In this paper we report the development and test of a high sensitivity fiber Bragg grating (FBG) seismic sensor system for intrusion detection application. A high sensitivity FBG seismic sensor is designed and its sensitivity is above 1000pm/g. Unbalanced Michelson interferometer and Phase generated carrier(PGC) algorithm are adopted by the demodulation system. The system noise is below 10-3 pm/ √Hz and the minimum detectable seismic signal is below 1μg√Hz . The FBG seismic sensor has a maximum detection range of about 70 meters for human and about 200 meters for small wheeled vehicle.
We propose an ultra-high sensitive temperature sensor based on multimode fiber (MMF) Mach-Zehnder interferometer (MZI). The multimode fiber MZI is composed of a short section of MMF inserting between another two pieces of MMFs with large lateral offset. The sensing head is packaged in a capillary which is filled with glycerol-water solution. At the offset splice interface, part of the light in the lead-in MMF is coupled into the glycerol-water solution around the sensing MMF and the remainder propagates along the cladding of the sensing MMF. Due to the large thermo-optic coefficient of the glycerol-water solution, the transmission spectrum of the MMF-based MZI shift quickly with temperature variation. Experimental results show that the temperature sensitivity is as high as 8.23 nm/°C.
A refractive index insensitive temperature sensor based on coaxial dual-waveguide optical fiber was proposed and demonstrated. The coaxial fiber contains a central core along the fiber axis and an annular core between the inner/outer claddings. By inserting the coaxial fiber in between two single mode fibers through core-offset splicing, cladding modes are excited at the splice point and therefore a modal Mach-Zehnder interferometer is achieved. The effective refractive index of the inner cladding mode is independent of the external refractive index due to the existence of the annular core. Owing to the large thermo-optic coefficient difference between the coaxial fiber's core and cladding, the modal interferometer has high temperature sensitivity. Such an interferometer is extremely suitable for temperature measurement in wet or liquid environment.
A novel composite interferometer sensor is presented and its sensing characteristics are investigated. Based on the infiber
integrated Michelson interferometer, a quartz tube is used to encapsulate the ends of the twin-core fiber and single
mode fiber to form the dual extrinsic FP cavities. Thereby, the Michelson and FP configurations are integrated into a
single fiber, which we call it Michelson-FP composite interferometer sensor. The novel sensor can respond to the axial
strain and radial bending simultaneously. We have derived and analyzed the interferometer principle of the new structure.
The analysis results show that the interferometer sensor could be considered as the superposition of Michelson
interferometer and FP interferometer. Moreover, we establish a testing system and conduct a series of experiments to
investigate the strain and bending characteristics. We measure the reflection spectra with the spectrum analyzer. The
spectral response of the composite interferometer sensor presents two pattern fringes with different frequencies due to
the respective optical path interferometers. The experimental results indicate that the composite interferometer sensor is
very sensitive to the strain and bending characteristics, and the presented sensor has different strain and bending
sensitivity coefficients. Due to these characteristics, the presented sensor might be able to measure the strain and bending
characteristics simultaneously. In conclusion, the presented novel interferometer sensor is of compact structure, high
integration and good strain and bending sensing characteristics. Thus, many types of fiber-optic sensors may be built
based on it.