Light advancement and delay by linear filters with close to zero resonant transmittance

Filters operated close to the critical coupling were reported to be capable of an increase and decrease of the group velocity. The published examples are ring resonators (feedback-based filter) and Mach-Zehnder filters (no feedback filters). Here we show that these linear filters have, within certain bandwidth, identical filtering characteristics, although they are based on completely different filtering principles. Further, we demonstrate experimentally a long period fiber grating as another no-feedback filter that is capable to perform similar filtering characteristics when operated close to the critical coupling condition. This scheme may be of interest due to inherently large bandwidth provided by long-period fiber grating based filters. In the example given here, filters compatible with systems operating at repetition rates up to 40 GHz are demonstrated.