All-fiber ring cavity Q-switched laser in the 2.8 µm band

We demonstrate the first, to the best of our knowledge, all-fiber ring-cavity Q-switched laser operating in the mid-infrared. The laser emits in the 2.8 μm band via a single-mode ZBLAN optical fiber coupler, an Er:ZBLAN gain fiber, and a polarization controller. The all-fiber components presented here are connectorized, enabling flexible design and easy insertion/removal while maintaining the advantages of all-fiber lasers. Single-walled carbon nanotubes trigger Q-switching as the saturable absorber. The optical fiber coupler provides different feedback paths, allowing the cavity to operate in two distinct regimes: one supporting low-threshold, short-pulse operation, and the other maximizing output power. Q-switching occurs at a pump power threshold of 0.30 W, producing pulses with a minimum duration of 0.77 μs, and a repetition rate of 71.8 kHz. The corresponding RF spectrum shows a signal-to-noise ratio of 52 dB, confirming stable pulsed operation.