Study On Sensing Characteristics And Applications Of Long Period Fiber Gratings

Fiber grating is one of the most important optical devices in domain of optical communication, sensing and optical information processing. This paper just focus on long period fiber grating, its radical characteristics and sensing application are studied. The following is the main content: A brief introduction of fiber grating and LPFG sensor is presented. According coupling-mode theory, coupling equations is deduced, which gives the relation between guided core mode and cladding mode. Then the effective index and coupling constant are calculated. Considering the physics effect that can be resulted from strain or temperature which affected on long-period fiber grating, such as elastic distortion, strain-optic effect, thermal expansion, thermo-optic effect et al., the general formula of strain and temperature sensitivities of it are obtained, and the relation of sensitivities with the period of LPFG, the order of mode, elastic-optic coefficient and thermo-optic coefficient are simulated. In additional, a novel design of biosensor based on long period fiber grating that can be used in detection or analysis of biological substance is proposed, the relation of resonance wavelength with the refractive index and thickness of the compound thin film are simulated by using a computer.Based on these works, the paper draws some conclusion: increasing the period of LPFG suitably or utilizing the lower order mode's coupling both have an advantage for strain and temperature sensitivities. When elastic-optic coefficient (or thermo-optic coefficient) of the cladding and the core are equal, the resonance wavelength has a little displacement and in proportion to central wavelength. By utilizing high efficient and special catalytic characteristic of the bioactive substances-where the cladding index and radium of the gratings is directly modulated, the detection or analysis of biological substance such as protein, enzyme et al can be carried out.