CN116817765A - A reflective blazed grating spectrometer - Google Patents

Abstract

The invention discloses a reflective blazed grating type spectrometer, which includes: a multimode optical fiber, a plano-convex single lens, a collimating spherical reflector, a planar reflective blazed grating, a focusing spherical reflector, a cylindrical lens, and a detector; the multimode optical fiber The emitted divergent light is condensed by a plano-convex single lens into divergent light with a small aperture angle, and then is collimated by a collimating spherical mirror and turned into parallel light. It is incident on the plane reflection blazed grating at an appropriate angle and is reflected by the plane. After first-order diffraction by the blazed grating, it is focused by a focusing spherical mirror. After further focusing by a cylindrical lens, the light in the non-dispersive direction is finally received by the linear array detector. The invention is suitable for spectral confocal sensing systems and can be used as its built-in spectrometer. It has the characteristics of high throughput, large numerical aperture, high spectral resolution, excellent imaging quality, compact structure and low cost.

Description

A reflective blazed grating spectrometer

Technical field

The invention relates to the optical path structure design of a reflective blazed grating type spectrometer, which is suitable for spectral confocal sensing systems and can be used as its built-in spectrometer.

Background technique

Spectral confocal displacement measurement is a non-contact, high-precision micro-displacement measurement method with outstanding features in terms of precision, accuracy, and portability. Spectral confocal measurement technology makes use of the longitudinal chromatic aberration in optics, that is, when a point on the optical axis is imaged by the imaging system, the position of the focus point will be different due to different wavelengths, thus matching the wavelength and displacement one by one. By detecting the wavelength corresponding to a certain displacement through a spectrometer and analyzing the information at that wavelength, the displacement data can be obtained. Among them, the spectrometer can be considered one of the most important parts. The accuracy of spectral confocal measurement has a direct positive correlation with the measurement accuracy of the spectrometer. Since the light entering the spectrometer is not directly incident from the light source but first passes through the confocal part of the spectrum before entering the spectrometer, the energy received by the spectrometer is greatly attenuated compared to the energy emitted by the light source. Therefore, the spectral confocal shift measurement system has high-throughput and high-resolution requirements for the built-in spectrometer.

For grating spectrometers, there is a natural contradiction between high resolution, high throughput, large numerical aperture and miniaturization, which poses considerable problems for the development of spectrometers. Although the transmission grating spectrometer can achieve a larger numerical aperture, if it wants to achieve high spectral resolution, it will inevitably conflict with the miniaturization of the spectrometer; although the Dyson spectrometer using a concave reflection grating as a dispersion element can also achieve a larger numerical aperture aperture, but the cost of its concave reflection grating is relatively high, and the incident slit is too close to the image plane, making the detector susceptible to stray light. The traditional Czerny–Turner structural spectrometer can achieve higher resolution and eliminate coma from the structure, but due to the influence of off-axis spherical mirror astigmatism, the imaging quality of the system is very poor. At present, there are many improved Czerny–Turner structural spectrometers that use various methods to correct the astigmatism caused by off-axis spherical mirrors, so that they have good imaging quality, but most of their numerical apertures are still less than 0.12.

Contents of the invention

In order to solve the above-mentioned shortcomings of existing spectrometers, the present invention provides a reflective blazed grating type spectrometer, which is expected to be used as a built-in spectrometer of a spectral confocal shift measurement system and can meet the requirements of high throughput, large numerical aperture, and high precision. It has the requirements of excellent spectral resolution, imaging quality, compact structure, and low cost, thereby solving the problems of long integration time and slow acquisition speed caused by energy attenuation caused by optical components in the confocal part of the spectrum.

In order to solve the above problems, the present invention provides the following technical solutions:

The characteristic of the reflective blazed grating type spectrometer of the present invention is that a multimode optical fiber, a plano-convex single lens, a collimating spherical reflector, a planar reflective blazed grating, a focusing spherical reflector, and a cylindrical lens are arranged in sequence according to the direction of the optical axis. ,detector;

The exit port S of the multi-mode optical fiber is the entrance slit of the spectrometer, and the core diameter a of the multi-mode optical fiber is the width of the entrance slit;

The distance _l0 between the plano-convex single lens and the multi-mode optical fiber is less than the focal length of the plano-convex single lens;

The plano-convex single lens is coated with an anti-reflection coating in the visible light band;

The collimating spherical reflector is coated with an anti-reflection coating in the visible light band;

The focusing spherical reflector is coated with an anti-reflective coating in the visible light band;

The cylindrical lens is coated with an anti-reflection coating in the visible light band;

The detector is a linear array charge-coupled device, and the inclination angle between the photosensitive surface of the detector and the plane of the cylindrical lens is

The light beam emitted from the exit port S of the multi-mode optical fiber is condensed by the plano-convex single lens and becomes divergent light with a smaller aperture angle, and is reflected by the collimating spherical reflector on the plano-convex single lens. The divergent light is collimated and turned into parallel light to achieve the lighting conditions of the planar reflection blazed grating;

The plane reflection blazed grating divides the parallel light from the collimating spherical reflector into multiple beams of monochromatic light according to wavelength, and each beam of monochromatic light is parallel light with different diffraction angles;

After the focusing spherical reflector converges the multiple beams of parallel light split by the plane reflection blazed grating, the cylindrical lens focuses the light in the non-dispersion direction from the focusing spherical reflector, so that the dispersion direction and The light in the non-dispersed direction is focused at the same position, and the light beam from the cylindrical lens is received by the detector.

The reflective blazed grating type spectrometer of the present invention is also characterized in that the distance l ₀ between the plano-convex single lens and the multi-mode optical fiber and the distance l _PC between the collimating spherical reflector are Follow these steps to confirm:

Step 1: Use equations (1) to (2) to perform optical path analysis on the plano-convex single lens, and obtain the image distance l _P and image-side aperture angle i ₂ of the plano-convex single lens;

In formulas (1) to (2), i ₀ and i ₁ are the object-side aperture angle and image-side aperture angle of the plano-convex single lens plane respectively, l ₀ and l' ₀ are respectively the plano-convex single lens The object distance and image distance of the plane, n _P is the refractive index of the plano-convex single lens, d represents the thickness of the plano-convex single lens; l' ₀ + d is the object distance of the spherical surface of the plano-convex single lens, i _1. The object-side aperture angle of the spherical surface of the plano-convex single lens, α ₁ and α ₂ are respectively the incident angle and the refraction angle of the spherical surface of the plano-convex single lens;

Step 2: Use equation (3) to constrain the image-side aperture angle i ₂ to constrain the distance l ₀ between the plano-convex single lens and the multi-mode optical fiber;

2f _C tan i ₂ <D _C cos I ₁ (3)

In formula (3), f _C and D _C are the focal length and diameter of the collimating spherical reflector respectively, and I ₁ is the off-axis angle of the collimating spherical reflector;

Step 3: Use equation (4) to determine the distance l _PC between the plano-convex single lens and the collimating spherical reflector;

f _C cos I ₁ = l _P + l _PC (4)

The characteristic of the reflective blazed grating type spectrometer of the present invention is that the distance l _FL between the cylindrical lens and the focusing spherical mirror and the distance l _LD between the cylindrical lens and the detector are determined by the following steps :

Step 1: Use equations (5) to (7) to analyze the optical path of the light passing through the collimating spherical reflector, planar reflection blazed grating, and focusing spherical reflector;

In formulas (5) to (7), l' _t1 and l' _s1 are the image distances of the meridional and sagittal planes of the collimating spherical reflector respectively; R ₁ and I ₁ are respectively the collimating spherical reflector. The radius of curvature and off-axis angle; l' _t2 and l' _s2 are the image distances of the meridional and sagittal planes of the plane reflective blazed grating respectively; i and θ are respectively the incident angle and diffraction angle of the plane reflective blazed grating; l' _t3 and l' _s3 are the image distances of the meridional and sagittal planes of the focusing spherical reflector respectively; R ₂ and I ₂ are the curvature radius and off-axis angle of the focusing spherical reflector respectively;

Step 2: Use equations (8) to (9) to conduct optical path analysis on the convex surface and flat surface of the cylindrical lens respectively;

In formulas (8) to (9), l' _t4 and l' _s4 are the image distances of the meridional plane and the sagittal plane of the convex surface of the cylindrical lens respectively; R ₃ is the radius of curvature of the convex surface of the cylindrical lens; n _L is the refractive index of the cylindrical lens; d _L is the center thickness of the cylindrical lens; l' _t and l' _s are the image distances of the meridional plane and the sagittal plane of the cylindrical lens plane, respectively, that is, the spectrometer The final image distance of the meridional and sagittal planes;

Step 3: Let the final image distances l' _t and l' _s of the meridional and sagittal planes be equal to obtain the distance l _FL between the cylindrical lens and the focusing spherical mirror, and l' _t and l' _s Both are equal to the distance l _LD between the cylindrical lens and the detector, so that both the meridional plane and the sagittal plane of the spectrometer are focused on the photosensitive surface of the detector.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention uses a plano-convex single lens to converge the divergent light from multi-mode optical fiber, solves the contradiction between high throughput and miniaturization of the reflection spectrometer, and avoids incomplete energy absorption when the spectrometer operates under a large numerical aperture. And the issue of waste;

2. The present invention uses a cylindrical lens to selectively focus the light from the focusing spherical mirror, and only focuses the light in the non-dispersion direction, which solves the problem of poor imaging quality and low energy transfer efficiency when the spectrometer works under a large numerical aperture. question;

3. This invention adds a plano-convex single lens and a cylindrical lens to the traditional Czerny–Turner structural spectrometer, so that it can meet the requirements of high resolution, high throughput, large numerical aperture and miniaturization, while also having production The advantages of low difficulty and low cost.

Description of the drawings

Figure 1 is a structural diagram of the optical path of the reflective blazed grating spectrometer of the present invention;

Figure 2 is an optical path structure diagram of a plano-convex single lens in the spectrometer of the present invention;

Figure 3 is a structural diagram of the optical path of the meridional plane and sagittal plane of the cylindrical lens in the spectrometer of the present invention;

Figure 4 is a comparison chart of the optical performance of the spectrometer of the present invention and the traditional Czerny–Turner structure spectrometer;

Numbers in the figure: 10 multimode optical fiber; S multimode optical fiber initial port; 20 plano-convex single lens; 30 collimating spherical reflector; 40 planar reflection blazed grating; 50 focusing spherical reflector; 60 cylindrical lens; 601 cylindrical lens Lens meridional section; 602 cylindrical lens sagittal section; 70 linear array detector; 701 linear array detector meridional section; 702 linear array detector sagittal section.

Detailed ways

In this embodiment, as shown in Figure 1, a reflective blaze grating type spectrometer is provided with a multimode optical fiber 10, a plano-convex single lens 20, a collimating spherical mirror 30, and a planar reflection blaze grating in sequence according to the direction of the optical axis. Grating 40, focusing spherical mirror 50, cylindrical lens 60, detector 70;

The input port of the multimode optical fiber 10 is monochromatic light obtained by confocal reflection from a white light source with a spectral range of 400nm to 700nm; the output port S of the multimode optical fiber 10 is used as the entrance slit of the spectrometer, and the core of the multimode optical fiber 10 The diameter a is used as the entrance slit width; the numerical aperture of the multimode optical fiber 10 is determined by the output numerical aperture of the spectral confocal system; the input numerical aperture NA of the spectrometer is not greater than the numerical aperture of the optical fiber, which is generally 0.2±0.02;

The distance l ₀ between the plano-convex single lens 20 and the multi-mode optical fiber 10 is less than the focal length of the plano-convex single lens 20;

The plano-convex single lens 20 is coated with an anti-reflection coating in the visible light band;

The collimating spherical reflector 30 is coated with an anti-reflection coating in the visible light band;

The focusing spherical reflector 50 is coated with an anti-reflective coating in the visible light band;

The cylindrical lens 60 is coated with an anti-reflection coating in the visible light band;

The detector 70 is a linear array charge-coupled device, and the inclination angle between the photosensitive surface of the detector 70 and the plane of the cylindrical lens 60 is

After the light beam emitted from the exit port S of the multimode optical fiber 10 is converged by the plano-convex single lens 20, it becomes divergent light with a smaller aperture angle, and the divergent light of the plano-convex single lens 20 is collimated by the collimating spherical reflector 30. Straight, making it become parallel light to achieve the lighting conditions of the plane reflective blazed grating 40;

The planar reflection blazed grating 40 divides the parallel light from the collimating spherical reflector 30 into multiple beams of monochromatic light according to wavelength, and each beam of monochromatic light is a parallel light with different diffraction angles;

After the focusing spherical reflector 50 converges the multiple parallel light beams split by the planar reflective blazed grating 40, the cylindrical lens 60 focuses the light in the non-dispersion direction from the focusing spherical reflector 50, so that the dispersion direction and the non-dispersion direction are The light in the direction is focused at the same position, and the detector 70 receives the beam from the cylindrical lens 60 .

According to the design requirements of the spectral resolution Δλ, the known core diameter a of the multimode optical fiber 10 and the grating groove spacing d of the planar reflection blazed grating 40, the focal length f _C of the collimating spherical reflector 30 is determined according to equation (1).

The structure of the plano-convex single lens 20 is shown in Figure 2. In order to ensure that all the divergent light energy collected by the plano-convex lens 20 is collimated by the collimating spherical reflector 30, the distance l between the plano-convex single lens 20 and the multi-mode optical fiber 10 is _0. The distance l _PC between 0 and the collimating spherical reflector 30 is determined according to the following steps:

Step 1: Use equations (2) to (3) to analyze the optical path of the plano-convex single lens (20), and obtain the image distance l _P and image-side aperture angle i ₂ of the plano-convex single lens (20);

In formulas (2) to (3), i ₀ and i ₁ are the object-side aperture angle and image-side aperture angle of the plano-convex single lens 20 plane respectively, l ₀ and l' ₀ are respectively the object-side aperture angle and image-side aperture angle of the plano-convex single lens 20 plane Object distance and image distance, n _P is the refractive index of the plano-convex single lens 20, d represents the thickness of the plano-convex single lens 20; l' ₀ + d is the object distance of the spherical surface of the plano-convex single lens 20, i ₁ plano-convex single lens The object-square aperture angle of the spherical surface 20, α ₁ and α ₂ are respectively the incident angle and refraction angle of the spherical surface of the plano-convex single lens 20;

Step 2: Use equation (4) to constrain the image-side aperture angle i ₂ to constrain the distance l ₀ between the plano-convex single lens 20 and the multimode optical fiber 10;

2f _C tan i ₂ <D _C cos I ₁ (4)

In formula (4), f _C and D _C are the focal length and diameter of the collimating spherical reflector 30 respectively, and I ₁ is the off-axis angle of the collimating spherical reflector (30);

Step 3: Use equation (5) to determine the distance l _PC between the plano-convex single lens 20 and the collimating spherical reflector 30;

f _C cos I ₁ = l _P + l _PC (5)

The structure of the cylindrical lens 60 is shown in Figure 3. In order to achieve good imaging quality and low RMS radius, the distance l FL between the cylindrical lens (60) and the focusing spherical mirror 50, and the distance l _FL between the detector 70 The distance l _LD is determined by the following steps:

Step 1: Use equations (6) to (8) to analyze the optical path of the light passing through the collimating spherical reflector 30, the planar reflection blazed grating 40, and the focusing spherical reflector 50;

In formulas (6) to (8), l' _t1 and l' _s1 are the image distances of the meridional and sagittal planes of the collimating spherical reflector 30 respectively; R ₁ and O ₁ are the curvatures of the collimating spherical reflector 30 respectively. radius and off-axis angle; l' _t2 and l' _s2 are the image distances of the meridional and sagittal planes of the plane reflective blazed grating 40 respectively; i and θ are respectively the incident angle and diffraction angle of the plane reflective blazed grating 40; l' _t3 and l' _s3 are the image distances of the meridional and sagittal planes of the focusing spherical reflector 50 respectively; R ₂ and I ₂ are the curvature radius and off-axis angle of the focusing spherical reflector 50 respectively;

Step 2: Use equations (9) to (10) to conduct optical path analysis on the convex surface and flat surface of the cylindrical lens 60 respectively;

In formulas (9) to (10), l' _t4 and l' _s4 are the image distances of the meridional plane and the sagittal plane of the convex surface of the cylindrical lens 60 respectively; R ₃ is the convex surface curvature radius of the cylindrical lens 60; n _L is The refractive index of the cylindrical lens 60; d _L is the center thickness of the cylindrical lens 60; l' _t and l' _s are the image distances of the meridional plane and the sagittal plane of the cylindrical lens 60 plane, respectively, that is, the meridional plane and sagittal plane of the spectrometer. The final image distance in the sagittal plane;

Step 3: Let the final image distances l' _t and l' _s of the meridional and sagittal planes be equal to obtain the distance l _FL between the cylindrical lens 60 and the focusing spherical mirror 50, and l' _t and l' _s are both equal to The distance l _LD between the cylindrical lens 60 and the detector 70 is equal, so that both the meridional plane and the sagittal plane of the spectrometer are focused on the photosensitive surface of the detector 70 .

The off-axis angle I ₁ and the radius of curvature R ₁ of the collimating spherical mirror 30, the incident angle i and the diffraction angle θ of the planar reflection blazed grating 40, the off-axis angle I ₂ and the radius of curvature R ₂ of the focusing spherical mirror 50 are given by the formula (11) Make constraints to eliminate coma.

After determining the length l _D of the detector 70, in order to ensure that all the light collected by the focusing spherical reflector 50 can be accepted by the detector 70, the maximum focal length of the focusing spherical reflector 50 is constrained by Equation (12).

In equation (12), λ ₂ and λ ₁ are the maximum wavelength and the minimum wavelength respectively.

Figure 4 shows a comparison chart of the optical performance of the spectrometer of the present invention and the traditional Czerny–Turner structure spectrometer. The root mean square radius of the image of the point light source is used as the standard for judging the optical performance. Since the traditional Czerny–Turner structural spectrometer corrects the astigmatism produced by the off-axis spherical mirror, its root mean square radius in the wide spectral range is greater than 200 μm; the reflective blazed grating type spectrometer designed in this embodiment has the average average in the wide spectral band. The overall square root radius is less than 11 μm, which shows that its optical performance is better.

Claims (3)

1. A reflective blaze grating type spectrometer, characterized in that a multimode optical fiber (10), a plano-convex single lens (20), a collimating spherical reflector (30), and a planar reflection blaze are arranged in sequence according to the direction of the optical axis. Grating (40), focusing spherical mirror (50), cylindrical lens (60), detector (70); The exit port S of the multimode optical fiber (10) is the entrance slit of the spectrometer, and the core diameter a of the multimode optical fiber (10) is the width of the entrance slit; The distance l ₀ between the plano-convex single lens (20) and the multimode optical fiber (10) is less than the focal length of the plano-convex single lens (20); The plano-convex single lens (20) is coated with an anti-reflection coating in the visible light band; The collimating spherical reflector (30) is coated with an anti-reflection coating in the visible light band; The focusing spherical reflector (50) is coated with an anti-reflective coating in the visible light band; The cylindrical lens (60) is coated with an anti-reflection coating in the visible light band; The detector (70) is a linear array charge-coupled device, and the inclination angle between the photosensitive surface of the detector (70) and the plane of the cylindrical lens (60) is After the light beam emitted from the exit port S of the multi-mode optical fiber (10) is converged by the plano-convex single lens (20), it becomes divergent light with a smaller aperture angle and is reflected by the collimating spherical reflector (30). ) collimate the divergent light of the plano-convex single lens (20) so that it becomes parallel light to achieve the lighting conditions of the planar reflective blazed grating (40); The plane reflection blazed grating (40) divides the parallel light from the collimating spherical reflector (30) into multiple beams of monochromatic light according to wavelength, and each beam of monochromatic light is parallel light with different diffraction angles; After the focusing spherical mirror (50) converges multiple parallel beams of light split by the planar reflection blazed grating (40), the non-linear light from the focusing spherical mirror (50) is focused by the cylindrical lens (60). The light in the dispersion direction is focused so that the light in the dispersion direction and the non-dispersion direction are focused at the same position, and the light beam from the cylindrical lens (60) is received by the detector (70). 2. The reflective blazed grating type spectrometer according to claim 1, characterized in that the distance l ₀ between the plano-convex single lens (20) and the multi-mode optical fiber (10) and the collimating sphere The distance l _PC between the surface reflectors (30) is determined as follows: Step 1: Use equations (1) to (2) to perform optical path analysis on the plano-convex single lens (20), and obtain the image distance l _P and image-square aperture angle i ₂ of the plano-convex single lens (20); In formulas (1) to (2), i ₀ and i ₁ are respectively the object-side aperture angle and image-side aperture angle of the plane of the plano-convex single lens (20), l ₀ and l' ₀ are respectively the plane The object distance and image distance of the plane of the convex single lens (20), n _P is the refractive index of the plano-convex single lens (20), d represents the thickness of the plano-convex single lens (20); l' ₀ + d is The object distance of the spherical surface of the plano-convex single lens (20), i ₁ is the object-side aperture angle of the spherical surface of the plano-convex single lens (20), α ₁ and α ₂ are respectively the spherical object distance of the plano-convex single lens (20) Angle of incidence and angle of refraction; Step 2: Use equation (3) to constrain the image-side aperture angle i ₂ to constrain the distance l ₀ between the plano-convex single lens (20) and the multi-mode optical fiber (10); 2f _C tan i ₂ <D _C cos l ₁ (3) In formula (3), f _C and D _C are the focal length and diameter of the collimating spherical reflector (30) respectively, and I ₁ is the off-axis angle of the collimating spherical reflector (30); Step 3: Use equation (4) to determine the distance l _PC between the plano-convex single lens (20) and the collimating spherical reflector (30); f _C cos I ₁ = l _P + l _PC (4) 3. The reflective blazed grating type spectrometer according to claim 1, characterized in that the distance l _FL between the cylindrical lens (60) and the focusing spherical mirror (50) and the detector The distance l _LD between (70) is determined by the following steps: Step 1: Use equations (5) to (7) to perform optical path analysis on the light passing through the collimating spherical reflector (30), plane reflection blazed grating (40), and focusing spherical reflector (50); In formulas (5) to (7), l' _t1 and l' _s1 are the image distances of the meridional and sagittal planes of the collimating spherical reflector (30) respectively; R ₁ and I ₁ are respectively the collimating spheres. The radius of curvature and off-axis angle of the surface mirror (30); l' _t2 and l' _s2 are the image distances of the meridional and sagittal surfaces of the plane reflection blazed grating (40) respectively; i and θ are respectively the plane reflection The incident angle and diffraction angle of the blazed grating (40); l' _t3 and l' _s3 are the image distances of the meridional and sagittal planes of the focusing spherical mirror (50) respectively; R ₂ and I ₂ are respectively the focusing spherical surfaces The radius of curvature and off-axis angle of the reflector (50); Step 2: Use equations (8) to (9) to conduct optical path analysis on the convex surface and flat surface of the cylindrical lens (60) respectively; In formulas (8) to (9), l' _t4 and l' _s4 are the image distances of the meridional plane and the sagittal plane of the convex surface of the cylindrical lens (60) respectively; R ₃ is the cylindrical lens (60) The radius of convex curvature of the cylindrical lens (60); n _L is the refractive index of the cylindrical lens (60); d _L is the center thickness of the cylindrical lens (60); l' _t and l' _s are respectively the cylindrical lens (60) 60) The image distance between the meridional plane and the sagittal plane of the plane, that is, the final image distance between the meridional plane and the sagittal plane of the spectrometer; Step 3: Let the final image distance l' _t and l' _s of the meridional plane and the sagittal plane be equal to obtain the distance l _FL between the cylindrical lens (60) and the focusing spherical mirror (50), and l ' _t and l' _s are both equal to the distance l _LD between the cylindrical lens (60) and the detector (70), so that both the meridional plane and the sagittal plane of the spectrometer are focused on the detector (70 ) on the photosensitive surface.