阅读说明：本技术 一种光谱仪光学系统及其设计方法 (Spectrometer optical system and design method thereof ) 是由 张佳伦 郑玉权 蔺超 纪振华 是逸 韩艳雪 于 2021-08-31 设计创作，主要内容包括：本发明提供了一种光谱仪光学系统设计方法,包括搭建全反射式光学系统为所述光谱仪光学系统的基础结构；选择球面反射镜作为光谱仪的主镜；选择凸表面为变形光阑的凸面光栅作为所述光谱仪的次镜,所述变形光阑的子午轴不与所述光谱仪的主光轴重合；选择自由曲面反射镜作为所述光谱仪的三镜；利用该方法设计的自由曲面光谱仪具有子午光瞳与弧矢光瞳不等的椭圆形光瞳,相比于传统设计方法,该设计结果可以提高能量收集能力40％以上；在不增加任何光学元件的基础上获得同时实现大视场和大数值孔径的设计需求。(The invention provides a design method of a spectrometer optical system, which comprises the steps of constructing a total reflection type optical system as a basic structure of the spectrometer optical system; selecting a spherical reflector as a main mirror of the spectrometer; selecting a convex grating with a convex surface as a deformation diaphragm as a secondary mirror of the spectrometer, wherein the meridian axis of the deformation diaphragm is not overlapped with the main optical axis of the spectrometer; selecting a free-form surface reflector as a three-mirror of the spectrometer; the free-form surface spectrometer designed by the method has elliptical pupils with different meridional pupils and sagittal pupils, and compared with the traditional design method, the design result can improve the energy collection capacity by more than 40%; the design requirement of simultaneously realizing a large field of view and a large numerical aperture is achieved on the basis of not adding any optical element.)

1. A spectrometer optical system is characterized by comprising an incident slit, a spherical reflector, a convex grating, a free-form surface reflector and a detector;

the incident slit is arranged at a light inlet of a spectrometer and used for providing an object image for the spectrometer;

the spherical reflector is positioned behind the entrance slit and used for reflecting the parallel light incident through the entrance slit;

the convex grating is positioned on a reflection light path of the spherical reflector, and comprises a deformation diaphragm which is used for increasing the numerical aperture of the optical system of the spectrometer;

the free-form surface reflector is positioned on a reflection light path of the convex grating and is used for increasing the field of view of the optical system of the spectrograph and correcting and compensating the aberration of the optical system of the spectrograph;

the detector is positioned at the image surface of the optical system of the spectrometer and used for receiving the light beam reflected by the free-form surface reflector and imaging.

2. The spectrometer optical system as in claim 1, wherein the convex grating is a rectangular convex grating.

3. The spectrometer optical system as claimed in claim 1, wherein the anamorphic diaphragm is an elliptical diaphragm, and the direction of the long axis of the anamorphic diaphragm is the same as the direction of the entrance slit.

4. A spectrometer optical system as claimed in claim 1 or 3, wherein the direction of the entrance slit is a sagittal direction.

5. A method of designing a spectrometer optical system, the method comprising:

building a total reflection type optical system as a basic structure of the optical system of the spectrometer;

selecting a spherical reflector as a primary mirror of the optical system of the spectrometer;

selecting a convex grating with a convex surface being a deformation diaphragm as a secondary mirror of the optical system of the spectrometer, wherein the meridian axis of the deformation diaphragm is not superposed with the main optical axis of the optical system of the spectrometer;

and selecting a free-form surface reflecting mirror as a three-mirror of the optical system of the spectrometer.

6. The spectrometer optical system design method as claimed in claim 5, wherein the anamorphic diaphragm design method comprises the steps of:

determining the non-blocking distance between the lower edge of the spectrometer optical system in the meridian direction and aperture rays, wherein the aperture rays are light beams converged by the spherical reflector;

determining the light passing area of the deformation diaphragm according to the non-blocking distance of the aperture ray to obtain deformation diaphragm parameters, wherein the deformation diaphragm parameters comprise a meridian upper half shaft aR, a meridian lower half shaft bR and a sagittal long half shaft cR;

and step three, setting the vignetting coefficient of the deformation diaphragm according to the deformation diaphragm parameters, and finishing the bias design of the deformation diaphragm so that the meridian axis of the deformation diaphragm is not superposed with the main optical axis of the optical system of the spectrometer.

7. The design method of the spectrometer optical system according to claim 5, wherein the free-form surface reflector is an xy polynomial free-form surface.

Technical Field

The invention belongs to the technical field of spectrometers, and particularly relates to a free-form surface spectrometer optical system with a large field of view and a large numerical aperture and a design method thereof.

Background

The spectrometer in the prior art is a technical approach to a compact spectrometer with a large numerical aperture or a large field of view. Typical technical means include the following:

first, a large numerical aperture design is achieved by adding a meniscus lens in the Offner optical path. As shown in fig. 2, the meniscus lens is positioned in front of the spectrometer slit and the image plane, and penetrates through the incident arm and the converging arm. The use of the meniscus lens can realize the design of large numerical aperture, the meniscus lens on the incident arm plays the role of diverging light path, and the incident angle of light on the primary mirror is reduced. Meanwhile, the addition of the meniscus lens can provide three effective variables to achieve the effect of correcting system aberration. However, the use of a large-sized meniscus adds significant weight to the system, while adding complexity to the mechanical installation. Multiple reflections easily occur in such an optical path, thereby forming ghost images. In actual design, the position of the meniscus lens is often coincided with the grating, so that the purpose of simplifying the structure is achieved. However, this location is not the optimal location and such design results are a compromise choice. This construction is not useful in designs where the system size and weight requirements are high.

Second, a convex grating with a complex profile is used to achieve a large numerical aperture design. The basic surface of the traditional convex grating is a spherical surface, and the basic surface of the complex-surface convex grating is formed by superposing complex surface types on the basis of the spherical surface. Convex gratings with complex surface profiles can provide more aberration correction than conventional convex gratings, and can achieve the best image quality when a compact design is achieved. However, the design of the complex surface type convex grating related by the method is in a confidential state, and the manufacturing of the complex surface type convex grating with high diffraction efficiency cannot be finished at home at present.

And thirdly, the large view field design is realized by dividing the view field by using an optical fiber or a micro lens array. The method can obtain a theoretically large field of view after field splicing. However, the scheme of dividing the view field by multiple lenses not only increases the volume and weight of the instrument, but also puts high requirements on the position precision of the lenses and the relative position between the lenses, increases difficulties for mechanical design and material design, and simultaneously solves the problems of consistency, uniformity and channel balance of triggering of multiple channels.

Fourth, a large field of view design is achieved using a free form surface. The severe aberration caused by a large field of view can be corrected by designing a free-form surface. But it is difficult to obtain a large numerical aperture at the same time due to the requirement of the unobstructed design of the Offner structure. It is inherently difficult for the free-form surface to provide aberration correction capability in both directions simultaneously.

Disclosure of Invention

The invention provides a spectrometer optical system and a design method thereof, aiming at solving the technical defect that the existing spectrometer cannot have a large field of view and a large numerical aperture at the same time. In order to achieve the purpose, the invention adopts the following specific technical scheme:

a spectrometer optical system comprises

The device comprises an incident slit, a spherical reflector, a convex grating, a free-form surface reflector and a detector;

the incident slit is arranged at a light inlet of the spectrometer and used for providing an object image for the spectrometer;

the spherical reflector is positioned behind the entrance slit and used for reflecting parallel light incident through the entrance slit;

the convex grating is positioned on a reflection light path of the spherical reflector and comprises a deformation diaphragm which is used for increasing the numerical aperture of the optical system of the spectrometer;

the free-form surface reflector is positioned on a reflection light path of the convex grating and is used for increasing the field of view of the optical system of the spectrometer and correcting and compensating the aberration of the optical system of the spectrometer;

the detector is positioned at the image surface of the optical system of the spectrometer and used for receiving the light beam reflected by the free-form surface reflector and imaging.

Preferably, the convex grating is a rectangular convex grating.

Preferably, the anamorphic diaphragm is an elliptical diaphragm, and the long axis direction of the anamorphic diaphragm is the same as the direction of the entrance slit.

Preferably, the direction of the entrance slit is a sagittal direction.

A spectrometer optical system design method, comprising:

building a total reflection type optical system as a basic structure of a spectrometer optical system;

selecting a spherical reflector as a main mirror of the spectrometer;

selecting a convex grating with a convex surface as a deformation diaphragm as a secondary mirror of the spectrometer, wherein the meridian axis of the deformation diaphragm is not superposed with the main optical axis of the spectrometer;

and selecting a free-form surface reflecting mirror as a three-mirror of the spectrometer.

Preferably, the method for designing the anamorphic diaphragm comprises the following steps:

determining the non-blocking distance between the lower edge of a spectrometer optical system in the meridian direction and aperture rays, wherein the aperture rays are light beams converged by a spherical reflector;

determining the light passing area of the deformation diaphragm according to the non-blocking distance of the aperture ray to obtain deformation diaphragm parameters, wherein the deformation diaphragm parameters comprise a meridian upper half shaft aR, a meridian lower half shaft bR and a sagittal long half shaft cR;

and step three, setting the vignetting coefficient of the deformation diaphragm according to the deformation diaphragm parameters, and finishing the offset design of the deformation diaphragm to ensure that the meridian axis of the deformation diaphragm is not superposed with the main optical axis of the optical system of the spectrometer.

Preferably, the free-form surface mirror is an xy polynomial free-form surface.

The invention can obtain the following technical effects:

1. the free-form surface spectrometer designed by the invention has elliptical pupils with unequal meridional pupils and sagittal pupils, and compared with the traditional design method, the design result can improve the energy collection capacity by more than 60%.

2. The free-form surface spectrometer of the invention fully plays the role of a free-form surface, redesigns the free-form surface which can correct both large field of view and large numerical aperture, and obtains the Offne type free-form surface spectrometer which can realize both large field of view and large numerical aperture on the basis of not increasing any optical element.

Drawings

FIG. 1 is a schematic diagram of a spectrometer optical system according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a conventional circular pupil;

FIG. 3 is a schematic view of an elliptically deformed pupil according to one embodiment of the present invention;

FIG. 4 is a schematic illustration of increased luminous flux for one embodiment of the present invention;

FIG. 5 is a schematic of the calculations of FIG. 4;

FIG. 6 is a schematic illustration of a anamorphic diaphragm pupil-altering optical track according to one embodiment of the invention;

FIG. 7 is a plot of the transfer function at 400nm for a spectrometer optical system in accordance with an embodiment of the present invention;

FIG. 8 is a graph of energy transfer at 400nm for one embodiment of the present invention;

FIG. 9 is a graph of the transfer function at 1000nm for one embodiment of the present invention;

FIG. 10 is a graph of energy transfer at 400nm for one embodiment of the present invention;

FIG. 11 is a schematic of a light trace based on a classical Offner spectrometer according to an embodiment of the present invention;

FIG. 12 is a flow chart of a spectrometer optics design method of an embodiment.

Reference numerals:

the device comprises a spherical reflector 1, a convex grating 2, a free-form surface reflector 3, an incident slit 4 and an image surface 5.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not to be construed as limiting the invention.

The invention aims to provide a spectrometer optical system and a design method thereof. The following will describe a spectrometer optical system and its design method in detail by specific embodiments.

Fig. 1 is a schematic structural diagram of an optical system of a spectrometer according to the present invention, and includes an entrance slit 4, a spherical mirror 1, a convex grating 2, a free-form surface mirror 3, and a detector located at an image plane 5. An incident slit 4 is arranged at a light inlet of the spectrometer, and a spherical reflector 1, a convex grating 2 and a free-form surface reflector 3 are sequentially arranged along the propagation direction of light beams.

As shown in fig. 1, light emitted from the entrance slit 4 passes through the primary mirror, i.e., the spherical reflector 1, and then is first folded to reach the secondary mirror; the secondary mirror is a convex grating 2, the light beam is subjected to secondary folding at the convex grating 2, meanwhile, due to the action of the convex grating 2, the light beam is subjected to dispersion, and the dispersion light reaches three mirrors, namely a free-form surface reflector 3; the dispersed light is refracted for the third time at the three mirrors and converged at the same time, and finally received by a detector positioned at the image surface 5.

The convex surface grating 2 comprises a deformation diaphragm which is used for increasing the numerical aperture of the optical system;

the free-form surface mirror 3 is used to increase the field of view of the optical system while correcting and compensating for aberrations of the optical system.

In a preferred embodiment of the present invention, as shown in fig. 4, the anamorphic diaphragm is an elliptical diaphragm, and since the direction of the entrance slit 4 in the optical system is set to be the X-axis direction, i.e. the direction of the sagittal, the whole optical system is symmetrical about the YZ plane, so the long axis direction of the anamorphic diaphragm is designed to be the same as the direction of the entrance slit 4, the light transmission area in the direction of the sagittal is increased, and the large numerical aperture design of the optical system is realized.

Meanwhile, the convex surface grating 2 is designed to be rectangular, so that the design requirement of the elliptical diaphragm is met, and unnecessary blocking of the light path of the optical system is avoided.

The following describes in detail a design method of a spectrometer optical system of the present invention, the design method including:

building a total reflection type optical system as a basic structure of a spectrometer optical system;

selecting a spherical reflector as a main mirror of an optical system of the spectrometer;

selecting a convex grating with a convex surface as a deformation diaphragm as a secondary mirror of the optical system of the spectrometer, wherein the meridian axis of the deformation diaphragm is not superposed with the main optical axis of the optical system of the spectrometer;

and selecting a free-form surface reflecting mirror as a three-mirror of the optical system of the spectrometer.

On the basis of the existing free-form surface spectrometer of the reflective optical system for realizing the large-field design, the invention creatively generates a variable pupil by designing an aperture diaphragm, namely a deformable diaphragm, and increases the light passing size of the optical system, thereby meeting the design requirement of increasing the numerical aperture of the optical system.

In a preferred embodiment of the present invention, since the spherical mirror has the characteristics of low cost and easy processing and detection, and the optical system only includes one free-form surface mirror, the difficulty of installation and adjustment of the optical system is greatly reduced, and the current manufacturability requirements are met, so that the spherical mirror is selected as the primary mirror.

Taking the conventional total reflection optical system of Offner structure as an example of the basic structure of the spectrometer optical system of the present invention, the Offner structure spectrometer often uses a circular aperture stop as shown in fig. 2, and the stop position is also often used as a coordinate reference of the optical system. Considering that the Offner structure spectrometer is a "meridional-limited system", that is, as shown in fig. 1, the direction of the entrance slit 4 in the default optical system is the X-axis direction (sagittal direction), the whole optical system is symmetrical about the YZ plane, and the aberration cost caused by the non-obstruction caused by increasing the size of the aperture stop in the meridional direction is very large.

Therefore, a deformation diaphragm is designed, namely the aperture in the meridian direction is ensured to be unchanged, and the function of increasing the numerical aperture of the system and improving the signal-to-noise ratio can be achieved by increasing the aperture in the sagittal direction. In the paper "Analysis method of the hyperspectral imaging spectrometer based on vector interference the term" discusses in detail that the distance between the tertiary mirror and the secondary mirror becomes an effective optimization variable when the system size of the Offner spectrometer becomes the highest priority constraint. Meanwhile, due to the existence of diffraction orders, the aperture diaphragm is a deformation diaphragm which is specially designed according to actual requirements and has a meridian off-axis (compared with the traditional form).

The deformed pupil generated by the deformable diaphragm is shown in fig. 3, the pupil is in an elliptical shape, the meridian direction of the pupil is the minor axis direction of the ellipse, and the sagittal direction is the major axis direction of the ellipse. Referring to fig. 4, a diagram of the optical trace of the deformed pupil generated by the anamorphic diaphragm of fig. 6, where a is the maximum pupil size that can be achieved by the conventional diaphragm, and B is the deformed pupil size proposed by the present invention, and the shaded portion is the increased light-passing area. It can be seen that the optical system still meets the technical requirement of the Offner structure with a large field of view without obstruction in the meridional direction, and the light passing size is greatly increased in the sagittal direction.

Fig. 12 shows a flow of a method for designing an anamorphic diaphragm according to the invention, comprising the following steps:

step one, determining the non-blocking distance l between the lower edge of the optical system in the meridian direction and the aperture ray₂The aperture light is a light beam converged by the spherical reflector;

step two, according to the non-blocking distance l of the aperture light₂Determining the light transmission area of the deformation diaphragm meeting the design requirement, and further obtaining deformation diaphragm parameters, wherein the deformation diaphragm parameters comprise a meridian upper half shaft aR, a meridian lower half shaft bR and a sagittal long half shaft cR;

and step three, setting the vignetting coefficient of the deformation diaphragm according to the deformation diaphragm parameters, and finishing the offset design of the deformation diaphragm to ensure that the meridian axis of the deformation diaphragm is not superposed with the main optical axis of the optical system of the spectrometer.

Because the reflection and dispersion of the convex grating, the incident angle and the diffraction angle of the light beam on the surface of the convex grating are different, the non-blocking distance l between the upper edge and the lower edge of the convex grating and the light beam₂Unequal, the anamorphic diaphragm needs to be designed in an offset way, so that the meridian axis of the anamorphic diaphragm is not superposed with the main optical axis of the optical system of the spectrometer, namely, an off-axis distance l exists₁。

In a preferred embodiment of the invention, the meridian axis of the anamorphic diaphragm is set to have an upward off-axis distance l from the Y axis of the optical system according to the diffraction order of the convex grating₁The phenomenon of 'aperture loss' caused by shielding of the back part of the light spot convex grating reflected to the spherical reflector is avoided.

Referring to fig. 5 in conjunction with the classical Offner-based optical track diagram shown in fig. 11, the method for obtaining the anamorphic diaphragm parameters is as follows:

and taking the radius R of the circular diaphragm as a reference, the meridional upper half shaft of the deformation diaphragm is aR, the lower half shaft of the deformation diaphragm is bR, and the size of the sagittal long half shaft is cR.

We can calculate the area S of the effective clear aperture of the anamorphic diaphragm₁：

Area S of clear aperture of circular diaphragm:

S＝πR² (2)

according to the formula (1) and the formula (2), a group of parameters of the deformation diaphragm can be obtained according to the light transmission area of the designed deformation diaphragm, and several groups of deformation diaphragm parameters meeting the requirements are selected according to the design requirements.

Unblocked distance l between lower edge of aperture diaphragm of Offner structure spectrometer in meridian direction and aperture ray₂The constraining nature is the systematic aberrations. Therefore, we can use this distance as a direct constraint. Guarantee oneFixed non-blocking distance l₂And selecting proper parameters of the deformation diaphragm according to the required light passing area of the deformation diaphragm.

A set of deformation diaphragm parameters meeting the requirements can be approximately obtained through ray tracing data of optical design software. And the designer sets the vignetting coefficient of the diaphragm according to the parameters of the deformation diaphragm to finish the bias design of the deformation diaphragm.

In a preferred embodiment of the present invention, since the spectrometer has high requirements on image quality for the optical system for analyzing the spectrum, and requires a continuous and smooth surface of the optical element, a polynomial characterization method is selected.

Zernike polynomials (Zernike polynomials) are free-form surface characterization forms with strong surface fitting capability and orthogonal characteristics, and each term corresponds to a specific aberration; the XY polynomial (X-Y polymodal) is another characterization form of the free-form surface, and the surface type of Zernike polymodal needs to be converted into the X-Y polymodal characterization form in the design process to realize processing. Therefore, the free-form surface is directly characterized by X-Ypolynomical in the design of the free-form surface of the invention.

Because the optical system is symmetrical about a YZ plane, for this reason, the X odd term in the X-Y polynomial is set to be 0, only the even term needs to be used in the optimization process, and the specific form is as follows:

wherein c is the curvature of the free-form surface reflector;

r is a radius coordinate under a free-form surface reflector unit;

k is a quadric coefficient;

a_iare coefficients of a monomial.

In another preferred embodiment of the invention, a spectral range is designed: 0.4-1 μm; meridional numerical aperture: 0.167; sagittal direction numerical aperture: 0.217; length of entrance slit: 20 mm; the spectral resolution is better than that of a 2.7nm spectrometer.

The effective light transmission capacity of the elliptical pupil newly generated according to the calculation is consistent with that of a circular pupil with the numerical aperture of 0.2. Compared with a circular pupil spectrometer system with the numerical aperture of 0.167, the information collection capacity of the optical system is improved by more than 40%. Meanwhile, the axial size of the optical system is 85mm, and a large view field design result of 20mm is realized.

According to the shape of the pupil, the difficulty in correcting the aberration of the pupil in the sagittal direction of the optical system is very high, the three mirrors in the large field of view bring serious off-axis phase difference, meanwhile, the optical system is in an off-axis asymmetric state, and the derived aberration makes the aberration field of the system more complex, so that the aberration of the optical system is corrected and compensated by means of the adjustment capability of the free-form surface reflector, and the optical system of the spectrometer has a large numerical aperture while realizing the large field of view.

Fig. 7-10 sequentially show MTF and energy curves of the spectrometer optical system at 400nm and 1000nm, respectively, and it can be observed that the MTF of the whole system meets the design requirements of the spectrometer system, the energy concentration ratio of the optical system is above 90%, and the optical system has better image quality.

Therefore, the spectrometer optical system of the invention enables the energy collection capacity of the optical system to reach more than 40% based on the design of the deformation diaphragm, simultaneously gives full play to the function of the free-form surface, redesigns the free-form surface which can correct both the large field of view and the large numerical aperture, and obtains the Offner type free-form surface spectrometer with the large field of view and the large numerical aperture.

In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.

Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.

The above embodiments of the present invention should not be construed as limiting the scope of the present invention. Any other corresponding changes and modifications made according to the technical idea of the present invention should be included in the protection scope of the claims of the present invention.