Limitations of Soliton Fiber Lasers

Soliton fiber lasers typically generate pulse durations in the range of several picoseconds, with soliton pulse energies at the picojoule level. For shorter pulse durations (corresponding to higher soliton pulse energies), the soliton period decreases rapidly. In addition, pulse formation becomes unstable due to the excessively large nonlinear phase shift accumulated during each cavity round trip.

When operating near this critical regime, the pulse spectrum exhibits pronounced Kelly sidebands, as shown in the figure below. For pulse durations significantly shorter than 1 ps, this effect becomes increasingly difficult to control, even when strong saturable absorbers are employed.

In principle, large-mode-area (LMA) fibers can be used to reduce nonlinear effects. However, this approach mainly increases the pulse energy rather than shortening the pulse duration, because the fundamental dependence of the soliton period on pulse width remains unchanged. Furthermore, the use of specialty fibers limits the availability of compatible fiber components, increasing the difficulty of developing an all-fiber laser system.

Therefore, achieving shorter pulse durations and higher pulse energies generally requires the adoption of more advanced and sophisticated mode-locking techniques.