In this example, Ansys Circuit and INTERCONNECT are used to perform an electro-optical signal integrity simulation of a 2.5D integrated optical transceiver. The transceiver consists of an electrical integrated circuit (EIC) and photonic integrated circuit (PIC) connected through an interposer.
Ansys Circuit is used to model the electrical part of the signal path and INTERCONNECT is used to model the optical part. Unidirectional signal transfer is used to connect the electrical and optical sections of the signal path. The signal path over the interposer is modelled using S-parameters calculated with 3D electromagnetic simulations in Ansys HFSS.
Overview
Understand the simulation workflow and key results
The transceiver signal path begins at the driver on the EIC which sends a 10 Gb/s NRZ signal over the interposer to a depletion-type ring modulator on the PIC. The modulated optical signal passes through an attenuator representing the channel losses and reaches a photodetector on the receiver. The photocurrent drives the received signal back over the interposer to a resistor on the EIC.
Step 1: Transmitter Electrical Circuit
Circuit is used to simulate the electrical portion of the transmitter signal path between the driver on the EIC and the ring modulator on the PIC.
The transmitter electrical circuit consists of a voltage source representing the modulator driver, a state space model element for the interposer, and the electrical equivalent circuit of the ring modulator. The interposer state space model is generated based on electrical S-parameters calculated by 3D electromagnetic simulations with Ansys HFSS.
The ring modulator electrical equivalent circuit consists of two resistors and a capacitor representing the resistance and capacitance of the modulator PN junction. The voltage across the junction capacitance in the equivalent circuit is saved in a text file and used as the input to the ring modulator’s optical model in the next step.
Step 2: Optical Channel
Lumerical INTERCONNECT is used to simulate the optical channel consisting of a laser source, transmitter, and receiver.
The voltage recorded in a text file in the previous step is read in by the “Signal Voltage” element and used to drive the ring modulator model in the transmitter. A 3 dB attenuation is used to model the losses in the optical channel between the modulator and the photodetector on the receiver. A low pass filter element is placed after the photodetector to model the carrier transit time limited bandwidth of the photodetector. The photocurrent generated by the photodetector on the receiver is saved as a text file by the “PD Current” element and used as an input to the next step.
Step 3: Receiver Electrical Circuit
Similar to the transmitter electrical circuit, Ansys Circuit is used to simulate the electrical section of the receiver. The receiver electrical circuit consists of the electrical equivalent circuit of the photodetector, the interposer state space model, and a resistor on the EIC acting as a transimpedance amplifier (TIA).
The photocurrent from the INTERCONNECT simulation is read in from a text file and used to drive a current source in the electrical equivalent circuit of the photodetector. The voltage generated across the load resistor on the EIC is recorded and used to create an eye diagram of the received signal.
Run and Results
Instructions for running the model and discussion of key results
Step 1: Transmitter Electrical Circuit
- Open Ansys Electronics Desktop and extract the archive file optical_transceiver.aedtz into the folder with the example files.
- Right click on the “Driver” circuit in the Project Manager and select Analyze to run the simulation.
- When the simulation has finished running, right click on Results > RM_Voltage and select Export…
- Save the results as a Comma delimited data file (*.csv) with the name RM_Voltage.csv in the folder with the example files.
The RM_Voltage result report records the voltage across the capacitor representing the junction capacitance of the ring modulator, as recorded by the Differential Eye Probe placed in parallel with the capacitor in the ring modulator electrical equivalent circuit.
This voltage is exported to a text file named RM_Voltage.csv so that it can be imported into INTERCONNECT in the next step.
Step 2: Optical Channel
- Open the simulation file optical_channel.icp in INTERCONNECT.
- Run the script process_voltage_data.lsf in INTERCONNECT to convert the voltage data from Circuit to the correct format.
- Run the simulation in optical_channel.icp .
The process_voltage_data.lsf script converts the voltage data in the RM_Voltage.csv file from the previous step into the correct format for INTERCONNECT by removing the file headers and converting the time units to seconds. The updated data is saved as a text file named RM_Voltage_processed.txt.
The processed voltage data is read in by the Piecewise Linear Import element “Signal Voltage” and used to drive the ring modulator. The Piecewise Linear Export element “PD Current” automatically exports the photodetector current signal into a text file named PIN_output.txt in the same folder as the simulation file. This file can be imported back into Ansys Circuit in the receiver circuit in the next step.
The Eye Diagram analyzer creates an eye diagram of the electrical signal generated by the photodetector.
Note: The script and simulation files in this step assume that they are placed in the working directory as the RM_Voltage.csv file from the previous step. You can set the current working directory using File > Change working directory in the toolbar. The filename property of the "Signal Voltage" must point to the RM_Voltage_processed.txt file created by the process_voltage_data.lsf script, which is placed in the current working directory by the script.
Step 3: Receiver Electrical Circuit
- Go back to the optical_transceiver.aedt project in Ansys Electronics Desktop.
- Double click on the “Receiver” circuit in the Project Manager to open the schematic of the receiver electrical circuit.
- Right click on the “Receiver” circuit in the Project Manager and select Analyze to run the simulation.
The I_PD piecewise linear current source will import the photodetector current from the PIN_output.txt text file saved in the INTERCONNECT simulation in the previous step. The VL voltmeter will record the voltage across the load resistor. An eye diagram of the voltage will be automatically plotted on the schematic.
Note: The PWLFILE property of the I_PD element in the Receiver circuit must point to the PIN_output.txt created by INTERCONNECT in the previous step. By default, it is set to point to the same location as the AEDT project file.
Important Model Settings
Description of important objects and settings used in this model
File Pathways for Data Exchange
In this workflow, the electrical signals are exchanged between AEDT and INTERCONNECT through text files. If the files are not in the expected locations the workflow will not work. The steps described in the Run and Results section above assume that all of the files for this workflow are placed in the same file location.
For the exchange from AEDT to INTERCONNECT, the filename and path of the generated file is determined by the dialog window that opens when you right click on Results > RM_Voltage and select Export… in the Project Manager . The file used as input to the script process_voltage_data.lsf is the file specified in line 8 of the script:
voltage_filename = "RM_Voltage";
By default this file is assumed to be in the current working directory of INTERCONNECT. The ".csv" file extension is appended to this name in line 10 of the script. The text file output by the script is given the same name with "_processed" appended to it and is placed in the same location as the input file.
The data in this file is then read in by the "Signal Voltage" element in the INTERCONNECT simulation, which is a Piecewise Linear Import element. The file read in by this element is set using the filename property of this element.
When the INTERCONNECT simulation is run, the "PD Current" element, which is a Piecewise Linear Export element, will automatically export the electrical signal values at its input port to the text file specified by its filename property.
The data format exported by Piecewise Linear Export elements is already suitable for import into AEDT so no editing is required. The data from this file is read in by the "I_PD" component in the Receiver circuit in AEDT, which is a Piecewise Linear Current Source. The file read in is specified using the PWLFILE property of this component.
Electrical Equivalent Models
The loading effect of optoelectronic components like modulators and photodetectors is modelled using electrical equivalent circuits for the components. These circuits typically consist of basic electrical components like resistors, capacitors, inductors, and diodes. The values used for these components can be calculated using simulations or extracted from experimental measurements.
In this workflow, relatively simple equivalent circuits are used for the modulator and photodetector to represent the resistance and capacitance of these devices. More detailed electrical equivalent circuits that include other parasitic electrical effects can be used, depending on the details of the device design.
Taking the Model Further
Information and tips for users that want to further customize the model
Simulation with Multiple Channels
This workflow can be extended to include multiple channels by copying the components used in this example. Each channel will have separate text files for its signal transfer between AEDT and INTERCONNECT, with separate corresponding elements in each circuit for exporting/importing the signals.
Additional Resources
Additional documentation, examples and training material
See Also
- Signal Integrity Analysis in a Co-packaged Optics System using RaptorX-Spectre-INTERCONNECT Interop
- Thermally aware photonic circuit simulation of a WDM transceiver – Icepak integration
- Layout-aware statistical yield analysis – WDM transceiver
- Wavelength division multiplexing