Apparatus and method for birefringent material crystal axis and surface roughness characterization

Abstract

The present application belongs to the field of optical precision measurement technology, and particularly relates to a device and method for characterizing the crystal axis and surface roughness of birefringent materials. The method for characterizing the crystal axis and surface roughness of birefringent materials is to obtain a plurality of sets of data between Stokes parameters and the rotation angle θ of a half-wave plate by adjusting the angle θ of the half-wave plate, to obtain a parabola by drawing a fitting graph, and to obtain the position of the optical axis of the birefringent material at the lowest point of the parabola. After obtaining a speckle image, a mathematical relationship graph between a plurality of sets of θ and is obtained by calculation, and the optical axis of the birefringent material is found at the lowest point of the fitting curve. In the measurement process, the present application is not affected by the error of the incident angle, the properties of the material to be measured, and the voltage. The determination of the optical axis of the birefringent material is realized by the combination of the half-wave plate and the circular polarization system. The roughness of the birefringent material can be obtained by drawing a scatter plot of C and N for fitting.

Description

Technical Field

[0001] This invention belongs to the field of optical precision measurement technology, specifically relating to an apparatus and method for characterizing the crystal axis and surface roughness of birefringent materials. Background Technology

[0002] Currently, roughness measurement methods are mainly divided into contact measurement and non-contact measurement. Since contact measurement can easily damage objects, non-contact measurement methods are gradually being applied to the roughness of target objects.

[0003] Optical crystals can be classified into homogeneous and heterogeneous materials based on their optical properties. Birefringence is the phenomenon where light rays are split into two beams and refracted in different directions when passing through anisotropic crystals. When light propagates in anisotropic materials, its propagation speed and refractive index typically change with the direction of vibration, and it may have more than one refractive index value. However, when light passes through anisotropic optical crystals from a specific direction, birefringence does not occur. This specific direction is the optical axis of the birefringent material. Birefringent materials have wide applications in lasers, medical imaging, optical communication, and many other fields. However, due to the anisotropy of birefringent materials, determining their optical axis and measuring their surface roughness becomes difficult.

[0004] Currently, the traditional method for determining the optical axis is based on scalar imaging, which can only characterize scalars and not polarization. This is because the roughness of general objects can be measured directly using non-contact methods, such as scattering and interferometry, but these methods have requirements on the incident angle and the properties of the material being measured. Summary of the Invention

[0005] The purpose of this invention is to provide an apparatus and method for characterizing the crystal axis and surface roughness of birefringent materials, so as to solve the problems of optical axis measurement and roughness measurement of birefringent materials.

[0006] This invention employs the following technical solution: a method for characterizing the crystal axis and surface roughness of birefringent materials, which obtains multiple sets of Stokes parameters by adjusting the half-wave plate angle θ. The data between the half-wave plate rotation angle θ and the curve can be used to plot a parabola. The lowest point of the parabola is the location of the optical axis of the birefringent material. After obtaining the speckle image, calculations can be performed... Multiple sets of θ and The mathematical relationship between them can be plotted by finding the lowest point after fitting the curve. Locate the optical axis of the birefringent material.

[0007] Furthermore, based on the determination of the optical axis of the birefringent material, the roughness of the birefringent material can be measured by the recorded speckle intensity contrast and the average number of speckles.

[0008] Furthermore, the device for characterizing the crystal axis and surface roughness of birefringent materials provided by the present invention comprises, in sequence, a first lens, a first linear polarizer, a half-wave plate, the birefringent material to be tested, a second lens, a circular polarizer system, and a camera arranged in the main optical path of a laser.

[0009] Furthermore, the main optical path of the aforementioned laser is sequentially provided with a first lens, a first linear polarizer, a half-wave plate, a second lens, a pupil, a third lens, and a camera.

[0010] Compared with the prior art, the beneficial effects of the present invention are:

[0011] 1. During the measurement process, it will not be affected by errors in the incident angle, the properties of the material being measured, or the voltage;

[0012] 2. The optical axis of a birefringent material is determined by combining a half-wave plate and a circular polarization system. After obtaining the speckle image, the polarization plane angle can be calculated. The scatter plot relationship between the two points can be fitted to obtain the optical axis of the birefringent material.

[0013] 3. In terms of roughness measurement, an optical pupil is added to calculate the autocorrelation function of the obtained speckle image, thereby obtaining the average number of speckles N and the contrast C. By fitting a scatter plot of N and C, the roughness of the birefringent material can be obtained. Attached Figure Description

[0014] Figure 1 This is a flowchart of an apparatus and method for characterizing the crystal axis and surface roughness of a birefringent material according to the present invention.

[0015] Figure 2 This invention provides an optical path diagram for measuring the optical axis of a birefringent material.

[0016] Figure 3 This is a graph used by the present invention to determine the optical axis of a birefringent material;

[0017] Figure 4 This invention provides an optical path diagram for measuring the roughness of birefringent materials.

[0018] Figure 5 The present invention presents a curve showing the average number of speckles N and the light intensity contrast C of speckles under different roughnesses of birefringent materials.

[0019] Among them, 1. laser, 2. first lens, 3. first linear polarizer, 4. half-wave plate, 5. birefringent material, 6. second lens, 7. pupil, 8. third lens, 9. circular polarization system, 10. camera. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0021] See Figure 1 First, the optical axis of the birefringent material is determined. Then, under the polarization system, the Stokes parameter S3 of the speckle intensity is obtained using Equation 2. By rotating a half-wave plate, polarization planes at different angles can be obtained. Then, curve fitting between the two can yield an upward-opening parabola, with the lowest point being the optical axis of the birefringent material. After determining the optical axis of the birefringent material, the contrast C of the speckle image can be obtained using formulas 3, 4, and 5. The average number of speckles N can be obtained by autocorrelation processing of the speckle image using formula 1. By fitting C and N, the roughness of the birefringent material can be obtained.

[0022] This invention provides a method for characterizing the crystal axis and surface roughness of birefringent materials, see [link to relevant documentation]. Figure 3 By adjusting the angle θ of the half-wave plate, multiple sets of... The data between the half-wave plate rotation angle θ and the curve can be used to plot a parabola. The lowest point of the parabola is the location of the optical axis of the birefringent material. After obtaining the speckle image, calculations can be performed... Multiple sets of θ and The mathematical relationship between them can be plotted by finding the lowest point after fitting the curve. Locate the optical axis of the birefringent material.

[0023] Based on the determination of the optical axis of the birefringent material, the roughness of the birefringent material under test is then measured based on the recorded speckle intensity contrast and the average number of speckles.

[0024] The apparatus for characterizing the crystal axes of birefringent materials, see [link to relevant documentation]. Figure 2 The main optical path of laser 1 is sequentially arranged with a first lens 2, a first linear polarizer 3, a half-wave plate 4, a second lens 6, a circular polarizer system 9, and a camera 10. When this device is used for crystal axis measurement, the birefringent material 5 to be measured is placed between the half-wave plate 4 and the second lens 6. The coherent beam emitted by laser 1 is expanded by the first lens 2 and then perpendicularly incident on the first linear polarizer 3 to become linearly polarized light. It is then perpendicularly incident on the half-wave plate 4 to adjust its polarization plane, and subsequently incident on the birefringent material 5 to form scattered light. The scattered light is collimated by the second lens 6 and then perpendicularly incident on the circular polarizer system 9 before entering the camera 10. The circular polarizer system 9, located between the second lens 6 and the camera 10, is used to detect the circularly polarized light generated by the birefringent material 5. The crystal axis of the birefringent material 5 is measured by adjusting the angle of the half-wave plate 4.

[0025] For the measurement of the optical axis of the birefringent material 5, the following conditions should be met: the distance between the birefringent material 5, the second lens 6, and the camera 10 is equal to the focal length of the second lens 6; for the measurement of the roughness of the birefringent material 5, the birefringent material 5, the second lens 6, the pupil 7, the third lens 8, and the camera 10 should satisfy a 4f system, and the resolution of the speckle can be changed by changing the size of f1 and f2.

[0026] The apparatus for characterizing the surface roughness of birefringent materials, see [link to relevant documentation]. Figure 4 The main optical path of laser 1 is sequentially arranged with a first lens 2, a first linear polarizer 3, a half-wave plate 4, a second lens 6, a pupil 7, a third lens 8, and a camera 10. This measurement is based on a measurement system that determines the optical axis of the birefringent material, but removes the circular polarization system 9 and adds the pupil 7 and the third lens 8. For roughness measurement, the birefringent material 5 to be measured is placed between the half-wave plate 4 and the pupil 7. After determining the crystal axis of the birefringent material 5, the direction of the incident light is adjusted to align with the crystal axis of the birefringent material 5. At this point, the scattered field is a single linearly polarized light scattered field. This scattered light passes through the birefringent material 5 and forms a 4f system with the camera 10 before entering the camera 10. The average number of speckles can be adjusted by changing the size of the pupil 7. Based on the recorded intensity contrast of the speckles and the average number of speckles, the roughness of the birefringent material 5 is measured.

[0027] The experiment obtained different speckle images by changing the birefringent material 5 and adjusting the size of the aperture. The light intensity contrast C and the average number of speckles N corresponding to different speckle images are different. By plotting the scatter plots of N and C and fitting them, the roughness value can be estimated.

[0028] The relevant formulas of this invention are explained below:

[0029]

[0030]

[0031]

[0032]

[0033]

[0034]

[0035] Γ I : Autocorrelation function of speckle light intensity;

[0036] S3: Stokes parameter.

[0037] I: Indicates the light intensity of the speckle pattern;

[0038] The average value of the speckle light intensity;

[0039] σ I Standard deviation of speckle light intensity;

[0040] C: Contrast of speckle pattern;

[0041] I: Indicates the light intensity at each point of the speckle;

[0042] A k The area of ​​the entire image;

[0043] A a Area of ​​a single speckle;

[0044] N: The average number of speckle patterns after autocorrelation;

[0045] After obtaining a scatter plot of the relationship between the average number of speckles N and the light intensity contrast C of the speckles through experiments, an NC curve was obtained by fitting the plot. Figure 5 The theoretical curve is used to match the experimental curve with the theoretical curve to obtain the roughness of the birefringent material 5.

[0046] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Any modifications or substitutions made by those skilled in the art within the scope of the technology disclosed in this invention should be included within the scope of this invention.

Claims

1. A method for characterizing the crystal axes of birefringent materials, characterized in that: A birefringent material crystal axis characterization device is used. The device includes a laser (1). A first lens (2), a first linear polarizer (3), a half-wave plate (4), a second lens (6), a circular polarizer system (9), and a camera (10) are arranged sequentially on the main optical path of the laser (1). When measuring the crystal axis, the birefringent material (5) to be measured is placed between the half-wave plate (4) and the second lens (6). The characterization method is as follows: by adjusting the angle of the half-wave plate. Multiple sets of Stokes parameters were obtained. and half-wave plate rotation angle The data between these points can be used to plot a parabola, and the lowest point of the parabola is the location of the crystal axis of the birefringent material. After obtaining the speckle image, calculations can be performed... Multiple sets were obtained. and The mathematical relationship between them can be plotted by finding the lowest point after fitting the curve. Find the crystal axis of the birefringent material; When characterizing the crystal axis of the birefringent material (5), the following conditions must be met: the distance between the birefringent material (5) and the second lens (6), and the distance between the second lens (6) and the camera (10) are both equal to the focal length of the second lens (6).

2. A method for characterizing the surface roughness of birefringent materials, based on the method for characterizing the crystal axes of birefringent materials as described in claim 1, characterized in that: A surface roughness characterization device for birefringent materials is adopted. This device replaces the circular polarizer system (9) in the crystal axis characterization device for birefringent materials with a pupil (7) and a third lens (8). When used for roughness measurement, the birefringent material (5) to be measured is placed between the half-wave plate (4) and the pupil (7). The characterization method is as follows: After determining the crystal axis of the birefringent material (5), the direction of the incident light is adjusted to be consistent with the direction of the crystal axis of the birefringent material (5). At this time, the scattering field is a single linearly polarized light scattering field. The scattered light passes through the birefringent material (5) to the camera (10) to form a 4f system and then enters the camera (10). The average number of speckles is adjusted by changing the size of the pupil (7). The scatter plot between the average number of speckles N and the light intensity contrast C of the speckles is obtained through experiments. The NC curve is fitted and matched with the theoretical curve to obtain the roughness of the birefringent material (5).

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