Mid-Infrared Grating Couplers for Silicon-on-Sapphire Waveguides

Theoretical and experimental results of mid-infrared (mid-IR) gratings for coupling between transverse-electric (TE)/transverse-magnetic (TM) mode silicon-on-sapphire (SOS) waveguides and zirconium, barium, lanthanum, aluminum, and sodium fluoride (ZBLAN) fibers are presented. The shallow-etched uniform grating, full-etched subwavelength grating, and apodized grating are analyzed theoretically. TE mode shallow-etched apodized gratings with coupling efficiency of 80.6% and TM mode full-etched apodized subwavelength gratings with coupling efficiency of 39.6% are predicted at the wavelength of 2.75 $\mu\hbox{m}$ by finite-difference time-domain (FDTD) simulation. An $\hbox{Er}^{3+} \ndash \hbox{Pr}^{3+}$ co-doped ZBLAN fiber laser is set up to be used as a mid-IR light source at 2.75 $\mu\hbox{m}$. Coupling efficiencies of 32.6% and 11.6% to TE mode and TM mode SOS waveguides, respectively, were obtained experimentally with uniform grating couplers. The dependence of coupling efficiency on etch depth, grating period, fill factor, and incident angle are also studied.