Silicon Modulators
Forward and reverse bias modulators of many geometries
Introduction
The start of Photon Design’s design flow for simulating silicon modulators is with software Harold.
As the market’s leading laser simulator, Harold is equipped to model the diffusion and accumulation of carriers due to forward or reverse bias, the effect this has on a material’s refractive index through the plasma dispersion effect, and find how a supported mode’s effective index changes with applied voltage.
Photon Design tools can design modulators entirely with component simulation, dynamics, and layout (see end of page).
Various Si modulator designs in Harold with optical mode contour plot.
Top: Capacitive carrier accumulation modulator with electric field
Left: Simple Si rib pn-junction modulator
Right: MOS capacitor Si modulator
Key Results
Phase Shift Under Reverse Bias
The key result for modulator design, Harold excellently matches the results of [1] showing accurate calculation of charge distribution, implementation of the plasma diffusion effect, and calculations of the resultant optical mode.
Harold’s material files include doping diffusion coefficients, the important effect of which is shown in these results.
This thoroughly simulated model can calibrate a surrogate model used in efficient simulations during later design flow steps designing full modulator circuits (see end of page).
IV Characteristic
Results from this Harold simulation include the band gap narrowing effect due to dopant density and the free carrier density which can grow significantly under forward bias.
Harold simulations of the LI curve agree with experimental results to comparable standard as those in source [2] shown in the insert.
Differential Capacitance
The recorded capacitance of this carrier accumulation modulator of 2.33pF at 2V agrees well with the simulated result of 2.15pF reported in [3].
Combining this capacitance result with a simulated resistance (calculated in a variety of ways depending on device) provides a response time used in dynamic modulation simulations in PICWave.
Design Flow Next Steps
The Harold results of neff(V) can be imported as a surrogate model for simulations of entire 3D modulators:
Components + Layout
Rigorous simulations of waveguide components (splitters and bends) can be natively combined in a layout environment using MT-FIMMPROP; Harold’s model is used to define modulator sections to see the effect of applied voltage on the optical simulation. Simulations in the EME engine are length independent suiting the long modulator sections in typical designs.The result is a full 3D optical simulation of the MZM with a full layout.
Full 3D Mach Zehnder Modulator (TFLN)
Unique - MT-FIMMPROP simulates full modulator rigorously in layout environment
Dynamic Simulations
Circuit simulator PICWave provides dynamic simulations of the modulator using traveling wave electrodes.
Using the electrical characteristics computed by Harold, a sophisticated electrical surrogate model is created. This reproduces the modulator dynamics including electrical travelling wave effects which can then be included in the PICWave optoelectronic model to study the high speed characteristics of the modulator.
Oscilloscope tools provide eye diagrams/ constellation diagrams with symbol error rate for a given input.
Left: MZM set up in PICWave using model from Harold
Right: Eye diagram of result (26 Gbps)
Traveling Wave Electrode – TFLN Modulator
Simulation of a travelling-wave modulator
References
[1] D. Thomson et al., "50-Gb/s silicon optical modulator", 2012 IEEE Photonics Technology Letters.
[2] G. Zhou et al. "Effect of carrier lifetime on forward-biased silicon Mach–Zehnder modulators", 2008 Opt. Express.
[3] A. Abraham, S. Olivier, D. Marris-Morini, and L. Vivien, "Evaluation of the performances of a silicon optical modulator based on a silicon-oxide-silicon capacitor", 2014 Int. Conf. Group IV Photonics.