Ray tracing is a widely applicable technique for modeling the propagation of light through an optical system. However ray tracing is not appropriate for all modeling tasks. Rays are incoherent in the sense that the path a ray takes during propagation is not affected by the presence or absence of other rays.
Physical Optics is the most comprehensive way to describe the transport of coherent radiation through an optical system. As the coherent wavefront travels through free space or glass, each part of the wavefront coherently interferes with all the other parts. Modeling this coherent propagation is called physical optics. Physical Optics Propagation (POP) is the ability of Zemax to use diffraction calculations to propagate the beam through the optical system surface by surface, including the transfer of the beam through any Zemax sequential surface type.
When using POP, the beam is modeled using an array of points. At each point, the complex amplitude of the electric field is stored. To propagate the beam from one surface to another, either a Fresnel diffraction propagation or an angular spectrum propagation algorithm is used. Zemax automatically chooses the algorithm that yields the highest numerical accuracy. The diffraction propagation algorithms yield correct results for any propagation distance, for any arbitrary beam. As the beam propagates, Zemax automatically scales the dimensions of the array to properly fit the beam size.
POP is based on explicit diffraction calculation at every surface in a sequential optical system, not just at the exit pupil. This allows proper consideration of diffraction from lens apertures, and at small apertures such as a pinhole near the focus of an aberrated beam. Any TEMx,y, multimode, top hat or user-defined beam can be propagated, and Zemax computes all beam parameters, such as M2 at every surface. Zemax also supports the OptiWave .f3d file format, so that complex amplitude distributions from integrated optic waveguides can be imported and launched though Zemax optical systems.
The difficulty in implementing a diffraction propagation capability is not the free space beam propagation algorithm. The hard part is propagating through arbitrary optical surfaces, especially if the surface is hit at some non-zero angle. When the beam reaches an optical surface between two media, Zemax computes a transfer function between the object and image space side of the surface. The transfer function accounts for all the effects a surface may have on the
Phase imparted to the wavefront, including all aberrations
Amplitude transmittance of the surface, including polarization effects
Diffraction by gratings, binary optics, or other phase surfaces
Change in beam size, due to obliquity and/or diffraction
Vignetting by arbitrary apertures on surfaces
Zemax also supports simple paraxial Gaussian beam analysis and a more complex skew Gaussian analysis. Zemax has the most comprehensive treatment of coherent beam propagation of any lens design code. And, of course, all POP parameters can be optimized and toleranced.
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