<title>Computer-aided modeling of beam propagation effects in diffraction-critical spaceborne instruments</title>

Abstract

This talk concerns applications of a ray-trace model to the computation of the effect of diffraction on beam propagation. It reports the use of the technique in the design of apertures for space-borne instruments having critical diffraction properties.The modelling technique used is that of gaussian beam decomposition a numerical beam propagation technique incorporated in a commercially available ray-trace program. The result is the powerful capability to model the optical field at any point, in systems of any geometry, with any amount of aberration.The technique is particularly useful for design problems where 'non-imaging' effects are important, and examples of its use will be given. Although the computation requirements for such detailed analysis may seem daunting, the continuing increase in readily available computing power is now overcoming this drawback.The application here is to certain 'diffraction-critical' situations, where the design of correctly sized apertures is needed for the control of unwanted diffraction effects. Three recent design studies are illustrated:1 . Millimetre wave imaging with off-axis reflectors. Analysis of the effects of aberration on coherent detection efficiency. 2. Long-distance beam propagation in space-borne laser interferometry. This involves the analysis of coherent detection efficiency in the presence of aberrated gaussian beams. 3. Design of a Lyot stop system for an infra-red radiometer which is to view the Earth's limb from space. Here the critical (and unwanted) diffraction is that from the bright Earth disc, lying just outside of the instrument field of view.The analysis technique is explained, and examples given of diffracted energy patterns analysed at progressive stages in the system. It is shown how these aid the design and analysis of the systems. The aim is to show the range problems in which this method is useful, and to hopefully learn from others at the conference about other cases where such techniques have been used or else might be useful.

Application of a generalized diffraction analysis to the design of nonstandard Lyot-stop systems for earth limb viewing radiometers

“…As well as use for diffraction stray light design, other fields of use are antenna design, laser cavity design, and beam-propagation problems. 22,25,32 Although the formulation of such detailed optical analyses can be lengthy, they have been made practical by the large computer powers now available, along with the development of advanced ray-trace programs incorporating beam analysis methods. This enables powerful techniques, such as Gaussian beam superposition, to be readily combined with real system geometries, as modeled via other computer-aided engineering programs.…”