a power feedback quantity and a kHz-MHz level larger bandwidth locking loop to realize suppression of medium and high frequency noise. The two are matched to effectively suppress the noise power spectrum in a MHz level bandwidth. In particular, the application has a very efficient suppression level for low frequency noise in a Hz level bandwidth, and thus greatly improves the long-term stability of the pulse type OPO.

[0043] Secondly, the expected income and commercial value of the technical scheme of the application after transformation are:

[0044] The technical scheme of the application uses the pulse width in the time domain waveform as a feedback quantity, uses the ultra-short pulse measurement technology to improve the signal-to-noise ratio and intensity of the error signal, and is expected to realize an OPO pulse light source with a higher locking level. This not only simplifies the system design, but also improves the stability and performance of the system. It will bring significant optical application and control application benefits, and has a wide market prospect and commercial value in the fields of infrared spectrum measurement, biomedical research, atmospheric environment detection, laser radar, optical communication, quantum information processing and infrared light frequency precision measurement.

[0045] (2) The technical scheme of the application fills the technical gap at home and abroad:

[0046] The technical scheme of the application fills an important gap in the cavity length locking technology of an optical parametric oscillator (OPO) at home and abroad. The traditional method mainly relies on external modulation, while this scheme innovatively uses the pulse width as a feedback quantity, which is the inverse Fourier transform item of the parasitic light pulse spectrum, without external modulation. In combination with the progress of the ultra-short pulse measurement technology, real-time high sensitivity and high resolution measurement are realized, the signal-to-noise ratio and signal intensity of the error signal are effectively improved, and a new way is provided for an OPO pulse light source with a higher locking level.

[0047] (3) The technical scheme of the application solves the technical problems that people have been eager to solve but have failed to succeed:

[0048] The technical scheme of the application solves the technical problem that the signal-to-noise ratio and signal intensity of the error signal are limited when the OPO resonance peak is locked, and realizes an OPO pulse light source with a higher locking level by using the pulse width as a new feedback quantity.

[0049] (4) The technical scheme of the application overcomes technical prejudice:

[0050] The technical prejudice overcome by the technical solution of the present application mainly lies in the traditional cognition of the OPO resonance peak top locking mode. In the past, the technical workers may be more inclined to rely on external modulation means to achieve the locking target, and consider that the parasitic light pulse spectral power as a feedback quantity has limitations, such as being limited by spectral shape and detection sensitivity. However, the new scheme successfully realizes higher level of OPO cavity length locking by innovatively using the inverse Fourier transform item (i.e. pulse width) of the parasitic light pulse spectrum as a new feedback quantity, and with the progress of ultra-short pulse measurement technology, breaking this traditional technical prejudice. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 is the cavity length locking system structure diagram of the pulse type OPO provided by the embodiment of the present application;

[0052] Figure 2 is the schematic diagram of the optical parametric oscillator provided by the embodiment of the present application;

[0053] Figure 3 is the schematic diagram of the pulse width measuring device (autocorrelator) provided by the embodiment of the present application. Wherein, (a) Michelson interference structure; (b) Mach-Zehnder interference structure;

[0054] Figure 4 is the cavity length locking method flow chart of the pulse type OPO provided by the embodiment of the present application;

[0055] Figure 5 is the related OPO leakage spectrum obtained by the technical effect of the embodiment of the present application;

[0056] Figure 6 is the response curve between the parasitic pulse pulse width variable, power variable and cavity length variable obtained by the technical effect of the embodiment of the present application. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical scheme and advantages of the present application more clear and understandable, the present application will be further described in detail below with examples. It should be understood that the specific examples described here are only used to explain the present application, and are not used to limit the present application.

[0058] As shown in Figure 1 , the present application proposes a cavity length locking system of pulse type OPO. The system mainly includes OPO pulse light source, pulse width feedback locking loop and auxiliary locking loop based on power.

[0059] 1) OPO pulse light source, including seed pulse light source and OPO resonant cavity;

[0060] Furthermore, the seed pulse source can be a solid-state or fiber pulsed laser, whose output pulsed laser is coupled into the OPO resonant cavity, where parametric conversion is performed inside the nonlinear crystal to generate the required resonant pulse, and parasitic light pulses are generated in a non-phase-matched manner.

[0061] 2) such as Figure 3 As shown, the pulse width feedback locking loop consists of a pulse width measuring device, a signal processing circuit, and a cavity length control element;

[0062] The pulse width measurement device is used to measure the pulse width of the parasitic optical pulses output by the OPO in real time and convert it into a digital voltage signal. The signal processing circuit compares the pulse width voltage signal with a reference voltage signal to obtain an error signal, and then uses a digital proportional-integral-derivative (PID) circuit to obtain a servo signal, which is fed back to the cavity length control element. The cavity length control element adjusts the cavity length according to the servo voltage signal output by the signal processing circuit.

[0063] Furthermore, the pulse width measurement device includes a filtering optical path and an autocorrelator. The filtering optical path, based on waveplates, filters, and mirrors, is used to extract parasitic light pulses from the OPO leakage spectrum. The autocorrelator, based on a Michelson or Mach-Zehnder interferometer, measures the pulse width in real time, achieving power sensitivity at the microwatt level and numerical resolution at the femtosecond level.

[0064] Furthermore, the signal processing circuit mainly consists of a digital PID circuit and a digital-to-analog converter circuit. The digital PID circuit has high gain and low bandwidth feedback loop characteristics, mainly targeting low-frequency noise suppression within the Hz-level bandwidth of the signal. The digital-to-analog converter circuit converts the servo digital signal into an analog signal.

[0065] The cavity length control element can be a piezoelectric ceramic or a stepper motor, which will actively control the OPO cavity length according to the output voltage of the signal processing circuit.

[0066] 3) The power-based auxiliary locking loop consists of a power signal measurement device, a signal processing circuit, and a cavity length control element;

[0067] The power signal measurement device includes a filtered optical path and a low-noise power detector. The filtered optical path, based on waveplates, filters, and mirrors, extracts parasitic light pulses from the OPO leakage spectrum. The low-noise power detector features a kHz bandwidth and a low-noise detection level.

[0068] Furthermore, the signal processing circuit mainly consists of a comparator circuit and an analog PID circuit;

[0069] The comparator circuit compares the power sensing voltage with a reference voltage to obtain an error signal. This comparator circuit has a wide bandwidth. The analog PID circuit has wide feedback loop characteristics, primarily for effectively suppressing noise in the kHz-MHz frequency band.

[0070] The cavity length control element is a piezoelectric ceramic or a stepper motor, which is driven by a servo voltage to actively control the OPO cavity length.

[0071] 4) such as Figure 4 As shown, the locking steps of this system include:

[0072] After OPO resonance, two leakage beams are used to construct a power-based auxiliary locking loop and a pulse-width feedback locking loop, respectively. First, the power-based auxiliary locking loop is operated. A signal measurement device measures the power variable related to the cavity length jitter of the parasitic optical pulse, and a comparison circuit obtains an error voltage signal. This error voltage signal is then processed by an analog PID circuit to obtain a servo voltage signal, which drives the cavity length control element to achieve closed-loop locking of one cavity length. This feedback loop has a bandwidth in the kHz-MHz range and actively suppresses noise in the kHz-MHz frequency band of the OPO pulse. Next, the pulse-width feedback locking loop is operated. An autocorrelation meter obtains the pulse width parameters of the parasitic pulse, and a signal processing circuit obtains a high signal-to-noise ratio, high-intensity error signal, and a high-gain, low-bandwidth servo signal. This signal drives the other cavity length control element to achieve closed-loop locking of the other cavity length. This feedback loop has a bandwidth in the Hz range and actively suppresses noise in the Hz frequency band of the OPO pulse. The two loops operate simultaneously, ultimately achieving stable locking of the OPO cavity length.

[0073] This invention can be widely applied in fields requiring high-precision and high-stability light sources, such as infrared spectroscopy measurement, biomedical research, atmospheric environment monitoring, lidar, optical communication, quantum information processing, and infrared optical frequency precision measurement. By improving the OPO locking performance, it provides more reliable and powerful pulsed light source support for these fields.

[0074] Taking high-energy ytterbium-doped fiber pulse synchronous pumping of MgO:PPLN crystal OPO as an example, the OPO has a pump pulse center wavelength of 1040 nm, a pulse width of 200 fs, and a peak intensity density of approximately 1 GW / cm². 2 A 1700nm signal light and a 2660nm idler light were generated using an MgO:PPLN crystal (temperature controlled at 60℃) with a length of 1mm and a period of 31.3μm. The OPO cavity mirror coating ensured single resonance of the signal light pulse. The leaked spectrum after final resonance is shown below. Figure 5Besides the pump light (p), signal light (s) and idler light (i), the parasitic lights of pump and signal sum frequency (p+s), pump and idler sum frequency (p+i), signal frequency doubling (2s) and pump frequency doubling (2p) are also included.

[0075] The pump and signal sum frequency parasitic light is used to obtain the error signal. By adjusting the cavity length, the response curves of the parasitic light pulse width, power and cavity length variable are measured in turn, as shown in Figure 6 It can be seen that there are linear response intervals for both, indicating that both can be used as feedback for locking the cavity length. After removing the dimensional interference, if normalized, the response coefficient of the pulse width is higher than that of the power, which shows that in the appropriate working interval of the OPO, compared with the power feedback, the pulse width feedback can obtain higher signal-to-noise error signal. And because the measurement resolution of the autocorrelator can reach an optical period (2fs at 630nm), the measurement light intensity reaches the micro-watt level. This measurement accuracy can further improve the intensity and signal-to-noise ratio of the measured error signal.

[0076] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any modification, equivalent replacement and improvement within the technical range disclosed by the present application and within the spirit and principle of the present application should be covered within the protection scope of the present application.

Claims

1. A cavity length locking system for a pulsed optical parametric oscillator, characterized by, The OPO pulse light source comprises a seed pulse light source and an OPO resonant cavity. The OPO pulse light source comprises a seed pulse light source and an OPO resonant cavity. The pulse width feedback locking loop comprises a pulse width measurement device, a signal processing circuit and a cavity length control element. The pulse width measurement device is used for measuring the pulse width of the parasitic light pulse output by the OPO in real time and converting the measurement result into a digital voltage signal. The signal processing circuit compares the pulse width voltage signal with a reference voltage signal to obtain an error signal and generates a servo signal through a digital proportional-integral-differential circuit, which is fed back to the cavity length control element.

2. The cavity length locking system for a pulsed optical parametric oscillator of claim 1, wherein, The cavity length control element adjusts the cavity length according to the servo voltage signal output by the signal processing circuit.

3. The cavity length locking system for a pulsed optical parametric oscillator of claim 1, wherein, The power-based auxiliary locking loop comprises a power signal measurement device, a signal processing circuit and a cavity length control element.

4. The cavity length locking system for a pulsed optical parametric oscillator of claim 1, wherein, The power signal measurement device comprises a filtering optical path and a low-noise power detector.

5. The cavity length locking system for a pulsed optical parametric oscillator of claim 1, wherein, The filtering optical path comprises a wave plate, a filter and a mirror, and extracts the parasitic light pulse in the OPO leakage spectrum.

6. The cavity length locking system for a pulsed optical parametric oscillator of claim 1, wherein, The low-noise power detector has a kHz bandwidth and a low-noise detection level. The seed pulse light source is a solid-state or fiber pulse laser, and the output pulse laser is coupled into the OPO resonant cavity to generate the required resonant pulse through parametric conversion in the nonlinear crystal inside the cavity, and the parasitic light pulse is generated by non-phase matching. The pulse width measurement device comprises a filtering optical path and an autocorrelator.

7. A cavity length locking method of a pulsed optical parametric oscillator using the cavity length locking system of the pulsed optical parametric oscillator according to any one of claims 1 to 6, characterized by, The filtering optical path comprises a wave plate, a filter and a mirror, and is used to extract the parasitic light pulse in the OPO leakage spectrum. The autocorrelator is based on a Michelson interferometer structure or a Mach-Zehnder interferometer structure. The signal processing circuit of the pulse width feedback locking loop comprises a digital PID circuit and a digital-to-analog conversion circuit. The digital PID circuit has a high-gain and low-bandwidth feedback loop characteristic, and is used to suppress low-frequency noise in the Hz-level bandwidth.

8. The cavity length locking method of claim 7, wherein, The digital-to-analog conversion circuit converts the servo digital signal into an analog signal. The cavity length control element is a piezoelectric ceramic or a stepper motor, which actively controls the OPO cavity length according to the output voltage of the signal processing circuit. The signal processing circuit of the power-based auxiliary locking loop comprises a comparison circuit and an analog PID circuit. The comparison circuit compares the power detection voltage with a reference voltage to obtain an error signal. The analog PID circuit has a wide-band feedback loop characteristic and is used to effectively suppress kHz-MHz-level noise. The cavity length control element of the power-based auxiliary locking loop is a piezoelectric ceramic or a stepper motor, which is driven by the servo voltage to actively control the OPO cavity length. The method comprises the following steps: (1) Extracting the parasitic light pulse output by the OPO, measuring the pulse width in real time through a pulse width measurement device, and converting the measurement result into a digital voltage signal; (2) Comparing the pulse width voltage signal with a reference voltage signal, and generating a servo signal through a digital proportional-integral-differential (PID) circuit; (3) Adjusting the cavity length of the OPO resonant cavity according to the servo signal through a cavity length control element to realize real-time locking of the pulse width. In step (1), the method for extracting the parasitic light pulse of the OPO comprises: The parasitic light pulse in the OPO leakage spectrum is extracted by a filtering light path composed of a wave plate, a filter and a mirror; The autocorrelator is used to measure the pulse width based on the Michelson interference structure or the Mach-Zehnder interference structure, and the measurement sensitivity reaches the micro-watt level and the resolution reaches the femtosecond level.

9. A power-assisted cavity length locking method using the cavity length locking system of the pulsed optical parametric oscillator according to any one of claims 1 to 6, characterized by, The method comprises the following steps: (1) extracting the OPO parasitic light pulse, and measuring the power signal thereof in real time by a low-noise power detector; (2) comparing the power signal with a reference power signal, and generating a servo signal by an analog PID circuit; (3) dynamically adjusting the cavity length of the OPO resonant cavity according to the servo signal by using a piezoelectric ceramic or a stepping motor, so as to realize the power-assisted cavity length locking.

10. The power assist based cavity length locking method of claim 9, wherein, In step (1), the low-noise power detector has a kHz bandwidth and a low-noise detection level, can realize high-sensitivity detection when the parasitic light pulse power is weak, and converts the power signal into a voltage signal for subsequent processing.

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