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Harmonic diffractive optical design theory and applied research

Author: LouDi
Tutor: YangGuoGuang
School: Zhejiang University
Course: Measuring Technology and Instruments
Keywords: Harmonic diffractive optical elements Diffractive optical Hybrid optical system Micro-optical glasses Multi-band optical system Diamond cutting
Type: PhD thesis
Year: 2008
Downloads: 565
Quote: 4
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The conventional DOE with the +1st diffracted order used in IR spectrum has been matured both in design and fabrication. The reversed chromatic dispersion and athermal properties of DOE make it very effective to design achromatic & athermal optical systems. Diffractive optical element (DOE) and refractive/diffractive hybrid optical systems have been widely used in the field of contemporary optics. Optical systems are currently developing towards the aim of larger NA, shorter wavelengths and broader spectrum. But it makes the DOE fabrication much more difficult for larger NA and shorter wavelengths. This disadvantage bottlenecked the development of diffractive optics. In addition, the effective Abbe number of DOE is usually very small and it means the amount of reversed dispersion is very large; and the diffractive efficiency drops dramatically when the wavelength is shifted from the center wavelength, resulting that the stray lights from the unexpected diffracted orders adversely affect the imaging performance of optical system. It also prevents DOE from being applied in the broadband/multiband optical systems.In order to solve the limitations above, we propose that harmonic diffractive elements (HDOE) be applied to increase the critical linewidth of DOE micro-structure, and to facilitate the fabrication. Thus, it will make the fabrication of hybrid optial systems with larger NA and shorter wavelengths possible. At the same time, HDOE can also solve the problems of diffractive efficiency drop at the shifted wavelength and the large amount of reversed dispersion in the broadband/multiband optical systems. Through theoretical design and practical fabrication & test, we have made some exploration in HDOE.In this dissertation, we firstly research into the principal theories of DOE, including scalar diffractive theory, vector diffractive theory and geometrical optics theory. Based on scalar diffractive theory, detailed analyses on HDOE and binary optical element (BOE) have been implemented. Harmonic diffractive optical element (HDOE) is the generalized form of DOE. It increases the etching depth of DOE surface relief, making the phase shift 2mn. HDOE can theoretically diffract 100% of energy to the different diffracted order at its correspondent harmonic wavelength, and all these diffracted orders have the same optical power. According to this feature, we can design a harmonic achromat in broadband or a harmonic multiband hybrid optical system. Further analysis indicates that, lights of higher diffracted order have different imaging properties from the 1st diffracted order. Also, the selection of diffracted order is discussed.After the theoretical analysis of HDOE, we provide the design procedures of HDOE-used systems, including single-piece harmonic achromats and harmonic multiband hybrid optical systems.The novel ideas and works of this dissertation are as below:We designed and fabricated a refractive/harmonic diffractive optical system to substitute the standard spherical lenses in the digital wavefront interferometer. This new optical system structurally simplifies the old one and has the root mean square (RMS) wavefront measurement precision of A/20. Experimentally, we testified the practicability of using HDOE in the visible spectrum. To further the research, we fabricated a harmonic micro-optical spectacle with continuous surface relief and optical power of +6.0D. The optical power test data show good agreements to theoretical design. Another important application of HDOE is based on the "863 subject" of "Triple-band Optical System Research and Development". The adoption of HDOE with 16-step surface relief made two IR bands (4.5um-band at the 2nd order and 9.0um-band at the 1st order) optically coincident and focused to the same detecting plane. The MTF tests indicate that the harmonic triple-band optical system has very good imaging performance in each band.

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