The field of silicon photonics is attracting a lot of attention due to the prospect of low-cost and compact circuits that integrate photonic and microelectronic elements on a single chip. Such silicon chips have applications in optical transmitter and receiver circuits for short-distance communications as well as for long-haul optical transmissions. Silicon photonics has proven to be a successful platform for many functional elements such as low-loss waveguides, filters, multiplexers/demultiplexers, optical modulators and Ge-on-Si photodiodes. On-going developments for advanced photonic integrated circuits include compact and energy-efficient silicon modulators, temperature-insensitive passive devices and hybrid III-V on Silicon lasers.
The European COSMICC project gathers key industrial and research partners in the field of silicon photonics, CMOS electronics, printed circuit board packaging, optical transceivers and datacenters, aiming at developing low-cost and low-energy consumption 50 Gb/s 4-wavelength coarse wavelength division multiplexing optical transceivers that will be packaged on-board. Combining CMOS electronics and Si-photonics with innovative high-throughput fiber attachment techniques, the developed solutions will be scalable beyond 1 Tb/s to meet the future data-transmission requirements in data-centers and super computing systems.
In this paper, the simulation, design and fabrication of a back-side coupling (BSC) concept for silicon photonics, which targets heterogeneous hybrid III-V/Si laser integration is presented. Though various demonstrations of a complete SOI integration of passive and active photonic devices have been made, they all feature multi-level planar metal interconnects, and a lack of integrated light sources. This is mainly due to the conflict between the need of planar surfaces for III-V/Si bonding and multiple levels of metallization. The proposed BSC solution to this topographical problem consists in fabricating lasers on the back-side of the Si waveguides using a new process sequence. The devices are based on a hybrid structure composed of an InGaAsP MQW active area and a Si-based DBR cavity. The emitted light wavelength is accordable within a range of 20 nm around 1.31μm thanks to thermal heaters and the laser output is fiber coupled through a Grating Coupler (GC). From a manufacturing point of view, the BSC approach provides not only the advantages of allowing the use of a thin-BOX SOI instead of a thick one; but it also shifts the laser processing steps and their materials unfriendly to CMOS process to the far back-end areas of fabrication lines. Moreover, aside from solving technological integration issues, the BSC concept offers several new design opportunities for active and passive devices (heat sink, Bragg gratings, grating couplers enhanced with integrated metallic mirrors, tapers…). These building boxes are explored here theoretically and experimentally.