Optical system for crystal thermal lens imaging

Abstract

The utility model discloses an optical system for crystal thermal lens imaging. The optical system comprises a background image, a thermal lens, a bifocal focusing lens and a CCD (Charge Coupled Device) which are arranged in sequence, the focal lengths of the CCD and the bifocal focusing lens are located at the position of a background image, the background image is imaged on the CCD, and an optical element in the thermal lens is arranged on a light path of the CCD for collecting the background image. The crystal thermal lens has different depth of field, so that the background image and the crystal thermal lens can be clearly imaged.

Description

Technical Field

[0001] The utility model relates to the field of optical measurement, in particular to an optical system used for crystal thermal lens imaging. Background Art

[0002] Thermal lens effect is a common problem in high-power laser technology. Optical components, such as laser crystals, nonlinear crystals, etc., have a small but non-negligible absorption rate of incident lasers due to their own crystal quality problems. When an optical component is irradiated by a laser beam for a long time, it locally absorbs the laser energy and produces a temperature rise, accompanied by increased internal stress and deformation of the surface. The temperature field inside the optical component forms a distribution with high center and low surroundings, and the refractive index of the material changes with temperature, causing the optical component to form an effect similar to a converging lens or a diverging lens, which is called a thermal lens. Thermal lensing will reduce the beam quality of the transmitted laser, change the direction of the transmitted laser, affect the spatial mode matching of the laser, and in severe cases even cause damage to the optical material.

[0003] Thermal lens measurement of laser crystals is already a mature technology. The most commonly used method is to estimate the thermal lens by measuring the output power of the laser resonator or the spot shape of the output laser. For example, the application document with application number 201610072071.1 is a method and device for online real-time measurement of the focal length of a laser crystal thermal lens, the application document with application number 201610328103.X is a method for measuring the focal length of a nonlinear crystal thermal lens, the application document with application number 201610328935.1 is a method for measuring the absorption coefficient of a nonlinear crystal, the application document with application number 201710041976.7 is a device and method for measuring the focal length of a nonlinear crystal thermal lens, the application document with application number 201910094646.3 is a method for measuring the thermal focal length of a solid-state laser, the application document with application number 201911212703.X is a device and method for measuring the thermal load of energy transmission upconversion in a laser gain crystal, and the application document with application number 201911344114.7 is a method and device for measuring the focal length of a laser crystal thermal lens using a knife-edge method. This method requires a complete laser, and places the laser crystal into a resonant cavity to generate laser. The thermal lens is indirectly measured by measuring the output laser. The method is complex and slow, and can only measure the focal length parameters of the thermal lens. It cannot give effective results for thermal lenses that produce spatial distortion.

[0004] Background schlieren technique is a transparent medium refractive index observation method developed on the basis of schlieren technique. It is often used for dynamic observation of refractive index of gas and liquid. For example, NASA has used BOS technology to observe the acoustic compression field generated by supersonic aircraft. K.Hayasaka et al. used BOS technology to study the transmission of acoustic waves in liquids (Measurement of alaser-induced underwater shock wave by the optical-flow-basedbackground-orientedschlieren technique, 18th InternationalSymposium on the Application of Laserand Imaging Techniquesto Fluid Mechanics·LISBON|PORTUGAL·JULY 4–7,2016). The principle of BOS is to place a background plate behind the transparent object to be measured, with a background pattern with clear boundaries and complex shapes on the background plate. CCD and lens are placed in front of the background plate. When there is a refractive index change in the optical path, the image of the background pattern on the CCD will shift and deform. By calculating the translation amount of the image, the refractive index change can be imaged. The BOS technology is simple to use and can achieve imaging speeds of seconds, but it is not very sensitive and is not easy to measure when the refractive index changes slightly. In addition, BOS is mainly designed for flow fields and backgrounds that are far away. The imaging system has a focal length and the depth of field of the lens is shallow, which can easily cause crystal and background image blurring, resulting in blurring of the image and flow field and reduced resolution. Utility Model Content

[0005] Purpose of the utility model: In order to solve the problems existing in the above-mentioned prior art, the utility model provides an optical system for crystal thermal lens imaging.

[0006] The technical solution of the utility model: The utility model provides an optical system for crystal thermal lens imaging, which comprises a background image, a thermal lens, a double-focus lens, and a CCD arranged in sequence, wherein the distance between the background image and the thermal lens is L1, the distance between the thermal lens and the double-focus lens is L2, and the distance between the double-focus lens and the CCD is L3, wherein L1>L3>L2; the focal lengths of the CCD and the double-focus lens are adjusted to the position of the background plate, the background image is imaged on the CCD, and the optical elements in the thermal lens are arranged on the optical path of the CCD for collecting the background image.

[0007] Furthermore, the thermal lens includes an optical element and a laser, and the laser is arranged above the optical element.

[0008] Furthermore, the optical material is a laser crystal.

[0009] Furthermore, the double-focus lens adopts a Fresnel lens with double focal lengths, and the surface of the Fresnel lens has two curvature radii r1 and r2, r1 corresponds to a diameter D1, r2 corresponds to a diameter D2, and D2>2D1.

[0010] Beneficial effects: The utility model proposes a dual-focal-length Fresnel lens with different depths of field, so that both the background image and the crystal thermal lens can be imaged clearly. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a basic structural diagram of this embodiment;

[0012] Figure 2 is a structural diagram of a Fresnel lens with double focal lengths in this embodiment;

[0013] Figure 3 It is the image of the background image by the small depth of field imaging system;

[0014] Figure 4 It is the image of the background image by the large depth of field imaging system;

[0015] Figure 5 It is the imaging diagram of the background image in this implementation. DETAILED DESCRIPTION

[0016] The utility model is further described in detail below in conjunction with the accompanying drawings.

[0017] The basic structure of the utility model is as follows Figure 1 As shown, it includes CCD, bifocal lens, optical element (laser crystal is used in this embodiment), and background image. The CCD and bifocal lens constitute a camera (or camera head), the laser crystal is placed in front of the bifocal lens, and the background image is placed at a farther position. The background image is composed of patterns with high contrast (e.g. black and white), clear boundaries, and complex shapes.

[0018] The background image is at lens L1, the laser crystal is at lens L2, and the lens distance is CCDL3. This constitutes an imaging system including object distance u, image distance v, and focal length f. The imaging system satisfies the following relationship:

[0019]

[0020] The focal length of the CCD and the double-focus lens is adjusted to the background image position, and the background image is clearly imaged on the CCD. An optical element is placed in front of the double-focus lens. The two laser light-transmitting end faces of the optical element are polished to facilitate the transmission of the laser; the observation surface of the optical element is polished to image the thermal lens from the side of the thermal lens. Figure 1 As shown in the figure, when the laser is focused and passes through a long optical element, the optical element absorbs the heat of the laser, forming a thermal lens at the position where the laser passes through the optical element, or an uneven distribution of the refractive index. The refractive index is higher at the center of the laser and lower at the edge of the laser, and a lens-like effect is also produced when observed from the side.

[0021] In this embodiment, the image and refractive index change of the thermal lens are calculated as follows: first, the image on the background plate is photographed without passing the laser through the optical element; then, the image on the background plate is photographed after passing the laser through the crystal to generate the thermal lens. The through-light algorithm compares the two images and calculates the image and refractive index change of the thermal lens.

[0022] This embodiment proposes a dual-focus lens, which can be composed of wavelength dual focus, polarization dual focus, metamaterial, two-dimensional optical diffraction material (DOE), Fresnel surface structure, etc. This embodiment is preferably as follows Figure 2 The Fresnel surface structure shown constitutes a double-focus lens. The Fresnel lens is composed of two surface structures, one of which has a curvature radius of r1 and the other has a curvature radius of r2. The focal length of the lens is determined by the following formula:

[0023]

[0024] Therefore, the two radii of curvature correspond to two focal lengths f1 and f2, and the lens constitutes a double-focus lens with two focal lengths.

[0025] The above-mentioned dual-focus lens has different apertures D1 and D2. D1 is much smaller than D2; specifically: D2>2D1, the purpose of this is to form two imaging systems with different depths of field. The imaging system with a small depth of field has clear images for objects at a certain distance, but blurry images for objects at other distances, and the farther the distance, the blurrier the image. Figure 3 As shown in , if the depth of field of the imaging system is small, when the background image at a distance is clear, the image of the crystal at a distance is blurred; Figure 4 As shown in the figure, if the depth of field of the imaging system is large, both the distant background and the close crystal can be imaged clearly. In this way, accurate and clear thermal lens imaging effect can be obtained.

[0026] The dual focal length lens of this embodiment has different depths of field, wherein D1 corresponds to a small aperture, a small depth of field, and a small focal length, and is responsible for imaging the crystal. D2 corresponds to a large aperture, a large depth of field, and a large focal length, and is responsible for imaging the background, such as Figure 5 As shown, the bifocal lens of the present invention can form clear images of the background and the crystal at the same time.

[0027] This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. An optical system for crystal thermal lens imaging, characterized in that: include: The background image, thermal lens, double-focus lens and CCD are arranged in sequence; the focal lengths of the CCD and the double-focus lens are located at the background image position, the background image is imaged on the CCD, and the optical element in the thermal lens is arranged on the optical path of the CCD collecting the background image.

2. An optical system for crystal thermal lens imaging according to claim 1, characterized in that: The double-focus lens adopts a Fresnel lens with double focal lengths. The surface of the Fresnel lens has two curvature radii r1 and r2. The diameter corresponding to r1 is D1, the diameter corresponding to r2 is D2, and D2>2D1.

3. The optical system for crystal thermal lens imaging according to claim 1, characterized in that: The thermal lens includes an optical element and a laser, wherein the laser is arranged above the optical element.

4. The optical system for crystal thermal lens imaging according to claim 1, characterized in that: The optical element adopts a laser crystal.

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