Studies On Characteristics Of Isotropic And Anisotropic Dynamic Gratings

Dynamic grating in Er-doped fiber has a significant application value in the field of optical fiber lasers and optical fiber information technology due to their characteristics of ultra-narrow bandwidth、real-time light-operation and self-adaption.And it hase drawn extensive attention.In tranditional applications,two mutually coherent laser waves of the same polarization are needed to form a standing wave,then isotropic dynamic grating of absorption is induced through the saturable absorption effect.And in practical applications dynamic grating of this type has negative effect on optical fiber devices and can lead to instability in optical fiber lasers because of its relative low grating efficiency and sensibility to polarization.In recent years,reports shows that dynamic gratings of gaining may have a higher grating efficiency.And a new effect has been found by our research group independently that anisotropic dynamic gratings can be induced by two orthogonally polarized waves in ordinary Er-doped fiber without birefringence utilizing Faraday rotating mirror,which deserves a deep-going research.This dissertation will do research on these two kinds of dynamic gratings.And a method called “Strong light and weak light alternation” is developed to measure the transient characteristics of dynamic grating.The main contents and results of the dissertation are as follows:1.The uniform four wave mixing nonlinear differential equaltion is deduced inEr-doped fiber to describe the reflectance spectrum of dynamic with or withoutpumping.Numerical analysis is done to the uniform nonlinear equaltion,and we findthat dynamic gratings of gaining and absorption can exist in Er-doped fibersimultaneously,and we measured this phenomenon.Experiments are conducted tostudy the intensity of writing light and pumping dependence of characteristics ofisotropic dynamic gatings.And the results shows that the new nonlinear differentialequaltion can describe the reflectance spectrum characteristics of dynamic gratings.2.Base on the light frequency modulation method,a new method called “strong light and weak light alternation” is proposed to measure the characteristic time of dynamic grating.An experiment was conducted to show that this method can measure the characteristic time effectively3.a contrast experiment was conducted between isotropic dynamic gratings and anistropic dynamic gratings.The result shows that anistropic dynamic grating resembles the istropic one in characteristic time、bandwidth、peak to valley ratio、reflectivity of central frequency of their reflectance spectrum under different writing light power,The result shows that the main difference between them is the peak to valley ratio.The max peak to vally ratio of anistropic dynamic grating reflectance spectrum reaches 2.5%,which is approximately half of that of the isotropic one.