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Title: Dynamic and static Dammann gratings and their potential applications in optical testing
Other Titles: Dong tai yu jing tai Daman guang ji qi zai guang xue ce liang zhong de ying yong
Authors: Zhao, Shuai (趙帥)
Department: Dept. of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Diffraction gratings
Optical instruments -- Design and construction
Optical measurements
Notes: CityU Call Number: TA1750.Z43 2007
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2007
xiv, 180 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: Dynamic diffractive optical elements can significantly improve a lot of applications. Some applications benefit by the possibility to change the optical behavior in real-time without mechanical motion. In this dissertation, a novel digital speckle shearing interferometer, using an electrically addressed liquid crystal spatial light modulator (SLM) as shearing device, is developed. A binary phase 1×2 Dammann grating realized by the SLM creates two displaced images of an object on the CCD camera. The optical phase of two interference beams can be shifted by digitally shifting the grating pattern and no calibration is therefore needed. To reduce the effects of vibration, a simultaneously phase-shifting shearing interferometer is proposed. The lateral shear is introduced by a double grating structure. The novelty of this method consists in the phase-shift Dammann grating, which is divided into four sub 1×2 Dammann gratings. They have a lateral shift of 1/8 period in turn, which generates a phase shift of π/2 between two first-order diffraction beams. Four resulting interference patterns, representing the partial derivative of the wave fronts, are recorded by a CCD camera simultaneously. Another effort of this dissertation is to propose a circular Dammann grating (CDG) as a counterpart of traditional Dammann grating. The Fourier spectrum of a circular sine (or cosine) grating consists of a circular impulse ring with a radius equal to the grating frequency. If a circular phase grating can be decomposed into a linear combination of circular sine functions or cosine functions with different frequency, its Fourier spectrum will consist of a series of impulse rings with different radius and amplitude. Through choosing appropriate phase profile of the grating, the circular equal intensity diffraction pattern can be achieved in far field analogous to the rectangular pattern produced by traditional Dammann grating. The design process and simulation results for some orders are presented. A sample of three-order CDG is fabricated to verify our design. Several kinds of fabrication errors are also considered in analytical or numerical ways. CDGs have potential applications in a variety of fields. In this dissertation, a simple focal length measurement technique and a collimation testing technique are proposed based on the unique diffraction characteristics of CDG. The main maximum in the Fourier spectrum of the CDG will split into two rings due to the limited grating aperture. When the observed plane has a longitudinal displacement from the focal plane of a lens, the radial separation between the two rings will increase. Through searching for the position where the separation is minimum, the focal point of the lens can be located. The back focal length can then be determined. Such a radial separation also depends on the illumination beam. When the beam deviating from collimation, the separation rapidly increases. Therefore, through monitoring the separation, the degree of the collimation can be evaluated. When the feature size of diffractive optical elements (DOE) is of the order of or less than the optical wavelength, the classical scalar diffraction theory is no longer valid and cannot be applied to the analysis and design of such elements. The finite-difference time- domain (FDTD) method applicable to structure with axial symmetry is used in the dissertation to simulate the diffraction behavior of the subwavelength CDGs. A new technique on rigorous coupled-wave theory is also proposed for the subwavelength CDGs design.
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