Electrode design for high speed silicon optical modulator

Abstract

Due to their limited speed and high power dissipation copper wires can no longer meet the demands of future high speed computing devices. While many different approaches are being tried to overcome the challenge, the one based on a high-speed optical link looks promising. By using photons to transfer information one is able to eliminate the bottlenecks associated with electronic based interconnects. Where optical alternatives usually suffer from high costs and a cost effective approach that is also capable of high volume production is required. In this regard, silicon photonics offers a solution due to its CMOS compatibility.

Over the last decade remarkable progress has been made in the field of silicon photonics where photodetectors, optical sources (lasers) and modulators have been the focus of the study. The electro-optic modulator, which converts electrical signal into optical signal, is a fundamental building block of silicon photonics. Designers have previously concentrated more on optimizing the optical design of the modulator but the modulator’s bandwidth can significantly improve if its electrode design is optimized. The aim of the project was to optimize the modulator electrode to maximize the bandwidth performance of the electrode.

Each element of the electrode has been studied separately. These include bends, tapers and two different dimensions of the coplanar line. Simulation results of square bend, mitred bend, round bend, exponential taper, Klopfenstein taper, triangular taper and two different dimensions of the coplanar line losses are presented. The fabrication process, analysis of aluminium surface morphology and etching techniques are discussed. As a result, characterization of different types of bends, tapers and electrodes are shown where discontinuities like bends and tapers reveal insignificant added loss between 0GHz and 67GHz. The final characterized electrode for a 0.5mm long modulator has a 3dB point above 67GHz whereas previous designs were limited to 42.8GHz.
Since velocity mismatch between optical and electrical signals also limits the modulator’s performance, an effective technique to slow the electrical signal to match its velocity to that of the optical signal for slow wave modulators has successfully been demonstrated with the potential to achieve very high slowing down factors. The slow wave structure for electrode consisted of silicon dioxide and floating metal strips. The characterised slow wave modulator with slow wave electrode confirms a slowing down factor of 2. Not only will the slow wave structure slow down the electrical signal to reduce the mismatch with optical signal velocity, it will also reduce the substrate loss. This shows the potential of slow wave modulator bandwidth to double if optimized further.

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