Realizing a Watt-Level 1.72 µm Femtosecond Fiber Laser by a Single Frequency Laser Seeded Stimulated Raman Amplification

In this work, we propose a method of efficiently generating a watt-level 1.72 µm ultrafast fiber laser by using a Raman amplifier seeded by a single frequency continuous wave (CW) laser. The Raman amplifier is pumped by an ultrafast laser with a designed operating wavelength at 1.605 µm. The 1.72 µm Raman wavelength and the 1.605 µm pump wavelength can match the maximum Raman gain at a ∼440 cm⁻¹ Raman shift of the nonlinear fiber we choose. Benefiting from ultrafast response time of stimulated Raman scattering (SRS), the nonlinear fiber pumped by the ultrafast laser has a periodic nonlinear gain modulation, which is able to transform the single-frequency CW laser in the Raman amplifier to a pulsed laser. We construct a high power 1.605 µm pump system including a chirped Soliton-self-Frequency-Shift (SSFS) seed and a large-mode-field (LMF) amplification system. The LMF amplification system can amplify this specially-designed pump seed to several watts and meanwhile maintain a high suppression to C-band spontaneous emission. The high power 1.605 µm pump guarantees enough pump power to get a watt-level 1.72 µm ultrafast Raman laser. The home-made single frequency CW laser is achieved by taking a classical method of inserting a piece of un-pumped Tm-doped fiber to form a narrow dynamic induced grating. When the 1.605 µm pump source and the single-frequency CW laser are injected together into the Raman amplifier, we obtain a ∼9.175 ps 1.72 µm ultrafast Raman laser with a maximum power of ∼1.1 W. The highest conversion efficiency is close to 70%. This picosecond (ps) 1.72 µm laser is further compressed to ∼561 fs. In comparison with conventional Tm chirped pulse amplification (CPA) systems, we believe our work provides a more efficient way to generate a high power (>1 W) 1.7 µm ∼ 1.8 µm femtosecond (fs) laser.