Generation of bright-dark pulses in a Q-switched random fiber laser based on enhanced distributed backscattering

In this paper, a passively Q-switched random fiber laser (RFL) with a short linear open cavity based on enhanced distributed feedback in a fiber ring is proposed. The theoretical analysis of the fiber ring shows that the power accumulation happens at the phase matching condition and the maximum enhancement increases with the decreasing of coupling coefficient k and insert loss γ . As a result, the enhanced distributed feedback in the fiber ring promotes the generation of the Q-switching process in RFL. Under the combined action of enhanced distributed backscattering in the fiber ring and broadband reflection in the fiber loop mirror (FLM), bright-dark pulses with tunable frequency and pulse width are generated, and the pulse shape of bright-dark pulse pairs is controllable by inserting a length of SMF in the RFL. Moreover, when a random phase shift Bragg grating (RPS-FBG) or fiber Bragg grating (FBG) is used to replace the FLM, the dark pulses disappear, which indicates that the appearance of dark pulses is the result of the combined action of broadband reflection of FLM and enhanced distributed backscattering of the fiber ring.