Abstract
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. Related Research Topics
Related Projects
|
|
