Holographic Diffractive Components for Beam Coupling

Abstract

Optical devices for non-parallel transmission and processing of information have been designed as 3-dimensional structures up to now. In such arrangements optical beams as streams of photons carry information. Beams are transformed by means of classical or diffractive optical elements in 3-D space. However, such optical structures occupy a lot of volume, their discrete components must be positioned using an appropriate mechanical cage or frame, and adjustment is often very tedious and complicated. Therefore there is an evident effort to convert 3-D structures, at least in part, to 2-dimensional components where there are opportunities for technology integration. This 2-D technology better allows automated production of the components in massive quantities. 2-D or planar optics can be divided in to two branches according to the nature of light propagation inside a dielectric layer or a plate where the light field is confined, unlike propagation in 3-D free space. If the thickness of the plate is much larger than the wavelength of light, zigzag propagation takes place due to substrate modes. On the other hand, if the thickness of the layer is comparable with the light wavelength, guided waves arise and guided modes must be taken into account. In both cases, there must be a way to provide coupling between freely propagating beams in 3-D space and beams propagating in the planar component. Classical coupling elements for this purpose are prisms. However, prism couplers are discrete elements which must be fixed to the surface of 2-D optics. Their production is tedious and they are not compatible with the planar integration. On the other hand, gratings1 offer many more advantages for coupling, because they are compatible with planar optics and can be produced large numbers.