This video is taken from the HEAT 100 course on Lumerical University.

## Transcript

In this unit, we will review solver settings that are important when running transient

simulations.

Transient simulation controls determine the time step used in the simulation.

The “min time step” defines the smallest time step that will be used in the transient

simulation and is always the initial time step used by the solver at the start of simulation.

However, if changes in simulation variables are below a certain limit, the solver will

increase the time step (up to the “max time step” value) to make the simulation faster.

“absolute lte limit” and “relative lte limit” determine the absolute and relative

amount of tolerance for change in simulation variables below which the solver will increase

the time step.

Using larger values for transient simulation controls will make simulation faster at the

expense of less accuracy.

Larger values will also increase the chance of divergence issues.

Downsampling is used to reduce the amount of data saved during the simulation, particularly

when using a very small step size.

There are two downsampling modes available.

“Interval” and “count”.

Having a downsampling with count value “n” means that only the results at every n points

in the simulation will be saved.

On the other hand, with downsampling at an interval of delta T, simulation results will

be recorded every delta T femtoseconds and if no result is available at that exact time,

closest results to that moment in time will be recorded.

The global Source shutter settings will apply a time-dependent shutter to all the source

objects in the simulation.

If enabled, there are two shutter modes available.

Step and pulse.

Each one can be set in an initially ON or initially OFF mode.

The on and off times for the shutter can then be specified.

SHUTTER TSLEW sets a slew in the stepping of the shutter for a gradual change in source

power rather than abrupt.

This feature is useful in helping the solver to converge where a sharp step in the shutter

can make the simulation diverge.

The SHUTTER SLEW FUNCTION can be linear for a linear transition between ON and OFF states,

or logarithmic for an exponential change.

SHUTTER SLEW CUTOFF is available when the shutter uses the logarithmic function.

The change in source power will be exponential from this cut-off value to 1 (for turning

on) and from 1 to this value (for turning off).

The change from zero to the slew cutoff value (and back) is abrupt.

SOURCE POWER SCALING option can be used to scale the amplitude of the source.

By default the value is set to '1' which means there is no scaling.

In addition to the settings available for sources and the solver, when performing transient

simulations, some of the boundary conditions offer a transient mode which can be used to

make the boundary condition variable in time.

These settings are available from the property editor window for each boundary condition

object.