Silicon Ring Modulator

Overview

Silicon photonic electro-optical ring modulators are key components in short-range optical interconnects due to their low operating voltage, compact size, and compatibility with CMOS circuit drivers. This application note demonstrates the complete creation of a silicon ring modulator with Synopsys’ Photonic Solutions tools. First, RSoft Photonic Device Tools will be used to create custom PDK models for the ring modulator’s building blocks: a quarter-ring passive component and a three-quarter ring curved phase shifter component. These PDKs are then imported into one of Synopsys’ Photonic System tools, OptSim Circuit, for circuit-level simulation and optimization of the ring modulator. 

The constituent parts of the ring modulator: a) the quarter ring in the RSoft CAD | Synopsys
The constituent parts of the ring modulator: b) the phase shifter cross-section in the RSoft CAD | Synopsys
The constituent parts of the ring modulator: c) the OptSim Circuit schematic for the ring modulator using the two custom PDK blocks | Synopsys

Figure 1: The constituent parts of the ring modulator: a) the quarter ring in the RSoft CAD, b) the phase shifter cross-section in the RSoft CAD, and c) the OptSim Circuit schematic for the ring modulator using the two custom PDK blocks.

Device Level PDK Generation

First, the RSoft Photonic Device Tools’ PDK generation utility is used with FullWAVE and RSoft’s Multi-Physics Utility to automatically create a custom, parametric PDK model for the quarter ring and the phase shifter, so that these components can be used directly in the OptSim Circuit circuit-level simulation & optimization tool. The quarter-ring PDK model is parametrized for the coupling gap and ring radius; the curved phase shifter is parametrized over the radius which enables the calculation of bending modes for the specified radius and a bending PDK model. The length of the phase shifter is set in OptSim circuit.

S-matrix for quarter-ring (left) | Synopsys
Neff vs. Lambda for phase shifter (right) | Synopsys

Figure 2: S-matrix for quarter-ring (left), Neff vs. Lambda for phase shifter (right).

Circuit-Level Simulation & Optimization

After generating the component PDKs in the RSoft Photonic Device Tools, OptSim Circuit can then be used for the simulation & optimization of the ring modulator. The first parameter to optimize is the gap of the coupler; selecting the resonance near 1550nm, scan results show that a gap of 0.2um gives optimum performance.

Gap scan results for resonance near 1550nm | Synopsys

Figure 3: Gap scan results for resonance near 1550nm.

Once the optimal gap is chosen, the voltage can be optimized. For a resonance near 1550nm, maximum modulation is obtained for a voltage of V=-2V. 

Bias scan results for resonance near 1550nm | Synopsys

Figure 4: Bias scan results for resonance near 1550nm.

After determining the optimum gap and wavelength, the eye performance of the ring can be examined. The eye results show the transient response of the ring modulator.

Calculated eye diagram for (left) input electrical signal, (middle) after electrical filter, and (right) after ring modulator | Synopsys

Figure 5: Calculated eye diagram for (left) input electrical signal, (middle) after electrical filter, and (right) after ring modulator.