Resolving the build-up of femtosecond mode-locking with single-shot spectroscopy at 90 MHz frame rate

Mode-locked lasers have enabled some of the most precise measurements ever performed, from attosecond time-domain spectroscopy to metrology with frequency combs. However, such extreme precision belies the complexity of the underlying mode-locking dynamics. This complexity is particularly evident in the emergence of the mode-locked state, an intrinsically singular, non-repetitive transition. Many details of mode-locking are well understood, yet conventional spectroscopy cannot resolve the nascent dynamics in passive mode-locking on their natural nanosecond timescale, the single pulse period. Here, we capture the pulse-resolved spectral evolution of a femtosecond pulse train from the initial fluctuations, recording similar to 900,000 consecutive periods. We directly observe critical phenomena on timescales from tens to thousands of roundtrips, including the birth of the broadband spectrum, accompanying wavelength shifts and transient interference dynamics described as auxiliary-pulse mode-locking. Enabled by the time-stretch transform, the results may impact laser design, ultrafast diagnostics and nonlinear optics.