Nanosecond-Pulsed Hybrid Thulium-Raman Fiber Amplifier at 2.1 $\mu\mathrm{m}$

Laser sources operating around 2.1 μm are attractive for a range of applications in areas such as remote sensing, free-space optical communication, defense and medicine. Most commonly, holmium-doped fibers pumped by thulium-doped fiber lasers at 1.95 μm are used to reach this spectral regime, but several studies have shown that holmium-doped fiber amplifiers have a much lower efficiency than theoretically possible [1]. Coincidentally, the wavelength 2.1μm lies close to the peak Raman shift (~ 13 THz in silica) of the 1.95μm pump. This means that amplifiers can be developed where 2.1 μm light (called Stokes) could be amplified through stimulated Raman scattering (SRS) in a passive fiber instead of through stimulated emission in a Ho-doped fiber.