High Efficiency Raman Soliton Generation in Passive Silica Fiber

Pulsed laser sources in the spectral window of 2 μm find many applications in engineering and science [1] . Thulium doped fiber lasers (TDFL) are especially attractive for this purpose with their emission in the 1.8-2.1 µm spectral range. The emission spectrum of a TDFL beyond 2.1 µm is also attainable with nonlinear approaches such as soliton self-frequency shift (SSFS) [2] , [3] . Previously, TDFL systems using SSFS have been demonstrated from thulium-doped fiber amplifiers (TDFA) pumped by tens of watts of continuous wave signal, leading to high-power femtosecond pulses (~100 kW, ~100 fs) [4] , [5] . However, such high-power systems are bulky and require external cooling mechanisms, as well as they lead to SSFS for a specific wavelength and pulse intensity. On the other hand, reports of TDFL with SSFS from passive fibers such as Ge-doped fibers require custom-made dispersion engineered fibers [6] . Here, we present a simple, all-fiber system that extends the wavelength reach of a TDFL from a passive, commercially available silica fiber. The system is tunable over 310 nm with an energy conversion efficiency up to 84.6%. To the best of our knowledge, this is the highest energy conversion efficiency ever reported in any SSFS system based on a passive fiber.