Sub-60 fs Pulse Generation and Complex Dynamics in a GHz-Fundamental-Repetition-Rate Linear-Cavity Yb:Fibre Laser

We present a Yb-doped mode-locked fibre laser (MLFL) with a linear cavity structure designed to generate sub-100 femtosecond (fs) pulses at a high repetition rate. The laser relies on a semiconductor saturable absorber mirror (SESAM) for self-starting mode-locking and integrates a multifunctional gain module. Operating at a fundamental repetition rate of 1.079 gigahertz (GHz), the laser produces near-transform-limited 58-fs pulses with 16 mW of output power. Numerical simulations corroborate the experimental findings, providing insights into intracavity pulse-shaping mechanisms. Significantly, spectral compression is observed during intracavity pulse propagation, which is accompanied by a complex temporal reshaping process. Notably, Gaussian pulses evolve into a parabolic profile through nonlinear dynamics, followed by asymmetric temporal stretching. Subsequent dispersion compensation yields the compression of the parabolic pulses into sech2profiles, whereas asymmetric pulses are reshaped into Gaussian profiles. To our knowledge, this interplay of spectral compression and multistage temporal evolution, which spans Gaussian, parabolic, and sech2regimes, is the first demonstration in MLFLs. Furthermore, the sub-60 fs pulse duration represents the smallest value reported for GHz-repetition-rate MLFLs with a linear cavity. This compact GHz fs-scale laser offers a robust platform for advanced photonic applications.