Fuzhong Bai

Two-shot point-diffraction interferometer with an unknown phase shift

On the basis of a modified Mach–Zehnder interferometer, this paper proposes a two-shot point-diffraction interferometer (PDI), by which two π-shifted interferograms are created according to the principle of half-wave loss and are simultaneously captured by a single CCD camera. After an arbitrary unknown phase shift is induced, a second and final shot captures these last two patterns. Then a novel algorithm based on statistical principles is developed to extract the actual phase shift and the wavefront phase from four interferograms. The combination of two required shots and a robust algorithm allows the proposed PDI to be implemented efficiently and accurately. Related simulation and experiments are conducted to prove the correctness of the proposed method.

Absolute measurement of the generated aberrations from liquid crystal spatial light modulator using a common-path interferometry

Abstract A compact common-path interferometry is proposed to measure the wavefront aberration generated from liquid crystal spatial light modulator (LC-SLM). The LC-SLM is encoded with an aberration pattern and illuminated with a linearly polarized light oriented at ±45° with respect to the fast axis of liquid crystal, which is vertically oriented. The horizontal polarization component of the incident beam is not affected by the driving signal, while the vertical polarization component is modulated to the aberration loaded to the LC-SLM. By imposing a quarter-wave plate and a rotating analyzer, these two waves create four frames of phase-stepped interferograms. The aberration to be measured can be retrieved, and the result does not include any systematic error such as the substrate error of LC-SLM. Therefore, this method can implement absolute measurement, and help us to evaluate perfectly the fitting accuracy of the LC-SLM.

Analysis on measured signal retrieval approaches in non-modulation pyramid wavefront sensor

Pyramid wavefront sensor (PWFS) without modulation is prevailing over one with modulation. So far how to describe measured signals of non-modulation PWFS needs deeply research. In this paper, the theory of the non-modulation PWFS is briefly presented according to wave optics. This paper analyses the existing four approaches in theory. By numerical simulation this paper further verifies the performance of four approaches under the experiment condition. The result shows that the approach with total intensity of pixels conjugate to the same spot in the pupil as signal denominator is the best choice for the non-modulation PWFS in closed-loop correction.

Workpiece angle measurement based on Hough transform

To overcome the difficulty in the non-contact angle measurement especially for large-scaled workpieces, the paper presents a new angle measurement method based on Hough transform. The realization process of the method consists of workpiece image acquisition & preprocessing and angle measurement. With the preprocessing one can obtain an ideal binarized edge image of the workpiece image. The angle measurement is then carried out by employing the classical Hough transform. Except the discussion of measurement principle, both practical examples and simulated experiments are introduced to demonstrate the performance of the method proposed in the paper. The experiment results indicate that this method can be applied to measure the workpiece angle under many occasions and can achieve the aim of the non-contact angle measurement. At the same time, it does still possess great advantages in terms of reliability and noise rejection capability.