Group delay analysis of chirped fiber Bragg gratings using photon counting

We present a novel measurement method for the relative group delay of chirp ed fiber Bragg gratings, based on the time-of-flight of single photons. In contrast to existing techniques, the light source is a pulsed broad-band LE D. This simple and inexpensive source allows for measurements over a large spectral range with a high spectral resolution (8 pm) obtained from a tunab le Fabry-Perot filter. The photons are detected by an InGaAs avalanche phot odiode operated in photon-counting mode, resulting in a precision for the g roup delay of 15 ps, We observed an unusual asymmetry in the group delay cu rves of one of the commercial gratings measured with our method. This asymm etry can be qualitatively understood, taking into account a nonmonotonous v ariation of the grating period.