Ion Milled Facets for Direct Coupling to Optical Waveguides

Low loss coupling to optical waveguides is one of the on-going challenges with integrated photonics. Edge coupling of fibers or fiber arrays allows for in principle low loss coupling but strongly depends on the optical facet quality. We demonstrate an innovative strategy utilizing ion milling for polishing photonic integrated circuit edge facets for direct optical coupling to waveguides. Specifically, the authors created a 750 mu m wide by 130 mu m deep polished facet for coupling SM300 fiber to AlN waveguides on Al2O3 substrates; all capped with an index matched, but highly stressed, SiON cladding. Ion milling avoids the lateral shear forces that can delaminate a stressed film, resulting in scattering sites at the tapered edge coupler/facet interface. The authors demonstrate that a mechanical polish produced chipped facets that scattered the light away from the waveguide, thus requiring reprocessing of the chip. After ion milling, the authors coupled light into the waveguides and demonstrate critical coupling into AlN microring resonators between 390 and 395 nm.