Study On Relative Humidity Sensors Based On Fiber Bragg Gratings

Fiber Bragg gratings (FBGs) have received great interests in recent yearsdue to their unique characteristics, such as electrically-passive operation, immunity toelectromagnetic interference, small size, low insertion loss, high sensitivity etc. Theyare widely employed in numerous areas, such as civil structures, aerospace systems,ocean survey, oil and electricity industry, chemical and biological industry, andmedical instrument. This work studied on relative humidity sensors based on FBGs.Following are the main contents：1. The development of FBG sensor technology and its applications are introduced.Fiber grating fabrication approaches and the fabrication system developed byourselves are introduced. Then, the sensing principle of FBGs is studied. The currentsituation and the development of fiber humidity sensing technology are reviewed.2. A hybrid fiber grating coated with polyvinyl alcohol is proposed anddemonstrated experimentally for relative humidity (RH) sensor. The hybrid fibergrating including a FBG and a TFBG was fabricated in a H2-loaded B/Ge-codopedphotosensitive fiber with a244-nm frequency-doubled Argon laser and the phasemask technique. The length of TFBG is2.0cm and the tilted angle is about8o. A0.3cm-long FBG was then superimposed at the rear part of the TFBG. Optical power ofthe reflected signal by the FBG changes with refractive index of the moisture sensitivepolyvinyl alcohol, and humidity measurement is therefore realized. Experimentalresults show that the measurement range is30%RH-95%RH and the maximumsensitivity is0.737nW/%RH. The average response time is~2s and the measurementis nearly insensitive to temperature.3. An optical fiber relative humidity sensor is proposed and experimentallydemonstrated with a multimode-fiber taper coated with a layer of polyvinyl alcoholcascading with a fiber Bragg grating. The sensor operation is based on the interactionbetween the optical evanescent field and the coating along the taper waist. Humiditychanges the refractive index of the extended coating, thus changes transmission lossof the light signal. Humidity measurement is therefore realized. Moreover, theproposed humidity sensor is temperature-insensitive. 4. An novel all-fiber sensor for simultaneous temperature and humiditymeasurement is proposed based on a photonic crystal fiber (PCF) spliced between asingle-mode fiber and a FBG. Due to the different responses of the modal-interferenceand the FBG on the changes of temperature and humidity, temperature and humiditywere measured simultaneously. Experimental results show that humidity sensitivity of0.0995nm/%RH and sensing resolution of±0.133%RH are achieved, andtemperature sensing resolution is±1.04°C. The maximum errors are1.13°C and0.24%RH for temperature and humidity respectively, by using matrix inversionmethod.