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

## Transcript

For Transient Sample Mode simulations, the sample rate must be sufficiently high for

accurate results.

In the following, we'll provide some practical tips for choosing an appropriate sample rate.

For optical and electrical sources, it is recommended to inherit the sample rate from

the Root element, as was discussed in the Time Domain Simulation Setup, Sources unit.

For impulse response simulations with the Optical Network Analyzer, the sample rate

is defined by the frequency range property.

One way to check whether your sample rate is sufficiently high, is by comparing the

results from frequency and time domain optical network analyzer simulations.

Connect your element to an ONA with the analysis type set to scattering data.

Next connect your element to an ONA with analysis type set to impulse response.

Run the simulation and compare the plots of the group delay and transmission.

The frequency domain results are accurate and independent of the sample rate.

Increase the sample rate until there is good agreement between the frequency and time domain

simulations over an acceptable frequency range.

Here is an example of this sample rate check for a Straight waveguide with a length of

10 microns and a group index of 4.

The frequency domain results are plotted in blue and the time domain results are plotted

in green.

The two left-most plots show the group delay and real part of the transmission for a sample

rate of 20 THz.

The values agree at the center frequency of the simulation, but begin to deviate away

from it.

The two right-most plots show the results when the sample rate is increased to 80 THz.

Now the frequency range over which the time domain simulations are accurate is much larger.

When the time delay of the element is critical, special care must be taken to set the sample

rate.

In general, the sample period must be less than the group delay for accurate results.

This means that the sample rate should be larger than the inverse of the group delay.

When the delay compensation of an element is a non-zero value, the sample rate must

be greater than the delay compensation divided by the group delay.

If this condition is not met, a warning message will appear, as we'll see shortly in the demo.

Let's take a quick look at the ring modulator.

The length of each waveguide is pi times the radius and so the group delay for each is

pi R times the group index divided by the speed of light.

One waveguide has a delay compensation of 2, while the other is 3.

This means that the sample rate should be greater than 3 divided by the group delay,

as 3 is the larger delay compensation.