In this technical publication, we provide information about using ASAP® (Advanced Systems Analysis Program) from Breault Research Organization (BRO) to perform wave-optics calculations. This topic may seem beyond the scope of a geometric raytracing program, but it is not. With the addition of a few new tools and utilities, you can use the basic non-sequential, ray-tracing engine at the heart of ASAP to model interferometry, diffraction, partial coherence, and other wave phenomena.
In geometrical ray optics, the rays can be thought of as representing the local wavefront normals. ASAP traces these geometric rays through optical systems. While this is all that is required for the analysis of many imaging and non-imaging systems, we have consistently ignored the phase of these rays.
ASAP overcomes these limitations through a method known as Gaussian beam summation. This is discussed in more detail in the following sections, but the essence of the method is relatively simple. The Gaussian beam is a solution to the paraxial wave equation, and is a good description of many laser beams propagating in free space. A Gaussian beam has its narrowest beam radius at its waist and expands as it propagates. The propagation of a Gaussian beam is well understood, and easily characterized by a few simple parameters. Furthermore, we see that a Gaussian beam can, within certain limitations, be traced through optical systems by geometric ray-trace methods.
But can the simplicity of Gaussian beam propagation be exploited to model more general sources? Laser beams, after all, represent a small subset of interesting sources displaying wave characteristics. The answer is yes. Any complex field can be represented as the superposition of Gaussian beams, and this observation is the basis for investigating wave phenomena with ASAP.
ASAP includes two types of wave optics propagation. The method in longest use is Gaussian Beam Propagation, and the newest method is the Beam Propagation Method (BPM). BPM was added to handle microstructures, which traditional Gaussian beam methods can not adequately address. Both methods are addressed in this document.
Summary of document changes
This document received its annual technical review for accuracy and completeness. Changes were made for clarification. Equations were redone to improve viewing quality.