Passively Q-switched thulium fluoride fiber laser operating in S-band region using N-doped graphene saturable absorber

A passively Q-switched S-band fiber laser is proposed and demonstrated using a thulium-doped fluoride fiber as laser gain medium. The proposed design possesses significant advantages including broader tuning wavelength range as compared to previously reported Q-switched S-band lasers. The laser cavity is configured in a simple forward-pumped ring design with a 1400 nm single upconversion pumping scheme. Q-switching is achieved using a polyvinyl alcohol/nitrogen-doped graphene nanosheet (PVA–N-Gns)-based thin-film saturable absorber. Stable Q-switched pulses are obtained at a center wavelength of 1504.7 nm over a pump power range from 101.8 to 280.6 mW. The generated pulses have a maximum pulse repetition rate of 36.85 kHz and minimum pulse width of 4.8 μs at the pump power of 280.6 mW. The maximum pulse output power of 1.30 mW indicates a laser efficiency of approximately 0.63% while a signal-to-noise ratio of 57 dB shows pulse stability. The ability of the proposed cavity design to produce broad wavelength tunability range from 1450 to 1510 nm make it highly suitable for various application in the S and S + band regions.