Low-threshold broadband-tunable cascaded Raman fiber laser at 2.3 μm based on highly GeO-doped silica and AsS fibers.

We demonstrate a low-threshold, broadband-tunable cascaded Raman fiber pulsed laser utilizing highly GeO-doped silica and AsS fibers as Raman gain media. As a result of the high Raman spectral purity of the first-order Raman laser and the high Fresnel reflection coefficient of the AsS fiber resonant cavity, a second-order nanosecond pulsed Raman laser with a center wavelength of 2335 nm is achieved, with a peak threshold power of 95 W with a 1970nm nanosecond pump source and corresponding to 8.4 W with a 2150 nm first-order Raman laser as the pump source. The maximum single-pulse energy and spectral purity of the second-order Raman laser are measured at 5.1 μJ and 47.2%, respectively. Furthermore, through precise tuning of the central wavelength in the gain-modulated pulsed source, the second-order Raman laser achieved a wavelength tuning range spanning 74 nm, from 2303 nm to 2377 nm. To the best of our knowledge, this achievement establishes the broadest wavelength tuning range reported to date for cascaded Raman fiber lasers operating in the 2.3 μm spectral region.