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Copying and Reusing Boundary Mode Analysis Results
When using the Beam Envelopes or the Electromagnetic Waves, Frequency Domain interfaces, one can often have multiple Port boundary conditions that have identical structure, but are at different locations in space. It is often the case, especially for optical and photonics models, that multiple different modes are of interest at these ports, and these modes need to be numerically computed. It is possible to compute all of the modes of interest using just a single Port boundary condition, of type Numeric, and to copy and reuse these solutions to all of the other ports in the model, which will be User-Defined Ports. The supporting example is shown below: an optical ridge waveguide that does not change shape along the length. A mode of a particular polarization is launched, but other modes can exist due to anisotropic properties that exist in part of the substrate. For this example, the first two modes of propagation are considered. A uniform waveguide with two ports that support multiple modes. Precomputing All of the Modes of the Waveguide This technique begins with first defining a Numeric Port that will be used to compute all of the modes of interest. Since the Port will not be used directly in the ultimate analysis, give it a Port number of 0 to clearly differentiate it. Apply the Port boundary condition to the boundaries describing the waveguide. For optical waveguides, ensure that the width of the cladding region is large enough that the fields fall off to nearly zero.
Visualization of the two modes of interest. Applying the Precomputed Modes to the Complete Model Begin by defining the excitation port, Port 1, on the same set of boundaries where Port 0 was defined. Set the Type of port to User defined and, within the Port Mode Settings section, use the withsol operator to refer back to the eigenvalue solutions previously computed. The third argument to withsol sets the index of the eigenvalue, lambda, to an integer that indexes into all of the computed modes. Use parameters M1=1, M2=2 to simplify setup. Populate both the electric mode field and the propagation constant with the data computed from Study 1, which has the name 'sol1', which can be seen when the study is expanded. So, for example, the previously computed propagation constant to fill in is the expression: withsol('sol1',ewbe.beta_0, setind(lambda,M1))
Next, copy the boundary mode data from the input boundaries to the boundaries defining the output ports. To do so, introduce a General Extrusion operator, with settings as shown in the screenshot below. The input and output ports are at identical xy-coordinates, so no change to the mapping of these is needed. The z-expression of z-Length is used here because the output boundaries are offset in the z-direction by the Global Parameter Length. This operator will make the data computed on the input boundaries available on the output boundaries. An alternative approach is to leave the z-expression entirely empty, in both the Source and Destination Map settings. This will extrude the data available on the xy-plane on the source boundaries everywhere along the z-axis.
![]() When solving, an optional Study Reference can be used to recompute the Port 0 modes before solving the 3D model. Disable Port 0 within the Physics and Variables Selection section, as shown in the screenshot below.
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