Dirk Taillaert

An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides and single-mode fibers

We have designed and fabricated an out-of-plane coupler for butt-coupling from fiber to compact planar waveguides. The coupler is based on a short second-order grating or photonic crystal, etched in a waveguide with a low-index oxide cladding. The coupler is optimized using mode expansion-based simulations. Simulations using a 2-D model show that up to 74% coupling efficiency between single-mode fiber and a 240-nm-thick GaAs-AlO/sub x/ waveguide is possible. We have measured 19% coupling efficiency on test structures.

Grating Couplers for Coupling between Optical Fibers and Nanophotonic Waveguides

Nanophotonic waveguides and components are promising for use in the large-scale integration of photonic circuits. Coupling light between nanophotonic waveguides and a single-mode fiber is an important problem and many different solutions have been proposed and demonstrated in recent years. In this paper, we discuss a grating coupler approach. Grating couplers can be placed anywhere on a circuit and can easily be integrated. We have experimentally demonstrated >30% coupling efficiency with a 1 dB bandwidth of 40 nm on standard wafers. Theoretically, the coupling efficiency can be improved to >90% using an optimized grating design and layer stack. The fabrication of the couplers in silicon-on-insulator and in indium phosphide membranes is also discussed.

Compact efficient broadband grating coupler for silicon-on-insulator waveguides

We have designed a high-efficiency broadband grating coupler for coupling between silicon-on-insulator (SOI) waveguides and optical fibers. The grating is only 13 microm long and 12 microm wide, and the size of the grooves is optimized numerically. For TE polarization the coupling loss to single-mode fiber is below 1 dB over a 35-nm wavelength range when using SOI with a two-pair bottom reflector. The tolerances to fabrication errors are also calculated.

Low-loss SOI photonic wires and ring resonators fabricated with deep UV lithography

We demonstrate single-mode photonic wires in silicon-on-insulator with propagation loss as low as 2.4 dB/cm, fabricated with deep ultraviolet lithography and dry etching. We have also made compact racetrack and ring resonators functioning as add-drop filters, attaining Q values larger than 3000 and low add-drop crosstalk.

A compact two-dimensional grating coupler used as a polarization splitter

We demonstrate a novel polarization splitter based on a two-dimensional grating etched in a silicon-on-insulator waveguide. The device couples orthogonal modes from a single-mode optical fiber into identical modes of two planar ridge waveguides. The extinction ratio is better than 18 dB in the wavelength range of 1530-1560 nm and the coupling efficiency is approximately 20%. The device is very compact and couples light only to transverse-electric modes of the planar waveguides. Therefore, it may be used in a polarization diversity configuration to implement a polarization insensitive photonic integrated circuit based on photonic crystal waveguides.

Compact and Highly Efficient Grating Couplers Between Optical Fiber and Nanophotonic Waveguides

We present high-efficiency grating couplers for coupling between a single-mode fiber and nanophotonic waveguides, fabricated both in silicon-on-insulator (SOI) and InP membranes using BenzoCycloButene wafer bonding. The coupling efficiency is substantially increased by adding a gold bottom mirror to the structures. The measured coupling efficiency to fiber is 69% for SOI grating couplers and 56% for bonded InP membrane grating couplers

Compact Wavelength-Selective Functions in Silicon-on-Insulator Photonic Wires

We present a number of compact wavelength-selective elements implemented in silicon-on-insulator (SOI) photonic wires. These include arrayed waveguide gratings (AWGs), Mach-Zehnder lattice filters (MZLFs), and ring resonators. The circuits were fabricated with deep UV lithography. We also address the sensitivity of photonic wires to phase noise by selectively broadening the waveguides, and demonstrate this in a compact AWG with -20 dB crosstalk and an insertion loss of 2.2 dB for the center channels

Basic structures for photonic integrated circuits in Silicon-on-insulator

For the compact integration of photonic circuits, wavelength-scale structures with a high index contrast are a key requirement. We developed a fabrication process for these nanophotonic structures in Silicon-on-insulator using CMOS processing techniques based on deep UV lithography. We have fabricated both photonic wires and photonic crystal waveguides and show that, with the same fabrication technique, photonic wires have much less propagation loss than photonic crystal waveguides. Measurements show losses of 0.24dB/mm for photonic wires, and 7.5dB/mm for photonic crystal waveguides. To tackle the coupling to fiber, we studied and fabricated vertical fiber couplers with coupling efficiencies of over 21%. In addition, we demonstrate integrated compact spot-size converters with a mode-to-mode coupling efficiency of over 70%.

A polarization-diversity wavelength duplexer circuit in silicon-on-insulator photonic wires

We present a wavelength duplexer based on a compact arrayed waveguide grating (AWG) in silicon-on-insulator photonic wire waveguides. Polarization insensitive operation is achieved through a special polarization diversity approach in which we use 2-D grating fiber couplers as integrated polarization splitters. To mitigate the effects of process variations, we propagated both polarizations in opposite directions through the same AWG with a mere 600x350microm(2) footprint. This resulted in an on-chip insertion loss between -2.1dB and -6.9dB, crosstalk of -15dB, and only 0.66dB polarization dependent loss. This is the first demonstration of a functional polarization-diversity circuit implemented in SOI nanophotonic waveguides, including interfaces to single-mode fiber.

Compact Focusing Grating Couplers for Silicon-on-Insulator Integrated Circuits

We report experimental results on compact and broadband focusing grating couplers, both in silicon-on-insulator (SOI) and gold on SOI. An eight-fold length reduction of the coupling structure from fiber to photonic wire in SOI, as compared to a linear grating and adiabatic taper, is obtained, without performance penalty. A proof of principle is given for a focusing grating coupler in gold on SOI, with 20% fiber-to-focus efficiency.