The Research Of Sensor And All Optical Switching Based On Long Period Fiber Grating

Long period grating fiber (LPFG) is a new type of optical passive device which is developing rapidly in recent years. It promotes coupling between the propagating core mode and co-propagating cladding modes. LPFG has been studied by many researchers all around the world for its unique advantages, such as its easiness to be fabricated, low insert loss, no back reflection, sensitive to the changes of environments, small size, and the capability to be easily integrated or connected with other optical communication devices. It has many important applications both in fiber communications areas and fiber sensing systems. In this paper, based on the couple mode theory of LPFG, the performance of sensors and optical switching based on LPFG were analyzed and investigated theoretically. The main research contents are as follows.Firstly, from the analysis of the dispersion relation and mode fields of the core mode and cladding modes in a single mode fiber, the equations about bar and cross transmission, resonance wavelength and bandwidth are derived combined with the coupled mode theory, and the spectral characteristics of uniform LPFG are also discussed. Then, based on Maxwell's equations, the nonlinear coupled mode equations about LPFG are derived. All these researches offer a theoretical platform for the research of LPFG.Secondly, the sensitivities of LPFG to temperature, axial stress and refractive index are investigated theoretically. Analysis shows that the sensitivities of temperature, axial stress and refractive index about resonance wavelength are dependent upon the composition of the fiber, the period of grating and the order of the cladding mode. And then the recent researches on the applications of LPFG sensors are reviewed, including temperature sensing, stress sensing, refractive index sensing, bending sensing, torsion sensing and current sensing.Finally, by using the standard split-step Fourier techniques and the fourth-order Runge–Kutta method in time, the two coupled nonlinear Schr?dinger equations are resolved numerically. The switching rate of LPFG in two typical cases: on-resonance and off-resonance is analyzed. It is found that in the both cases, the on-off ratio of all-optical switching in long period fiber gratings can be improved remarkably by using the higher order super-Gaussian pulse, and the on-off ratio of the switching increases as the order of the super-Gaussian pulse increases.