Deeply etched MMI-based components on 4 μm thick SOI for SOAbased optical RAM cell circuits

We present novel deeply etched functional components, fabricated by multi-step patterning in the frame of our 4 μm thick Silicon on Insulator (SOI) platform based on singlemode rib-waveguides and on the previously developed rib-tostrip converter. These novel components include Multi-Mode Interference (MMI) splitters with any desired splitting ratio, wavelength sensitive 50/50 splitters with pre-filtering capability, multi-stage Mach-Zehnder Interferometer (MZI) filters for suppression of Amplified Spontaneous Emission (ASE), and MMI resonator filters. These novel building blocks enable functionalities otherwise not achievable on our SOI platform, and make it possible to integrate optical RAM cell layouts, by resorting to our technology for hybrid integration of Semiconductor Optical Amplifiers (SOAs). Typical SOA-based RAM cell layouts require generic splitting ratios, which are not readily achievable by a single MMI splitter. We present here a novel solution to this problem, which is very compact and versatile and suits perfectly our technology. Another useful functional element when using SOAs is the pass-band filter to suppress ASE. We pursued two complimentary approaches: a suitable interleaved cascaded MZI filter, based on a novel suitably designed MMI coupler with pre-filtering capabilities, and a completely novel MMI resonator concept, to achieve larger free spectral ranges and narrower pass-band response. Simulation and design principles are presented and compared to preliminary experimental functional results, together with scaling rules and predictions of achievable RAM cell densities. When combined with our newly developed ultra-small light-turning concept, these new components are expected to pave the way for high integration density of RAM cells.