Light coupling in dimension mismatch waveguides for silicon photonic integrated circuits

TLDR: In this article, an ultra-subwavelength grating coupler has been developed with an engineered grating structure which exhibits high coupling efficiency and bandwidth without the need for bottom mirrors.

Abstract: In recent years silicon photonics has become a considerable mainstream technology, especially in telecommunications fields to overcome the limitations imposed by copper-based technology. Nanoscale photonic technologies have attracted a lot of attention to co-develop photonic and electronic devices on silicon (Si) to provide a highly integrated electronic–photonic platform. Silicon-on-insulator (SOI) technology that relies heavily on the contrasted indices of Si and SiO2, enables the design and integration of these photonic devices in submicronic scales, similar to the devices produced by a standard CMOS fabrication platform in the electronics industry. One of the key challenges with these submicronic waveguide devices is to enable efficient coupling with fibre, which is mainly due to the mode-field differences between fibre and the waveguide, and their relative misalignments. To overcome this challenge, various techniques including prism, butt and grating coupling have been proposed. Among them, although butt coupling is an elegant solution for low loss and wideband operation, it often requires post-processing for accurate polishing and dicing to taper the waveguide edges. Therefore, it is not suitable for wafer-scale testing. Grating couplers, which mostly perform out of the plane coupling between a fibre and a waveguide, are also an attractive solution as light can be coupled in and out everywhere on the chip, opening the way for wafer-scale testing. However, despite such advantages, grating couplers often exhibit low coupling efficiency (CE) due to downward radiation of light that propagates towards substrate through buried oxide (BOX) which comprises 35%-45% of total incident light. Grating couplers are also very sensitive to the wavelength of the light as different wavelengths exhibit specific diffraction properties at the grating, which cause a narrow coupling bandwidth. In this thesis we have studied various techniques to improve the coupling efficiency and coupling bandwidth of the grating couplers. We have used the finite difference time domain (FDTD) and Eigenmode Expansion (EME) methods to study the interaction of light with grating. The directionality of the coupler which determines the coupling efficiency has been improved by means of silicon mirrors in the BOX layer that essentially redirect the light propagates toward substrate. For improvement of directionality, an ultra-subwavelength grating coupler has also been developed with an engineered grating structure which exhibits high coupling efficiency and bandwidth without the need for bottom mirrors. The grating coupler only converts vertical dimension into nano scale, leaving the lateral width in micrometre range typically >15 μm. In order to connect the grating coupler with a nanophotonic waveguide, the grating structure needs to be matched in dimensions both vertically and laterally. Conventionally, to meet the requirement the width of grating structure is gradually tapered to nano scale. The coupling efficiency relies highly on the taper length, which is typically hundreds of micrometres. Such a long taper waveguide causes an unnecessarily large footprint of the photonic integrated circuits. In order to minimise the length of the taper while retaining high coupling efficiency, we have designed two different types of tapered waveguides. One of them is a partially overlaid tapered waveguide and the other is a hollow tapered waveguide.