A wavelength calibration method for a spectrometer based on grating dispersion and spatial filtering

Abstract

This invention relates to the field of spectroscopic instrument calibration technology, and discloses a spectral wavelength calibration method based on grating dispersion and spatial filtering, comprising: 1. incident collimated light from a broadband light source onto a grating; 2. setting a spatial filter that can move along the dispersion direction near the image plane of the dispersive optical path, and selecting narrowband light with different center wavelengths from the continuous spectrum by moving the filter; 3. for each filter position, measuring the center wavelength of the selected narrowband light using a high-precision spectrometer, and capturing a spot image using a detector array to be calibrated; 4. extracting the centroid pixel coordinates of the spot from the image; 5. fitting the wavelength and pixel coordinates to establish a calibration equation between the pixel coordinates and the wavelength. This invention does not overly rely on discrete standard light sources, can actively and continuously acquire high-density calibration points, the process is easy to automate, and the calibration accuracy is high, making it suitable for online calibration of miniaturized and integrated spectrometers.

Description

Technical Field

[0001] This invention relates to the field of precision measurement and spectroscopic instrument technology, specifically to a wavelength calibration method for grating dispersive spectrometers. Background Technology

[0002] Wavelength calibration is fundamental for quantitative and qualitative analysis in spectroscopic instruments, and its accuracy directly determines the authenticity and reliability of spectral data reconstruction. In grating dispersive spectrometers, the core physical process involves using a grating to spatially expand the composite light onto the detector image plane according to wavelength. Therefore, the essence of calibration is to establish a precise mapping relationship between the position of each pixel on the detector image plane and the wavelength of the incident light.

[0003] Currently, calibration methods widely used in industry and laboratories mainly rely on standard discharge light sources such as mercury lamps, neon lamps, and argon lamps. These light sources can emit several known and stable characteristic spectral lines of atoms or molecules. During calibration, the spectrometer to be calibrated is illuminated with a standard lamp, and the pixel coordinates corresponding to these known wavelength spectral lines on the area array detector are recorded, thereby obtaining several discrete wavelength and pixel position data points. Then, a calibration curve for the entire wavelength band is established using methods such as polynomial fitting.

[0004] However, the aforementioned traditional calibration methods suffer from drawbacks such as sparse and unevenly distributed calibration points and reliance on external standard components. With the development of spectral sensing technology towards rapid on-site detection, embedded systems, and the Internet of Things (IoT), there is an urgent need for automated, online, and high-precision spectrometer calibration methods. Therefore, a novel wavelength calibration method is required that does not excessively rely on external discrete standard light sources and can actively and continuously generate dense calibration points. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the defects of the existing wavelength calibration methods mentioned above, and to provide a wavelength calibration method for spectrometers based on grating dispersion and spatial filtering, so as to effectively reduce the dependence on professional standard light sources and meet the strict requirements of modern spectrometers in large-scale production and online monitoring. This is of great significance to promoting the development of spectroscopic analysis technology towards intelligence and embeddedness.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The wavelength calibration method for a spectrometer based on grating dispersion and spatial filtering, as described in this invention, is applied to an optical system. This optical system includes: a broadband light source, a collimating optical path, a blazed grating, a spatial filter, a mirror, a convex lens, a transmission grating, a high-precision spectrometer, and a CMOS camera. The wavelength calibration method for the spectrometer includes the following steps: Step 1: After the output light of the broadband light source is collimated by the collimating optical path, it is incident into the blazed grating at the incident angle to form a dispersed light that is continuously spread in space according to wavelength. A convex lens is placed near the image plane of the dispersed light to collimate the dispersed light and form collimated dispersed light. Step 2: Set a spatial filter that can move along the dispersion direction in the propagation direction of the collimated dispersed light, and control the movement of the spatial filter so that the collimated dispersed light passes through the light transmission window of the spatial filter in sequence, and filters out narrow band light with continuously changing center wavelength in the light transmission window in sequence. After being reflected by the mirror, the narrow band light passes through the light transmission window again and enters the blazed grating to form inverse diffraction light. Step 3: Guide the inverse diffracted light into the transmission grating, and spatially split the inverse diffracted light to form zero-order diffracted light. and first-order diffraction light ; Step 4: Simultaneously acquire zero-order diffraction light using a high-precision spectrometer. The center wavelength at each sampling point location of the spatial filter And obtain first-order diffraction light The light spot image group formed on the CMOS camera ,in, This represents the center wavelength at the i-th sampling position. Let represent the i-th light spot image; N represents the total number of light spot images; Step 5, Extraction Pixel coordinates of the center of the spot in the dispersion direction To constitute the first Group calibration data pairs ; Step 6: Use the least squares method to... Curve fitting is performed on the calibration data pairs to establish pixel coordinates. With center wavelength The mapping equation between them is used to complete the wavelength calibration.

[0007] The wavelength calibration method for a spectrometer based on grating dispersion and spatial filtering described in this invention is characterized in that, in step 2, the spatial filter includes a light-passing window whose width determines the bandwidth of the narrowband light selected, and the moving step size of the spatial filter is controlled by a precision displacement stage, which determines the density of sampling points.

[0008] Furthermore, in step 2, by finely adjusting the angle of the reflector, the light beam reflected by it and incident again on the blazed grating is spatially offset from the light beam incident on the blazed grating in step 1, so as to achieve optical path separation.

[0009] Furthermore, step 3 also includes: setting a lens group in the optical path of the reverse diffracted light, wherein the lens group performs beam collimation on the diverging light in the reverse diffracted light.

[0010] Furthermore, the pixel coordinates are extracted. The process is as follows: First, locate the linear region of the spectral lines in the spot image, and then analyze the i-th spot image within this region. The grayscale matrix is ​​projected and summed along the vertical direction perpendicular to the dispersion direction to obtain a one-dimensional intensity distribution curve; Then, the one-dimensional centroid algorithm is applied to calculate the one-dimensional intensity distribution curve to obtain... Pixel coordinates of the center of the spot .

[0011] The present invention provides an electronic device, including a memory and a processor, characterized in that the memory is used to store a program supporting the processor in performing the method described therein, and the processor is configured to execute the program stored in the memory.

[0012] The present invention discloses a computer-readable storage medium storing a computer program, characterized in that the computer program is executed by a processor to perform the steps of the method described thereon.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. To address the problem that wavelength calibration of spectrometers relies on external discrete light sources and has sparse calibration points, this invention adopts an active continuous sampling method based on grating dispersion and spatial filtering. By moving the filter, narrowband light is selected sequentially from the continuous spectrum. This method can obtain high-density calibration points while ensuring the accuracy of the calibration principle, thereby significantly improving the overall accuracy and reliability of the calibration curve.

[0014] 2. For the precise mapping of calibration point wavelength and image plane position, this invention constructs a dual-channel data acquisition system by synchronously triggering a high-precision spectrometer and an area array detector. Then, the light spot coordinates are extracted based on a one-dimensional grayscale projection and centroid positioning algorithm. This mapping method can effectively obtain the correspondence between wavelength and position.

[0015] 3. Regarding the establishment and practicality of the final calibration equation, this invention uses the least squares method to perform nonlinear polynomial fitting on the high-density "wavelength-position" data pairs obtained. The resulting calibration equation has high accuracy and good reproducibility, and the entire process is easy to automate, which meets the requirements of modern industry for intelligent online detection. Attached Figure Description

[0016] Figure 1 This is a simulation diagram of the Zemax optical system of this invention; Figure 2 This is a schematic diagram of the overall optical system of the present invention; Figure 3 This is a schematic diagram of the aperture stop filter of the present invention; Figure 4 This is a beam splitting and acquisition diagram of the transmission grating of the present invention; Figure 5 The images shown are the light spot detection images and corresponding spectral images of this invention. Figure 6 This is a scatter plot of the spectral line bending of the present invention; Figure 7 This is a scatter plot of the linear region of the spectral lines in this invention; Figure 8 This is a scatter plot showing the location of the light spot and the corresponding wavelength of the present invention. Detailed Implementation

[0017] In this embodiment, a wavelength calibration method for a spectrometer based on grating dispersion and spatial filtering is a wavelength calibration method capable of actively, continuously, and with high density acquiring calibration points. Its complete optical system is simulated using Zemax as follows: Figure 1 As shown. This optical system mainly includes: a broadband light source, a collimator, a blazed grating, a spatial filter, a plane mirror, a lens group, a transmission grating, a high-precision spectrometer, and a CMOS camera. The overall process of the calibration method is as follows: Figure 2 As shown, the specific steps are as follows.

[0018] Step 1: Broadband optical collimation and grating dispersion: A broadband near-infrared light source with a center wavelength of 850 nm is selected. Its output light is coupled via optical fiber to a fixed aspherical collimator, producing collimated parallel light with a spot diameter of approximately 2.4 mm. This collimated light illuminates the surface of a blazed grating with a grating density of 1200 lines / mm at an incident angle of approximately 30 degrees. Because the incident angle is close to the Littrow angle of the grating, the diffraction efficiency can reach 80%, improving the system's luminous flux and signal-to-noise ratio. The choice of the incident angle must satisfy the grating equation. , where d is the grating constant, α is the incident angle, β is the diffraction angle, and m is the diffraction order. In this example, m=1 and the center wavelength is 850nm.

[0019] Under the diffraction of the grating, the incident broadband light is dispersed and spread out in the horizontal plane. Different wavelength components exit at different angles, forming a dispersive light path that is spatially distributed according to wavelength. A convex lens with a focal length of 200mm is placed in the direction of propagation of the dispersive light path to collimate the light path, forming collimated dispersive light. The function of the convex lens is to re-collimate the divergent dispersive light. The choice of its focal length f determines the spatial length of the dispersive light rays. The longer the focal length, the smaller the range of the same wavelength range spread on the image plane, i.e., the smaller the dispersion rate, and the higher the spatial resolution, but the larger the system size requirement. In this example, a focal length of 200mm is a balance between size and performance.

[0020] Step 2, Spatial Filtering and Ray Beam Combining: A spatial filter, movable along the dispersion direction, is positioned 50 mm away from the convex lens along the propagation direction of the collimated dispersed light. The spatial filter is a variable aperture stop, but can be replaced with a fixed-width vertical slit or light-blocking plate as needed. In this example, a variable aperture stop is used. The stop is mounted on a one-dimensional precision displacement stage, and its movement is controlled by a computer.

[0021] During calibration, the control stage is gradually moved to move the aperture, so that the collimated dispersed light passes sequentially through the light transmission window of the spatial filter. For example... Figure 3 As shown, since the aperture of the aperture only allows a small segment of the spectrum near its opening to pass through, a narrowband beam corresponding to the center wavelength is extracted from the continuous spectrum each time it moves to a new position. The step size of the displacement stage determines the wavelength sampling interval; to obtain high-density calibration points, the step size only needs to be reduced. The aperture size of the aperture determines the spectral bandwidth of the narrowband light; the smaller the aperture, the narrower the bandwidth, the better the selected monochromaticity, but the lower the transmitted light power. In actual operation, the aperture should be minimized as much as possible to obtain higher wavelength resolution while ensuring that the back-end detector can obtain a sufficient signal-to-noise ratio. The narrowband light continues to propagate forward and illuminates a plane mirror. In order to spatially separate the returning beam from the initial incident beam, the pitch or yaw angle of the mirror needs to be finely adjusted. By precisely controlling the deflection of the mirror, the reflected beam passes through the aperture again and returns along a path slightly deviating from the original optical path to illuminate the surface of the blazed grating, with its incident point 10 mm horizontally away from the first incident point on the grating surface. According to the principle of optical reversibility, this narrow-band light undergoes inverse diffraction on the blazed grating, and the diffracted light of different wavelengths merges back into a beam with high collimation. However, due to the influence of the mirror angle offset, the beam diverges to a certain extent.

[0022] Step 3: Beam collimation and beam splitting: The inverse diffracted light is guided into the transmission grating, and the inverse diffracted light is spatially split to form zero-order diffracted light. and first-order diffraction light To address the beam divergence problem and improve subsequent reception efficiency, a beam-constricting collimating lens group consisting of two biconvex lenses, each with a focal length of 200mm, is placed in the optical path of the back-diffracted light. The first lens focuses the diverging back-diffracted light, and the second lens, positioned appropriately behind the focal point, recollides the focused, diverging beam into a parallel beam with a diameter of approximately 2.4mm, matching its reception parameters with those of the downstream collimator. Figure 4 As shown, the beam, after being collimated and focused, is incident on a transmissive planar grating (with a line density of 1200 lines / mm). The incident angle of the grating is adjusted so that 90% of the light rays diffract at the zeroth order. The light is directly transmitted and enters the collimator into a high-precision spectrometer, with 10% of the light diffracting as first-order diffraction. The form of dispersion is deposited into the image sensor of the CMOS camera due to zero-order diffraction. and first-order diffraction Since the two rays originate from the same source, they have the same spectrum. It can be used as information about the location of the light spot, and Can be used as a counterpart Wavelength information.

[0023] Step 4: Synchronize data acquisition: High-precision spectrometer synchronously acquires zero-order diffraction light The center wavelength at each sampling point location of the spatial filter And obtain first-order diffraction light The light spot image group formed on the CMOS camera ,in, This represents the center wavelength at the i-th sampling position. Let represent the i-th light spot image; N represents the total number of light spot images; For each sampling position of the aperture, the system simultaneously performs dual-channel data acquisition.

[0024] First, wavelength reference acquisition, using the zero-order diffraction light from the transmission grating. The signal is received by a collimator and transmitted via optical fiber to a traceable, high-precision commercial spectrometer. This spectrometer accurately measures the center wavelength of the current narrowband light. , as the wavelength reference for calibration.

[0025] Second, image information acquisition, the first-order diffraction light of the transmission grating. Illumination is projected onto the photosensitive surface of the CMOS camera to be calibrated, forming a light spot image. And it is saved by the computer.

[0026] The optical aperture is driven by a computer-controlled displacement stage to scan in the dispersion direction, and the above acquisition process is repeated to obtain a total of [number missing] samples. Wavelength data groups at each sampling location and the corresponding light spot image group ,like Figure 5 The image shown is a captured light spot image. and corresponding wavelength data As can be seen, the spectrum exhibits the peaks characteristic of narrow-bandwidth light.

[0027] Step 5: Spot image processing and coordinate extraction: extract Pixel coordinates of the center of the spot in the dispersion direction To constitute the first Group calibration data pairs ; For each light spot image The pixel coordinates of the light spot center in the dispersion direction (horizontal direction) are extracted by combining vertical grayscale projection with the one-dimensional centroid method. The specific steps are as follows: For light spot images The data is divided horizontally every 5 rows of pixels from bottom to top. For each row of pixels cut, a one-dimensional centroid is found. The resulting centroid positions are as follows: Figure 6 As shown, the vertical axis represents the pixel coordinates of the centroid position, revealing a spectral bending phenomenon, meaning that the position of dispersive light of the same wavelength is offset at different spatial heights. However, the data also shows that, for example... Figure 7 As shown, between row 520 and pixel 590, the spectral line curvature is not obvious, remaining basically at the 1085 pixel position, which exhibits a linear relationship. Therefore, the spot image can be further processed between row 520 and pixel 590. The two-dimensional grayscale matrix is ​​summed column by column along the vertical direction (perpendicular to the dispersion direction) to obtain a one-dimensional horizontal intensity distribution curve. This operation effectively suppresses random noise through the integral effect and simplifies the two-dimensional localization problem into a one-dimensional problem.

[0028] Applying the centroid algorithm to the one-dimensional intensity distribution curve, the centroid position of its grayscale distribution is calculated. This position is the coordinate of the spot center with sub-pixel precision. The centroid method makes full use of all the grayscale information of the light spot. Even if pixel saturation occurs in the central area of ​​the light spot, as long as the overall distribution of the light spot remains symmetrical, it can still provide a stable center position.

[0029] After performing the above processing on all light spot images, pixel coordinate data sets corresponding one-to-one with wavelength data are obtained. ,constitute Group calibration data pairs .

[0030] Step 6: Curve fitting and calibration equation establishment: Using the least squares method Curve fitting is performed on the calibration data pairs to establish pixel coordinates. With center wavelength The mapping equation between them is used to complete the wavelength calibration.

[0031] Will get Group calibration data pairs Plot it as a scatter plot. For example... Figure 8 As shown, observing the data distribution trend reveals a strong linear correlation between pixel coordinates and wavelength, but with a slight systematic curvature. This is because the system employs a simplified configuration of a "grating-planar detector," whose "wavelength-position" mapping relationship is theoretically a nonlinear function.

[0032] Therefore, a second-order polynomial model is used for least squares fitting, and the fitting equation is in the form of: The fitting results show that the coefficient of determination for the second-order polynomial fit is... The accuracy can reach above 0.9999, the residual distribution is uniform and there is no systematic trend, and the fitting accuracy is significantly better than that of the linear model.

[0033] In this embodiment, the pixel coordinates of the spectrometer system With wavelength The exact mapping equation: In practical measurement applications, simply substituting the pixel coordinates of the light spot to be measured into the equation will yield the corresponding precise wavelength value. In this embodiment, the formula achieves a measurement accuracy of 0.02 nm.

[0034] In this embodiment, an electronic device includes a memory and a processor. The memory stores a program that supports the processor in executing the above-described method, and the processor is configured to execute the program stored in the memory.

[0035] In this embodiment, a computer-readable storage medium stores a computer program, which is executed by a processor to perform the steps of the above method.

Claims

1. A method for wavelength calibration of a spectrometer based on grating dispersion and spatial filtering, characterized in that, This is applied to an optical system, which includes: a broadband light source, a collimating optical path, a blazed grating, a spatial filter, a mirror, a convex lens, a transmission grating, a high-precision spectrometer, and a CMOS camera. The wavelength calibration method for the spectrometer includes the following steps: Step 1: After the output light of the broadband light source is collimated by the collimating optical path, it is incident into the blazed grating at the incident angle to form a dispersed light that is continuously spread in space according to wavelength. A convex lens is placed near the image plane of the dispersed light to collimate the dispersed light and form collimated dispersed light. Step 2: Set a spatial filter that can move along the dispersion direction in the propagation direction of the collimated dispersed light, and control the movement of the spatial filter so that the collimated dispersed light passes through the light transmission window of the spatial filter in sequence, and filters out narrow band light with continuously changing center wavelength in the light transmission window in sequence. After being reflected by the mirror, the narrow band light passes through the light transmission window again and enters the blazed grating to form inverse diffraction light. Step 3: Guide the inverse diffracted light into the transmission grating, and spatially split the inverse diffracted light to form zero-order diffracted light. and first-order diffraction light ; Step 4: Simultaneously acquire zero-order diffraction light using a high-precision spectrometer. The center wavelength at each sampling point location of the spatial filter And obtain first-order diffraction light The light spot image group formed on the CMOS camera ,in, This represents the center wavelength at the i-th sampling position. Let represent the i-th light spot image; N represents the total number of light spot images; Step 5, Extraction Pixel coordinates of the center of the spot in the dispersion direction To constitute the first Group calibration data pairs ; Step 6: Use the least squares method to... Curve fitting is performed on the calibration data pairs to establish pixel coordinates. With center wavelength The mapping equation between them is used to complete the wavelength calibration.

2. The method for wavelength calibration of a spectrometer based on grating dispersion and spatial filtering according to claim 1, characterized in that, In step 2, the spatial filter includes a light-passing window whose width determines the bandwidth of the narrowband light selected. The moving step size of the spatial filter is controlled by a precision displacement stage, and the moving step size determines the density of sampling points.

3. The method for wavelength calibration of a spectrometer based on grating dispersion and spatial filtering according to claim 1, characterized in that, In step 2, by finely adjusting the angle of the reflector, the light beam reflected by the reflector and then incident on the blazed grating is spatially offset from the light beam incident on the blazed grating in step 1, so as to achieve optical path separation.

4. The method for wavelength calibration of a spectrometer based on grating dispersion and spatial filtering according to claim 3, characterized in that, Step 3 further includes: setting a lens group in the optical path of the inverse diffracted light, wherein the lens group performs beam collimation on the diverging light in the inverse diffracted light.

5. The method for wavelength calibration of a spectrometer based on grating dispersion and spatial filtering according to claim 1, characterized in that, In step 5, the pixel coordinates are extracted. The process is as follows: First, locate the linear region of the spectral lines in the spot image, and then analyze the i-th spot image within this region. The grayscale matrix is ​​projected and summed along the vertical direction perpendicular to the dispersion direction to obtain a one-dimensional intensity distribution curve; Then, the one-dimensional centroid algorithm is applied to calculate the one-dimensional intensity distribution curve to obtain... Pixel coordinates of the center of the spot .

6. An electronic device, comprising a memory and a processor, characterized in that, The memory is used to store a program that supports a processor in executing the method of any one of claims 1-5, the processor being configured to execute the program stored in the memory.

7. A computer-readable storage medium storing a computer program thereon, characterized in that, The computer program is executed by the processor to perform the steps of the method according to any one of claims 1-5.

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