Development And Applications Of Wavelength Modulated Fiber-Optic Sensors

Fiber-based devices have become one of the most important optoelectronic components with the development of fiber optics as well as the driving of the industry of optical fiber sensors and communications.The common fiber devices include fiber interferometers,fiber Bragg gratings(FBG),long-period fiber gratings (LPFG),and photonic crystal fibers(PCF) etc.In recent years,those fiber devices attracted a lot of research attentions because of their special sensing capabilities. Among various techniques,the wavelength modulated sensing scheme can avoid intensity variation issues induced by e.g.light-source power instability,fiber attenuation etc.In this thesis,we introduce our research work on the development of wavelength modulated fiber sensors and their application researches.We firstly present a new fiber modal interferometer based on thin-core optical fiber,whose cutoff wavelength is around three times shorter than standard single-mode fiber,as a refractive-index sensor.It was found from the experiments that the high-order cladding modes can be efficiently excited due to the core mismatch,and interfere with the guided core mode to form a high extinction-ratio filter(＞30 dB).Its interference spectrum dip exhibits high sensitivity to a small change of external RI(＞104.3 nm/R.I.U) but low sensitivity to the change of temperature(＜0.015 nm/℃).Such a fiber device is with extremely simple structure, but excellent refractive-index sensor properties,and thus is an ideal candidate for bio-chemical sensing applications.Fiber grating is a special fiber-optic device based on resonant scattering.It has been well developed into a compact and high resolution sensor due to their good properties,such as electrically passive operation,electromagnetic immunity,high sensitivity,compactness,and flexible multiplexing capability.We have developed a high-temperature and high-pressure hydrogen-loading system for the KrF laser based grating fabrication system.Moreover,we present a CO₂ laser based LFPG fabrication system,and successfully fabricated LPFGs.Based on the above two systems,we successfully produced high-temperature FBG sensors named as chemical composition grating(CCG),which can work up to 1100℃with sensitivity of 15pm/℃.Finally we demonstrated a low-cost FBG sensing system for coal mine security monitoring.With coherence multiplexing(CM) and spatial division muhiplexing (SDM) techniques,the hybrid sensing network can support more than forty sensors for quasi-distributed sensing schemes.It was experimentally demonstrated that the multiplexed signal of each sensor can be clearly demodulated.Methane concentration is experimentally determined with a maximum sensitivity of 10.92% (intensity variation)/1%(concentration variation) using the designed sensor structure.The system is also tested by measuring strain and temperature and showed good experimental results.