Theoretical Investigating On The Bistable Characteristics Of Nonlinear Fiber Gratings

The nonlinear optical bistable switching has considerable potentials in optical gate operations, signal reshape and regeneration, optical time-division demulplexing, optical memory, all-optical wavelength convertion and so on. The related research results have showed that, optical bistability in nonlinear fiber gratings is limited by high switching-on power; The introduction of the phase shift is located at the center of the grating will reduce switching-on threshold 2 or further orders of magnitude. In practice, considering the cascading or integration of the grating with other photoelectric devices, the joint probably exists some reflectivity which certainly affects switching characteristics of the nonlinear fiber gratings. Also, the unstable states such as self-pulsation and chaos may emerge in the upper branch of the hysteresis under certain conditions due to the influence of nonlinearity. At present, many efforts have been made to improve the characteristics of the bistability of nonlinear fiber gratings. Based on the above considerations, in this paper, the bistable characteristics of nonlinear fiber gratings have been exploited as follows.Based on the coupled mode theory, by using the reversely recursive transmission matrix method, the switching characteristics of theλ/4 phase-shifted fiber grating and the influence of introducing chirp on the switching characteristics have been numerically studied when the reflection of the transmission facet is taken into consideration. The results show: for coherent superposition reflection strengthening, it can reduce the threshold switching energy of theλ/4 phase-shifted grating, but the switching contrast will decline; with the introduction of negative chirp inλ/4 phase-shifted grating, the switching contrast will be greatly improved, but the threshold switching energy will increase; with the introduction of positive chirp inλ/4 phase-shifted grating, the switching threshold can be further reduced, but the switching contrast will decline; and the hysteresis loop width will be influenced obviously by the introducing chirp.Starting from the coupled mode theory, by utilizing the modified time-domain transfer matrix method, the bistable characteristics of nonlinear fiber gratings can be analyzed numerically. The simulations demonstrate that the periodic self-pulsation may emerge extremely easily in nonlinear fiber gratings under the dynamic conditions. With the introduction of taper, the corresponding taper parameter can effect the frenquency of the self-pulsation. As the input power is increased gradually, the self-pulsation can be converted into chaos.The above results can provide an instructive insight to optimize the nonlinear fiber gratings bistable switching.