Slow light in fiber sensors

This paper examines the sensitivity improvements that have been achieved by making use of slow light in a variety of fiber sensors. We show in particular that slow light can have dramatically different impacts depending on its nature (material or structural) and on the parameter that is being sensed. In a fiber optic gyroscope measuring an absolute rotation for example, structural slow light does not enhance the maximum sensitivity achievable for a given loss and sensing area compared to a non-resonant structure such as a Sagnac-based fiber optic gyroscope. However, it does reduce the length of fiber required to achieve this sensitivity. For fiber sensors relying on the measurement of absorption, such as gas detectors, structural slow light improves the sensitivity because it increases the effective path length through the absorber and therefore the level of absorption. Material slow light, on the other hand, has been measured to have no impact on the sensitivity. For many other parameters besides rotation and absorption, the sensitivity is expected to be enhanced by either type of slow light, by orders of magnitude with suitable configurations. We illustrate this enormous potential with two configurations of strain sensors utilizing a fiber Bragg grating (FBG) as the sensing and slow-light medium. In properly designed FBGs supporting light with a group index in the range of 50 to 130, we measured a maximum sensitivity of 1.7-3.14 105 strain-1 and a record minimum detectable strain of 820-880 fε/√Hz. This value is ~730 lower than the previous record using conventional light in a passive FBG sensor, in accord with predictions. Further enhancements are expected with straightforward improvements in FBG design.

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