Characterization and analysis of graded index optical waveguides for the realization of low-power, high-density, and high-speed optical link

This paper describes an advanced optical link model composed of multimode waveguide that is used to aid the development of low-power, high-density, and high-speed multi-channel interconnects. The model consists of a VCSEL, a pair of multi-channel rectangular step-index (SI) or graded-index (GI) type optical waveguides, a graded-index multimode fiber (GI MMF), and a photo detector. Here we assume that each waveguide is integrated on a printed circuit board (PCB), and these two PCBs are connected by the GI MMF ribbon (board-to-board interconnection). Then, we focus on the connection of these link components. For optical links with low-power consumption, the link penalty should be minimized. In this paper, the benefits of GI waveguides over SI waveguides are investigated, particularly about the coupling losses. We start the analysis using the fundamental ray op tics. The rays emit from a VSCEL with Gaussian angular intensity distribution. Both between the laser source and the waveguide (Tx side), and between the waveguide and the photodiode (Rx side), a 50 m gap is assumed, which is filled with a uniform medium with similar refractive index to the core center for the purpose of reducing the Fresnel reflection loss. Furthermore, the two waveguides are connected by a GI MMF, which guides the light from the Tx side to the Rx side. The characteristics such as near field pattern (NFP) and connection loss are addressed. The calculated results show the GI waveguides confine the lightwave intensity near the core center more tightly than the SI waveguide, which result in lower coupling loss (0.46 dB for GI waveguide vs. 1.35 dB for the SI counterpart) between the 35 ìm core size waveguides and the 35 ìm diameter photo diode (PD). This calculation helps us to characterize the high performance optical link with a more reliable model..