A high-precision reconstruction system of laser Brillouin scattering spectrum based on image

Abstract

A high-precision reconstruction system of laser Brillouin scattering spectrum based on image, comprising a back echo beam expansion and shaping unit, a frequency discrimination unit, an optical path switching device, an image sensor, an image acquisition and data processing unit. The back echo beam expansion and shaping unit shoots the scattered echo after beam expansion and collimation into the frequency discrimination unit; the frequency discrimination unit carries out multi-beam interference on the scattered echo after beam expansion and collimation, forms linear interference fringes and then emits; the optical path switching device reflects the linear interference fringes to the image sensor and photosensitizes on the image sensor; the image acquisition and data processing unit acquires the photosensitization result, processes by an interference spectrum extraction algorithm to obtain the device function and wedge angle corresponding to the interference fringes. The application adopts the image sensor to replace the multi-channel photoelectric detector to receive the calibration interference fringes, improves the spatial resolution of calibration reception and improves the calibration accuracy.

Description

Technical Field

[0001] This invention belongs to the field of laser hyperspectral detection and relates to a spectral reconstruction system for lasers, which provides calibration input for spectral identification systems and improves the measurement accuracy of underwater Brillouin scattering spectra. Background Technology

[0002] The blue-green wavelength range of 470–580 nm represents a transmission window with a relatively low attenuation coefficient for seawater. Blue-green lasers in this band can undergo various scattering processes with water. Among these, by receiving and analyzing the inelastic scattering echoes generated by the laser and seawater molecules—namely, Brillouin scattering and Raman scattering—it is possible to invert two marine environmental parameters: temperature and salinity. Furthermore, because Brillouin scattering has a narrower bandwidth, it is less susceptible to background signals and possesses higher detection potential. Various types of spectral identification systems for inverting seawater temperature and salinity using Brillouin scattering retrieve this information by acquiring two spectral parameters: the frequency shift of the Brillouin scattering relative to the emitted laser frequency, and the Brillouin scattering spectral lines.

[0003] The spectral discrimination system combining a Fiszo interferometer with a multi-channel photodetector is characterized by its high real-time performance and ability to acquire complete Brillouin spectral lines. However, in practical applications, the resolution of this system is limited by several factors. Specifically, if prior information about the Fiszo interferometer itself, such as the interferometer wedge angle and interferometer instrument functions, cannot be accurately determined, systematic errors will be introduced during the extraction of Brillouin scattering spectral line parameters.

[0004] In existing Brillouin spectral identification systems employing a Fezo interferometer, only multi-channel photodetectors used to detect echo signals can be used to perform system self-calibration. While multi-channel photodetectors can detect weak signals and have high temporal resolution, they have a small number of channels and low spatial resolution. Summary of the Invention

[0005] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide an interferometer parameter calibration system for a spectral identification system combining a Fiszo interferometer with a multi-channel photodetector. By adding an image sensor receiving channel to the calibration process of the spectral identification system, high spatial resolution image information of the interference fringes is acquired, thereby improving the wedge angle and device function calibration accuracy of the Fiszo interferometer and obtaining more reliable Brillouin divergence radio frequency shift and linewidth extraction accuracy.

[0006] The technical solution of this invention is: a high-precision reconstruction system for laser Brillouin scattering spectra based on images, comprising an echo beam shaping unit, a frequency discrimination unit, an optical path switching device, an image sensor, an image acquisition unit, and a data processing unit, wherein:

[0007] Echo beam spreading and shaping unit: collimates the scattered echo beam and then directs it into the frequency discriminator unit;

[0008] Frequency discrimination unit: Performs multi-beam interference on the beam-expanded and collimated scattered echo to form linear interference fringes before emission;

[0009] Optical path switching device: used to control the reflection of the linear interference fringes to the image sensor and the light-sensing on the image sensor;

[0010] Image acquisition and data processing unit: Acquires and stores the photosensitized results of the image sensor, and processes them through an interference spectrum extraction algorithm to obtain the device function and wedge angle corresponding to the interference fringes.

[0011] Furthermore, a beam shaping unit is also included between the frequency discrimination unit and the optical path switching device to expand or shrink the beam passing through the frequency discrimination unit to match the size of the image sensor.

[0012] Preferably, after the echo beam-expanding and shaping unit collimates the scattered echo beam, the divergence angle of the beam entering the frequency discriminator unit is on the order of milliradians.

[0013] Preferably, the frequency discrimination unit is a Fiszo interferometer.

[0014] Furthermore, the direction of the linear interference fringes is parallel to the column direction of the image sensor pixels.

[0015] Furthermore, the image acquisition and data processing unit obtains the device function and wedge angle corresponding to the interference fringes through an interference spectrum extraction algorithm, specifically:

[0016] (61) The brightness values ​​of each row of pixels in the image acquired by the image sensor are averaged to compress the two-dimensional interference fringes into a one-dimensional interference spectrum; the image acquired by the image sensor is obtained by irradiating a diffuse reflection target with a fixed frequency laser emitted by a laser, and the echo signal of the diffuse reflection target is obtained after passing through a frequency discrimination unit and an optical path switching device in sequence.

[0017] (62) Near the brightness peak of the one-dimensional interference spectrum, obtain the center position x0 of the interference fringes in pixels;

[0018] (63) Change the frequency of the laser emitted by the laser and repeat steps (61) to (62) to obtain the relationship data with the center position of the interference fringes as the independent variable and the frequency offset of the laser emitted by the laser as the dependent variable. Obtain the fitting slope value k by fitting a linear function.

[0019] (64) Calculate the wedge angle α of the frequency discriminator unit using the absolute value of the fitted slope value |k|.

[0020]

[0021] Where n is the refractive index of the medium between the two reflecting planes of the frequency discriminator, λ0 is the laser wavelength, and v FSR The free spectral range of the frequency discrimination unit;

[0022] (65) Normalize the one-dimensional interference spectrum, then adjust the abscissa of the one-dimensional interference spectrum to have relative frequency as the independent variable, and make the relative frequency corresponding to the fringe center position 0; perform function fitting on the one-dimensional interference spectrum to obtain the fitted spectral linewidth Γ. x Using the spectral linewidth Γ x The device function Γ of the frequency discriminator is obtained by calculating the absolute value of the fitted slope value |k|. v =Γ x *|k|.

[0023] Furthermore, obtaining the center position x0 of the interference fringes in pixels specifically involves: representing the signal intensity vector of each column within the selected region of the one-dimensional interference spectrum as... Record the pixel position of each column as Calculate the position of the centroid as

[0024]

[0025] Where the centroid x p The dimension is length, x0 = d0x p d0 is the pixel size of the image sensor.

[0026] Furthermore, obtaining the center position x0 of the interference fringes in pixels specifically involves: fitting the centroid x using the Lorentz function. p The Lorentz function is in the form of

[0027]

[0028] Where x p Γ represents the pixel row position corresponding to the peak intensity of the one-dimensional interference spectrum. x Let c0 be the full width at half maximum (FWHM) of the function fitted to the interference spectrum, and let I be the dark level and the amount of background signal boost. x Indicates the signal strength; x0 = d0x p d0 is the pixel size of the image sensor.

[0029] Preferably, the frequency of the laser emitted by the laser is changed in steps of 0.2 to 1.0 GHz.

[0030] Preferably, the image sensor is a CCD or CMOS device, and the size of the photosensitive surface is larger than the coverage area of ​​the linear interference fringes.

[0031] The advantages of this invention compared to the prior art are:

[0032] (1) This invention addresses the problem of low calibration accuracy of the wedge angle and device function information in traditional spectral discrimination systems combining a Fiszo interferometer with a multi-channel photodetector. It proposes using an image sensor instead of a multi-channel photodetector to receive the calibration interference fringes, thereby improving the spatial resolution of the calibration reception and enhancing calibration accuracy. Higher calibration information input reduces the systematic error in retrieving Brillouin scattering spectral line parameters when the Fiszo interferometer is combined with a multi-channel photodetector.

[0033] (2) The present invention introduces an optical path switching device, which enables the interferometer transmission signal to switch between the image sensor used in the present invention and the original multi-channel photodetector in the spectral identification system, and can be integrated into the existing mature spectral identification system without destroying its original detection function. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the system composition of the present invention;

[0035] Figure 2 This is a schematic diagram illustrating an embodiment of the spectral identification system of the present invention, which includes the system of the present invention.

[0036] Figure 3 This is a schematic diagram of two-dimensional interference fringes and one-dimensional interference spectrum in an embodiment of the present invention;

[0037] Figure 4 This is a schematic diagram of the photosensitive surface of the multi-channel image sensor in the embodiment;

[0038] Figure 5 This is a flowchart illustrating the generation and processing of simulated signals in the embodiment.

[0039] Figure 6 This is a comparison diagram of the fitting effect of the stripe center position in the embodiment;

[0040] Figure 7 This is a comparison chart of the stripe linewidth fitting effect in the embodiments;

[0041] Figure 8 This is a comparison chart of the fitting results for the frequency-space mapping slope |k| in the embodiments. Detailed Implementation

[0042] In the calibration process of a Fezo interferometer, the echo energy is significantly stronger than the scattered signal in the detection application, and there is no requirement for temporal resolution. Therefore, image sensors with high spatial resolution, lower temporal resolution, and poorer ability to detect weak signals are a more suitable option for signal acquisition and processing in the calibration process.

[0043] This invention enhances the self-calibration capability of the existing Brillouin scattering spectral identification system by adding an optical path switching device and an image monitoring channel for calibrating relevant parameters of the Fiszo interferometer, along with corresponding image processing algorithms. After calibration measurements, the optical path switching device is adjusted to switch the interference fringes back from the image monitoring channel to being received by the multi-channel photodetector, allowing the original measurement process to continue. Therefore, this invention's system can work collaboratively with existing Brillouin spectral identification systems that combine a Fiszo interferometer with a multi-channel detector.

[0044] Common image sensors have pixel sizes on the order of several micrometers. Compared to multi-channel photodetectors with pixel sizes on the order of millimeters, the spatial resolution is improved by two orders of magnitude, enabling the reception of finer interference fringe information and thus improving the calibration accuracy of interferometer parameters.

[0045] like Figure 1 As shown, the system of the present invention includes an echo beam shaping unit 1, a frequency discrimination unit 2, an optical path switching device 3, an image sensor 4, and an image acquisition and data processing unit 5. An optional beam shaping unit 6 is also available.

[0046] The echo beam expanding and shaping unit 1 expands and collimates the scattered echo used for Fiszo interferometer calibration, then directs it into the frequency discrimination unit 2 at a divergence angle on the order of milliradians. The echo enters the frequency discrimination unit 2, undergoes a multi-beam interference process, forming linear interference fringes. These fringes are then reflected by the optical path switching device 3 and introduced into the image sensor 4. The interference fringes are photosensitive on the image sensor 4, acquired and stored by the image acquisition and data processing unit 5, and processed by an interference spectrum extraction algorithm to obtain information such as the device function and wedge angle corresponding to the interference fringes. When the image sensor 4 and the input beam size are mismatched, i.e., the size of the formed interference fringes is significantly larger or smaller than the photosensitive surface size of the image sensor 4, an additional beam shaping unit 6 can be inserted between the frequency discrimination unit 2 and the optical path switching device 3 to expand or shrink the beam passing through the frequency discrimination unit 2 to match the size of the image sensor 4. If the beam size does not need adjustment, the beam shaping unit 6 is not required.

[0047] The echo beam expanding and shaping unit 1 can adopt different configurations depending on the input signal. When the echo beam expanding and shaping unit 1 receives an echo signal coupled from an optical fiber, this unit consists of an optical fiber flange, a collimating beam expanding optical lens, and an aperture. When the echo beam expanding and shaping unit 1 receives a free-space input, this unit consists of a collimating beam expanding optical lens and an aperture. After passing through the echo beam expanding and shaping unit 1, the laser spot exactly covers the aperture of the frequency discriminator unit 2, and the emission divergence angle should be as small as possible, reaching the milliradian level.

[0048] The reflective cavity of frequency discriminator 2 consists of two reflective planes with a tiny included angle, on the order of microradians. This structure is commonly referred to as a Fizeau interferometer. The reflective planes are coated with a partially reflective film composed of multiple layers of dielectric or metal, and the space between the two planes is formed by gas or solid optical glass. The cavity length and included angle of the reflective cavity can be designed as a non-adjustable structure or as a structure that can be adjusted by mechanical motion. When the scattered echo excited by the narrow-linewidth laser is shaped into a near-parallel beam and perpendicularly incident on the Fizeau interferometer, it is reflected multiple times within the reflective cavity and then emitted from the rear surface of the Fizeau interferometer, where they coherently superimpose to form periodically changing bright and dark linear interference fringes with the fringe direction perpendicular to the direction of the tiny included angle. The fringe-shaped laser spot generated by frequency discriminator 2 has the direction parallel to the fringe spot representing the energy distribution, and the direction perpendicular to the fringe spot corresponding to the spectral distribution of the incident light. By adjusting the angle between the front and rear reflection planes of the frequency discrimination unit 2, the direction of the linear stripe light spot is made parallel to the column direction of the pixels of the image sensor 4. At this time, the positions of different columns of the same row of pixels correspond to the spectrum of the echo signal.

[0049] The optical path switching device 3 consists of a reflector and a mechanical adjustment mechanism. Its function is to move the reflector into or out of the optical path so that the interference fringes formed after passing through the frequency discriminator 2 are respectively injected into different subsequent units.

[0050] Image sensor 4 uses a CCD or CMOS device. Its photosensitive surface size should be larger than the coverage area of ​​the interference fringes obtained by the system.

[0051] Image acquisition and data processing unit 5 calibrates and obtains the interferometer instrument function and wedge angle. The specific calibration process is as follows:

[0052] During the preparation phase, the diffuse reflection target needs to be placed at the center of the receiving system's field of view and illuminated by the beam emitted by the monostable laser. After the diffuse reflection signal is received, it will sequentially pass through the echo beam shaping unit 1, the frequency discrimination unit 2, the optical path switching device 3, and the image sensor 4. The image sensor 4 will then collect the linear interference fringes formed by the diffuse reflection echo. The angle between the front and rear reflection planes of the frequency discrimination unit 2's reflection cavity is adjusted so that the direction of the interference fringes is parallel to the column direction of the image sensor 4 and the long side direction of the multi-channel detector 7. Since the echo signal comes from the laser directly diffusely reflected by the hard target plate, its spectral distribution is consistent with the emitted laser. Data processing is then performed according to the following procedure.

[0053] S1. Average the brightness values ​​of each row of pixels in the image acquired by image sensor 4 to compress the two-dimensional interference fringes into a one-dimensional interference spectrum. Select a region near the brightness peak of the one-dimensional interference spectrum and obtain the center position x0 of the fringes in pixels by finding the centroid position or fitting the Lorentz function.

[0054] (a) The method for finding the centroid is as follows:

[0055] The signal intensity vector of each column within the selected region of the one-dimensional interferogram is represented as follows: Record the pixel position of each column as

[0056] The position of the centroid is represented as

[0057]

[0058] Where the centroid x p The dimension of is length, and a unit length of 1 corresponds to the pixel size of the image sensor, that is, the side length d0 of each pixel.

[0059] (b) The Lorentz function has the following form.

[0060]

[0061] Where x p Γ represents the pixel row position corresponding to the peak intensity of the one-dimensional interference spectrum. x Let c0 be the full width at half maximum (FWHM) of the function fitted to the interference spectrum, and let I be the dark level and the amount of background signal boost. x Indicates the strength of the signal.

[0062] Then, based on the pixel size d0 of the image sensor, the center position x of the stripes, in pixels, is determined. p Converting to length units, the stripe center position x0 is x0 = d0x p .

[0063] S2. Continue the calibration process by changing the frequency of the laser emission. Tune the emission frequency in increments of approximately 0.2–1.0 GHz. The center position of the fringes will shift accordingly. Repeat the acquisition of interference fringe images and execute step S1 to obtain the center position of the fringes. Finally, using the fringe center position (mm) as the independent variable and the laser output frequency shift (GHz) as the dependent variable, perform a linear function fitting to obtain the fitting slope value k. The absolute value of this fitting slope value |k| gives the correspondence between the relative shift v of the laser frequency and the spatial position x of the fringe:

[0064] v=x*|k|

[0065] Due to the periodicity of the interference fringes, when the frequency of the monochromatic light incident on the interferometer is changed, the maxima of the linear interference fringes will periodically repeat. The difference in optical frequency between two adjacent fringe maxima is the free spectral range (FSR) of the interferometer, denoted as v. FSRIts size can be calculated by measuring the distance between the two reflective surfaces of the frequency discrimination unit 2.

[0066] Given the free spectral range (FSR) of the interferometer, the relationship between the magnitude of the fitting slope |k| and the interferometer wedge angle α can be given by the following relationship:

[0067]

[0068] Where n is the refractive index of the medium between the reflective planes of frequency discrimination unit 2, and λ0 is the laser wavelength.

[0069] S3. Acquire a two-dimensional interference image in the manner described in step S1, average each row to obtain a one-dimensional interference spectrum, and normalize and adjust its maximum value to 1.

[0070] Substituting the pixel size d0 of the image sensor, and then the slope value obtained from fitting in step S2, the abscissa of the one-dimensional interference spectrum is adjusted to have relative frequency as the independent variable. The abscissa is then shifted so that the relative frequency corresponding to the fringe center position is 0.

[0071] The one-dimensional interferometric spectrum was then fitted with a function, using the Lorentz line type, to obtain the linewidth Γ of the fitted spectral line. x

[0072]

[0073] Since the spectral linewidth of the laser itself is much smaller than the transmission linewidth of the interferometer instrument function, the fitted linewidth can be approximated as the linewidth value of the Fiszo interferometer instrument function.

[0074] The linewidth Γ obtained by fitting the one-dimensional interferometric spectrum x With a unit of length, the slope |k| obtained in step S2 can be used to determine the interference spectral linewidth Γ. x Mapped to device function Γ in units of frequency. v .

[0075] Γ v =Γ x *|k|

[0076] like Figure 2As shown, the link consisting of frequency discrimination unit 2, beam shaping unit 6 (optional), optical path switching device 3, image sensor 4, image acquisition, and data processing unit 5 is the calibration link of the spectral identification system designed in this invention. The link consisting of frequency discrimination unit 2, beam shaping unit 6 (optional), multi-channel detector 7, and acquisition and processing unit 8 is an existing Brillouin spectral identification system. When both work together, the optical path switching device 3 is first adjusted so that the echo enters the image sensor 4. The wedge angle and device function calibration results of the frequency discrimination unit 2 are obtained according to the aforementioned condition preparation method and data processing flow S1-S3. Then, the optical path switching device 3 is adjusted so that the echo enters the multi-channel detector 7. At this time, the Brillouin scattering signal of the water body can be collected on the photosensitive surface of the photodetector 7. The image sensor 4 introduced in this invention does not work when the system performs the scattering signal acquisition function.

[0077] Example

[0078] To explain why this invention, compared to the existing spectral identification system, can provide more accurate calibration information for the Fiszo interferometer wedge angle and device functions even when the received signal is subject to noise interference, a simulation experiment is designed below to compare the calibration results of this invention and the existing spectral identification system.

[0079] The simulation conditions, simulation process, and simulation results are described below.

[0080] The echo signal 1 incident on the calibration system is approximated as monochromatic light with a wavelength λ0 of 532 nm. The frequency discrimination unit 2 uses air as the filling medium between the reflecting mirrors, with a refractive index of 1.0. The free spectral range of the Fezo interferometer is known to be 25 GHz.

[0081] After monochromatic light passes through frequency discriminator 2, it forms linear interference fringes, and its illumination area can completely fill the spectrum. Figure 2 The image sensor 4 and the multi-channel photodetector 7 are shown. The resulting two-dimensional interference fringes on the photosensitive surfaces of the image sensor 4 and the multi-channel photodetector 7 are shaped as follows: Figure 3 As shown on the left. The peripheral dimensions of the photosensitive surfaces of both the image sensor 4 and the multi-channel photodetector 7 are set to 15.8 mm wide and 16 mm long. Within this area, it is assumed that the illumination intensity incident on the frequency discrimination unit 2 is uniform. Averaging the two-dimensional interference fringes along the column direction yields the one-dimensional interference spectrum as shown below. Figure 3 As shown on the right.

[0082] Under noise-free conditions, the line shape of the one-dimensional interference spectrum is set to Lorentz line shape. Since the simulation approximates the calibration echo as reflected light with negligible linewidth, its linewidth in the frequency domain is equivalent to the true linewidth Γ0 = 1.0 GHz of the device function of the interferometer, and the maximum amplitude of the interference spectrum is set to 1.

[0083]

[0084] The multi-channel photodetector structure in the simulation settings is as follows: Figure 4 As shown, the unit is mm. The photosensitive surface specifications used in the simulation are based on the relevant structures of common multi-anode photomultiplier tubes on the market. This multi-channel photodetector 7 has 16 effective photosensitive surfaces. Each photosensitive area is 0.8 mm wide and 16 mm long. There is a dead zone of 0.2 mm wide between adjacent photosensitive areas, which is considered to have no response to incident light.

[0085] The image sensor in the simulation setup has a 4-pixel size of 3.45µm. A coverage width of 15.8mm requires 4580 columns of pixels, assuming that all pixels have the same response characteristics to the incident signal.

[0086] The process of generating simulation signals is as follows: Figure 5 As shown.

[0087] ① Generation of the one-dimensional interferometric spectrum: First, a Lorentz line shape with a true linewidth of 1.0 GHz is generated as the input for the one-dimensional interferometric spectrum in the simulation. The center position of the spectral lines is set to a specific location. Since the relative frequency coordinates of the interferometric spectrum have a linear relationship with the spatial position coordinates of the interferogram, this one-dimensional interferometric spectrum can simultaneously have two sets of abscissas. The range of the relative frequency coordinates is set to -12.5 GHz to 12.5 GHz, corresponding to the complete free spectrum range of the frequency discriminator 2. The range of the spatial coordinates is set to 0 to 15.8 mm, corresponding to the photosensitive surface width of the image sensor 4 and the multi-channel photodetector 7. The frequency-space mapping slope determined by the wedge angle of the interferometer is therefore set to |k| = 25 GHz / 15.8 mm.

[0088] The simulation aims to recover the following three calibration parameters from a noisy input signal by processing the simulated signal, and to evaluate the statistical difference between the recovered results and the true values:

[0089] The true linewidth of the interferometer's instrument function is Γ0 = 1.0 GHz, and the center position of the interference fringes is x0 = 7.9 mm. The frequency-space mapping slope is |k| = 25 GHz / 15.8 mm.

[0090] ② Calculation of theoretical transmittance

[0091] The theoretical transmittance of each channel of the multi-channel photodetector 7 and the image sensor 4 can be modeled as a piecewise integral over a specific interval of the one-dimensional interference spectrum.

[0092] The multi-channel photodetector 7 divides the horizontal axis of the one-dimensional interference spectrum from 0 to 15.8 mm into 16 discrete segmented integrals, while the image sensor 4 divides the horizontal axis of the one-dimensional interference spectrum from 0 to 15.8 mm into 4580 closely adjacent segmented integrals. The length of the integration interval of each pixel is 3.45 μm, which corresponds to the pixel width.

[0093] ③ Generate noisy detector signals

[0094] Then, a multi-channel incident signal of the same length as the 7 channels of the multi-channel photodetector is generated. The signal consists of two parts added together. One part is a DC signal with a constant amplitude of 1. The other part is a random number following a Gaussian distribution, with a signal-to-noise ratio (SNR) set to 20. The variance of the random noise is 1 / SNR, and the mean and variance are the same.

[0095] ④ Inversion of interferometric spectrum parameters - multi-channel detector signal

[0096] Next, the center position and linewidth of the original one-dimensional interferometric spectrum are estimated using the generated noisy detector signal, and the spectral parameters are inverted using a nonlinear least squares problem. The principle is to set the linewidth and center position of the interferometric spectrum as the quantities to be fitted, and calculate the 16-channel detector signal vector under noise-free conditions. The calculation result is compared with the input of the noisy signal. Subtraction, using nonlinear optimization methods to search for... The minimum center position and linewidth of the interference spectrum are used as estimates of the true spectral line parameters of the one-dimensional interference spectrum.

[0097] ⑤ Inversion of interferometric spectral parameters - image sensor signal

[0098] When processing the signal from image sensor 4, since there are enough pixels, a non-linear curve fitting method is used to fit the input signal.

[0099] The fitted curve is set to a Lorentz line type with an added bias:

[0100]

[0101] In this model, p1 fits the amplitude of the spectral line, p2 fits the center position of the spectral line, p3 fits the full width at half maximum (FWHM) of the spectral line, and p4 fits the bias introduced by noise. p2 and p3 are the parameter values ​​to be inverted, with length units (mm), representing the spatial characteristics of the one-dimensional interference spectrum. Assuming the frequency-space mapping slope |k| is known, the center position and FWHM of the spectral line can be mapped to the frequency domain.

[0102] ⑥ Results Analysis

[0103] With the signal-to-noise ratio set to 20, the simulation described in steps ③-⑤ was repeated 100 times to generate 100 sets of interference spectral parameters obtained by inverting signals from multi-channel detectors and image sensors.

[0104] The true value of the frequency-space mapping slope |k| = 25GHz / 15.8mm is taken as known information, and the true linewidth of the device function Γ0 = 1.0GHz is taken as unknown information for analysis.

[0105] The inversion results for the center position and linewidth of the interference fringes are as follows: Figure 6 and Figure 7 As shown.

[0106] The mean deviation of the center position retrieved using the multi-channel detector signal is -6.2937MHz, and the standard deviation is 99.9709MHz.

[0107] The mean deviation of the center position retrieved using the image sensor signal is -0.4769MHz, and the standard deviation is 9.8423MHz.

[0108] Using standard deviation as an evaluation index for the stability of device function linewidth calibration, under a signal-to-noise ratio of 20, the image sensor introduced in this invention reduces the error by an order of magnitude compared to calibration using a multi-channel detector.

[0109] When calibrating the frequency-space mapping slope |k|, it is necessary to change the emission frequency of the calibration monochromatic light, thereby changing the position of the center of the bright stripe on the photosensitive surface accordingly.

[0110] Under the condition that the frequency change of the calibrated monochromatic light is known, the center position of the interference fringe is inverted according to the method described in steps ④-⑤ above. The frequency change of the monochromatic light is used as the vertical axis and the inversion result of the fringe center position is used as the horizontal axis for linear least squares fitting. The slope value obtained by fitting is the estimate of the frequency-space mapping slope |k|.

[0111] To calibrate the frequency-space mapping slope |k|, simulations were performed as follows. The signal-to-noise ratio was kept at 20, and the true linewidth remained Γ0 = 1.0 GHz. The relative center frequency position of the one-dimensional interferometric spectrum was changed, and simulations were conducted under five different conditions: 0, 0.5 GHz, 1 GHz, 1.5 GHz, and 2 GHz. Under each condition, steps ①-⑤ were repeated 100 times, resulting in a total of 500 data sets.

[0112] At five different center frequencies, experimental data were collected once each. The estimated value of the frequency-space mapping slope |k| was calculated using the fitting results (in mm) of the fringe center spatial positions. This constituted 100 independent estimates of the slope |k|, and the results are shown below. Figure 8 As shown.

[0113] The mean deviation of the slope from the true value obtained by using the multi-channel detector signal inversion is -0.02476 (GHz / mm), and the standard deviation is 0.1016 (GHz / mm).

[0114] The mean deviation of the center position retrieved using the image sensor signal is -0.001300 (GHz / mm), and the standard deviation is 0.009512 (GHz / mm).

[0115] Using standard deviation as an evaluation index for the stability of device function linewidth calibration, under a signal-to-noise ratio of 20, the calibration error of the frequency-space mapping slope |k| is reduced by an order of magnitude compared to calibration using a multi-channel detector when using the image sensor introduced in this invention.

[0116] After obtaining the frequency-space mapping slope |k|, the current wedge angle of the interferometer's reflecting plane can be calculated according to the following relationship.

[0117]

[0118] ⑦ Simulation Summary

[0119] The above simulations demonstrate that, under the given conditions, the method of calibrating the parameters of the Fezo interferometer using an image sensor, as introduced in this invention, reduces the calibration error of the interferometer's function linewidth, interference fringe center position, and frequency-space mapping slope |k| by approximately one order of magnitude compared to using the original multi-channel detector as the calibration data source. This proves the effectiveness of the invention.

[0120] The contents not described in detail in this specification are common knowledge to those skilled in the art.

Claims

1. A high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image, characterized in that: It comprises echo beam expanding and shaping unit (1), frequency discrimination unit (2), optical path switching device (3), image sensor (4), image acquisition and data processing unit (5), wherein: The echo beam expanding and shaping unit (1) collimates the scattered echo after beam expanding and then injects it into the frequency discrimination unit (2); The frequency discrimination unit (2) carries out multi-beam interference on the scattered echo after beam expanding and collimation, forms linear interference fringes and then emits them; The optical path switching device (3) is used for controlling the reflection of the linear interference fringes to the image sensor (4) and the photosensing on the image sensor (4); The image acquisition and data processing unit (5) acquires and stores the photosensing results of the image sensor (4) and obtains the device function and wedge angle corresponding to the interference fringes through interference spectrum extraction algorithm; The direction of the linear interference fringes is parallel to the column direction of the image sensor (4) pixels; The image acquisition and data processing unit (5) obtains the device function and wedge angle corresponding to the interference fringes through interference spectrum extraction algorithm, specifically: (61) The brightness values of each row of pixels of the image acquired by the image sensor (4) are averaged, and the two-dimensional interference fringes are compressed into one-dimensional interference spectrum; the image acquired by the image sensor (4) is obtained by using a laser to emit fixed frequency laser to irradiate a diffuse reflection target, and the echo signal of the diffuse reflection target is obtained in turn after the frequency discrimination unit (2) and the optical path switching device (3); (62) In the vicinity of the brightness peak value of the one-dimensional interference spectrum, the center position x0 of the interference fringes is obtained in pixel units; (63) The frequency of the laser emitted by the laser is changed, and steps (61) to (62) are repeated to obtain the relationship data of the center position of the interference fringes as the independent variable and the frequency offset of the laser emitted by the laser as the dependent variable, and the fitting slope value k is obtained by linear fitting; (64) The absolute value |k| of the fitting slope value is used to calculate the wedge angle α of the frequency discrimination unit (2), where n is the refractive index of the medium between the two reflecting planes of the frequency discriminator (2), λ0is the laser wavelength, v FSR is the free spectral range of the frequency discriminator (2); (65) The one-dimensional interference spectrum is normalized, and then the abscissa of the one-dimensional interference spectrum is adjusted to be the relative frequency as the independent variable, and the relative frequency corresponding to the center position of the stripe is 0; the one-dimensional interference spectrum is functionally fitted to obtain the spectral line width Γ x , the device function Γ x of the frequency discrimination unit (2) is calculated using the spectral line width Γ v and the absolute value |k| of the fitted slope value x = Γ x *|k|.

2. The high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image according to claim 1, characterized in that: The beam shaping unit (6) is further included between the frequency discrimination unit (2) and the optical path switching device (3), which expands or shrinks the light beam passing through the frequency discrimination unit (2) to match the size of the image sensor (4).

3. The high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image according to claim 1 or 2, characterized in that: The divergence angle of the scattered echo injected into the frequency discrimination unit (2) after beam expanding and collimation by the echo beam expanding and shaping unit (1) is in the order of milliradians.

4. The high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image according to claim 1 or 2, characterized in that: The frequency discrimination unit (2) is a Fizeau interferometer.

5. The high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image according to claim 1, characterized in that: The center position x0 of the interference fringe in pixels is obtained, specifically, a signal intensity vector of each column in a selected region of the one-dimensional interference spectrum is represented as The pixel position of each column is recorded as The centroid position is calculated as where the centroid x p has the dimension of length, x0= d0x p , d0being the pixel size of the image sensor (4).

6. The high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image according to claim 5, characterized in that: The center position x0 of the interference fringes in pixels is obtained, specifically: adopting a Lorentz function to fit the centroid x p The form of the Lorentz function is where x p represents the pixel row position corresponding to the peak intensity of the one-dimensional interference spectrum, Γ x is the full width at half maximum occupied by the function fit on the interference spectrum, c0represents the dark level, the amount of uplift of the background signal, I x represents the intensity of the signal; x0=d0x p , d0is the pixel size of the image sensor (4).

7. The high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image according to claim 5, characterized in that: The frequency of the laser emitted by the laser is changed by 0.2-1.0 GHz.

8. The high-precision reconstruction system of a laser Brillouin scattering spectrum based on an image according to claim 1 or 2, characterized in that: The image sensor (4) uses CCD or CMOS devices, and the photosensitive surface size is larger than the coverage range of the linear interference fringes.

Images