A dynamic gap measurement method based on fast sparse frequency scanning interferometry

Abstract

The invention discloses a dynamic gap measurement method based on fast sparse swept frequency interference, and the steps are as follows: the swept frequency light source sends an optical signal to an optical fiber probe through a circulator, a part of the optical signal is reflected at the end face of the optical fiber probe and propagates back to the circulator, and is reflected and propagates back; after the other part of the optical signal is emitted from the optical fiber probe, it is reflected and Doppler frequency shifted, and is re-coupled into the optical fiber probe as a measuring light, and the two beams of light interfere in the circulator to generate a beat signal; the photoelectric detection module detects the beat signal, converts the beat signal into a swept frequency interference signal s(t), and transmits it to a data acquisition and storage module. The Doppler correction coefficient is used to suppress the Doppler error, so as to obtain an optimized dynamic gap value. When the sweep frequency rate is more than 20 times the gap movement rate, Fourier transform can be directly used for demodulation. The complex phase demodulation of Hilbert transform is avoided, and the calculation is simple, convenient and fast.

Description

Technical Field

[0001] The invention relates to the field of gap measurement, and in particular to a dynamic gap measurement method based on fast sparse frequency sweeping interference. Background Art

[0002] Sweep frequency interferometry gap measurement has the advantages of no ranging ambiguity, strong anti-interference ability, high ranging accuracy and can be used for non-cooperative target measurement.

[0003] However, in the actual measurement process, the error caused by the Doppler effect due to the relative displacement of the target to be measured can be dozens of times the actual gap change. In order to solve this problem, experts and scholars at home and abroad have done a lot of research on the correction of this error from the perspective of system structure and algorithm.

[0004] The earliest method used was triangular wave frequency modulation, which used the frequency difference between forward and reverse sweeps to offset the Doppler frequency shift introduced by the Doppler effect, thereby accurately extracting the absolute distance information of the target. However, this method assumes that the target's speed remains unchanged during the modulation cycle, which is often not the case in reality. Siemens in Germany, Oxford University in the UK, and Xi'an Jiaotong University in my country have eliminated Doppler errors based on this principle, but their specific implementation methods have their own characteristics.

[0005] Existing measurement techniques based on swept frequency interferometry, such as Figures 1 to 4 , additional optical path structures will be introduced to obtain parameters for eliminating Doppler errors, which makes the system structure and algorithm quite complicated, resulting in low system reliability.

[0006] In summary, the current gap measurement system based on swept frequency interferometry has the following problems:

[0007] The measurement system structure is complex. In order to achieve accurate measurement in dynamic conditions, many research teams use complex system structures to eliminate dynamic errors, which means that the cost of the entire system is greatly increased, more unknown error factors are introduced, and subsequent maintenance is also very troublesome;

[0008] The demodulation algorithm is complex. In order to reduce the complexity of the system structure, some research groups spend a lot of time studying complex algorithms, but complex algorithms mean high computing costs and low computing efficiency. Summary of the invention

[0009] The object of the present invention is to provide a dynamic gap measurement method based on fast sparse swept frequency interferometry, comprising the following steps:

[0010] 1) Build a dynamic gap measurement system based on fast sparse frequency sweep interferometry, including PC control terminal, frequency sweep light source, fiber optic probe, photoelectric detection module, data acquisition and storage module;

[0011] 2) The swept frequency light source sends an optical signal to the optical fiber probe through a circulator, and a portion of the optical signal is reflected at the end face of the optical fiber probe and propagates back to the circulator, and then propagates back; the other portion of the optical signal is reflected and Doppler shifted after exiting the optical fiber probe, and then is re-coupled into the optical fiber probe as the measuring light, and the two beams of light interfere in the circulator to generate a beat frequency signal;

[0012] 3) The photoelectric detection module detects the beat frequency signal, converts the beat frequency signal into a swept frequency interference signal s(t), and transmits it to the data acquisition and storage module;

[0013] 4) The data acquisition and storage module transmits the swept frequency interference signal s(t) to the PC control terminal;

[0014] 5) The PC control end processes the swept frequency interference signal s(t) using Fourier transform demodulation to obtain the dynamic gap L d ;

[0015] 6) The Doppler correction coefficient is used to suppress the Doppler error, thereby obtaining an optimized dynamic gap value.

[0016] Furthermore, at the beginning of each frequency sweep cycle, the PC control terminal sends instructions to the light source module (2) and the data acquisition and storage module, thereby causing the light source module and the data acquisition and storage module to start working.

[0017] Furthermore, the optical fiber probe outputs an optical signal to the end face of the gap to be measured.

[0018] Furthermore, the frequency sweeping light source (2) transforms a low-speed frequency sweeping light source into a high-speed frequency sweeping light source through fast sparse frequency sweeping.

[0019] Furthermore, the swept frequency interference signal s(t) is as follows:

[0020]

[0021] In the formula, f α (t) and Δf 0 They represent the starting frequency and sweep bandwidth of the light source respectively; c is the speed of light. l(t) is the actual value of the gap to be measured; v(t) is the movement speed of the gap reflection end face; φ 0 , T is the frequency sweep period; t is the time; φ 0 is the initial phase.

[0022] Furthermore, the dynamic clearance L d As shown below:

[0023]

[0024] In the formula, fα f(t) and B represent the starting frequency and sweep bandwidth of the light source respectively; v(t) is the movement speed of the gap reflection end face, which is a function of time t. scan is the light source sweep speed, which is the inverse of the sweep period.

[0025] Further, the PC control terminal stores an error compensation model;

[0026] The error compensation model is used to perform error compensation on the actual value of the gap to be measured, and the steps include:

[0027] 1) The actual value l(t) of the gap to be measured is as follows:

[0028]

[0029] Where a(t) is acceleration; v is velocity; v 0 is the initial velocity;

[0030] 2) In a frequency sweep period T, let the acceleration be a constant, and the velocity of the gap reflection end face increases from v(t) to (v(t)+aT). At this time, the dynamic Fourier transform result L d As shown below:

[0031]

[0032] Where l(0) is the initial value of the gap to be measured;

[0033] 3) Define the Doppler correction coefficient G, that is:

[0034]

[0035] 4) Update the Fourier transform result L d (t), we get:

[0036]

[0037] 5) Fourier transform result L d (t) Doppler error compensation is performed to obtain:

[0038]

[0039] Where, L d ′(t) is the gap to be measured after Doppler error compensation;

[0040] 6) Calculate the residual error Δx after compensation, that is:

[0041]

[0042] In the formula, Δv and Δt are the speed difference and time difference.

[0043] Furthermore, the output parameters of the frequency sweeping light source are controlled by a PC control terminal;

[0044] The output parameters include the frequency sweep rate, frequency sweep step length, and start time of the frequency sweep of the light source.

[0045] Furthermore, the number of sampling points N of the photoelectric detection module satisfies the following constraints:

[0046]

[0047] Where L is the dynamic clearance value.

[0048] Furthermore, the beat frequency f of the dynamic beat signal bd As shown below:

[0049]

[0050] In the formula, f L (t), f doppler (t) is the frequency and Doppler shift generated by the real gap; L(t) is the length of the gap to be measured at time t; c is the speed of light;

[0051] When the relative static condition is met, the beat frequency f of the dynamic electrical signal bd As shown below:

[0052]

[0053] Further, in step 5), when the ratio ρ of the dynamic beat frequency to the real beat frequency under dynamic conditions approaches a stable value, the sweeping speed of the sweeping light source reaches a relatively static state relative to the gap change frequency, and the PC control end demodulates the swept interference signal s(t) using a Fourier transform demodulation method; the standard for approaching a stable value is: the difference between the two ratios before and after is less than a preset threshold;

[0054] The ratio ρ of the dynamic beat frequency to the actual beat frequency in dynamic state is as follows:

[0055]

[0056] In the formula, f b 、f bd They are the dynamic beat frequency and the real beat frequency under dynamic conditions respectively.

[0057] Furthermore, the gap length L(t) to be measured at time t satisfies the following formula:

[0058]

[0059] Where, L 0 is the initial gap.

[0060] Among them, the changing speed v(t) of the dynamic gap length is as follows:

[0061]

[0062] Where L is the dynamic gap length; A is the amplitude; ω is the gap change frequency, and t is time.

[0063] The technical effect of the present invention is unquestionable. The present invention proposes to increase the frequency sweep rate and reduce the Doppler error; further define the Doppler coefficient and compensate for the Doppler error in the dynamic gap, which can theoretically reduce the Doppler error to the nanometer level.

[0064] When the frequency sweep rate is more than 20 times the gap motion rate, the gap motion is relatively static relative to the frequency sweep. The fast sparse frequency sweep spectrum is demodulated using Fourier transform to solve the gap quantity.

[0065] The system consists of a sweeping laser with adjustable sweeping rate, a single detector and a data acquisition module, and has a simple structure.

[0066] When the frequency sweep rate is more than 20 times the gap motion rate, Fourier transform can be used directly for demodulation, avoiding the complex phase demodulation of Hilbert transform, and the calculation is simple, convenient and fast. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 This is a simplified diagram of the distance measurement system based on swept frequency interferometry;

[0068] Figure 2 Eliminate Doppler error for forward and reverse frequency sweep of dual lasers;

[0069] Figure 3 A dynamic error elimination method based on dual acousto-optic modulators;

[0070] Figure 4 It is a Doppler error correction method based on common-path heterodyne interferometry;

[0071] Figure 5 A dynamic gap measurement system based on fast sparse swept frequency interferometry

[0072] Figure 6 It is the flow chart of the fast sparse frequency-sweeping interferometry dynamic gap measurement algorithm;

[0073] Figure 7 is the error curve changing with the sweep rate;

[0074] Figure 8 Fourier transform of interference spectrum at different scanning speeds;

[0075] Fig. 9 is the relative static coefficient at different sweep rates;

[0076] Fig.10 The gap changes in a sinusoidal form;

[0077] Fig.11 (a)-(b) are the simulation restoration and error of the gap length changing in a sinusoidal form;

[0078] Fig.12 To take the light source sweep bandwidth as 5THz, the gap motion end surface changes in cosine form, the change frequency is 100Hz, 250Hz, 500Hz and 1kHz, the initial value of the gap l(0) = 1000μm, and the velocity phase is π / 2 (the acceleration reaches the maximum) simulation results;

[0079] In the figure, there are a PC control terminal 1, a frequency sweeping light source 2, a fiber optic probe 3, a photoelectric detection module 4, and a data acquisition and storage module 5. DETAILED DESCRIPTION

[0080] The present invention is further described below in conjunction with the embodiments, but it should not be understood that the above subject matter of the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, various substitutions and changes are made according to the common technical knowledge and customary means in the art, which should all be included in the protection scope of the present invention.

[0081] Embodiment 1:

[0082] See also Figures 5 to 12 , a dynamic gap measurement method based on fast sparse frequency sweeping interferometry, comprising the following steps:

[0083] 1) Building a dynamic gap measurement system based on fast sparse frequency sweeping interferometry, including a PC control terminal 1, a frequency sweeping light source 2, a fiber optic probe 3, a photoelectric detection module 4, and a data acquisition and storage module 5;

[0084] 2) The swept frequency light source 2 sends an optical signal to the optical fiber probe 3 through the circulator, and a part of the optical signal is reflected at the end face of the optical fiber probe 3 and propagates back to the circulator, and then propagates back; the other part of the optical signal is reflected and Doppler shifted after emitting the optical fiber probe 3, and then is re-coupled into the optical fiber probe as the measuring light, and the two beams of light interfere in the circulator to generate a beat frequency signal;

[0085] 3) The photoelectric detection module 4 detects the beat frequency signal, converts the beat frequency signal into a swept frequency interference signal s(t), and transmits it to the data acquisition and storage module 5;

[0086] 4) The data acquisition and storage module 5 transmits the swept frequency interference signal s(t) to the PC control terminal 1;

[0087] 5) The PC control terminal 1 processes the swept frequency interference signal s(t) using Fourier transform demodulation to obtain the dynamic gap L d ;

[0088] 6) The Doppler correction coefficient is used to suppress the Doppler error, thereby obtaining an optimized dynamic gap value.

[0089] At the beginning of each frequency sweep cycle, the PC control terminal 1 sends instructions to the light source module (2) and the data acquisition and storage module 5, so that the light source module 2 and the data acquisition and storage module 5 start working.

[0090] The optical fiber probe 3 outputs an optical signal to the end face of the gap to be measured.

[0091] The frequency sweeping light source 2 converts a low-speed frequency sweeping light source into a high-speed frequency sweeping light source by fast sparse frequency sweeping.

[0092] The swept frequency interference signal s(t) is shown below:

[0093]

[0094] In the formula, f α (t) and Δf 0 They represent the starting frequency and sweep bandwidth of the light source respectively; c is the speed of light. l(t) is the actual value of the gap to be measured; v(t) is the movement speed of the gap reflection end face; φ 0 , T is the frequency sweep period; t is the time; φ 0 is the initial phase.

[0095] Dynamic clearance L d As shown below:

[0096]

[0097] In the formula, f α f(t) and B represent the starting frequency and sweep bandwidth of the light source respectively; v(t) is the movement speed of the gap reflection end face, which is a function of time t. scan is the light source sweep speed, which is the inverse of the sweep period.

[0098] The PC control terminal 1 stores an error compensation model;

[0099] The error compensation model is used to perform error compensation on the actual value of the gap to be measured, and the steps include:

[0100] a) The actual value l(t) of the gap to be measured is as follows:

[0101]

[0102] Where a(t) is acceleration; v is velocity; v 0 is the initial velocity;

[0103] b) In a frequency sweep period T, let the acceleration be a constant, and the velocity of the gap reflection end face increases from v(t) to (v(t)+aT). At this time, the dynamic Fourier transform result L d As shown below:

[0104]

[0105] Where l(0) is the initial value of the gap to be measured;

[0106] c) Define the Doppler correction factor G, that is:

[0107]

[0108] d) Update the Fourier transform result L d (t), we get:

[0109]

[0110] e) Fourier transform result L d (t) Doppler error compensation is performed to obtain:

[0111]

[0112] Where, L d ′(t) is the gap to be measured after Doppler error compensation;

[0113] f) Calculate the residual error Δx after compensation, that is:

[0114]

[0115] In the formula, Δv and Δt are the speed difference and time difference.

[0116] The output parameters of the frequency sweeping light source 2 are controlled by the PC control terminal 1;

[0117] The output parameters include the frequency sweep rate, frequency sweep step length, and start time of the frequency sweep of the light source.

[0118] The number of sampling points N of the photoelectric detection module 4 satisfies the following constraints:

[0119]

[0120] Where L is the dynamic clearance value.

[0121] The beat frequency f of the dynamic beat signal bd As shown below:

[0122]

[0123] In the formula, f L (t), f doppler (t) is the frequency and Doppler shift generated by the real gap; L(t) is the length of the gap to be measured at time t; c is the speed of light;

[0124] When the relative static condition is met, the beat frequency f of the dynamic electrical signal bd As shown below:

[0125]

[0126] In step 5), when the ratio ρ of the dynamic beat frequency to the real beat frequency under dynamic conditions approaches a stable value, the sweeping speed of the sweeping light source reaches a relatively static state relative to the gap change frequency, and the PC control terminal 1 demodulates the swept interference signal s(t) using the Fourier transform demodulation method; the standard for approaching a stable value is: the difference between the two ratios before and after is less than a preset threshold;

[0127] The ratio ρ of the dynamic beat frequency to the actual beat frequency in dynamic state is as follows:

[0128]

[0129] In the formula, f b 、f bd They are the dynamic beat frequency and the real beat frequency under dynamic conditions respectively.

[0130] The measured gap length L(t) at time t satisfies the following formula:

[0131]

[0132] Where, L 0 is the initial gap.

[0133] Among them, the changing speed v(t) of the dynamic gap length is as follows:

[0134]

[0135] Where L is the dynamic gap length; A is the amplitude; ω is the gap change frequency, and t is time.

[0136] Embodiment 2:

[0137] A dynamic gap measurement system based on fast sparse frequency sweeping interferometry, comprising:

[0138] PC control terminal 1. Controls the output mode of the frequency sweeping light source, including the frequency sweeping rate, frequency sweeping step length, and the start of the frequency sweeping.

[0139] Sweep frequency light source 2. The core of this system can control the rate and scanning step of the output sweep frequency laser through the PC, and at the beginning of each sweep frequency cycle, a trigger pulse will be output to trigger the data acquisition system to start working.

[0140] The optical fiber probe 3 is used to output laser light to the target to be measured and collect the reflected light carrying the gap information to be measured.

[0141] Photoelectric detection module 4 is used to convert the reflected optical signal carrying the information of the target to be detected into an electrical signal for subsequent further processing.

[0142] Data acquisition and storage module 5. Controlled by the PC and the frequency sweeping light source. This module is mainly used to collect the output signal of the photoelectric detection module and upload the collected signal to the PC for further processing.

[0143] The light emitted by the swept frequency light source enters the fiber optic probe through the circulator. Part of the light returns at the end face of the probe, and the other part of the light returns to the fiber after reaching the end face of the gap to be measured. The two beams of light meet again in the fiber and interfere with each other. The interference signal is detected by the photoelectric detector, and then the data is collected by the data acquisition module and sent to the computer for processing. The sweep rate of the swept frequency light source can be controlled and adjusted by the computer.

[0144] Figure 2 Where l(t) is the actual value of the gap to be measured, L b is the Doppler error. L is obtained by Fourier transform d After that, error correction is performed to reduce the error L b , to achieve accurate measurement of the gap.

[0145] Embodiment 3:

[0146] A dynamic gap measurement method based on fast sparse frequency sweeping interferometry comprises the following steps:

[0147] ① The light emitted by the frequency-sweeping light source controlled by the PC reaches the fiber probe through the optical circulator. Part of the light is reflected and propagates in the opposite direction as the reference light. The other part of the light is emitted from the fiber probe, reflected at the target to be measured, and Doppler shifted before being recoupled into the fiber probe as the measurement light. The two beams of light will interfere to generate a beat frequency signal. After mathematical modeling and analysis, the frequency-sweeping interference signal detected by the detector is s(t).

[0148]

[0149] In the formula, T is the frequency sweep period of the light source, c is the speed of light, and since the actual frequency sweep light source satisfies the requirement that the starting frequency of the frequency sweep is much larger than the frequency sweep bandwidth, f can be taken as α(t) is the center frequency of the light source, l(t) is the real gap, v(t) is the speed of the moving end face, which is a function of time t. is the initial phase of the frequency.

[0150] ② Use the data acquisition system to trigger the acquisition of the swept frequency interference signal and perform gap calculation; the measurement system is as follows: Figure 5 Shown

[0151] ③ Use Fourier transform demodulation method to obtain dynamic gap L from s(t) d ;

[0152]

[0153] The light source sweep speed is defined as the inverse of the sweep period:

[0154]

[0155] ④ Suppress Doppler error by high-speed frequency sweep of the light source and definition of Doppler correction coefficient;

[0156] ⑤ The low-speed frequency sweeping light source is transformed into a high-speed frequency sweeping light source through the fast sparse frequency sweeping of the light source.

[0157] In step ③, when the gap variation frequency reaches a relatively static state relative to the frequency sweeping speed of the light source, a simple Fourier transform demodulation method can be used to complete the dynamic gap resolution.

[0158] The relative static coefficient ρ is defined as the ratio of the real beat frequency to the dynamic beat frequency:

[0159]

[0160] The light source sweep bandwidth is 5THz, the gap motion end surface changes in cosine form, the change frequency is 100Hz, 250Hz, 500Hz and 1kHz respectively, the initial value of the gap l(0) = 1000μm, the velocity phase is π / 2 (the acceleration reaches the maximum), and the above formula is simulated to obtain: Fig.12 Results shown.

[0161] In general, when the sweep speed is about 20 times the gap change frequency, it can be considered that the gap motion is in a relatively static state relative to the sweep frequency of the sweep light source. At this time, from the frequency domain point of view, the corresponding frequencies are merged into a peak value, which meets the applicable conditions of the Fourier demodulation algorithm.

[0162] In step ④, it is known that:

[0163]

[0164] When the light source sweep speed is large enough, the motion acceleration a of the moving end surface can be considered to be a constant value, and the initial gap is v 0 The real gap is

[0165]

[0166] In the period T, the velocity is v 0 Increase to (v 0 +aT).

[0167] Then the dynamic L d It can be expressed as:

[0168]

[0169] To compensate for the Doppler error, the Doppler correction coefficient G is defined, and then:

[0170]

[0171] At this time, L d (t) can be expressed as:

[0172]

[0173] Then to L d (t) Doppler error compensation can be obtained:

[0174]

[0175] The residual error after compensation can be expressed as:

[0176]

[0177] It can be found that the corrected error is inversely proportional to the light source sweep speed, which can reduce the error to the nm level.

[0178] In step ⑤, by adjusting the sweep step length of the swept frequency light source, the number of scanning points in a single cycle is reduced, thereby reducing the scanning cycle of the light source and increasing the sweep speed of the light source.

[0179] The amount of sparse frequency sweep data required must satisfy the Nyquist theorem.

[0180] The swept frequency interference signal is known to be s(t)

[0181]

[0182] The beat frequency in dynamic state is

[0183]

[0184] When the relative static condition is met, and t approaches T, the beat frequency can be simplified to

[0185]

[0186] If the light source samples a total of N points within a scanning period T, the sampling rate at this time can be expressed as:

[0187]

[0188] According to the Nyquist theorem, when sampling a signal, in order to ensure the accuracy of the signal, the sampling frequency must be no less than twice the frequency of the signal to be sampled. Then the number of scanning points should satisfy:

[0189]

Claims

1. A dynamic gap measurement method based on fast sparse frequency scanning interferometry, It is characterized in that The following steps are involved: 1) Building a dynamic gap measurement system based on fast sparse frequency sweeping interferometry, comprising a PC control terminal (1), the frequency sweeping light source (2), an optical fiber probe (3), a photoelectric detection module (4), and a data acquisition and storage module (5); 2) The swept frequency light source (2) sends an optical signal to the optical fiber probe (3) through a circulator, a portion of the optical signal is reflected at the end face of the optical fiber probe (3) and propagates back to the circulator, and then propagates back; another portion of the optical signal is reflected and Doppler shifted after exiting the optical fiber probe (3), and then is re-coupled into the optical fiber probe as measurement light, and the two beams of light interfere in the circulator to generate a beat frequency signal; 3) The photoelectric detection module (4) detects the beat frequency signal, converts the beat frequency signal into a swept frequency interference signal s(t), and transmits it to the data acquisition and storage module (5); 4) the data acquisition and storage module (5) transmits the swept frequency interference signal s(t) to the PC control terminal (1); 5) The PC control end (1) processes the swept frequency interference signal s(t) using Fourier transform demodulation to obtain the dynamic gap L d ; 6) Using the Doppler correction coefficient to suppress the Doppler error, thereby obtaining an optimized dynamic gap value; The frequency sweeping light source (2) transforms the low-speed frequency sweeping light source into a high-speed frequency sweeping light source through fast sparse frequency sweeping; The swept frequency interference signal s(t) is shown below: In the formula, f α and Δf 0 They represent the starting frequency and sweep bandwidth of the light source respectively; c is the speed of light; l(t) is the actual value of the gap to be measured; v(t) is the movement speed of the gap reflection end face; T is the sweep period; t is time; φ 0 is the initial phase; Dynamic clearance L d (t) is as follows: In the formula, f α and Δf 0 They represent the starting frequency and sweep bandwidth of the light source respectively; v(t) is the movement speed of the gap reflection end face, which is a function of time t; f scan is the light source sweep speed, is the inverse of the sweep period; l(t) is the actual value of the gap to be measured; The PC control terminal (1) stores an error compensation model; The error compensation model is used to perform error compensation on the actual value of the gap to be measured, and the steps include: s1) The actual value l(t) of the gap to be measured is as follows: In the formula, a is acceleration; v is velocity; v 0 is the initial velocity; s2) In a frequency sweep period T, the acceleration is constant a, and the motion speed of the gap reflection end face increases from v(t) to (v(t)+aT). At this time, the dynamic Fourier transform result L d As shown below: Where l(0) is the initial value of the gap to be measured; s3) Define the Doppler correction coefficient G, that is: s4) Update the Fourier transform result L d (t), we get: s5) Fourier transform result L d (t) Doppler error compensation is performed to obtain: Where, L d ′(t) is the gap to be measured after Doppler error compensation; s6) Calculate the residual error Δx after compensation, that is: In the formula, Δv and Δt are speed difference and time difference; The beat frequency f of the dynamic beat signal bd As shown below: Where L(t) is the dynamic gap length; c is the speed of light; When the relative static condition is met, the beat frequency f of the dynamic beat signal bd As shown below: Where, L(t) is the dynamic gap length; When the ratio ρ of the dynamic beat frequency to the real beat frequency under dynamic conditions approaches a stable value, the sweeping speed of the sweeping light source reaches a relatively static state relative to the gap change frequency, and the PC control end (1) demodulates the swept interference signal s(t) using a Fourier transform demodulation method; the standard for approaching a stable value is: the difference between the two ratios is less than a preset threshold; The ratio ρ of the dynamic beat frequency to the actual beat frequency in dynamic state is as follows: In the formula, f b 、f bd are the real beat frequency under dynamic conditions and the beat frequency of the beat signal under dynamic conditions respectively; At the beginning of each frequency sweep cycle, the PC control terminal (1) sends instructions to the light source module (2) and the data acquisition and storage module (5), so that the light source module (2) and the data acquisition and storage module (5) start working; The number of sampling points N of the photoelectric detection module (4) satisfies the following constraints: Where, L(t) is the dynamic gap length; The dynamic gap length L(t) at time t satisfies the following formula: Where, L 0 is the initial gap; Among them, the changing speed v(t) of the dynamic gap length is as follows: Where l(t) is the dynamic gap length; A is the amplitude; ω is the gap change frequency, and t is time.

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