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

Abstract

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.

O-Band Grating Couplers Using Silicon Nitride Structures

“…Mode matching refers to ensuring that the guided mode in the Si 3 N 4 waveguide is appropriately coupled to the desired SMF mode for efficient light transmission. To achieve this goal, the Bragg condition [1] must be fulfilled, which can be achieved by carefully optimizing the grating parameters, such as the grating period, the coupling angle from the SMF, and etch depth, and the GC can enhance the coupling efficiency, enabling the effective transmission of light into and out of the Si 3 N 4 waveguide.…”

“…The diffraction behavior of a GC can be described using the Bragg condition, which is also referred to as the phase matching condition. This reveals the relationship between the wave-vector k 0 of a light beam through the waveguide and propagation constant β of the corresponding coupled light beam out of the waveguide to the SMF, and the Bragg condition is expressed as [1]:…”

“…Grating couplers (GC) based on silicon (Si 3 N 4 ) are now widely used to couple the light between a single-mode fiber (SMF) and photonic integrated circuits (PICs) [1,2]. The use of GC in the field of PICs has brought significant advantages by enabling the avoidance of expensive processing and packaging steps [3].…”