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Physical Optics | ||||||
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Physical Optics Propagation (POP) is the ability of ZEMAX to use diffraction calculations to propagate a beam through the optical system surface by surface, including transfer of the beam through any ZEMAX surface type. Diffraction that occurs at lens apertures, and as the beam propagates between lenses is considered. The beam may be defined with user selectable power per area units, for example, in watts/centimeter squared. ZEMAX supports user selectable array sizes, and the X and Y direction sampling and point to point spacing may be different. The output includes irradiance and phase surface plots, cross-section plots, encircled energy, and fiber coupling. Axial and non-axial skew beams can be computed. This powerful feature is in the ZEMAX-EE edition. Frequently Asked QuestionsWhat
is POP? What is POP?The
propagation of light is a coherent process. As the wavefront
travels through free space or glass, each part of the wavefront
coherently interferes with all the other parts. Modeling this
coherent propagation is the realm of 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 transfer of the beam through any
ZEMAX surface type. What about ray tracing?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. The modeling of beam
propagation via ray tracing is commonly called geometrical optics.
Doesn’t ZEMAX already consider diffraction?ZEMAX does support some diffraction calculations, such as the Diffraction PSF and MTF. However, these calculations are based upon geometric ray tracing. Rays are used to propagate through the entire optical system, and the path length of the rays is used to reconstruct the wavefront in image space. A single Fraunhofer diffraction step is then used to compute the PSF or MTF. When using the geometric optics model, all of the diffraction is assumed to occur in just the last propagation, from the exit pupil to the image. Diffraction that occurs at the lens apertures, and as the beam propagates between the lenses, is ignored. For many optical systems, including most imaging lenses, this simplified model is adequate. For other systems, it is not. How does POP work?When using POP, the beam is modeled using an array of points. At each point, the complex amplitude of the electric field is stored. The phase of these complex values determines the phase of the wavefront relative to a reference surface. The amplitude of the values determines the power of the beam in user selectable power per area units, for example, in watts/centimeter squared. The array size is user selectable, in dimension, sampling, and aspect ratio. ZEMAX supports user selectable array sizes, and the X and Y direction sampling and point to point spacing may be different. Both dimensions change dynamically to best fit the beam during propagation. 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. To minimize phase errors, ZEMAX finds the best surface to use for reference of the phase. What types of beams are supported?The diffraction propagation and surface transfer algorithms are accurate for any arbitrary beam; they are not limited to simple Gaussian beams. ZEMAX supports defining the initial beam as Gaussian, “top hat”, or by data files or external DLL programs. Any amplitude and phase distribution is supported. Once the beam is defined, the beam may be inserted at any surface in the optical system, and the beam may be aligned with any chief ray from any field position. POP is not limited to axial propagation, it works with non-axial systems and skew beams. What type of data is produced?ZEMAX can display beam irradiance or phase in correct dimensions and units at any surface in the optical system, in surface, contour, grey scale, false color, or cross section plots. The beams may also be stored as data files for later use. The propagation can be done once, and the beam file at every surface stored for quick analysis and review. Sample Output
Are fiber coupling calculations supported?Yes. ZEMAX can determine the coupling efficiency by computing the overlap integral between the beam and any arbitrary fiber mode. What happens at surfaces?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. 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 beam, including:
Once the surface transfer function is applied, the beam may then propagate to the next optical surface. The surface by surface propagation proceeds through the entire optical system. The coordinate system of the beam travels along the chief ray, and rotates as required at coordinate breaks or tilts. The transfer function may be calculated for all ZEMAX surface types, even user-defined and diffractive surfaces. Are Non-Sequential surfaces supported?Surfaces such as gradient index and non-sequential are handled by ray tracing. The surface transfer function concept is extended to span multiple surfaces at once, so a range of surfaces is described by a single transfer function. Groups of surfaces to be handled by ray tracing rather than diffraction propagation are user defined. Is polarization supported?Yes. Beams may be polarized or unpolarized. The surface transfer function incorporates polarization ray tracing to correctly include the effects of angle of incidence and optical coatings on the polarization, phase, and transmission of the beam. How is POP integrated with ZEMAX?POP is fully integrated with ZEMAX. Just use an existing ZEMAX lens file, define the initial beam parameters, and the propagation proceeds. What are the typical applications for POP?
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