CN103900688A - Imaging spectrometer beam splitting system based on free-form surface - Google Patents

Abstract

The invention discloses an imaging spectrometer beam splitting system based on a free-form surface. The system is composed of an entrance slit, a free-form surface collimation reflector, a plane diffraction grating, a free-form surface imaging reflector and an image surface detector. The imaging spectrometer beam splitting system is characterized in that the high-freedom-degree non-rotation-symmetrical free-form surface is used in cooperation with the plane grating which is high in performance and mature in technology, and the beam splitting system which is simple and compact in structure, low in aberration, low in distortion and high in efficiency is realized. In the system, the mode that the hole diameter deviates from the axis is adopted to achieve the purpose of no light obstruction, relative angle rotation does not exist among all optical elements, installation and adjustment can be realized conveniently and easily, and the development time can be shortened. The beam splitting system based on the free-form surface realizes good spectral imaging at the 8-12.5-micrometer long wave infrared spectrum, and on the premise that it is guaranteed that the system is simple and compact, spherical aberration, comatic aberration, astigmatism, spectral distortion, spectral line bending and the like are well corrected. The imaging spectrometer beam splitting system can be applied to the field of aerospace and aviation hyperspectral imaging.

Description

A kind of imaging spectrometer beam splitting system based on free form surface

Technical field

The invention belongs to imaging spectrometer design field, be specifically related to a kind of optical design of the novel plane grating beam splitting system based on free form surface, embodiments of the invention spectral coverage is LONG WAVE INFRARED, and it is also applicable to the design of the spectrum imaging system spectrum groupware of other spectral coverages such as visible near-infrared or medium-wave infrared.

Background technology

Imaging spectrometer is the combination of imaging technique and spectral technique, it can obtain two-dimensional space information and the one dimension spectral information of target simultaneously, form data cube, be widely used in the numerous areas such as social production, military activity, universe and astrosurveillance and Aero-Space remote sensing.Its optical system is generally made up of preposition telescopic system and spectrum beam splitting system, and beam splitting system is determining the key property of imaging spectrometer, the key of innovation routine spectra instrument, because beam splitting system directly affects performance, the complexity of structure, the weight and volume of whole imaging spectrometer.In numerous light splitting modes, optical grating diffraction light splitting is because its structure is high without dynamic component, technology maturation, performance, good linearity, have wide spectral range and high resolution and become the light splitting means of existing imaging spectrometer main flow.The principal mode of the grating splitting system of widespread use has in the world: adopt the Czerny-Turner structure of the Offner structure of convex grating, the Dyson structure that adopts concave grating, employing plane grating etc., also have some to adopt the beam-splitting structure of gratings and prism combination.

Offner structure is total-reflection type, not limited by wave band, and its spectrum groupware is convex grating, simple and compact for structure; Dyson structure adopts transmission-type, can realize larger relative aperture and little volume, and its spectrum groupware is concave grating.Opposite planar grating, these two kinds of grating development are used more late, and the technology such as its processing, detection are relatively immature, the manufacturing process difficulty of counting grating especially for the ultralow groove of thermal infrared spectral coverage is larger, cause such high performance device to manufacture difficult, cost is high, and design limitations is large.

Plane grating is lower as a kind of processing technology thereof maturation, cost, obtain the spectrum groupware that channel is relatively unimpeded, is the most general spectrum groupware of application now.But for convex grating and concave grating, adopt the Czerny-Turner structure of plane grating light splitting to be subject to the restriction of aberration correction, be difficult to realize large relative aperture, and along with elongated the caused chromatic variation of distortion of slit and Spectral line bend also increase acutely, there is the problem that aberration correcting capability is poor, resolution is lower.In order to make up these defects and to proofread and correct various aberrations, Czerny-Turner structure based on plane grating is through being usually used in the spectrum imaging system of the poor large F number of statuette, and in order to increase the degree of freedom of balance aberration, often all there is rotation and bias in various degree in each element, to having relatively high expectations of designing, debug, the lead time is longer.

In addition, other spectrometer beam splitting system based on plane grating generally increases the degree of freedom of balanced system by increasing the mode of component, system is comparatively complicated, need to introduce multiple lens or prism, the small skew of each prism or lens and the eccentric picture quality of system of can giving are brought serious impact, what must cause whole system debugs and tests difficulty, and the lead time is long and development cost is high.This UV light will just can reach detector through repeatedly reflecting and reflecting, and reflects at every turn and reflects and all bring light loss of energy, will inevitably cause the system efficiency of light energy utilization low.

Summary of the invention

The problem existing based on above-mentioned existing spectrometer light splitting technology, in order to solve preferably the contradiction between beam splitting system high-performance, simplification and low cost, fundamental purpose of the present invention is to be referred from existing Offner structure and Czerny-Turner structure, get its advantage separately, keep away its shortcoming separately, taking plane blazed grating as light-splitting device, the non-rotating symmetrical free form surface that is equipped with high-freedom degree collimates and imaging, designs the novel beam splitting system based on free form surface.This system is simple, compact, debugs and is easy to can realize little aberration, low distortion and high-level efficiency simultaneously.

Beam splitting system of the present invention adopts the version of approximate Offner, and system is total-reflection type, only utilizes reaching and avoid the object of blocking from axle of aperture, does not have relative angle rotation between each optical element.System utilizes respectively a free form surface collimating mirror and a free form surface imaging lens to replace the collimating mirror and the imaging lens that in conventional planar grating splitting system, are made up of multiple lens, based on the large degree of freedom of freeform optics element and non-rotating symmetry, can proofread and correct well the advantages such as multiple off-axis aberration and non-rotating symmetrical aberration, to reach the design that improves spectrometer beam splitting system image quality, simplifies system, the processing of reduction system, resetting difficulty, reduce the volume of spectral instrument, the object of deadweight.

As shown in Figure 1, it comprises entrance slit 1, collimation free-form surface mirror 2, diffraction grating 3, imaging free-form surface mirror 4 to beam splitting system structure of the present invention, and entrance slit 1 is positioned on front telescopical focal plane, is the picture of front telescope infinite distance target; Diffraction grating 3 is plane gratings, and free-form surface mirror collimating mirror 2 and imaging free-form surface mirror 4 are the free form surface without Rotational Symmetry form; The aperture diaphragm of system is arranged on plane diffraction grating 3, and collimation free-form surface mirror 2 and imaging free-form surface mirror 4 arrange with respect to grating 3, and the center of curvature of the two overlaps, and and the center of grating on same straight line; The light sending from entrance slit 1 reflexes to diffraction grating 3 through collimation free-form surface mirror 2, and the light after diffraction grating 3 diffraction light splitting is reflected onto imaging free-form surface mirror 4, is finally reflexed in the image planes of laying detector 5 by free-form surface mirror 4.

Collimation free-form surface mirror 2 and imaging free-form surface mirror 4 are the free form surface of non-rotating symmetry, and its expression formula is shown in formula (1), wherein: centered by c, put O ₁and O ₂curvature, k is quadratic coefficients, C ₁-C ₃₅for every coefficient, (x, y, z) is the coordinate of each point on free form surface.。

In this system, aperture diaphragm is arranged on diffraction grating 3, collimation free-form surface mirror 2 and imaging free-form surface mirror 4 all arrange with respect to diffraction grating 3, the center of collimation free-form surface mirror 2, diffraction grating 3 and imaging free-form surface mirror 4 is all located on the same line, system adopts aperture to avoid blocking of light from the mode of axle, does not have relative angle rotation between each optical element.

$\begin{array}{c}z(x,y)=\frac{c(x^2+y^2)}{1+\sqrt{1-(1+k)\left((x^2+y^2)c^2\right)}}+\\ C_{1}x+C_{2}y+\\ C_3{x}^2+C_4\mathrm{xy}+C_5{y}^2+\\ C_6{x}^3+C_7{x}^{2}y+C_{8}x{y}^2+C_9{y}^3+\\ C_{10}{x}^4+C_{11}{x}^{3}y+C_{12}{x}^2{y}^2+C_{13}x{y}^3+C_{14}{y}^4+\\ C_{15}{x}^5+C_{16}{x}^{4}y+C_{17}{x}^3{y}^2+C_{18}{x}^2{y}^3+C_{19}x{y}^4+C_{20}{y}^2+\\ C_{21}{x}^6+C_{22}{x}^{5}y+C_{23}{x}^4{y}^2+C_{24}{x}^3{y}^3+C_{25}{x}^2{y}^4+C_{26}x{y}^5+C_{27}{y}^6+\\ C_{28}{x}^7+C_{29}{x}^{6}y+C_{30}{x}^5{y}^2+C_{31}{x}^4{y}^3+C_{32}{x}^3{y}^4+C_{33}{x}^2{y}^5+C_{34}x{y}^6+C_{35}{y}^7+\\ ......\end{array}---\left(1\right)$

Taking LONG WAVE INFRARED spectral coverage, (8 μ m-12.5 μ beam splitting system m) is as design example, feasibility of the present invention and practicality are set forth, due to the singularity of infrared imaging system design, the development threshold of thermal infrared wave band imaging spectrometer is far above visible ray/near-infrared band, the Development Techniques of thermal infrared wave band imaging spectrometer only has a few advanced country to grasp at present, and therefore the design's example has more real Practical significance.The parameter index of design example is as shown in table 1, and under the prerequisite of the system of guarantee simplicity, indices all reaches the suitable level of beam splitting system of utilizing convex surface or concave grating, and this is that traditional plane grating beam splitting system is difficult to accomplish.Every coefficient of collimation free-form surface mirror 2 and imaging free-form surface mirror 4 is respectively as shown in table 2 and table 3, it is worth mentioning that, under existing single point diamond machining technical merit, the free form surface of mentioning in the present invention using copper or aluminum metal under the prerequisite of material, its machining precision can reach 1/4 λ@632.8nm, and this is enough to meet the request for utilization under thermal infrared spectral coverage.

The invention has the beneficial effects as follows: system operation maturation, cost are lower, function admirable and obtain beam splitter plane grating that channel is unimpeded and be equipped with the free-form surface mirror of non-rotating symmetry, realize little aberration, low distortion, high efficiency beam splitting system; System architecture is simple, compact, has less optical element, and collimating mirror and imaging lens be by monolithic catoptron composition, has reduced the optical energy loss causing due to multiple reflections or refraction; In system, between each optical element, there is not relative angle rotation, and the center of three critical optical elements (collimation free-form surface mirror 2, diffraction grating 3 and imaging free-form surface mirror 4) is on same straight line, this has reduced the difficulty of debuging largely, has shortened widely the cycle of development; Last the present invention utilizes under the prerequisite of free form surface simplied system structure, the performance that ensured system.

Table 1

Parameter	Size
		Spectral range	8-12.5μm
Field angle (FOV)	11°
		F#	2.6
Slit sizes	6mm×30μm
		Objective focal length	30mm
System size	100mm×100mm
		Spatial resolution	1mrad
Detector	30μm×30μm
		Spectrum sample interval	25nm

Table 2

X 1Y 0	X 0Y 1	X 2Y 0	X 1Y 1	X 0Y 2	X 3Y 0
						0	-9.27E-6	1.021E-3	0	1.192E-3	0
X 2Y 1	X 1Y 2	X 0Y 3	X 4Y 0	X 3Y 1	X 2Y 2
						-1.53E-6	0	2.97E-6	-3.18E-7	0	-6.7E-7
X 1Y 3	X 0Y 4	X 5Y 0	X 4Y 1	X 3Y 2	X 2Y 3
						0	-2.64E-7	0	-2.4E-9	0	-5.53E-10
X 1Y 4	X 0Y 5	X 6Y 0	X 5Y 1	X 4Y 2	X 3Y 3
						0	-2.13E-9	7.66E-11	0	1.54E-10	0
X 2Y 4	X 1Y 5	X 0Y 6	X 7Y 0	X 6Y 1	X 5Y 2
						2.82E-10	0	-1.55E-11	0	7.22E-13	0
X 4Y 3	X 3Y 4	X 2Y 5	X 1Y 6	X 0Y 7	X 8Y 0
						3.47E-12	0	3.65E-12	0	-3.36E-13	-5.6E-14

Table 3

X 1Y 0	X 0Y 1	X 2Y 0	X 1Y 1	X 0Y 2	X 3Y 0
						0	-1.15E-5	1.17E-3	0	9.83E-4	0
X 2Y 1	X 1Y 2	X 0Y 3	X 4Y 0	X 3Y 1	X 2Y 2
						1.02E-6	0	1.12E-6	-3.73E-7	0	-7.2E-7
X 1Y 3	X 0Y 4	X 5Y 0	X 4Y 1	X 3Y 2	X 2Y 3
						0	-3.05E-7	0	2.92E-9	0	-3.54E-10
X 1Y 4	X 0Y 5	X 6Y 0	X 5Y 1	X 4Y 2	X 3Y 3
						0	-8.93E-10	4.04E-11	0	1.6E-10	0
X 2Y 4	X 1Y 5	X 0Y 6	X 7Y 0	X 6Y 1	X 5Y 2
						2.03E-10	0	6.2E-11	0	-1.42E-12	0
X 4Y 3	X 3Y 4	X 2Y 5	X 1Y 6	X 0Y 7	X 8Y 0
						-2.5E-12	0	-1.84E-12	0	-4.26E-13	9.9E-15

Brief description of the drawings

Fig. 1 is the structural representation of beam splitting system.

Fig. 2-4 are respectively the point range figures under short-wavelength limit 8 μ m, centre wavelength 10.25 μ m and long wavelength threshold 12.5 μ m.

Fig. 5-7 are respectively the transfer curves under short-wavelength limit 8 μ m, centre wavelength 10.25 μ m and long wavelength threshold 12.5 μ m.

Fig. 8 is respectively the light spectrum image-forming schematic diagram of short-wavelength limit, centre wavelength and long wavelength threshold skew 50nm.

Embodiment

Below in conjunction with drawings and Examples, the present invention is described in further detail:

As shown in Figure 1, spectrometer beam splitting system of the present invention is taking high-performance, convex grating Offner structure simple in structure and the Czerny-Turner structure based on plane grating as prototype, get its two advantage, keep away its shortcoming, realize the high-performance light splitting under components and parts and constructional simplicity prerequisite.Beam splitting system comprises slit 1, collimation free-form surface mirror 2, diffraction grating 3, imaging free-form surface mirror 4 and image planes detector 5.The focal plane that slit 1 is front telephotolens, images in slit from the incident beam of distant object through telescope, as the object plane of beam splitting system; Collimation free-form surface mirror 2 is collimating mirror, after the light beam of slit is reflected mirror 2 collimation, incide and on diffraction grating 3, carry out the loose light splitting of diffraction colour, the light of different wave length after grating 3 diffraction with different emergence angle outgoing, be imaged on the diverse location that free-form surface mirror 4 is imaged on image planes detector 5, realize light spectrum image-forming.

The design procedure of beam splitting system is:

(1), the initial configuration of system calculates, collimating mirror 2 and imaging lens 4 separate from centre with respect to grating, the center of curvature overlaps, and so both can avoid re-diffraction and diffraction repeatedly, facilitates again debuging and processing of catoptron simultaneously.The radius-of-curvature of the two is r ₁=r ₂=r, for aplanasia, according to Rayleigh criterion, radius-of-curvature need meet shown in formula (2).Wherein, the diameter that D is catoptron, the radius-of-curvature that r is catoptron, the F number that F is system

$\frac{{(D/2)}^4}{{8\mathrm{<figure-callout\; id="4"\; label="r"\; filenames="HDA0000483571240000011.png"\; state="\{\{state\}\}">r</figure-callout>}}^4}\&le;\frac{\mathrm{\&lambda;}}{4}\&DoubleRightArrow;f=r/2\&le;256\mathrm{\&lambda;}{F}^4---\left(2\right)$

Correspondingly, slit is r/2 to the distance of collimating mirror, imaging lens to the distance of image planes is-r/2, the simultaneously version of similar Offner, collimation mirror set one with respect to y axle from axle amount, to avoid blocking of light, at this moment can generally obtain incident angle i and the diffraction angle of grating, simultaneously according to the dispersion equation of grating, as shown in Equation (3), the groove that can roughly calculate every millimeter, grating according to centre wavelength, incident angle and angle of diffraction etc. is counted 1/d.

d(sini-sinθ)＝mλ _B （3）

(2), obvious above initial configuration can not obtain good light spectrum image-forming quality, next distance between radius-of-curvature, each element of collimating mirror and imaging lens is set, is variable from the groove number of axle amount and grating etc., and tentatively adjust these variablees, noting controlling blocking of light (can be by arranging the form of virtual surface, control the coordinate figure of light in virtual surface), through calculating and optimization repeatedly, make the result of design approach step by step the designing technique index of system.

(3), the optimized variable of step (2) is very limited, for traditional plane grating Czerny-Turner structure, the rotation of each element has not been can optimized variable, and the design's method based on relative aperture and spectral coverage scope etc. be also better than widely conventional Czerny-Turner structure, utilize step (2) can not overcome the picture element bringing due to plane grating, the problem such as chromatic variation of distortion and Spectral line bend, so generally can't meet the needs of use, also will be according to the width of dispersion dimension on detector, the width of space dimension, the situation of Spectral line bend and spectrum distortion and system are further optimized the parameters of system the requirement of imaging pixel quality.Arranging under the prerequisite that collimating mirror and imaging lens are the described free form surface of formula (1), pitch angle of groove number, detector and the optical axis of its front 36 coefficients and central point radius-of-curvature, quadratic coefficients, grating etc. is set to variable, system is carried out to further optimal design, make final design can meet the demand of the property indices of system.Here it is worth mentioning that, if the visual field of slit is set to along the setting of x direction of principal axis, about y rotational symmetry, two free-form surface mirrors also should have symmetry about y axle so, so when two free form surfaces are optimized to design, the index of its x is that the coefficient of the item of odd should be set to 0, and the coefficient of the index of only optimizing x is even item can improve the efficiency of software Automatic Optimal so widely.

Finally, according to above design procedure, the present invention is with the embodiment that is designed to of LONG WAVE INFRARED beam splitting system, provide design example according to the design parameter index of table 1, feasibility of the present invention and practicality are set forth, the collimation free form surface of using in beam splitting system and every coefficient of imaging free form surface are as shown in table 2 and table 3, and each component parameters of system arranges as shown in table 4.Ensureing under the prerequisite of system simplicity, indices all reaches domestic other quite levels of the spectrometer of complex configuration more, and spectrum distortion is controlled in 1/10 pixel, and Spectral line bend is controlled in 2/3 pixel.

Table 4

Type	Radius/mm	Spacing/mm	Quadratic coefficients	Other
					Slit
1	Plane	93.559	0
					Coordinate interrupts		0	—	The eccentric 21.2043mm of Y-axis
Collimation free form surface 2	-130.6827	-93.559	-5.9288
					Grating 3	Plane	93.559	0	12.5 lines are right/mm
Imaging free form surface 4	-123.2412	-79.844	-5.8417
					Coordinate interrupts		0	—	-19.2171 ° of X-axis rotations
Image planes
	5	Plane	—	0

Claims (2)

1. the imaging spectrometer beam splitting system based on free form surface, it comprises entrance slit (1), collimation free-form surface mirror (2), diffraction grating (3), imaging free-form surface mirror (4), it is characterized in that: entrance slit (1) is positioned on front telescopical focal plane, is the picture of front telescope infinite distance target; Diffraction grating (3) is plane grating, and collimation free-form surface mirror (2) and imaging free-form surface mirror (4) are the free form surface without Rotational Symmetry form; It is upper that the aperture diaphragm of system is arranged on plane diffraction grating (3), and collimation free-form surface mirror (2) and imaging free-form surface mirror (4) arrange with respect to grating (3), the center O of the two ₁and O ₂overlap, and and the center of grating on same straight line; The light sending from entrance slit (1) reflexes to diffraction grating (3) through collimation free-form surface mirror (2), light after diffraction grating (3) diffraction light splitting is reflected onto imaging free-form surface mirror (4), is finally reflexed in the image planes of laying detector (5) and is realized light spectrum image-forming by free-form surface mirror (4).

2. a kind of imaging spectrometer beam splitting system based on free form surface according to claim 1, is characterized in that: described collimation free-form surface mirror (2) and the non-rotating symmetrical free form surface of imaging free-form surface mirror (4) are:

$\begin{array}{c}z(x,y)=\frac{c(x^2+y^2)}{1+\sqrt{1-(1+k)\left((x^2+y^2)c^2\right)}}+\\ C_{1}x+C_{2}y+\\ C_3{x}^2+C_4\mathrm{xy}+C_5{y}^2+\\ C_6{x}^3+C_7{x}^{2}y+C_{8}x{y}^2+C_9{y}^3+\\ C_{10}{x}^4+C_{11}{x}^{3}y+C_{12}{x}^2{y}^2+C_{13}x{y}^3+C_{14}{y}^4+\\ C_{15}{x}^5+C_{16}{x}^{4}y+C_{17}{x}^3{y}^2+C_{18}{x}^2{y}^3+C_{19}x{y}^4+C_{20}{y}^2+\\ C_{21}{x}^6+C_{22}{x}^{5}y+C_{23}{x}^4{y}^2+C_{24}{x}^3{y}^3+C_{25}{x}^2{y}^4+C_{26}x{y}^5+C_{27}{y}^6+\\ C_{28}{x}^7+C_{29}{x}^{6}y+C_{30}{x}^5{y}^2+C_{31}{x}^4{y}^3+C_{32}{x}^3{y}^4+C_{33}{x}^2{y}^5+C_{34}x{y}^6+C_{35}{y}^7+\\ ......\end{array}$

Wherein: the curvature of putting centered by c, k is quadratic coefficients, C ₁-C ₃₅for every coefficient, (x, y, z) coordinate for putting on free form surface.

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