RSoft Application Gallery Note: Edge Spot-Size Converter

Tool Used: BeamPROP

A leading telecommunications company in Japan had plans to develop an ultra-low loss Spot Size Converter (SSC) for edge coupling of rib-type Si wire waveguides using an inverted taper with an ultra-narrow tip.

The Challenge

The project goal was to develop an effective SSC between a silica fiber and a rib Si-wire waveguide with ultra-low coupling loss. In addition, the design was required to minimize the converter footprint while reducing manufacturing and production costs. In order to evaluate multiple SSC configurations, an efficient tool to model the structure was needed. RSoft™ BeamPROP™ is the ideal tool for this design challenge.

Figure 1. Spot Size Converter (SSC). Image source: Tsuchizawa, Tai, et al. Selected Topics in Quantum Electronics, IEEE Journal of 17.3 (2011)

The Solution

BeamPROP provides rigorous, fast simulations of light propagation in the taper structure. FDTD is too slow due to large taper size. (BeamPROP is ~1300x faster than FDTD in simulating this SSC.) Eigenmode expansion requires modes to be computed at a large number of junctions.

Figure 2. BeamPROP optimized results for the SSC design

The Result

BeamPROP yielded the following results:

  • Obtained SSC output field distribution and coupling loss
  • Optimized the structure to achieve <0.22dB fiber-to-waveguide coupling loss with a 300um taper length and 80nm tip width 
    • Produced dimensions compatible with CMOS processes to reduce fabrication costs 
    • Coupling loss was seen to increase for reduced taper lengths