In this demo, we will learn about the importance of the sample rate in INTERCONNECT simulations
with multiple sources.
The sample rate of the optical and electrical signals in an INTERCONNECT simulation is determined
by source properties.
For example, the ONA acts as the optical source in this
frequency domain simulation and the sample rate of this optical signal is defined by
the "frequency range" property of the ONA.
Right now, we only have the ONA in our simulation and the "frequency range"
property of the ONA is that it defines the range over which the frequency
domain simulation will run.
The number of frequency points available in the calculated transmission is set by the
"number of points" parameter and we have set this point to 1000 to ensure that the peaks
in the transmission spectrum can be properly resolved.
Sure enough when we run the simulation we can see that the simulated transmission spectrum
has a range of 1 THz and that the peaks are nicely resolved.
Now we'll see what happens when we add an electrical source to our circuit.
We will now edit the ring and turn it into a ring modulator driven by a DC electrical
To do this, we will first add an "Optical Modulator Measured" element from the Element
It can be found in the "Modulators" folder inside the "Optical" subfolder.
To place this element inside the ring, we will first break the connection between waveguide
2 and waveguide coupler 1 by selecting the connection and pressing "DELETE", and then
connect port 1 of the modulator to port 4 of the waveguide coupler 1 and port 2 of the
modulator to port 1 of waveguide 2.
Next, we will add a DC source from the "Source" folder, and "Electrical" subfolder in the Element
Connect the output of the DC source to the modulation port of the optical modulator.
We will now set the properties of the Optical Modulator Measured element.
If we assume that 80% of the ring is active then we can set the length parameter as 0.8
times 2 pi times the radius, recalling that the ring has a radius of 40 micron.
The next step is to set the measurement data for the modulator.
The optical modulator measured element is used in places where measurement data are
available for the modulator.
The measurement data can be either in the form of effective index or in the form of
absorption and phase as a function of bias voltage.
As we can see, the element comes populated with data.
To make sure that the change in index is enough to give us a significant shift in the transmission
spectrum, we will edit this data.
For the DC source we will keep the amplitude to 0 for now.
Note that the sample rate of the DC source is set to be equal to the sample rate property
of the Root Element by default using the expression.
The sample rate is currently set to 1.6 THz.
If we run the simulation as it is, we can see that the simulation fails and gives us
an error message that says that the sample rate of the optical signal at port 1 of the
modulator is wrong.
This is because the sample rate of the electrical signal coming from the DC source is set to
1.6 THz while the sample rate of the ONA is set to 1 THz.
During the preliminary parameter extraction step, the optical modulator tries to calculate
the S-matrix by considering both the optical and electrical signals but fails since the
sample rate is different between the two.
The simplest way to solve this problem is to set the frequency range of the ONA to be
equal to the sample rate of the Root Element using an expression, similar to the DC source.
The advantage of setting the frequency range this way is that now if we want to change
the frequency range of the ONA to calculate the transmission spectrum over a larger range,
we only have to change it once in the Root Element and the sample rate of both the ONA
and the DC source will get updated.
Running the simulation again we can see that the error has been resolved and the simulation
finishes successfully. Let's plot the absolute value squared of the transmission.
We will now change the bias applied to the modulator to minus five volts and see how it changes the spectrum.
Let's plot the transmission in the same window.
We can see that by applying a bias voltage we have shifted the transmission spectrum of the ring modulator.