A device for measuring the working distance of a mid-infrared convex diffraction grating

Abstract

The present invention relates to the technical field of diffraction element measurement, and in particular to a device for measuring the working distance of a mid-infrared convex diffraction grating. The device comprises a semiconductor laser, a polarization device, a collimating beam expansion optical component, a concave standard mirror, a plane scraper mirror, a convex grating to be measured, a focusing optical component and a CCD camera, which are sequentially arranged along an optical path: wherein the visible measurement laser emitted by the semiconductor laser becomes linearly polarized light after passing through the polarization device; the linearly polarized light outputs the expanded beam through the collimating beam expansion optical component, and then passes through the central holes of the concave standard mirror and the plane scraper mirror to irradiate the convex grating to be measured; the diffracted light returns along the original optical path to the reflective focusing optical component that passes through the concave standard mirror and the plane scraper mirror in sequence, and the focused beam is then irradiated into the CCD camera. The present invention obtains the working distance of the convex grating to be measured by measuring the spacing between the concave standard mirror and the convex grating to be measured, and the measurement is simple and intuitive.

Description

Technical Field

[0001] The invention relates to the technical field of diffraction element measurement, and in particular to a device for measuring the working distance of a mid-infrared convex diffraction grating. Background Art

[0002] Mid-infrared lasers have important applications in laser spectroscopy, laser ranging, environmental monitoring, optoelectronic countermeasures, and medical and health care. Some gas lasers such as HF / DF lasers, CO lasers, and CO 2 Lasers and other devices all work in this band. Most of these lasers achieve laser radiation through molecular vibration-rotation transitions, so they are multi-wavelength light-emitting systems with very rich output spectra. In many laser applications, lasers are required to output single wavelength or narrow linewidth lasers, so laser line selection technology is needed to select the spectra of these lasers. Diffraction gratings are a good laser wavelength selection device.

[0003] A diffraction grating is a diffraction element with a periodic or quasi-periodic structure. It can modulate the amplitude or phase of the incident light and diffract the incident light of different wavelengths to different directions, thereby separating the single-wavelength light from the multi-wavelength light. In a line-selective laser, a diffraction grating is usually used as a cavity mirror in the laser resonant cavity, so that the light of the required output wavelength oscillates and amplifies in the resonant cavity, and the light of other wavelengths diffracts out of the resonant cavity, thereby achieving the purpose of single-wavelength laser output. In a laser using a stable cavity, a plane diffraction grating can be used as the cavity mirror of the resonant cavity, while in an unstable cavity laser, a convex or concave diffraction grating is required as the cavity mirror of the resonant cavity, which requires determining the working distance of the diffraction grating. For spherical reflectors, the radius of curvature can be measured by a contact or non-contact profilometer to obtain its focal length. For diffraction gratings, because their surface cannot be touched and they cannot respond to the light source of conventional measuring equipment, traditional contact or non-contact measurement methods are not applicable. For concave gratings, a parallel light beam can be used to illuminate them, and their working distance can be determined by the position of the convergence point of the diffracted light. For convex gratings, since their diffraction of light is divergent, the working distance of the grating cannot be directly obtained.

[0004] From the above analysis, in order to meet the requirements of line selection resonant cavity of mid-infrared gas laser, a device is needed that can accurately, effectively and simply measure the working distance of mid-infrared convex diffraction grating. Summary of the invention

[0005] The purpose of the present invention is to overcome the above-mentioned technical problems and provide a device for measuring the working distance of a mid-infrared convex diffraction grating. The device uses a visible light source to directly measure the working distance of the convex grating. It is simple, effective and accurate, and has important practical significance in the application of grating line selection in mid-infrared gas lasers.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The invention provides a device for measuring the working distance of a mid-infrared convex diffraction grating, comprising a semiconductor laser, a polarizing device, a collimating beam expanding optical component, a concave standard mirror, a plane scraper mirror, a convex grating to be measured, a focusing optical component and a CCD camera which are sequentially arranged along an optical path, wherein the visible measuring laser emitted by the semiconductor laser is converted into linearly polarized light through the polarizing device, and then outputs the expanded beam light through the collimating beam expanding optical component, the expanded beam light sequentially passes through the central holes of the concave standard mirror and the plane scraper mirror to irradiate the convex grating to be measured, after being diffracted by the convex grating to be measured, the diffracted light returns along the original optical path, passes through the central hole of the plane scraper mirror again, and then irradiates the reflecting surface of the concave standard mirror, is reflected by the reflecting surfaces of the concave standard mirror and the plane scraper mirror, and then is focused by the focusing optical component and irradiates the CCD camera.

[0008] The polarization device includes a polarization cube and a half-wave plate arranged in sequence along the optical path. The measuring laser becomes linearly polarized light after passing through the polarization cube. The half-wave plate is rotated to make the polarization direction of the linearly polarized light consistent with the polarization direction of the incident light required by the convex grating to be measured.

[0009] The collimating and beam expanding optical component comprises a reflector I, a reflector II, an off-axis concave mirror I and an off-axis concave mirror II which are sequentially arranged along the optical path. The linearly polarized light output by the polarizing device is sequentially reflected and redirected by the reflector I and the reflector II, and then sequentially collimated and expanded by the off-axis concave mirror I and the off-axis concave mirror II to become parallel light.

[0010] A pinhole is arranged between the off-axis concave mirror I and the off-axis concave mirror II, and the focal points of the off-axis concave mirror I and the off-axis concave mirror II coincide with each other and are located at the center of the pinhole.

[0011] The convex grating to be measured is placed at a self-collimation angle, and after reflecting the incident parallel light, it returns along the original light path, passes through the central hole of the plane scraper mirror, and irradiates the mirror surface of the concave standard mirror.

[0012] The focusing optical component includes an off-axis concave mirror III and an off-axis convex mirror arranged in sequence along the optical path. The light reflected by the plane scraper mirror is reflected and focused by the off-axis concave mirror III and the off-axis convex mirror in sequence, and then irradiated into the CCD camera.

[0013] The shape and size of the central hole of the concave standard mirror are the same as the shape and size of the projection of the surface area to be measured of the convex grating to be measured on a plane perpendicular to the optical axis;

[0014] The shape and size of the center hole on the plane scraper mirror projected on a plane perpendicular to the optical axis are the same as the shape and size of the center hole on the concave standard mirror.

[0015] The convex grating to be tested is placed on a grating adjustment frame, and the grating adjustment frame is placed on a three-dimensional electric displacement platform.

[0016] The number of light spot pixels on the image plane of the CCD camera is measured by moving the concave standard mirror along the direction of the incident light optical axis; when the light spot shape is circular and the number of pixels reaches the minimum, the virtual focus of the convex grating to be measured coincides with the focus of the concave standard mirror, and the distance between the center of the concave standard mirror and the center of the convex grating to be measured is L, then

[0017] f 2 =f 1 -L

[0018] Among them, f 1 is the focal length of the concave standard mirror, f 2 is the working distance of the convex grating to be measured.

[0019] The concave standard mirror is placed on a translation stage which is movable along the optical axis of the incident light.

[0020] The present invention has the following advantages and beneficial effects:

[0021] 1. The present invention forms a reflective confocal optical system with a convex grating and a concave standard mirror. By adjusting the distance between the two optical elements, the working distance of the convex grating is calculated according to the focal length of the concave standard mirror. The method is simple and intuitive.

[0022] 2. The present invention uses visible light as the measuring light source, the processing and coating of optical elements in the optical system are very mature, and the adjustment of the optical system is also more convenient.

[0023] 3. The main optical element used in the measuring device of the present invention is a reflective element, which avoids the influence of element chromatic aberration. For measuring the working distance of the convex grating of different diffraction wavelengths, it is only necessary to replace the visible light source of the corresponding wavelength. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a schematic structural diagram of a device for measuring the working distance of a mid-infrared convex diffraction grating.

[0025] In the figure: 1 is a laser, 2 is a polarization cube, 3 is a half-wave plate, 4 is a reflector I, 5 is a reflector II, 6 is an off-axis concave mirror I, 7 is a pinhole, 8 is an off-axis concave mirror II, 9 is a concave standard mirror, 10 is a plane scraper mirror, 11 is a convex grating to be measured, 12 is an off-axis concave mirror III, 13 is an off-axis convex mirror, and 14 is a CCD camera. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] As attached Figure 1 As shown, a device for measuring the working distance of a mid-infrared convex diffraction grating comprises a semiconductor laser 1, a polarizing device, a collimating beam expanding optical component, a concave standard mirror 9, a plane scraper mirror 10, a convex grating to be measured 11, a focusing optical component and a CCD camera 14 which are arranged in sequence along an optical path, wherein the semiconductor laser 1 is used to emit a visible measuring laser; the measuring laser becomes linearly polarized light after passing through the polarizing device, and then outputs the expanded light through the collimating beam expanding optical component, the expanded light passes through the central holes of the concave standard mirror 9 and the plane scraper mirror 10 in sequence and irradiates the convex grating to be measured 11, after being diffracted by the convex grating to be measured 11, the diffracted light returns along the original optical path, passes through the central hole of the plane scraper mirror 10 again and irradiates the reflecting surface of the concave standard mirror 9, after being reflected by the concave standard mirror 9, it is incident on the reflecting surface of the plane scraper mirror 10 and is reflected, and the reflected light is focused by the focusing optical component and irradiates the CCD camera 14.

[0028] In an embodiment of the present invention, the polarization device includes a polarization cube 2 and a half-wave plate 3 arranged in sequence along the optical path. The measuring laser becomes linearly polarized light after passing through the polarization cube 2. The half-wave plate 3 is rotated so that the polarization direction of the linearly polarized light is consistent with the polarization direction of the incident light required by the convex grating 11 to be measured.

[0029] In an embodiment of the present invention, the collimating and expanding optical component includes a reflector I4, a reflector II5, an off-axis concave mirror I6 and an off-axis concave mirror II8 arranged in sequence along the optical path. The linearly polarized light output by the polarization device is reflected and redirected by the reflector I4 and the reflector II5 in sequence, and then the linearly polarized light is adjusted in incident direction and then collimated and expanded by the off-axis concave mirror I6 and the off-axis concave mirror II8 in sequence to become parallel light.

[0030] Furthermore, a pinhole 7 is provided between the off-axis concave mirror I6 and the off-axis concave mirror II8, the focal points of the off-axis concave mirror I6 and the off-axis concave mirror II8 overlap, the distance between the center of the off-axis concave mirror I6 and the center of the off-axis concave mirror II8 is equal to the sum of the focal length of the off-axis concave mirror I6 and the focal length of the off-axis concave mirror II8; the overlapping focal point of the off-axis concave mirror I6 and the off-axis concave mirror II8 is located at the center of the pinhole 7. The parallel light beam emitted by the off-axis concave mirror II8 passes through the central hole of the concave standard mirror 9 and the central hole of the plane scraper mirror 10 and irradiates onto the convex grating 11 to be measured.

[0031] Specifically, the convex grating 11 to be measured is placed at a self-collimation angle, and after reflecting the incident parallel light, it returns along the original light path through the central hole of the flat scraper mirror 10 and irradiates the mirror surface of the concave standard mirror 9 .

[0032] In the embodiment of the present invention, the focusing optical assembly includes an off-axis concave mirror III 12 and an off-axis convex mirror 13 arranged in sequence along the optical path. The light reflected by the plane scraper mirror 10 is reflected and focused by the off-axis concave mirror III 12 and the off-axis convex mirror 13 in sequence, and then irradiated to the CCD camera 14. The image plane of the CCD camera 14 is on the focal plane of the optical system composed of the off-axis concave mirror III 12 and the off-axis convex mirror 13.

[0033] Specifically, the light emitted by the semiconductor laser 1 is visible light, and its wavelength is the Nth frequency of the wavelength of the diffraction of the convex grating to be measured; the incident angle of the convex grating 11 to be measured corresponds to the -Nth order diffraction angle of the convex grating 11 to be measured to the visible light. The shape and size of the central hole of the concave standard mirror 9 are the same as the shape and size of the surface area to be measured of the convex grating 11 to be measured projected on a plane perpendicular to the optical axis; the incident angle of the plane scraper mirror 10 is 45°, and the shape and size of the central hole on the plane scraper mirror 10 projected on a plane perpendicular to the optical axis are the same as the shape and size of the central hole on the concave standard mirror 9.

[0034] In the embodiment of the present invention, the convex grating 11 to be measured is placed on a grating adjustment frame, which is placed on a three-dimensional electric translation stage. The concave standard mirror 9 is placed on a translation stage that is movable along the optical axis of the incident light.

[0035] During operation, the number of light spot pixels on the image plane of the CCD camera 14 is measured by moving the concave standard mirror 9 along the direction of the incident light optical axis; when the light spot shape is circular and the number of pixels reaches the minimum, the virtual focus of the convex grating 11 to be measured coincides with the focus of the concave standard mirror 9, and the distance between the center of the concave standard mirror 9 and the center of the convex grating 11 to be measured is L, then

[0036] f 2 =f 1 -L

[0037] Among them, f 1 is the focal length of the concave standard mirror 9, which can be directly determined by the profilometer, f 2 is the working distance of the convex grating 11 to be measured. Therefore, the working distance f of the convex grating 11 to be measured can be obtained from the above formula 2 .

[0038] The present invention provides a device for measuring the working distance of a mid-infrared convex diffraction grating, and its specific working principle is: a visible measuring laser emitted by a semiconductor laser 1 passes through a polarization cube 2 and a half-wave plate 3 in sequence, is reflected by a reflector I4 and a reflector II5, is reflected by an off-axis concave mirror I6, passes through a pinhole 7, is reflected by an off-axis concave mirror II8, passes through the center hole of a concave standard mirror 9 and the center hole of a plane scraper mirror 10 in sequence, and is irradiated onto a convex grating 11 to be measured; after diffraction by the convex grating 11 to be measured, the diffracted light returns along the original optical path, passes through the center hole of the plane scraper mirror 10 again, and is irradiated onto a reflecting surface of the concave standard mirror 9; after reflection by the concave standard mirror 9, it is incident on the reflecting surface of the plane scraper mirror 10 and is reflected; the reflected light is reflected by an off-axis concave mirror III 12 and an off-axis convex mirror 13 in sequence, and is irradiated onto a CCD camera 14.

[0039] The diffraction wavelength of the convex grating 11 to be measured is set to λ 1 , the measurement laser wavelength is set to λ 2 , according to the grating equation

[0040] Λ(sinα+sinβ)=Nλ

[0041] Differentiating the above formula gives the following formula:

[0042] ΔΛ(sinα+sinβ) / Nλ=Δλ / λ

[0043] Where α is the incident angle, β is the diffraction angle, Λ is the grating period, N is the diffraction order, and λ is the wavelength. 2 =λ 1 / N, the convex grating to be measured 11 pairs of measuring laser λ 2 The Nth order diffraction imaging and the convex grating to be measured 11 pairs of diffraction wavelength λ 1 Therefore, the convex grating 11 to be measured is used to measure the laser λ 2 The working distance of the convex grating 11 to be measured is measured by imaging the Nth order of diffraction.

[0044] When the working distance of the convex grating 11 to be measured is measured, the convex grating 11 to be measured is placed on the grating adjustment frame, the grating adjustment frame is placed on the three-dimensional electric translation stage, and the three-dimensional electric translation stage and the grating adjustment frame are adjusted so that the optical axis of the incident light passes through the geometric center of the grating surface of the convex grating 11 to be measured. The incident angle of the convex grating 11 to be measured is adjusted to correspond to the diffraction wavelength λ 1 The self-collimation angle, that is, for λ 1 is the first-order diffraction, for λ 2 is the Nth order diffraction. The concave standard mirror 9 is placed on a translation stage, which can move along the optical axis direction of the light passing through the central hole of the concave standard mirror 9. The semiconductor laser 1 emits a visible measurement laser, and the wavelength of the measurement laser is λ2 , the measuring laser becomes linearly polarized light after passing through the polarization cube 2, and the half-wave plate 3 is rotated to make the polarization direction of the linearly polarized light consistent with the polarization direction of the incident light required by the convex grating 11 to be measured. This beam of linearly polarized light is reflected by the reflector Ⅰ4 and the reflector Ⅱ5, and after adjusting the incident azimuth, it is incident on the off-axis concave mirror Ⅰ6, reflected and focused by the off-axis concave mirror Ⅰ6, passes through the pinhole 7, and then expanded into a parallel beam after reflection by the off-axis concave mirror Ⅱ8. The off-axis concave mirror Ⅰ6 and the off-axis concave mirror Ⅱ8 are confocal, forming a reflective beam expansion system. The parallel light beam emitted by the off-axis concave mirror Ⅱ8 passes through the central hole of the concave standard mirror 9 and the central hole of the plane scraper mirror 10, and is irradiated onto the convex grating 11 to be measured. The focal length of the concave standard mirror 9 is f 1 , the working distance of the convex grating 11 to be measured is f 2 . After the measuring light is diffracted by the Nth order of the convex grating 11 to be measured, it becomes a divergent spherical wave and returns along the original incident light path. This beam of diffracted light passes through the central hole of the plane scraper mirror 10 again, is transmitted and irradiated onto the reflecting surface of the concave standard mirror 9. After the light reflected and converged by the concave standard mirror 9 is transmitted to the plane scraper mirror 10 and reflected by it, it is reflected and focused by the off-axis concave mirror III 12 and the off-axis convex mirror 13 and incident on the CCD camera 14. The image plane of the CCD camera 14 is located at the focus of the focusing optical component composed of the off-axis concave mirror III 12 and the off-axis convex mirror 13. Observe the size of the light spot on the image plane of the CCD camera 14, move the position of the translation stage on which the concave standard mirror 9 is placed along the direction of the optical axis of the incident light, and measure the number of light spot pixels on the image plane of the CCD camera 14. When the spot shape is circular and the number of pixels reaches the minimum, it indicates that the light beam emitted by the plane scraper mirror 10 is approximately a plane wave. At this time, the virtual focus of the convex grating 11 to be measured almost coincides with the focus of the concave standard mirror 9. By measuring the distance L between the center of the reflection surface of the concave standard mirror 9 and the center of the grating surface of the convex grating 11 to be measured, and according to the focal length of the concave standard mirror 9, the working distance f of the convex grating 11 to be measured can be calculated. 2 The working distance f of the convex grating 11 to be measured 2 It is the difference between the focal length of the concave standard mirror and the distance between the convex grating 11 to be measured and the concave standard mirror 9.

[0045] The present invention provides a device for measuring the working distance of a mid-infrared convex diffraction grating. The device utilizes the characteristic that the first-order diffraction of the incident light by the grating is consistent with the N-order diffraction wave aberration of its N-fold frequency light. By using a visible light source, the distance between the grating to be measured and the concave standard mirror is changed so that the two form a confocal optical system, thereby directly measuring the working distance of the grating to be measured. The device has a simple structure, is quick to use, and has the characteristics of being effective, accurate, and intuitive. It has important practical significance in the application of grating line selection in mid-infrared gas lasers.

[0046] The above description is only an embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, expansion, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A device for measuring the working distance of a mid-infrared convex diffraction grating, characterized in that: The invention comprises a semiconductor laser (1), a polarizing device, a collimating beam expanding optical component, a concave standard mirror (9), a plane scraper mirror (10), a convex grating to be measured (11), a focusing optical component and a CCD camera (14) which are sequentially arranged along an optical path, wherein the visible measuring laser emitted by the semiconductor laser (1) is converted into linearly polarized light by the polarizing device, and then outputs expanded light by the collimating beam expanding optical component, the expanded light sequentially passes through the central holes of the concave standard mirror (9) and the plane scraper mirror (10) and is irradiated onto the convex grating to be measured (11), after being diffracted by the convex grating to be measured (11), the diffracted light returns along the original optical path, passes through the central hole of the plane scraper mirror (10) again and is irradiated onto the reflection surface of the concave standard mirror (9), is reflected by the reflection surfaces of the concave standard mirror (9) and the plane scraper mirror (10), and the reflected light is focused by the focusing optical component and is irradiated onto the CCD camera (14); The number of light spot pixels on the image plane of the CCD camera (14) is measured by moving the concave standard mirror (9) along the direction of the incident light optical axis; when the light spot shape is circular and the number of pixels reaches a minimum, the virtual focus of the convex grating (11) to be measured coincides with the focus of the concave standard mirror (9), and the distance between the center of the concave standard mirror (9) and the center of the convex grating (11) to be measured is L , then: f 2 = f 1- L in, f 1 is the focal length of the concave standard mirror (9), f 2 is the working distance of the convex grating (11) to be measured.

2. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 1, characterized in that: The polarization device comprises a polarization cube (2) and a half-wave plate (3) which are arranged in sequence along the optical path. The measuring laser is converted into linearly polarized light after passing through the polarization cube (2). The half-wave plate (3) is rotated so that the polarization direction of the linearly polarized light is consistent with the polarization direction of the incident light required by the convex grating (11) to be measured.

3. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 1, characterized in that: The collimating and beam expanding optical component comprises a reflector I (4), a reflector II (5), an off-axis concave mirror I (6) and an off-axis concave mirror II (8) which are arranged in sequence along the optical path. The linearly polarized light output by the polarizing device is reflected and redirected by the reflector I (4) and the reflector II (5) in sequence, and then collimated and beam expanded by the off-axis concave mirror I (6) and the off-axis concave mirror II (8) in sequence to become parallel light.

4. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 3, characterized in that: A pinhole (7) is provided between the off-axis concave mirror I (6) and the off-axis concave mirror II (8), and the focal points of the off-axis concave mirror I (6) and the off-axis concave mirror II (8) coincide with each other and are located at the center of the pinhole (7).

5. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 4, characterized in that: The convex grating (11) to be measured is placed at a self-collimation angle, and after reflecting the incident parallel light, returns along the original light path, passes through the central hole of the flat scraper mirror (10), and irradiates the mirror surface of the concave standard mirror (9).

6. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 1, characterized in that: The focusing optical component comprises an off-axis concave mirror III (12) and an off-axis convex mirror (13) which are arranged in sequence along the optical path. The light reflected by the plane scraper mirror (10) is reflected and focused by the off-axis concave mirror III (12) and the off-axis convex mirror (13) in sequence, and then irradiated onto the CCD camera (14).

7. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 1, characterized in that: The shape and size of the central hole of the concave standard mirror (9) are the same as the shape and size of the projection of the surface area to be measured of the convex grating (11) to be measured on a plane perpendicular to the optical axis; The shape and size of the center hole on the plane scraper mirror (10) projected on a plane perpendicular to the optical axis are the same as the shape and size of the center hole on the concave standard mirror (9).

8. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 1, characterized in that: The convex grating (11) to be measured is placed on a grating adjustment frame, and the grating adjustment frame is placed on a three-dimensional electric displacement platform.

9. The device for measuring the working distance of a mid-infrared convex diffraction grating according to claim 1, characterized in that: The concave standard mirror (9) is placed on a translation stage that is movable along the optical axis of the incident light.

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