Investigation On Theories And Applications Of Fiber Laser Sensing Based On Micro/Nano-structures

In earthquake precursor monitoring field, temperature of earth’s crust, lithostatic stress,rare gas density, and mineral concentration in underground water are some criticalparameters which need accurate, real time and distributed detection. As a result, temperaturestrain and refractive index (RI) sensors with high sensitivity, low cost and multiplexingcapacity are highly desirable. As a category of novel sensors, optical fiber sensors have gotrapid development for sensing a variety of physical parameters, such as temperature, strain,vibration, acceleration, RI, concentration of solution, hydroacoustics, etc, in the past20years.Compared with the traditional electronic sensors, fiber sensors have many distinctadvantages, including immunity to magnetic interference, the capability to work in harshenvironment, the durability and the convenience for sensor networking. Thus, fiber sensorsare good candidates for earthquake precursor monitoring.In this thesis, we focus on investigating novel fiber sensors based on micro-structuresfor applications of earthquake precursor sensing. High sensitivity sensing for temperature,strain and RI are realized. The main research achievements are listed as follows:Firstly, the history and current status of optical fiber sensors are reviewed. Two kinds ofmicro-structured optical fibers, which are fiber gratings and micro/nano-fibers, areintroduced. Based on Optical Waveguide Theories, the optical properties of fiber gratingsand micro/nano-fibers are discussed through numerical simulation and comprehensiveanalysis.Secondly, based on a multi-mode fiber laser, a longitudinal mode number encodedstrain sensor is demonstrated. Strain applied on fiber Bragg grating (FBG) influences thenumber of longitudinal modes in the output laser, which can be counted via its beat spectrum.Strain sensitivity of0.02με per mode change is experimentally demonstrated, showing astrain resolution with two orders of magnitude higher than an optically interrogated fiberstrain sensor.Thirdly, an optical fiber microstructure by combining fiber gratings and microfiber isproposed. By taper drawing a uniform FBG, a length microfiber was derived at its centersection and the original FBG is separated into two shorter ones, and consequently amicrofiber Fabry-Perot interferometer (MFPI) is generated. Theoretical study indicates that the MFPI inherits a wavelength-selective property from FBG and sensitivity for ambient RIfrom microfiber. Thus, it has certain response to temperature, strain and ambient RI. Weexperimentally realize simultaneous temperature and RI sensing based on the MFPI throughspectrum analysis. Assisted by frequency domain processing and Gaussian fitting of thereflection spectrum of MFPI, the measurement resolution of wavelength shift is improvedone order of magnitude.Fourthly, beat frequency interrogation of the MFPI based dual-wavelength fiber laser asan RI sensor is investigated. The MFPI is used as a reflector of a linear cavity fiber laser.Dual-wavelength lasing is derived at two adjacent reflection peaks of the MFPI. Thus, thebeat frequency of two wavelengths is equivalent to the free spectral range of MFPI, anddepends on ambient RI. RI sensitivity of911MHz/RIU is experimentally achieved andcorresponding high resolution up to10-6RIU can be realized.Finally, we present another optical fiber micro-structure, which is formed by twocascaded Sagnac loops based on microfiber. The Sagnac MFPI is fabricated bymicro-operating a length of microfiber with a probe and then constructing two cascadedSagnac loops. Laser and sensor characteristics of such structure (Sagnac MFPI) are studied.Its reflection spectrum has uniform periodical interference patterns and high sensitivity toambient RI. By incorporating it as a reflector of a linear cavity fiber laser, dual-wavelengthlasing at two adjacent reflection peaks of Sagnac MFPI is achieved. By measuring the beatfrequency of the two coherent lasing with different wavelengths, which is equivalent to thefree spectral range of the Sagnac MFPI, RI sensing capability is investigated and confirmedby experiment.