An absolute distance measurement method and device of a dual optical comb differential time domain signal

Abstract

The application relates to an absolute distance measurement method of a double optical comb differential time domain signal, wherein a hetero-frequency optical frequency comb is combined by a fiber combiner and a one-to-two optical fiber beam splitter to form mixed signal light and mixed local light; the mixed signal light passes through a half-reflective half-transmissive lens and a target corner pyramid to generate mixed reference signal light and mixed measurement signal light, and then the mixed reference signal light and the mixed measurement signal light are combined by a polarization beam splitter together with the mixed local light; the combined light is frequency-doubled twice to generate a second harmonic signal with a fixed time delay, and the second harmonic signal is input into a double-port balanced differential detector; the differential operation of the detector generates interference signals with opposite slopes, and the absolute distance is obtained through slope separation, ambiguity number judgment and formula simultaneous solution.The application has the advantages that a rough measurement device and optical comb modulation are not needed, a single measurement breaks through the non-ambiguity range limit, the extraction of an attosecond time interval is realized by relying on differential detection, the distance measurement precision is high, the optical path is modularized, the assembly and adjustment are simple, the anti-interference performance is strong, the dynamic measurement has no step error judgment, and the application is suitable for precise distance measurement requirements in multiple scenes.

Description

Technical Field

[0001] This invention belongs to the field of precision measurement technology, specifically relating to a method and apparatus for measuring the absolute distance of a dual-comb differential time-domain signal. Background Technology

[0002] Optical frequency combs, with their evenly spaced frequency comb teeth, establish a precise connection between optical and microwave frequencies, providing a high-precision, real-time technical means for absolute distance measurement. They hold significant application potential in numerous fields, including industrial precision machining, aerospace assembly, geodesy, and metrology standards. Utilizing their high time resolution and high frequency stability, they can achieve high-precision, large-scale absolute distance measurements.

[0003] However, several technical challenges remain in large-scale spatial multi-target coordinate measurement based on femtosecond optical frequency combs. In femtosecond optical frequency comb absolute distance measurement methods, the measurement results are closely related to the unambiguous range, which depends solely on the repetition frequency of the measuring optical frequency comb. In practical measurements, the distance measurements within integer multiples and the unambiguous length range jointly determine the final measurement result. To address this issue, existing technologies typically employ additional coarse measurement devices to estimate integer multiples, or perform secondary measurements by changing the repetition frequency of the optical frequency comb to determine this multiple. However, both introducing additional coarse measurement devices and changing the frequency for secondary measurements increase the complexity of the measurement system structure and reduce the efficiency of distance measurement. Especially in dynamic measurement scenarios, multiple measurements can easily lead to errors in integer multiple measurements, thus affecting the accuracy of the measurement results.

[0004] Patent CN117130005A discloses a blind-zone-free, large-unambiguity-range dual-comb ranging device and method. By setting up a dual-channel dual-comb ranging optical path, it receives dual-comb beams emitted by a dual-comb light source for testing the target to obtain reference and measurement interference signals. A coarse ranging module combines the emitted test light with the signal light emitted from the dual-channel dual-comb ranging optical path to form a coarse ranging beam for testing the target and obtaining coarse measurement results. A signal processing module calculates the dual-comb ranging results based on the reference and measurement interference signals and fuses the dual-comb ranging results with the coarse ranging results to obtain the final ranging result. This effectively solves the problem of measurement failure caused by the overlap of reference and measurement interference signals in the time domain, eliminating the "blind zone" in ranging over a large dynamic range. However, in practical applications, an additional coarse ranging module is required, which leads to a complex system structure, high hardware costs, and reliance on the fusion of fine and coarse measurement results to solve fuzzy numbers. The measurement process is cumbersome and inefficient. In dynamic measurements, the synchronization problem between coarse and fine measurements can easily cause errors in the judgment of integer multiples, resulting in poor dynamic adaptability.

[0005] Patent CN119064946A discloses an absolute distance measurement system and method based on an on-chip crossed dual optical comb. This invention generates two sets of microcavity soliton optical combs with slightly different repetition frequencies and misaligned comb teeth by setting up an on-chip dual optical comb generation module, a ranging module, a detection module, and a data processing module. The two optical combs are combined for dispersive interferometry ranging, and then the spectrum containing distance information and the repetition frequencies of the two optical combs are detected and obtained. Data processing is performed by the data processor to obtain the absolute distance. Combining the two optical combs achieves a significant expansion of the unambiguous range and precise distance calculation. However, it still relies on spectral detection and complex data calculation using dispersive interferometry. It requires splitting the aliased dispersive interferometry spectrum and iteratively solving multiple sets of formulas. The signal processing is cumbersome, the detection equipment requires high precision, and it does not achieve high-precision time-domain extraction of the measurement signal, limiting the time resolution and making it difficult to further improve the accuracy of the ranging.

[0006] Therefore, there is an urgent need for a better method for measuring absolute distance to improve the accuracy of distance measurement and simplify the measurement process. Summary of the Invention

[0007] The present invention aims to solve the problems of insufficient accuracy and cumbersome measurement process in current absolute distance measurement methods.

[0008] The present invention solves the above-mentioned technical problems through the following technical means:

[0009] An absolute distance measurement method for dual-comb differential time-domain signals includes the following steps:

[0010] Step S1: The two femtosecond optical frequency combs emitted from the two sets of femtosecond laser sources are mixed by an optical fiber combiner to form a hybrid optical frequency comb;

[0011] Step S2: Use a 1-to-2 fiber optic beam splitter to split the hybrid optical frequency comb into two beams, which are denoted as hybrid signal light and hybrid local light.

[0012] Step S3: The mixed signal light is split by a half-reflective lens. The part reflected in the original path is recorded as the mixed reference signal light, and the remaining part that is directed toward the target and reflected is recorded as the mixed measurement signal light.

[0013] Step S4: After combining the mixed reference signal light, the mixed measurement signal light and the mixed local light, the combined light is passed through the first and second type frequency doubling crystal to generate the second harmonic signal light and part of the combined light that has not undergone frequency doubling effect, and the second harmonic signal light is input into port a of the balanced differential detector.

[0014] Step S5: Pass the remaining combined light that has not undergone frequency doubling through the second type II frequency doubling crystal to generate the second harmonic signal light again, and input the second harmonic signal light into port b of the balanced differential detector.

[0015] Step S6: The balanced differential detector performs difference calculation on the signals from ports a and b to generate two sets of mixed dual-comb differential time-domain interference signals with opposite positive and negative slopes at the center zero point. The slope characteristics of the two sets of interference signals are separated and identified, the ambiguity multiple relationship is determined, and the distance is calculated by solving the formulas simultaneously.

[0016] As the core method of this application, this method innovatively proposes a technical solution of "hybrid dual optical comb generation + two second harmonic generation + balanced differential detector dual-port input + slope feature separation signal + simultaneous ambiguity calculation". It does not require additional coarse measurement device and optical frequency comb repetition frequency modulation, and realizes single direct measurement of large absolute distance. At the same time, it achieves accurate separation of two sets of dual optical comb signals by using the slope characteristics of the hybrid differential interference signal, thus breaking through the non-ambiguity range limitation of traditional dual optical comb ranging from the methodological level.

[0017] Preferably, the two sets of femtosecond optical frequency combs in step S1 have different frequencies, namely... and ,and In the formula This indicates the repetition frequency of the first femtosecond laser source output. This indicates the repetition frequency of the second femtosecond laser source output. This represents the difference in output repetition frequency between the first femtosecond laser source and the second femtosecond laser source.

[0018] Based on engineering experience, the range of frequency difference was determined, which is consistent with the principle of optical asynchronous sampling and lays the foundation for the formation and recognition of two sets of dual optical comb signals.

[0019] Preferably, in step S4, before the combined light enters the first type II frequency doubling crystal, it is further focused by a first convex lens; after the combined light passes through the first type II frequency doubling crystal, the outgoing light is further collimated by a second convex lens.

[0020] Focusing operations can enhance the light field intensity within the crystal, meeting the requirements of nonlinear optical effects in the generation of second harmonics and ensuring efficient generation of second harmonics. Collimation operations can restore the converged light emitted from the crystal to parallel light, ensuring stable signal transmission in subsequent optical paths. This refines the technical details of first-order second harmonic generation and makes the method more practical.

[0021] Preferably, in step S4, the outgoing light passing through the second convex lens includes a second harmonic signal light and a portion of the combined light that has not undergone frequency doubling. The second harmonic signal light and the portion of the combined light that has not undergone frequency doubling pass through a high-pass dichroic mirror. The second harmonic signal light is reflected by the high-pass dichroic mirror and enters the balanced differential detector. The combined light that has not undergone frequency doubling passes through the high-pass dichroic mirror.

[0022] This technology achieves precise separation of the optical paths of the first second harmonic signal light and the remaining fundamental frequency light, ensuring that the second harmonic signal light is directionally input into the a port of the balanced differential detector, while also allowing the remaining fundamental frequency light to be transmitted to the subsequent optical path without interference to generate the second second harmonic, thus making the technical solution for second harmonic generation and transmission more complete.

[0023] Preferably, in step S5, before the combined light without frequency doubling effect enters the second type II frequency doubling crystal, it is further focused by a third convex lens; after the combined light without frequency doubling effect passes through the second type II frequency doubling crystal, the outgoing light is further collimated by a fourth convex lens.

[0024] The optical path design for generating the first second harmonic forms a unified "focusing-collimation" system, which not only ensures the efficient generation of the second second harmonic but also keeps the second harmonic signal light parallel and stable, effectively improving the practicality of the method and the stability of signal transmission.

[0025] Preferably, in step S5, a fixed time delay is introduced into the second type II frequency doubling crystal. τ 0.

[0026] This feature is the physical basis for generating hybrid differential time-domain interference signals and a key technical means to achieve opposite positive and negative slopes at the center zero points of two sets of hybrid dual-comb differential time-domain interference signals. It makes the generation principle of hybrid differential interference signals clearer and is directly related to the core innovation of this invention, "separating and identifying hybrid dual-comb signals through slope polarity features," which is fundamentally different from traditional dual-comb ranging methods.

[0027] Preferably, in step S6, the distance result calculated from the dual-comb signal formed by the local light is: The distance calculated from the dual-comb signal formed by the measurement signal light and the local light is as follows: Using the differential time-domain interference signal formula, it is found that when the overlap directions of the two sets of optical frequency comb signals are different, the slopes of the signal center zeros are opposite. The expression is as follows: In the formula, c is the speed of light; m 1, m 2 represents an integer multiple of their respective unfuzzy ranges; n g Δ is the refractive index of air. t 1. Δ t 2 represents the time interval between the reference signal light and the measurement signal light for each dual-comb signal measurement. The distance to the target being measured. The repetition frequency of the first femtosecond laser source. This is the repetition frequency of the second femtosecond laser source. This indicates the unambiguous range of the first group of dual optical comb signals. This indicates the unambiguous range of the second set of dual optical comb signals. This represents the fractional distance within the ambiguity range obtained from the measurement of the first set of dual optical comb signals. This represents the fractional distance within the ambiguity range obtained from the measurement of the second set of dual optical comb signals; and These are the peak intensities of the two light pulses. and These are the pulse widths of a single reference signal light or measurement signal light and a single local light, respectively. This represents the relative time offset between the two pulses. This is a fixed time delay generated by the second type of frequency doubling crystal.

[0028] A complete technical chain of "optical operation - signal characteristics - mathematical solution" was established, providing quantitative theoretical support for the method steps. At the same time, the core solution parameters such as integer multiples of the unambiguous range, the time interval between the reference / measurement signal light, and the unambiguous range were clarified, and the fractional distance and unambiguous range of the two sets of dual optical comb signals were distinguished. This further demonstrates the technical advantage of the present invention in eliminating the unambiguous range limitation through simultaneous solution, ensuring the accuracy and repeatability of the measurement method.

[0029] Preferably, the method further includes an apparatus for measuring the absolute distance of a dual-comb differential time-domain signal, comprising a light source and beam splitting system, a signal transmission and beam combining system, a differential time-domain interferometry system, and a signal processing and calculation system. The light source and beam splitting system includes a rubidium atomic clock, a first femtosecond laser source, a second femtosecond laser source, an optical fiber combiner, and a 1-to-2 optical fiber beam splitter. The rubidium atomic clock is connected to the first femtosecond laser source and the second femtosecond laser source, respectively. The first femtosecond laser source and the second femtosecond laser source are respectively connected to different entrance optical fibers of the optical fiber combiner. The optical fiber combiner is connected to the optical fiber of the 1-to-2 optical fiber beam splitter. The signal processing and calculation system is used to receive the mixed differential time-domain interferometry signal output by the balanced differential detector, separate and identify the two sets of dual-comb ranging signals, extract the time interval, and solve the ranging formula to obtain the absolute distance of the target being measured.

[0030] The device is divided into four functional modules: light source and beam splitting system, signal transmission and beam combining system, differential time-domain interferometry system, and signal processing and calculation system. The functional positioning and technical synergy of each module are clearly defined. The rubidium atomic clock provides time reference for the femtosecond laser light source, and the signal processing and calculation system completes the core functions of signal separation and distance calculation. It achieves a high degree of correspondence between "measurement method and measurement device". The device structure has no redundant modules and does not require the addition of coarse measurement modules. It breaks through the technical bottleneck of the complex structure of traditional dual-comb ranging devices. The technical solution is significantly innovative.

[0031] Preferably, the signal transmission and beam combining system includes an optical fiber circulator, an optical fiber collimator, a semi-reflective mirror, a target pyramid, a quarter-wave plate, and a polarizing beam splitter; the optical fiber collimator includes a first optical fiber collimator, a second optical fiber collimator, and a third optical fiber collimator; the quarter-wave plate includes a first quarter-wave plate and a second quarter-wave plate; the one-to-two optical fiber beam splitter is optically connected to the optical fiber circulator and the third optical fiber collimator; the optical fiber circulator is optically connected to the first optical fiber collimator and the second optical fiber collimator; one end of the semi-reflective mirror is connected to the optical path of the first optical fiber collimator, and the other end is connected to the target pyramid. The optical path of the target cone is connected; one end of the first quarter-wave plate is connected to the optical path of the second fiber collimator, and the other end is connected to the optical path of the polarizing beam splitter; one end of the second quarter-wave plate is connected to the optical path of the third fiber collimator, and the other end is connected to the optical path of the polarizing beam splitter; the mixed signal light is directed to the half-reflecting half-lens after passing through the fiber circulator and fiber collimator, and is split into a mixed reference signal light and a mixed measurement signal light directed to the target cone. After the mixed reference signal light and the mixed measurement signal light are returned through the fiber circulator, they are combined with the mixed local light that has passed through the fiber collimator and quarter-wave plate at the polarizing beam splitter.

[0032] Preferably, the differential time-domain interferometry system includes a convex lens, a second-class frequency-doubling crystal, a high-pass dichroic mirror, a reflector, and a balanced differential detector; the convex lens includes a first convex lens, a second convex lens, a third convex lens, and a fourth convex lens; the second-class frequency-doubling crystal includes a first second-class frequency-doubling crystal and a second second-class frequency-doubling crystal; the reflector includes a first reflector, a second reflector, and a third reflector; one end of the first convex lens is connected to a polarizing beam splitter, and the other end is connected to the first second-class frequency-doubling crystal; one end of the second convex lens is connected to the optical path of the first second-class frequency-doubling crystal, and the other end is connected to the optical path of the high-pass dichroic mirror; one end of the third convex lens is connected to the optical path of the high-pass dichroic mirror, and the other end is connected to the optical path of the second second-class frequency-doubling crystal; the fourth convex lens... One end of the second mirror is connected to the optical path of the second type II frequency doubling crystal, and the other end is connected to the optical path of the first mirror. One end of the second mirror is connected to the optical path of the first mirror, and the other end is connected to the optical path of the third mirror. One end of the third mirror is connected to the optical path of the second mirror, and the other end is connected to one entrance optical path of the balanced differential detector. The other entrance of the balanced differential detector is connected to the optical path of the high-pass dichroic mirror. The combined beam passes through the first set of convex lenses and the first type II frequency doubling crystal to generate the first second harmonic, which is then reflected by the high-pass dichroic mirror and input into one entrance of the balanced differential detector. The remaining fundamental frequency beam passes through the second set of convex lenses and the second type II frequency doubling crystal to generate the second second harmonic with a fixed time delay, which is then guided by multiple mirrors and input into the other entrance of the balanced differential detector.

[0033] The advantages of this invention are:

[0034] (1) Precise signal processing and significantly improved measurement accuracy: By generating two second harmonics in a step-by-step manner and using a balanced differential detector for dual-port differential detection, a hybrid dual-comb differential time-domain interference signal with opposite positive and negative slopes at the center zero point is generated. The time resolution near the zero point reaches the attosecond level, which can extract the time interval between the reference / measurement signal light with high precision. At the same time, relying on the slope polarity characteristics, the two sets of hybrid dual-comb signals are accurately separated and identified, avoiding the calculation error caused by signal aliasing, and greatly improving the accuracy and reliability of distance measurement.

[0035] (2) Innovation in ranging system, breaking through the bottleneck of traditional technology: abandoning the coarse measurement device and optical frequency comb repetition modulation method that traditional dual optical comb ranging relies on, and through the innovative system of hybrid dual optical comb construction + differential time domain interference signal slope feature recognition, the integer multiple of non-fuzzy range can be directly calculated in a single measurement, which fundamentally breaks through the non-fuzzy range limitation of traditional technology, realizes the direct measurement of absolute distance in large space, and solves the core pain points of traditional schemes such as complex system and low measurement efficiency;

[0036] (3) The system structure is simplified and the hardware cost and installation difficulty are reduced: The modular optical path design is adopted. Two femtosecond optical frequency combs with fixed repetition frequency are mixed by only using an optical fiber combiner. Combined with a differential time domain interference system composed of conventional devices such as frequency doubling crystals, convex lenses, and high-pass dichroic mirrors, signal separation and distance calculation can be achieved without the need to add an extra coarse measurement module. This simplifies the optical system structure, reduces the hardware procurement cost and optical path installation difficulty, and is more suitable for industrial-scale applications.

[0037] (4) Strong dynamic adaptability and suitable for complex measurement scenarios: Two sets of dual optical comb ranging signals are acquired synchronously through a single measurement. The non-fuzzy range integer multiple association is automatically determined by combining the time interval difference relationship. No multiple measurements or frequency scanning are required. This effectively avoids the problem of misjudgment of integer multiple step due to target movement in dynamic measurement, improves the stability of measurement results, and can be adapted to dynamic / multi-target measurement scenarios such as industrial precision machining and aerospace assembly.

[0038] (5) The method and device are highly coordinated, and the implementation and scalability are strong: the measurement method and the supporting device form a closed-loop design. The method steps clearly define the core optical operation and mathematical calculation logic. The device adopts a modular hierarchical structure of light source beam splitting, signal transmission beam combining, differential time domain interference, and signal processing calculation. The functions of each module are coordinated and the optical path design is scientific. This not only ensures the actual operability of the technical solution, but also reserves space for subsequent technical optimization and integrated improvement. It has good engineering promotion value. Attached Figure Description

[0039] Figure 1 This is a flowchart of an absolute distance measurement method for dual-comb differential time-domain signals according to a first embodiment of the present invention;

[0040] Figure 2 Two optical frequency comb waveforms are shown in the first embodiment of the present invention for an absolute distance measurement method of a dual optical comb differential time domain signal;

[0041] Figure 3 This is a simulation data diagram of differential time-domain interferometry signals for an absolute distance measurement method of dual-comb differential time-domain signals according to the first embodiment of the present invention.

[0042] Figure 4 This is a graph showing the dynamic changes of parameters caused by the dynamic changes of the target cone in an absolute distance measurement method for dual-comb differential time-domain signals according to the first embodiment of the present invention.

[0043] In the picture:

[0044] 1. First femtosecond laser source; 2. Second femtosecond laser source; 3. Fiber optic combiner; 4. 1-to-2 fiber optic splitter; 5. Fiber optic circulator; 6. First fiber optic collimator; 7. Target pyramid; 8. Second fiber optic collimator; 9. First quarter-wave plate; 10. Rubidium atomic clock; 11. Third fiber optic collimator; 12. Second quarter-wave plate; 13. Polarizing beam splitter; 14. First convex lens; 15. First type II frequency doubling crystal; 16. Second convex lens; 17. High-pass dichroic mirror; 18. Third convex lens; 19. Second type II frequency doubling crystal; 20. Fourth convex lens; 21. First reflecting mirror; 22. Second reflecting mirror; 23. Third reflecting mirror; 24. Balanced differential detector; 25. Semi-reflective mirror. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] Example 1:

[0047] This embodiment provides an absolute distance measurement method for dual-comb differential time-domain signals, specifically an absolute distance measurement method based on hybrid dual-comb differential time-domain interference signals.

[0048] See Figure 1This application presents an absolute distance measurement method for dual-comb differential time-domain signals based on a hybrid dual-comb differential time-domain interferometric signal absolute distance measurement device. To facilitate understanding of the method, this embodiment first provides a preliminary analysis of the device. The absolute distance measurement device based on hybrid dual-comb differential time-domain interferometric signals in this embodiment includes: a light source and beam splitting system, a signal transmission and beam combining system, a differential time-domain interferometric system, and a signal processing and calculation system. See details in the attached document. Figure 1 The light source and beam splitting system includes a rubidium atomic clock 10, a first femtosecond laser source 1, a second femtosecond laser source 2, an optical fiber combiner 3, and a one-to-two optical fiber beam splitter 4; the signal transmission and beam splitting system includes an optical fiber circulator 5, a first optical fiber collimator 6, a second optical fiber collimator 8, a third optical fiber collimator 11, a semi-reflective mirror 25, a target pyramid 7, a first quarter-wave plate 9, a second quarter-wave plate 12, and a polarizing beam splitter prism 13; the differential time-domain interferometry system includes a first convex lens 14, a second convex lens 16, a third convex lens 18, a fourth convex lens 20, a first type II frequency doubling crystal 15, a second type II frequency doubling crystal 19, a high-pass dichroic mirror 17, a first reflector 21, a second reflector 22, a third reflector 23, and a balanced differential detector 24.

[0049] The rubidium atomic clock 10 is connected to the first femtosecond laser source 1 and the second femtosecond laser source 2. The first femtosecond laser source 1 and the second femtosecond laser source 2 are connected to different entrance fibers of the fiber combiner 3. The fiber combiner 3 is connected to the fiber splitter 4. The fiber splitter 4 is connected to the fiber circulator 5 and the third fiber collimator 11. The fiber circulator 5 is connected to the first fiber collimator 6 and the second fiber collimator 8. One end of the semi-reflective mirror 25 is connected to the optical path of the first fiber collimator 6, and the other end is connected to the optical path of the target pyramid 7. One end of the first quarter-wave plate 9 is connected to the optical path of the second fiber collimator 8, and the other end is connected to the optical path of the polarizing beam splitter 13. One end of the second quarter-wave plate 12 is connected to the optical path of the third fiber collimator 11, and the other end is connected to the optical path of the polarizing beam splitter 13. One end of the first convex lens 14 is connected to the optical path of the polarizing beam splitter 13, and the other end is connected to the optical path of the first type II frequency doubling crystal 15; one end of the second convex lens 16 is connected to the optical path of the first type II frequency doubling crystal 15, and the other end is connected to the optical path of the high-pass dichroic mirror 17; one end of the third convex lens 18 is connected to the optical path of the high-pass dichroic mirror 17, and the other end is connected to the optical path of the second type II frequency doubling crystal 19; one end of the fourth convex lens 20 is connected to the optical path of the second type II frequency doubling crystal 19, and the other end is connected to the optical path of the first reflecting mirror 21; one end of the second reflecting mirror 22 is connected to the optical path of the first reflecting mirror 21, and the other end is connected to the optical path of the third reflecting mirror 23; one end of the third reflecting mirror 23 is connected to the optical path of the second reflecting mirror 22, and the other end is connected to one entrance optical path of the balanced differential detector 24; the other entrance of the balanced differential detector 24 is connected to the optical path of the high-pass dichroic mirror 17.

[0050] This embodiment presents an absolute distance measurement method based on hybrid dual-comb differential time-domain interferometry signals, comprising the following steps:

[0051] S1. The two femtosecond optical frequency combs emitted from the two sets of femtosecond laser sources are mixed by an optical fiber combiner to form a hybrid optical frequency comb.

[0052] The specific process of step S1 includes: the first femtosecond laser source 1 and the second femtosecond laser source 2 are externally synchronized with the rubidium atomic clock 10 and are in a stable working state; the output repetition frequency of the first femtosecond laser source is... =99.8MHz femtosecond optical frequency comb, the second femtosecond laser source output repetition frequency is =100MHz femtosecond optical frequency comb ( It should be much smaller But if If the refresh rate is too small, the dynamic measurement results will be poor. If the refresh rate is too high, the sampling rate requirements for the signal acquisition equipment will also increase. Based on experimental experience, a suitable refresh rate should be selected. In the embodiments of the present invention =200kHz); both optical frequency combs have stable optical power and femtosecond-level pulse widths, exhibiting an equally spaced comb tooth structure in the frequency domain and an equally spaced ultrashort pulse sequence in the time domain, without frequency drift or intensity fluctuations. The fiber output end of the first femtosecond laser source 1 is precisely connected to port b of the fiber combiner 3, and the fiber output end of the second femtosecond laser source 2 is connected to port c of the fiber combiner 3 to the same specifications. During the connection process, the fiber end faces are ensured to be clean, free of scratches and deflections, avoiding optical signal reflection, loss, or time / frequency domain distortion. The single-path hybrid optical frequency comb, after mixing by the fiber combiner 3, is output from port a of the combiner 3. This hybrid optical frequency comb provides a unified optical signal source for the 1-to-2 fiber splitter 4 in the subsequent step S2. The two repetition frequencies of the optical frequency combs are uniformly distributed in the mixed light, without spatial separation or signal aliasing distortion.

[0053] Step S1 integrates two different frequency femtosecond optical frequency combs through the fiber combiner 3, abandoning the traditional dual-comb ranging mode of "independent optical path transmission of local light and signal light". This allows the mixed signal light and mixed local light after beam splitting to simultaneously contain optical frequency combs with two repetition frequencies. This lays the foundation for the innovative design of "each optical frequency comb and the other optical frequency comb serving as both local optical frequency comb and signal optical frequency comb" and is a necessary technical prerequisite for generating two sets of dual-comb ranging signals and solving the integer multiples of the non-ambiguity range.

[0054] S2. Use a 1-to-2 fiber optic beam splitter to split the hybrid optical frequency comb into two beams, which are denoted as hybrid signal light and hybrid local light.

[0055] The specific process of step S2 includes: the single-channel hybrid optical frequency comb output from step S1 has been transmitted through optical fiber to the input port of the 1-to-2 fiber beam splitter 4, and this hybrid optical frequency comb also contains the first femtosecond laser source 1. The second femtosecond laser source 2 Two optical frequency comb sequences with different repetition frequencies, maintaining the inherent characteristics of femtosecond-level narrow pulse width, equal time-domain spacing, and stable optical power, can ensure that the two optical signals after beam splitting are free from characteristic distortion. The hybrid optical frequency comb output from port b of the 1-to-2 fiber beam splitter 4 is defined as the hybrid signal light. This light serves as the signal source for subsequently generating hybrid reference signal light and hybrid measurement signal light, and is transmitted through optical fiber to the fiber circulator 5 of the subsequent signal transmission and beam combining system. The hybrid optical frequency comb output from port c of the 1-to-2 fiber beam splitter 4 is defined as the hybrid local light. This light serves as the local fundamental frequency light that is subsequently combined with the reference / measurement signal light and undergoes a nonlinear frequency doubling effect, and is transmitted through optical fiber to the third fiber collimator 11 of the subsequent signal transmission and beam combining system.

[0056] Step S2 uses a 1-to-2 fiber beam splitter 4 to achieve homogeneous and homogeneous beam splitting of the hybrid optical frequency comb. The generated hybrid signal light and hybrid local light are "homogeneous dual-branch" optical signals—ensuring that the signal characteristics of the two light paths are completely consistent, and establishing independent transmission links for the "signal light branch" and the "local light branch," laying the optical path foundation for the subsequent splitting of the hybrid signal light (reference / measurement light) and the precise beam combining of the hybrid local light and the reference / measurement light. At the same time, since both light paths retain a complete dual-comb sequence, it also provides a signal basis for the subsequent dual-comb interference of "mutual local light / signal light."

[0057] S3. The mixed signal light is split by a half-reflective lens. The part reflected in the original path is recorded as the mixed reference signal light, and the remaining part that is directed toward the target and reflected is recorded as the mixed measurement signal light.

[0058] The specific process of step S3 includes: the mixed signal light output from port b of the 1-to-2 fiber optic beam splitter 4 (including...) and The optical signal is first transmitted through an optical fiber to port a of the optical fiber circulator 5, and then output from port b of the circulator 5, precisely connected to the first optical fiber collimator 6. The first optical fiber collimator 6 converts the optical field transmitted in the optical fiber into a parallel collimated spatial beam, ensuring that the mixed signal light is directed towards the subsequent half-reflective lens 25 in a uniform, non-divergent spatial optical path, and that the optical axis of the collimated beam is strictly coaxially aligned with the optical centers of the half-reflective lens 25 and the target pyramid 7. The parallel mixed signal light output from the first optical fiber collimator 6 is incident perpendicularly onto the beam splitting surface of the half-reflective lens 25. Part of the incident light is directly reflected along the original path by the beam splitting surface of the half-reflective lens 25 to form a mixed reference signal light. After being split by the half-reflective lens 25, most of the remaining mixed signal light passes through the half-reflective lens 25 in a transmission manner, maintaining a parallel optical path and continuing to be directed along the optical axis towards the target pyramid 7 at the target being measured. After the transmitted mixed signal light is incident on the target pyramid 7, it is reflected back along the original optical path and passes through the semi-reflective lens 25 again (the transmitted light does not split again when returning along the original path and passes directly through), precisely hitting the first fiber collimator 6 to form a mixed measurement signal light. The mixed reference signal light and the mixed measurement signal light are coupled back into the fiber by the first fiber collimator 6 and then merge into the same fiber, synchronously transmitted to port b of the fiber circulator 5; constrained by the unidirectional transmission characteristics of the fiber circulator 5, the two lights are output uniformly from port c of the fiber circulator 5 and transmitted to the subsequent second fiber collimator 8, preparing for the next step of combining with the mixed local light.

[0059] This step utilizes an integrated design of single beam splitting and two-path reflection to simultaneously generate a reference light and a measurement light carrying distance information on the same optical path. The two lights share the same origin and characteristics, effectively eliminating systematic errors caused by two independent optical paths and significantly improving the accuracy of ranging. The mixed reference signal light serves as the ranging time-domain reference, providing the time interval for subsequent extraction of the two sets of dual-comb signals. , A zero-point reference is provided, and the optical path difference information carried by the mixed measurement signal light is used to calculate the measured distance. The core physical quantity.

[0060] S4. After combining the mixed reference signal light, the mixed measurement signal light, and the mixed local light, the combined light is passed through the first and second type frequency doubling crystals to generate the second harmonic signal light and part of the combined light that has not undergone frequency doubling effect. The second harmonic signal light is then input into port a of the balanced differential detector.

[0061] The specific process of step S4 includes: the mixed reference signal light and mixed measurement signal light output from port C of the fiber optic circulator 5 in step S3 are transmitted through the fiber to the second fiber optic collimator 8, which converts the fiber light into parallel spatial light and directs it perpendicularly to the first quarter-wave plate 9. The first quarter-wave plate 9 completes the precise adjustment of the polarization state so that the polarization direction of the light path matches the beam combining requirements of the polarization beam splitter 13; the mixed local light output from port C of the one-to-two fiber optic beam splitter 4 in step S2 is transmitted through the fiber to the third fiber optic collimator 11, which converts the fiber light into parallel spatial light and directs it perpendicularly to the second quarter-wave plate 12 to complete the polarization state adaptation adjustment and ensure that the polarization state is orthogonal to the mixed reference signal light and the mixed measurement signal light. The mixed reference signal light, mixed measurement signal light, and mixed local light, after their polarization states have been adjusted by the first quarter-wave plate 9 and the second quarter-wave plate 12, are respectively incident on the two incident surfaces of the polarization beam splitter 13. Lossless, high-coupling polarization beam combining is achieved inside the polarization beam splitter 13, outputting a combined beam containing the mixed reference signal light, mixed measurement signal light, and mixed local light; this combined beam simultaneously retains the polarization of all three beams. and The dual-comb sequence features, and the optical axis is transmitted strictly along the optical path direction of the differential time-domain interference system, providing a unified optical signal source for the subsequent frequency doubling effect.

[0062] The combined beam output from the polarizing beam splitter 13 is directed along the optical axis toward the first convex lens 14. After being focused by the first convex lens 14, the light field energy is compressed into the effective working region of the first type II frequency doubling crystal 15. The focused combined beam passes through the first type II frequency doubling crystal 15, and part of the mixed local light undergoes nonlinear frequency doubling with the mixed reference signal light and the mixed measurement signal light, respectively, to generate the first second harmonic signal light (the wavelength of which is 1 / 2 of the original fundamental frequency light). The remaining fundamental frequency combined beam (mixed local light + mixed reference signal light + mixed measurement signal light) that has not undergone frequency doubling effect is emitted from the first type II frequency doubling crystal 15 together with the second harmonic signal light. The emitted light (second harmonic signal light + undoped fundamental frequency light) from the first and second type frequency doubling crystal 15 is directed towards the second convex lens 16 and collimated by the second convex lens 16, then converted into parallel spatial light. The collimated parallel light is incident perpendicularly onto the high-pass dichroic mirror 17, which achieves high reflection of the short-wavelength first and second harmonic signal light and high transmission of the long-wavelength undoped fundamental frequency light. The reflected first and second harmonic signal light is precisely directed along the optical path to port a of the balanced differential detector 24, completing the directional input of the first harmonic signal. The undoped fundamental frequency combined beam is transmitted through the high-pass dichroic mirror 17 and continues to be transmitted along the optical axis into the optical path of step S5.

[0063] S5. Pass the remaining combined light that has not undergone frequency doubling through the second type II frequency doubling crystal to generate second harmonic signal light again, and input the second harmonic signal light into port b of the balanced differential detector 24.

[0064] The specific process of step S5 includes: the undoped fundamental frequency combined beam that has passed through the high-pass dichroic mirror 17 in step S4 is directed along the optical axis toward the third convex lens 18. After being focused by the third convex lens 18, the light field energy is compressed into the effective working region of the second type II frequency doubling crystal 19, satisfying the light intensity requirements of the nonlinear frequency doubling effect; the focused combined beam passes through the second type II frequency doubling crystal 19, and the remaining mixed local light undergoes nonlinear frequency doubling again with the mixed reference signal light and the mixed measurement signal light, respectively, to generate the second second harmonic signal light; at the same time, the second type II frequency doubling crystal 19 introduces a fixed time delay τ0 for this harmonic signal. This delay is the core physical feature for the subsequent generation of mixed differential time-domain interference signals and the realization of signal slope polarity differentiation. A small amount of fundamental frequency light that has not undergone frequency doubling is emitted together with the second harmonic signal light. The light emitted from the second type II frequency doubling crystal 19 is directed towards the fourth convex lens 20. After being collimated by the fourth convex lens 20, it is converted into parallel spatial light. The collimated parallel light first strikes the first reflecting mirror 21, and after being reflected by the first reflecting mirror 21, the light path direction is changed and it strikes the second reflecting mirror 22. After being reflected by the second reflecting mirror 22, it strikes the third reflecting mirror 23. Through the continuous light path guidance of the three reflecting mirrors, the transmission optical path of the second harmonic signal light is precisely adjusted to the incident direction of the b-port of the balanced differential detector 24, and the optical axis is ensured to be perpendicular to the b-port detection surface of the balanced differential detector 24 (at this time, a small amount of undoped fundamental frequency light has no effective impact on the detector detection due to its extremely weak light intensity, and can be ignored).

[0065] S6. The balanced differential detector performs difference calculations on the signals from ports a and b, generating two sets of mixed dual-comb differential time-domain interference signals with opposite positive and negative slopes at the center zero point. The slope characteristics of the two sets of interference signals are separated and identified, the ambiguity multiple relationship is determined, and the distance is calculated by solving the formulas simultaneously.

[0066] Step S6 specifically includes: the balanced differential detector 24 simultaneously receives the first second harmonic signal light input from port a and the second second harmonic signal light input from port b with a fixed time delay τ0, and performs photoelectric conversion on the light intensity signals of the two harmonic signals to convert the light signals into electrical signals; the balanced differential detector 24 performs real-time difference calculation on the two converted electrical signals to eliminate common-mode interference (such as light intensity fluctuations and environmental noise) in the two signals, and outputs a voltage difference signal, which is the hybrid dual-comb differential time-domain interference signal; the amplitude change of this signal is related to the relative time offset of the dual-comb pulse. The signals are correlated. The signal processing and resolution system receives the hybrid dual-comb differential time-domain interferometric signal output from the balanced differential detector 24, and performs preprocessing such as noise reduction and filtering on the signal. The preprocessed signal is correlated with... Figure 3The simulation diagram of the differential time-domain interferometric signal is highly consistent and perfectly matches the mathematical expression of a single differential time-domain interferometric signal: In the formula and These are the peak intensities of the two light pulses. and These are the pulse widths of a single reference signal light or measurement signal light and a single local light, respectively. This represents the relative time offset between the two pulses. This represents the fixed time delay generated by the second-order frequency doubling crystal. In the expression, the exponential term determines the "bell-shaped envelope" characteristic of the signal, and the cosine term determines the periodic interference fluctuations of the signal; together, they constitute... Figure 3 The standard "S"-shaped curve shape. According to the mathematical expression, when the two optical comb pulses completely overlap (τ=0), cos(0)=1, the signal amplitude I(0)=0, corresponding to... Figure 3 The center zero point of the signal (the position where the amplitude is 0); this point is the time-domain zero point of the reference signal light, and is the time interval for subsequent extraction. , The benchmark is completely consistent with its physical meaning and mathematical derivation.

[0067] See Figure 2 The system first determines the slope polarity. Based on the extracted slope polarity, the signal processing and decomposition system precisely separates the mixed differential time-domain interference signals: signals with a negative slope at the center zero point are separated into the first group of dual-comb differential time-domain interference signals (corresponding to...). Figure 2 (a) in the middle, signal light and Local light generation), separating the signal with a positive slope at the center zero point into a second set of dual-comb differential time-domain interferometric signals (corresponding to Figure 2 (b) in the middle signal light and (Local light). Next, the reference and measurement interference signals are decomposed. Each set of dual-comb interference signals is further decomposed into a reference differential time-domain interference signal (without the optical path difference of the measured distance, corresponding to the interference between the mixed reference signal light and the local light) and a measurement differential time-domain interference signal (carrying the optical path difference of the measured distance, corresponding to the interference between the mixed measurement signal light and the local light). The time-domain offset of these two sets of signals is the core parameter for subsequent calculations—that is, the time interval. (Group 1) (Second group). In actual measurement, the measuring device of the present invention synchronously detects, acquires, and outputs the above two groups of differential time-domain interference signals through the balanced differential detector 24. The two groups of signals are not displayed independently, but are presented in the same time-domain coordinate system in a parallel superposition form, forming a... Figure 2 (c) in the diagram presents the measurement signal of the parallel dual optical comb. Figure 2 The core function of (c) is to visually present the temporal distribution of the two mixed dual-comb signals, enabling the rapid separation and identification of the two dual-comb ranging signals from the parallel signal diagram by using the positive and negative characteristics of the slope at the center zero point, and thus extracting their respective values. and This provides a basis for solving the measured distance using simultaneous formulas and eliminating the limitations of non-fuzzy range.

[0068] See Figure 3 The system extracts two core features of the signal: ① Center zero-point feature: accurately locates the center zero-point of the signal (the position where the amplitude is 0). This point is the time-domain zero point of the reference signal light, corresponding to the pulse synchronization position of the mixed reference signal light, and is the subsequent extraction time interval. , The baseline; ② Linear features near the zero point: Extract the signal interval near the center zero point. This interval is a steep linear change region, and the signal amplitude and the relative time offset τ within the linear interval have a one-to-one linear relationship. This feature ensures the attosecond resolution of the time interval extraction, which is the core theory and data support for the high-precision ranging of this invention; At the same time, based on this linear region, the slope polarity (positive / negative) of the center zero point of each group of signals is calculated, providing a quantitative judgment basis for the separation of dual optical comb signals.

[0069] Extracted , Numerical changes follow Figure 4 The dynamic change pattern of distance: Figure 4 (a) Figure 4 (b) in the middle are respectively , The periodic variation curves of the measured target distance show a fixed periodic offset relationship, and the magnitude of the numerical relationship is the core basis for determining the integer multiples of the non-ambiguous range; the system is based on the design feature of dual optical comb repetition frequency ( much smaller ), combined Figure 4 Based on the dynamic pattern, automatically determine the integer multiples of the non-fuzzy range corresponding to the two sets of signals. , :when At that time, the judgment (correspond Figure 4 The stage where the mid-range does not cross the integer period of the non-fuzzy range); when At that time, the judgment (correspond Figure 4 (The step phase of the non-ambiguous range integer period in the mid-range); This determination process does not require an additional coarse measurement device and is directly based on the time domain characteristics of the optical signal, effectively avoiding misjudgment of integer multiples caused by target motion in dynamic measurement.

[0070] Formula combination and solution: from repetition frequency Measurement signal light and repetition frequency The distance calculated from the dual-comb signal formed by local light is as follows: ; based on repetition frequency Measurement signal light and repetition frequency The distance calculated from the dual-comb signal formed by local light is as follows: The signal processing and decoding system will extract , and judgment , And by combining the two formulas above, we get:

[0071]

[0072] In the formula The speed of light; m 1, m 2 represents an integer multiple of their respective unfuzzy ranges; n g Δ is the refractive index of air. t 1. Δ t 2 represents the time interval between the reference signal light and the measurement signal light for each dual-comb signal measurement. The distance to the target being measured. The repetition frequency of the first femtosecond laser source. This is the repetition frequency of the second femtosecond laser source. This indicates the unambiguous range of the first group of dual optical comb signals. This indicates the unambiguous range of the second set of dual optical comb signals. This represents the fractional distance within the ambiguity range obtained from the measurement of the first set of dual optical comb signals. This represents the fractional distance within the ambiguity range obtained from the measurement of the second set of dual optical comb signals.

[0073] The solution obtained Matching required Figure 4 The dynamic change characteristic of (c) in the figure: continuous change with the distance to the measured target. It exhibits a continuous, non-fluctuating linear change with no periodic aliasing error. After verifying the accuracy of the solution results, the system outputs the final absolute distance of the measured target (i.e., the straight-line distance between the semi-reflective mirror 25 and the target pyramid 7), completing the entire process of hybrid dual-comb differential time-domain interferometric absolute distance measurement.

[0074] This method eliminates the need for additional coarse measurement devices and optical comb repetition frequency modulation, resolving ambiguity numbers in a single measurement and overcoming range limitations. It utilizes a balanced differential detector 24 to extract attosecond-level time intervals, and combines the positive and negative slope characteristics of interference signals generated by the first and second type frequency doubling crystals 15 and 19 to achieve precise signal separation, significantly improving ranging accuracy. The entire process employs modular optical path connections via fiber combiners 3, 1-to-2 fiber splitters 4, and fiber circulators 5, resulting in a simple process and low assembly / adjustment difficulty. Furthermore, it simultaneously acquires two sets of signals in a single measurement, automatically determining the ambiguity number relationship, avoiding dynamic measurement step errors, and improving dynamic adaptability and stability. This enables efficient and high-precision measurement of absolute distances in large spaces, adapting to the needs of various industrial applications.

[0075] Specifically, in this embodiment, the first femtosecond laser source 1 adopts a mode-locked femtosecond laser structure (or other suitable optical frequency comb pulse sequence, including mode-locked femtosecond lasers of different technical routes and other femtosecond lasers with optical frequency comb output capability), comprising a laser gain medium, a resonant cavity, a phase-locked module, and an optical fiber output end, with a pulse width in the femtosecond range (fs), generating a fixed repetition frequency of... A 99.80MHz femtosecond optical frequency comb pulse signal serves as one of the core optical signal sources for dual-comb ranging. It works in conjunction with the heterofrequency optical comb of the second femtosecond laser source 2 to form a hybrid dual-comb system where each source is both the local light and the signal light. This system provides stable output power and a spectral range suitable for the nonlinear effects of subsequent frequency doubling crystals. The second femtosecond laser source 2 and the first femtosecond laser source 1 are the same type of mode-locked femtosecond laser with identical structures, differing only in their repetition frequency parameters. This ensures matching of the output optical pulse characteristics (pulse width, optical power, and spectrum) between the two sources, avoiding signal incompatibility due to device differences. In this embodiment, the second femtosecond laser source 2 generates a fixed repetition frequency of... MHz femtosecond optical frequency comb pulse signal ( It should be much smaller Based on practical experience In the embodiments of the present invention =200kHz), a dual-comb pair that forms a small frequency difference with the first femtosecond laser source 1. It is the core technical feature carrier for realizing two sets of dual-comb ranging signals and solving integer multiples of the non-ambiguous range.

[0076] In this embodiment, the rubidium atomic clock 10 adopts a miniaturized rubidium atomic frequency standard structure, which has low frequency drift and high anti-interference capability, making it suitable for precision measurement scenarios in industry and laboratories. During use, after the second femtosecond laser source 2 and the first femtosecond laser source 1 are externally synchronized with the rubidium atomic clock 10, power is applied and the clock is started. Parameters such as output repetition frequency and optical power are set on the operation interface. After the laser output stabilizes, its fiber optic output end is precisely connected to port b of the fiber optic combiner 3 to ensure lossless and unbroken fiber connections, guaranteeing efficient transmission of the optical comb signal.

[0077] In this embodiment, the fiber optic combiner 3 includes two fiber optic input ports (port b and port c of the fiber optic combiner 3) and one fiber optic output port (port a of the fiber optic combiner 3). During use, ports b and c of the fiber optic combiner 3 are connected to the fiber optic output ends of the first and second femtosecond laser sources, respectively, and port a of the fiber optic combiner 3 is connected to port a of the 1-to-2 fiber optic splitter 4. During connection, ensure the fiber end faces are clean and free of scratches to avoid optical signal reflection and loss, and ensure that the two optical combs are fully mixed within the combiner. Its core function is to achieve lossless, high-coupling-efficiency combining of the two different-frequency femtosecond optical combs output from the first and second femtosecond laser sources within the fiber, outputting a single mixed optical frequency comb, so that the subsequent mixed signal light and mixed local light after beam splitting simultaneously contain... and The two optical comb sequences provide the basis for dual-comb ranging, where each serves as both local light and signal light.

[0078] In this embodiment, the 1-to-2 fiber optic bundle splitter 4 includes one fiber optic input port (i.e., port a of the 1-to-2 fiber optic bundle splitter 4) and two fiber optic output ports (i.e., port b and port c of the 1-to-2 fiber optic bundle splitter 4). In use, port a of the 1-to-2 fiber optic bundle splitter 4 is connected to port a of the fiber optic bundle combiner 3. Port b of the 1-to-2 fiber optic bundle splitter 4 is connected to the fiber optic circulator 5 of the subsequent signal transmission and bundle combining system through a single-mode fiber. Port c of the 1-to-2 fiber optic bundle splitter 4 is connected to the third fiber optic collimator 11 of the subsequent signal transmission and bundle combining system through a single-mode fiber. After connection, it is necessary to ensure that the output optical power of the two paths is stable. Its core function is to split the single-path hybrid optical frequency comb output from the fiber combiner 3 into two identical hybrid optical frequency combs according to a preset power ratio (equal power splitting in this embodiment), which are defined as hybrid signal light (output from port b of the 1-to-2 fiber beam splitter 4) and hybrid local light (output from port c of the 1-to-2 fiber beam splitter 4). The hybrid signal light is used to generate the reference / measurement signal light, and the hybrid local light is used to interfere with the reference / measurement signal light to generate the second harmonic. It is a key component for realizing the optical path branching of dual-comb interferometric ranging.

[0079] Specifically, in this embodiment, the fiber optic circulator 5 adopts a three-port fiber optic circulator structure, including three fiber optic ports: a, b, and c. The optical signal is transmitted unidirectionally only along the a→b and b→c directions. In use, port a of the fiber optic circulator 5 is connected to port b of the 1-to-2 fiber optic beam splitter 4 to input the mixed signal light; port b of the fiber optic circulator 5 is connected to the fiber input end of the first fiber optic collimator 6 to output the mixed signal light to the spatial optical path; port c of the fiber optic circulator 5 is connected to the fiber input end of the second fiber optic collimator 8 to receive and output the combined mixed reference signal light and mixed measurement signal light. Its core function is to achieve unidirectional directional transmission of the mixed signal light, and to achieve combined transmission and echo isolation of the mixed reference signal light and mixed measurement signal light; to prevent the reflected light of the mixed reference signal light and mixed measurement signal light from returning to the front-end femtosecond laser source, causing unstable light source output; and to achieve the combined transmission of the two split signal lights into a single path, simplifying the subsequent optical path structure.

[0080] Specifically, in this embodiment, the fiber input end of the first fiber collimator 6 is connected to port b of the fiber circulator 5, and the spatial light output end is aligned with the beam splitting surface of the semi-reflective mirror 25 along the optical axis, with the outgoing light incident perpendicularly to the beam splitting surface. During installation, it is ensured that the optical axis is coaxial with the semi-reflective mirror 25 and the target pyramid 7, without offset or tilt, to ensure coupling efficiency and optical path accuracy. Its core function is to achieve efficient coupling conversion of fiber light to parallel spatial light, converting the mixed signal light output from the fiber circulator 5 into non-divergent parallel spatial light, ensuring uniform beam splitting by the subsequent semi-reflective mirror 25 and accurate reflection by the target pyramid 7; at the same time, it receives the mixed reference signal light reflected by the semi-reflective mirror 25 and the mixed measurement signal light returned by the target pyramid 7, realizing reverse coupling of parallel spatial light to fiber light.

[0081] Specifically, in this embodiment, the target pyramid 7 is installed at the target position, with the light entrance of the pyramid aligned with the direction of transmitted light from the semi-reflective mirror 25 along the optical axis, and the optical axis coinciding with the center of the pyramid; in this invention, the straight-line distance between the semi-reflective mirror 25 and the target pyramid 7 is the absolute distance to be measured. During installation, ensure that the target cone 7 is not tilted or wobbled to ensure stable return of the reflected light. Its core function is to accurately reflect the mixed signal light transmitted through the semi-reflective lens 25 back along the original optical path, generating a mixed measurement signal light carrying the distance information to be measured; its cone structure can eliminate optical path deviations caused by its own slight positional offset, ensuring the consistency of the reflected light's optical path and improving ranging accuracy.

[0082] Specifically, in this embodiment, the fiber input end of the second fiber collimator 8 is connected to port c of the fiber circulator 5, and the spatial light output end is aligned with the light transmission surface of the first quarter-wave plate 9 along the optical axis, with the emitted light perpendicular to the light transmission surface; the optical axis is strictly coaxial with the subsequent first quarter-wave plate 9 and polarization beam splitter 13. Its core function is to complete the secondary conversion of the fiber light mixed with the reference signal light and the measurement signal light into parallel spatial light, converting the converged signal light output from the fiber circulator 5 into parallel spatial light, ensuring the precise adjustment of the polarization state of the subsequent first quarter-wave plate 9, and the efficient beam combining of the polarization beam splitter 13; its structure and performance are consistent with the first fiber collimator 6, realizing a symmetrical optical path design.

[0083] Specifically, in this embodiment, the first quarter-wave plate 9 is installed along the optical axis between the second fiber collimator 8 and the polarization beam splitter 13, with its light-transmitting surface perpendicular to the incident optical axis and the optical axis coinciding with the center of the device. The polarization state can be adjusted by rotating the wave plate until the polarization state of the mixed reference signal light and the mixed measurement signal light matches the beam combining requirements of the polarization beam splitter 13. Its core function is to precisely adjust the polarization state of the mixed reference signal light and the mixed measurement signal light output from the second fiber collimator 8, converting them into polarized light that meets the beam combining requirements of the polarization beam splitter 13, ensuring lossless and high-coupling beam combining with the mixed local light within the polarization beam splitter 13.

[0084] Specifically, in this embodiment, the fiber input end of the third fiber collimator 11 is connected to port C of the 1-to-2 fiber beam splitter 4, and the spatial light output end is aligned with the light transmission surface of the second quarter-wave plate 12 along the optical axis, with the emitted light perpendicular to the light transmission surface; the optical axis is strictly coaxial with the second quarter-wave plate 12 and the polarization beam splitter 13. Its core function is to realize the coupling conversion of the mixed local light fiber light into parallel spatial light, converting the mixed local light output by the 1-to-2 fiber beam splitter 4 into non-divergent parallel spatial light, ensuring the subsequent polarization state adjustment of the second quarter-wave plate 12 and the precise beam combining of the polarization beam splitter 13; it is a device of the same specification as the first and second fiber collimators, ensuring the uniformity of the spatial light characteristics of the three beams.

[0085] Specifically, in this embodiment, the second quarter-wave plate 12 and the first quarter-wave plate 9 are of the same specification and are installed along the optical axis between the third fiber collimator 11 and the polarization beam splitter 13. The light-transmitting surface is perpendicular to the incident optical axis, and the optical axis coincides with the center of the device. By rotating the waveplate to adjust the polarization state, in conjunction with the first quarter-wave plate 9, the polarization states of the two beams meet the beam combining requirements of the polarization beam splitter 13. Its core function is to precisely adjust the polarization state of the mixed local light output from the third fiber collimator 11, making its polarization state orthogonal to the polarization states of the mixed reference signal light and the mixed measurement signal light, and adapting it to the beam combining logic of the polarization beam splitter 13, ensuring that the two beams achieve efficient polarization beam combining within the polarization beam splitter 13 without polarization loss.

[0086] Specifically, in this embodiment, the two incident surfaces of the polarization beam splitter prism 13 are directly opposite the output optical axes of the first quarter-wave plate 9 and the second quarter-wave plate 12, respectively, and the optical axes of the two incident beams are perpendicular to the incident surfaces and coincide with the center of the prism. The output surface of the combined beam is directly opposite the subsequent first convex lens 14 along the optical axis, ensuring that the combined beam is accurately input into the differential time-domain interferometry system. Its core function is to achieve lossless, high-coupling polarization beam combining of mixed reference signal light, mixed measurement signal light, and mixed local light, integrating two spatially parallel beams with matched polarization states into a single beam with coaxial optical axes and consistent characteristics, which is then output to the subsequent differential time-domain interferometry system. During the beam combining process, all the dual-comb characteristics of the two beams are preserved, and there is no signal distortion.

[0087] Specifically, in this embodiment, the first convex lens 14 is installed along the optical axis between the polarizing beam splitter 13 and the first type II frequency doubling crystal 15. Its light-transmitting surface is perpendicular to the incident optical axis, and its focal point is precisely located at the center of the first type II frequency doubling crystal 15. During installation, it is ensured that the optical axis is coaxial with the crystal center and the subsequent second convex lens 16, without any offset. Its core function is to precisely focus the combined light output from the polarizing beam splitter 13, compressing the parallel spatial light into the effective operating area of ​​the first type II frequency doubling crystal 15, significantly increasing the light field intensity within the crystal, meeting the requirements of the nonlinear optical effect generated by the second harmonic, and providing light intensity assurance for the efficient generation of the first and second harmonics.

[0088] Specifically, in this embodiment, the first and second type frequency doubling crystal 15 is a nonlinear optical crystal, installed between the first convex lens 14 and the second convex lens 16 along the optical axis. The light-transmitting surface is perpendicular to the incident optical axis, and the crystal center coincides with the optical axis. During installation, it is necessary to ensure that the phase matching direction of the crystal is consistent with the polarization state of the light field to maximize the frequency doubling efficiency. Its core function is to utilize nonlinear optical effects to cause part of the mixed local light in the combined beam to undergo frequency doubling conversion with the mixed reference signal light and the mixed measurement signal light, respectively, to generate the first second harmonic signal light (the wavelength of which is 1 / 2 of the original fundamental frequency light). It is one of the core devices for realizing differential time-domain interference signal generation.

[0089] Specifically, in this embodiment, the second convex lens 16 is installed along the optical axis between the first type II frequency doubling crystal 15 and the high-pass dichroic mirror 17. The light-passing surface is perpendicular to the incident optical axis, and the optical axis is strictly coaxial with the first convex lens 14 and the high-pass dichroic mirror 17. This ensures that the light field emitted from the crystal is collimated and then incident perpendicularly onto the beam-splitting surface of the high-pass dichroic mirror 17. Its core function is to re-collimate the diverging light emitted from the first type II frequency doubling crystal 15 (including the first second harmonic light + the undoped fundamental frequency light) into parallel spatial light, ensuring the beam splitting effect of the subsequent high-pass dichroic mirror 17 and avoiding beam divergence-induced uneven beam splitting or signal loss. Its focal length matches that of the first convex lens 14, forming a "focusing-collimating" optical pairing.

[0090] Specifically, in this embodiment, the high-pass dichroic mirror 17 adopts a flat-panel dichroic beam splitter structure, installed at a 45° angle between the second convex lens 16 and the third convex lens 18, with the beam splitting surface facing the incident optical axis to ensure that the fundamental frequency light is perpendicular to the incident beam splitting surface; the reflected light direction is aligned with port a of the balanced differential detector 24, and the transmitted light direction is aligned with the third convex lens 18 along the optical axis, ensuring that the optical axis is coaxial with subsequent devices during installation. Its core function is to achieve precise separation of the first second harmonic light and the un-frequency-doubled fundamental frequency light, achieving high reflection of the short-wavelength second harmonic light (1 / 2 the fundamental frequency wavelength) and high transmission of the long-wavelength fundamental frequency light. It ensures that the first second harmonic light is directionally input into port a of the balanced differential detector 24, and allows the un-frequency-doubled fundamental frequency light to enter the second frequency-doubled branch without interference, making it a key beam splitting device for the two frequency-doubled branches.

[0091] Specifically, in this embodiment, the third convex lens 18 is installed along the optical axis between the high-pass dichroic mirror 17 and the second type II frequency doubling crystal 19. The light-transmitting surface is perpendicular to the incident optical axis, and the focal point is precisely located at the center of the second type II frequency doubling crystal 19. The optical axis is coaxial with the high-pass dichroic mirror 17, the second type II frequency doubling crystal 19, and the fourth convex lens 20. Its core function is to perform secondary focusing on the undoped fundamental frequency light transmitted by the high-pass dichroic mirror 17, compressing the parallel fundamental frequency light into the effective working area of ​​the second type II frequency doubling crystal 19, increasing the light intensity within the crystal, satisfying the nonlinear optical effect requirements of the second harmonic generation, and forming a symmetrical frequency doubling focusing design with the first convex lens 14.

[0092] Specifically, in this embodiment, the second type II frequency doubling crystal 19 and the first type II frequency doubling crystal 15 are nonlinear optical crystals of the same specification. They are installed along the optical axis between the third convex lens 18 and the fourth convex lens 20, with the light-transmitting surface perpendicular to the incident optical axis and the crystal center coinciding with the optical axis. During installation, the phase matching direction is ensured to be consistent with the polarization state of the fundamental frequency light, while simultaneously ensuring the stability of the fixed time delay τ0 to avoid signal characteristic distortion caused by delay drift. Its core function is to perform a second frequency doubling on the undoped fundamental frequency light transmitted through the high-pass dichroic mirror 17, generating a second second harmonic signal light. Simultaneously, a fixed time delay τ0 is introduced, which is the core physical basis for the two sets of second harmonic signals to form opposite slopes at their center zero points, providing key features for subsequent signal separation and identification.

[0093] Specifically, in this embodiment, the fourth convex lens 20 is installed along the optical axis between the second type II frequency doubling crystal 19 and the first reflector 21. Its light-transmitting surface is perpendicular to the incident optical axis, and its optical axis is coaxial with the third convex lens 18 and the first reflector 21. This ensures that the collimated parallel light is incident perpendicularly onto the reflecting surface of the first reflector 21. Its core function is to re-collimate the diverging light emitted from the second type II frequency doubling crystal 19 (including the second harmonic light and a small amount of undoped fundamental frequency light) into parallel spatial light, ensuring the optical path guidance accuracy of the subsequent reflector and avoiding signal loss or spot shift when the beam diverges. It forms a "focusing-collimating" pair with the third convex lens 18 and an overall symmetrical optical path design with the first and second convex lenses.

[0094] Specifically, in this embodiment, the first reflector 21, the second reflector 22, and the third reflector 23 all adopt a planar reflector structure. The first reflector 21 is installed at a 45° angle between the fourth convex lens 20 and the second reflector 22, the second reflector 22 is installed at a 45° angle between the first reflector 21 and the third reflector 23, and the third reflector 23 is installed at a 45° angle between the second reflector 22 and the balanced differential detector 24. Its core function is to adjust the optical path to a direction perpendicular to the b-port of the balanced differential detector 24 through multiple optical path redirections, thereby achieving the directional incidence of the second harmonic light.

[0095] Specifically, in this embodiment, the balanced differential detector 24 adopts a dual-input balanced photodetector structure, including two independent detection channels, a and b, and a built-in differential amplifier circuit. It can perform real-time subtraction of the two electrical signals and output a voltage difference signal. During installation, ensure that the detection surface of port a of the balanced differential detector 24 faces the direction of reflected light from the high-pass dichroic mirror 17, and the detection surface of port b of the balanced differential detector 24 faces the direction of reflected light from the third reflecting mirror 23. Both incident lights are perpendicular to the detection surface, and the center of the light spot coincides with the center of the detection surface. The signal output terminal of the detector is connected to the signal processing and calculation system via a signal line to ensure stable transmission of the differential signal. Its core function is to achieve synchronous photoelectric conversion and differential operation of the two second harmonic lights, converting the optical signal into an electrical signal. Through differential operation, it eliminates common-mode interference (environmental noise, light intensity fluctuations) and outputs a pure hybrid dual-comb differential time-domain interference signal. It is the core detection device connecting optical and electrical signals.

[0096] Specifically, in this embodiment, the semi-reflective mirror 25 adopts a non-polarizing flat beam splitter structure, installed at a 45° angle or vertically (vertical incidence in this invention), with the beam splitting surface facing the output optical axis of the first fiber collimator 6, and the optical axis coinciding with the center of the beam splitting surface. Its core function is to perform fixed-ratio beam splitting and reflection on the parallel mixed signal light output from the first fiber collimator 6, generating mixed reference signal light (reflected along the original path), and allowing the remaining light to be transmitted to the target pyramid 7 to generate mixed measurement signal light.

[0097] The device described in this application has the following advantages:

[0098] ① Innovative hybrid dual-comb architecture breaks through the limitation of unambiguous range: The first femtosecond laser source 1 and the second femtosecond laser source 2, synchronized with the rubidium atomic clock 10, generate a small frequency difference optical comb. After being mixed by the fiber combiner 3, a dual-comb system that is "mutually local light / signal light" is realized. No additional coarse measurement device or modulation of the optical comb repetition frequency is required. By solving the slope characteristics of the two sets of interference signals, the limitation of the unambiguous range of traditional dual-comb ranging can be directly broken through, realizing a single direct measurement of the absolute distance in a large space.

[0099] ② Differential time-domain interferometry design for ultra-high precision signal extraction: A differential time-domain interferometry system of "double frequency doubling + differential detection" is constructed. The first and second type frequency doubling crystals 15 and 19 generate the second harmonic in steps. With the help of the high-pass dichroic mirror 17 for precise beam splitting and the reflector for directional guidance, the balanced differential detector 24 receives the signal through two ports and performs the difference operation, effectively suppressing common-mode interference. Relying on the linear characteristics of the interference signal at the attosecond level near the zero point, the time interval is realized. , High-precision extraction provides core support for ranging accuracy.

[0100] ③ Modular optical path integration simplifies structure and facilitates assembly and adjustment: The entire device is divided into four modular systems: light source and beam splitter, signal transmission and beam combiner, differential time domain interference, and signal processing and calculation. Each system is connected to the spatial optical path through standardized optical fibers. The core components are all general-purpose optical components. The coaxial design of the optical axis reduces the difficulty of assembly and adjustment. No complicated calibration process is required, making it suitable for large-scale application and maintenance in industrial scenarios.

[0101] ④ Strong dynamic adaptability and stable and reliable measurement: Two sets of dual-comb interference signals are acquired simultaneously through a single measurement, based on... , The numerical relationship is automatically determined to be an integer multiple relationship within the non-fuzzy range, avoiding step misjudgment caused by target movement during dynamic measurement; the unidirectional transmission characteristics of the fiber optic circulator 5 isolate echo interference, and the precise reflection of the target pyramid 7 ensures optical path consistency. The end-to-end design improves the measurement stability and reliability of the device in dynamic and complex environments.

[0102] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Terms such as "upper," "lower," "left," "right," "front," and "rear" used in the invention are merely for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0103] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for measuring the absolute distance of a dual-comb differential time-domain signal, characterized in that, Includes the following steps: S1. The two femtosecond optical frequency combs emitted from the two sets of femtosecond laser sources are mixed through an optical fiber combiner to form a hybrid optical frequency comb; S2. Use a 1-to-2 fiber optic beam splitter to split the hybrid optical frequency comb into two beams, which are denoted as hybrid signal light and hybrid local light. S3. The mixed signal light is split by a half-reflective lens. The part reflected in the original path is recorded as the mixed reference signal light, and the remaining part that is directed toward the target and reflected is recorded as the mixed measurement signal light. S4. After combining the mixed reference signal light, the mixed measurement signal light and the mixed local light, the combined light is passed through the first and second type frequency doubling crystal to generate the second harmonic signal light and part of the combined light that has not undergone frequency doubling effect, and the second harmonic signal light is input into port a of the balanced differential detector. S5. Pass the remaining combined light that has not undergone frequency doubling through the second type II frequency doubling crystal to generate the second harmonic signal light again, and input the second harmonic signal light into port b of the balanced differential detector; and introduce a fixed time delay in the second type II frequency doubling crystal. τ 0; S6. The balanced differential detector performs difference calculations on the signals from ports a and b, generating two sets of hybrid dual-comb differential time-domain interferometric signals with opposite slopes at their center zero points. The slope characteristics of the two sets of interferometric signals are used to separate and identify the ambiguity multiple relationship, and a simultaneous formula is used to calculate the distance. This simultaneous formula includes the distance calculated from the dual-comb signal formed by the local light. The distance calculated from the dual-comb signal formed by the measurement signal light and the local light: The distance to the measured target is obtained by simultaneous calculation as follows: ; In the formula The speed of light; m 1 indicates an integer multiple of the unambiguous range corresponding to the first group of dual optical comb signals. m 2. Integer multiples of the non-ambiguity range corresponding to the second group of dual optical comb signals. n g Δ is the refractive index of air. t 1 represents the time interval between the reference signal light and the measurement signal light obtained from the first set of dual-comb signal measurements, Δ t 2. The time interval between the reference signal light and the measurement signal light obtained from the second set of dual-comb signal measurements. The distance to the target being measured. The repetition frequency of the first femtosecond laser source. This is the repetition frequency of the second femtosecond laser source. This indicates the unambiguous range of the first group of dual optical comb signals. This indicates the unambiguous range of the second set of dual optical comb signals. This represents the fractional distance within the ambiguity range obtained from the measurement of the first set of dual optical comb signals. This represents the fractional distance within the ambiguity range obtained from the measurement of the second set of dual optical comb signals.

2. The absolute distance measurement method for dual-comb differential time-domain signals according to claim 1, characterized in that, In step S1, the two sets of femtosecond optical frequency combs have different frequencies, namely and ,and .

3. The absolute distance measurement method for dual-comb differential time-domain signals according to claim 1, characterized in that, In step S4, before the combined light enters the first type II frequency doubling crystal, it is further focused by the first convex lens; after the combined light passes through the first type II frequency doubling crystal, the outgoing light is further collimated by the second convex lens.

4. The absolute distance measurement method for dual-comb differential time-domain signals according to claim 3, characterized in that, In step S4, the outgoing light passing through the second convex lens includes the second harmonic signal light and a portion of the combined light that has not undergone frequency doubling. The second harmonic signal light and the portion of the combined light that has not undergone frequency doubling pass through a high-pass dichroic mirror. The second harmonic signal light is reflected by the high-pass dichroic mirror and enters the balanced differential detector. The combined light that has not undergone frequency doubling passes through the high-pass dichroic mirror.

5. The absolute distance measurement method for dual-comb differential time-domain signals according to claim 1, characterized in that, In step S5, before the combined light without frequency doubling effect enters the second type II frequency doubling crystal, it is further focused by a third convex lens; after the combined light without frequency doubling effect passes through the second type II frequency doubling crystal, the outgoing light is further collimated by a fourth convex lens.

6. The absolute distance measurement method for dual-comb differential time-domain signals according to claim 1, characterized in that, In step S6, using the differential time-domain interference signal formula, it is found that when the overlapping directions of the two sets of optical frequency comb signals are different, the slopes of the signal center zeros are opposite. The expression is as follows: In the formula and These are the peak intensities of the two light pulses. and These are the pulse widths of a single reference signal light or measurement signal light and a single local light, respectively. This represents the relative time offset between the two pulses. This is a fixed time delay generated by the second type of frequency doubling crystal.

7. An apparatus for measuring the absolute distance of a dual-comb differential time-domain signal according to any one of claims 1 to 6, characterized in that, The system includes a light source and beam splitting system, a signal transmission and beam combining system, a differential time-domain interferometry system, and a signal processing and calculation system. The light source and beam splitting system includes a rubidium atomic clock, a first femtosecond laser source, a second femtosecond laser source, an optical fiber combiner, and a 1-to-2 optical fiber beam splitter. The rubidium atomic clock is connected to both the first and second femtosecond laser sources, which are respectively connected to different input optical fibers of the optical fiber combiner. The optical fiber combiner is connected to the optical fiber of the 1-to-2 optical fiber beam splitter. The signal processing and calculation system receives the differential time-domain interferometry signal output from the balanced differential detector, separates and identifies two sets of dual-comb ranging signals, extracts the time interval, and combines them with the ranging formula to calculate the absolute distance to the target.

8. The absolute distance measurement device for dual-comb differential time-domain signals according to claim 7, characterized in that, The signal transmission and beam combining system includes an optical fiber circulator, an optical fiber collimator, a semi-reflective mirror, a target pyramid, a quarter-wave plate, and a polarizing beam splitter. The optical fiber collimator includes a first, second, and third optical fiber collimator; the quarter-wave plate includes a first quarter-wave plate and a second quarter-wave plate; the fiber splitter is optically connected to the optical fiber circulator and the third optical fiber collimator; the optical fiber circulator is optically connected to the first and second optical fiber collimators; one end of the semi-reflective mirror is connected to the optical path of the first optical fiber collimator, and the other end is connected to the target pyramid. The cone optical path is connected; one end of the first quarter-wave plate is connected to the optical path of the second fiber collimator, and the other end is connected to the optical path of the polarizing beam splitter; one end of the second quarter-wave plate is connected to the optical path of the third fiber collimator, and the other end is connected to the optical path of the polarizing beam splitter; the mixed signal light is directed to the half-reflecting half-lens after passing through the fiber circulator and fiber collimator, and is split into a mixed reference signal light and a mixed measurement signal light directed to the target cone. After the mixed reference signal light and the mixed measurement signal light are returned through the fiber circulator, they are combined with the mixed local light that has passed through the fiber collimator and quarter-wave plate at the polarizing beam splitter.

9. The absolute distance measurement device for dual-comb differential time-domain signals according to claim 7, characterized in that, The differential time-domain interferometry system includes a convex lens, a second-order frequency-doubling crystal, a high-pass dichroic mirror, a reflector, and a balanced differential detector. The convex lens includes a first convex lens, a second convex lens, a third convex lens, and a fourth convex lens. The second-order frequency-doubling crystal includes a first second-order frequency-doubling crystal and a second second-order frequency-doubling crystal. The reflector includes a first reflector, a second reflector, and a third reflector. One end of the first convex lens is connected to a polarizing beam splitter, and the other end is connected to the first second-order frequency-doubling crystal. One end of the second convex lens is connected to the optical path of the first second-order frequency-doubling crystal, and the other end is connected to the optical path of the high-pass dichroic mirror. One end of the third convex lens is connected to the optical path of the high-pass dichroic mirror, and the other end is connected to the optical path of the second second-order frequency-doubling crystal. One end of the fourth convex lens is connected to... The second type II frequency doubling crystal has its optical path connected, with one end connected to the optical path of the first reflector; one end of the second reflector is connected to the optical path of the first reflector, and the other end is connected to the optical path of the third reflector; one end of the third reflector is connected to the optical path of the second reflector, and the other end is connected to one entrance optical path of the balanced differential detector; the other entrance of the balanced differential detector is connected to the optical path of the high-pass dichroic mirror; the combined beam passes through the first set of convex lenses and the first type II frequency doubling crystal to generate the first second harmonic, which is then reflected by the high-pass dichroic mirror and input into one entrance of the balanced differential detector; the remaining fundamental frequency light passes through the second set of convex lenses and the second type II frequency doubling crystal to generate the second second harmonic with a fixed time delay, which is then guided by multiple reflectors and input into the other entrance of the balanced differential detector.

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