Silicon Ring Modulator | Synopsys

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 photonic ring modulator with Synopsys’ Photonic Solutions tools. First, RSoft Photonic Device Tools will be used to create abstracted models for the ring modulator’s building blocks: a quarter-ring passive part and a three-quarter ring phase shifter part. These building blocks are then imported into OptSim Circuit, for circuit-level simulation and optimization of the complete ring modulator. The Custom PDK Utility will be used to automate the step from physical design with RSoft Photonic Device Tools to the creation of the abstracted models for OptSim Circuit.

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 RSoft CAD, b) the phase shifter cross-section in RSoft CAD, and c) the OptSim Circuit schematic representation for the complete ring modulator using the two building blocks.

Device Level Building Block Generation

First, the RSoft Photonic Device Tools’ Custom PDK Utility is used together with FullWAVE FDTD and the Multi-Physics Utility to automatically create a custom, parametric model for the passive part of the ring and the phase shifter part, so that these components can be used directly in the OptSim Circuit circuit-level simulation & optimization tool. The quarter-ring model is parametrized for the coupling gap and ring radius; the curved phase shifter is parametrized for the radius which enables the calculation of bending modes for the specified radius and a bending abstracted model. The length of the phase shifter will be 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 building blocks with 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 to obtain resonance near a wavelength of 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 driving 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 driving voltage, 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.