  Mode-locked lasers exhibit complex nonlinear dynamics. Precise observation of
these dynamics will aid in understanding of the underlying physics and provide
new insights for laser design and applications. The starting dynamics, from
initial noise fluctuations to the mode-locking regime, have previously been
observed directly by time-stretched transform-based real-time spectroscopy.
However, the regime transition dynamics, which are essential processes in
mode-locked lasers, have not yet been resolved because regime transition
process tracking is very challenging. Here we demonstrate the first insight
into the regime transition dynamics enabled by our design of a real-time
programmable mode-locked fibre laser, in which different operating regimes can
be achieved and switched automatically. The regime transition dynamics among
initial noise fluctuations, Q-switching, fundamental mode-locking and harmonic
mode-locking regimes have been observed and thoroughly analysed by both
temporal and spectral means. These findings will enrich our understanding of
the complex dynamics inside mode-locked lasers.
