Interface positioning and film thickness measuring device based on sweep frequency light source

Abstract

The invention relates to the field of frequency modulation continuous wave laser ranging, and discloses an interface positioning and film thickness measuring device based on a sweep-frequency light source, which comprises a sweep-frequency laser source S-source, a first optical coupler OC1, an optical fiber circulator CIR, a laser collimator FC, a semi-transparent and semi-reflective mirror PM, a second optical coupler OC2, a balanced photoelectric detector BPD and a spectrum analyzer SA which are connected through optical fibers. By means of the device, based on the sweep frequency interference principle, interface positioning and film thickness measurement can be achieved through four stages of laser beam splitting, double-path laser transmission, interference integration and signal analysis. The interface positioning and film thickness measuring device meets the requirements of high precision and high resolution in related fields, and has remarkable advantages in the application fields of industrial manufacturing, optical fiber communication and the like.

Description

Technical Field

[0001] This invention relates to the field of frequency-modulated continuous wave laser ranging, and more particularly to an interface positioning and film thickness measurement device based on a frequency-sweeping light source, which can be widely used in fields requiring high-precision and high-resolution position detection, such as non-contact film thickness measurement, precise positioning, and radar detection. Background Technology

[0002] There is an urgent need for high-precision, high-resolution position detection and thickness measurement devices in many fields, such as optical device inspection, industrial precision manufacturing, and dual-use radar detection. However, traditional optical position detection methods have many limitations in terms of anti-interference and integration, making it difficult to meet the ever-increasing technical requirements of these fields.

[0003] In practical applications across different scenarios, the limitations of existing methods become even more pronounced: In thin film thickness measurement within the field of optical device inspection, existing contact-based inspection equipment is prone to abrasion of the film and is unsuitable for outdoor operations; camera-based non-contact inspection is susceptible to ambient light, leading to errors. In radar detection, traditional electronic radar has limited bandwidth and low resolution, making it difficult to achieve accurate thickness detection of minute targets such as ultrathin films and multilayer composite films. Furthermore, in the field of optical coherence tomography (OCT), time-domain low-coherence interferometry (TD-OCT) suffers from insufficient system signal detection sensitivity and a significantly reduced signal-to-noise ratio due to limitations in light source linewidth and inherent defects in the signal processing link, further restricting measurement accuracy.

[0004] Therefore, existing optical position detection methods and technologies are insufficient to simultaneously meet the comprehensive requirements of multiple fields for detection devices in terms of high resolution, anti-interference capability, and high integration. Developing a novel interface positioning and film thickness measurement device based on a swept-frequency light source is of significant practical importance for solving these problems. Summary of the Invention

[0005] The present invention aims to provide an interface positioning and film thickness measurement device based on a swept frequency light source, so as to solve the shortcomings of existing position detection devices in related application fields, meet the needs of multiple fields for high-precision, non-contact measurement, and improve the overall system performance and detection accuracy.

[0006] To achieve its objectives, the present invention employs the following technical solution: This invention first discloses an interface positioning and film thickness measurement device based on a swept-frequency light source, which includes a swept-frequency laser source S-source, a first optical coupler OC1, a fiber optic circulator CIR, a laser collimator FC, a semi-transparent mirror PM, a second optical coupler OC2, a balanced photodetector BPD, and a spectrum analyzer SA. All components are connected via optical fibers. The first optical coupler OC1, the fiber optic circulator CIR, the laser collimator FC, the semi-transparent mirror PM, the second optical coupler OC2, and the balanced photodetector BPD form an interference optical path, responsible for laser beam splitting, transmission, interference, and preliminary signal processing, providing a stable interference signal for subsequent film thickness calculation and positioning.

[0007] The device of this invention is based on the principle of swept-frequency interferometry. It achieves interface positioning and film thickness measurement through four stages: laser beam splitting, dual-path laser transmission, interferometric integration, and signal analysis. After beam splitting, one path of the swept-frequency laser serves as the reference beam for the reference arm, while the other path, after processing by the sample arm, forms a zero-point calibration beam and two reflection beams from the front and rear surfaces of the sample, carrying information about the sample under test. These beams are superimposed and interfere with the reference beam of the reference arm. Through signal processing and frequency domain feature extraction, the film thickness and interface position information are calculated. The complete operation process is as follows: 1. Laser beam splitting stage The continuous linear sweeping laser output from the sweeping laser source S-source is first input to the first optical coupler OC1. The first optical coupler OC1 splits the sweeping laser into two beams according to a power ratio of 50:50. One beam enters the "sample arm" for detection of the sample under test, and the other beam enters the "reference arm" as an interference reference signal.

[0008] 2. Dual-path laser transmission stage Reference arm transmission: The swept laser from the reference arm is directly transmitted to the second optical coupler OC2, without any additional optical components. Its optical path length is... To maintain the stability of the optical signal, it serves as the reference light for subsequent interference.

[0009] Sample arm transmission: The swept laser beam from the sample arm is first directionally transmitted through a fiber optic circulator (CIR) to avoid backlash interference, and then enters a laser collimator (FC) for directional calibration, converting the divergent laser beam into parallel light. The parallel light is then incident on a semi-transparent mirror (PM), where it is split again into reflected and transmitted light. Due to their different transmission paths, the two beams have different optical path lengths, and both ultimately return to the second optical coupler (OC2) along their original optical paths, as detailed below: The reflected light from the semi-transparent mirror PM does not pass through the sample under test and returns directly to the second optical coupler OC2 along the original optical path. Its total optical path is... (The total optical path from the first optical coupler OC1 → fiber optic circulator CIR → laser collimator FC → semi-transparent mirror PM → laser collimator FC → fiber optic circulator CIR → second optical coupler OC2) This light is used to calibrate the system's measurement zero point and eliminate inherent optical path errors such as fiber length differences. It is denoted as the zero-point calibration light.

[0010] The transmitted light from the semi-transparent mirror PM illuminates the surface of the sample under test. After reflection from the front and rear surfaces of the sample, it returns along the original optical path to the second optical coupler OC2, forming reflected light from the front and rear surfaces of the sample. The total optical path of the reflected light from the front surface of the sample is... (Total optical path from first optical coupler OC1 → fiber optic circulator CIR → laser collimator FC → semi-transparent mirror PM → front surface of the sample under test → semi-transparent mirror PM → laser collimator FC → fiber optic circulator CIR → second optical coupler OC2); The total optical path of the light reflected from the rear surface of the sample under test is (Total optical path from first optical coupler OC1 → fiber optic circulator CIR → laser collimator FC → semi-transparent mirror PM → front surface of sample under test → rear surface of sample under test → front surface of sample under test → semi-transparent mirror PM → laser collimator FC → fiber optic circulator CIR → second optical coupler OC2); Both beams of sample information light carry the position and film thickness information of the sample under test. When the sample under test is a multilayer film, the front and rear surfaces of the sample under test can be regarded as the upper and lower surfaces of each layer, thereby obtaining the film thickness and position of each layer.

[0011] 3. Intervention and Integration Phase The reference beam from the reference arm and the three reflected beams from the sample arm (zero-point calibration beam, beam reflected from the front surface of the sample under test, and beam reflected from the rear surface of the sample under test) are superimposed at the second optical coupler OC2 to form three independent interference signals. These interference signals are then input to the balanced photodetector BPD, which performs differential processing to eliminate the inherent noise of the light source and enhance the signal-to-noise ratio, ultimately outputting a stable beat frequency electrical signal containing the interference information of the three beams.

[0012] 4. Signal Analysis Stage The spectrum analyzer SA receives the beat frequency electrical signal output by the balanced photodetector BPD, extracts the frequency domain features through discrete Fourier transform (DFT), and completes the measurement calculation as follows: Let the coherence length of the swept-frequency laser source S-source be To ensure that all three reflected beams can effectively interfere with the reference beam of the reference arm, the following interference conditions must be met: Optical path difference between the zero-point calibration light and the reference light of the reference arm = - < ; Optical path difference between the reflected light from the front surface of the sample and the reference light of the reference arm = - < ; Optical path difference between the reflected light from the back surface of the sample and the reference light of the reference arm = - < ; All of the aforementioned optical path differences must simultaneously satisfy the laser interference phase matching condition, which is determined by the sweep frequency range of the sweep laser source S-source.

[0013] The relationship between the frequency of the beat frequency electrical signal and the optical path difference: Let the frequency change rate of the swept-frequency laser source S-source be... The beat frequency electrical signal frequencies corresponding to the zero-point calibration light, the light reflected from the front surface of the sample, and the light reflected from the rear surface of the sample are respectively... , , Then, the frequency of the beat frequency electrical signal (i.e., the frequency in the time-domain intensity diagram of the series of beat frequency lights formed by the interference light) within one sweep frequency cycle satisfies the following relationship with the optical path difference: Where c is the speed of light in a vacuum. They can correspond to each , , , Corresponding to their respective optical path differences , , When the interference light intensity reaches its maximum value, the corresponding optical path difference is exactly an integer multiple of the wavelength of a certain frequency light within the frequency sweep range.

[0014] Film thickness calculation: By performing a discrete Fourier transform on the superimposed beat frequency electrical signal output by the balanced photodetector (BPD), its frequency domain characteristic peaks can be obtained. The three frequency domain characteristic peaks correspond to the three interference signals, thereby obtaining the beat frequency electrical signal frequencies corresponding to the zero-point calibration light, the light reflected from the front surface of the sample under test, and the light reflected from the rear surface of the sample under test, respectively. , , Using the formula relating beat frequency electrical signal frequency and optical path difference Calculate each optical path difference Thus, the net optical path difference between the front and back surfaces of the sample under test can be obtained. - The net optical path difference between the front and rear surfaces of the sample under test is equal to the round-trip optical path of light in the sample under test. , where n is the refractive index of the sample to be tested, The actual film thickness of the sample under test is used to obtain the final optical film thickness of the sample under test. : If the refractive index n of the sample to be tested is known, the actual film thickness of the sample can also be obtained. : .

[0015] Interface positioning: based on the obtained optical path difference , , Using the interface calibration formula Precisely measure the position of each surface of the sample to be tested. ,in for or , To measure the refractive index of the propagation medium outside the optical fiber in the optical path.

[0016] In the interface positioning and film thickness measuring device of the present invention: The type of swept-frequency laser source (S-source) is selected according to the application scenario and can be any of the following types: semiconductor tunable laser, swept-frequency fiber laser, tunable external cavity semiconductor laser, and broadband light source spectrometer. To ensure continuous and stable linear frequency sweeping, the rate of change of instantaneous frequency over time must remain stable within the desired accuracy.

[0017] The first optical coupler OC1 splits the swept laser beam from the swept laser source into two beams: one for the sample arm and one for the reference arm. The second optical coupler OC2 integrates the swept laser beam from the reference arm with the swept laser beam reflected from the sample arm, where interference occurs. Furthermore, both the first and second optical couplers OC1 and OC2 are single-mode fiber couplers of the same mode to ensure efficient beam splitting / combining. Alternatively, the first and second optical couplers OC1 and OC2 can be replaced by an optical waveguide chip PLC or other devices or optical paths that can force the optical field into the same spatial mode.

[0018] The laser collimator (FC) is used to calibrate the angle of the emitted light, converting the divergent output light into parallel light and ensuring the spatial mode consistency of the emitted beam. In the sample arm, the laser collimator (FC) and the semi-transparent mirror (PM) can be integrated into the same unit to reduce the optical path volume. The laser collimator (FC) needs to be matched with the fiber's numerical aperture and focal length to ensure parallel light quality. The splitting ratio of the semi-transparent mirror (PM) can be adjusted according to the reflectivity of the sample to avoid excessively weak or oversaturated signals.

[0019] The function of a balanced photodetector (BPD) is to eliminate light source noise and enhance interference signals, making the obtained beat frequency electrical signals more stable and clear, thereby improving the measurement accuracy of film thickness and positioning. Other devices with this function can be used as substitutes.

[0020] This invention relates to an interface positioning and film thickness measurement device based on a swept-frequency light source. It utilizes an all-fiber structure, employing a first optical coupler OC1 and a second optical coupler OC2 to achieve beam splitting and integration of the optical path. A semi-transparent, semi-reflective mirror (PM) is used for zero-point calibration, and a balanced photodetector (BPD) optimizes signal quality. This device meets the high-precision, non-contact position detection requirements of related fields and can address shortcomings in precision measurement applications, offering significant advantages in industrial manufacturing and fiber optic communication. Specifically, compared to existing technologies, the beneficial effects of this invention are reflected in: 1. The device of the present invention outputs linearly swept-frequency laser through a high-sweep-rate light source. Combining the principle of swept-frequency interference with precise optical path difference calculation, it can achieve micron-level distance resolution (the measurement error of a standard optical film thickness of 10 μm in actual measurement is only 1.3‰). It has significant advantages in the detection of small targets. For example, in the scenario of biological tissue detection, it can clearly identify micron-level cell structures or nano-level biological membrane thickness, providing more accurate data support for related fields.

[0021] 2. This invention uses a balanced photodetector (BPD) to process interference signals, effectively canceling inherent noise from the light source and improving the signal-to-noise ratio. This solves the problem of camera-based non-contact detection being susceptible to ambient light interference, ensuring stable signal output even in strong outdoor light or complex industrial electromagnetic environments. Furthermore, by performing a discrete Fourier transform on the beat frequency electrical signal, multiple frequency domain characteristic peaks can be extracted from the frequency domain graph. Each peak corresponds to a reflection interface. Combined with the optical path difference and film thickness conversion formula, the positional information of each interface in a multilayer film can be accurately distinguished, solving the problems of traditional contact detection's inability to distinguish multilayer structures and electronic radar's low resolution making it difficult to identify thin layers.

[0022] 3. This invention, by adjusting the type and parameters of the swept-frequency laser source (S-source), can meet the accuracy and range requirements of different application scenarios, exhibiting strong adaptability and flexibility. Simultaneously, the device has a simple structure, is easy to integrate and expand, reduces system costs, and improves overall performance. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below.

[0024] Figure 1 This is a schematic diagram of the interface positioning and film thickness measurement device based on a swept frequency light source according to the present invention.

[0025] Figure 2 This is a graph showing the frequency variation of the light source in the interface positioning and film thickness measurement device based on the swept-frequency light source of the present invention.

[0026] Figure 3This is the output time-domain diagram of the interface positioning and film thickness measurement device based on the swept frequency light source of the present invention.

[0027] Figure 4 This is a frequency variation diagram obtained after Fourier transform of the interference light of the interface positioning and film thickness measurement device based on the swept frequency light source of the present invention. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0029] The structure of the interface positioning and film thickness measurement device based on a swept frequency light source constructed in this invention is as follows: Figure 1 As shown. The measuring device includes a swept-frequency laser source S-source, a first optical coupler OC1, a fiber optic circulator CIR, a laser collimator FC, a semi-transparent mirror PM, a second optical coupler OC2, a balanced photodetector BPD, and a spectrum analyzer SA. The sample in the figure is the sample to be tested. Specifically, the selection and connection methods of each component are as follows: The swept-frequency laser source S-source is a semiconductor tunable laser with a center wavelength of 1310 nm (supporting linear frequency sweeping); the first optical coupler OC1 and the second optical coupler OC2 are both single-mode fiber 50:50 beam splitters; the fiber circulator CIR is a 3-port polarization-maintaining circulator, including port 1, port 2, and port 3; the laser collimator FC has a numerical aperture of 0.0005 and a focal length of 50 mm; the semi-transparent mirror PM has a splitting ratio of 50:50, a reflectivity of 50%, and a transmittance of 50%; the balanced photodetector BPD has a bandwidth of 20 GHz and a signal-to-noise ratio of 40 dB; the spectrum analyzer SA has a sampling rate of 30 GS / s and a frequency range of 0-10 GHz. All components are connected by single-mode fiber fusion splicing, with the fiber length adjusted according to the experimental scenario to ensure no significant optical loss. In this embodiment, the reference arm fiber is 1 m long, and the sample arm fiber is 1 m long.

[0030] To ensure the device meets the requirements for high-precision measurement, the parameters of the swept-frequency laser source (S-source), the interferometric optical path, and the detection end must be adjusted according to the following requirements: 1. Adjustment of S-source parameters for the frequency-sweeping laser source The output optical signal must meet the "continuously stable linear frequency sweep" characteristic, with the instantaneous frequency change rate fluctuating within a range of ≤100MHz over time, ensuring the stability and linearity of the output optical signal. In this embodiment, the specific parameters are set as follows: Sweep rate: set to 10 5THz / s, based on the theory that "the higher the sweep rate, the higher the detection accuracy", combined with the actual performance of the light source, balances accuracy and signal stability, and avoids frequency fluctuations caused by excessively high rates; Frequency sweep range: set to 30 THz, with 1310 nm as the center wavelength (corresponding frequency ≈ 229 THz), covering the measurement requirements of the film thickness to be measured (e.g., 5-100 μm); Sweep period: set to T=300 μs, derived from sweep range and sweep rate (T=sweep range / sweep rate).

[0031] Since the sweep frequency laser source S-source has an inherent signal rising edge (about 300 μs) and falling edge (about 10 ns), considering the response time, the actual sweep frequency range is smaller than the set input sweep frequency range, and the actual sweep frequency period is longer than the input sweep frequency period. It is necessary to monitor and record the calibration value with an oscilloscope for subsequent calculations.

[0032] Coherence Constraint: Given that the instantaneous linewidth of the swept-frequency laser source S-source is 100 MHz and the coherence length is... =0.95 m, it is necessary to ensure that when interference occurs in the frequency-sweeping laser in the optical path, the optical path difference between the reference light of the reference arm and the zero-point calibration light of the sample arm, the reflected light from the front surface of the sample under test, and the reflected light from the rear surface of the sample under test is 0.95 m. < , < , < To avoid interference signal attenuation, the optical path difference is achieved by adjusting the fiber lengths of the sample arm and the reference arm.

[0033] 2. Optimization of interference optical path parameters To ensure the normal operation of the interference optical path, the position of the semi-transparent mirror PM is adjusted to avoid strong interference signals masking other interference signals when the optical path difference between the reference arm and the sample arm is zero, thus ensuring that the output is a stable interference beat frequency electrical signal.

[0034] Simultaneously install and debug the laser collimator (FC) to ensure sufficient collimation of the swept-frequency laser irradiating the sample. The laser collimator (FC) within the sample arm affects the spatial mode and morphology of the output light. A fiber collimator with appropriate numerical aperture and focal length should be selected to ensure the collimation of the output pulse. A laser collimator (FC) with a numerical aperture of 0.0005 and a focal length of 50 mm has a divergence angle of 2 mrad.

[0035] Ideally, when the reflected light from the semi-transparent mirror PM interferes with multiple reflected beams from the surface of the sample, the resulting interference signal is a superposition of a series of beat frequency electrical signals, which is significantly affected by the optical path difference. Therefore, the position of the semi-transparent mirror PM can be reasonably adjusted according to the specific application of the system in different scenarios. In this embodiment, the semi-transparent mirror PM is adjusted to account for the optical path difference. =20μm.

[0036] 3. Adjustment of detection parameters To improve the quality of the interference signal, in addition to selecting a suitable spectrum analyzer (SA), a suitable balanced photodetector (BPD) should also be selected at the detection end. The balanced photodetector (BPD) is used to process the reading of interference signals under low power and high noise environments, and performs differential processing on the detected light intensity signal to significantly improve the signal-to-noise ratio. In this embodiment, the balanced photodetector (BPD) has a bandwidth of 20 GHz and a signal-to-noise ratio of 40 dB; the spectrum analyzer (SA) has a sampling rate of 30 GS / s and a frequency range of 0-10 GHz.

[0037] In addition to the above requirements, the output signal of the sweeping laser source S-source can be optimized according to specific application needs, such as in interface positioning applications, based on the length of the sensing fiber and the detection accuracy requirements.

[0038] The system's performance testing and verification methods are as follows: The swept laser output from the S-source was analyzed in the time domain and in the spectrum using equipment such as an oscilloscope and a spectrum analyzer (SA) to verify its time domain characteristics and frequency diversity.

[0039] Based on the above requirements, set up as follows Figure 1 The device shown inputs to the first optocoupler OC1. Figure 2 The frequency sweep signal shown.

[0040] Specifically, the swept-frequency laser source S-source uses k=10 5 A linear frequency sweep of 30 THz was performed with a sweep rate of THz / s and a center wavelength of 1310 nm. The optical thickness of the sample film under test was 10 μm, and the refractive index was 1.57.

[0041] The output time-domain result obtained after processing by the balanced photodetector (BPD) is as follows: Figure 3 As shown, theoretically it should be the single sweep time of the frequency-sweeping laser source S-source. =300 μs is the large period, multiple beat frequency signals correspond to multiple small periods, and the superposition results in a periodic beat frequency electrical signal. The frequency domain diagram obtained after the discrete Fourier transform of the interferometric light is as follows. Figure 4As shown, the frequency of the beat frequency electrical signal formed by the interference of the reflected light from each reflective surface and the reference light of the reference arm corresponds to a peak value. From left to right, these are the beat frequency electrical signal frequencies corresponding to the zero-point calibration light. =6.66 kHz, the beat frequency electrical signal frequency corresponding to the reflected light from the front surface of the sample under test. =66.67 kHz, the beat frequency electrical signal frequency corresponding to the reflected light from the back surface of the sample under test. =73.33 kHz. Theoretically, when the optical film thickness is 10 μm, the frequency difference between the two interference peaks is 6.66 kHz.

[0042] More specifically, based on the measured beat frequency electrical signal frequency, according to Calculated =19.996μm =199.871μm =219.846μm.

[0043] According to the formula for calculating optical film thickness The measured optical film thickness of the sample was 9.987 μm, with an error of only 1.3‰ compared to the actual optical film thickness of 10 μm. Simultaneously, according to the interface calibration formula... The measurements are taken in the air. The front surface of the sample to be tested is obtained. for 89.952 meters from zero This enabled precise positioning of the thin film.

[0044] This invention can be widely applied in position detection engineering. Taking a thin film thickness measurement experiment as an example, an optical thin film is first added to the sample arm, and a swept-frequency laser source is injected into it. The film thickness is read by detecting and demodulating the returned interference beat frequency electrical signal. Simultaneously, based on the experimental test results, the parameters and structure of the swept-frequency laser source are optimized to improve its performance and reliability.

[0045] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An interface positioning and film thickness measurement device based on a swept-frequency light source, characterized in that: It includes a swept-frequency laser source S-source, a first optical coupler OC1, an optical fiber circulator CIR, a laser collimator FC, a semi-transparent and semi-reflective mirror PM, a second optical coupler OC2, a balanced photodetector BPD, and a spectrum analyzer SA, with each component connected by optical fiber. The interference optical path consists of a first optical coupler OC1, an optical fiber circulator CIR, a laser collimator FC, a semi-transparent mirror PM, a second optical coupler OC2, and a balanced photodetector BPD.

2. A method for interface positioning and film thickness measurement based on a swept-frequency light source, characterized in that, Using the device described in claim 1, based on the principle of frequency sweeping interferometry, interface positioning and film thickness measurement are achieved through four stages: laser beam splitting, dual-path laser transmission, interference integration, and signal analysis. After the frequency sweeping laser is split, one path becomes the reference light of the reference arm, and the other path is processed by the sample arm to form a zero-point calibration light and a front surface reflection light and a rear surface reflection light of the sample carrying information about the sample to be measured. Each light is superimposed and interfered with the reference light of the reference arm. After signal processing and frequency domain feature extraction, the film thickness and interface position information are calculated.

3. The method according to claim 2, characterized in that, The complete operation process is as follows: (1) Laser beam splitting stage The continuous linear sweeping laser output from the sweeping laser source S-source is first input to the first optical coupler OC1. The first optical coupler OC1 splits the sweeping laser into two beams according to a power ratio of 50:

50. One beam enters the sample arm to detect the sample under test, and the other beam enters the reference arm as an interference reference signal. (2) Dual-path laser transmission stage Reference arm transmission: The swept laser from the reference arm is directly transmitted to the second optical coupler OC2, and its optical path is... , serving as the reference light for subsequent interference; Sample arm transmission: The swept laser beam from the sample arm is first directionally transmitted through a fiber optic circulator (CIR), then enters a laser collimator (FC) for direction calibration, converting the divergent laser beam into parallel light. Subsequently, the parallel light is incident on a semi-transparent mirror (PM), where it is split again into reflected and transmitted light. The optical path lengths of the two beams are as follows: The reflected light from the semi-transparent mirror PM returns directly to the second optical coupler OC2 along the original optical path, and its total optical path is... This light is used to calibrate the system's zero point measurement and is denoted as the zero-point calibration light; The transmitted light from the semi-transparent mirror PM illuminates the surface of the sample under test. After being reflected by the front and rear surfaces of the sample, it returns to the second optical coupler OC2 along the original optical path, forming the reflected light from the front and rear surfaces of the sample under test. The total optical path of the light reflected from the front surface of the sample under test is The total optical path of the light reflected from the back surface of the sample to be tested is Both beams of light carrying information about the sample's position and film thickness are used to determine the sample's location. (3) Intervention and integration stage The reference light from the reference arm, the zero-point calibration light returned from the sample arm, the light reflected from the front surface of the sample under test, and the light reflected from the rear surface of the sample under test are superimposed at the second optical coupler OC2 to form three independent interference signals. These signals are input to the balanced photodetector BPD, which performs differential processing to eliminate the inherent noise of the light source and enhance the signal-to-noise ratio. Finally, a stable beat frequency electrical signal containing the interference information of the three beams is output. (4) Signal analysis stage The spectrum analyzer SA receives the beat frequency electrical signal output by the balanced photodetector BPD, extracts the frequency domain features through discrete Fourier transform, and calculates the film thickness information.

4. The method according to claim 3, characterized in that: Let the coherence length of the swept-frequency laser source S-source be To ensure that the zero-point calibration light returned from the sample arm, the light reflected from the front surface of the sample under test, and the light reflected from the rear surface of the sample under test can all effectively interfere with the reference light of the reference arm, the following interference conditions must be met: Optical path difference between the zero-point calibration light and the reference light of the reference arm = - < ; Optical path difference between the reflected light from the front surface of the sample and the reference light of the reference arm = - < ; Optical path difference between the reflected light from the back surface of the sample and the reference light of the reference arm = - < .

5. The method according to claim 4, characterized in that: Let the sweep rate of the sweep laser source S-source be The beat frequency electrical signal frequencies corresponding to the zero-point calibration light, the light reflected from the front surface of the sample, and the light reflected from the rear surface of the sample are respectively... , , Then, the frequency of the beat frequency electrical signal and the optical path difference within one sweep cycle satisfy the following relationship: Where c is the speed of light in a vacuum, when Corresponding to , , hour, Corresponding to their respective optical path differences , , ; The method for calculating and obtaining film thickness information is as follows: A discrete Fourier transform is performed on the superimposed beat frequency electrical signal output by the balanced photodetector (BPD) to obtain its frequency domain characteristic peaks. These three frequency domain characteristic peaks correspond to three interference signals, thereby obtaining the beat frequency electrical signal frequencies corresponding to the zero-point calibration light, the light reflected from the front surface of the sample under test, and the light reflected from the rear surface of the sample under test, respectively. , , Using the formula relating beat frequency electrical signal frequency and optical path difference Calculate each optical path difference , , Thus, the net optical path difference between the front and back surfaces of the sample under test can be obtained. - The net optical path difference between the front and rear surfaces of the sample under test is equal to the round-trip optical path of light within the sample. , where n is the refractive index of the sample to be tested, The actual film thickness of the sample under test is used to obtain the optical film thickness of the sample under test. Compared with the actual film thickness of the sample to be tested : ; .

6. The method according to claim 5, characterized in that, The method for interface positioning is as follows: Based on the obtained optical path difference , , Using the interface calibration formula Accurately measure the position of each surface of the sample to be tested, among which for or , To measure the refractive index of the propagation medium outside the optical fiber in the optical path.

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