Sensing With Slow Light in Fiber Bragg Gratings

Abstract

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 f Hz, the lowest value reported for a passive FBG sensor.

Enhancing optical field intensities in Gaussian-profile fiber Bragg gratings

“…2 to its measured group delay, we estimate the rate of absorption for light in the FBG region to be 5.4 1.7 × 10 7 s −1 . Assuming that the same absorption applies to wavelengths of light passing through the 1.2 cm FBG without interacting with the grating, we estimate the broadband, nonresonant absorption induced in the fiber core by the grating writing process to be α 0.25 0.17 m −1 (0.057 0.040 dB∕cm), which is in keeping with FBGs with similar Δn in SMF-28 [10]. Figure 4(a) shows a close up of the six lowest-order transmission peaks simulated by the transfer matrix method with (black) and without (red) this estimated absorption.…”

“…Experimentally these fine resonant structures were first reported in 1993 [6] and have been shown to exhibit group delays of several nanoseconds [10]. In this work we investigate the relationship between the group delay and FIE of these resonant modes, which could be used to enhance nonlinear optical phenomena in fiber [5].…”

Single-Crystal Silicon Photonic-Crystal Fiber-Tip Pressure Sensors

“…Thus, optical fiber sensors have been used for pressure sensing [1]; strain sensing [2]; gyroscopes [3]; acoustic sensing [4], [5]; and temperature sensing [6].…”

Enhancing the light-matter interaction using slow light: towards the concept of dense light

“…It has also been theoretically and experimentally proved that the extreme dispersion of slow light can lead to an enhancement of the spectral sensitivity of interferometers 4 . The role of slow light in gyroscopes has also been theoretically investigated 5 and the possibility to use fiber Bragg gratings to realize ultrasensitive strain sensors has been experimentally proved 6 . Recent works 7,8 have theoretically demonstrated that BeerLambert-Bouguer (BLB) absorption can be increased while light transmits through a special kind of microstructured photonic crystal (PC) cuvette.…”