Commercial aspects of diffractive optics

The purpose of this article is to provide the reader with an up-to-date status on the commercial applications of diffractive optical elements. Rather than merely treating these as individual components, this article will also focus on the their applications from a systems point-of-view. Diffractive optics − an introduction Diffractive optics, as the name implies, are based on the principle of diffraction contrary to conventional optics that are based on refraction and reflection. This of course, also is reflected in the physical structure that determines the diffractive optical element. The common properties of such elements can be summarised as follows: • A diffractive optical element consists of a periodic fringe pattern, typically with fringe spacing compatible with the wavelength of the light used. For visible light sources this figure would be in the range 0.5-2 μm. • Depending on the application, recording material and the grating type, such structures will typically have a thickness between 0.1 and 100 μm. • The function of the diffractive optical element is exclusively determined by the geometry and spatial variations in the width and depth of its fringe pattern. • The periodic pattern is recorded as variations in the refractive index of the film and hence it is the phase of the light that is affected by the element. Diffractive optics is a generical term and often this is replaced by a term that either describes the fabrication method or the way the fringe pattern is encoded. One of the terms used rather indiscriminately is that of holographic optical elements (HOEs). This term is only correct when the diffractive optics has been created by interferometrical means. Other frequently used terms are: • Binary optics, for surface relief microstructures with square-wave profile. • Digital optics or computer generated holograms (CGHs), for surface relief microstructures synthesised by a computer and created with high-resolution laser plotters or e-beam writers. • Bragg gratings, for grating structures formed inside a film, e.g., photo polymers or dichromated gelatine (DCG). Also referred to as volume holograms. • Lippmann gratings, special cases of Bragg gratings with the fringe pattern parallel to the surface like an interference filter. The advantages of diffractive optical elements (DOEs) are their ability to generate arbitrary complex wavefronts from a piece of optical material that is essentially flat. A very powerful property is that of spatial and angular multiplexing found in volume phase holograms like dichromated gelatin (DCG) or photopolymers. On the other hand, the surface relief microstructures generated in photo resists lend themselves to mass production techniques like embossing and injection moulding. In the case of surface relief structures, they may even be combined with liquid crystals to form electrically controlled diffractive optical elements. Two American companies are actively pursuing this area, viz. Digilens and Meadowlark Optics. Diffraction gratings as components Diffraction gratings, ruled as well as holographic, have been offered by a number of companies for the past 30 years and are regarded as an off-the-shelf component today. The major applications of these gratings are still as spectral dispersion elements in spectroscopic applications. More exotic applications include grating pairs for pulse compression and wavelength selectors on pulsed dye lasers. The proliferation of ion-etching techniques has made it possible to “mill” diffraction gratings onto a number of optical materials covering the range from UV and VIS (quartz) to mid-IR (ZnSe, Si, Ge and GaAs). These gratings have been used for beam sampling or attenuation of high-power lasers. Companies offering these products for sale are Lasiris and Gentec both in Canada. One of the key technologies in optical communication is dense wavelength demultiplexing (DWDM). The prerequisite for DWDM, among other things, is the fabrication of fibre Bragg gratings. The most widespread method for doing this is the use of phase masks. Phase masks are diffraction gratings made to diffract two equal 1 order beams with total suppression of the zero order beam. Fig. 1. Exposure scheme for formation of a Bragg grating inside an optical fibre. Drawing provided by ADC Denmark.