Researches On Interferometric Microstructured Fiber-optic Sensors And Their Applications

Optical fiber sensors,as a means of measuring and monitoring environmental information,have great value in the field of scientific and industrial applications,and show great potential in the modern IoT and AI industry.Compared with other types of fiber-optic sensors,the interferometric sensors have attracted greater attention due to the high sensitivity,simple fabrication,flexible structures,and large dynamic measuring range.However,there are two major challenges for conventional interferometric fiber-optic sensors.Firstly,for the conventional optical fibers,the manipulation of the light field is limited,which degrades the sensitivity.Secondly,conventional sensing units are complex and require difficult processes to prepare,which decreases the device repeatability and robustness.On the other hand,due to unique optical properties,microstructured fibers provide an extremely attractive solution for achieving high sensitivity and robustness of sensors.In this thesis,we focus on the microstructured fiber-optic sensors and make extensive research.We establish a set of theoretical models,efficient optimization methods and fabrication procedures for interferometric microstructured fiber sensors.Firstly,we establish the theoretical model of dispersion turning point(DTP).Compared with conventional fiber refractometers,the sensitivity of DTP-based refractometers can be improved by at least one order of magnitude.Secondly,we propose a method to enhance the fabrication tolerance and broaden the operation bandwidth of high sensitivity by introducing a microstructure into the fiber,which eases the strict requirement for fabrication and addresses the detection difficulties for conventional refractometer based on DTP theory.In this way,the waveguide dispersion can be flexibly manipulated,and the wavelength sensitivity of G can be reduced,thus the high-sensitivity region can be significantly broadened.Additionally,the fiber diameter dependence of G is reduced,and therefore the fabrication tolerance of the microstructured fiber can be improved.Thirdly,a low-cost one-step drawing process is proposed,which can achieve the flexible control of diameter and taper,paving the way for the fabrication of microstructured fiber.The main research contents and progress are as follows:(1)We demonstrate a tapered PANDA-air-hole fiber(PAHF)operating near the birefringent DTP,and present an ultrasensitive enhanced fabrication-tolerance refractometer utilizing the polarimetric interference in a Sagnac loop.To decrease fabrication error sensitivity of conventional DTP-based microfiber,we optimize the waveguide dispersion profiles by using the PAHF-based microfiber which is specially designed by introducing the double air holes into the cladding.Due to the tunable birefringent dispersion,the diameter dependence of the group birefringence difference is reduced.In this way,compared with all the reported microfibers,the workable diameter range is doubled,indicating larger fabrication tolerance and higher device repeatability.Besides,the ultrasensitive bandwidth of PAHF-based microfiber is dramatically enhanced for at least 600 nm.(2)We introduce a low-cost one-step drawing process to fabricate the PAHF-based microfiber.The critical factor of fabricating the microfiber with the desired diameter while maintaining the internal microstructure is the controlling of pulling time and pulling distance.We experimentally verify the ultrahigh refractometer of 47223 nm/RIU using the PAHF with a diameter of 3.6?m.Our proposed PAHF-based refractometer provides an effective solution for water quality monitoring.(3)We propose a tapered single stress-applying fiber(SSAF)operating near the DTP,and demonstrated an ultrasensitive broadband compact refractometer in the form of an In-Line modal interferometer.To promote the detection efficiency of conventional DTP-based microfiber,we optimize the waveguide dispersion profiles by using a tapered single stress-applying fiber(SSAF)which is specially designed by introducing the stress-applying part(SAP)into the cladding.The wavelength sensitivity of the group effective refractive index difference can be significantly reduced.In this way,the operation bandwidth is dramatically broadened for 500 nm in comparison with that of conventional microfiber with the same diameter based on DTP,indicating a broad range of optional probing wavelength,a liberal fiber-length condition,and thus high fabrication tolerance.(4)We use the proposed one-step drawing process to fabricate the SSAF-based microfiber with the abrupt taper angle,and the higher order mode can be effectively excited and guided.We experimentally verify the ultrahigh sensitivity and broadband operation using an SSAF with a diameter of 2.3 ?m,and the maximum sensitivity of 30563 nm/RIU is obtained.Our proposed SSAF-based refractometer shows great potential in microfluidic chemical and biochemical sensing.(5)We present a single stress-applying fiber(SSAF)and demonstrated an assemble-easy compact cost-effective vector bending sensor based on modal interference,overcoming the low device repeatability of conventional vector bending sensors.The SSAF is specially designed by introducing a stress-applying part(SAP)into the cladding to achieve asymmetrical geometry.The asymmetric cladding mode can be effectively excited,which is sensitive to the bending and has orientation-dependence.(6)We experimentally demonstrate an in-line vector bending sensor with 3 mm-long SSAF as the sensing unit,with the maximum sensitivity of 2.04 nm/m-1.Additionally,the temperature crosstalk is relatively low.Our proposed SSAF-based vector bending sensor is promising in structural health monitoring.