Liu, XuemingPopa, DanielAkhmediev, Nail2020-06-032020-06-030031-9007http://hdl.handle.net/1885/204789Q switching (QS) and mode locking (ML) are the two main techniques enabling generation of ultrashort pulses. Here, we report the first observation of pulse evolution and dynamics in the QS-ML transition stage, where the ML soliton formation evolves from the QS pulses instead of relaxation oscillations (or quasicontinuous-wave oscillations) reported in previous studies. We discover a new way of soliton buildup in an ultrafast laser, passing through four stages: initial spontaneous noise, QS, beating dynamics, and ML. We reveal that multiple subnanosecond pulses coexist within the laser cavity during the QS, with one dominant pulse transforming into a soliton when reaching the ML stage. We propose a theoretical model to simulate the spectrotemporal beating dynamics (a critical process of QS-ML transition) and the Kelly sidebands of the as-formed solitons. Numerical results show that beating dynamics is induced by the interference between a dominant pulse and multiple subordinate pulses with varying temporal delays, in agreement with experimental observations. Our results allow a better understanding of soliton formation in ultrafast lasers, which have widespread applications in science and technology.This Letter was partially supported by the National Natural Science Foundation of China under Grants No. 61525505 and No. 11774310.application/pdfen-AU© 2019 American Physical SocietyRevealing the Transition Dynamics from Q Switching to Mode Locking in a Soliton Laser201910.1103/PhysRevLett.123.0939012019-12-19