RESUMO
We report on the spectral-temporal characterization of a 1.8 µm wavelength and high power picosecond pulse Raman source. It is generated via frequency conversion to the first-order Stokes of a 27 ps chirped pulse Yb-doped fiber laser inside a molecular hydrogen-filled Kagome hollow-core photonic crystal fiber (HC-PCF). Depending on the average power and chirp of the pump laser, the average power of this Raman source can be as high as 9.3 W, and its pulse duration can be as short as â¼17 ps. In agreement with stimulated Raman scattering under the very high gain transient regime, the experimental results show the Stokes spectral structure to change following a three-stage sequence when the average pump power is increased. For a pump with a chirp corresponding to a bandwidth of 200 GHz, we found that for a pump power lower than 7 W, the Stokes spectrum is generated from the blue side of the pump spectrum, and then it exhibits a spectral replica of the pump spectrum for 7-14 W pump power range. Finally,the Stokes spectrum is chiefly generated from the red side of the pump spectrum when the pump power is further increased. Conversely, the Stokes pulse temporal profile shows a strong dependence with the pump power. For a low pump power range, the Stokes pulse exhibits a single peak with a full width at half-maximum of â¼17 ps. For higher pump powers, the Stokes pulse presents a double-peak structure with each peak having a duration of less than 15 ps. The present results can be used to develop compact and efficient frequency down-convertors to the increasingly widespread Yb-based picosecond lasers.
