We design tapered waveguide junctions for coupling between photonic crystal and traditional dielectric waveguides and evaluate their transmission efficiency. While the transmission efficiency is less than 60% using no taper,the tapered couplers have over 90% power transmission. We show that different types of couplers are needed for efficient coupling into and out of photonic crystal waveguides.
Attila Mekis and J. D. Joannopoulos, "Tapered Couplers for Efficient Interfacing Between Dielectric and Photonic Crystal Waveguides," J. Lightwave Technol. 19, 861- (2001)