Up-chirped nonlinear thulium fiber amplifier delivering sub-100 fs high-energetic pulses

Nonlinear fiber amplifiers allow for scaling the pulse energy, while the pulse properties can be tailored by the interplay of dispersion and Kerr-nonlinearity, aiming to significantly shorten the input pulses. Here, we describe a monolithic polarization-maintaining nonlinear fiber amplifier based on thulium, relying solely on single-mode step-index fused silica fibers. During the amplification of up-chirped pulses with an initial transform-limited pulse duration of 2.3 ps in an anomalous dispersive gain fiber, monotonic spectral broadening occurs and is subsequently extended in a normal dispersive fiber. The setup generates pulses with ultra-broadband spectra of up to 119 nm at pulse energies of up to 74 nJ. The pulses can be well compressed to sub-100 fs duration, equivalent to a compression factor of 24. Even though massive nonlinear spectral broadening is observed, the noise performance characterized by the relative intensity noise remains almost unaffected. Fundamentals for design and limitations of the output parameters are discussed by a numerical model.