Sensing With Slow Light in Fiber Bragg Gratings

On the edge of the bandgap in a fiber Bragg grating (FBG) narrow peaks of high transmission exist at frequencies where light interferes constructively in the forward direction. In the vicinity of these transmission peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. Since the sensitivity of a phase sensor to most external perturbations is proportional to the reciprocal of group velocity, in these slow-light regions the sensitivity of an FBG is expected to be significantly enhanced over traditional FBG sensors operated around the Bragg wavelength. In this paper, we describe means of producing and operating FBGs that support structural slow light with a group index that can be in principle as high as several thousand. We present simulations elucidating how to select the FBG parameters, in particular index modulation, length, and apodization, to generate such low group velocities, and quantify the very large improvement in strain and temperature sensitivities resulting from these new slow-light configurations. As a proof of concept, we report an FBG with a group index of 127, or a group velocity of ~2,360 km/s. This is by far the lowest group velocity reported to date in an FBG. Used as a strain sensor, this slow-light FBG is shown to be able to detect a strain as small as 880 fε/ √Hz , the lowest value reported for a passive FBG sensor.