Analysis Of LPFG's Sensitivity In Different Resonant Modes

Optic communication network plays a very important role in modern communication system. As the demand for network capacity is increasing, it is becoming more and more important to study the switching abilities in network node. Due to the advantages in small size, low loss and economic cost, fiber optic gratings have great potential in optic network applications, including the possible applications in switching nodes.Long-period fiber grating (LPFG), as one kind of fiber optic gratings, has been widely used in tunable wavelength selective optical switch for its high sensitivity to surrounding refractive index. This thesis mainly studies the coupling characteristics and the sensitivity to external surroundings of LPFG under different cladding radius and cladding modes. The object of the study is to find an effective way to excite coupling between different modes to realize optical devices with high sensitivity.Firstly, we analyzed the coupling principle in LPFG, in which, the effective refractive index of cladding modes were calculated. At the same time, the filed distributions of different cladding modes were emulated by IFO Gratings, a kind of computer emulation software.Then, theoretical analysis and experiments were presented to study the resonant wavelength's shift of different cladding modes with the change of external refractive index. The experiment showed that when the surrounding changed from air to ethanol absolute, resonant wavelength of LP04 mode shifted from 1577.6nm to 1570nm ; LP03 mode shifted from 1591.2nm to 1589.6nm ; While LP02 mode has been stable at 1454nm . The result showed that higher order cladding modes got larger shift than lower order modes.After that, experiments were designed to validate the above methods by reducing claddings radius and by choosing higher order modes, so as to increase LPFG's sensitivity to external surroundings. Experiment showed that when the cladding radius was 62.5μm, LP04 mode's shift was 1.6nm; LP03 mode was 1.6nm; LP02 mode didn't change. When the cladding radius was reduced to 45μm, LP04 mode's shift increased to 8nm; LP03 mode increased to 2.4nm; while LP02 mode still barely moved. Higher order cladding modes get better enhancement by reducing cladding radius.Finally, we introduced a high refractive index (HRI) layer into LPFG to understand the abilities of LPFG. Theoretical study and emulation showed that when HRI layer's thickness changed from 0.1μm to 0.5μm, the resonant wavelength shift of LP06 mode increased from 3nm to 9.5nm ; when HRI layer's refractive index changed from 1.5 to 1.52, shift of LP06 mode increased from 9.5nm to 30.6nm ; While if we kept HRI layer's refractive index and thickness unchanged and reduced the cladding radius from 62.5μm to 40μm, shift of LP06 mode increased from 9.5nm to 45.4nm. As a result, increasing the coating thickness and its refractive index, reducing cladding radius can facilitate the coupling between different modes, and increase the sensitivity to external environment.By the above researches, it can be concluded that through reducing cladding radius, setting higher surrounding refractive index changing point and choosing higher order coupling modes, we can enhance the sensitivity of LPFG to surrounding refractive index.