Some Studies Of LPG Based Fiber Optical Sensors And Multiplexing Technologies

With the development of the fiber optics, fiber gratings have become the key compo-nents for a vast number of applications in optical communication systems and optical sens-ing area. There are two kinds of fiber grating, fiber Bragg gratings (FBG) and long period gratings (LPG). Compared with FBGs' practical applications in the industry, the applica-tions of LPGs are not intensively developed. With the copropagating mode coupling me-chanism, LPG can be utilized as a gain flattening filter or a chromatic dispersion compasa-tion. Since the cladding mode is sensitive to the surrounding refractive index, LPG is an excellent candidate for a fiber-optic bio-chemical sensor.In this thesis, we introduced our research work on the design and fabrication of LPG based fiber optic sensing probes and building up two multiplexing sensing system for LPG-type sensors.Firstly, a brief overview was given about the basic techniques for fiber gratings, in-cluding the classification, fabrication and application of fiber gratings in sensing region. The characteristic of LPG was theoretically discussed with the coupled mode theory and transfer matrix method. A fiber grating writing system based on UV laser in our lab was in-troduced here.We proposed three novel structures of sensors based on LPG working in reflection mode. 1) The LPG based sensor with reflection mirror at the end of the fiber forms a Mi-chelson interferometer, whose resolution is improved by the interfering fringe; 2) the sens-ing probe with a buble scattering can reflect the resonance light of the LPG without chang-ing the sensing characteristic of the grating; 3) the sensor with a cladding-mode-selective fiber end mirror make the structure of the refletion LPG type sensor more compact. These three LPG based sensors were fabricated by metal sputtering, lithography technology and the probes were tested to show their good quality to surrounding refractive index. By self-assembly technique some special chemical polymer is coated on the LPG, the experi- ment of measuring pH value was also given in this thesis.Despite the good performance in single-point sensing, LPGs are rarely reported as dis-tributed sensors. Since LPG works in transmission mode and occupies several wide loss-bands in spectrum, those well-developed multiplexing techniques, like wavelength di-vision multiplexing (WDM) and time division multiplexing (TDM), are not suitable for LPG-type sensors. We proposed two multiplexing system based on optical low coherent reflectrometry (OLCR) and optical frequency domain reflectometry (OFDR) techniques to make LPGs work as distributed sensors. Detailed characteristics of these two novel multip-lexing systems were experimentally studied and corresponding principles were physically explained here.OLCR technique utilizes different coherence conditions by broad band light source to distinguish sensing points in the network. It provides a demodulation method for the mul-tiplexing sensors. Despite the simple principle, the low cost gives the OLCR technique more potential commercial value than WDM and TDM system. We designed and built up a LPG multiplxing system based on OLCR technique and measured strain, temperature and bending signals. Additionally, multiplexing FBG pairs by our OLCR system was also in-troduced here.OFDR technique, from radar system into optical fiber region, is wildly used to demo-dulate reflective fiber sensors. By modifying the traditional OFDR system, we designed and fabricated a real-time interrogation system based on OFDR technique to multiplexing LPG type distributed sensors to meaure bending signal. We also experimentally studied and dis-cussed the principle of the traditional OFDR system with FBG distributed sensors.