Binary Optics Technology: Theoretical Limits on the Diffraction Efficiency of Multilevel Diffractive Optical Elements

Abstract : Diffractive optical elements are being considered as potential solutions to a number of optical design problems that are difficult or impossible to solve with conventional refractive and reflective elements. Two unique characteristics of diffractive elements can be exploited; the first is teh dispersion property. Diffractive structures bend light rays of longer wavelengths more than those of shorter wavelengths, which is the reverse of refractive materials; therefore, diffractive structures minimize or eliminate the dispersive effects of refractive materials. The second unique characteristic is the relative ease with which arbitrary phase profiles can be implemented. Advances in both diamond turning technology and the use of semiconductor fabrication equipment have made possible the construction of a variety of diffractive elements. Diamond turning technology allows fabricating diffractive surfaces over large areas in a relatively short period of time. However, there are limitations: the phase profile has to be circularly symmetric, and the accuracy with which a diffractive profile can be made is dependent on the tip size of the diamond turning tool. Using semiconductor fabrication equipment to make diffractive elements has become a powerful technique. This particular approach produces a stepped approximation, referred to as a multilevel structure, to the ideal profile.