High-spectral-flatness mid-infrared supercontinuum generated from a Tm-doped fiber amplifier.

Broadband mid-infrared supercontinuum pulses were generated directly from a short piece of active fiber in a single-mode Tm-doped fiber amplifier. The broadband mid-infrared pulses have an extremely high spectral flatness with ~600 nm FWHM bandwidth (from 1.9 μm to 2.5 μm), >15 kW peak power, and >20 GW/cm(2) laser peak intensity. This new approach exhibits a significantly different physical mechanism from other supercontinuum generation demonstrations in the literature, in which usually a piece of passive fiber was used for nonlinear spectral broadening. The physical mechanism for the broadband mid-infrared supercontinuum generation in this approach has been attributed to a combined effect of two superradiative processes of Tm(3+) ions (i.e., the (3)F(4)-(3)H(6) transition covering the 1.8~2.1  μm spectral region and the (3)H(4)-(3)H(5) transition covering the 2.2~2.5  μm spectral region), and also nonlinear optical processes as well in the Tm-doped gain fiber. The spectra of the mid-infrared supercontinuum pulses were further broadened in a 2 m chalcogenide fiber with 20 dB bandwidth ~1100 nm and a 3 m fluoride fiber with 20 dB bandwidth ~2600 nm.