This video is taken from the varFDTD Learning Track on Ansys Innovation Courses.

## Transcript

The varFDTD solver is ideal for designing photonic integrated circuit devices where

there is no light coupled out of the device in the vertical direction and there is negligible

coupling between different slab modes of the device.

Since it only takes about the equivalent amount of time and memory of a 2D FDTD simulation,

but it can give results that are comparable to a 3D FDTD simulation, it's good for simulating

large devices whose size makes it difficult or unfeasible to simulate using 3D FDTD, and

for optimization of designs where many simulations with different design parameters need to be

performed.

We will go over some examples of devices and simulation results from varFDTD including

the star coupler, taper, MMI coupler, ring modulator, and soliton propagation in a nonlinear

slab.

These examples can also be found in our online Knowledge Base, and the links to the examples

can be found below this video.

Star couplers have several input and output ports with a free space propagation region

between.

They can be used to split a single input signal into many outputs, or combine multiple inputs

into one output.

Star couplers are a component of arrayed waveguide gratings which are used for wavelength division

multiplexing in optical networks.

These devices can be on the order of millimeters in size, and the large size of these devices

can make the memory requirements prohibitively large to be simulated using 3D FDTD, but they

can be simulated in a reasonable amount of time using varFDTD.

Additionally, the propagation in the free space region of the slab can be represented

very accurately using the effective slab material in varFDTD.

Using a single broadband simulation, we can get the field profile of the light at different

wavelengths, and using mode expansion monitors, we can obtain the power transmitted to each

output, and the fraction of power which is travelling in the fundamental mode of each

of the output waveguides.

When designing a taper with a set length, we can optimize the shape of the taper to

maximize the average transmission over the operational bandwidth.

The shape of the taper in this example is defined by an equation with variable "m".

Using the built-in parameter sweep tool, we can sweep the value of m and collect the average

transmission to determine the optimal value of m.

This requires running several simulations, so it's much more time efficient to run the

simulations using varFDTD compared to 3D FDTD.

Similarly to the star coupler, the multimode interference coupler, or MMI coupler, can

be used to split or combine signals, and the propagation in the interference region can

be simulated accurately using the varFDTD method.

This is an example of a 1 by 2 MMI coupler which can be used to split light from 1 input

port to 2 output ports.

This type of device can also be simulated using the EME solver in MODE Solutions.

The EME solver is efficient for sweeping the length of the device, however, using the varFDTD

solver can be preferable to in the case where you have a wide interference region which

can support many modes, since you would need to calculate a large number of modes with

EME in order to be able to accurately represent the light propagation in the interference

region.

Using varFDTD, we can obtain the field profile and transmission through each port over a

broadband range from one simulation.

This example shows a ring modulator which is a tunable ring resonator.

The permittivity of the ring is modulated by an electrical input signal, and we can

study the transient response due to a given electrical signal.

The transient response requires a time-domain simulation method to characterize.

In this example, we simulate a step function electrical signal which takes the ring out

of resonance.

A custom material is used to represent the step function change in the material permittivity

as function of time.

This custom material is created using Lumerical's flexible material plugin framework.

A time monitor records the output time signal at the through port of the device.

You can see the initial fluctuations as the ring resonance is being established, followed

by the low signal until the step change in the material permittivity where the signal

overshoots at the leading edge of the step.

Nonlinear effects can be simulated using varFDTD.

This example shows a simple slab of nonlinear material with Kerr and Raman effects.

The nonlinear material effects can be tuned to counteract the linear dispersion in the

material to allow a pulse to maintain its shape as it propagates, forming a soliton.

We can simulate a pulse propagating in the slab and record the pulse shape after different

propagation distances.

These are just a few of the types of devices that can be simulated, and results that can

be obtained using the varFDTD solver.