Retrieving Internal Phase Dynamics of Soliton Molecules in the Normal Dispersion Regime

The current phase retrieval methods are challenging to retrieve the internal phase of soliton molecules (SMs) in the normal-dispersion regime of mode-locked lasers because the interference-like spectrum of such SMs is extremely dense. Here, we first retrieve phase evolutions of such SMs, by virtue of a novel phase retrieval method (Nat. Commun. 10, 830 (2019)). Three types of phase evolutions of SMs are revealed. Our work paves the way towards revealing various SM dynamics in ultrafast fibe lasers.