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Custom PDKs: Ring Resonator
Tools Used: OptSim Circuit and FullWAVE FDTD
Although photonic foundries continue to enhance support for wider arrays of active and passive photonic PDKs based on different technologies, there are times when the ability to generate custom, proprietary PDKs and use them in circuit simulations and mask generation are important. Whether to generate intellectual property (IP) or to supplement an existing PDK, OptSim Circuit together with RSoft device modeling tools offer virtually limitless ways to create custom components and PDKs and deploy them in PICs.
This application note uses output from the S-Matrix/PDK Generation Utility of RSoft device modeling tools in OptSim Circuit. For illustration, we consider a case of S-matrix generation of a half ring component in FDTD software tool FullWAVE as shown in Figure 1, and use it in OptSim Circuit as custom PDK for creating and simulating a full ring resonator circuit.
The ring structure is built from standard silicon wire waveguides. Simulation options in FullWAVE FDTD set grid size, simulation domain, boundary condition, etc. A pulse temporal excitation is used. The launch field is automatically moved to each port by the S-Matrix/PDK Generation Utility to generate respective transfer function data.
Figure 1. The half ring structure with ports labeled for clarity
Figure 2 shows setup options for the S-Matrix and PDK Generation Utility. The ¡°Output Mode¡± is set to ¡°S-Matrix + PDK¡± to generate S-matrix data file, an OptSim Circuit *.dta file, a GDS II mask file, and an *.spt file for OptoDesigner.
Figure 2. Setup options for the S-Matrix and PDK Generation Utility
To learn more about this utility and its options, please refer to Chapter 12 of RSoft device tools CAD manual.
To convert the output of the S-Matrix and PDK Generation Utility into an OptSim Circuit custom PDK, launch OptSim Circuit¡¯s User Libraries Organizer and select the *.dta file to be added as a custom PDK entity as shown in Figure 3.
Figure 3. User Libraries Organizer in OptSim Circuit
The PDK entity will show up in OptSim Circuit¡¯s models¡¯ library (Figure 4) as a custom PDK element that can be dragged and dropped in to the schematic like any other OptSim Circuit model in the models¡¯ tree.
Figure 4. Custom PDK available in the OptSim Circuit GUI for use in schematic like any other Circuit model
To use the half-ring custom PDK in OptSim Circuit as a building block to make a full ring resonator circuit, we start with creating a compound component, placing two half-ring PDKs and making connections as shown in Figure 5.
Fig. 5 An OptSim Circuit compound component using two half-ring PDKs to create a ring resonator circuit
Each half-ring PDK element uses S-Matrix data automatically generated by the S-Matrix and PDK Generation Utility in FullWAVE.
The S-matrix data file format follows the file format of OptSim Circuit¡¯s bi-directional optical multiport device model.
To generate a PDAFlow *.spt file for use with OptoDesigner, with the schematic of Figure 5 open, from the OptSim Circuit GUI, click on Utilities¨¤Generate PDAFlow Netlist (Figure 6).
Figure 6. Generating a PDAFlow netlist (*.spt file) for the schematic of Figure 5
Chapter 1 of the OptSim Circuit Interface to Mask Layout Software manual describes the interface and setup options used to create mask in OptoDesigner from the OptSim Circuit PDAFlow netlist. Figure 7 shows the mask in OptoDesigner of the OptSim Circuit schematic of Figure 5.
Figure 7. Mask in OptoDesigner from OptSim Circuit PDAFlow netlist (*.spt file) for the schematic of Figure 5
To carry out circuit simulation of the custom PDK-based ring resonator, create a new project file and load the compound component of Figure 5. This is shown in Figure 8.
Fig. 8 Schematic of the example project file
A wideband source launches light to the input port 1 and analyzers at all four ports are used to measure power spectra. Of particular interest in case of a ring resonator are the spectra at the through and drop ports, as shown on the top and bottom respectively in Figure 9.
(a)
(b)
Figure 9. Power spectra at the (a) through and (b) drop ports of the ring resonator circuit of Figure 11
The procedure described in this application note can be applied to passive, linear photonic components modeled in RSoft device modeling tools. Circuit level behavior of such components can be fully described by their S-parameters. The S-Matrix/PDK Generation Utility from the RSoft device modeling tools automates the task of measuring transfer matrix and creating necessary output files that the ¡°User Libraries Organizer¡± of OptSim Circuit creating and organizing custom PDKs. The custom PDKs, as shown in this application note, can be used in OptSim Circuit to build and simulate photonic circuits, and to create PDAFlow netlist for generating mask via interface to OptoDesigner.