Analogy of harmonic modelocked pulses to trapped Brownian particles improves laser performance

High-repetition-rate ultrafast lasers are needed for diverse applications. Harmonic modelocking, where multiple identical, equidistant pulses circulate in the cavity, reaches beyond practical limitations of reducing cavity length. However, it suffers from stochastic deviations that manifest as supermodes radio-frequency spectrum and difficulties maintaining same harmonic state, often coupled with trade-offs pulse energy, duration, or noise performance. Here, we first show temporal positions contribute disproportionately more to than their amplitudes. Then, argue these fluctuations analogous those trapped Brownian particles. This analogy reveals suppressed by stronger spectral filtering, which corresponds fluid viscosity, higher energy reduces noise, akin lower temperature. Guided this intuitive picture, construct a Yb-fibre laser incorporating strong filtering high intracavity energies limiting nonlinear polarisation evolution short section ordinary fibre. The rest all-fibre comprises polarisation-maintaining fibre, additionally improves environmental robustness. We report record-high supermode suppression ratios, reaching 80 dB, excellent long-term stability, characteristics similar fundamentally modelocked lasers.