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Ring Modulator - Time Domain

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cover_picture_ring_modulator_time_domain_zoom33This example is the continuance of the Ring Modulator example. With the ring modulator compact model built in the above-mentioned example, time domain simulation is then performed after the validation of the model and measurements such as the eye diagram and modulator S21 response can be retrieved. To simplify the design, we replaced the MODE Waveguides in the ring modulator model by Straight Waveguides.

Files and Required Products

 

download-example

 

INTERCONNECT

 

Minimum product version: 2019a r3

Contents

Overview

Run and results

Important model settings

Updating the model with your parameters

Taking the model further

Additional resources

Overview

Understand the simulation workflow and key results

workflow_ring_modulator_time_domain_new_zoom66

It is always recommended to do a validation of the ring modulator model using the Optical Network Analyzer (ONA) before putting it into a compact model. Then the compact model can be used in a time domain simulation for further measurements such as eye diagram and time signal.

Step 1: simulation with ONA (validation)

Ring modulator validation

The impulse response (time domain) and scattering analysis (frequency domain) results of the ring modulator are measured in this step by using the ONA. Delay compensation and fractional delay are enabled in the Straight Waveguide models to makes sure the time domain result matches the frequency domain result in the bandwidth of interest.

 

Create compound (compact model)

Once the ring modulator has been validated, we can create a compound out of it to build a compact model for further use in the ensuing simulations.

Step 2: Simulation with laser source

After the compact model has been validated, it can be used in time domain simulations. In this example, we use a laser source and an electrical signal generated based on a Pseudo-Random Bit Sequence (PRBS) to drive the ring modulator and measure its output and the signal eye diagram.

Run and results

Instructions for running the model and discussion of key results

Step 1: Time & frequency domain results comparison (with ONA)

1. Open the ring_modulator_time_domain_validation.icp file in INTERCONNECT.

2. Open and run the ring_modulator_time_domain_validation.lsf script file to generate the comparison plot. Alternatively, take the following steps to generate the same plot.

3. Set the ONA “analysis type” to “scattering analysis” and the DC_1 “amplitude” to 0.

4. Run the simulation.

5. In the Result View window, plot the “input 1/ mode 1/ gain” result by

a. Right-clicking the result entry and selecting “Visualize” or “Add to visualizer…”

b. Double-clicking the result entry

6. Return to design mode (by clicking on the run button) and set the ONA “analysis type” to “impulse response” and rerun the simulation.

7. In the Result View window, plot the “input 1/ mode 1/ gain” result by right-clicking the result entry and selecting “Add to visualizer 1”.

8. Return to design mode and set the DC_1 “amplitude” from to 1, and rerun the simulation.

9. In the Result View window, plot the “input 1/ mode 1/ gain” result by right-clicking the result entry and selecting “Add to visualizer 1”.

 

Resonance Gain curve

The following plot shows the resonance gain curves of the ring modulator model under scattering analysis and impulse response for different driving voltages. With fractional delay and delay compensation in the waveguides accounted for (as will be shown in the next section), the time domain gain curve matches that of the frequency domain in the band of interest (central frequency band).

The inset shows the enlarged view of the gain curves at one of the ring resonance peaks (@ ~1550 nm) and the shift of the curve under different driving voltages forms the modulation effect of the ring modulator.

ring_modulator_time_domain_gain_zoom33

 

Create compact model

To create the ring modulator compact model for further use in the next step of time domain simulations, follow the steps below:

1. Select all elements except the DC source.

2. Right click and select “Create” then “Compound element”.

3. Edit the compound and add 4 optical ports, 2 on left side and 2 on right side. Then auto-arrange the ports location by clicking on the “Arrange” button.

4. Expand the compound and connect the Relays to the output ports of the ring modulator.

For further information about creating Compound Element, see here.

Step 2: Time domain simulation (with laser source)

1. Open the ring_modulator_time_domain_laser.icp file.

2. Run the simulation. Results can be explored manually in the Result View window of the analyzers by

a. Right-clicking the result entry and selecting “Visualize” or “Add to visualizer…”

b. Double-clicking the result entry

 

Eye diagram

An eye diagram is the signal waveform display in which the signal is repetitively sampled and overlaid on top of each other. Some useful system quality criterion can be measured from the eye diagram such as Quality-factor (Q-factor), bit error rate (BER) and time jitter. The following eye diagram shows a clear open eye with slight noise from the system.  

ring_modulator_time_domain_eye_zoom25

 

Time signal

The following time signal shows the response of the photodetector at steady-state (well past the turn-on stage of the photodetector). The shape of the time signal matches that of the input signal, but with a lower power/amplitude level and noises accumulated in the system.

ring_modulator_time_domain_time_signal_zoom25

 

EO response (Net 3dB bandwidth)

1. Open the ring_modulator_time_domain_EO_response.icp file.

2. Open and run the script file ring_modulator_time_domain_EO_response.lsf.

 

The script ring_modulator_time_domain_EO_response.lsf analyzes the output signal from the modulator driven by a step source. The step source is biased at the linear region of the modulator (0.6 V in this example) and has an amplitude of a small perturbation (small signal, and in this example as 0.1 V). The output signal goes through a derivative to get the impulse response then a Fourier transform to get the EO response. Note that this measures the net bandwidth of the modulator, which includes both electrical bandwidth and optical bandwidth.

ring_modulator_time_domain_eo_zoom25

Important model settings

Description of important objects and settings used in this model

Delay compensation & fractional delay: The delay compensation compensates the artificial delay caused by the bidirectional ports in the circuit. Fractional delay allows a non-integer multiple of sample period delay in a waveguide element. Together with the delay compensation, it guarantees the accuracy of delay in waveguide elements. These two setting are typically important in ring models and are always enabled in time domain simulations. For more details on these settings, please visit this page.

ring_modulator_time_domain_delay_zoom50

 

Sample rate & ONA frequency range: When doing impulse response analysis using ONA and when there is other signal source(s) than the ONA in the circuit, the simulation “sample rate” needs to match the ONA “frequency range”. This is because the element that has both signal inputs (Optical Modulators (OMs) in this circuit) need the two signals to be synchronized for calculation.

 

A recommended way to set the two properties with same value is to set the expression of the ONA “frequency range” to the simulation “sample rate”.

ring_modulator_time_domain_freq_zoom45

Updating the model with your parameters

Instructions for updating the model based on your device parameters

For component model update of the ring modulator, for example the ring radius and ring-bus gap, visit the component design Ring modulator example. For time domain simulation:

Set the wavelength of interest in the Laser Source and Local oscillator.

Set the bit rate.

Set the electrical signal level (amplitude) of the non-return to zero Pulse Generator (NRZ).

Taking the model further

Information and tips for users that want to further customize the model

For component model design, visit the component design Ring modulator example. For time domain simulation design:

 

Other modulation formats

This example uses the on-off keying (OOK) signal to drive the ring modulator, and this can be easily scale up to other modulation formats driving signals. For advanced modulation formats, please visit this page.

 

Circuit with compact models

This example uses primitive elements to build the circuit. For more physical realistic circuits, replace the existing primitive elements models based on real components. For more information on compact model libraries, please visit the Lumerical Compact Model Library.

Additional resources

Additional documentation, examples and training material

Ring Modulator

INTERCONNECT Ring Modulator Model

PIN Mach-Zehnder

Digital filter setting

ONA frequency/time domain measurements

Lumerical Compact Model Library

Lumerical University courses:

Course_INT100

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