Realization and optimization of phase-shifted distributed feedback fiber Bragg grating Raman lasers

Single-frequency laser sources have found a great number of applications, but are difficult to implement and suffer from poor robustness, poor quality (linewidth and stability), and are generally expensive to fabricate. One solution for a cheaper and simpler single-frequency source is a π-phase-shifted distributed feedback (DFB) fiber Bragg grating (FBG) based laser. Typically, such a laser usually uses a fiber with rare-earth dopants as an active medium for gain. However, its operating wavelength is limited to the emission bandwidth of the rare-earth dopant in the fiber. A proposed solution to overcome this limitation is to use Raman gain. Raman DFB fiber lasers have been successfully demonstrated, and a few simulations have been undertaken and reported. However, a thorough study of parameters and careful optimization has not been reported due to the long computation time and difficulty in the fabrication of long FBGs with known parameters. We demonstrate here, with the aid of a fast but exact method, a detailed optimization study on phase-shifted Raman DFB fiber lasers. These theoretical results are compared with the experimental operation of many fabricated FBGs thanks to a newly developed fabrication technique for the replication of FBGs. We show that fabricated lasers have poor performance compared to simulations of ideal lasers. We also show that the difference in performance is due to the high internal optical intensity induced nonlinear thermal gradient along the FBG.