In this demo, we are going to revisit the ring modulator circuit from the frequency
domain simulation section.
This time we are going to run a time domain simulation using the transient sample mode.
To do so, we need to edit the element properties.
First click on the ONA, scroll down to the Numerical properties, and set analysis type
to impulse response.
We are going to run a TE simulation, so let's set the orthogonal identifier to 1.
Now let's update the numerical properties of the waveguides.
Use the mouse to select both of the waveguides and scroll down to the Numerical properties.
The group delay is critical for the waveguides, so we will set digital filter to true, single
tap filter to false, and number of taps estimation to group delay.
The total delay compensation required is 5, as there are five connections along the ring.
For Straight Waveguide 1, set the delay compensation to 2, and for Waveguide 2, set the delay compensation
Now use the mouse to select both Waveguide Coupler 1 and 2.
We are going to treat these as point couplers with no delay.
To do so, set the number of fir taps to 1.
Let's use a DC source amplitude of 0.
The last thing we need to set is the sample rate.
Click on the ONA, and recall that the frequency range currently inherits the Root Element
We will use the script prompt to calculate the minimum sample rate based on the recommendations
in the last unit then update the sample rate.
The minimum sample rate is delay compensation, 3, times the speed of light, c, divided by
the group index, which is 3.9 for the TE mode, times pi R, where R is 40 microns.
Here 3 is the larger of the two delay compensations.
The question mark is used to display the result.
The minimum sample rate is 1.83e12.
Click outside of the circuit to get the Root Element properties.
We'll set the sample rate 2e12.
The simulation completed successfully.
Now let's set the sample rate to 1e12, which is below the minimum value.
Run the simulation.
We see that we get a warning message that says to increase the sample rate to 2e12.
Let's return it back to 2e12 and run.
We are now going to compare the free spectral range to the value predicted analytically,
which is the speed of light, c, divided by the group index, 3.9, times the total ring
length, 2 *pi* radius.
The free spectral range is predicted to be 0.306 THz.
Before we look at the simulated FSR, let's first plot the transmission for the drop channel,
input 2, to make sure we've captured multiple peaks.
Set the transmission to Abs^2.
Now that we know we've captured multiple peaks, let's plot the free spectral range.
It is close to the expected value of 3.06e11 Hz.
Note that if we set the delay compensation of the waveguide to 0, then when we plot the
free spectral range, it drops to around 1.7e11 THz, which is incorrect.