Three-stage dynamics of nonlinear pulse amplification in an ultrafast mid-infrared fiber amplifier with anomalous dispersion.

Nonlinear pulse amplification in optical fiber, with the capability of breaking the gain-bandwidth limitation, is a key technique for high-energy, ultrafast pulse generation. In the longer wavelength region (including 1.55 μm, 2 μm and 2.8 μm) where the gain fiber has normally strong anomalous dispersion, the nonlinear amplification process over fiber exhibits more complicated dynamics than that of its 1-μm counterpart, and the underlying mechanism of the nonlinear pulse propagation process in high-gain anomalous fiber is still elusive so far. Here, we demonstrate an in-depth study on the nonlinear amplification process in high-gain ultrafast mid-infrared fiber, providing a clear physical understanding on the debate of adiabatic soliton compression. We unveil that under the high-gain condition, the ultrafast pulse launched into the anomalous gain fiber experiences successively three distinct stages, named as the balance between linear and nonlinear chirp, high-order-soliton-like pulse compression and soliton fission due to high-order effects. While a relatively clean ultrafast pulse can be obtained immediately after the high-order-soliton-like compression stage, excessive gain fiber length could hardly enhance further the pulse peak power due to soliton fission. Our findings can provide several critical guidelines for designing high-power ultrafast fiber amplifiers at near- and mid-infrared wavelengths.