Hongjun Wang

Study on key algorithm for scanning white-light interferometry

Determining the location of the zero order fringes is one of the key aspects in scanning white-light interferometry. The measurement principle of the scanning white-light interferometry is introduced at first; then the location of the zero order fringes as calculated by four different algorithms; namely Weight-center, Phase-shift, Frequency Domain Analysis (FDA), and Coherent Correlation are compared. Finally, numerical simulations on random generated surfaces are done to reach conclusions by comparing and analyzing the results. The research is important in the understanding and development of white-light interferometry.

Optical subsurface damage evaluation using LSCT

The evaluation of subsurface damage (SSD) has become a key problem in optical engineering field. Laser scanning confocal technology (LSCT) is one of the potential SSD non-destructive evaluation (NDE) methods. Under three assumptions, an innovative and credible scattering model, which is to detect subsurface defect through LSCT, is built base on the classical Mie light scattering theory. Aimed to the model, three important parameters are discussed respectively that is N.A., wave length and relatively refraction index. In order to verify the reasonableness of the model, an effective measurement is done for a usual polished optical surface, and the survey result is in accord with the model. At last, several helpful conclusions about subsurface damage assessment using LSCT are given.

Identification and quantification of non-commonpath error in lateral shearing interferometry

The lateral shearing interferometry based on four step phase shifting has been widely used for aspherical surface measurement, but the non-commonpath error is the main factor that directly affects the measurement results. Based on the structure of lateral shearing aspherical surface interferometer with phase shifting, the non-commonpath components, such as the beam splitter cubes, shearing plate and two right angle prisms, are adopted to produce the displacement of wave-front of under test aspherical surface, because of these components with manufacture errors, the under test wavefront will become deformation when it goes through them, and these wave-front deformations will be performed in the shearing interferograms, therefore, the measurement results obtained by the shearing interferograms compromise the non-commonpath error. In this paper, through analyzing four step phase shifting interferograms with a shearing value and four step phase shifting interferograms with no displacement, the identification and quantification methods of noncommonpath error were put forward, then, the non-commonpath error can be subtracted to correct the measurement results, finally, the simulation and experiment results demonstrate the feasibility of this approach to improve measurement accuracy.

Research on encode technology for aspherical surface measurement based on real-time hologram

The aspherical measuring technology that based on computer-generated hologram (CGH) was introduced. The advantage of this method is that the phase shifts can be controlled digitally, no any mechanical moving and rotating element. By changing CGH coding which displayed on the Liquid Crystal Display (LCD), the wavefront and phase shifts in measuring system were induced. Based on the characteristics of aspherical measurement and LCD structure, the CGH encode technology used in LCD was discussed. Then a new encode method which applied to aspherical measurement was put forward. In this method, the LCD modulates functions of amplitude and phase was coexistent, and the character of LCD diffraction frequency spectrum was considered, and phase hologram was applied. This aspherical measuring technology is more flexible than usual method. In this paper, the hologram encode method based on LCD were illuminated in detail. In order to verify the correction of encode technology, the aspherical surface with standard wavefront was generated by coaxial hologram reconstruct system when hologram encode image was displayed on SONY LCX023 LCD, it interfere with the standard spherical wavefront, then the interferogram was sampled to computer by Charge Coupled Device (CCD) and A/D transfer, the wavefront of hologram reconstruct was obtained by image process finally. All calculation is completed by Matlab. An aspherical measuring system based on LCD was built experimentally. Both the theoretical analysis and experimental results demonstrate the feasibility of this approach.

Research on the calibration method of reference flat based on gravity deformation

In high-precision vertical large-caliber flat interferometry, the gravity deformation of the reference flat will lead to a large error of the surface shape measured. Based on the analysis of the finite element method, this paper combines the three flats test method with Zernike polynomial fitting to compensate and calibrate the error of the reference flat. For the diameter of 300 mm and a thickness of 90 mm of the fused silica reference flat, the results of analysis show that the PV deformation under the T-bracing is 0.021 λ. (λ = 632.8nm). The simulation test result of the influence of deformation on the three-flats test shows that the reference flat the gravity deformation not only affects the detection of self surface, but also has a great influence on the surface detection results of the undeformed flat. And the PV of the surface shape detection results residual is close to 0.011λ. In the end, the new calibration method base on compensate reference deformation for three flats test was proposed, and the validity was verified by simulation.

Analysis of noise reduction performance with the rotated diffuser in Fizeau interferometer of a large aperture

Since the coherent noise affected the quality of the Fizeau’s interferograms in the large aperture, the coherence of the beam was changed by rotated diffuser to reduce the noise of the interfering system. The relationships between the frequency of the rotated diffuser, the contrast of the fringes and the SNR of the system were simulated. Then, the control parameters of rotated diffuser would be required in the optimum interference fringe. The interference images were obtained under different control parameters, and the fringe contrast and system SNR of each image were analyzed. The results showed that the contrast can be reduced by choosing the proper frequency of the rotated diffuser in a certain extent, but the SNR can be improved effectively and it was convenient to process the interference image later.

Microstructure measurement of digital holography

Miniaturization is a development trend of electronics, machinery and information systems, while micro structure brought a large amount of new development for industrial and research applications, whereas, slow measuring speed and two-dimensional results of traditional micro measurement could not meet our needs, it’s urgent to find a matching testing techniques with more sensitivity, more effectiveness and better to have a real-time three-dimensional display. Digital holography applied to the measurement of micro structure, it made up for the lacking of traditional micro structure measure systems of too much time consuming, poor immunity, easily damaging samples with its simple structure, high accuracy, non contact features and three-dimensional reproduction. This paper analyzed the key factors of digital holographic recording and reproducing process.In order to solve the low quality of holograms captured by traditional recording system, holograms and pre-processing algorithm was combined for real-time, by observing the holograms and delicate adjusting the system, to ensure that the collected holograms with full use of CCD width while convenient for subsequent processing. In the processing of reproduction,the influence of spectrum choice, reconstruction wavelength and algorithms and unwrapping algorithm was been studied, and finally obtained an accurate three-dimensional topography of the object. The improved rerecording system and reconstruction algorithm mentioned above solved the low holography quality, much noise and not clear shortcomings of the reconstructed image. Experiment on a raster, compared with traditional system and algorithm results, results showed that the recording system and determine algorithms can reproduce the three-dimensional topography of the object with high precision and has a broader applicability.

Influence of shearing displacement error study on two dimensional lateral shearing interferometry

Lateral shearing interference detection is a economically efficient method which is used to the online testing process of aspheric surface. Whereas, when shear plate is used in lateral shearing interference to detect large aperture aspherical surface, the real moving shear plate can produce shearing error, which brings error into the two-dimensional wavefront reconstruction of aspheric surface. So, for shearing error, Zernike polynomial fitting method is used to study shearing displacement error on the influence of two-dimensional wavefront information reconstruction by computer simulation. For the same measured non-spherical surface, different shearing displacement error influences on the precision of wavefront reconstruction are compared by computer simulation，and finally build the corresponding relations between wave information integrity and shearing displacement error

Study on polarization properties of particle's scattered light

It is important for particle detection technology to research the polarization properties of particle’s scattered light and explore the relationship between particle size and polarization degree of its scattered light. Particle scattering model was established and polarization properties of spherical particle’s scattered light with different sizes and different scattering angles was simulated using the finite element method. The polarization degree of particle scattered light were explored with different sizes, also were the polarization degree with different scattering angles. Results show that relationship curves between polarization degree and scattering angles can be described as parabolas. Polarization degree increases when scattering angles increase in the range of 0°-90° and reached the maximum value at 90°. Polarization degree decreases when the size of particle increase and this change more obvious gradually. The polarization degree of particle scattered light reached 1 at 90°when the particle size is much smaller than the wavelength and the maximum value of polarization degree decreases gradually when particle size increase further. In order to verify the correctness of the finite element simulation model, the polarization degrees were calculated in different particle sizes and different scattering angles by classical Mie scattering theory. Polarization simulation model of particle scattered light is proved to be correct and effective by making comparisons between simulation results and theoretical calculation results. This model lays a foundation for the further research on polarization characteristics of particle scattered light with different morphology and distribution.

Characterization of Liquid Crystal Display Phase Modulation

The LCD is proposed as a new phase shifter with the advantage that the phase shifting is controlled digitally with no any mechanical moving and rotating elements. Phase shifting can be readily introduced by changing the coded image displayed on the LCD. Hence the LCDs phase modulation characteristics are analyzed theoretically and experimentally. A short overview of different calibration methods is made and a new one is proposed. It relies on cross correlation technology that compare phase shifting image and standard image which grey levels is increasing linearly from the value of 0 to 255. Preliminary experimental results are also presented.