Chapter 6 - Diffractive Optical Elements for Optical Computers

Abstract

This chapter presents the fabrication, the theory, and the applications of diffractive optical elements (DOEs). An important quantity of a DOE is its diffraction efficiency, ηd. Diffraction efficiency is defined as the amount of light intensity that goes into a particular diffraction order (or array of orders, as in the case of a Dammann grating) compared to the sum of intensities in all the diffraction orders. However, the diffraction efficiency is not all that matters when one considers the power budget of an optical system because other sources for loss have to be considered. Diffraction efficiency describes only losses as a result of light being diffracted into unwanted orders. On the other hand, light efficiency is an overall efficiency that includes diffraction losses but also losses as a result of reflections, absorption, and scattering from the DOE. Under ideal circumstances, the value of the diffraction efficiency depends only on the number of phase levels. However, various fabrication errors exist that result in a reduction of the diffraction efficiency and in losses as a result of scattering from the optical element. These errors can be subdivided into mask errors and processing errors. The fact that DOEs are surface relief structures allows for their replication by using molding techniques. The replication of diffractive optics is not new; commercially available blazed gratings, as they are often used in spectrometers, are usually replicas.