What is EME?

Date	Category
2025-05-27 18:52:21	EME

The EigenMode Expansion (EME) method is a frequency-domain technique useful for simulating very long waveguide-based structures. Its main advantage is that uniform sections of the structure require only a single cell for computation, while varying sections can be efficiently approximated using a limited number of cells. This approach can significantly reduce computational costs compared to FDTD method, while delivering highly comparable results.

Key capabilities of the Tidy3D EME solver include:

• Bidirectional propagation — Computes the full bidirectional scattering matrix, accounting for reflections and backward-propagating modes at every interface.

• Passivity and unitarity constraints — Optional constraints ensure physically meaningful scattering matrices at cell interfaces (see the constraint parameter of tidy3d.EMESimulation).

• Bent waveguides — Simulates curved structures via bend_radius in tidy3d.EMEModeSpec.

• Anisotropic materials — Supports diagonally anisotropic media (tidy3d.AnisotropicMedium).

• Fast parameter sweeps — Efficiently sweeps cell lengths, number of modes, and number of periodic repetitions without recomputing modes (see tidy3d.EMELengthSweep, tidy3d.EMEModeSweep, tidy3d.EMEPeriodicitySweep).

• Broadband frequency interpolation — Modes are interpolated across frequencies to reduce cost (see interp_spec in tidy3d.EMEModeSpec).

• Diagnostics — Access mode coefficients, interface S matrices, overlaps, and propagation indices via tidy3d.EMECoefficientMonitor.

Some common application examples include MMIs, tapers and couplers, and bent waveguides.