Refractometry using evanescent field based fiber loop ringdown spectroscopy

An analysis of fiber loop ring down spectroscopy for refractive index sensing with side polished fiber has been carried out in this paper. Side polished fiber as an evanescent field based sensor element has been fabricated with a low insertion loss of 0.2055 dB. In second part, a refractometric sensor using evanescent field based fiber loop ring down spectroscopy (FLRDS) has been implemented in which the output is represented in the form of a time constant. The detection sensitivity is increased manifold due to the multipass nature of the ringdown technique. Refractive index based detection of various concentrations of sodium chloride and glucose has been demonstrated with FLRDS technique using two fiber loops of length 5 m and 15 m. A sensitivity of 1.4 × 10−6 s/RIU change is recorded and minimum RI change of 3.5 × 10−5 is detectable.