Hongzhen Jiang

Influence of pitting defects on quality of high power laser light field

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.

Reconstruction of on-axis lensless Fourier transform digital hologram with the screen division method

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.

Conoscopic polarized interference applied in measuring uniaxial axis direction of electro-optic crystal

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.

The microstructure measurement of surface defects of optical component based on digital image-plane holographic microscopy

In order to measure the three-dimensional microstructure of surface defects on optical component, a novel measuring method based on digital image-plane holographic microscopy (DIPHM) is proposed in this paper. The experimental system has been designed and built to measure the microstructure of optical component’s surface defects. The object light wavefront can be reconstructed by using the algorithm based on the angular spectrum theory, and the technique of phase correction is contributive to eliminate the system error. There is a definite relationship between the object light wavefront and the surface topography, so the 3D microstructure of surface defects can be measured. This measuring technique is helpful to judge the damage degree of the optical component and analysis the influence of the surface defects, and it is of great significance to ensure the laser system security running.

Measurement of z-axis deviation angle of electro-optic crystal by conoscopic interference

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.

Principal and error analysis of mirror symmetry method in three-flat test

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.

Analyses of multiple surfaces transform interferometry in parallel plate measurement

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.

Influence analysis of measuring beam's divergence on optics transmissivity testing

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.

Simulation of the atmospheric turbulence image reconstruction based on compressed sensing

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.