Diffractive Optical Element Design Theory And Its Application In Hybrid Optical System

Diffractive optical elements (DOEs) have been shown to be very popular in a wide variety of applications for its features such as the effects of wavefront complex amplitude modulation to balance axial aberrations, wavelength dispersion to correct the chromatic aberration, athermal property and considerable reduction in the apparent size, weight and number of elements of the optical system. But only the rays in the design order are useful, while one beam of incident light was diffracted into different orders by DOE. Though we can make all of the rays at design wavelength concentrated on the design order, which means the diffraction efficiency is 100%, the diffraction efficiency will falls as the light wavelength deviates from the design one, unexpected diffracted light in the other orders increases and becomes stray light. Based on scalar diffraction theory, much works focus on the improvement of diffraction efficiency of DOE in wide spectrum application in this dissertation.Firstly, the theory of diffraction optics such as scalar diffraction theory, vector diffraction theory and geometric optical model are studied in detail and summarized. And the method of microstructure DOE based on sphere as substitute for aspheric surface was clearly described. Spherical surface takes the main focusing power and diffractive profile compensates the phase difference from aspheric surface to reference sphere. The reference wavelength and the minimum feature size were introduced into the limiting condition to make the minimum distance between DOE rings greater than the minimum feature size. With this method all the lens surfaces of optical systems can be fabricated in sphere. We proved the rationality in theory and simulated on computer to illuminate the feasibility of this method.Secondly, according to the characteristics of using phase polynomial and phase transform function to describe DOE in scalar theory, a new type of diffractive optical element with stratified multi-layer surface microstructure was introduced in this dissertation. More than two types of materials with different dispersion characters are taken into the construction of multi-layer DOE to increase the number of variable factors in the phase distribution function. The phase difference is or nearly be integer times of 2π to each wavelengths of design waveband. The diffraction efficiencies of the whole waveband are improved and the influence of stray lights of non-design diffractive orders is decreased. For example, in the visual band from 0.4～0.8μm, the diffraction efficiency of two layer DOE are greater than 96%.Finally, a long focal-length objective lens was designed and manufactured with a two-layer DOE used for large aperture fluorite lens for apochromatic. The performance of this hybrid optical system was tested. And the manufacture techniques of spherical DOE are discussed.