Low-Loss and Broadband Cantilever Couplers Between Standard Cleaved Fibers and High-Index-Contrast Si $_{3}$N $_{4}$ or Si Waveguides

We demonstrate a novel facet coupler for coupling between 10.4-
$\mu$
m mode-diameter cleaved fibers and high-index-contrast Si
$_{3}$
N
$_{4}$
or Si waveguides. By creating a cantilevered glass waveguide surrounding an inverse taper and injecting a low-index cladding around the cantilever, we demonstrate coupling loss as low as 0.7 dB/facet to 500 nm
$\,\times\,$
400 nm Si
$_{3}$
N
$_{4}$
waveguides, with only 0.2-dB polarization-dependent loss and 0.4-dB wavelength-dependent loss from 1480 to 1580 nm. Coupling loss of 1.5–2.0 dB/facet to 500 nm
$\,\times\,$
220 nm Si waveguides at 1550 nm is also demonstrated.