CN114112037A - Variable spectral resolution spectrometer and design method - Google Patents

Variable spectral resolution spectrometer and design method Download PDF

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CN114112037A
CN114112037A CN202111161612.5A CN202111161612A CN114112037A CN 114112037 A CN114112037 A CN 114112037A CN 202111161612 A CN202111161612 A CN 202111161612A CN 114112037 A CN114112037 A CN 114112037A
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grating
oac
spectrometer
gratings
spectral resolution
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CN114112037B (en
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常军
胡瑶瑶
纪钟晔
陈蔚霖
李轶庭
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Beijing Institute of Technology BIT
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
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Abstract

The invention discloses a variable spectral resolution spectrometer and a design method, which are characterized in that the design is improved on the basis of the structure of a classical spectrometer, the initial structure of the variable spectral resolution spectrometer based on a plurality of OAC gratings is obtained, and the design efficiency is greatly improved; the invention takes the classical Offner type spectrometer as a starting point, only the convex grating is replaced by a plurality of OAC gratings, and other elements are finely adjusted, compared with the existing variable spectral resolution spectrum system, the system structure designed by the method is lighter and is easy to assemble and adjust; the invention adopts a plurality of OAC gratings to replace convex gratings in the structure of a classical spectrometer, the number and the basic parameters of the OAC gratings can be set according to the requirements of users, the applicability is wide, and the design is flexible and simple; the spectral system designed by the invention has higher spectral resolution, and when the size of the detector is increased, the ratio of the variable spectral resolution is increased.

Description

Variable spectral resolution spectrometer and design method
Technical Field
The invention relates to a variable spectral resolution spectrometer and a design method thereof, which can be used for identifying and classifying various complex objects and belongs to the technical field of optical design.
Background
Higher demands are made on the speed of spectral data acquisition and processing, and in addition, the improvement of large field of view and high resolution performance is also a target which is constantly pursued by manufacturers. Thus, the spectral imaging system will produce a huge image data cube, which tends to hinder the efficiency of data processing and transmission. However, in practical application, only the spectral information of the region of interest needs to be obtained with emphasis, and the rest of the spectral information does not need high resolution, so that only the interested part in the whole scene (in a large field range) needs to be imaged with high resolution, and the rest of the fields are imaged with lower resolution, so that the use requirement of the system can be met, and the storage and transmission quantity of the spectral data cube can be greatly reduced. Therefore, there is a strong need for a variable spectral resolution spectral imaging technique that meets the above needs.
The prior patent CN107655569A for realizing a spectrometer with variable spectral resolution by using a digital micro mirror array modulates a light beam at a slit to realize modulation of spatial resolution and spectral resolution, and this way can only obtain spectral information in one modulation state per scanning, i.e. cannot obtain spectral information with different spectral resolutions in one scanning. And the mode is limited by the size and the material of the digital micro reflector array, the spectral resolution and the spatial resolution can not break through the pixel size of the digital micro reflector, and the mode is not suitable for wave bands except visible light.
Disclosure of Invention
The invention aims to provide a variable spectral resolution spectrometer based on a plurality of off-axis convex (OAC) gratings and a design method thereof, namely, a plurality of OAC gratings are used for replacing the convex gratings in the traditional Offner structure, each light beam is diffracted from the OAC sub-grating and then forms a sub-spectral image on an image plane, the spectral resolution of the spectral images is different according to the change of the parameters of the OAC sub-grating, and the purpose of obtaining different spectral resolution spectrum information by one-time scanning can be realized.
A variable spectral resolution spectrometer is based on the structure of an Offner type spectrometer and comprises a slit, a main mirror, a convex grating, three mirrors and an image surface, wherein the convex grating comprises a plurality of off-axis convex gratings, and each off-axis convex grating generates sub-spectral images which have different spectral resolutions and are not overlapped with each other on the image surface.
Preferably, the primary mirror and the tertiary mirror are in different planes.
Preferably, the primary mirror and the tertiary mirror have different curvatures.
A method of designing a variable spectral resolution spectrometer, comprising:
designing a classical Offner spectral system, and determining an initial structure;
step two, taking any light ray at the slit for ray tracing, and calculating the relation between the grating density of the convex grating and the coordinates of the light ray reaching the image surface;
step three, designing a plurality of OAC gratings to replace convex gratings in the Offner spectrum system in the step 1; according to the calculation conclusion in the step two, designing and optimizing basic parameters of a plurality of OAC gratings to obtain an initial system structure of a plurality of OAC grating spectrometers, which specifically comprises the following steps:
s301, setting the grating density and off-axis eccentricity of each OAC grating, and obtaining the incidence angle of the central light of each OAC grating according to the calculation conclusion of the step one;
s302, calculating the coordinate of the central light of each OAC grating on the image plane according to the wavelength range and the central wavelength, thereby obtaining the sub-spectrum image width formed on the image plane by diffraction of each OAC grating;
s303, judging whether the following conditions are met:
the sum of the widths of the sub-spectrum images of all the OAC gratings is smaller than or equal to the effective detection target surface size of the image surface, and the sub-spectrum images of the adjacent OAC gratings are not overlapped;
if the conditions are met, the grating density and off-axis eccentricity of the OAC grating meet the requirements at the moment, and a plurality of OAC grating spectrometer initial system structures are obtained;
if the condition is not met, adjusting the grating density of the OAC grating, recalculating the incidence angle and the coordinate on the image surface, further obtaining the sub-spectrum image width, and judging whether the condition is met again; and repeating the steps until the conditions are met, and outputting the initial system structure of the plurality of OAC grating spectrometers.
Preferably, based on the initial system of the plurality of OAC grating spectrometers obtained in the step three, the primary mirror and the three mirrors are separated to enable the primary mirror and the three mirrors to have different curvatures, smile distortion and keystone distortion are controlled simultaneously, and the system is optimized until the system achieves the imaging performance meeting the requirements.
Preferably, the off-axis eccentricity value of the OAC grating is adjusted based on the initial system of the plurality of OAC grating spectrometers obtained in the step three, so that the utilization rate of the detector is maximized.
Preferably, the density of each grating is more than 40g/mm and less than 1000 g/mm.
The invention has the following beneficial effects:
the invention improves the design on the basis of the structure of the classical spectrometer, obtains the initial structure of the variable spectral resolution spectrometer based on a plurality of OAC gratings and greatly improves the design efficiency.
The invention takes the classical Offner type spectrometer as a starting point, only the convex grating is replaced by a plurality of OAC gratings, and other elements are finely adjusted.
The invention adopts a plurality of OAC gratings to replace convex gratings in the structure of the classical spectrometer, the number and the basic parameters of the OAC gratings can be set according to the requirements of users, the applicability is wide, and the design is flexible and simple.
The spectral system designed by the invention has higher spectral resolution, and when the size of the detector is increased, the ratio of the variable spectral resolution is increased.
Drawings
FIG. 1 is a block diagram of a classical Offner spectrometer in an embodiment of the present invention;
FIG. 2(a) is a schematic view of the structure of an OAC in an embodiment of the present invention in a YZ plane, and FIG. 2(b) is a schematic view of the structure of an OAC grating in an embodiment of the present invention in an XY plane;
FIG. 3 is a flow chart of a multi-block OAC grating spectrometer optimization in an embodiment of the present invention;
FIG. 4 is a block diagram of a multi-block OAC grating spectrometer according to an embodiment of the present invention;
FIG. 5 is a full field of view field plot of an image plane of a system according to an embodiment of the present invention;
FIG. 6 is a graph of spectral resolution of a system according to an embodiment of the present invention;
FIG. 7 is a graph of the system spectral resolution zoom ratio according to an embodiment of the present invention;
FIG. 8 is a distortion plot for a multi-block OAC grating spectrometer according to an embodiment of the present invention;
the three-dimensional imaging system comprises a slit 1, a primary mirror 2, a convex grating 3, a three-mirror 4 and an image plane 5.
Detailed Description
The invention is described in detail below with reference to the figures and the specific examples.
The design idea of the invention is as follows: an improvement is made on the structure of a classical Offner spectrometer. And deducing the relation among the image plane coordinates, basic parameters of the multi-OAC grating and the system wave band through ray tracing. Each light diffracted from the OAC grating will produce a sub-spectral image at the image plane. However, each sub-spectral image is overlapped, an automatic optimization procedure is designed to avoid the overlapping of the sub-spectral images, and an optimal initial structural parameter is calculated. And moreover, the optimization freedom degree of the initial system is increased, the main mirror and the three mirrors of the original system are separated, the three mirrors are not in the same plane and have different curvatures, smile distortion and keystone distortion of the system are controlled, and the system is further optimized until better imaging quality and smaller distortion are obtained. Finally, the off-axis eccentricity of the OAC grating is slightly adjusted to maximize the utilization rate of the detector.
The design process is mainly divided into two parts: and calculating initial structure parameters and optimizing the initial structure, wherein the design is realized through a design program and optical design software.
Designing initial structural parameters:
(1) and deducing the relation among the image plane coordinates, basic parameters of the multi-OAC grating and the system wave band by performing ray tracing on the classical Offner spectral structure:
the light beam emitted from the slit 1 is reflected by the primary mirror 2 and then diffracted at the convex grating 3. The three mirrors 4 focus the dispersed beam onto an image plane 5. In fig. 1, any one of the light rays emitted from the slit 1 is traced. (0, y)0,z0),(xp,yp,zp),(xg,yg,zg),
Figure BDA0003290399260000041
And
Figure BDA0003290399260000042
the coordinates of the light emission point, the main mirror 2, the grating 3, the three mirrors 4 and the image surface 5 are correspondingly represented respectively.
Figure BDA0003290399260000043
And
Figure BDA0003290399260000044
the vectors of the light rays from the slit 1 to the main mirror 2, from the main mirror 2 to the convex grating 3, from the convex grating 3 to the three mirrors 4, and from the three mirrors 4 to the image plane 5 are shown.
Figure BDA0003290399260000045
And the unit normal vector of the light passing through the primary mirror 2, the convex grating 3 and the three mirrors 4 is shown. Light emitted from the slit 1
Figure BDA0003290399260000046
The projection on the YZ plane forms an included angle theta with the Z axis, and the projection on the XY plane forms an included angle with the X axis
Figure BDA0003290399260000047
According to the space geometric relationship, the following steps are carried out:
Figure BDA0003290399260000048
Figure BDA0003290399260000049
the primary mirror 2 has a curvature of cpSpherical, the base of the grating 3 being of curvature cgSpherical surface, D is the distance from the primary mirror 2 to the convex grating 3, then:
Figure BDA00032903992600000410
Figure BDA00032903992600000411
Figure BDA00032903992600000412
and
Figure BDA00032903992600000413
on one plane, according to the law of reflection:
Figure BDA00032903992600000414
Figure BDA00032903992600000415
wherein the content of the first and second substances,
Figure BDA00032903992600000416
and
Figure BDA00032903992600000417
available y0,z0,xp,yp,zp xg,yg,zgAnd cpExpressed as the following equation:
Figure BDA00032903992600000418
Figure BDA00032903992600000419
Figure BDA0003290399260000051
in a similar manner to that described above,
Figure BDA0003290399260000052
can be expressed as:
Figure BDA0003290399260000053
the incident angle θ of the convex grating 3 according to the formulas (1) to (10)iCan be obtained in formula (11).
Figure BDA0003290399260000054
According to the grating formula, the incident angle thetaiAnd angle of diffraction
Figure BDA0003290399260000055
Can be determined by the grating density T and the wavelength λnAnd the diffraction order M represents:
Figure BDA0003290399260000056
Figure BDA0003290399260000057
Figure BDA0003290399260000058
three mirrors 4 having a curvature ctThe spherical surface of (2), then:
Figure BDA0003290399260000059
according to the law of reflection:
Figure BDA00032903992600000510
Figure BDA00032903992600000511
wherein the content of the first and second substances,
Figure BDA00032903992600000512
and
Figure BDA00032903992600000513
available xg,yg,zg,
Figure BDA00032903992600000514
And ctExpressed as the following formula
Figure BDA00032903992600000515
Figure BDA00032903992600000516
Figure BDA00032903992600000517
From equation (12), when T is determined, it can be derived
Figure BDA00032903992600000518
Further, according to equations (13) - (20), it can be found that
Figure BDA00032903992600000519
In summary, using the model composed of equations (1) - (20), the initial structure of interest can be finalized once the relevant limiting factors are entered into the model.
(2) Designing a plurality of OAC gratings:
the multiple OAC gratings are composed of m sub-gratings (g)1…gm-1gm) And (4) forming. As shown in FIG. 2, (a) is a structural diagram of OAC in YZ plane, and an OAC grating is an off-axis off-center d-side cross-sectional grating of the convex surface of the substrate. m sub-gratings coupled at a curvature of cgOn the spherical surface of (a). The light-transmitting aperture of multiple OAC gratings is phiyThe coordinate values of the edge ray and the principal ray passing through the grating are ypmax,ypminAnd ypo. FIG. 2(b) is a schematic diagram of the structure of the OAC grating in the XY plane, the grating frequency T of each OAC gratingmDifferent. The light beam is diffracted by each sub-grating to generate two-dimensional spectral images with different spectral broadening.
The convex grating in the classical Offne structure is replaced by an OAC grating. Each beam of light is at different grating density TmDiffraction occurs at the OAC of (a). Diffraction angle of each OAC grating according to equation (12)
Figure BDA0003290399260000061
Subject wavelength lambdanAnd the grating frequency TmThe influence of (c). Further, according to equations (13) - (20), the coordinates of the light beam on the image plane also depend on λnAnd Tm。λ1～λnThe light beam in the wave band passes through an OAC grating gmSpectral width Δ p formed on image plane by diffractionmComprises the following steps:
Figure BDA0003290399260000062
wherein the content of the first and second substances,
Figure BDA0003290399260000063
and
Figure BDA0003290399260000064
is gmSpectral line lambda generated at the image planenAnd λ1The value of the Y coordinate. Similarly, gm-1Resulting spectral width Δ pm-1Expressed as:
Figure BDA0003290399260000065
wherein
Figure BDA0003290399260000066
And
Figure BDA0003290399260000067
is gm-1Spectral line lambda generated at the image planenAnd λ1The value of the Y coordinate.
gmAnd gm-1Offset Δ h between atlas images produced at image planem,m-1Expressed as:
Figure BDA0003290399260000068
wherein
Figure BDA0003290399260000069
And
Figure BDA00032903992600000610
is gmAnd gm-1Central spectral line lambda generated at the image plane0The value of the Y coordinate. To ensure that no overlap occurs between sub-image spectral imaging, Δ pm,Δpm-1And Δ hm,m-1The following formula is satisfied:
2Δhm,m-1≥Δpm-1+Δpm (24)
from the above derivation, Δ pm,Δpm-1And Δ hm,m-1Can be represented by Y coordinate on image surface, and Y coordinate is represented by lambdanAnd TmAnd (6) determining. Thus, in place of an OAC grating in a conventional Offner spectrometer system, whether sub-spectral imaging overlaps light primarily from the OAC grating or notGrid frequency TmAnd radiation spectrum lambda1～λnAnd (6) determining.
(3) The initial structure optimization process of the spectrum structure of a plurality of OAC gratings comprises the following steps:
the OAC grating was replaced on the basis of the classical Offner spectrometer initial system. The convex grating is composed of m OAC gratings with respective grating density of T1…Tm-1,TmOff-axis eccentricity is respectively
Figure BDA0003290399260000071
The optimization procedure is shown in fig. 3.
The parameters of the initial system are first entered in the calculation program. For ease of manufacture, the grating density T1…Tm-1,TmIt should be greater than 40g/mm and less than 1000 g/mm. In order to ensure sub-image spectrum imaging energy balance, the light transmission aperture of each OAC grating should be close to the same, so the off-axis eccentricity is
Figure BDA0003290399260000072
Should be uniformly arranged according to the clear aperture size of each OAC grating.
Second, the spectral range and center wavelength are input to the calculation program. The bandwidth Δ p of each sub-spectrum imaging according to equations (21) - (23)1…Δpm-1,ΔpmAnd Δ h12…Δhm-1,mCan be obtained. And then judging whether the sub-image spectrum images are overlapped or not according to a formula (24). And (3) according to the size L of the target surface effectively detected by the detector, the sum of the bandwidths of all the sub-image spectrum imaging needs to satisfy the formula (25), and if the formulas (24) and (25) are not satisfied, the program returns to reset the grating density. If yes, outputting
Figure BDA0003290399260000073
And
Figure BDA0003290399260000074
wherein the spectral resolution is scaled by a factor betaDThis is shown in equation (26).
Figure BDA0003290399260000075
And all dispersion spectral lines of the image surface occupy Y-axis space. Through the calculation process, data meeting the requirements are output, and the optimal initial parameters of a plurality of OAC grating spectrometers are selected from the data.
Figure BDA0003290399260000076
Figure BDA0003290399260000077
Optimizing an initial structure: optimizing the obtained initial system of the plurality of OAC grating spectrums, increasing optimization variables, separating a main mirror and three mirrors to enable the main mirror and the three mirrors to have different curvatures, and simultaneously controlling smile distortion and keystone distortion to optimize the imaging performance meeting the requirements. And finally, adjusting the off-axis eccentricity value of the OAC grating so as to maximize the utilization rate of the detector.
Example (b):
according to a specific embodiment, a classical Offner spectrometer is designed with a wavelength range of 460nm to 900nm divided into 13 sampling wavelengths with a central wavelength of 680 nm. The Numerical Aperture (NA) was set to 0.145, a detector with a pixel number of 2048 × 2048 was selected, and the pixel size was 6.5 μm X6.5.5 μm. The convex grating has a grating density of 150 lines per millimeter, using a first positive diffraction order. The basic parameters of a classical Offner spectrometer are shown in table 1.
TABLE 1 basic parameters of a classical Offner spectrometer
Figure BDA0003290399260000078
Figure BDA0003290399260000081
The OAC grating was replaced on the basis of the classical Offner spectrometer initial system. In the example, the convex grating is composed of 3 OAC gratings, and the off-axis eccentricity is preliminarily set to
Figure BDA0003290399260000082
Through the optimization process, 5 sets of data meeting the requirements shown in table 2 are output. In the data of the first group and the fourth group, the variable power ratio of the spectral resolution is higher than 4, but the Y-axis space occupied by all dispersion spectral lines of the image plane in the data of the first group is 8.45mm, which is far smaller than the effective detection target surface size of the detector by 13mm, and the utilization rate of the detector is not facilitated. The fifth set of data represents higher detector utilization but lower spectral resolution. The third set of data is finally selected as the initial parameters for the multi-block OAC grating spectrometer.
TABLE 2 five sets of data satisfying the requirements
Figure BDA0003290399260000083
After further optimization of the multi-block OAC grating spectrometer, the system structure is shown in fig. 4. The system basic parameters are shown in table 3 and the multi-block OAC grating parameters are shown in table 4.
TABLE 3 Multi-block OAC Grating spectrometer basic parameters
Figure BDA0003290399260000084
TABLE 4 Multi-block OAC Grating basic parameters
Figure BDA0003290399260000091
The multi-block OAC grating spectrometer of the present embodiment is evaluated by using the following two evaluation indexes:
1. spectral resolution
Fig. 5 shows a full field of view spot diagram at the image plane. The X direction represents the imaging of the slit and the normalized field of view represents the coordinate values. The Y direction represents the dispersion line of the system and the colors represent different dispersion bands. The figure shows that the system forms a broadening of Δ p in the image plane Y direction1,Δp2,Δp3The three partial dispersion lines of (2). The spectral resolution of the system is shown in the graph of fig. 6. The spectral resolution of OAC grating 1 diffraction imaging is limited by RMS spot radius at 460-636 nm and 900nm and between 0.46nm and 0.49 nm; at 665nm-871nm, the size of the detector pixel is limited to 0.45 nm. The spectral resolution of the OAC grating 2 diffraction imaging is limited to RMS spot radius at 841-900 nm and between 0.90nm-0.93 nm; the spectral resolution of 0.89nm OAC grating 3 diffraction imaging is limited by the pixel size of the detector at 460nm-812nm and is limited by the pixel size of the detector at 460nm-900nm, which is 1.91 nm.
The spectral resolution ratio beta of the system is shown in FIG. 7, and is between 489nm and 607nm, and beta is between 3.86 and 3.94; the other spectrum band, beta is larger than the spectrum resolution zoom ratio of the basic structure in section 3.2, 4.02, and the highest value is 4.27.
2. Distortion of
Figure 8 shows smile distortion and keystone distortion of the system, respectively. The maximum smile distortion occurs at the end of the slit at a wavelength of 900nm, produced by the OAC grating 1, which is less than 3.5 microns, about half of a pixel. The maximum keystone distortion occurs at the end of the 900nm slit, produced by the OAC grating 1, less than 3.2 microns, on the order of half a pixel.
The multi-OAC grating spectrometer has excellent image quality and has the potential of continuously increasing the numerical aperture and the potential of improving the spectral resolution.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it will be understood by those skilled in the art that various changes, substitutions and alterations can be made herein without departing from the invention, and these are intended to be within the scope of the invention.

Claims (7)

1. A variable spectral resolution spectrometer is based on the structure of an Offner type spectrometer and comprises a slit, a main mirror, a convex grating, three mirrors and an image surface.
2. The variable spectral resolution spectrometer of claim 1, wherein the primary mirror and the tertiary mirror are in different planes.
3. The variable spectral resolution spectrometer of claim 1, wherein the primary mirror and the tertiary mirror have different curvatures.
4. A method of designing a variable spectral resolution spectrometer as claimed in claim 1, comprising:
designing a classical Offner spectral system, and determining an initial structure;
step two, taking any light ray at the slit for ray tracing, and calculating the relation between the grating density of the convex grating and the coordinates of the light ray reaching the image surface;
step three, designing a plurality of OAC gratings to replace convex gratings in the Offner spectrum system in the step 1; according to the calculation conclusion in the step two, designing and optimizing basic parameters of a plurality of OAC gratings to obtain an initial system structure of a plurality of OAC grating spectrometers, which specifically comprises the following steps:
s301, setting the grating density and off-axis eccentricity of each OAC grating, and obtaining the incidence angle of the central light of each OAC grating according to the calculation conclusion of the step one;
s302, calculating the coordinate of the central light of each OAC grating on the image plane according to the wavelength range and the central wavelength, thereby obtaining the sub-spectrum image width formed on the image plane by diffraction of each OAC grating;
s303, judging whether the following conditions are met:
the sum of the widths of the sub-spectrum images of all the OAC gratings is smaller than or equal to the effective detection target surface size of the image surface, and the sub-spectrum images of the adjacent OAC gratings are not overlapped;
if the conditions are met, the grating density and off-axis eccentricity of the OAC grating meet the requirements at the moment, and a plurality of OAC grating spectrometer initial system structures are obtained;
if the condition is not met, adjusting the grating density of the OAC grating, recalculating the incidence angle and the coordinate on the image surface, further obtaining the sub-spectrum image width, and judging whether the condition is met again; and repeating the steps until the conditions are met, and outputting the initial system structure of the plurality of OAC grating spectrometers.
5. The method of claim 4 wherein based on the initial system of the multi-OAC grating spectrometer obtained in step three, the primary mirror and the tertiary mirror are separated to have different curvatures, while smile distortion and keystone distortion are controlled to optimize the system until the system achieves the required imaging performance.
6. The method of claim 5 wherein the off-axis eccentricity of the OAC grating is adjusted to maximize the detector utilization based on the initial system of the multi-OAC grating spectrometer obtained in step three.
7. A method of designing a variable spectral resolution spectrometer as claimed in claim 4, 5 or 6 wherein the density of each grating is greater than 40g/mm and less than 1000 g/mm.
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