Low-loss and Broadband Cantilever Couplers Between Standard Cleaved Fibers and High-index-contrast Si3N4 or Si Waveguides

We demonstrate a novel facet coupler for coupling between standard cleaved single-mode fibers and high-index- contrast Si3N4 or Si waveguides. By creating a cantilevered glass waveguide surrounding a Si3N4 or Si inverse taper, and by further injecting a low-index cladding around the cantilever, we substantially delocalize the optical mode to match with that of standard cleaved fibers. The coupler design requires no additional deposition / lithography steps, and is implemented on 200-mm wafers with 248-nm deep ultraviolet photolithography. We demonstrate coupling loss as low as 1.3 dB/facet for Si3N4 waveguides of 500 nm x 400 nm. The coupler exhibits a polarization dependent loss of ~0. 3 dB, and a wavelength dependent loss of ~0.2 dB from 1480 nm to 1580 nm. Coupling loss of 1.5~1.7 dB/facet for transverse- electric polarization to Si waveguides of 500 nm x 220 nm is also demonstrated.