The compact FBG-based humidity sensor setup

Abstract

This paper presents a concept of novel intensity-modulated humidity optical sensor with cascaded and spectrally overlapped fiber Bragg gratings (FBGs). Basically, the sensor consists of broadband light source, circulator, FBG sensitive to humidity and temperature (polymer-coated FBG), standard FBG (compensation of temperature influence) and optical power meter. In this setup the light is propagating through circulator to these FBGs and the whole reflected light is then directed to detector. Both standard FBG and polymer-coated FBG are assumed to be placed serially. This approach provides that the value of detected optical power is a complex function of the reflections from FBGs. Although polymer-coated FBG is sensitive not only to humidity, but also to temperature, the proposed sensor is an attempt of overcoming this problem. Overlapping spectra of FBGs and similar temperature sensitivities of FBGs imply that the sensor is able to response to change of relative humidity (RH), simultaneously having minimized temperature sensitivity. The sensor setup was verified experimentally in a temperature range from 25°C to 50°C and relative humidity from 30%RH to 70%RH. Instead of usage of high cost and complex devices, such as optical spectrum analyzer, the concept of alternative intensity-modulated FBG-based optical sensor for humidity measurement is clearly presented.