Demonstration of a low loss, highly stable and re-useable edge coupler for high heralding efficiency and low g^(2) (0) SOI correlated photon pair sources

TL;DR

This paper presents a stable, low-loss, and re-usable edge coupler for silicon photonic chips that enhances coupling efficiency and stability, enabling high heralding efficiency and low g^(2)(0) in correlated photon pair sources.

Contribution

The authors introduce a monolithic on-chip tapered waveguide and small core fiber coupler that simplifies fabrication and improves stability and efficiency in fiber-chip coupling.

Findings

Coupling loss of -0.64 dB achieved.

Stable coupling with ±0.1 dB fluctuation over ten days.

High heralding efficiency of 21.3% and g^(2)(0) as low as 0.0004.

Abstract

We report a stable, low loss method for coupling light from silicon-on-insulator (SOI) photonic chips into optical fibers. The technique is realized using an on-chip tapered waveguide and a cleaved small core optical fiber. The on-chip taper is monolithic and does not require a patterned cladding, thus simplifying the chip fabrication process. The optical fiber segment is composed of a centimeter-long small core fiber (UHNA7) which is spliced to SMF-28 fiber with less than -0.1 dB loss. We observe an overall coupling loss of -0.64 dB with this design. The chip edge and fiber tip can be butt coupled without damaging the on-chip taper or fiber. Friction between the surfaces maintains alignment leading to an observation of +-0.1 dB coupling fluctuation during a ten-day continuous measurement without use of any adhesive. This technique minimizes the potential for generating Raman noise in the fiber, and has good stability compared to coupling strategies based on longer UHNA fibers or fragile lensed fibers. We also applied the edge coupler on a correlated photon pair source and observed a raw coincidence count rate of 1.21 million cps and raw heralding efficiency of 21.3%. We achieved an auto correlation function g^(2) (0) as low as 0.0004 at the low pump power regime.