RF Mode Analysis

RF Mode Analysis#

`tidy3d.rf.MicrowaveModeSpec`	The `MicrowaveModeSpec` class specifies how quantities related to transmission line modes and microwave waveguides are computed.
`tidy3d.rf.AutoImpedanceSpec`	Specification for fully automatic transmission line impedance computation.
`tidy3d.rf.CustomImpedanceSpec`	Specification for custom transmission line voltages and currents in mode solvers.
`tidy3d.rf.MicrowaveModeMonitor`	`Monitor` that records amplitudes from modal decomposition of fields on plane.
`tidy3d.rf.MicrowaveModeSolverMonitor`	`Monitor` that stores the mode field profiles returned by the mode solver in the monitor plane.
`tidy3d.ModeSimulation`	Simulation class for solving electromagnetic eigenmodes in a 2D plane with translational invariance in the third dimension.

The MicrowaveModeSpec extends the standard ModeSpec to include automatic characteristic impedance calculation for transmission line modes. This is particularly useful for microwave and RF applications where the characteristic impedance \(Z_0\) is a critical parameter.

Unlike the standard ModeSpec, MicrowaveModeSpec includes an impedance_specs field that defines how voltage and current path integrals are computed for each mode. These integrals are then used to calculate the characteristic impedance of the transmission line.

# Create a microwave mode specification with automatic impedance calculation
mode_spec = MicrowaveModeSpec(
    num_modes=2,
    target_neff=1.5,
    impedance_specs=AutoImpedanceSpec()  # Automatic path integral setup
)

# Use in a microwave mode monitor
monitor = MicrowaveModeMonitor(
    center=(0, 0, 0),
    size=(2, 2, 0),
    freqs=[1e9, 2e9, 3e9],
    mode_spec=mode_spec,
    name='mode_monitor'
)

Automatic Impedance Calculation

The AutoImpedanceSpec automatically determines appropriate voltage and current integration paths based on the mode field distribution. This is the recommended approach for most use cases, as it eliminates the need to manually define integration paths.

# Using automatic impedance specification (recommended)
mode_spec_auto = MicrowaveModeSpec(
    num_modes=1,
    impedance_specs=AutoImpedanceSpec()
)

Custom Impedance Calculation

For more control over the impedance calculation, you can specify custom voltage and current integration paths using CustomImpedanceSpec. This allows you to define exactly where and how the voltage and current are computed.

# Define custom voltage path (line integral)
voltage_spec = AxisAlignedVoltageIntegralSpec(
    center=(0, 0, 0),
    size=(0, 0, 1),  # Vertical line
    sign="+"
)

# Define custom current path (contour integral)
current_spec = AxisAlignedCurrentIntegralSpec(
    center=(0, 0, 0),
    size=(2, 1, 0),  # Rectangular loop
    sign="+"
)

# Create custom impedance specification
custom_impedance = CustomImpedanceSpec(
    voltage_spec=voltage_spec,
    current_spec=current_spec
)

# Use in mode specification
mode_spec_custom = MicrowaveModeSpec(
    num_modes=1,
    impedance_specs=custom_impedance
)

Multiple Modes

When solving for multiple modes, you can provide different impedance specifications for each mode:

# Different impedance specs for each mode
mode_spec_multi = MicrowaveModeSpec(
    num_modes=2,
    impedance_specs=(
        AutoImpedanceSpec(),  # Auto for first mode
        custom_impedance,         # Custom for second mode
    )
)

# Or use a single spec for all modes (will be duplicated)
mode_spec_shared = MicrowaveModeSpec(
    num_modes=2,
    impedance_specs=AutoImpedanceSpec()  # Applied to both modes
)

Mode Solver Monitors

There are two types of monitors for microwave mode analysis:

MicrowaveModeMonitor: Records mode amplitudes and transmission line parameters (voltage, current, impedance) on a monitor plane during a full 3D FDTD simulation.
MicrowaveModeSolverMonitor: Stores the complete 2D mode field profiles along with transmission line parameters from a standalone mode solver calculation.

# Monitor for mode amplitudes in full simulation
mode_monitor = MicrowaveModeMonitor(
    center=(0, 0, 0),
    size=(2, 2, 0),
    freqs=[1e9, 2e9],
    mode_spec=mode_spec,
    name='mode_monitor'
)

# Monitor for mode field profiles (mode solver only)
mode_solver_monitor = MicrowaveModeSolverMonitor(
    center=(0, 0, 0),
    size=(2, 2, 0),
    freqs=[1e9, 2e9],
    mode_spec=mode_spec,
    name='mode_solver_monitor'
)

Mode Simulation

For standalone mode solving (without a full 3D FDTD simulation), use ModeSimulation:

import tidy3d.web as web

# Create a mode simulation for transmission line analysis
mode_sim = ModeSimulation(
    size=(10, 10, 0),
    grid_spec=GridSpec.auto(wavelength=0.3),
    structures=[...],  # Your transmission line structures
    mode_spec=mode_spec,  # MicrowaveModeSpec defining impedance calculation
    freqs=[1e9, 2e9, 3e9],
)

# Run the mode simulation
mode_sim_data = web.run(mode_sim, task_name='mode_analysis')

# Access impedance results from the modes
Z0 = mode_sim_data.modes.transmission_line_data.Z0