Christy K. Y. Fung

Apodized Waveguide Grating Couplers for Efficient Coupling to Optical Fibers

We describe a shallow-etched diffractive waveguide grating coupler which achieves efficient coupling between single-mode fiber and a silicon-on-insulator optical waveguide. By the appropriate choice of waveguide/grating thicknesses and varying the coupling strength of the grating coupler via tailoring its fill factor to optimize the mode matching, a coupling loss of only 1.2 dB was obtained for each fiber/silicon waveguide interface. The coupling strength and effective refractive index of grating with different fill factors were also characterized to confirm the design simulations. The back reflection was suppressed by varying the coupling strength, leading to further enhancement of the coupling efficiency.

Wideband subwavelength gratings for coupling between silicon-on-insulator waveguides and optical fibers.

We propose and experimentally demonstrate a novel subwavelength grating coupler on silicon-on-insulator, for coupling to optical fibers with a wide optical bandwidth. Theoretical analysis and design optimization of the coupler are described. About 73 nm 1 dB bandwidth was experimentally demonstrated with -5.6  dB coupling efficiency. Better than -3.4  dB efficiency with 86 nm 1 dB bandwidth is predicted for these structures with optimized buried oxide thickness.

Focusing subwavelength grating coupler for mid-infrared suspended membrane waveguide.

A mid-infrared (mid-IR)-focusing subwavelength grating (SWG) coupler and suspended membrane waveguide (SMW) on a silicon-on-insulator wafer are studied. For a transverse-electric mode uniform SWG, finite-difference time-domain simulation predicts 44.2% coupling efficiency with 1 dB bandwidth of about 220 nm and backreflection of 0.78% at 2.75 μm. Then the uniform SWG is curved to a focusing SWG using a phase-matching formula. The SMWs are analyzed by the finite element method and fabricated. An Er3+-Pr3+ co-doped mid-IR fiber laser is used for device characterization. The fabricated mid-IR SWG coupler has 24.7% coupling efficiency.

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 by finite-difference time-domain (FDTD) simulation. An co-doped ZBLAN fiber laser is set up to be used as a mid-IR light source at 2.75 . 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.

Design and applications of silicon waveguide grating couplers

We review our recent work on waveguide grating couplers, including an apodized grating coupler with engineered coupling strength to achieve Gaussian-like output profile, which greatly improves the fiber-chip coupling efficiency. We will also discuss a new class of grating couplers involving the use of sub-wavelength nanostructures to engineer the optical properties. Effective medium theory can be used in the design of sub-wavelength structures, which, when properly engineered, can offer broadband coupling and polarization independence. Other applications of waveguide gratings, for example bi-wavelength two dimensional gratings coupler for (de-)multiplexing two different wavelengths, fiber-waveguide hybrid lasers and mid-infrared grating couplers on silicon-on-sapphire wafer will also be briefly discussed.

Mid-infrared suspended membrane waveguides on silicon-on-insulator

We describe a platform for mid-infrared silicon photonic devices based on focusing subwavelength grating coupler (SWG) and suspended membrane waveguides. The focusing SWG was designed for 2.75 μm wavelength. 24.7% coupling efficiency was measured. The suspended membrane waveguide was optimized in terms of holes size and etch depth.

Mid-infrared micro-ring resonator on silicon-on-sapphire characterized by thermal tuning

A silicon-on-sapphire (SOS) micro-ring resonator was fabricated and characterized at 2.75μm by thermal tuning. The measured extinction ratio in the drop port is about 10dB and the Q factor is about 2000.

Subwavelength waveguide grating coupler for fiber-to-chip coupling on SOI with 80nm 1dB-Bandwidth

We propose and experimentally demonstrate a novel subwavelength grating coupler on SOI for coupling between fiber and nanophotonic waveguide with a wide bandwidth. 80nm 1dB-bandwidth was experimentally demonstrated.

Silicon waveguide side-cladding distributed Bragg reflector hybrid laser

We report the design, simulation and experimental results of a hybrid silicon laser with side-cladding Bragg grating as wavelength selective component and erbium doped fiber as gain medium. Over 35dB side-mode-suppression-ratio (SMSR) is achieved.

Dual-wavelength hybrid laser with micro-ring resonator and erbium-doped fiber amplifier

We report a dual-wavelength hybrid erbium fiber-silicon waveguide ring laser. A micro-ring resonator in the silicon waveguide served as the wavelength selective component. Simultaneous lasing at two wavelengths with over 50 dB side mode suppression ratio was achieved.