One of the problems to be solved before photonic crystal based photonic integrated circuits can become useful devices is the interface to the outside world. Efficient coupling of light between a fiber and a photonic crystal waveguide is a challenge because of the small dimensions of the photonic crystal waveguides. To obtain a good efficiency and alignment sensitivity, some kind of spot-size converter between the photonic crystal waveguide and the fiber is required. Our work is focussed on coupling to 2-D photonic crystal waveguides etched in a semiconductor slab. We use a grating and a lateral taper as spot-size converter. The grating couples light from an out-of-plane fiber into a planar ridge waveguide. This waveguide is tapered into a narrow waveguide that can be connected to a photonic crystal waveguide. We use a grating of only 10um x 10um and a high vertical-index-contrast slab structure (SOI or GaAs/AlOx). Compared to a traditional grating coupler, our grating is very short (approximately 20 periods). This allows butt-coupling of the fiber and results in a larger bandwidth. Also a 10um wide waveguide can be adiabatically tapered into a narrow waveguide over a relatively short (few 100um) distance. We have designed and optimized the grating using 2D-simulations. For a simple 2nd order grating (grating period = wavelength) the coupling efficiency to fiber is limited to approximately 20%. By using a bottom reflector (2 pair DBR) this can be increased to 40%. More complicated gratings result in 75% calculated efficiency, but the fabrication tolerances become very critical. Experimentally, we have demonstrated 20% coupling efficiency from a standard single-mode fiber to a planar GaAs/AlOx waveguide (with bottom reflector). This coupling efficiency is comparable to the maximum efficiency achievable using edge coupling with tapered lensed fibers. But the alignment tolerances of the out-of-plane coupler are much better.
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