Directly gain control of loosely bound pulsating soliton pairs in a mode-locked fiber laser.

The formation of multi-pulses with analogous characteristics in fiber lasers is due to the soliton energy quantization. The self-assembly behaviors of pulses akin to particles imply the potential for synchronized manipulation. However, for multi-pulses with separations on a large scale, also known as loosely bound soliton pairs (LBSP), it remains challenging to control muti-pulses precisely and reversibly. Here, we introduce a direct method to control LBSP through electronic modulation of the pump source. By adjusting the gain level, we demonstrate that quasi-steady-state LBSPs, composed of varying soliton numbers, can achieve continuous transitions between defined states and exhibit strong robustness. Real-time spectral measurements disclose the asynchronous pulsations of LBSPs under steady-state gain conditions and the quasi-periodical transition processes during transient modulation. Numerical simulations well reproduce the experimental observations. These findings highlight the controllability of multi-soliton states and the predominant role of gain competition in manipulation processes, offering what we believe to be a novel avenue for the tailored and dynamic manipulation of LBSP patterns.