Engineered π-Phase-Shifted Fiber Bragg Gratings for Efficient Distributed Feedback Raman Fiber Lasers

Distributed feedback Raman fiber lasers with pi-phase-shifted uniform gratings are modeled and simulated in the steady state, to optimize their performance. Using the parameters of realistic devices, it is found that the position change of the pi-phase-shift in a constant-strength uniform grating has a significant impact on the laser performance including right and left-hand-side output power and emission frequency, and linewidth. Also, it is shown that the optimum phase-shift position to maximize the laser uni-directionality is dependent on pump power and fiber loss value. A new design approach based on an engineered pi-phase-shifted step-like-strength uniform grating is presented demonstrating that even for a high-loss fiber (0.1 dB/m), the laser power and linewidth can be efficiently increased and decreased by similar to 25% and similar to 45%, respectively, for the same total device length and pumping condition, just by the proper choices of the phase-shift position and two coupling coefficients.