Research on multi-soliton spectral-temporal dynamics in mode-locked fiber lasers

Abstract

Solitons are localized structures in nonlinear systems, appearing in diverse fields ranging from fluid dynamics to plasma physics to nonlinear optics. In optics, mode-locked fiber laser is an ideal platform for soliton generation. Mode-locked fiber laser has advantages of small size, high energy and extensive wavelength range, and it has been widely applied in various fields in last few decades, including biological imaging, optical communication and optical spectroscopy. On the other hand, as a dissipative nonlinear system, it also serves as a good testbed for soliton dynamics study. Many dynamic phenomena have been explored on this platform, such as soliton explosion, pulsating soliton and soliton rain. It should be noted that owing to the high nonlinearity caused by the small fiber core size, mode-locked fiber lasers can easily run into multi-soliton regime. The soliton interactions in this regime can lead to more complicated multi-soliton dynamic behaviors compared with dynamics in single-soliton regime. The investigations on multi-soliton dynamics can not only shed new insights into the complex dissipative soliton dynamics, but also benefit the design and optimization of the ultrafast mode-locked lasers for practical applications. In this thesis, the multi-soliton spectral-temporal dynamics in mode-locked fiber lasers are explored, and schemes for multi-soliton pattern design are also demonstrated. Benefiting from the powerful roundtrip-resolved dispersive Fourier transform (DFT) technique and the well-developed photodetector (PD) and real-time oscilloscope, spectral and temporal evolutions of several multi-soliton dynamics have been studied. Firstly, the build-up of multi-soliton mode-locking is investigated. The whole build-up process of individual soliton is observed, including intensity fluctuation, spectral broadening and spectral beating. In addition, ultraweak soliton interaction between two bounded solitons during their build-up processes is also observed. Secondly, the soliton explosions in multi-soliton regime is studied. It is found that the explosion of one soliton can be induced by that of another soliton through the soliton interaction mediated by transient gain response of gain fiber, which is denoted as “mutually-ignited soliton explosions”. Interestingly, two different kinds of explosive behaviors are observed in the same cavity simultaneously. Finally, a special multi-soliton state in a net-normal-dispersion bidirectional mode-locked fiber laser is observed. In this laser, the dissipative solitons propagate in opposite directions in the cavity and collide in every roundtrip. It is revealed that the dissipative solitons from different directions have similar temporal and spectral behaviors in both stable mode-locking state and dynamic states. Based on the complex interactions in the multi-soliton mode-locked fiber lasers, two multi-soliton manipulation schemes are proposed. Firstly, by controlling nonsoliton lightwave in the laser cavity either through increasing the pump power or injecting the amplified spontaneous emission (ASE) signal, the temporal separation between dissipative solitons can be tuned in a range of up to 1.5 ns. Secondly, in the net-normal-dispersion bidirectional mode-locked fiber laser based on carbon nanotube (CNT), dissipative solitons always appear in pairs and collide in the CNT. Therefore, flexible and precise multi-soliton manipulation can be achieved by finely designing the positions of the CNTs in the laser cavity.