Introduction to Geometrical Optics Modeling
Geometrical optics provides a complement to other electromagnetics modeling techniques such as the finite element method. It can be used to efficiently model electromagnetic wave propagation when the wavelength is much smaller than the smallest geometric entity in the model. While propagating, waves are assumed to be locally plane; that is, the surfaces of constant electric field are either planar, or if they are curved, their radii of curvature are much larger than the wavelength. The effects of diffraction at edges and corners in the geometry are also neglected. When these assumptions are valid, geometrical optics provides an efficient means of modeling wave propagation through optically large systems.
The Geometrical Optics Interface is used to compute ray trajectories. For each ray, a set of two first-order differential equations are solved for each component of the position and wave vector of the ray, for a total of six degrees of freedom per ray in 3D or four degrees of freedom per ray in 2D. A Time Dependent study or the Ray Tracing study, which is a variant of the Time Dependent study, is used to solve these equations.
In order to compute ray trajectories with the Geometrical Optics interface, at least the following must be present:
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At least one release feature, such as the Release, Inlet, or Release from Grid node. Release features are used to specify the initial position and direction of rays. If other ray properties such as intensity are computed, they are initialized by release features as well. The Geometrical Optics interface also includes dedicated release features to release solar radiation and to release reflected or refracted rays from an illuminated boundary.
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Boundary conditions, such as the Material Discontinuity and Wall nodes, that determine how rays interact with their surroundings. By default, the Material Discontinuity boundary condition is applied to all boundaries to model reflection and refraction between adjacent domains. The Wall feature can be used to reflect or absorb rays at selected boundaries. Specialized boundary conditions are also available to model optical devices such as polarizers and wave retarders.
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The Ray Properties node, which is present by default and cannot be removed. The Ray Properties node can be used to specify the frequency or free-space wavelength of the rays. If the Allow frequency distributions at release features check box is selected in the Settings window for the physics interface, the frequency is instead specified in the settings for the ray release features and can be assigned a different value for each ray, but the Ray Properties node still cannot be removed.
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The Medium Properties node, which is used to specify the refractive indices of the media through which rays propagate. The refractive index may either be a real or complex quantity, depending on whether the rays propagate through an absorbing medium.
In addition to the required functionality listed above, the Geometrical Optics interface also includes tools for coupling the results to other physics interfaces. Dedicated nodes called Accumulator nodes can be used to transfer information from rays to the domains they pass through or the boundaries they hit. The variable computed by an Accumulator node can represent any physical quantity and can be expressed in terms of variables that exist on rays, domains, and boundaries.
In addition, The Ray Heating Interface includes a dedicated multiphysics node that computes the heat source resulting from the attenuation of rays in absorbing media. This allows for easy coupling between the Geometrical Optics interface and a heat transfer interface, such as the Heat Transfer in Solids interface. Because the Geometrical Optics interface is compatible with moving meshes, it is possible to expand ray heating models to account for thermal expansion as well.