Single-pulse operation in actively Q-switch erbium-doped fibre lasers

Active Q-switching is a common technique to enforce a fibre laser (FL) to oscillate in the regime of short powerful pulses. It is known that if FL cavity is long enough and rise-time of a Q-switch cell (an acousto-optic modulator, AOM) is comparable or shorter than the photon round-trip time, a Q-switch pulse spreads into multiple sub-pulses (sometimes referred to as “self-mode-locking” operation), which significantly deteriorates the laser performance. To mitigate the multi-pulse structure of Q-switch pulses we propose here two special schemes permitting FL to operate in the regime of lone Q-switch pulses with duration less than AOM´s rise-time. In both experimental arrangements, AOM rise-time was fixed (50 ns) whereas the active fiber length was chosen such that spurious lasing (usually arising as the result of small internal reflections from a Q-switch cell) still does not occur, thus making the laser operating in a “pulse-on-demand” regime and in the meantime protecting the laser against turning into one of the nonlinear-dynamics regimes. In either scheme, FL cavity was formed by a pair of highly and lowly reflective fiber Bragg gratings (FBGs).