Microfiber coated with gold nanorods as saturable absorbers for 2 μm femtosecond fiber lasers

We demonstrate an all-fiber thulium-doped femtosecond laser by using microfiber coated with gold nanorods (MF-GNRs) as a saturable absorber (SA). The MF-GNR SA exhibits a broadband absorption from 1000 nm to 3000 nm. By placing the MF-GNR SA into a thulium-doped fiber laser (TDFL) cavity, a stable passively mode-locked laser with a central wavelength of ~1943.5 nm is achieved for a threshold pump power of ~870 mW. Furthermore, a femtosecond laser with a pulse width of ~404 fs, a maximum average power of ~45.5 mW, and a repetition rate of ~25.66 MHz is obtained for a pump power of 2.1 W. The maximum average power of the laser is increased by ten times and the pulse width is reduced from several picoseconds to 404 fs compared to previously reported 2 μm modelocked lasers based on GNR SA because of the weak photothermal effect and high laser damage threshold of MF-GNRs SA. To the best of our knowledge, this is the first time allfiber thulium-doped femtosecond lasers based on MF-GNR SAs have been reported. These results show that MF-GNRs are promising SAs for constructing 2 μm femtosecond fiber lasers.