The optical surface microcosmic defects will influence the safe working of high power laser system, which is an important factor of limiting the system output energy and resulting in the damage of optics. The quantitative measurement for the three dimensional (3D) structure of microcosmic defects on macroscopic optical surface is of great importance for researching the influence of defects to the capability of high power laser system. The present structure measurement methods, such as white-light interference surface profiler and atom-force microscopy, have the disadvantages of slow measurement speed and small measurement field and therefore aren't suitable for the 3D measurement of microcosmic defects on macroscopic optical surface. In order to solve this problem, an effective approach for measuring the structure of microcosmic defects on optical surface is proposed based on digital holographic microscopy (DHM). The wavefront aberration induced by the defects is recorded and then the 3D structure of defects is calculated according to the relationship between the wavefront aberration and optical path. This approach will have potential application in the quantitative measurement for the microcosmic defects on optical surface and is helpful for the further research and understanding the influence of surface microcosmic defects on high-power laser system.
With the split-step-Fourier-transform method for solving the nonlinear paraxial wave equation, the intensity distribution of the light field when the pits diameter or depth change is obtained by using numerical simulation, include the intensity distribution inside optical element, the beam near-field, the different distances behind the element and the beam far-field. Results show that with the increase of pits diameter or depth, the light field peak intensity and the contrast inside of element corresponding enhancement. The contrast of the intensity distribution of the rear surface of the element will increase slightly. The peak intensity produced by a specific location element downstream of thermal effect will continue to increase, the damage probability in optics placed here is greatly increased. For the intensity distribution of the far-field, increase the pitting diameter or depth will cause the focal spot intensity distribution changes, and the energy of the spectrum center region increase constantly. This work provide a basis for quantitative design and inspection for pitting defects, which provides a reference for the design of optical path arrangement.
An effective approach for reconstructing on-axis lensless Fourier Transform digital hologram by using the screen division method is proposed. Firstly, the on-axis Fourier Transform digital hologram is divided into sub-holograms. Then the reconstruction result of every sub-hologram is obtained according to the position of corresponding sub-hologram in the hologram reconstruction plane with Fourier transform operation. Finally, the reconstruction image of on-axis Fourier Transform digital hologram can be acquired by the superposition of the reconstruction result of sub-holograms. Compared with the traditional reconstruction method with the phase shifting technology, in which multiple digital holograms are required to record for obtaining the reconstruction image, this method can obtain the reconstruction image with only one digital hologram and therefore greatly simplify the recording and reconstruction process of on-axis lensless Fourier Transform digital holography. The effectiveness of the proposed method is well proved with the experimental results and it will have potential application foreground in the holographic measurement and display field.
The crystal can be used to be electro-optic switch because of its electro-optic modulation. Generally the uniaxial axis of electro-optic crystal is perpendicular to the light injection surface. Due to the manufacturing precision, the uniaxial axis direction has a little angle with the normal of the light injection surface, which affects the electro-optic modulation ability. In conoscopic polarized inference, due to birefraction the ordinary ray and extraordinary ray from crystal interferes after the polarizer. The interference pattern of crystal component is circle fringes with dark cross. The center of interference pattern has relation to the uniaxial axis direction. Using digital camera to capture the pattern and the center position of interferogram can be determinate by image processing program. In repeatability experiments the rms of center position is around 1 pixel. To measure the uniaxial axis direction, the normal direction of the crystal component should also be accurately determinate. Michelson interference method is introduced to determinate the normal direction. If rotate the crystal component around the normal direction in conoscopic polarized interference, the track of interferogram center is a circle theoretically. The circle center is related to the normal direction of crystal component, and the radii is related to the angle uniaxial axis, which can be determinate by least square fitting method. Experiment result shows that the measuring precision can achieves several tens of microradians.
Properties of plasma electrode pockels cell is directly affected by the Z-axis deviation angle of the electro-optic crystal. Therefore, high precision measurement of the Z-axis deviation angle is indispensable. By using conoscopic interference technique, a measurement system for Z-axis deviation angle of electro-optic crystal is introduced. The principle of conoscopic interference method is described in detail, and a series of techniques are implied in this measurement system to improve the accuracy. High-precision positioning method of the crystal based on Michelson interference is proposed to determine the normal consistency of crystal, which can ensure the high positioning repeatability of crystal in the measurement process. The positioning comparison experiment of the crystal shows that the standard deviation of our method is less than 1pixel, which is much better than the traditional method (nearly 4pixels). Moreover, melatope extraction algorithm of optical axis based on image matching technique is proposed to ensure the melatope can be extracted in high precision. Calibration method of the normal of transmission surface of crystal is also proposed. The experiment results show that the PV and rms of Z-axis deviation angle is less than 0.05mrad and 0.02mrad, respectively. The repeatability accuracy is less than 0.01mrad.
In this paper, a method which can effectively achieve the accurate estimation of the focusing reconstruction distance of recorded object in digital holography (DH) is proposed. By analyzing the variance variety of the reconstructed intensity images corresponding to different reconstruction distances, the accurate focusing reconstruction distance of the recorded object is obtained and thus the legible reconstructed image in-focus is acquired. The effectiveness of the proposed method is successfully validated with both pure phase objects and pure amplitude objects in experiment. This method will have great potential application foreword in digital holographic measurement field.
Three-flat test can separate the reference surface error from the test part surface in the surface measurement by interferometry. The solution based on mirror symmetry of three-flat test is compact and highly accurate. In practice, the error will be introduced when the position of three flats are misalignment or the relative rotation angle are not accurate. The influence of rotation angle error are simulated and discussed. Then the experiment was carried out on three reference flats and the flat surface profiles were derived by mirror symmetry method. The experiment results show good agreement and the difference is in a nanometer level.
KEYWORDS: Digital holography, Holograms, 3D image reconstruction, Digital recording, Superposition, Digital imaging, Charge-coupled devices, Image resolution, Optical resolution, Fourier transforms
Synthetic aperture digital holography can effectively increase the recording area of digital hologram, which is propitious to extend the range and improve the resolution of the reconstruction image. However, the area of synthetic aperture digital hologram is usually very large, and thus if it is directly reconstructed, the numerical reconstruction process may can’t progress in order for the limitation of the disposal capability of computer. Therefore, a screen-division reconstruction method for synthetic aperture digital Fresnel hologram is proposed in the paper. Relatively to the direct reconstruction method, the screen division reconstruction method can effectively reduce the area of the hologram participant in the numerical operation process and thus make it possible to reconstruct the synthetic aperture digital Fresnel hologram which area exceeds the disposal capability of computer. The synthetic aperture digital Fresnel hologram with large area is acquired by the precise control for the removal of CCD array and then reconstructed by the proposed screen division reconstruction method. The experimental results show that, the introduced numerical reconstruction method can well correct the position and phase distribution of the sub-reconstructed-images and obtain accurate synthetic numerical reconstruction image.
Multiple surfaces transform interferometery is a preferred technology for surface profile and index homogeneity measurement using a Fourier based analysis method combined with phase-shifting interferometer. As a four-surface cavity for example, the surface form and index inhomogeneity of the parallel plate are deduced by extracting the information from the corresponding interference frequency. The errors of surface form and index homogeneity are simultaneously simulated and analyzed with different sampling buckets. The results show the feasibility and high precision of this approach compared with traditional methods.
As the development of high powered laser system, the measuring accuracy of optic’s transmissivity need to be improved in order to guarantee all the optics’ quality. This paper analyses all the factors which largely influence the testing accuracy of optics transmissivity when using the spectralphotometer. Experiments are carried out to verify all the deduced results. The results show that the accuracy of wavelength, error of incident angle and divergence of measuring beam would influence the testing result of transmissivity, and the divergence of measuring beam would contrubite to the largest error. Therefore, the testing accuracy could be greatly increased by decreasing the divergence angle of measuring beam.
For long distance imaging in the atmosphere environment, the image quality is affected by the atmospheric turbulence. In this paper, the image quality is analyzed, while the atmospheric turbulence is simulated through the spectrum inversion method of the Fourier transform. The target images affected by both weak and strong atmospheric turbulence environments are reconstructed respectively, employing the compressed sensing algorithm. Results show that the compressed sensing algorithm inhibits the atmospheric turbulence effect to some extent, but not that brilliant in the strong turbulence conditions. Outstanding performance of the compressed sensing algorithm in image noise reduction is also confirmed.
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