A resonant cavity enhanced non-cooperative target laser sweep frequency carrier ranging method and system

Abstract

The application discloses a resonant cavity enhanced non-cooperative target laser sweep frequency carrier ranging method and system and belongs to the technical field of laser precision measurement. Sweep frequency detection laser output by an external cavity tunable laser is output to a non-cooperative target, and diffuse reflection feedback light returns to a resonant cavity and interferes with intrinsic light in the cavity, target distance information is modulated on output light frequency to generate a laser sweep frequency carrier, the carrier is demodulated by a Mach-Zehnder interferometer to output an interference signal, and the interference signal is mixed to separate sum frequency and difference frequency components, the two components are respectively subjected to equal optical frequency resampling to correct sweep frequency nonlinearity, and the resampled signals are subjected to dispersion compensation to solve the target distance. The application realizes high-precision and high-sensitivity ranging of the non-cooperative target, and the system structure is compact.

Description

Technical Field

[0001] This invention belongs to the field of laser precision measurement technology, specifically relating to a non-cooperative target laser sweep frequency carrier ranging method and system enhanced by resonant cavity. Background Technology

[0002] Lasers, due to their high monochromaticity, high directionality, and non-contact measurement capabilities, are widely used in ranging, vibration measurement, and velocity detection. Among these, frequency-swept interferometry (FSI) ranging technology, with its stable measurement process, high accuracy, and high resolution, has become one of the mainstream technologies for precise absolute distance measurement. It can achieve high-precision detection of physical quantities such as absolute distance, micro-vibrations, and velocity of the target, and has broad application prospects in industrial precision measurement, large-size component assembly, biomedical testing, and gravitational wave detection. However, the precision measurement achieved by FSI technology depends on the target surface having high reflectivity to ensure that the target feedback laser has high power, thereby forming a high-quality interference signal with the reference light. In practical engineering scenarios such as industrial measurement and biological detection, a large number of the targets being measured are non-cooperative targets, including black oxide surfaces, rough curved surfaces, cylindrical surfaces, liquids, and non-metallic porous materials. Such targets cannot be fitted with high-reflectivity mirrors, and the measurement light undergoes diffuse reflection and scattering on the target surface, resulting in extremely low return target feedback light power. Traditional FSI technology struggles to obtain high-quality interference signals and cannot achieve stable and high-precision measurements.

[0003] To address the challenge of measuring weak feedback light from non-cooperative targets, existing technologies primarily employ three approaches for improvement. These are detailed below: The first category is hardware performance enhancement solutions, which increase the output optical power by using high-power external cavity tunable lasers or optical power amplifiers, or by using high-sensitivity, high-gain detectors to detect weak feedback optical signals. In some applications of laser ranging for space targets, laser energy multi-stage amplification technology is also used to increase the output energy. However, such solutions can only improve the measurement effect to a certain extent and cannot fundamentally improve the signal-to-noise ratio of the interference signal. At the same time, they significantly increase the system cost and structural complexity, which is not conducive to engineering implementation and miniaturization applications.

[0004] The second category is signal processing algorithm optimization schemes, which process interference signals through algorithms such as frequency domain fitting, time domain filtering, and high-frequency modulation and demodulation. Examples include fast FSI signal detection methods based on Sparse Fourier Transform (SFT) and s-Rife algorithms, and residual nonlinearity compensation methods based on optical frequency inversion. These methods can achieve micrometer-level ranging accuracy for non-cooperative targets within a range of 12 to 25 meters. Equal optical frequency interval sampling technology is also widely used to eliminate the influence of light source sweep frequency nonlinearity on the system. However, these methods are limited by the signal quality of the measurement system itself. When the feedback light power is extremely low, the effect of algorithm optimization on improving measurement accuracy is extremely limited and cannot solve the fundamental problem.

[0005] The third category is laser feedback interferometry, such as Frequency-Swept Feedback Interferometry (FSFI). FSFI utilizes the laser feedback effect to return the feedback light to the resonant cavity of the external cavity tunable laser, achieving spontaneous amplification. When the beat frequency signal resonates with the relaxation oscillation of the external cavity tunable laser, the modulation contrast gain can reach 10. 4 This technology achieves absolute distance measurement of non-cooperative targets at a range of hundreds of meters under sub-milliwatt probe optical power. However, the existing FSFI technology system has a complex structure and requires the use of a frequency-stabilized external cavity tunable laser and an acousto-optic modulator for measurement result compensation. At the same time, it is limited by the bandwidth and frequency sweep speed of the external cavity tunable laser, which cannot meet the requirements of high-precision measurement scenarios. Furthermore, traditional solutions mostly use amplitude modulation (AM) detection methods, and their weak signal detection capabilities still have considerable room for improvement.

[0006] While laser feedback interferometry using frequency modulation detection has improved the response and detection capabilities for weak signals, the measurement resolution and accuracy are limited by the contradiction between resolution and dispersion mismatch caused by a large-bandwidth swept-frequency laser source.

[0007] In summary, existing technologies cannot simultaneously meet the core requirements of low-light detection of non-cooperative targets, high measurement accuracy, high resolution, and compact and low-cost system structure. There is an urgent need to develop a new laser ranging technology to improve the quality of measurement signals from the perspectives of physical mechanisms and algorithm optimization, so as to achieve high-precision, high-sensitivity, and high-resolution ranging of non-cooperative targets. Summary of the Invention

[0008] The purpose of this invention is to provide a resonant cavity-enhanced non-cooperative target laser sweep frequency carrier ranging method and system, which solves the technical problems in the prior art, such as the extremely low feedback light power of non-cooperative targets leading to poor measurement signal quality, the difficulty of achieving stable and high-precision measurement with traditional FSI technology, and the inability of existing improved schemes to balance weak light detection capability, measurement accuracy and system structure compactness.

[0009] This invention is achieved through the following technical solution: This invention discloses a resonant cavity-enhanced non-cooperative target laser sweep carrier ranging method, comprising the following steps: S1: The frequency sweeping probe laser output by the external cavity tunable laser changes linearly with time and is incident on the surface of the non-cooperative target being measured through the probe optical path. S2: The diffuse reflection feedback light from the surface of the non-cooperative target being tested returns to the resonant cavity of the external cavity tunable laser along the original detection optical path. The diffuse reflection feedback light interferes with the intrinsic sweep frequency laser in the resonant cavity, forming an intracavity interference signal carrying target distance information. The intracavity interference signal modulates the carrier concentration of the gain medium of the external cavity tunable laser and performs phase modulation on the instantaneous frequency of the output light of the external cavity tunable laser to generate a laser sweep frequency carrier carrying target distance information. S3: The laser sweep carrier is output to the demodulation optical path via the pigtail of the external cavity tunable laser, and is frequency-modulated and demodulated by the Mach-Zehnder interferometer to output an interference signal containing target distance information; the length of the delay fiber in the Mach-Zehnder interferometer is less than the distance to the target being measured; S4: Acquire the interference signal containing the target distance information, preprocess the interference signal to obtain a preprocessed signal; perform frequency mixing on the preprocessed signal to separate the sum frequency component and the difference frequency component; Using the interference signal containing target distance information as a frequency scale with equal optical frequency intervals, the separated sum frequency component and difference frequency component are resampled at equal optical frequencies to obtain resampled interference signals of the sum frequency component and difference frequency component; dispersion compensation is performed on the resampled interference signals of the sum frequency component and difference frequency component to obtain dispersion-compensated sampling point sequences of the sum frequency component signal and the difference frequency component signal; spectral analysis is performed on the dispersion-compensated sampling point sequences of the sum frequency component signal and the difference frequency component signal to calculate the absolute distance value of the target.

[0010] Furthermore, in S3, the interference signal containing target distance information, after being converted by a balanced photodetector, has a time-domain spectrum containing the following three frequency components: The main carrier frequency is determined by the fixed delay of the delay fiber of the Mach-Zehnder interferometer and the sweep rate of the external cavity tunable laser. The sum-frequency component is generated by superimposing the main carrier frequency and the target characteristic frequency. The difference frequency component is generated by subtracting the target characteristic frequency from the main carrier frequency; The target characteristic frequency is proportional to the time delay of the probe light to and from the target, and is a frequency component carrying the absolute distance information of the target.

[0011] Furthermore, in S4, the equal-frequency resampling adopts a dual-path parallel resampling architecture. The interference signal generated by the Mach-Zehnder interferometer is used as the frequency scale of equal-frequency intervals. By configuring the resampling factor, the Mach-Zehnder interferometer delay fiber with a length smaller than the distance to the target being measured is used as a reference to perform equal-frequency resampling on the separated difference frequency component and sum frequency component respectively.

[0012] Furthermore, in S4, dispersion compensation is performed on the resampled interference signal of the sum frequency component and the difference frequency component. The dispersion compensation method specifically includes the following steps: Hilbert transforms are performed on the two resampled interference signals respectively to construct analytic signals and extract the instantaneous phase; After unenveloping the instantaneous phase, a second-order polynomial is fitted using the least squares method to obtain the phase fitting curve; the expression of the phase fitting curve is: ; in, The coefficient of the quadratic term; The coefficient of the linear term, For constant terms; The coefficient of the quadratic term As a dispersion compensation factor, the dispersion compensation factor is applied to the complex signal of the resampled interference signal to obtain the dispersion-compensated complex interference signal. Taking its real part yields the dispersion-compensated interference signal, expressed as: ; in, This is the interference signal after dispersion compensation; The complex analytic signal is constructed by applying the Hilbert transform to the target characteristic frequency components; Index of resampling points; These are the dispersive second-order phase coefficients obtained by second-order fitting of the instantaneous phase curve; For the first The second nonlinear phase that needs to be canceled at each resampling point; It is the unit mode compensation factor that is opposite to the direction of the dispersive phase.

[0013] Furthermore, the delay fiber length of the Mach-Zehnder interferometer ranges from 1m to 5m.

[0014] Furthermore, in S4, the interference signal is preprocessed, including the following sub-steps: The acquired interference signal is subjected to DC blocking processing to filter out the DC component; Perform bandpass filtering to remove out-of-band environmental noise and circuit noise; The filtered signal is normalized to eliminate the amplitude instability caused by fluctuations in the output power of the external cavity tunable laser and changes in optical path loss.

[0015] Furthermore, in step S4, spectral analysis is performed on the sampled sequence of the sum-frequency component signal and the sampled sequence of the difference-frequency component signal after dispersion compensation to calculate the absolute distance value of the target. Specifically: Fast Fourier transform is performed on the sampling point sequence of the sum frequency component signal and the sampling point sequence of the difference frequency component signal after dispersion compensation, respectively, to obtain the spectrum of the sum frequency component and the spectrum of the difference frequency component. The sampling point index value of the peak of the difference frequency component spectrum is determined in the spectrum of the difference frequency component, and the sampling point index value of the peak of the sum frequency component spectrum is determined in the spectrum of the sum frequency component. Based on the sampling point index value of the peak of the compensated difference frequency component spectrum and the sampling point index value of the peak of the compensated sum frequency component spectrum, the distance value corresponding to the difference frequency component and the distance value corresponding to the sum frequency component are calculated using the distance calculation formula, and the average value of the two is taken as the final absolute distance value of the target under test. The distance calculation formula is as follows: ; ; in, This represents the distance value corresponding to the difference frequency component. P1 is the distance value corresponding to the sum-frequency component; P2 is the sampling point index value corresponding to the peak value of the main carrier frequency component in the spectrum after resampling; P3 is the sampling point index value corresponding to the peak value of the target characteristic frequency component in the spectrum after resampling. This is the index value of the peak point of the difference frequency component spectrum. L is the index value of the peak point of the sum-frequency component spectrum; N is the system preset resampling factor, L MZI The fixed length of the delay fiber in the Mach-Zehnder interferometer; This represents the total number of sampling points in the spectrum after resampling the difference frequency component. This represents the total number of sampling points in the spectrum after resampling the sum-frequency components.

[0016] Furthermore, the detection optical path includes a polarization controller and an adjustable optical attenuator arranged sequentially along the optical transmission direction; the polarization controller is used to adjust the polarization state of the detection laser and the feedback laser to improve the signal-to-noise ratio of the interference signal; the adjustable optical attenuator is used to adjust the output power of the detection laser to avoid the feedback light power being too high and affecting the working state of the external cavity tunable laser resonator.

[0017] Furthermore, the demodulation optical path also includes a fiber Bragg grating trigger branch. The fiber Bragg grating trigger branch splits a portion of the optical power from the pigtail of the external cavity tunable laser through an optical fiber coupler. A high-speed photodetector is connected in series at the rear end of the fiber Bragg grating. When the instantaneous wavelength of the laser sweeps across the center wavelength of the fiber Bragg grating, the high-speed photodetector generates a spike electrical pulse as a trigger signal. The trigger signal is used to: lock the start and end intervals of a single complete sweep frequency cycle, distinguish between the forward and reverse sweep frequency intervals of the external cavity tunable laser, and trigger the data acquisition card to start hardware synchronous sampling, so as to achieve complete alignment between the sweep frequency action of the external cavity tunable laser and the clock of data acquisition.

[0018] This invention discloses a resonant cavity-enhanced non-cooperative target laser sweep carrier ranging system, comprising: An external cavity tunable laser is used to output a probe laser. Its resonant cavity is used to receive the target feedback light and interact with the intrinsic light in the cavity to generate a laser sweep carrier carrying target distance information. The probe optical path module is set on the output optical path of the external cavity tunable laser, and includes a polarization controller and a tunable optical attenuator arranged sequentially along the optical transmission direction. It is used to transmit the probe laser to the surface of the non-cooperative target under test and to make the target feedback light return to the resonant cavity of the external cavity tunable laser along the original path. The demodulation optical path module includes an optical fiber coupler, a Mach-Zehnder interferometer, and a balanced photodetector. The Mach-Zehnder interferometer has a fixed-length delay fiber for receiving the laser sweep carrier and performing frequency modulation and demodulation, and outputting an interference signal containing target distance information. The data acquisition and processing module includes a data acquisition card and a signal processing unit, which is used to acquire the interference signal containing the target distance information and perform signal processing. The signal processing includes at least: preprocessing, frequency mixing, equal-frequency resampling, dispersion compensation and distance calculation, and outputs the absolute distance value of the target being measured.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects: This invention combines an equal-frequency resampling method with a phase-corrected dispersion compensation method to decouple sweep frequency nonlinearity error from fiber dispersion error. Specifically, the nonlinear sweep frequency signal is first converted into an equal-frequency sampling sequence using the EFRS method to suppress the influence of sweep laser tuning nonlinearity on range demodulation. Then, the instantaneous phase of the target range component is extracted based on Hilbert transform, and a dispersion compensation factor is obtained through second-order polynomial fitting, thereby correcting the quadratic phase error caused by dispersion. This method effectively avoids spectral peak broadening and peak shift caused by the coupling of sweep frequency nonlinearity and fiber dispersion, improving the accuracy of range peak positioning.

[0020] This invention eliminates the need for an additional, precisely length-matched physical dispersion compensation fiber; instead, it achieves dispersion compensation through software algorithms. Since the dispersion delay in MZI delay fibers is related to the frequency scanning range and the fiber length, traditional physical compensation methods require precise matching of the compensation fiber length and dispersion parameters, resulting in complex system structures and high debugging difficulty. This invention eliminates the main dispersion error by estimating and inversely compensating for the quadratic term in the phase of the interference signal, thereby simplifying the optical path structure and reducing system assembly and adjustment complexity and implementation costs.

[0021] The dispersion compensation algorithm employed in this invention has low computational complexity and is suitable for rapid distance demodulation. The target interference signal affected by dispersion can be represented as containing linear and quadratic phase terms, where the linear phase term corresponds to the target distance information and the quadratic phase term corresponds to the fiber dispersion error. This invention constructs an analytical signal using Hilbert transform, extracts the instantaneous phase, and then fits it using a least-squares second-order polynomial to directly obtain the dispersion compensation factor. Compared to compensation methods relying on search-based algorithms such as fractional Fourier transforms, this invention eliminates the need for extensive parameter searches, simplifies the calculation process, and accelerates processing, making it more suitable for real-time or near-real-time applications in engineering surveying.

[0022] This invention effectively compresses the spectral peak broadening caused by fiber dispersion, improving ranging accuracy and resolution. Fiber dispersion causes the instantaneous frequency of the MZI output signal to change with the sampling sequence, resulting in broadening of the originally concentrated target frequency components in the spectrum, thus reducing the accuracy of spectral peak positioning. This invention applies inverse quadratic phase compensation to the analytical signal, restoring the target range components to an approximately single-frequency signal, thereby refocusing the spectral energy, making the spectral peaks sharper, and the peak positions more accurate, thereby improving the system's distance calculation accuracy and resolution.

[0023] This invention shortens the length of MZI delay fiber, reducing dispersion errors and environmental disturbances introduced by long fibers. Since dispersion delay increases with MZI delay fiber length, excessively long delay fibers exacerbate spectral leakage and harmonic interference; conversely, excessively short delay fibers result in insufficient modulation depth of the target distance component and reduced signal-to-noise ratio. This invention utilizes the EFRS method to flexibly set the resampling factor, freeing the MZI delay fiber length from the strict limitations of traditional resampling methods. It allows for the use of shorter delay fibers while ensuring clear differentiation of the target spectral components, thereby achieving a balance between signal-to-noise ratio and dispersion suppression, and improving system stability.

[0024] This invention is applicable to complex interferometric signals with multiple frequency components coexisting in LFSC systems. After demodulation by MZI, the LFSC signal simultaneously contains the main optical frequency beat frequency component, the target range-related component, and the mixing component introduced by dispersion. This invention separates the target-related frequency component through filtering and normalization, and independently performs phase extraction, second-order fitting, and dispersion compensation on this component. This enables accurate recovery of target range information under multi-frequency interferometric conditions, improving the demodulation reliability of the system in complex signal environments.

[0025] This invention further suppresses system-level errors by calculating the distance for both the sum-frequency and difference-frequency components separately and averaging the results. Since fiber dispersion, temperature variations, or slight drift in MZI delay can produce opposite-direction errors in the sum-frequency and difference-frequency paths, averaging the two distance results cancels out common-mode errors. Furthermore, when one spectral component is affected by circuit noise, mechanical vibration, or stray light interference, the other can still provide effective distance information, thus enhancing the system's anti-interference capability. In addition, when the difference-frequency component is close to zero and difficult to detect, the sum-frequency component remains clear, which helps overcome measurement dead zones and achieves more continuous and reliable full-range ranging.

[0026] This invention uses a fiber optic grating (FBG) to generate sweep cycle trigger pulses, which simplifies the cycle calibration structure and reduces costs. When the sweep laser wavelength sweeps across the FBG reflection peak, the FBG outputs a steep trigger signal, which can be used to lock each sweep cycle. Compared to schemes using a gas absorption cell as a wavelength scale, the FBG scheme does not require the introduction of a precision gas cell; only a fiber optic grating and detector are needed to achieve cycle marking, resulting in a more compact structure, lower cost, and simpler signal processing.

[0027] This invention combines FBG trigger markers with a windowed data capture strategy to avoid the effects of mechanical oscillations or jitter in the frequency sweep transition region. By using the FBG pulse peak as a period reference and selecting a stable region at the center of the interference signal for distance calculation, it avoids the impact of signal quality degradation near the motor's forward and reverse scanning transition points on the ranging results, thereby improving the quality of effective data and the stability of distance calculation. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the ranging optical path system of the present invention; Figure 2 This is a schematic diagram of the theoretical model of the resonant cavity for generating the Laser Frequency-Swept Carrier (LFSC) according to the present invention. Figure 3 This represents the ideal laser frequency sweep curve. Figure 4 The laser sweep curve is affected by dispersion mismatch; Figure 5 The laser sweep curve is affected by dispersion mismatch and nonlinearity; Figure 6 This is a schematic diagram of the signal processing method of the present invention; Figure 7 Dispersion delay for different fiber lengths. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0030] The detailed description of the embodiments of the present invention provided in the following figures is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the figures and embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0031] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0032] This invention proposes a laser sweep carrier ranging method based on resonant cavity enhancement. The core of this method is to utilize the resonant cavity gain amplification effect to improve the signal-to-noise ratio of weak feedback light, modulate the target distance information onto the output optical frequency of an external cavity tunable laser to form a laser sweep carrier, extract the distance information through frequency modulation and demodulation technology, and correct the sweep frequency nonlinearity through an equal optical frequency resampling algorithm. This method overcomes the limitations of the Nyquist sampling theorem and achieves high-precision and high-sensitivity ranging of non-cooperative targets.

[0033] Please see Figure 1 The ranging optical path system of this invention comprises four core units: an external cavity tunable laser (ECTL), a probe optical path module, a demodulation optical path module, and a data acquisition and processing module. The overall optical path structure of the system is as follows: External cavity tunable laser (ECTL): As the sweep laser source of the system, it can achieve mode-hopping linear sweep. The center wavelength of the external cavity tunable laser is 1550nm, the tuning range is 34nm (1530nm~1564nm), the tuning rate is 68nm / s, the linewidth measured by the delayed self-interference method is 160kHz, and the theoretical coherence length is 1.875km. The zero-order diffraction light of the external cavity tunable laser grating is used as the probe laser output, and the laser output from the pigtail is a laser sweep frequency carrier (LFSC) carrying target distance information for subsequent demodulation.

[0034] The probe optical path module consists of a polarization controller (PC) and a tunable optical attenuator (VOA) arranged sequentially along the probe laser transmission direction. The polarization controller is used to adjust the polarization state of the probe laser and the feedback laser to improve the signal-to-noise ratio of the interference signal. The tunable optical attenuator is used to adjust the output power of the probe laser to avoid the feedback light power being too high and affecting the working state of the external cavity tunable laser resonator. It is also used to test the weak light detection limit of the system.

[0035] Demodulation optical path module: A first fiber coupler connected to the pigtail of the external cavity tunable laser. The output end of the first fiber coupler is provided with two branches, namely an interference branch and a fiber Bragg grating triggering branch.

[0036] The fiber Bragg grating triggering branch includes a circulator, a fiber Bragg grating (FBG), and a first balanced photodetector (BPD1) connected in sequence. The interference branch includes a second fiber coupler, a Mach-Zehnder interferometer (MZI), and a second balanced photodetector (BPD2) connected in sequence. The first fiber coupler has a splitting ratio of 10:90. 10% of the optical signal enters the fiber Bragg grating to calibrate the frequency sweep range of the external cavity tunable laser, providing a reference for frequency sweep nonlinear correction. 90% of the optical signal enters the Mach-Zehnder interferometer, which uses a 1.5m delay fiber as the reference interferometer length and serves as a high-sensitivity detection device for the frequency-modulated signal, demodulating the LFSC. The interference signal output from the Mach-Zehnder interferometer is connected to the balanced photodetector to complete the conversion from optical to electrical signals.

[0037] Data acquisition and processing module: It uses a data acquisition card (DAQ) to acquire the electrical signal output by the balanced photodetector. It has a built-in signal processing algorithm unit to complete signal filtering, mixing, equal-frequency resampling, spectrum analysis and distance calculation, and finally outputs the absolute distance value of the measured target.

[0038] Specifically, the first balanced photodetector is a high-speed photodetector.

[0039] The resonant cavity-enhanced non-cooperative target laser sweep carrier ranging method of the present invention specifically includes the following steps: Step 1: Power on and warm up, complete the parameter configuration of the external cavity tunable laser, set the sweep frequency range, sweep frequency period and sampling rate, and complete the initial adjustment of the polarization controller and tunable optical attenuator; Step 2: Using a fiber Bragg grating, calibrate the actual sweep frequency range of the external cavity tunable laser to provide a wavelength reference for subsequent sweep frequency nonlinear correction; Step 3: The external cavity tunable laser outputs a probe laser, which is incident on the non-cooperative target being measured through the probe optical path. The target feedback light returns to the resonant cavity of the external cavity tunable laser along the original optical path. After being amplified by the resonant cavity gain, it interferes with the probe laser inside the cavity, modulating the output optical frequency of the external cavity tunable laser and generating a laser frequency sweep carrier (LFSC) carrying target distance information. Step 4: The LFSC is output to the demodulation optical path via the external cavity tunable laser pigtail. Frequency modulation and demodulation are completed by the Mach-Zehnder interferometer, and the output interference signal containing the target distance information is converted into an electrical signal by the balanced photodetector. Step 5: The data acquisition card acquires electrical signals and transmits them to the data processing module, which sequentially performs bandpass filtering, mixing, secondary bandpass filtering, equal-frequency resampling, dispersion compensation, and FFT spectrum analysis to calculate and output the absolute distance value of the target.

[0040] Please see Figure 2 The laser frequency swept optical carrier (LFSC) of the present invention is formed as follows: First, a sweeping laser beam with a strictly linearly varying frequency over time is emitted from an external cavity tunable laser of the Littman-Metcalf structure. This beam serves as the probe light and illuminates the surface of the non-cooperative target being measured. The extremely weak laser beam is reflected back along its original emission path to the internal resonant cavity of the external cavity tunable laser; this reflected light is the feedback light. Because the probe light takes time to travel to and from the target, the feedback light, compared to the intrinsic laser within the cavity, exhibits a round-trip time delay that strictly corresponds to the target distance. The farther the target, the greater the time delay; this delay is the core carrier of distance information, and the feedback light becomes the signal light carrying this target distance information.

[0041] When the round-trip phase of the feedback light exactly satisfies the phase matching condition of an integer multiple of 2π, it will undergo constructive interference with the intrinsic laser oscillating in the cavity. At the same time, the feedback light will oscillate back and forth in the resonant cavity and be continuously amplified by the gain medium. The originally weak feedback light at the nanowatt level will be exponentially enhanced through stimulated emission of the gain medium to obtain sufficiently high optical power.

[0042] The amplified feedback light and the intrinsic linearly swept laser within the resonant cavity form a stable interference signal due to the time delay difference between the round trip to the target. The frequency of this interference signal is strictly related to the round trip time delay of the target distance. The target distance information is encoded into this interference signal within the resonant cavity, and this interference signal directly acts on the gain medium of the external cavity tunable laser.

[0043] The output frequency of an external cavity tunable laser is primarily determined by the gain characteristics of the resonant cavity and the carrier concentration in the gain medium. The intracavity interference signal, generated in the previous stage and carrying distance information, directly alters the carrier concentration in the gain medium: the strength of the interference signal changes periodically, and the stimulated emission efficiency and carrier concentration of the gain medium also change periodically in sync. Ultimately, this interference signal carrying distance information modulates the laser frequency output by the external cavity tunable laser.

[0044] The following describes the scan cycle marking process: Please see Figure 1 This system also provides hardware-level periodic reference and sampling synchronization for the entire ranging process. An independent fiber Bragg grating (FBG) trigger branch is constructed in the system's optical path: the swept-frequency optical carrier output from the external cavity tunable laser (FBG) pigtail passes through a low-splitting-ratio fiber coupler (the first fiber coupler), splitting a small amount of optical power into a narrowband FBG branch with a center wavelength of 1550nm. A high-speed photodetector is connected in series at the FBG's rear end. The output of the high-speed photodetector is synchronously connected to the external trigger interface of the data acquisition card (DAQ) and the FBG motor drive controller. During the continuous linear frequency sweep of the FBG, when the instantaneous wavelength of the laser precisely sweeps across the 1550nm center wavelength of the FBG, the FBG generates a narrowband strong reflection peak. The high-speed photodetector converts this optical pulse into a spike electrical pulse with an extremely steep rising edge and an extremely narrow pulse width; this pulse is the trigger signal for the frequency sweep cycle. After the laser wavelength sweeps past 1550nm, the output of the high-speed photodetector returns to the baseline level, waiting for the next frequency sweep cycle to generate the trigger pulse again. Using the rising edge of the trigger signal as an absolutely fixed optical frequency reference zero point, three core marking actions are completed: First, the start and end intervals of a single complete sweep cycle are locked to ensure that the sweep bandwidth and tuning range intercepted in each measurement are completely consistent; second, the forward / reverse sweep intervals of the external cavity tunable laser are distinguished to compensate for motor backlash and hysteresis errors; and third, the DAQ is triggered to start hardware synchronous sampling, so as to achieve complete alignment between the sweep operation of the external cavity tunable laser and the clock of data acquisition, and at the same time provide a unified reference alignment point for multi-cycle repeated measurements. Even in low-light scenarios where the feedback light power of non-cooperative targets is as low as nanowatts, stable cycle locking and synchronous sampling can still be achieved.

[0045] The following describes frequency-modulated optical carrier interferometry demodulation based on MZI: The diffuse reflection feedback light from the non-cooperative target returns to the resonant cavity of the external cavity tunable laser along the original optical path. There, it resonates, amplifies, and coherently interferes with the intrinsically swept-frequency laser within the cavity, forming an intracavity interference signal carrying the target's round-trip time delay information. This interference signal directly modulates the carrier concentration in the gain medium of the external cavity tunable laser, thereby modulating the instantaneous frequency of the laser's output light. This transforms the originally linearly swept-frequency laser into a frequency-modulated swept-frequency optical carrier carrying the target's distance information. This frequency-modulated optical carrier, output from the external cavity tunable laser's pigtail, enters a Mach-Zehnder interferometer (MZI) to convert the frequency-modulated signal to an amplitude-modulated signal. Upon entering the MZI, the optical carrier is split into two paths: one is transmitted through a straight-through arm, and the other is transmitted through a fixed-length delay fiber arm, generating a known fixed time delay. The two paths eventually converge and coherently interfere. Since the input light is a frequency-modulated optical carrier, the time delay difference between the two light sources will convert the modulation information on the optical frequency into a phase difference between the two light sources. The intensity of the interfering light changes with the phase difference, and finally generates a time-domain interferometric electrical signal that can be collected by a balanced photodetector.

[0046] Specifically, the interference signal naturally contains three independent frequency components, and each component has a definite physical meaning: The first type is the main carrier frequency A. dis (t)ω1, determined by the time delay of the MZI fixed-delay fiber and the sweep rate of the external cavity tunable laser, is a known intrinsic parameter of the system, independent of the target being measured, and is the benchmark of the signal, as well as the main peak with the highest signal-to-noise ratio in the spectrum. The second type is the sum-frequency component A. dis (t)(ω1+ω2); The third type is the difference frequency component A. dis (t)|ω1-ω2|.

[0047] Among them, A dis (t) represents the signal amplitude modulation coefficient; the two sideband frequencies are generated by superimposing and subtracting the main carrier frequency and the target characteristic frequency ω2. The target characteristic frequency ω2 is strictly proportional to the time delay of the probe light to and from the target and is the only frequency component that carries the absolute distance information of the target.

[0048] Please see Figure 6For the raw MZI interferometric electrical signal acquired through synchronous sampling, preprocessing operations are performed sequentially: First, DC blocking and bandpass filtering are performed to remove the DC component, out-of-band environment, and circuit noise from the signal, retaining only the effective signal range containing the three frequency components; then, amplitude normalization is performed on the filtered signal to eliminate amplitude instability caused by fluctuations in the output power of the external cavity tunable laser and changes in optical path loss; finally, a Fast Fourier Transform (FFT) is performed on the preprocessed signal to complete the time-domain to frequency-domain conversion, obtaining the signal's spectral distribution, and accurately identifying and separating the main carrier frequency A. dis (t)ω1, sum frequency component A dis (t)ω1+ω2 and the difference frequency component |A dis (t)ω1-ω2| Three frequency components.

[0049] The sweep frequency operation of external cavity tunable lasers is affected by factors such as uneven motor rotation and carrier drift, resulting in inherent sweep frequency nonlinearity. This causes the laser frequency to change nonlinearly over time, and direct calculation will lead to problems such as spectrum broadening, insufficient peak extraction accuracy, and increased ranging error. At the same time, the traditional resampling method based on zero-crossing point and peak-valley extraction is limited by the Nyquist sampling theorem. Long-distance measurement requires a long-delay reference interferometer that matches the measurement distance, resulting in a bulky system structure and low integration.

[0050] This invention employs an equal-frequency resampling algorithm (EFRS) to correct laser sweep frequency nonlinearity. Using the interference signal generated by the MZI in the system as a frequency scale with equal optical frequency intervals, the acquired interference signal (i.e., a non-uniform time-domain sampled signal) is converted into a uniform optical frequency-domain sampled signal, fundamentally correcting the frequency non-uniformity problem caused by laser sweep frequency nonlinearity and avoiding spectral broadening. Specifically, a dual-path parallel resampling architecture is used to perform equal-frequency resampling on the difference frequency component and the sum frequency component separately. The fixed delay length L of the MZI is utilized. MZI A uniform resampling factor N is set to convert the two signals into a sequence of sampling points with uniform optical frequency spacing, mapping ω1 and ω2 in the frequency domain to P1 and P2 in the signal sampling point domain, thus completely eliminating the influence of laser frequency sweep nonlinearity. Simultaneously, by configuring the resampling factor and using an MZI delay fiber with a length much smaller than the distance to the target as a reference, the separated difference frequency component and sum frequency component are resampled at equal optical frequencies to complete the equal optical frequency resampling of the interference signal for ultra-long-distance targets. This overcomes the limitation of the Nyquist sampling theorem on the length of the reference interferometer, eliminating the need for a long delay fiber and significantly simplifying the system structure. After resampling, the resampled interference signal of the sum frequency component and difference frequency component affected by the dispersion of the MZI delay fiber is obtained, i.e., the sum frequency characteristic component signal sampling point sequence A. dis (t) (P1+P2) and the sampling point sequence of the difference frequency characteristic component signal |A dis (t)P1 P2∣.

[0051] However, under wide-bandwidth frequency sweep conditions, material dispersion still exists in the delay fiber of the Mach-Zehnder interferometer. Because the group velocity of lasers of different wavelengths differs in the delay fiber, the propagation delay of the MZI changes nonlinearly with the laser wavelength, resulting in nonlinear phase shifts caused by fiber dispersion in the sum and difference frequency components after equal-frequency resampling. These nonlinear phase shifts cause spectral peak broadening in the sum and difference frequency components, reducing the accuracy of peak position identification and consequently decreasing the system's distance resolution and measurement accuracy. To address these issues, this invention employs a phase linearization correction method to compensate for the dispersion error caused by the MZI delay fiber. By pre-calibrating the dispersion coefficient of the MZI delay fiber, the nonlinear phase shift caused by dispersion is obtained. During resampling, linear compensation is performed on the phase to counteract the spectral broadening effect caused by dispersion, resulting in the dispersion-compensated sum component signal sampling point sequence P1+P2 and difference frequency component signal sampling point sequence |P1. P2∣.

[0052] Then, the sampling point sequence P1+P2 of the dispersion-compensated sum frequency component signal and the sampling point sequence |P1 of the difference frequency component signal were respectively processed. P2| Performs an FFT transform to obtain the sum-frequency component spectrum and the difference-frequency component spectrum. Determine the index value of the sampling point where the peak of the difference-frequency component spectrum is located within the difference-frequency component spectrum. Determine the index value of the sampling point where the peak of the sum-frequency component spectrum is located in the sum-frequency component spectrum. .

[0053] Based on the sampling point index value where the compensated difference frequency component spectral peak is located The index value of the sampling point where the sum-frequency component spectrum peak is located after compensation. Combined with the system's preset resampling factor N and the fixed length of the Mach-Zehnder interferometer's delay fiber... The total number of sampling points in the spectrum after resampling the difference frequency component The total number of sampling points in the spectrum after resampling the sum-frequency components Substitute into the distance calculation formula and Calculate the distance values ​​corresponding to the difference frequency component and the sum frequency component. , The average of the two It is the actual distance to the target.

[0054] in, This represents the distance value corresponding to the difference frequency component. P1 represents the distance value corresponding to the sum-frequency component; P2 represents the sampling point index value corresponding to the peak value of the main carrier frequency component in the spectrum after resampling; and P3 represents the sampling point index value corresponding to the peak value of the target characteristic frequency component in the spectrum after resampling.

[0055] The dispersion compensation mentioned above will be explained in detail.

[0056] I. Analysis of Dispersion Problems To improve the system's measurement resolution, the tuning range of the frequency-scanning laser source must be expanded. However, this expansion causes dispersion mismatch in the interference signal generated by the LFSC as it propagates through the MZI, resulting in spectral broadening or peak shift.

[0057] The dispersion error caused by the delay in the fiber segment is not negligible. Fiber dispersion leads to frequency-dependent group velocity delay: within one scan cycle of the ECTL, different frequency components propagate at different group velocities, with high-frequency components propagating more slowly and low-frequency components propagating more quickly.

[0058] like Figure 3 This demonstrates that under ideal conditions, the frequency sweep law of the ECTL laser changes linearly, such as... Figure 4 This indicates that the frequency sweep law of ECTL lasers under the influence of dispersion mismatch has a fixed time delay. However, in actual measurement environments, laser frequency sweep is mostly affected by both dispersion mismatch and nonlinearity, as shown in the frequency sweep law curve. Figure 5 As shown, to obtain accurate distance measurements of the target, correction and compensation are necessary. First, the mathematical expressions of the LFSC signal and interference signal obtained under the combined effects of dispersion mismatch and nonlinearity are analyzed. The scanning curve of the delayed optical path is not parallel to the scanning curve of the reference optical path, and the delay amount... It will increase over time. This delay can be expressed as:

[0059] in, The second-order dispersion coefficient, Frequency scanning range, This refers to the length of the MZI delay fiber.

[0060] Instantaneous frequency of LFSC signal transmitted via MZI delay line It can be represented as:

[0061]

[0062] in, Indicates the linear optical frequency scanning rate. The initial optical frequency for laser scanning. , This indicates the delay of the LFSC through the MZI fiber delay line. This indicates the time delay caused by the distance to the target. This represents the beat frequency corresponding to the distance to the target, where c is the speed of light.

[0063] The LFSC signal output from the ECTL fiber pigtail passes through the MZI, and the resulting interference signal contains components consisting of the main optical frequency and the carrier wave. The phase difference between these components can be expressed as:

[0064] in, This refers to the beat frequency formed by the main optical frequency in the LFSC passing through the MZI fiber delay line.

[0065] The phase of the interference signal can be expressed as:

[0066] in, , , .

[0067] because The LFSC interferometric signal after MZI detection contains three frequency components affected by dispersion: , , The interference signal was then filtered to separate the sum-frequency components. Interference signal and difference frequency component Interference signals.

[0068] Then, the sum-frequency component signal Taking the frequency component as an example, the compensation mechanism for fiber dispersion error is explained. Phase demodulation is performed to eliminate components. Carrier phase, obtaining only the target characteristic frequency The target interference signal. Subsequently, based on the MZI reference signal, the target interference signal is resampled at the same optical frequency to obtain the frequency domain. Mapped to P2 in the signal sampling point domain, a resampled target interferometric signal containing the linear target phase and the second-order dispersion phase is obtained. The corresponding interference signal can be represented as:

[0069] Among them, item This indicates the phase error caused by dispersion mismatch. The time delay variation caused by dispersion mismatch is mainly related to the length of the MZI delay fiber and the optical frequency tuning range. Therefore, while increasing the optical frequency tuning range to improve resolution, the length of the MZI delay fiber should be shortened as much as possible. The equal-frequency resampling method used in this system can overcome the shortcomings of traditional resampling methods. It does not require the length of the delay fiber of the auxiliary interferometer to be several times that of the measurement distance. By simply selecting a reasonable resampling factor, the length of the MZI delay fiber can be made much smaller than the measurement distance.

[0070] However, shortening the fiber length cannot completely eliminate the dispersion effect; therefore, further dispersion compensation is required.

[0071] Step 1: The group velocity delay of the resampled interferometric signal affected by the dispersion of the delayed fiber introduces a residual nonlinear phase error. To separate this error, the resampled interferometric signal is first subjected to a Hilbert transform to construct an analytic signal, and the instantaneous phase containing the dispersion distortion is extracted.

[0072]

[0073]

[0074] Step 2: Perform quadratic polynomial fitting on the unwrapped instantaneous phase curve using the least squares method. The expression for the phase fitting curve of the interference signal is: ; in, For resampling point index, The coefficient of the quadratic term, The coefficient of the linear term, For constant terms; This represents the phase error. Since the sweep nonlinearity has been eliminated in the early stages of the system, the remaining nonlinear phase error is mainly caused by dispersion. Mathematically, this dispersion error is concentrated in the quadratic term of the phase, therefore the coefficient of the quadratic term is... It accurately characterizes the degree of dispersion mismatch.

[0075] Step 3: Based on the above physical mapping relationship, directly extract the quadratic coefficients of the phase fitting curve as the dispersion compensation factor. : ; The core function of this factor is to serve as a quantization benchmark for reconstructing a compensating phase that is equal in magnitude and opposite in direction to the original dispersion distortion in the next step.

[0076] Step 4: Substitute the dispersion compensation factor to construct the conjugate compensation phase (i.e., This is then compensated for on the complex signal of the original interference signal. Essentially, this step involves directly "subtracting" the nonlinear quadratic phase error in the complex domain. Taking its real part yields the pure time-domain signal, which eliminates the dispersion broadening effect and retains only the true target range and frequency information—the dispersion-compensated interference signal. ; in, This is the interference signal after dispersion compensation; The complex analytic signal is constructed by applying the Hilbert transform to the target characteristic frequency components; Index of resampling points; These are the dispersive second-order phase coefficients obtained by second-order fitting of the instantaneous phase curve; For the first The second nonlinear phase that needs to be canceled at each resampling point.

[0077] The interference signal after dispersion compensation It is the sum-frequency component signal sampling point sequence P1+P2 after dispersion compensation, and the difference-frequency component... The dispersion compensation method is similar.

[0078] II. Confirmation of the Optimal MZI Delay Fiber Length Increasing the fiber length prolongs the delay time, thereby enhancing the inherent nonlinear characteristics of the interference signal and making it easier to identify and correct. However, it also exacerbates fiber dispersion, potentially leading to spectral leakage and harmonic interference. Conversely, if the MZI delay fiber is too short, it will not generate sufficient modulation amplitude for the target interference signal, resulting in a reduced signal-to-noise ratio and affecting accurate demodulation. Furthermore, weak nonlinear characteristics can hinder effective nonlinear correction, ultimately reducing peak extraction accuracy and overall system resolution. Therefore, determining the optimal MZI delay fiber length is crucial.

[0079] The time delay variation caused by dispersion within a single frequency scanning period is as follows: Figure 7 As shown. At any given moment, the longer the MZI delay fiber, the corresponding... The larger the value, the more significant the effect of fiber dispersion on the broadening of the interference signal spectral lines.

[0080] This invention analyzes the LFSC interferometric signal spectrum through MZI. When the delay fiber length is 1 meter, the spectral component corresponding to the main optical frequency has a high signal-to-noise ratio (SNR); while the component related to the target distance is weak and almost indistinguishable from the noise floor. This indicates that a delay length of 1 meter is insufficient for accurate target demodulation. When the delay length increases to 5 meters, both the main optical frequency component and the target distance component exhibit high SNR and are clearly distinguishable. When the fiber length exceeds 5 meters, the SNR of both spectral components begins to decrease, and spectral broadening is particularly pronounced due to enhanced fiber dispersion. Under these conditions, accurate extraction of distance information becomes infeasible. When selecting the MZI delay fiber length, a balance should be struck between maximizing the target signal SNR and minimizing dispersion effects. As long as the target spectral peaks can be clearly identified, a shorter delay fiber is more suitable. This strategy ensures accurate extraction of the target-related signal and effective correction of nonlinear effects while minimizing the impact of fiber dispersion on measurement resolution. Therefore, a delay fiber length in the range of 1 to 5 meters is most suitable.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A non-cooperative target laser sweep carrier ranging method enhanced by resonant cavity, characterized in that, Includes the following steps: S1: The frequency sweeping probe laser output by the external cavity tunable laser changes linearly with time and is incident on the surface of the non-cooperative target being measured through the probe optical path. S2: The diffuse reflection feedback light from the surface of the non-cooperative target being tested returns to the resonant cavity of the external cavity tunable laser along the original detection optical path. The diffuse reflection feedback light interferes with the intrinsic sweep frequency laser in the resonant cavity, forming an intracavity interference signal carrying target distance information. The intracavity interference signal modulates the carrier concentration of the gain medium of the external cavity tunable laser and performs phase modulation on the instantaneous frequency of the output light of the external cavity tunable laser to generate a laser sweep frequency carrier carrying target distance information. S3: The laser sweep carrier is output to the demodulation optical path via the pigtail of the external cavity tunable laser, and is frequency-modulated and demodulated by the Mach-Zehnder interferometer to output an interference signal containing target distance information; the length of the delay fiber in the Mach-Zehnder interferometer is less than the distance to the target being measured; S4: Acquire the interference signal containing the target distance information, preprocess the interference signal to obtain the preprocessed signal; The preprocessed signal is mixed to separate the sum frequency component and the difference frequency component; Using the interference signal containing target distance information as a frequency scale with equal optical frequency intervals, the separated sum frequency component and difference frequency component are resampled at equal optical frequencies to obtain the resampled interference signal of the sum frequency component and the difference frequency component. Dispersion compensation is performed on the resampled interference signals of the sum frequency component and the difference frequency component respectively to obtain the dispersion-compensated sampling point sequence of the sum frequency component signal and the sampling point sequence of the difference frequency component signal; spectral analysis is performed on the dispersion-compensated sampling point sequence of the sum frequency component signal and the sampling point sequence of the difference frequency component signal to calculate the absolute distance value of the target.

2. The method according to claim 1, characterized in that, In S3, the interference signal containing target distance information, after being converted by a balanced photodetector, has a time-domain spectrum containing the following three frequency components: The main carrier frequency is determined by the fixed delay of the delay fiber of the Mach-Zehnder interferometer and the sweep rate of the external cavity tunable laser. The sum-frequency component is generated by superimposing the main carrier frequency and the target characteristic frequency. The difference frequency component is generated by subtracting the target characteristic frequency from the main carrier frequency; The target characteristic frequency is proportional to the time delay of the probe light to and from the target, and is a frequency component carrying the absolute distance information of the target.

3. The method according to claim 1, characterized in that, In S4, the equal-frequency resampling adopts a dual-path parallel resampling architecture. The interference signal generated by the Mach-Zehnder interferometer is used as the frequency scale of equal-frequency interval. By configuring the resampling factor, the Mach-Zehnder interferometer delay fiber with a length smaller than the distance to the target being measured is used as a reference to perform equal-frequency resampling on the separated difference frequency component and sum frequency component respectively.

4. The method according to claim 1, characterized in that, In S4, dispersion compensation is performed on the resampled interference signal of the sum frequency component and the difference frequency component. The dispersion compensation method specifically includes the following steps: Hilbert transforms are performed on the two resampled interference signals respectively to construct analytic signals and extract the instantaneous phase; After unenveloping the instantaneous phase, a second-order polynomial is fitted using the least squares method to obtain the phase fitting curve; the expression of the phase fitting curve is: ； in, For resampling point index, The coefficient of the quadratic term; The coefficient of the linear term, For constant terms; The coefficient of the quadratic term As a dispersion compensation factor, the dispersion compensation factor is applied to the complex signal of the resampled interference signal to obtain the dispersion-compensated complex interference signal. Taking its real part yields the dispersion-compensated interference signal, expressed as: ; in, This is the interference signal after dispersion compensation; The complex analytic signal is constructed by applying the Hilbert transform to the target characteristic frequency components; Index of resampling points; These are the dispersive second-order phase coefficients obtained by second-order fitting of the instantaneous phase curve; For the first The second nonlinear phase that needs to be canceled at each resampling point; It is the unit mode compensation factor that is opposite to the direction of the dispersive phase.

5. The method according to claim 1, characterized in that, The delay fiber length of the Mach-Zehnder interferometer ranges from 1m to 5m.

6. The method according to claim 1, characterized in that, In S4, the interference signal is preprocessed, including the following sub-steps: The acquired interference signal is subjected to DC blocking processing to filter out the DC component; Perform bandpass filtering to remove out-of-band environmental noise and circuit noise; The filtered signal is normalized to eliminate the amplitude instability caused by fluctuations in the output power of the external cavity tunable laser and changes in optical path loss.

7. The method according to claim 4, characterized in that, In step S4, spectral analysis is performed on the sampled sequence of the sum-frequency component signal and the sampled sequence of the difference-frequency component signal after dispersion compensation to calculate the absolute distance value of the target. Specifically: Fast Fourier transform is performed on the sampling point sequence of the sum frequency component signal and the sampling point sequence of the difference frequency component signal after dispersion compensation, respectively, to obtain the spectrum of the sum frequency component and the spectrum of the difference frequency component. The sampling point index value of the peak of the difference frequency component spectrum is determined in the spectrum of the difference frequency component, and the sampling point index value of the peak of the sum frequency component spectrum is determined in the spectrum of the sum frequency component. Based on the sampling point index value of the peak of the compensated difference frequency component spectrum and the sampling point index value of the peak of the compensated sum frequency component spectrum, the distance value corresponding to the difference frequency component and the distance value corresponding to the sum frequency component are calculated using the distance calculation formula, and the average value of the two is taken as the final absolute distance value of the target under test. The distance calculation formula is as follows: ； ； in, This represents the distance value corresponding to the difference frequency component. P1 is the distance value corresponding to the sum-frequency component; P2 is the sampling point index value corresponding to the peak value of the main carrier frequency component in the spectrum after resampling; P3 is the sampling point index value corresponding to the peak value of the target characteristic frequency component in the spectrum after resampling. This is the index value of the peak point of the difference frequency component spectrum. L is the index value of the peak point of the sum-frequency component spectrum; N is the system preset resampling factor, L MZI The fixed length of the delay fiber in the Mach-Zehnder interferometer; This represents the total number of sampling points in the spectrum after resampling the difference frequency component. This represents the total number of sampling points in the spectrum after resampling the sum-frequency components.

8. The method according to claim 1, characterized in that, The detection optical path includes a polarization controller and an adjustable optical attenuator arranged sequentially along the optical transmission direction; the polarization controller is used to adjust the polarization state of the detection laser and the feedback laser to improve the signal-to-noise ratio of the interference signal; the adjustable optical attenuator is used to adjust the output power of the detection laser to avoid the feedback light power being too high and affecting the working state of the external cavity tunable laser resonator.

9. The method according to claim 1, characterized in that, The demodulation optical path also includes a fiber Bragg grating trigger branch. The fiber Bragg grating trigger branch splits part of the optical power from the pigtail of the external cavity tunable laser through an optical fiber coupler. A high-speed photodetector is connected in series at the rear end of the fiber Bragg grating. When the instantaneous wavelength of the laser sweeps across the center wavelength of the fiber Bragg grating, the high-speed photodetector generates a spike electrical pulse as a trigger signal. The trigger signal is used to: lock the start and end intervals of a single complete sweep frequency cycle, distinguish between the forward and reverse sweep frequency intervals of the external cavity tunable laser, and trigger the data acquisition card to start hardware synchronous sampling, so as to achieve complete alignment between the sweep frequency action of the external cavity tunable laser and the clock of data acquisition.

10. A resonant cavity-enhanced non-cooperative target laser sweep carrier ranging system, characterized in that, include: An external cavity tunable laser is used to output a probe laser. Its resonant cavity is used to receive the target feedback light and interact with the intrinsic light in the cavity to generate a laser sweep carrier carrying target distance information. The probe optical path module is set on the output optical path of the external cavity tunable laser, and includes a polarization controller and a tunable optical attenuator arranged sequentially along the optical transmission direction. It is used to transmit the probe laser to the surface of the non-cooperative target under test and to make the target feedback light return to the resonant cavity of the external cavity tunable laser along the original path. The demodulation optical path module includes an optical fiber coupler, a Mach-Zehnder interferometer, and a balanced photodetector. The Mach-Zehnder interferometer has a fixed-length delay fiber for receiving the laser sweep carrier and performing frequency modulation and demodulation, and outputting an interference signal containing target distance information. The data acquisition and processing module includes a data acquisition card and a signal processing unit, which is used to acquire the interference signal containing the target distance information and perform signal processing. The signal processing includes at least: preprocessing, frequency mixing, equal-frequency resampling, dispersion compensation and distance calculation, and outputs the absolute distance value of the target being measured.

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