Some Optical Sensors Based On Novel Structured Fibers And Their Applications

With the economic development and social progress, environmental pollution and engineering quality have become the important factors to restrict the healthy and harmonious development of our society. To develop new sensors for environmental and engineering monitoring is a hot topic in both academic and engineering areas. Compared to the electrical counterpart, the optical fiber sensors have many advantages:small size, low weight, electromagnetic immunity, multiplex and remote sensing capability. Therefore optical fiber sensors have attracted more and more research interests. In this thesis, we propose some optical sensors based on novel structured fibers for environmental and engineering monitoring applications.We propose a novel in-fiber modal interferometer, which is based on the modal mismatch between a standard single-mode fiber and a thin-core fiber. The high-order cladding mode excited by the modal mismatch will interfere with the core mode, and the generated interference signal can be used as the sensing signal. The thin-core fiber modal interferometer (TCFMI) has a high refractive-index (RI) sensitivity of140nm/R.l.U.(refractive index unit:R.I.U.) and a low temperature sensitivity of15pm/℃. Thus, the TCFMI is a promising chemical and biological sensor. After coating the TCFMI by electrostatic self-assembly technology, the sensor can be used for pH sensing, relative humidity (RH) sensing, metal ion concentration sensing, etc. The pH sensor has a monotonic, highly sensitive, fast, reversible response through optimizing the self-assembled coating, i.e., increasing the coating thickness, generating nanoporous in the coating, etc. To measure the RH and temperature simultaneously, we fabricate a fiber Bragg grating in the thin-core fiber. Our proposed RH sensor based on TCFMI with self-assembled coating has many advantages, such as low cost, high sensitivity (97.2pm/1%RH), fast response (10s), etc. In metal ion sensing area, our proposed sensor can measure Cu2+, Fe2+and Zn2+with a high sensitivity. With a stronger metal chelator (ethylene-diamino-tetraacetic acid), the reusability of this sensor can be achieved.Strain is an important parameter in engineering monitoring. In this paper, we propose two types of fiber optics strain sensors. One is an interferometric sensor based on all-solid birefringent hybrid photonic crystal fiber (PCF). The stress-induced birefringent fiber is known to offer the maximum strain sensitivity, but also to suffer from temperature crosstalk. The birefringent sensor based on the all-solid hybrid PCF offers a high strain sensitivity of23.8pm/με and a high temperature sensitivity of-1.12nm/℃. To eliminate the cross sensitivity to temperature, we propose two cascaded Sagnac interferometers, one is for strain sensing, and the other is for temperature compensation. Experimental results show that the sensor can suppress the cross sensitivity to temperature to-9pm/℃, while still proving a high strain sensitivity of25.6pm/με. The other type is based on four-wave mixing (FWM) in a PCF. All the conventional strain sensors rely on either specialty fibers or postprocessing of the fibers and use the linear properties of the fiber. In this thesis, we demonstrate a nonlinear fiber-optic strain sensor for the first time, which uses the shifts of FWM Stokes and anti-Stokes peaks caused by the strain-induced changes in the structure and refractive index of the PCF. Experimental and simulation results are presented. The strain sensitivity can be improved by optimizing the pump wavelength and power.