Ultra-high sensitivity strain sensor using slow light in fiber Bragg gratings

Slow-light strain sensors based on fiber Bragg gratings (FBGs) have the potential to offer ultra-high sensitivity for high-end commercial applications. In the research presented in this dissertation, we study how to generate extremely slow light in fiber Bragg gratings and how to use slow light for strain sensing applications.;On the edge of the bandgap in an FBG there exist narrow peaks of high transmission. In the vicinity of these transmission peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. In this thesis we exploit such group delay for sensor applications. First, 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 to generate such low group velocities. The main parameters that need to be adjusted are the index modulation of the grating, which must be increased in order to increase the resonator finesse; the loss coefficient, which must be kept as low as possible, as in any resonator structure; the length, which must be optimized for this particular loss coefficient; and the degree of apodization, which introduces Fabry-Perot resonances that can enhance the group index over a uniform grating. Utilizing the best compromise between loss and index modulation we are aware of at the moment, which is a loss coefficient of 0.02 m-1 for a Deltan of 1x10-3 in an FBG fabricated with ultrafast pulses, we predict a maximum group index of about 5000 (with 2% transmission) in a uniform grating. 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 a factor of 25 lower than previously reported measured group velocities in an FBG. Second, we demonstrate two configurations of strain sensors utilizing an FBG as the sensing and slow-light medium: a phase-based and a power-based sensor. In properly designed FBGs supporting light with a group index in the range of 50 to 130, we measured a maximum sensitivity of 3.14x105 to 2.1x106 strain -1 and a record minimum detectable strain of 280-820 fepsilon/✓Hz. This lowest minimum detectable strain is a factor of 18 lower than the previous record in a passive FBG sensor. Further enhancements are expected with straightforward improvements in FBG design.