Study On F-P Interferometers With In-fiber Micro-cavity And Their Sensing Applications

Optical fiber sensors have attracted the attention of researchers or users because of their intrinsic safety,anti-electromagnetic interference and other outstanding characteristics,and are widely used in temperature,humidity,stress and gas sensing fields.In particular,the in-fiber Fabry-Perot(F-P)interferometer has become a research hotspot in the field of optical fiber sensing due to its compact structure and high sensitivity.This paper introduces two types of F-P interferometers based on capillary fusion and femtosecond laser direct writing micro-cavities in fibers.The response characteristics such as temperature,hydrogen concentration and air relative humidity are tested and analyzed,and the sensing applications of these two interferometers are discussed preliminarily.(1)A tip FPI based on a sandwiched capillary was proposed,and its temperature and hydrogen concentration sensing characteristics were experimentally studied.To fabricate the interferometer,a short segment of capillary is inserted between two singlemode optical fibers(SMFs)and then filled with liquid.Owing to the high thermaloptical coefficient(TOC)of the liquid,the interferometer has a high temperature sensitivity of-0.324nm/?.Furthermore,Pt-loaded tungsten trioxide(Pt-WO3)was coated on the surface of the interferometer,and the thermal effect of its reaction with hydrogen was used to achieve high-sensitivity measurement of hydrogen concentration.The sensor has a compact structure and a small size(total size<100?m).Experiments have shown that its sensitivity can reach up to 4.969 nm/%(vol.%)in the concentration range of 0 to 4.0%.And it also exhibits some advantages of good selectivity,repeatability,stability,and immunity to air humidity.(2)Another FPI with an in-fiber micro-cavity fabricated by fs-laser direct writing technology is presented and experimentally demonstrated for refractive index(RI)and air relative humidity(RH)sensing.The Fs laser pulse is focused into the optical fiber through the objective lens,causing the RI in the focusing area to change,then a microcavity is created in the fiber core.Combined with optical fiber cutting technology,a composite F-P interferometer is constructed.The experimental results agree with the theoretical analysis.This interferometer is sensitive to the surrounding RI with a maximum sensitivity of-497.2dB/RIU.In further,the humidity sensitive material of agarose is coated on the end-face of the FPI for RH sensing,and a maximum sensitivity of 2.03dB/%RH is obtained near 90%RH.Experimental results show that the sensor has an extremely short response and recovery time with good stability and repeatability.Finally,it is preliminarily tested for monitoring human breath.