The slowing down of light presents useful possibilities for applications in photonic integrated circuits. But efficient coupling of light into, e.g., slow-light photonic crystal channel guides from a ridge waveguide (butt-coupling) remains a substantial challenge, given the mismatch between the two guided modes [1]. Here we propose to use photonic crystal (PhC) continuous tapers [2] to improve this coupling. In our simulations (2D finite difference time domain), we used a reduced-width PhC channel guide (of width 0.8a{the square root of}3 between the hole centres - see inset in Fig. 1) to obtain a slow-light operating region close to 1.52 μm. The length of the slow-light section is 16a, a being the period (= 425 nm) - and the area filling-factor is 0.35. Firstly we performed an optimisation of butt-coupled launching by varying the input waveguide width. The best transmission is given in Fig. 1 - and shows that the level of transmission significantly decreases towards the band-edge, which is the spectral region associated with low group velocity. We then added PhC tapers at the input and output. They start from a channel width of a{the square root of}3, are linear and of length 8a. The ridge waveguide entry width was also optimised and the results for the transmission can be seen in Fig. 1. The transmission remains flat-top until very close to the band-edge, where it drops suddenly. For wavelengths close to this band-edge, where the group index is close to 100 (determined by the time of flight of a pulse), the transmission can be improved from 3% to 96%. This result shows the clear improvement that can be obtained by using a transition device such as a continuous PhC taper that has a simple linear shape and short length (only a few micrometres).
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