In the research work of fiber Bragg grating(FBG) based lasers, it was found out that the lasing light shifted away from the Bragg wavelength of the FBG. The lasing wavelength can either be longer or shorter than the Bragg wavelength of the FBG. Further study on erbium- and ytterbium-doped fiber grating lasers with both ring and linear cavity has shown that, the phenomenon of wavelength shift was caused by the anisotropy of the laser cavity. The maximum wavelength shifting value was limited mostly by the bandwidth of the fiber grating. Through the online polarization controller, the lasing wavelength can be controlled within the bandwidth of the FBG.