4.A.3. Boundary Conditions

*FemSIM* needs to define how the field is treated at the boundary of the simulation domain. The supported boundary conditions are:

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PML

A PML, or perfectly matched layer, boundary consists of several mesh points which are added to the edge of the domain. Its width is defined by the PML Width field in the *FemSIM* Simulation Parameters dialog.

Currently, the use of a PML is enabled by using a non-zero value for the PML Width.

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Symmetric (Neumann)

This condition sets the normal derivative of the field to zero at the boundary and implies an symmetric field pattern.

To use this boundary condition, use the symmetry options in the FEM Mesh Parameters dialog described above. This option can be set for X and Y independently.

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AntiSymmetric (Dirichlet)

This condition sets the field to zero at the boundary and implies an antisymmetric field pattern.

To use this boundary condition, use the symmetry options in the FEM Mesh Parameters dialog described above. This option can be set for X and Y independently.

When solving for transverse modes, it is important to consider the effect of the symmetry on the polarization. The above symmetric/antisymmetric conditions apply to only one of the field components. This field component is defined via the variable polarization in the symbol table. A value of 0 (default) means the field symmetry refers to E_{x}, and E_{y} has the opposite symmetry. A value of 1 means the field symmetry refers to E_{y}, and E_{x} has the opposite symmetry.