Mode size converter designs for fabrication of ultra-low loss optical interconnects

Over the past decade, the silicon photonics industry has made fantastic progress in implementing a wide range of optical and electro-optical devices using infrastructure and low-cost, high-volume, manufacturing technologies from the microelectronics industry. Many technical challenges have been solved. However, ultra-low loss light coupling at the interface between a high-index, sub-micron silicon waveguide and single mode fiber (SMF) remains problematic, and has attracted world-wide efforts to propose new designs and integration processes. Two widely used strategies have been reported in the literature: out-of-plane grating couplers and in-plane edge couplers with mode size converters (MSCs). Compared with most grating couplers, mode size converters have the advantage of broader bandwidth (> 100nm) and more design freedom to achieve good light coupling efficiency. In this paper, we will focus on designs for high coupling efficiency using inverse tapered mode size converters that will adiabatically transform the small mode size of a silicon waveguide to match the large mode size of a single mode fiber. Our designs involve mode size matching by the use of vertically coupled waveguides. Large scale fabrication of mode size converters at low cost and high repeatability is desired. Mode size converters can for example, be made of silicon, silicon nitride, or polymer materials. However, mode size converters with 3D gradually tapered structures cannot be fabricated in a straightforward manner by classic CMOS manufacturing processes. In this paper, a one-step, gray scale photolithography-based high efficiency mode size converter fabrication flow will be discussed. We will apply this methodology for the design of both a silicon nitride based mode size converter and also to a polymer based mode size converter. Notably, gray scale photolithography can be readily integrated into multi-mask CMOS processes to provide one-step manufacturing for mode size converters at extremely low cost and accurate positioning.