Sensing Characteristic Study Of Long Period Fiber Gratings In Single Core And Two-core Hollow Eccentric Fibers

The long period fiber gratings (LPGs) have rapidly attracted more and more attentions of researchers and specialist for their unique characteristics. In this thesis, the LPGs based on the single core hollow eccentric fiber (SCHF) and two-core hollow eccentric fiber (TCHF) have been investigated. The parameters of high-frequency CO2 laser have been discussed for obtaining higher quality gratings. Furthermore, the sensing characteristics of temperature,axial strain, and bending have been tested, and the coupling characteristics of the SCHF-LPGs and TCHF-LPGs have been also simulated and analyzed, respectively.Firstly, we fabricated the LPGs in the SCHF by using the grating fabrication system which is composed of the high-frequency CO2 laser,fiber supporters and computer. Secondly,the modes' field distributions in the SCHF-LPGs have been simulated and analyzed to research its coupling characteristics by combining the coupled mode theory (CMT) with finite element method (FEM). The results of numerical calculation indicate that the resonant wavelength of the SCHF-LPGs is from the coupling between cladding mode of LP91 and the core mode of LP01 at the wavelength of 908nm, which agrees well with the experimental data.Finally, the characteristics for temperature, bending, axial strain and polarization have been tested. The sensitivities of SCHF-LPGs to temperature, bending, and axial strain are 55 pm/?,1.02 nm/m-1 and 0.3pm/??, respectively. The results show that the SCHF-LPGs can distinguish the specific bending directions. The resonant peak shifts linearly with the increasing curvatures but its sensitivity is lower than that of the LPGs in single mode fiber(SMF). Furthermore, the effects of exposure directions on the bending features have been also investigated. The exposure direction has a great influence on bending characteristics because the core of the SCHF deviate geometrical center of the fiber.Analogously, for the TCHF-LPGs, we have studied the influences of grating period and exposure direction on the resonance peaks and also researched the coupling and sensing characteristics of the TCHF-LPGs. The resonant peak is mainly caused by the coupling between the core mode LP01 and cladding mode LP81 at the wavelength of 860nm. The experimental results agree well with the simulated results. Compared with the SMF-LPGs, the experimental results indicate that the sensitivity of the TCHF-LPGs to bending curvature is low and even very small at some bending directions (0.03nm/m-1 at the bending direction of 90°). In addition, TCHF-LPGs are insensitive to the axial strain (0.09 pm/??) while sensitive to the temperature (55 pm/0C). Therefore, the proposed TCHF-LPGs can efficiently sense the changing temperature that is independent of the strain.Compared with the SCHF, the sensitivities of TCHF-LPGs to bending and strain are much lower than that of SCHF-LPGs,which highlights the advantage of temperature sensing.On the other hand, the TCHF possesses two cores which are separated by the big air hole and independent of each other, so the TCHF-LPGs can offer more advantages over SCHF-LPGs in enlarging the signal capacity of fiber and application to the two-channel filters without signal crosstalk.