Frequency-domain field monitors collect the field profile in the frequency domain from simulation results across some spatial region within the simulation in the FDTD, varFDTD solvers.
Note: There are two very similar types of frequency domain field monitors: 'Frequency domain field profile' and 'Frequency domain field and power' monitors. These monitors are identical except for one advanced setting (the spatial interpolation setting). In most situations, we recommend using the 'field and power' monitor. This monitor 'snaps' to the nearest mesh cell, which minimizes the amount of interpolation required, generally leading to more accurate data. The 'profile' monitor does not snap to the nearest mesh cell. Instead, it records the data exactly where the monitor was located. This can be useful in a few situations, but the extra interpolation required can slightly reduce the accuracy of the data.
Tips: Memory and computation time
Frequency domain field monitors can require large amounts of memory when recording data over a large spatial domain. When possible, use 1D or 2D rather than 3D monitors. Similarly, try to minimize the number of frequency points recorded. It is also possible to use spatial downsampling to record less spatial resolution. Finally, it is possible to control which field components are recorded on the Data to record tab. If you are only interested in the power flux, you can select the OUTPUT POWER and disable everything else.
Generally, frequency monitors don't have a large effect on the simulation time, except when recording a very large amount of data. To determine the effect on the simulation speed, simply disable the monitor and re-run the simulation.
Simulation type: Record the type of simulation data, default setting is ALL
Override global monitor settings:
A toggle to override the global monitor settings. If checked, the user can specify the frequency range and number of points at which frequency-domain information will be recorded (using the options described below). If unchecked the options below are set from the global monitor settings.
SAMPLE SPACING: This combo-choice parameter determines how the sample frequency/wavelength will be selected. The three sample options are "uniform", "chebyshev" and "custom".
- USE WAVELENGTH SPACING: By default, data is recorded at certain spaced points with respect to frequency. Selecting this option spaces data at certain spaced points with respect to wavelength.
- USE SOURCE LIMITS: When checked these monitors use the source limits. When unchecked, the frequencies/wavelengths at which to record data can be set using the pull-down menus and boxes below them.
- FREQUENCY POINTS: Set to choose the number of frequency points at which to record data.
When sample spacing is selected to be "custom", all the above settings will be disabled and a "custom frequency samples" table will be shown. Properties of the table are:
- ADD: Adds an entry above the selected entry to the "frequency (THz)" table
- REMOVE: Removes the selected entry from the "frequency (THz)" table
- SORT: Sorts the table with frequency from low to high
- set global monitor settings: Access to global properties
Monitor type: The monitor type and orientation, this option will control the available of spatial setting below
- X, Y, Z: The center position of the simulation region
- X MIN, X MAX: X min, X max position
- Y MIN, Y MAX: Y min, Y max position
- Z MIN, Z MAX: Z min, Z max position
- X SPAN, Y SPAN, Z SPAN: X, Y, Z span of the simulation region
The DOWN SAMPLE X, Y, Z option is used to set the spatial downsampling performed by the monitor. A down sample value of N corresponds to sampling (recording) the data every Nth grid points. Setting the down sample value to 1 gives the most detailed spatial information (i.e. information at each grid point).
Data to record
- STANDARD FOURIER TRANSFORM: The monitor outputs data at specific frequencies.
- PARTIAL SPECTRAL AVERAGE:The monitor outputs the partial spectral average power through a monitor surface, normalized to the partial spectral average of the source.
- TOTAL SPECTRAL AVERAGE: The monitor outputs the total spectral average power through a monitor surface, normalized to the total spectral average of the source.
- OUTPUT EX, EY, EZ, HX, HY, HZ, PX, PY, PZ: A set of fields with which the user can select what field components (EX, EY, EZ, HX, HY, HZ) or Poynting vector (PX, PY, PZ) to measure. In 2D simulations, only some components are non-zero (i.e. only EX, EY, and HZ are applicable for TE simulations). All the field quantities remain active to facilitate easy change between TE and TM simulations.
- OUTPUT POWER: For surface monitors (3D simulation) and line monitors (2D simulation) only. You can calculate the integrated power over the monitor surface. This requires much less memory after the simulation is completed and is particularly suitable for large parallel simulations where only the integrated power across a surface is required.
Spectral averaging and apodization tab
- PARTIAL SPECTRAL AVERAGING, DELTA: FWHM of the Lorentzian weighting function.
- APODIZATION: Specifies the window function of the apodization. Options include none, start (i.e. beginning of time signature apodized), end (i.e. end of time signature apodized, or full (i.e. both start and end). Note that apodization will, in general, invalidate any source normalization performed and is therefore not suitable for accurate power measurements.
- APODIZATION CENTER, TIME WIDTH, FREQ WIDTH: See the diagram below for the definition of these terms. The FREQ WIDTH corresponds to the effective bandwidth of the full apodization window, as described under APODIZATION.
Note: Apodization functions
FULL apodization involves windowing the time-domain data on both the start and end side. The resulting “windowed” data is then processed to produce frequency-domain information.
START apodization involves windowing the front side of the time-domain data. This can be useful to exclude the initial source excitation from the frequency-domain data.
END apodization involves windowing the last part of the simulation. This can be useful for ramping down the time-domain signal in devices where the radiation lives a long time, like cavities.
For more information, please visit the apodization page.
|WARNING: This tab includes options that should only be changed if you are quite familiar with the meshing algorithm and techniques used.
- SPATIAL INTERPOLATION: In FDTD generally, the electromagnetic field components are not recorded at the same point in space. Instead, each component of the vectorial electric and magnetic fields are recorded at different locations in the Yee cell. In order to properly calculate the Poynting vector and electromagnetic energy density, the fields are interpolated to the same location in the Yee cell. This setting controls how the fields are interpolated. With the NEAREST MESH CELL option (default for 'field and power' monitors), the fields are interpolated to the nearest FDTD mesh cell boundary. With the SPECIFIED POSITION option (default for 'profile' monitors), the fields are recorded exactly where the monitor is located. With NONE, no interpolation is performed and each electromagnetic field component is recorded at a different position within the Yee cell.
Note: Spatial interpolation - NONE setting
Disabling the spatial interpolation is a very advanced feature. Only expert users that are very familiar with the FDTD method should consider using this feature. Most standard analysis functions (such as the transmission script function, the data visualizer, etc) assume that the spatial interpolation is enabled. They may not give the most accurate result when used to analyze such monitor data. All analyses must be done manually.
- RECORD DATA WITHIN PML: Collect monitor data within the PML boundary condition region. This is a very advanced option that should rarely be used. Contact Lumerical support before enabling this option. The simulation region - EXTEND STRUCTURE THROUGH PML option must be disabled when using this option.
OVERRIDE ADVANCED GLOBAL MONITOR SETTINGS: When this option is selected MIN SAMPLING PER CYCLE can be set. The other options cannot be altered, they are there to display settings.
- MIN SAMPLING PER CYCLE: This parameter determines the minimum amount of sampling per optical cycle that can be used. By default, it is set at 2 (the Nyquist limit) for optimum efficiency.
- DESIRED SAMPLING: This converts the minimum points per optical cycle into an actual sampling rate in Hz.
- NYQUIST LIMIT: The Nyquist sampling limit is calculated based on the maximum frequencies that may be present in the simulation volume.
- ACTUAL SAMPLING: The actual sampling rate is the rate that will actually be used for the discrete Fourier transforms (DFTs), taking into account the desired sampling rate, the Nyquist limit, and the time step, dt.
- DOWN SAMPLE TIME: This is the time step downsampling.
- E: Electric field data as a function of position and frequency/wavelength.
- H: Magnetic field data as a function of position and frequency/wavelength.
- P: Poynting vector as a function of position and frequency/wavelength.
- T: Transmission as a function of frequency/wavelength.
- FARFIELD: Farfield data can be obtained. For details about farfield settings, see the Simple far-field projection example.
- POWER: Time-averaged power as a function of frequency. This data is only returned when a surface monitor (3D simulation) or line monitor (2D simulation) is used. For more information or an example using this data, see Parseval's theorem.