# Mode Troubleshooting

Editor: messages after Check Simulation, and the results of a Mode project · Python: td.ModeSimulation

Each entry starts with what the editor shows. The messages come from the check that Check Simulation runs (see Run and Cost).

# Not ready to run (why?)

Cause: The check found at least one error. The simulation cannot run until you fix it.

Fix: Click the text to read the errors. Fix each one, then click Check Simulation again.

# "'ModeSolver.freqs' cannot be empty (size 0)."

Cause: The project has no wavelength to solve at, for example an empty Custom list.

Fix: In Setup › Mode Specifications, under Set the wavelength range, choose Enter range and set λmin, λmax and Number of points, or add values to the Custom list.

# "A large number (…) of modes requested … This can lead to solver slow-down and increased cost."

Cause: Number of modes is very large.

Fix: Ask for fewer modes. To find the modes you need, set Target effective index close to their effective index.

Related: Mode Specifications

# A mode has an effective index below the cladding index

Cause: A guided mode has an effective index between the index of the cladding (the material around the core) and the index of the core. A mode below the cladding index is not guided: it is radiation, or a mode of the boundaries.

Fix: Ignore that mode, or ask for fewer modes. Set Target effective index close to the core index, or use Polarization Filtering in See more to keep the modes you need.

# The modes come in an unexpected order

Cause: By default, the modes are ordered by their effective index, highest first. A TE mode and a TM mode can swap places between waveguides of different sizes.

Fix: Use Polarization Filtering in See more to filter or sort the modes, for example by their TE fraction.

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