  Mode-locked lasers, which produce ultrashort pulses in the picosecond and
femtosecond range, have enabled some of the most precise measurements. However,
despite significant recent progress, resolving the temporal behavior of their
short pulses is still a challenge. State-of-the-art oscilloscopes with tens of
picosecond resolution prevent time-resolved observations in mode-locked lasers
and limit the real-time pulse evolution tracking of ultrafast lasers. Here,
using the time-lens technique with a Raman amplifier, we implement an ultrafast
single-shot time-microscope (TM) with a high temporal magnification factor of
355 and a time measurement window of 1 millisecond that contains ~1.8*10^4
consecutive pulses. We use this TM to characterize the temporal evolution of
mode-locked lasers and reveal a temporal sideway oscillation (winding)
behavior, a previously unobserved feature of lasers in both theory and
experiment. Our experimental observations confirm that the winding behavior is
an essential feature in the operation of mode-locked lasers. We theoretically
and experimentally found that the winding characteristic evolution originates
from gain-induced fluctuations for relatively high gain energies, while
Q-switched modulations being the main cause for lower energies. Our findings
based on advanced real-time measurements open up new insights into ultrafast
and transient optics and may impact future laser designs, modern ultrafast
diagnostics, and influence progress in nonlinear optics in general.
