Ring resonator design for hybrid III-V on silicon tunable laser.

The silicon photonics platform is the realization of photonic integrated circuits compatible with the CMOS process and it useful for reducing costs and energy consumption of optical transceivers used in optical interconnect, while maintaining high communication speed and the amount of data traffic. A benefit of using silicon is that it allows on-chip integration with active devices such as transistors and photodiodes and take advantage of the well established CMOS technology. In recent years, industries have been focusing on this platform, in fact various passive optical devices have been successfully integrated, but an efficient laser source integrated on silicon wafers is still missing, because silicon is an indirect band-gap material and therefore spontaneous and stimulated light emission is difficult to obtain. A possible technological solution is the hybrid integration which consists in the coupling between a III-V reflective semiconductor optical amplifier (RSOA) and an external photonic integrated circuit (PIC) in silicon, designed as the external mirror of the laser cavity, and it is formed by optical waveguides and ring resonators. The aim of this thesis is to develop the blocks for the design of silicon micro-ring resonators to be employed in silicon photonic integrated circuits as laser mirrors. This project was carried on in the frame of a research collaboration between Politecnico di Torino and CISCO Systems. The design of the micro-rings has the aim of minimizing non linear optical loss and self-heating which occurs when the optical power in the micro-rings gets higher then a few mW. First of all I present the general theory of the main components of our PIC: waveguides and ring resonators. Subsequently, the theory of the non-linear effects of silicon is exposed, namely Two-photon Absorption (TPA) and Free Carrier Absorption (FCA) which are phenomena that generate power losses that are transformed into heat. All this background sets the basis to develop the modelling blocks required for the design of silicon micro-ring resonators. In this thesis I have focused on the development of three modelling blocks. These are: 1) the calculation of the optical waveguide effective area, 2) the calculation of the ring thermal impedance and 3) the calculation of photo-generated carrier distribution across the optical waveguide. The first parameter is obtained by the electromagnetic simulations of different sizes and shapes of waveguide cross sections and ring resonators, through the Finete Element Method. Thermal analysis is performed with the Comsol Multiphysic tool to obtain the thermal impedance of the ring. Comsol Multiphysic tool has also been used to calculate the carrier diffusion in silicon ridge waveguide once carriers are photogenerated due to TPA in the waveguide core. In conclusion during my thesis I have developed three modelling blocks that are required to quantify the impact of non-linear effect on optical loss in the micro-ring. I have employed these blocks to design optimized waveguide cross section to minimize non-linear optical absorption. These blocks have also been employed in the research group to simulate the non linear transmission coefficient of the silicon microrings, it compare with the experimental results then to design optimized wave-guide cross sections.