Theoretical investigation of active fiber Bragg grating

Active uniform fiber Bragg grating (FBG) written in the Er-doped or Er:Yt-co-doped fiber is theoretically investigated. We found that when pumped, two symmetric maximums arise in the reflectivity spectrum of such an FBG near the edges of its bandwidth. By increasing the pumping rate these peaks grow, and at a critical pumping value they diverge, which indicates lasing onset at wavelengths that correspond to the peaks. By further increasing pumping, lasing ceases at the first wavelengths and begins at another pair of wavelengths. Thus, lasing wavelengths change through a set of discrete values depending on the pumping rate. At the same time, conventional negative phase slope of reflective function becomes positive in the regions around the lasing wavelength. Proposed active FBG can serve as a narrowband filter or multi-wavelength switchable laser in DWDM technique. Positive phase slope in such active FBG can be used in novel approaches to increase the performance of interferometric sensors.