High sensitivity humidity sensor based on cladding-etched optical fiber and lossy mode resonances

In this work a high sensitivity optical fiber humidity sensor (OFHS) is presented. The configuration chosen for this purpose is a cladding-etched single mode optical fiber (CE-SMF) coated with a thin film of tin oxide (SnO₂). The etching has been made using hydrofluoric acid (HF) and the coating has been fabricated by means of sputtering. Tin oxide was used to build the nano-coating which produces the Lossy Mode Resonance (LMR) and works as sensitive material. Theoretical and experimental results are shown and compared. The device was tested using a climatic chamber in order to obtain the response of the OFHS to relative humidity. Changes greater than 130 nm have been obtained for relative humidity varying from 20% to 90%, which gives a sensitivity of 1.9 nm/%RH.