#### 5.3.3.3. Coordinate Systems

The OM3p simulation always uses Cartesian coordinates. In the OM2p simulation, choose XY or RZ in the simulation properties dialog box.

• By choosing XY, you indicate that the 2D geometry is to be treated as a plane in Cartesian coordinates. The structure will lie in the x-y plane, and the modal solution will be identical to the solution you would get if the structure extended to infinity in both directions in z. The eigenvector is assumed to be invariant in z.

• By choosing RZ, you indicate that the 2D geometry is to be treated as a radial slice of a cylindrical structure, where the x-axis is the cylindrical axis. Then, the numerical solution is appropriate for a radially symmetric structure where your geometry defines half the structure cross-section (in other words, your geometry is the half of the cross-section in the positive y plane.) If you choose RZ coordinates and your structure crosses the x-axis, the geometry is invalid. If your structure is adjacent to the x-axis, it is not necessary to explicitly define a boundary condition along the axis.

Define the field behavior with azimuthal angle φ by setting the Azimuthal Mode Number in the simulation properties dialog box. For azimuthal mode number m, the field varies with cos(mφ) or sin(mφ), depending on the mode.