A carbon nanotube-embedded fiber-optic tunable coupler for flexible repetition rate control of a passively Q -switched fiber laser

We propose a carbon nanotube (CNT)-embedded fiber-optic tunable coupler as a passive Q-switch that can provide flexible control of the output pulse repetition rate in passively Q-switched fiber lasers, and experimentally demonstrate its feasibility. The device is implemented by sandwiching a thin CNT layer between two side-polished fibers. Using the coupler, it was experimentally shown that repetition-rate-tunable Q-switched pulses can readily be produced from an erbium-doped fiber ring cavity by adjusting the cross-coupling ratio through control of the core alignment mismatch angle between the two side-polished fibers. Flexible tuning of the pulse repetition rate is shown to be achievable from 20.96 to 12.98 kHz at a fixed pump power by adjusting the core alignment mismatch angle while the pulse width is varied from 4.98 to 5.44 μs.