varFDTD solver - Simulation object

General tab

• BACKGROUND INDEX: The refractive index of the surrounding, background medium in the simulation region.
• SIMULATION TIME: The maximum duration of the simulation to be performed. The actual simulation may be shorter if the autoshutoff criteria are satisfied before this maximum simulation time is exceeded. For more information about how to choose the simulation time see set the simulation region section of the introduction in the Getting Started guide.

Geometry tab

• 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

Effective index tab

Slab mode position

• The x,y coordinate of the single point (core region) used by the effective index method to calculate the reference vertical slab mode used to generate the effective materials.
• Effective index method:The method used to calculate the effective index of the waveguide slab, see Propagator Physics in the Solver section for more information on each method.
• CAN OPTIMIZE MESH ALGORITHM FOR EXTRUDED STRUCTURES: This option allows the meshing algorithm to run much more efficiently when the geometry is made of objects that are extruded along the z axis with purely vertical sidewalls. Note that if the geometry contains more complex shapes (ie. when the sidewalls are not purely vertical) this option must be unchecked.
• CLAMP VALUES TO PHYSICAL MATERIAL PROPERTIES: The effective index treatment may lead to generated materials that have properties that are unphysical (for example, having an artificial negative imaginary index). This checkbox provides the option of restricting the range of generated indices to the min/max values defined by the physical material properties of the original materials.

Simulation bandwidth

• NARROWBAND: the material properties are only taken at the center frequency of the simulation. This option should be used for single frequency simulations (or when the frequency range of interest is very narrow), since only a single (n,k) value is generated over the entire frequency span of the simulation.
• BROADBAND: a dispersive material model will be created to fit the effect index materials based on the following settings:
  • maximum number of materials: the maximum number of different materials that are created from the effect index method
  • number of samples: the number of sample points to use for fitting the materials
  • max coefficients, fit tolerance, imaginary weight, improve stability, make fit passive: parameters used to fit the generated sample data (see Material explorer)

Test

The x,y coordinates of the test points used to study the materials created in those regions. The generated test materials corresponding to each test point can be accessed in the material explorer. Note that the coordinates of the test points are relative to the center position of the solver region.

Polarization

The polarization of the slab mode. If "user select" is chosen, the "Select mode" button is enabled. This button will open a mode solver to calculate the slab modes for the user to choose from.

Plot

The type of field components, refractive index (real or imaginary) to plot.

• PLOT CURRENT MODE: Plot the current mode.
• PLOT IN NEW WINDOW: Plot the current mode in a new window.

Mesh settings tab

Mesh type

Three types of mesh generation algorithms are available, as described below

• Auto non-uniform (default):

A non-uniform mesh is automatically generated based on the mesh accuracy slider bar.  It is strongly recommended to start with a mesh accuracy of 1-2 for initial simulations to make the simulations run quickly. Higher mesh accuracies can be used for convergence testing if necessary.

The MESH ACCURACY parameter is an integer from 1-8, where 1 is low accuracy, and 8 is high accuracy (smaller mesh).  Many factors go into the meshing algorithm, including source wavelength, material properties and the structure geometry. The number of mesh points per wavelength (ppw) is a major considerations for the meshing algorithm. Accuracy 1 corresponds to a target of 6 ppw. Acc 2 ->10 ppw, Acc 3 ->14ppw, and so on, in increments of 4 ppw per point on the slider bar. It is important to remember that wavelength is inversely proportional to the refractive index. In high index materials, the effective wavelength is smaller, meaning that the meshing algorithm will use a smaller mesh in high index materials.

• Custom non-uniform:

This setting allows the user to additional options to customize how the non-uniform mesh is generated. If setting the mesh cells using wavelength, the default setting of 10 is sufficient in general, but may be reduced to 6-8 for coarse simulations.

The grading factor determines the maximum rate at which the mesh can be modified. For example, if dx(i+1) = a*dx(i), then 1/(GRADING FACTOR) <= a <= GRADING FACTOR. The grading factor should be between 1 and 2. The default setting is sqrt(2).

• Uniform:

A uniform mesh is applied to the entire simulation volume, regardless of any material properties. If a mesh override region is used in conjunction with this option, the override region will force the mesh size everywhere, not just within the override region (afterall, the mesh is uniform).

Mesh Refinement

Mesh Refinement: Mesh refinement can give sub-cell accuracy for a simulation. See the Mesh refinement options page for more information.

Time Step:

• DT STABILITY FACTOR: A setting which determines the size of the time step used during the simulation, defined as a fraction of the Courant numerical stability limit. A larger number will result in faster simulation times, and a smaller number will result in slower simulation times. The Courant stability condition requires that this setting must be less than 1 for the FDTD algorithm to remain numerically stable.
• DT: The time step of the FDTD/Propagator simulation. This is determined by the values of the spatial grid to ensure numerical stability and cannot be directly set by the user.

Minimum mesh step settings

Min Mesh Step: The MIN MESH STEP sets the absolute minimum mesh size for the entire solver region. This overrides all other mesh size settings, including mesh override regions.

Boundary conditions tab

The boundary conditions that are supported by FDTD/MODE Solutions are listed below.

 PML

Perfectly matched layer (PML)1 boundaries absorb electromagnetic waves incident upon them. They essentially model open (or reflectionless) boundaries. In FDTD and varFDTD simulation regions, the user can directly specify all the parameters that control their absorption properties including the number of layers. To facilitate the selection of PML parameters, a number of profiles (or predefined sets of parameters) are available under the boundary conditions tab. In most simulation scenarios, the user only needs to choose one of the predefined profiles and fine tune the number of layers. PML boundaries perform best when the surrounding structures extend completely through the boundary condition region. This will be the default behavior of structures whether or not they were drawn to end inside or outside the PML region.

1 J. P. Berenger, Perfectly Matched Layer (PML) for Computational Electromagnetics. Morgan & Claypool Publishers, 2007.

 Metal

Metal boundary conditions are used to specify boundaries that behave as a Perfect Electric Conductor (PEC). The component of the electric field parallel to a metal (PEC) boundary is zero; the component of the magnetic field H perpendicular to a metal (PEC) boundary is also zero. Metal boundaries are perfectly reflecting, allowing no energy to escape the simulation volume along that boundary. In the FDE solver, metal BC is the default setting.

 PMC

Perfect Magnetic Conductor (PMC) boundary conditions are the magnetic equivalent of the metal (PEC) boundaries. The component of the magnetic field H parallel to a PMC boundary is zero; the component of the electric field perpendicular to a PMC boundary is also zero.

 Periodic

Periodic BC should be used when both the structures and EM fields are periodic. Periodic boundary conditions can be used in one or more directions (i.e. only in the x direction) to simulate a structure which is periodic in one direction but not necessarily other directions.

 Bloch (FDTD/varFDTD)

Bloch BC should be used when the structures and the EM fields are periodic, but a phase shift exists between each period. Bloch boundary conditions are used in FDTD and propagator simulations predominantly for the following two simulations:

• Launching a plane wave at an angle to a periodic structure – in this situation, accurate reflection and transmission data can be measured at a single frequency point for a given simulation.
• Calculating the bandstructure of a periodic object – in this situation, a broadband pulse is injected via a dipole source into a periodic structure.

Note: if you choose BFAST plane wave source, the Bloch BCs will be automatically overridden and use its built_in boundary conditions.

 Symmetric / Anti-Symmetric

Symmetric/anti-symmetric boundary conditions are used when the user is interested in a problem that exhibits one or more planes of symmetry; both the structure and source must be symmetric. Symmetric boundaries are mirrors for the electric field, and anti-mirrors for the magnetic field. On the other hand, antisymmetric boundaries are anti-mirrors for the electric field, and mirrors for the magnetic field. Careful consideration must be given to whether symmetric or anti-symmetric boundary conditions are required, given the vector symmetry of the desired solution. For meaningful results, the sources used must have the same symmetry as the boundary conditions. Further information about symmetric and anti-symmetric boundary conditions can be found in Choosing between symmetric and anti-symmetric BCs.

• ALLOW SYMMETRY ON ALL BOUNDARIES: Allows symmetric boundary conditions with periodic structures (this option is not available in the boundary condition tab of mode sources and mode expansion monitors). 

• XMIN, XMAX, YMIN, YMAX, ZMIN, ZMAX BOUNDARIES: These fields describe the boundary conditions to be applied along the perimeter of the simulation region. Symmetric and asymmetric boundary conditions should be applied to the lower boundary conditions.
• ALLOW SYMMETRY ON ALL BOUNDARIES: By default, symmetric and anti-symmetric conditions can only be used on the lower boundaries (x min, y min and z min). This box allows you to also use symmetry and anti-symmetric conditions on the upper boundaries in order to simulate periodic structures that exhibit symmetry.
• SET BASED ON SOURCE ANGLE: Bloch boundary conditions are often used to inject plane waves on angles into periodic structures. This option will determine the values kx, ky and kz for you based on the source in your current simulation. Note that if more than one source is defined, all sources must require consistent Bloch settings. By default, this box is checked. If you uncheck the box, you should set kx, ky and kz directly.
• BLOCH UNITS: Two types of units are allowed for specifying the values of kx, ky and kz:

• bandstructure: In these units kx,y,z are defined in units of (2p/ax,y,z) where ax,y,z is the x,y or z span of the FDTD simulation region. These units are very convenient for bandstructure calculations.
• SI: In SI units, kx,y,z are defined in units of m-1. This is generally more convenient for injection of plane waves on angles.

• KX, KY, KZ: The wavevector setting in the x, y and z direction for the Bloch boundary in the units specified above.

• PML SETTINGS - TYPE: Sets the type of PML boundary formulation to be used. The options are a PML based on a stretched coordinate formulation or a PML based on a uniaxial anisotropic material formulation (legacy option).
• PML SETTINGS - SAME SETTINGS ON ALL BOUNDARIES: When unchecked, this option allows users to set different PML settings for the XMAX, YMIN, YMAX, ZMIN and ZMAX boundaries. When checked, the same PML settings are used for all boundaries.
• PML SETTINGS - TABLE: Sets the PML profile to be used on each boundary. The section on PML boundary conditions provides a brief description of the intended use of each profile.
• PML SETTINGS - EXTEND STRUCTURE THROUGH PML: see extending structures through PML.

Advanced options tab

Simulation bandwidth

• SET SIMULATION BANDWIDTH: By default, the simulation bandwidth is inherited from the source bandwidth. Enabling this option allows the simulation bandwidth to be set directly. Simulation bandwidth affects many aspects of the simulation including mesh generation, material fits, monitor frequency ranges, etc.

Mesh settings

• FORCE SYMMETRIC X, Y, Z MESH: This will force a symmetric mesh about the x, y or z axis. When this option is enabled, the meshing algorithm ONLY considers objects in the positive half of the simulation region. The mesh in the negative half is simply a copy of the positive half mesh. All physical structures and mesh override regions in the negative half will not be considered by the meshing algorithm. This option also forces a mesh point at the center of the simulation region. Forcing a symmetric mesh ensures that the mesh does not change when going from a simulation with symmetry to a simulation without symmetry.

• OVERRIDE SIMULATION BANDWIDTH FOR MESH GENERATION: This option allows the simulation mesh to be generated following a custom wavelength or frequency range, rather than the simulation bandwidth (which is inherited from the source by default).
• SNAP PEC TO YEE CELL BOUNDARY: This option forces any structures defined as PEC to have interfaces that are aligned with the Yee cell boundaries. This ensures that all electric field components at the PEC interface are tangential to the interface. This setting avoids complications that can result in some circumstances if normal electric field components are set to 0 at a PEC interface. To achieve this, the specified PEC interface may be shifted by as much as dx/2 when creating the simulation mesh, where dx is the size of the Yee cell.
• ENABLE SLAB LAYER DETECTION (varFDTD only): Accurately resolve slab layer vertical thickness by using a very small mesh at vertical interfaces. There is little cost to using a small mesh in the vertical direction, so this option is enabled by default.

Miscellaneous

• ALWAYS USE COMPLEX FIELDS: This checkbox forces the algorithm to use complex fields during simulation. This will result in slower simulation times and increased memory requirements and should only be used when necessary. By default, complex fields are only used when Bloch boundary conditions are present.
• MAX SOURCE TIME SIGNAL LENGTH: This is the maximum length of data used by sources to store the “time” and “time_signal” properties of sources. If a large number of sources are used and the simulation time is on the order of 100ps (which is very rare), advanced users may want to reduce this to save memory. However, since the “time” and “time_signal” properties of sources are important for calculating the sourcepower, sourcenorm, and the normalization for the transmission functions, care must be taken with source normalization. In particular, in most cases, nonorm option should be used.

Advanced PML settings

• PML KAPPA: The normalized imaginary electric and magnetic conductivity used in the PML boundaries.
• PML SIGMA: The maximum normalized electric and magnetic conductivity used in the PML boundaries.
• MINIMUM PML LAYERS: The minimum number of cells which are used for PML boundary conditions. Increasing PML layers will reduce the back-reflection arising from the boundaries but will also increase time and memory requirements for simulations. Defaults to a setting of 12.
• MAXIMUM PML LAYERS: The maximum number of cells which are used for PML boundary conditions. Increasing PML layers will reduce the back-reflection arising from the boundaries but will also increase time and memory requirements for simulations. Defaults to a setting of 64.
• PML POLYNOMIAL: The polynomial power which determines how rapidly the electric and magnetic conductivity increases as radiation propagates at normal incidence into the PML. The default setting of 3 denotes a cubic increase of electric and magnetic conductivity with increasing depth into the PML.
• TYPE OF PML: The type of PML used can be STANDARD or STABILIZED. In some cases, STABILIZED PML provides more numerical stability, although the performance is slightly reduced.
• EXTEND STRUCTURE THROUGH PML: In most cases, we want the structures to extend through the PML so that fields exiting the simulation region are completely absorbed, otherwise there could be some unwanted back reflection. This option automatically extends structures through the region even if they were not drawn as such in the CAD. 

Auto shutoff

• USE EARLY SHUTOFF: This will automatically end the simulation when most of the energy has left the simulation volume.
• AUTO SHUTOFF MIN: The simulation will end when the total energy in the simulation volume drops to this fraction of the maximum energy injected. The simulation data will automatically save.
• USE DIVERGENCE CHECKING: This will automatically end the simulation when the total energy in the simulation volume is this many times larger than maximum energy injected.
• AUTO SHUTOFF MAX: The simulation will end when the total energy in the simulation volume rises to this many times the maximum energy injected. The simulation data will automatically save.
• DOWN SAMPLE TIME: Check the auto shutoff conditions every down sample time number of dT time steps

Parallel engine options

• SET PROCESS GRID: The division of of the simulation volume into sub-regions, which are then run as a separate processes, can be manually set for more efficient computation. Better performance will be achieved when the regions are square e.g. for 20 regions, 5x2x2 is usually more efficient than 20x1x1.
• NX, NY, NZ: The process layout for parallel simulations.
• SET DEFAULTS: Can be used to reset the settings back to their original default settings.