A coarse reference online self-calibration method based on dual-wavelength sinusoidal frequency modulation interference

Abstract

The application relates to an online self-calibration method for a coarse measurement reference based on double-wavelength sinusoidal frequency modulation interference and relates to the technical field of high-precision measurement of an absolute distance of a target based on double-wavelength interference. In the prior art, the optical path difference of an auxiliary interferometer is easily affected by temperature and structural changes, which leads to drift of the coarse measurement reference, and further causes inaccurate determination of the integer period of a synthetic wavelength. Therefore, the application proposes the following method: firstly, a unique coarse measurement value of the optical path difference of the auxiliary interferometer is obtained by using a large-bandwidth linear frequency modulation combined with a gas absorption chamber; secondly, sinusoidal frequency modulation is performed on two laser devices with different central wavelengths in an actual measurement process; thirdly, two interference signals are obtained through the auxiliary interferometer; fourthly, a periodic window function is constructed and mixed with a matching carrier; fifthly, filtering and quadrature demodulation are performed to extract constant phases of the two wavelengths; and finally, the real-time change amount of the optical path difference of the auxiliary interferometer is obtained by combining the coarse measurement optical path difference and the double-wavelength phase difference, so that the online updating of the coarse measurement reference is realized. The application is suitable for working scenes such as high-precision long-distance measurement and precise positioning.

Description

Technical Field

[0001] This relates to the field of high-precision measurement technology of absolute distance to a target based on dual-wavelength interferometry, specifically to an online self-calibration method for coarse measurement benchmarks based on dual-wavelength sinusoidal frequency-modulated interferometry. Background Technology

[0002] Frequency-modulated interferometry, as an important technique in precision optical ranging, has long been widely used in high-precision distance measurement scenarios such as precision manufacturing, optical assembly, and space exploration. This method modulates the frequency of a laser source, establishing a definite mapping between the frequency change of the interference signal and the measured optical path difference, thereby achieving quantitative acquisition of absolute distance. To further improve measurement accuracy and uncertainty control, dual-wavelength frequency-modulated interferometry has gradually become one of the mainstream solutions for high-precision ranging systems. This method uses two different wavelength frequency-modulated signals to synthesize a long composite wavelength, which can significantly amplify the phase change, enabling the system to maintain nanometer-level or even higher ranging capabilities over a large measurement range.

[0003] In sinusoidal frequency-modulated interferometry, the modulation method evolved from linear frequency sweep to sinusoidal frequency modulation because sinusoidal frequency modulation has advantages in suppressing modulation noise, controlling frequency changes, and facilitating the construction of matched demodulation algorithms. However, regardless of the modulation method used, the ranging process always relies on the "frequency modulation speed" as the measurement reference. In practical systems, since laser frequency modulation cannot be perfectly linear, directly using the frequency modulation speed as the reference would introduce significant nonlinear errors. Therefore, the optical path difference of the auxiliary interferometer is commonly used as a substitute for the frequency modulation speed as the actual measurement reference. Once the optical path difference of the auxiliary interferometer is stable and controllable, the frequency modulation nonlinearity can be compensated for through optical path difference mapping.

[0004] However, in real-world measurement environments, the optical path difference of the auxiliary interferometer is not constant but is highly susceptible to external factors such as temperature changes, structural thermal expansion and contraction, fluctuations in air refractive index, and mechanical perturbations. For example, when the external temperature changes by several degrees Celsius, the arm length difference of the auxiliary interferometer will exhibit a shift on the order of micrometers. When this shift is directly used for frequency modulation nonlinear compensation, it will be amplified into a more significant ranging error on the scale of the combined wavelength of the two wavelengths. Therefore, how to accurately calibrate the optical path difference of the auxiliary interferometer becomes a key factor affecting the accuracy of frequency modulation interferometric ranging.

[0005] To address the aforementioned issues, various reference control methods have been attempted in existing technologies. One approach employs hardware such as a constant temperature chamber and temperature-controlled structures to place the entire interferometer optical path in a strictly temperature-controlled environment to reduce thermal drift. However, this approach is not only costly but also difficult to achieve long-term, globally stable temperature control in practical engineering scenarios. Another approach utilizes gas absorption spectra to obtain the optical path difference of the auxiliary interferometer and calculates the reference length by comparing the absorption peak positions, suitable for narrow-bandwidth frequency modulation processes. However, in broadband frequency modulation systems, absorption spectra lack sufficient calibration capabilities, resulting in a limited calibration range. Yet another approach involves introducing high-precision external measuring instruments to synchronously monitor the optical path change of the auxiliary interferometer and then calculating the measurement reference in reverse. However, this approach cannot achieve online real-time measurement, is unsuitable for the dynamic modulation process in frequency modulation interferometry, and significantly increases system complexity and cost.

[0006] In a dual-wavelength sinusoidal frequency-modulated interferometry structure, the coarse measurement stage typically requires utilizing the constant phases corresponding to the two wavelengths. By establishing the correspondence between the phase difference and the synthesized wavelength, the integer period range of the optical path to be measured is determined. The optical path difference of the auxiliary interferometer serves both as a reference for frequency modulation nonlinearity compensation and as a fundamental parameter for phase difference judgment in the coarse measurement stage. If the optical path difference of the auxiliary interferometer shifts due to environmental changes, it will affect the accuracy of frequency modulation nonlinearity correction, potentially causing phase difference mapping deviations in the coarse measurement stage, and consequently affecting the stability and consistency of subsequent absolute distance calculations. To ensure the reliability of the coarse measurement reference, it is necessary to acquire the change in the optical path difference of the auxiliary interferometer in real time without increasing optical path complexity, excessively relying on external instruments, or introducing additional sensors. This online updating of the measurement reference is a crucial technical requirement for current frequency-modulated interferometric ranging systems.

[0007] In summary, existing technologies have the drawback of being unable to perform online real-time calibration of the optical path difference of the auxiliary interferometer while maintaining a simple optical path structure and without the need for additional measurement equipment. This results in drift of the coarse measurement reference and affects the accuracy of dual-wavelength sinusoidal frequency modulation ranging. Summary of the Invention

[0008] To address the shortcomings of existing technologies that fail to perform online real-time calibration of the optical path difference of the auxiliary interferometer while maintaining a simple optical path structure and eliminating the need for additional measurement equipment, thus leading to coarse reference drift and affecting the accuracy of dual-wavelength sinusoidal frequency modulation ranging, the present invention provides the following technical solution: An online self-calibration method for coarse measurement references based on dual-wavelength sinusoidal frequency-modulated interferometry includes: The steps for obtaining a wide-bandwidth linear frequency modulated interference signal and obtaining the optical path difference of the auxiliary interferometer through a gas absorption chamber are as follows: The steps of taking the coarse optical path difference as input, performing sinusoidal frequency modulation on two lasers with different center wavelengths, and obtaining wavelength one interference signal and wavelength two interference signal in an auxiliary interferometer; The steps are as follows: taking the wavelength one interference signal and the wavelength two interference signal as inputs, constructing periodic window functions and matched carriers respectively, and mixing the interference signals to obtain a mixed signal containing constant phase information; The steps involve taking the mixed signal as input and performing low-pass filtering and quadrature demodulation to obtain wavelength-constant phase and wavelength-two constant phase, respectively. The steps are as follows: taking the wavelength constant phase and the wavelength constant phase as input, calculating the phase difference, and combining the coarse optical path difference measurement with the dual-wavelength synthesis principle to obtain the real-time change of the optical path difference of the auxiliary interferometer. The step of using the real-time change in the optical path difference of the auxiliary interferometer as input to update the coarse measurement benchmark in real time, thereby realizing the online self-calibration of the coarse measurement benchmark.

[0009] Furthermore, a preferred embodiment is provided, in which the acquisition of a wide-bandwidth linear frequency-modulated interference signal and the obtaining of the optical path difference of the auxiliary interferometer through a gas absorption chamber include establishing an absolute correspondence between the optical path difference and the characteristic frequency positions corresponding to the gas absorption peaks using a wide-bandwidth linear frequency sweep signal.

[0010] Furthermore, a preferred embodiment is provided, in which sinusoidal frequency modulation is performed on two lasers with different center wavelengths, including passing the two laser beams with different center wavelengths sequentially through a polarization-maintaining isolator and an optical fiber coupler, and forming an optical path difference in the two interference arms of an auxiliary interferometer.

[0011] Furthermore, a preferred embodiment is provided, in which constructing a periodic window function and a matched carrier includes setting the window function period according to the signal modulation frequency and setting a matched carrier phase delay consistent with the phase delay of the interference signal to improve the constant phase energy concentration.

[0012] Furthermore, a preferred embodiment is provided, in which low-pass filtering and quadrature demodulation include obtaining a baseband signal by filtering out high-frequency components in the mixing result and obtaining a wavelength-constant phase and a wavelength-two constant phase by obtaining the phase of the quadrature components.

[0013] Furthermore, a preferred embodiment is provided, in which the phase difference calculation and the real-time change of the optical path difference of the auxiliary interferometer are obtained by combining the coarse optical path difference measurement, including obtaining the real-time change of the optical path difference by utilizing the mapping relationship between the phase difference of the dual wavelength constant and the synthesized wavelength.

[0014] Based on the same inventive concept, this invention also provides an online self-calibration device for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry, comprising: A module for acquiring a large-bandwidth linear frequency modulated interference signal and obtaining the optical path difference of the auxiliary interferometer through a gas absorption chamber; The module takes the coarse optical path difference as input, performs sinusoidal frequency modulation on two lasers with different center wavelengths respectively, and obtains the wavelength one interference signal and the wavelength two interference signal in the auxiliary interferometer; A module that takes the wavelength one interference signal and the wavelength two interference signal as inputs, constructs periodic window functions and matched carriers respectively, and mixes the interference signals to obtain a mixed signal containing constant phase information; The module takes the mixed signal as input, performs low-pass filtering and quadrature demodulation to obtain wavelength-constant phase and wavelength-two constant phase respectively; The module takes the wavelength constant phase and the wavelength constant phase as input, calculates the phase difference, and combines the coarse optical path difference measurement with the dual-wavelength synthesis principle to obtain the real-time change of the optical path difference of the auxiliary interferometer. The module uses the real-time change in the optical path difference of the auxiliary interferometer as input to update the coarse measurement benchmark in real time, thereby realizing the online self-calibration of the coarse measurement benchmark.

[0015] Based on the same inventive concept, the present invention also provides a computer storage medium for storing a computer program, wherein when the computer program is read by a computer, the computer executes the method described thereon.

[0016] Based on the same inventive concept, the present invention also provides a computer, including a processor and a storage medium, wherein when the processor reads a computer program stored in the storage medium, the computer executes the method described thereon.

[0017] Based on the same inventive concept, the present invention also provides a computer program product, which, when executed, implements the method described.

[0018] Compared with the prior art, the advantages of the technical solution provided by the present invention are as follows: Firstly, the offline calibration of the optical path difference of the auxiliary interferometer using a large-bandwidth linear frequency modulation combined with a gas cell brings uniqueness and high reliability to the optical path difference reference. Traditional methods, relying solely on sinusoidal frequency modulation or narrowband frequency modulation structures, cannot directly determine the absolute value of the optical path difference from the modulation signal. However, this scheme, through the combined effect of large-bandwidth linear frequency modulation and gas absorption spectral lines, obtains a unique coarse measurement value of the optical path difference of the auxiliary interferometer. This allows the integer period parameter required for subsequent online calibration to be clearly determined, avoiding the problems of non-unique integer periods and incorrect counting of synthesized wavelength periods in existing technologies, fundamentally improving the reliability of the coarse measurement reference.

[0019] The method of sinusoidally frequency modulating two lasers and simultaneously acquiring dual-wavelength auxiliary interferometric signals during actual measurement provides real-time availability of online phase information. Most existing methods use only a single-wavelength frequency-modulated signal, failing to achieve real-time estimation of reference changes. This scheme, however, uses simultaneous modulation of two wavelengths to output interference signals with independent carrier phase delays in the auxiliary interferometer, enabling real-time extraction of the two constant phases. This structure allows the system to continuously monitor the status of the auxiliary interferometer without altering the optical path or adding additional interferometer arms, providing a continuous data source for online optical path difference calculation—an effect difficult to achieve with single-wavelength frequency modulation methods.

[0020] The method of constructing a periodic window function and matching carrier for each interference signal and then performing frequency mixing results in a high signal-to-noise ratio phase extraction capability. Traditional sinusoidal frequency modulation demodulation, if directly using Fourier transform or peak detection, is easily affected by frequency modulation nonlinearity, noise, and phase drift, leading to unstable constant phase extraction. This scheme achieves time-domain shaping through a periodic window function and uses a matching carrier for coherent mixing, effectively highlighting the baseband component corresponding to the constant phase in the interference signal in the frequency domain. After low-pass filtering and quadrature demodulation, the constant phase of the interference signal can be extracted with high precision. This method significantly improves the noise immunity of the demodulation process and reduces the impact of frequency modulation nonlinearity on the phase output.

[0021] The method of subtracting the constant phases of the two lasers and combining the results with offline calibration to obtain the optical path difference variation of the auxiliary interferometer brings the ability to self-calibrate the optical path difference online. Through the "phase difference-optical path difference" mapping relationship constructed using dual wavelengths, minute changes in the optical path difference are amplified to the scale of the synthesized wavelength. Therefore, real-time monitoring of the dynamic changes in the optical path difference of the auxiliary interferometer with nanometer-level resolution is possible without adding any additional measurement equipment. This method overcomes the complex process of existing technologies that rely on external high-precision instruments or temperature control methods, enabling the system to maintain the stability of the measurement reference in an open environment.

[0022] The design approach, which involves performing an offline calibration only once during the system setup phase and then relying entirely on dual-wavelength sinusoidal frequency modulation for online updates, significantly simplifies the measurement system structure. This approach avoids the additional complex equipment commonly found in existing studies, such as temperature-controlled chambers, long-range frequency stabilization structures, and external calibration interferometers. This allows the measurement system to maintain high accuracy while significantly reducing size, cost, and optical path complexity. Furthermore, this design enhances the system's environmental adaptability, enabling it to continuously provide accurate auxiliary interferometer references even in scenarios with frequent temperature drift or minor structural changes—an effect that traditional one-time calibration systems cannot achieve.

[0023] It is suitable for high-precision absolute distance measurement work that requires real-time calibration of the optical path difference of the auxiliary interferometer and maintenance of a stable ranging reference in a broadband frequency modulated interferometric ranging system. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the optical path on which the high-precision ranging method based on dual-wavelength sinusoidal frequency-modulated interferometry is based; Figure 2 The absorption lines are from the HCN gas absorption chamber. Figure 3 This is the spectrum of the mixed signal; Figure 4 This is the demodulation result for the initial phase at wavelength 1; Figure 5 This is the demodulation result for the initial phase at wavelength 2. Detailed Implementation

[0025] To make the advantages and benefits of the technical solution provided by the present invention clearer, the technical solution provided by the present invention will now be described in further detail with reference to the accompanying drawings, specifically: Implementation Method 1: This implementation method provides an online self-calibration method for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry, including: The steps for obtaining a wide-bandwidth linear frequency modulated interference signal and obtaining the optical path difference of the auxiliary interferometer through a gas absorption chamber are as follows: The steps of taking the coarse optical path difference as input, performing sinusoidal frequency modulation on two lasers with different center wavelengths, and obtaining wavelength one interference signal and wavelength two interference signal in an auxiliary interferometer; The steps are as follows: taking the wavelength one interference signal and the wavelength two interference signal as inputs, constructing periodic window functions and matched carriers respectively, and mixing the interference signals to obtain a mixed signal containing constant phase information; The steps involve taking the mixed signal as input and performing low-pass filtering and quadrature demodulation to obtain wavelength-constant phase and wavelength-two constant phase, respectively. The steps are as follows: taking the wavelength constant phase and the wavelength constant phase as input, calculating the phase difference, and combining the coarse optical path difference measurement with the dual-wavelength synthesis principle to obtain the real-time change of the optical path difference of the auxiliary interferometer. The step of using the real-time change in the optical path difference of the auxiliary interferometer as input to update the coarse measurement benchmark in real time, thereby realizing the online self-calibration of the coarse measurement benchmark.

[0026] Obtaining a wide-bandwidth linear frequency-modulated interference signal and obtaining a coarse optical path difference for the auxiliary interferometer through a gas absorption chamber involves establishing an absolute correspondence between the optical path difference and the characteristic frequency positions corresponding to the gas absorption peaks using a wide-bandwidth linear frequency sweep signal.

[0027] Performing sinusoidal frequency modulation on two lasers with different center wavelengths involves passing the two laser beams with different center wavelengths sequentially through a polarization-maintaining isolator and an optical fiber coupler, and forming an optical path difference in the two interference arms of an auxiliary interferometer.

[0028] Constructing a periodic window function and a matched carrier involves setting the window function period based on the signal modulation frequency and setting a matched carrier phase delay consistent with the phase delay of the interference signal to improve the constant phase energy concentration.

[0029] Low-pass filtering and quadrature demodulation involve obtaining the baseband signal by filtering out high-frequency components from the mixing result and obtaining the phase from the quadrature components to obtain a wavelength-constant phase and a wavelength-two-constant phase.

[0030] The phase difference is calculated and combined with the coarse optical path difference to obtain the real-time change of the optical path difference of the auxiliary interferometer. This includes obtaining the real-time change of the optical path difference by using the mapping relationship between the phase difference of the dual wavelength constant and the synthesized wavelength.

[0031] A coarse measurement reference online self-calibration device based on dual-wavelength sinusoidal frequency-modulated interferometry is also provided, comprising: A module for acquiring a large-bandwidth linear frequency modulated interference signal and obtaining the optical path difference of the auxiliary interferometer through a gas absorption chamber; The module takes the coarse optical path difference as input, performs sinusoidal frequency modulation on two lasers with different center wavelengths respectively, and obtains the wavelength one interference signal and the wavelength two interference signal in the auxiliary interferometer; A module that takes the wavelength one interference signal and the wavelength two interference signal as inputs, constructs periodic window functions and matched carriers respectively, and mixes the interference signals to obtain a mixed signal containing constant phase information; The module takes the mixed signal as input, performs low-pass filtering and quadrature demodulation to obtain wavelength-constant phase and wavelength-two constant phase respectively; The module takes the wavelength constant phase and the wavelength constant phase as input, calculates the phase difference, and combines the coarse optical path difference measurement with the dual-wavelength synthesis principle to obtain the real-time change of the optical path difference of the auxiliary interferometer. The module uses the real-time change in the optical path difference of the auxiliary interferometer as input to update the coarse measurement benchmark in real time, thereby realizing the online self-calibration of the coarse measurement benchmark.

[0032] A computer storage medium is also provided for storing a computer program, which, when read by the computer, executes the method.

[0033] A computer is also provided, including a processor and a storage medium, wherein the computer executes the method when the processor reads a computer program stored in the storage medium.

[0034] A computer program product is also provided, which, when executed, implements the method described.

[0035] Implementation Method Two: This implementation method is a further detailed description of the technical solution provided in Implementation Method One, specifically: First, a wide-bandwidth linearly frequency-modulated (LFM) interference signal is acquired for offline calibration of the optical path difference of the auxiliary interferometer. LFM causes the laser frequency to change uniformly over time, thus forming frequency characteristics corresponding to the optical path difference in the interference signal. A gas absorption chamber is introduced into the optical path, causing significant intensity attenuation of the frequency-modulated signal at specific absorption peaks. By identifying the frequency positions corresponding to these absorption peaks and combining this with the LFM slope, a coarse measurement of the optical path difference of the auxiliary interferometer can be obtained. This coarse measurement is physically unique and serves as the initial reference for the entire online calibration process. The output of the offline calibration is the coarse measurement of the optical path difference of the auxiliary interferometer, which serves as the input for the next stage of dual-wavelength sinusoidal frequency-modulated measurement.

[0036] After obtaining the coarse optical path difference, sinusoidal frequency modulation is performed on both lasers, causing the laser frequency to change according to a sinusoidal function over time. Sinusoidal frequency modulation has the advantages of controllable frequency range and periodic repeatability of the modulation process, making it more suitable for subsequent signal processing using periodic window functions and matched carriers. The two lasers, with different center wavelengths, enter an auxiliary interferometer to generate interference signals, which are collected by a balanced detector as wavelength one interference signal and wavelength two interference signal, respectively. Since the two laser beams have different wavelengths, their corresponding interference signals contain different carrier phase information, which forms the basis for subsequent phase difference calculation and online update of the optical path difference. The output of the interference signal serves as the input to the next step of the signal processing chain.

[0037] The two acquired interference signals are introduced into a signal shaping step. To extract the constant phase, a periodic window function and a carrier signal matching the structure of the interference signals need to be constructed. The periodic window function is used to select the effective periodic portion of the interference signal and suppress the edge effects caused by nonlinear frequency modulation; the matched carrier is used to coherently superimpose the interference signal with it in the mixing operation, thereby transforming the constant phase portion of the original interference signal to an easily extractable position in the frequency domain. The interference signal is mixed with the periodic window function and the matched carrier respectively. The output signal after mixing contains baseband components and high-frequency components. The baseband components carry the main information of the constant phase and are the input required for the next step of filtering.

[0038] The mixed signal is input to a low-pass filter. By filtering out high-frequency components, only the baseband portion is retained, effectively preserving the constant phase information. The baseband signal contains real and imaginary components, which are separated through quadrature demodulation. The constant phase corresponding to the wavelength is then calculated using arctangent. Performing this demodulation step on the interference signals at wavelengths one and two respectively yields two sets of stable constant phase results. The output of the constant phase constitutes an important input for the next step of online optical path difference calculation.

[0039] After obtaining the constant phases of the two lasers, the two sets of constant phases are differentiated using dual-wavelength measurement theory. The wavelength difference leads to different equivalent optical path difference mappings for the two lasers, and there is a clear mathematical correspondence between this differential phase and the actual optical path difference change. Combining the coarse optical path difference value obtained from offline calibration, the phase difference is converted into a real-time change in the optical path difference of the auxiliary interferometer through the concept of synthesized wavelength. This real-time change directly reflects the optical path difference drift of the auxiliary interferometer under environmental changes without requiring the use of a gas cell or other external instruments. This real-time optical path difference output serves as the basis for updating the coarse measurement benchmark in the next stage.

[0040] By updating the coarse measurement reference using the online-derived optical path difference, the dual-wavelength sinusoidal frequency-modulated interferometric ranging system can accurately determine the integer period corresponding to the synthesized wavelength during the coarse measurement phase. Since determining the integer period is crucial for absolute distance measurement, any drift in the optical path difference of the auxiliary interferometer will lead to errors in the integer period calculation and cause error accumulation. This scheme continuously updates the optical path difference online, compensating in real time for reference offsets caused by changes in ambient temperature and mechanical micro-motions, ensuring that the coarse measurement results remain accurate and reliable. The updated coarse measurement reference is fed back to the main ranging link, enabling the system to maintain stable ranging accuracy during long-term operation.

[0041] Through the above-described step-by-step processing steps, this solution enables online real-time calibration of the optical path difference of the auxiliary interferometer without changing the optical path structure or adding additional calibration equipment, ensuring that the dual-wavelength sinusoidal frequency-modulated interferometric ranging system maintains high-precision operation in practical applications.

[0042] In the optical path section: The system employs a dual-wavelength frequency-modulated interferometric optical path structure, consisting of two lasers with different center wavelengths, a polarization-maintaining isolator, an optical fiber coupler, an auxiliary interferometer optical path, and two balanced detectors. The two lasers output narrow-linewidth continuous beams with different center wavelengths. These two beams, after modulation, are used to construct dual-wavelength auxiliary interference signals, thereby enabling subsequent constant phase extraction and online optical path difference calibration.

[0043] The laser output light first enters a polarization-maintaining isolator to prevent reflected light from returning to the laser cavity, ensuring modulation and output power stability. The light output from the polarization-maintaining isolator then enters a first-stage fiber coupler, which couples the laser output into a dual-path structure with stable optical power distribution. The two outputs of the coupler each output light in a 5:5 power ratio; one path enters the upper arm of the auxiliary interferometer, and the other enters the lower arm.

[0044] The auxiliary interferometer consists of two interferometer arms with different optical path lengths, formed by an extension fiber and a fixed fiber path. The two fiber arms have a preset difference in physical length, ensuring that interference signals are generated when the light propagates through the two arms and is re-combined. After propagating through the two arms of the auxiliary interferometer, the light enters a second-stage fiber coupler, where the two beams are re-combined to form an interference output containing information about the optical path difference. Since the system contains two lasers, each with its output passing through the same optical path structure, the two outputs of the second-stage fiber coupler will simultaneously contain interference signals from both wavelength one and wavelength two paths.

[0045] The two outputs of the second-stage fiber coupler are connected to two balanced detectors, enabling each detector to receive the interference signal from the combined beams of the two optical arms and detect the interference fringes differentially. Balanced detector one receives the interference signal from the first-stage output, and balanced detector two receives the interference signal from the second-stage output. Because the center wavelengths of the two lasers are different, the output signals of both detectors simultaneously contain interference components corresponding to both wavelength one and wavelength two. These components have different frequency characteristics and can be separated in subsequent signal processing.

[0046] The optical design of the entire optical path ensures that the output beams of both lasers pass through the same interferometer arm structure and interfere in the same combined coupler. Therefore, environmental disturbances, temperature changes, and structural micro-motions will act on both wavelength optical paths in the same way, ensuring consistent systematic error characteristics between the interference signals. Based on this, sinusoidal frequency modulation is used to form time-varying interference fringes between the two beams. These fringes are then detected by a balanced detector to obtain the wavelength one interference signal and the wavelength two interference signal. These two signals serve as the foundational inputs for subsequent construction of periodic window functions, matched carrier waves, mixing, filtering, and quadrature demodulation. They are used to extract the constant phase of the two wavelengths and further calculate the optical path difference change of the auxiliary interferometer through the phase difference, achieving online calibration.

[0047] This optical path structure uses only two stages of fiber couplers and a set of auxiliary interferometers to simultaneously perform dual-wavelength interferometry measurements, without introducing additional independent interferometers or additional probe links, thus maintaining a highly simple optical path. Since the two lasers share the exact same optical path structure, the systematic errors between the two wavelengths are naturally synchronized, allowing the optical path difference information to be mapped more stably and accurately through the phase difference. In actual measurements, this effectively suppresses errors caused by environmental drift, laying a physical foundation for online calibration of the optical path difference and improving the accuracy of dual-wavelength coarse measurements.

[0048] Implementation Method 3, in conjunction with Appendix Figure 1-5 This embodiment describes the technical solution provided above in further detail through specific examples. Specifically: This embodiment aims to design an online self-calibration method for coarse measurement references based on dual-wavelength sinusoidal frequency-modulated interferometry. To this end, the following description of the invention will briefly explain the optical path structure, measurement principle, and processing flow.

[0049] Figure 1 The optical path structure used in this embodiment is as follows: before sinusoidal frequency modulation interferometry, the optical path difference of the auxiliary interferometer is calibrated offline using a large-bandwidth linear frequency modulation combined with an air cell. Then, sinusoidal frequency modulation is performed on the two lasers to balance the auxiliary interference signal (auxiliary interferometer) of wavelength 1 received by detector 1. The auxiliary interference signal at wavelength 2 received by the balanced detector 2 (auxiliary interferometer) The mathematical expression for the signals received by balanced detectors 1 and 2 is as follows:

[0050]

[0051] In the formula, , These are the modulation bandwidths of the two lasers. It is the frequency of sinusoidal modulation. It is the time delay due to the difference in arm length of the auxiliary interferometer. , These are the carrier phase delays of the two lasers. , These represent the frequencies output by the two lasers. Since the interference signal processing methods for both lasers are the same, for ease of explanation, the processing method for the laser generating wavelength 1 will only be described in detail below. The product of is much less than 1, therefore we can consider Therefore, it can be rewritten as

[0052] The periodic window function and the matched carrier are constructed as shown in the following equation.

[0053]

[0054] In the formula Indicates the amplitude of the Gaussian curve. This represents the delay of the matched carrier phase delay. This indicates the matched carrier modulation depth. Mixing yields...

[0055] when , hour:

[0056] Applying low-pass filtering to the above equation to retain only the baseband signal, we get:

[0057] By taking the real and imaginary parts and performing orthogonal demodulation, the constant phase of the interference signal can be obtained. The same initialization method can also be used to obtain the constant phase of the interference signal from the laser emitting wavelength 2. Subtracting the two constant phases and combining them with the calibration results, the optical path difference of the auxiliary interferometer can be obtained.

[0058] In the formula The integer period representing the phase is uniquely determined by the gas chamber calibration results.

[0059] The above algorithm was simulated, with the output wavelengths of the two lasers set to 1533nm and 1560nm, a modulation frequency of 20kHz, and a modulation bandwidth of 15GHz. The length of the auxiliary interferometer with an optical path difference of (4m + 500nm) was calibrated, firstly using gas absorption peaks such as... Figure 2 As shown, a coarse measurement of the optical path difference of the auxiliary interferometer was obtained, which is (4m-5μm). Then, the coarse measurement was divided by the synthesized wavelength of 88μm to obtain an integer period of 45455. Finally, the interference signal was mixed with a periodic window function and a matched carrier. The spectrum of the mixed signal is shown in the figure. Figure 3 As shown, the constant phases of the two lasers are obtained through low-pass filtering and quadrature demodulation, and the results are as follows. Figure 4 and 5 Combining equation (9), the optical path difference of the auxiliary interferometer is (4m+502nm).

[0060] The simulation results demonstrate that this method can achieve high-precision online calibration of optical path difference in auxiliary interferometers.

[0061] This embodiment proposes an online self-calibration method for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry. First, offline calibration of the optical path difference of the auxiliary interferometer is performed using a large-bandwidth linear frequency modulated interferometer and an air cell. Then, during actual measurement, the constant phase is extracted from the interference signals generated by the auxiliary interferometer using two lasers. The optical path difference of the auxiliary interferometer is calibrated in real time using the dual-wavelength concept combined with the offline calibration value. In this method's measurement system, only one offline calibration of the optical path difference of the auxiliary interferometer is required; subsequent changes in the optical path difference can be measured online using only the dual-wavelength sinusoidal frequency-modulated interferometry method, thus making the system more concise. The above description of several specific embodiments further details the technical solution provided by the present invention in order to highlight the advantages and benefits of the technical solution provided by the present invention. However, the above-described specific embodiments are not intended to limit the present invention. Any reasonable modifications and improvements to the present invention, combinations of embodiments, and equivalent substitutions based on the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for online self-calibration of a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry, characterized in that, include: The steps for obtaining a wide-bandwidth linear frequency modulated interference signal and obtaining the optical path difference of the auxiliary interferometer through a gas absorption chamber are as follows: The steps of taking the coarse optical path difference as input, performing sinusoidal frequency modulation on two lasers with different center wavelengths, and obtaining wavelength one interference signal and wavelength two interference signal in an auxiliary interferometer; The steps are as follows: taking the wavelength one interference signal and the wavelength two interference signal as inputs, constructing periodic window functions and matched carriers respectively, and mixing the interference signals to obtain a mixed signal containing constant phase information; The steps involve taking the mixed signal as input and performing low-pass filtering and quadrature demodulation to obtain wavelength-constant phase and wavelength-two constant phase, respectively. The steps are as follows: taking the wavelength constant phase and the wavelength constant phase as input, calculating the phase difference, and combining the coarse optical path difference measurement with the dual-wavelength synthesis principle to obtain the real-time change of the optical path difference of the auxiliary interferometer. The real-time change in the optical path difference of the auxiliary interferometer is used as input to update the coarse measurement benchmark in real time, thereby realizing the online self-calibration of the coarse measurement benchmark. The construction of the periodic window function and the matching carrier includes setting the window function period according to the signal modulation frequency and setting the matching carrier phase delay consistent with the phase delay of the interference signal to improve the constant phase energy concentration.

2. The online self-calibration method for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry according to claim 1, characterized in that, Obtaining a wide-bandwidth linear frequency-modulated interference signal and obtaining a coarse optical path difference for the auxiliary interferometer through a gas absorption chamber involves establishing an absolute correspondence between the optical path difference and the characteristic frequency positions corresponding to the gas absorption peaks using a wide-bandwidth linear frequency sweep signal.

3. The online self-calibration method for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry according to claim 1, characterized in that, Performing sinusoidal frequency modulation on two lasers with different center wavelengths involves passing the two laser beams with different center wavelengths sequentially through a polarization-maintaining isolator and an optical fiber coupler, and forming an optical path difference in the two interference arms of an auxiliary interferometer.

4. The online self-calibration method for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry according to claim 1, characterized in that, Low-pass filtering and quadrature demodulation involve obtaining the baseband signal by filtering out high-frequency components from the mixing result and obtaining the phase from the quadrature components to obtain a wavelength-constant phase and a wavelength-two-constant phase.

5. The online self-calibration method for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry according to claim 1, characterized in that, The phase difference is calculated and combined with the coarse optical path difference to obtain the real-time change of the optical path difference of the auxiliary interferometer. This includes obtaining the real-time change of the optical path difference by using the mapping relationship between the phase difference of the dual wavelength constant and the synthesized wavelength.

6. An online self-calibration device for a coarse measurement reference based on dual-wavelength sinusoidal frequency-modulated interferometry, characterized in that, include: A module for acquiring a large-bandwidth linear frequency modulated interference signal and obtaining the optical path difference of the auxiliary interferometer through a gas absorption chamber; The module takes the coarse optical path difference as input, performs sinusoidal frequency modulation on two lasers with different center wavelengths respectively, and obtains the wavelength one interference signal and the wavelength two interference signal in the auxiliary interferometer; A module that takes the wavelength one interference signal and the wavelength two interference signal as inputs, constructs periodic window functions and matched carriers respectively, and mixes the interference signals to obtain a mixed signal containing constant phase information; The module takes the mixed signal as input, performs low-pass filtering and quadrature demodulation to obtain wavelength-constant phase and wavelength-two constant phase respectively; The module takes the wavelength constant phase and the wavelength constant phase as input, calculates the phase difference, and combines the coarse optical path difference measurement with the dual-wavelength synthesis principle to obtain the real-time change of the optical path difference of the auxiliary interferometer. The module uses the real-time change in the optical path difference of the auxiliary interferometer as input to update the coarse measurement benchmark in real time, thereby realizing the online self-calibration of the coarse measurement benchmark. The construction of the periodic window function and the matching carrier includes setting the window function period according to the signal modulation frequency and setting the matching carrier phase delay consistent with the phase delay of the interference signal to improve the constant phase energy concentration.

7. A computer storage medium for storing computer programs, characterized in that, When the computer program is read by the computer, the computer executes the method of claim 1.

8. A computer, comprising a processor and a storage medium, characterized in that, When the processor reads the computer program stored in the storage medium, the computer executes the method of claim 1.

9. A computer program product, as a computer program, is characterized by: When the computer program is executed, it implements the method of claim 1.

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