Research On Interferometric Microfiber Sensors

Taking advantage of microfibers as sensing elements, microfiber sensors provide us asensitive method to detect and measure a number of external environmental parameters due totheir strong evanescent field interaction. Compared with the conventional optical fiber sensors,microfiber sensors have presented advantages including high compactness, fast response andhigh sensitivity. Recently, how to develop fiber optic sensors with large dynamic range, highsensitivity and detection accuracy has received great interest. In this thesis, two novelinterferometric microfiber sensors are proposed and investigated, named “microfiberinterferometer based on cascaded long period gratings” and “micro-taper interferometer”,respectively. The external refractive index and relative humidity response of the proposedinterferometers are tested, respectively, showing that these two sensors have advantagesincluding large dynamic range, high sensitivity and stability, providing scientific and potentialapplications for biochemical measurements.The main content of the thesis is as follows:(1) Fabrication of microfibers through heat-and-draw approach. The diameter, length andshape of the microfiber are determined by the speeds of the stages, moving ranges and thepreheating time of flame. The mode properties of the microfiber are theoretically analyzed. Wefound that thinner microfibers are preferable for RI sensing due to the stronger evanescent-fieldsinteraction. In addition, the higher order modes are more sensitive to ambient RI change than thefundamental mode due to the stronger evanescent-fields. On the other hand, the characteristics oflight propagation over the transition regions with adiabatic and non-adiabatic conditions arediscussed, respectively. Low-loss transmission of fundamental mode can be achieved underadiabatic condition due to the slowly varying fiber cross sectional geometry. In contrast,fractional energy of fundamental mode couples into higher order modes under non-adiabaticcondition because of the abrupt change of fiber geometry along the fiber.(2) Microfiber interferometer based on cascaded long period gratings is proposed for themeasurement of external refractive index and fabricated by use of a CO2laser. The intensity oflaser irradiation and the scanning speed have been optimized. The mode coupling and spectral characteristic of the proposed interferometer are investigated while the interference spectrum iscalculated. The sensitivity to external refractive index and temperature are measured as2225.2nm/RIU and11.7pm/°C, respectively. The sensitivity to external refractive index is muchhigher than the previously reported interferometers based on cascaded long period gratings inSMFs. A further investigation on how to enhance the RI sensitivity for the detection of slightcomponential change of the surrounding medium is also carried out. The low temperaturesensitivity greatly reduces the effect of cross-sensitivity in the refractive index measurement. Thetotal length of the present device is only8.84mm showing excellent compactness.(3) Micro-taper interferometer is proposed for the measurement of relative humidity. Theenergy of fundamental mode couples to higher order modes (mainly HE12mode) over thetransition region which can act as a beam splitter/combiner. The interferometric fringes can beobserved as a result of the phase difference between the HE11and HE12modes. The sensitivity ofproposed interferometer to external relative humidity and temperature are measured as97.4pm/%RH and-4.74pm/°C, respectively. The low temperature sensitivity greatly reduces theeffect of cross-sensitivity in the relative humidity measurement. The sensor presents a responsetime less than188ms, much faster than the coated sensors.