Study on the spectral gain distribution and pulse self-compression in thulium-doped fiber Mamyshev oscillators

Abstract

We report on our recent results of the numeric evaluation of the gain evolution inside a thulium-doped fiber Mamyshev oscillator in the spectral 2 μm region, comparing the core pumping scheme with the double-clad pumping scheme in order to further optimize the output parameters in the experimental setup. By comparing the two pumping schemes, we find spectral gain channeling around 1950 nm for the double-clad pumping scheme owing to a three orders of magnitude lower pump intensity and an order of magnitude higher doping concentration. The found gain distribution is highly effective to suppress amplified spontaneous emission at the maximum emission cross section of thulium ions in silica glass and thus enables operation beyond the water absorption lines. Furthermore based on the gain evolution model, also a novel broadening mechanism inside the gain fiber of Mamyshev oscillators is numerically evaluated. The pulse evolution is determined by the interplay between the anomalous dispersion and self-phase modulation inside the gain fiber and allows to self-compress the pulse, while simultaneously monotonic spectral broadening arises during the amplification. This novel pulse evolution inside a Mamyshev oscillator shows nearly transform limited high peak power pulses with a pulse duration below 100 fs directly at the output without any additional compression stage.