Fabrication and characterization of fiber loop ringdown evanescent field sensors

Abstract

The evanescent field (EF) sensing mechanism has been widely implemented in the development of fiber optic sensors (FOSs). Challenges in the development of EF-FOSs include the enhancement of the sensor's sensitivity and implementing high precision control in the fabrication of a sensor head to allow the desired strength of the EF to be excited in the interface. We combine the EF sensing mechanism with the fiber loop ringdown (FLRD) sensing scheme to create FLRD-EF sensors. The multi-pass nature of the ringdown technique enables the detection sensitivity of the FLRD-EF sensors to be enhanced by more than ten-fold. The EF sensor heads are fabricated by etching the cladding of the single-mode fiber, and the process is monitored by the FLRD technique in real-time, on-line, and with a control precision of 0.1 µs in terms of ringdown time, which corresponds to a 0.5 µm precision in etched fiber diameters. Multiple evanescent field sensor heads consisting of various etched fiber diameters and fiber lengths are fabricated to form different FLRD-EF sensor units. The effect of the sensor head dimensions on the sensors' response time and detection sensitivity are investigated. New FLRD-EF scattering sensors based on index difference are demonstrated for the detection of water at 1515.25 nm.