Interface positioning and film thickness measuring device based on a swept source

Abstract

The application relates to the field of frequency-modulated continuous wave laser ranging, and discloses an interface positioning and film thickness measuring device based on a swept light source, which comprises a swept laser source S-source, a first optical coupler OC1, a fiber ring CIR, a laser collimator FC, a semi-transparent semi-reflective mirror PM, a second optical coupler OC2, a balanced photodetector BPD and a spectrum analyzer SA, and the components are connected through optical fibers. By using the above device, based on the swept interference principle, interface positioning and film thickness measurement can be realized 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 demand of high precision and high resolution in the related field, and has obvious advantages in application fields such as industrial manufacturing and fiber communication.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of frequency-modulated continuous wave laser ranging, in particular to an interface positioning and film thickness measuring device based on a swept light source, which can be widely applied in non-contact film thickness measurement, precise positioning, radar detection and other fields requiring high-precision and high-resolution position detection. BACKGROUND

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

[0003] In specific practical applications in different scenarios, the limitations of existing methods are more prominent: in the field of optical device detection, existing contact detection equipment is prone to wear on thin films and is not suitable for outdoor operations; non-contact detection based on cameras is easily affected by environmental light and has error problems. In the field of radar detection, traditional electronic radars have limited bandwidth and low resolution, making it difficult to achieve accurate thickness detection of ultra-thin films, multi-layer composite films and other small-sized targets. In the field of optical coherence tomography, time-domain low-coherence interferometry (TD-OCT) has insufficient system signal detection sensitivity due to the limitations of light source linewidth and inherent defects in signal processing chain, resulting in a significant reduction in signal-to-noise ratio and further restricting the measurement accuracy.

[0004] Therefore, existing optical position detection methods and technologies are difficult to meet the comprehensive needs of detection devices in high-resolution, anti-interference, high-integration and other aspects in multiple fields. Developing a new interface positioning and film thickness measuring device based on a swept light source has important practical significance in solving the above problems. SUMMARY

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

[0006] To achieve the purpose of the present application, the following technical solutions are adopted:

[0007] The application firstly discloses an interface positioning and film thickness measuring device based on a swept light source, which comprises a swept laser source S-source, a first optical coupler OC1, a fiber ring CIR, a laser collimator FC, a semi-transparent half-mirror PM, a second optical coupler OC2, a balanced photodetector BPD and a spectrum analyzer SA, and each component is connected through an optical fiber. Wherein, the first optical coupler OC1, the fiber ring CIR, the laser collimator FC, the semi-transparent half-mirror PM, the second optical coupler OC2 and the balanced photodetector BPD constitute an interference light path, which is responsible for completing laser beam splitting, transmission, interference and signal preliminary processing, and provides a stable interference signal for subsequent film thickness calculation and positioning.

[0008] The device of the application is based on the swept interference principle, and realizes interface positioning and film thickness measurement through four stages of laser beam splitting, two-way laser transmission, interference integration and signal analysis: after the swept laser is split, one way is the reference light of the reference arm, and the other way is the zero point calibration light, the front surface reflection light of the measured sample and the back surface reflection light of the measured sample after the sample arm processing, each light is superimposed and interfered with the reference light of the reference arm, and the film thickness and interface position information are calculated and obtained through signal processing and frequency domain feature extraction. The complete running process is as follows:

[0009] 1. Laser beam splitting stage

[0010] The continuous linear swept laser output by the swept laser source S-source is first input into the first optical coupler OC1, and the first optical coupler OC1 splits the swept laser into two beams according to the power ratio of 50:50, one beam enters the "sample arm" for detecting the measured sample, and the other beam enters the "reference arm" as the interference reference signal.

[0011] 2. Two-way laser transmission stage

[0012] Reference arm transmission: the swept laser of the reference arm is directly transmitted to the second optical coupler OC2, and there is no additional optical component in the whole process, and the optical path is , which maintains the stability of the optical signal and serves as the reference light for subsequent interference.

[0013] Sample arm transmission: the swept laser of the sample arm is first transmitted in a directional manner through the fiber ring CIR to avoid reverse interference of the optical signal, and then enters the laser collimator FC for direction calibration to convert the divergent laser into parallel light; then the parallel light is incident on the semi-transparent half-mirror PM and is split again into reflected light and transmitted light, and the two beams form different optical paths due to different transmission paths, and finally both return to the second optical coupler OC2 along the original optical path, as follows:

[0014] The reflected light of the semi-transparent half-mirror PM does not pass through the measured sample, and directly returns to the second optical coupler OC2 along the original optical path, and the total optical path is The total optical path (from the first optical coupler OC1→ fiber circulator CIR→ laser collimator FC→ half-transmission half-reflection mirror PM→ laser collimator FC→ fiber circulator CIR→ second optical coupler OC2) of the zero-point calibration light is used for calibrating the measurement zero point of the system and eliminating inherent errors of the optical path such as the fiber length difference, and is recorded as zero-point calibration light.

[0015] The transmitted light of the half-transmission half-reflection mirror PM irradiates the surface of the sample to be measured, is reflected by the front surface and the back surface of the sample to be measured, and then returns to the second optical coupler OC2 along the original optical path to form the front surface reflection light of the sample to be measured and the back surface reflection light of the sample to be measured. The total optical path (from the first optical coupler OC1→ fiber circulator CIR→ laser collimator FC→ half-transmission half-reflection mirror PM→ front surface of sample to be measured→ half-transmission half-reflection mirror PM→ laser collimator FC→ fiber circulator CIR→ second optical coupler OC2) of the front surface reflection light of the sample to be measured is The total optical path (from the first optical coupler OC1→ fiber circulator CIR→ laser collimator FC→ half-transmission half-reflection mirror PM→ front surface of sample to be measured→ back surface of sample to be measured→ front surface of sample to be measured→ half-transmission half-reflection mirror PM→ laser collimator FC→ fiber circulator CIR→ second optical coupler OC2) of the back surface reflection light of the sample to be measured is

[0016] 3. Interference integration stage

[0017] The reference light of the reference arm and the three beams of reflected light (zero-point calibration light, front surface reflection light of the sample to be measured, and back surface reflection light of the sample to be measured) returned by the sample arm complete optical field superposition at the second optical coupler OC2 to form three independent interference signals; these interference signals are then input into the balanced photodetector BPD, and the balanced photodetector BPD eliminates the inherent noise of the light source and enhances the signal-to-noise ratio of the signal through differential processing, and finally outputs a stable beat frequency electrical signal containing three beams of interference information.

[0018] 4. Signal analysis stage

[0019] 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 in the following manner:

[0020] Let the coherence length of the swept frequency laser source S-source be In order to ensure that the three beams of reflected light can form effective interference with the reference light of the reference arm, the following interference condition constraints need to be met:

[0021] The optical path difference between the zero-point calibration light and the reference light of the reference arm = - < ;

[0022] Optical path difference between the light reflected by the front surface of the sample under test and the reference light of the reference arm = - < ;

[0023] Optical path difference between the light reflected by the back surface of the sample under test and the reference light of the reference arm = - < ;

[0024] The above optical path differences all need to satisfy the laser interference phase matching condition, which is determined by the sweep range of the sweep laser source S-source.

[0025] Beat frequency electrical signal frequency and optical path difference: let the frequency variation 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 by the front surface of the sample under test, and the light reflected by the back surface of the sample under test are 、 、 respectively, then the beat frequency electrical signal frequency (i.e. the frequency in the time domain light intensity diagram of a series of beat frequency lights formed by the interference light) and the optical path difference within one sweep cycle satisfy the relationship: , where c is the speed of light in vacuum, may correspond to 、 、 , respectively. 、 、 When the interference light intensity reaches the maximum value, the corresponding optical path difference is exactly an integer multiple of the wavelength of a certain frequency light within the sweep range.

[0026] Film thickness calculation: the superimposed beat frequency electrical signal output by the balanced photodetector BPD is subjected to discrete Fourier transform, and its frequency domain characteristic peaks can be obtained, three frequency domain characteristic peaks respectively corresponding to three interference signals, so as to obtain the beat frequency electrical signal frequencies corresponding to the zero point calibration light, the light reflected by the front surface of the sample under test, and the light reflected by the back surface of the sample under test 、 、 respectively. The relationship formula between the beat frequency electrical signal frequency and the optical path difference is used to calculate the optical path differences ; and the net optical path difference of the front and back surfaces of the sample under test is obtained - The net optical path difference of the front and back surfaces of the sample to be measured is equal to the optical path of light in the sample to be measured n is the refractive index of the sample to be measured, is the actual film thickness of the sample to be measured, and finally the optical film thickness of the sample to be measured is obtained If the refractive index n of the sample to be measured is known, the actual film thickness of the sample to be measured can be obtained at the same time .

[0027] Interface positioning: according to the obtained optical path difference , , , the interface calibration formula is used to accurately measure the positions of the surfaces of the sample to be measured , wherein is or , is the refractive index of the propagation medium outside the optical fiber.

[0028] In the interface positioning and film thickness measurement device of the present application:

[0029] The type of the sweep laser source S-source is selected according to the application scenario, which can be any one of a semiconductor tunable laser, a sweep fiber laser, a tunable external cavity semiconductor laser, and a broadband light source spectrometer. In order to ensure continuous and stable linear sweep, the rate of change of instantaneous frequency over time is stable within the desired accuracy.

[0030] The first optical coupler OC1 is used to split the sweep laser output by the sweep laser source into two beams of sweep laser for the sample arm and the reference arm. The second optical coupler OC2 is used to integrate the sweep laser in the reference arm and the sweep laser reflected in the sample arm, and the two lasers interfere at this point. Further, the first optical coupler OC1 and the second optical coupler OC2 select the same mode single-mode fiber coupler to ensure the splitting / combining efficiency. The first optical coupler OC1 and the second optical coupler OC2 can also be replaced by a planar lightwave circuit (PLC) or other devices or optical paths that can force the light field into the same spatial mode.

[0031] The laser collimator FC is used to calibrate the angle of the outgoing light and convert the output divergent light into parallel light to ensure the consistency of the spatial mode of the outgoing beam. The laser collimator FC and the half-transmission half-reflection mirror PM in the sample arm can be integrated into the same unit to reduce the volume of the optical path. The laser collimator FC needs to match the numerical aperture and focal length of the optical fiber to ensure the quality of the parallel light. The splitting ratio of the half-transmission half-reflection mirror PM can be adjusted according to the reflectivity of the sample to be measured to avoid too weak or too saturated signals.

[0032] ​​The balanced photodetector BPD is used to eliminate light source noise and enhance the interference signal, so that the obtained beat frequency electric signal is more stable and clear, thereby improving the measurement accuracy of film thickness and positioning.

[0033] The interface positioning and film thickness measuring device based on a swept light source of the present application uses an all-fiber structure, realizes beam splitting and integration of the optical path through a first optical coupler OC1 and a second optical coupler OC2, realizes calibration of the measurement zero point in combination with a half-transmission half-reflection mirror PM, and optimizes the signal quality in combination with a balanced photodetector BPD. The device meets the needs of high-precision, non-contact position detection in related fields, can be used to solve the deficiencies existing in the application fields such as precision measurement, and has significant advantages in application fields such as industrial manufacturing and fiber communication. Specifically, the beneficial effects of the present application compared with the prior art are embodied in:

[0034] 1. The device of the present application outputs linearly swept frequency laser through a high swept frequency rate light source, and can realize micrometer-level distance resolution (the measurement error of a 10 μm standard optical film thickness is only 1.3‰ in actual measurement) in combination with the swept frequency interference principle and accurate optical path difference calculation, which has significant advantages in micro target detection, such as being able to clearly identify micrometer-level cell structures or nanometer-level biological film thickness in biological tissue detection scenarios, and provides more accurate data support for related fields.

[0035] 2. The balanced photodetector BPD is used to process the interference signal, which can effectively eliminate the inherent noise of the light source, improve the signal-to-noise ratio of the signal, and solve the problem that the non-contact detection based on the camera is easily disturbed by the ambient light. Even in strong light outdoors or complex electromagnetic environments in industry, the device can still stably output the measurement signal. At the same time, by performing discrete Fourier transform on the beat frequency electric signal, multiple frequency domain feature peaks can be extracted from the frequency domain graph, each peak value corresponds to a reflection interface, and the position information of each interface of the multi-layer film can be accurately distinguished in combination with the optical path difference and film thickness conversion formula, thereby solving the problems of "unable to distinguish multi-layer structure" of the traditional contact detection and "low resolution and difficult to identify thin layer" of the electronic radar.

[0036] 3. The device can meet the needs of precision and range in different application scenarios by adjusting the type and parameters of the swept laser source S-source, and has strong adaptability and flexibility. At the same time, the device has a simple structure, is easy to integrate and expand, reduces the system cost, and improves the overall performance. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed for the embodiment description will be briefly introduced as follows.

[0038] Figure 1 FIG. 1 is a structural schematic diagram of the interface positioning and film thickness measuring device based on a swept light source of the present application.

[0039] Figure 2 Frequency variation diagram of the light source of the interface positioning and film thickness measuring device based on the swept-frequency light source of the application.

[0040] Figure 3 Output time-domain diagram of the interface positioning and film thickness measuring device based on the swept-frequency light source of the application.

[0041] Figure 4 Frequency variation diagram obtained after Fourier transform of the interference light of the interface positioning and film thickness measuring device based on the swept-frequency light source of the application. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only some of the embodiments of the application, but not all the embodiments of the application.

[0043] The structure of the interface positioning and film thickness measuring device based on the swept-frequency light source of the application is shown in Figure 1 The measuring device includes a swept-frequency laser source S-source, a first optical coupler OC1, a fiber ring CIR, a laser collimator FC, a semi-transparent semi-reflective 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 measured. Specifically, the selection and connection mode of each component are as follows:

[0044] The swept-frequency laser source S-source is a semiconductor tunable laser with a center wavelength of 1310 nm (supporting linear sweep); the first optical coupler OC1 and the second optical coupler OC2 are both single-mode fiber 50:50 beam splitting couplers; the fiber ring CIR is a 3-port polarization maintaining ring, including a first port 1, a second port 2, and a third port 3; the laser collimator FC has a numerical aperture of 0.0005 and a focal length of 50 mm; the semi-transparent semi-reflective mirror PM has a splitting ratio of 50:50, a reflectivity of 50%, and a transmissivity 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. The components are connected by single-mode fiber fusion splicing, and the fiber length is adjusted according to the experimental scene to ensure that there is no obvious loss in the optical path. In this embodiment, the reference arm fiber length is 1 m, and the sample arm fiber length is 1 m.

[0045] To ensure that the device meets the high-precision measurement requirements, the swept-frequency laser source S-source, the interference light path, and the detection end need to be adjusted according to the following requirements:

[0046] 1. Swept-frequency laser source S-source parameter adjustment

[0047] The "continuous stable linear sweep" characteristic needs to be met, and the fluctuation range of the instantaneous frequency change rate in time is less than or equal to 100 MHz, which ensures the stability and linearity of the output light signal. In the embodiment, the parameters are specifically set as follows:

[0048] Sweep rate: set to 10 THz / s 10 5 THz / s, according to the theory that the higher the sweep rate, the higher the detection accuracy, combined with the actual performance of the light source, the balance of accuracy and signal stability, and the avoidance of high rate leading to frequency fluctuation;

[0049] Sweep range: set to 30 THz, with 1310 nm as the center wavelength (corresponding to a frequency of about 229 THz), covering the measurement requirements of the film thickness to be measured (such as 5-100 μm);

[0050] Sweep period: set to T = 300 μs, derived from the sweep range and the sweep rate (T = sweep range / sweep rate).

[0051] Because the sweep laser source S-source has inherent signal rising edge (about 300 μs) and falling edge (about 10 ns), considering the response time, the actual sweep range is less than the set input sweep range, and the actual sweep period is longer than the input sweep period, which needs to be monitored and recorded by an oscilloscope for subsequent calculation.

[0052] Coherence constraint: the instantaneous linewidth of the sweep laser source S-source is known to be 100 MHz, and the coherence length is = 0.95 m, which needs to ensure that when the sweep laser in the optical path interferes, the optical path difference between the reference arm of the reference light and the zero-point calibration light of the sample arm, the front surface reflection light of the sample to be measured, and the back surface reflection light of the sample to be measured , , , avoid interference signal attenuation, and the optical path difference is realized by adjusting the optical fiber length of the sample arm and the reference arm.

[0053] 2. Interference optical path parameter optimization

[0054] In order to ensure the normal work of the interference optical path, the position of the half-transmission half-reflection mirror PM is adjusted to avoid the strong interference signal when the optical path difference between the reference arm and the sample arm is zero, which covers the remaining interference signals, and ensures that the output is a stable interference beat signal.

[0055] ​​​Meanwhile, the laser collimator FC is installed and adjusted to ensure that the swept laser irradiating on the sample has enough collimation. The laser collimator FC in the sample arm affects the spatial mode and shape of the output light. The fiber collimator with appropriate numerical aperture and focal length should be selected to ensure the collimation of the output pulse. The divergence angle of the laser collimator FC with a numerical aperture of 0.0005 and a focal length of 50 mm is 2 mrad.

[0056] In an ideal case, when the reflected light of the half-mirror PM interferes with the multiple reflected lights of the sample surface, the obtained interference signal is the superposition of a series of beat signals, which is greatly affected by the optical path difference. Therefore, the position of the half-mirror PM can be adjusted according to the specific use of the system in different scenarios. In this embodiment, the half-mirror PM is adjusted to be at the optical path difference = 20 μm.

[0057] 3. Adjustment of detection end parameters

[0058] In order 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 in the detection end. The balanced photodetector BPD is used to process the reading of the interference signal in a low-power and high-noise environment, and the detected light intensity signal is differentially processed to significantly improve the signal-to-noise ratio. In the 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.

[0059] In addition to the above requirements, according to the specific application requirements, for example, in the interface positioning application, according to the length of the sensing fiber and the detection accuracy requirements, the output signal of the swept laser source S-source is optimized.

[0060] The performance test and verification method of the system is as follows:

[0061] The time domain analysis and spectrum analysis of the swept laser output by the swept laser source S-source are performed using an oscilloscope, a spectrum analyzer SA and other devices to verify the time domain characteristics and frequency diversity thereof.

[0062] According to the above requirements, the device as shown in Figure 1 is built, and the swept signal as shown in Figure 2 is input to the first optical coupler OC1.

[0063] Specifically, the swept laser source S-source performs linear sweeping with a sweeping rate of k = 10 5 THz / s and a center wavelength of 1310 nm in a frequency range of 30 THz. The optical film thickness of the sample film is 10 μm, and the refractive index is 1.57.

[0064] The output time-domain result obtained after processing the balanced photodetector BPD is shown in Figure 2. Figure 3 Theoretically, the single frequency-sweeping time of the frequency-sweeping laser source S-source should be a large period, and multiple beat signals correspond to multiple small periods, and the beat signals with periods are obtained after superposition. The frequency-domain diagram obtained after discrete Fourier transform of the interference light is shown in Figure 3. Figure 4 The beat signal frequency corresponding to the reflection light of each reflection surface and the reference light of the reference arm forms a peak value, and the beat signal frequencies corresponding to the reflection light of the front surface of the sample under test, the reflection light of the back surface of the sample under test and the zero-point calibration light from left to right are 6.66 KHz, 66.67 KHz and 73.33 KHz respectively. When the theoretical optical film thickness is 10 μm, the frequency difference between the two interference peaks is 6.66 KHz.

[0065] More specifically, according to the measured beat signal frequency, the optical film thickness of the sample under test is calculated according to = 19.996 μm, = 199.871 μm, = 219.846 μm.

[0066] According to the optical film thickness calculation formula the measured value of the optical film thickness of the sample under test is 9.987 μm, and the error with the actual optical film thickness 10 μm is only 1.3 ‰. At the same time, according to the interface calibration formula wherein the measurement is carried out in air, the distance between the front surface of the sample under test and the zero point is 89.952 μm, and the distance between the back surface of the sample under test and the zero point is 89.952 μm, and the distance between the back surface of the sample under test and the zero point is 89.952

[0067] The present application can be widely used in position detection engineering. Taking the film thickness measurement experiment as an example, first, an optical film is added to the sample arm, and the frequency-sweeping laser output by the frequency-sweeping laser source is injected into the optical film. By detecting and demodulating the returned interference beat signal, the film thickness is read. At the same time, according to the experimental test results, the parameters and structure of the frequency-sweeping laser source are optimized to improve its performance and reliability.

[0068] ​​​​The above-described specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present application, and it should be understood that the above-described specific embodiments are merely examples of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for interface positioning and film thickness measurement based on a swept source, characterized in that, The application discloses an interface positioning and film thickness measuring device based on a swept light source, which comprises a swept laser source S-source, a first optical coupler OC1, a fiber ring CIR, a laser collimator FC, a semi-transparent half mirror PM, a second optical coupler OC2, a balanced photodetector BPD and a spectrum analyzer SA, and each component is connected through an optical fiber; wherein the first optical coupler OC1, the fiber ring CIR, the laser collimator FC, the semi-transparent half mirror PM, the second optical coupler OC2 and the balanced photodetector BPD form an interference light path. The device is used to realize interface positioning and film thickness measurement through four stages of laser beam splitting, double-path laser transmission, interference integration and signal analysis based on the swept interference principle: after the swept laser is split, one path is reference light of a reference arm, and the other path is zero-point calibration light, front surface reflection light of a sample to be measured and back surface reflection light of the sample to be measured after sample arm processing; each light is superimposed and interfered with the reference light of the reference arm, and the film thickness and interface position information are calculated through signal processing and frequency domain feature extraction; the complete operation process is as follows: (1) Laser beam splitting stage The continuous linear swept laser output by the swept laser source S-source is first input into the first optical coupler OC1, and the first optical coupler OC1 splits the swept laser into two beams according to a power ratio of 50:50, one of which enters a sample arm to detect a sample to be measured, and the other of which enters a reference arm as an interference reference signal; (2) Double-path laser transmission stage Reference arm transmission: the swept laser of the reference arm is directly transmitted to the second optical coupler OC2, and the optical path is , as the reference light of subsequent interference; Sample arm transmission: the swept laser of the sample arm is first transmitted in a directional manner through the fiber ring CIR, then enters the laser collimator FC to perform direction calibration and convert divergent laser into parallel light; then the parallel light is incident on the semi-transparent half mirror PM and is split into reflected light and transmitted light again, and the optical paths of the two beams are as follows: The reflected light of the half-transmission half-reflection mirror PM directly returns to the second optical coupler OC2 along the original light path, and the total optical path is The light is used for calibrating the zero point of the system measurement, and is recorded as zero-point calibration light; The transmission light of the transflective mirror PM irradiates to the surface of the sample to be measured, and after being reflected by the front surface and the back surface of the sample to be measured, returns to the second optical coupler OC2 along the original light path to form the front surface reflection light of the sample to be measured and the back surface reflection light of the sample to be measured; the total optical path of the front surface reflection light of the sample to be measured is ; the total optical path of the back surface reflection light of the sample to be measured is ; both the two beams of sample to be measured information light carry the position and film thickness information of the sample to be measured; (3) Interference integration stage The reference light of the reference arm and the zero-point calibration light, the front surface reflection light of the sample to be measured and the back surface reflection light of the sample to be measured returned by the sample arm are respectively superimposed in a light field at the second optical coupler OC2, three independent interference signals are formed, and the three independent interference signals are input into the balanced photodetector BPD; the balanced photodetector BPD eliminates inherent noise of the light source and enhances signal signal-to-noise ratio through differential processing, and finally outputs a stable beat frequency electric signal containing three interference signals; (4) Signal analysis stage The spectrum analyzer SA receives the beat frequency electric signal output by the balanced photodetector BPD, extracts frequency domain features through discrete Fourier transform, and calculates film thickness information; Let the coherence length of the swept laser source S-source be In order to ensure that the zero-point calibration light returned by the sample arm, the light reflected by the front surface of the sample to be measured and the light reflected by the back surface of the sample to be measured can all form effective interference with the reference light of the reference arm, the following interference condition constraints need to be met: Zeroing the optical path difference of the reference light of the reference arm ​​​​ The optical path difference between the light reflected from the front surface of the sample under test and the reference light of the reference arm = - < ; The optical path difference between the light reflected from the back surface of the sample under test and the reference light of the reference arm .​​​​ 2. The method of claim 1, wherein: 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 front surface reflected light of the sample to be measured, and the back surface reflected light of the sample to be measured are respectively , , , the beat frequency electrical signal frequency and the optical path difference in a sweep cycle satisfy the relationship: , where c is the speed of light in vacuum, when corresponds to , , , corresponds to the respective optical path difference , , ; The method for calculating the film thickness information is: performing discrete Fourier transform on the superimposed beat frequency electrical signal output by the balanced photodetector BPD to obtain frequency domain characteristic peaks, and three frequency domain characteristic peaks correspond to three interference signals respectively, so as to obtain the beat frequency electrical signal frequencies corresponding to the zero point calibration light, the front surface reflected light of the sample to be measured and the back surface reflected light of the sample to be measured respectively 、 、 ; The relationship between the frequency of the beat electric signal and the optical path difference is used to calculate the optical path difference , 、 、 ; thus the net optical path difference of the front and back surfaces of the sample to be measured is obtained - , which is equal to the optical path of the light in the sample to be measured , n is the refractive index of the sample to be measured, is the actual film thickness of the sample to be measured, and the optical film thickness of the sample to be measured is obtained and the actual film thickness of the sample to be measured : ; .

3. The method of claim 2, wherein, The method for interface positioning is as follows: According to the obtained optical path difference , , , the positions of each surface of the sample to be measured are accurately measured using the interface calibration formula , wherein is or , is the refractive index of the propagation medium outside the optical fiber.

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