Low loss propagation in slow light photonic crystal waveguides at group indices up to 60

We have designed slow light photonic crystal waveguides operating in a low loss and constant dispersion window of Delta lambda = 2 nm around lambda = 1565 nm with a group index of n(g) = 60. We experimentally demonstrate a relatively low propagation loss, of 130 dB/cm, for waveguides up to 800 mu m in length. This result is particularly remarkable given that the waveguides were written on an electron-beam lithography tool with a writefield of 100 mu m that exhibits stitching errors of typically 10-50 nm. We reduced the impact of these stitching errors by introducing "slow-fast-slow" mode conversion interfaces and show that these interfaces reduce the loss from 320 dB/cm to 130 dB/cm at n(g) = 60. This significant improvement highlights the importance of the slow fast slow method and shows that high performance slow light waveguides can be realised with lengths much longer than the writing field of a given e-beam lithography tool. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.