# Microwave Mode Specification

Microwave Mode Specification defines how the mode solver finds transmission line modes and computes microwave waveguide quantities.

This object corresponds to tidy3d.rf.MicrowaveModeSpec (opens new window).

When the RF GUI solves modes from the shared setup, the solve frequencies come from Global Settings and are passed to ModeSolver.freqs (opens new window).

# Number of Modes

num_modes: Number of modes returned by the mode solver.

  • Type: integer
  • Constraint: greater than 0
  • Default: 1

# Target Effective Index

target_neff: Guess for the effective index of the mode.

  • Type: float number
  • Default: None

When enabled, the mode solver searches for modes closest to the target effective index.

# Precision

precision: Numerical precision used by the mode solver.

  • Options:
    • Auto: Use double precision when the simulation contains a good conductor, and single precision otherwise.
    • Single: Use single precision. This is faster and uses less memory, but can be less accurate.
    • Double: Use double precision. This is more accurate, but uses more memory and can be slower.
  • Default: Auto

# Group Index Step

group_index_step: Control whether the group index is computed alongside the effective index.

  • Options:
    • False: Do not compute group index.
    • True: Compute group index using the default fractional frequency step.
    • User value: Compute group index using the specified fractional frequency step.
  • Default: False

step value: Fractional frequency step used in the numerical differentiation of effective index.

  • Type: float number
  • Default: 0.005

# Bending Injection

angle_theta: Polar angle of the propagation axis from the injection axis.

  • Type: float number
  • Unit: degree
  • Default: 0

angle_phi: Azimuth angle of the propagation axis in the plane orthogonal to the injection axis.

  • Type: float number
  • Unit: degree
  • Default: 0

bend_radius: Curvature radius for simulation of waveguide bends.

  • Type: float number
  • Unit: m, mm (default unit), um
  • Default: None

bend_axis: Index into the two tangential axes defining the normal to the plane in which the bend lies.

  • Options: 0/1
  • Default: None

angle_rotation: Toggle the method used to compute modes when angle_theta is not zero.

  • Options:
    • False: Apply a coordinate transformation through the material tensors.
    • True: Rotate the structures first, solve at a reference plane, and rotate fields back to the mode plane.
  • Default: False

# Polarization Filtering

filter_pol: Polarization filter applied to the computed modes.

  • Options: None/TE/TM
  • Default: None

If a polarization filter is selected, modes with the selected polarization fraction greater than or equal to 0.5 are placed first.

# Mode Sorting

sort_spec: Specification for filtering, sorting, and tracking modes.

See Mode Sort Specification.

# Impedance and Calibration

impedance_specs: Specification for how voltage, current, and characteristic impedance are computed for each mode.

See Impedance and Calibration.