Silicon optical modulators for high data rate applications

Abstract

In this work we describe the carrier depletion MZI modulators, slow wave structures for modulation enhancement and the QCSE modulator which are under development in the European HELIOS project and the UK Silicon Photonics project. IntroductionHigh performance silicon optical modulators are key to many silicon based photonic applications. Over the previous decade the development seen in the performance of silicon optical modulators has been vast. Several routes to modulation have been used to overcome the lack of a strong electro optic effect in silicon. These include the plasma dispersion effect, III-V hybrid device fabrication, SiGe devices, Polymer and Strain induced electro-optic effects. Reported performances now regularly range from 10Gbit/s up to 40Gbit/s. HELIOS, which is a European FP7 funded project and the UK silicon photonics project (UKSP), funded by the EPSRC both involve the development of the different photonic components required to form photonic circuits with a range of functionality. Within both projects there is strong modulator activity with carrier depletion based modulation, QCSE modulation and structures to gain enhancement of the modulation effect under development. Carrier depletion modulationOptical modulators based upon free carrier depletion are widely regarded as being the simplest approach to achieve high performance modulation in silicon. They operate by reverse biasing a diode structure which is incorporated in or around an optical waveguide. The depletion of free carriers therefore interacts with the propagating light causing a change in phase through the plasma dispersion effect. Within the HELIOS project two phase modulators based upon this approach are under investigation using both PN and PIPIN diodes. Cross sectional diagrams of these devices are shown in figure 1. The first structure is based in silicon-on-insulator (SOI) of 220nm thickness. The waveguide section and the slab to one side is doped p type. The slab on the other side of the waveguide is then doped n type setting up a pn junction at the edge of the waveguide rib. The concentration of the n type doping is made larger than the p type doping such that the depletion region extends mainly into the waveguide during reverse bias. These lightly doped p and n type regions extend out to meet highly doped regions which in turn provide ohmic contacts to coplanar waveguide electrodes which are used to drive the device. The device