Single-tunable and dual-wavelength operation of a passive Q-switch double-Clad Er-Yb fiber laser based on the use of a titanium-oxynitride-coated fiber ball lens

We demonstrate generation of passive Q-switched laser pulses of a linear cavity Er-Yb double-clad fiber laser based on the use of a fiber ball lens coated with a thin film of titanium oxynitride (TiOxNy). The fiber ball lens is inserted within the laser cavity in a reflection configuration, alongside a reflecting mirror. Dual-wavelength and tunable single operations of the laser is obtained with the ball lens acting as an interference filter. At the same time, the ball lens coated with TiOxNy, deposited by DC reactive magnetron sputtering, allows saturable absorption suitable for generation of passive Q-switched laser pulses. Single wavelength laser generation tuned in a range of ~4.5 nm and simultaneous dual wavelength generation with separation of 5.1 nm are obtained at the 1.55 µm wavelength region. Pasive Q-switched pulses with maximum repetition rate of 124 kHz, minimum duration of 3.6 µs and peak power of 360 mW are achieved. The maximum pulse energy estimated is of 1.3 µJ.