Intermediate state between a solitary singlet and a molecule in lasers

As an important tool in science and industry, the ultrafast fiber laser has two paradigmatic pulse regimes of the single-soliton (SS) and the soliton molecule (SM), which have been individually researched thoroughly. However, the mutual transformation between two states is a fuzzy problem with significance in understanding the soliton dynamics and actively manipulating lasing states. Here, we reveal an intermediate state that spontaneously switches between SS and SM without external triggering in experiments and simulations. Two critical processes dominating the switching are the soliton splitting and SM partial annihilation, which are found to be induced by the high-order soliton effect and spontaneous symmetry breaking, respectively. Moreover, statistics of this intermediate state reveal the spectral rogue waves induced by the SM with high spectral brightness. This work sheds light on the switching dynamics between nonlinear states in the out-of-equilibrium systems, and has potential for intelligent manipulation of lasing states.