Dual-wavelength, passively Q-switched thulium-doped fiber laser with N-doped graphene saturable absorber

Demonstrations of dual-wavelength Q-switched fiber lasers in the wavelength regions of 1.0 μm and 1.55 μm have been widely reported but not in the longer wavelength of 2.0 μm region. In this paper, a dual-wavelength Q-switched thulium-doped fiber laser is proposed and demonstrated using an N-doped graphene film as the saturable absorber, while the dual-wavelength mechanism is achieved with the use of a tunable Mach-Zehnder filter. The dual-wavelength Q-switched laser produced consistent outputs at 1922.0 nm and 1961.0 nm with a wavelength spacing of 39.0 nm. The repetition rate of the pulses obtained covers a range from 17.68 kHz to 35.21 kHz corresponding to an increasing pump power of 112.55 mW until 147.78 mW. The narrowest pulse width obtained is 4.4 μs and a pulse energy as high as 25.56 nJ is generated. To the best of our knowledge, this is the first demonstration of a dual‐wavelength Q-switched thulium-doped fiber laser operating in the 2.0 μm wavelength region using a fiber ring cavity design.