Experimental observation of superluminal pulse reflection in a double-Lorentzian photonic band gap.

We report on the experimental observation of superluminal reflection of picosecond optical pulses at 1.5 microm using a specially designed 30-cm-long fiber Bragg grating (FBG) that realizes a spectral reflectivity profile given by the superposition of two closely spaced Lorentzian lines. Probing pulses of 380 ps duration tuned midway between the two Lorentzian lines are reflected without appreciable distortion with a measured peak pulse advancement of approximately 60 ps. The achievement of the negative group delay is due to the interference of the two resonance modes of the FBG structure and has a close connection to the phenomenon of negative group velocity for pulse propagation in an inverted medium possessing a doublet line.