Visualizing the "Invisible" Soliton Pulsation in an Ultrafast Laser

Pulsation is a universal phenomenon that exists in diverse fields. For nonlinear optics, the soliton pulsating behavior can be meaningful for fundamental physics and industrial purpose owing to its fruitful nonlinear dynamics and the possible detrimental effect of instability (or even route to chaos) during pulsating process. Herein, a novel type of soliton pulsation in an ultrafast laser is unveiled. The pulsating behavior features that the soliton experiences periodic peak power variation but with almost invariable pulse energy. This phenomenon is denoted as "invisible soliton pulsation" when referring to the routine diagnostic methods. However, the invisible soliton pulsation can be distinguished by recording the shot-to-shot spectra with real-time spectroscopy technique. It is found that the appearance of the invisible soliton pulsation is sensitive to the pump power level. Moreover, the phenomenon of invisible soliton pulsation is further revealed by numerical simulations. These findings can shed new insights into the complex nonlinear behavior of solitons in dissipative optical systems, and also enrich the performance diagnostic of ultrafast lasers for practical applications.