RSoft Enewsletter

April 2018

Elastic Connectors in OptSim Circuit

A typical schematic entry-driven photonic integrated circuit (PIC) design flow begins with implementing a photonic circuit idea as a schematic made up of compact models representing circuit elements [1]. A pictorial representation of the PIC design flow is shown in Fig. 1.

Figure 1. PIC design flow: From ideas to wafer runs

Typically, the circuit elements in a PIC schematic are foundry-specific process design kit (PDK) components¹. The process-specific characterization of PDK components for circuit simulations is provided by the foundries. PIC designers can also create custom PDKs for a specific foundry using the RSoft Component Design Suite’s S-Matrix/PDK Generation Utility [2] and use them in OptSim Circuit [3] for PIC simulations. Once the simulation results meet design objectives, the next step is to create a netlist in OptSim Circuit for creating its mask in OptoDesigner [4] and hand over the final graphic database system (GDS) to the foundry [5]. 

In OptSim Circuit, the bidirectional optical and electrical signal flow in the circuit is enabled by the connections between the circuit elements. From the circuit simulation perspective, these connections can be ideal (lossless, with no time- or phase delay) or physical (resulting in loss, time-delay and phase-shift). As an example, couplers and waveguides of appropriate dimensions provide physical circuit connections between adjacent photonic components. The connectivity information is used in tools such as OptSim Circuit for simulating physical-layer performance of the PIC. The connectivity information is also used in layout software such as OptoDesigner for placement, routing, and verification during generation of the layout. 

Constraints from packaging and design rule checks (DRCs) can impose restrictive routing requirements not foreseen during circuit simulation at the schematic level. Any changes by the layout tool to comply with the packaging and DRCs can potentially result in a PIC with dramatic deviation in its performance compared to the one predicted by the circuit simulation, which in turn affects the yield. 

This problem can be significantly alleviated by using so-called “elastic connectors,” also known as “pxConnectors”, at specific locations in the circuit schematic. Figure 2 shows some of the available elastic connectors in the OptSim Circuit model tree.

Figure 2. OptSim Circuit model tree showing elastic connectors 

These elastic connectors are placed in an OptSim Circuit schematic to create waveguide connections for which the exact geometries depend on the final circuit layout. The foundry-dependent shape and dimension of the elastic connectors are determined through tight integration with OptoDesigner, which communicates with OptSim Circuit to create equivalent compact models for the connectors during simulation. Figure 3 depicts this flow. 

Figure 3. Using an elastic connector in OptSim Circuit (top left), evaluating its geometry in OptoDesigner
(top right), and obtaining simulation results for the updated schematic (bottom)

OptSim Circuit’s simulation log file records information about each elastic connector used in the schematic that was updated through the integration with OptoDesigner. Figure 4 shows part of the OptSim Circuit simulation log file.

Figure 4. OptSim Circuit log file with information on updated dimensions of the elastic connectors (click for a larger image)


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¹Although compact models in a layout-ready schematic of the photonic circuits are usually PDKs, in principle, the compact models in a photonic circuit can be physical (i.e., characterized by analytical treatment of the underlying physics), behavioral (i.e., characterized by the behavior of the element, such as an electro-optic transfer function in the case of modulators), measurement-based (e.g., characterized by measured S-matrices for passive, linear photonic components, and measurements such as L-I curve, linewidth curve, RIN curve, amplitude- and frequency-modulation curves, etc. in the case of a laser), and process design kit or PDK-based (i.e., elements with process-specific characterization provided by the foundries).


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