# Nonlinear ultrafast fiber amplifiers beyond the gain-narrowing limit

## TL;DR

This paper introduces a novel fiber amplification regime that leverages a dynamically evolving gain spectrum to achieve ultrashort pulses with spectral broadening beyond the gain bandwidth, enabling high-energy, sub-50-fs pulses.

## Contribution

It demonstrates a new nonlinear amplification regime using a dynamic gain spectrum, supported by theory and experiments, to produce ultrashort, broad-spectrum pulses beyond traditional limits.

## Key findings

- Spectral broadening by nearly two orders of magnitude
- Pulses remain compressible to sub-50-fs durations
- Evidence of a nonlinear attractor governing pulse dynamics

## Abstract

Ultrafast lasers are becoming increasingly widespread in science and industry alike. Fiber-based ultrafast laser sources are especially attractive because of their compactness, alignment-free setups, and potentially low costs. However, confining short pulses within a fiber core leads to high intensities, which drive a host of nonlinear effects. While these phenomena and their interactions greatly complicate the design of such systems, they can also provide opportunities for engineering new capabilities. Here, we report a new fiber amplification regime distinguished by the use of a dynamically-evolving gain spectrum as a degree of freedom: as a pulse experiences nonlinear spectral broadening, absorption and amplification actively reshape both the pulse and the gain spectrum itself. The dynamic co-evolution of the field and excited-state populations supports pulses that can broaden spectrally by almost two orders of magnitude and well beyond the gain bandwidth, while remaining cleanly-compressible to their sub-50-fs transform limit. Theory and experiments provide evidence that a nonlinear attractor underlies the management of the nonlinearity by the gain. Further research into the mutual, pulse-inversion propagation dynamics may address open scientific questions and pave the way toward simple, compact fiber sources that produce high-energy, sub-30-fs pulses.

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Source: https://tomesphere.com/paper/1906.04141