Design of a silicon-on-calcium-fluoride-based compact and efficient polarization rotator for the mid-IR

Abstract

We report the design of a chip-scale and highly efficient polarization rotator (PR) based on an asymmetric directional coupler geometry involving a horizontal slot waveguide (WG) and a strip WG on a silicon-on-calcium-fluoride (SOCF) platform for the mid-IR regime. In particular, we have optimized it for rotations of both the polarizations at the operating wavelength of 4.47 µm in two configurations, which relied on single and double-slot WG geometries. Power coupling through appropriate phase matching between the quasi-TM mode of a horizontal slot WG and the quasi-TE mode of a strip WG has been exploited for realizing polarization rotation. Numerical simulations demonstrate that achievable maximum power coupling efficiency (Cm) is as high as ~95% (with a device length of ~0.57 mm) for the single slot WG geometry and ~97% (with an even shorter device length of ~0.47 mm) for the PR based on double-slot WG geometry for both the polarizations. Both the designed PRs exhibit relatively large bandwidth of 50 nm with reasonably high Cm of ~80%. A study on fabrication tolerances show that Cm remains ~80% for variation in width Δw from -2 to +3 nm and -6 to +5 nm for single and double-slot based PRs, respectively.