Optical tunable delay lines have many applications for high-performance optical switching and signal processing. Slowlight
has emerged as an enabling technology for achieving continuously tunable optical delays. The reconfigurable delay
opens up a whole new field of nonlinear signal processing using slow light. In this paper, we review recent advances in
slow-light-based optical signal processing, with a focus on the data fidelity after traversing the slow light elements. The
concept of slow-light-induced data-pattern dependence is introduced and is shown to be the main signal degrading effect.
We then propose and experimentally demonstrate phase-preserving slow light by delaying 10-Gb/s differential-phaseshift-
keying (DPSK) signals with reduced DPSK data pattern dependence. Spectrally-efficient slow light using advanced
multi-level phase-modulated format is further described. With this technique, doubled or even tripled bit-rate signals can
be transmitted through a bandwidth limited slow light element. We finally show several novel slow-light-based signal
processing modules. Unique features such as multi-channel operation and variable bit-rate capability are highlighted.