Study On Optical Fiber Sensors In Applications For Multi-parametric Measurement

Recently,optical fiber sensors have aroused a lot of attentions.Compared with conventional electrical sensors,optical fiber sensors possess a large number of advantages such as electrically passive,immunity to electromagnetic interference,high temperature and anti-corrosion resistance,lossless joint with optical communicative network,distributed and long-distanced measurement and ease to multiplex.Therefore,they are widely used in conventional measurements for temperature,strain and refractive index,but also in novel measurements for relative humidity,magnetic field and so on.In this work,there are five kinds of functionalized optical fiber sensor proposed.The main contents are as following:1.A simple magnetic field sensor by using an optical fiber Bragg grating?FBG?cascaded by a cleaved optical fiber end,which is surrounded with magnetic fluid?MF?,is experimentally demonstrated.Magnetic field changes Fresnel reflectivity of the fiber end face through refractive index change of the MF,leading to a modulation to the reflection background in the reflection spectrum of the FBG.By comparing intensity difference between the FBG reflection peak,which is not sensitive to the external magnetic field,and the reflection background,magnetic field measurement is achieved successfully.2.Nickel ions(Ni²⁺)probe with enhanced sensitivity is experimentally demonstrated by using a normal FBG cascaded with coating-functionalized fiber tip.The coating of chitosan/poly acrylic acid are formed directly onto the cleaved fiber tip via layer by layer technique.When the Ni²⁺ is absorbed by the coating,the Fresnel reflectivity of the fiber tip,which forms the noise background in the FBG's reflection spectrum,will be modulated due to Ni²⁺-caused refractive index variation of the coating.By tracing variation of the optical signal-to-noise ratio??R?,measurement of Ni²⁺ concentration is achieved successfully.3.A relative humidity?RH?sensor in reflection mode is proposed and experimentally demonstrated by using a polyvinyl alcohol?PVA?-coated tilted-fiber Bragg grating?TFBG?cascaded by a reflection-band-matched chirped-fiber Bragg grating?CFBG?.The sensing principle is based on the RH-dependent refractive index of the PVA coating which modulates transmission function of the TFBG.The CFBG is properly designed to reflect a broadband of light spectrally suited at the cladding mode resonance region of the TFBG,thus the reflected optical signal passes through and is modulated by the TFBG again.As a result,RH measurement is realized and demodulated in the range from 20%RH to 85%RH.4.A miniature all-fiber temperature sensor is demonstrated by using a Michelson interferometer formed with a short length of Germania-core,silica-cladding optical fiber?Ge-fiber?fusion-spliced to a conventional single-mode fiber?SMF?.Thanks to the large differential refractive index of the Ge-fiber sensing element,a reasonably small free spectral range?FSR?of 18.6 nm is achieved even with an as short as 0.9 mm Ge-fiber that may help us increase the measurement accuracy especially in point sensing applications.5.A smooth muscle cells?SMCs?traction force sensor is proposed by using a polarization-maintaining fiber?PMF?embedded into Polydimethylsiloxane?PDMS?microwall and connected with 50:50 single mode coupler to form Sagnac fiber loop.The cell traction force detection is achieved successfully by monitoring the wavelength shift by the polarization variation of the PMS with PDMS-microwall as mechanotransduction.