I. Introduction
FBG with different functions and applications in different situations will be penetrated into every corner of fiber communication and fiber sensing and play an irreplaceable role [1]. Due to its unique advantages, fiber Bragg grating sensor is widely used in modern industrial production and various fields in our daily life. Dynamic grating has important application potential in single frequency CW fiber laser, tunable narrow-band fiber filter, fiber sensor, adaptive interferometer and optical storage element [2]. The dynamic grating is usually formed by two coherent continuous laser waves with the same polarization propagating in the doped single-mode fiber [3]. The wavelength is written according to the absorption of rare earth ions [4].