## 7 Conclusion and Outlook  
In this review, we summarize the latest experimental researches on soliton molecules and other multisoliton states in mode-locked fibre lasers. As a promising equilibrium phase of multiple pulses potentially significant to developing high-capacity telecommunication, soliton molecules are stationary pulse pairs stemming from balanced forces by dissipation nature of fibre lasers, forming bound states with invariant interpulse temporal separation as well as discretely fixed phase relationship. Independent of cavity details for example working wavelength, dispersion scheme and mode-locking technique, soliton molecules exist in general as an intrinsic allowed status in fibre lasers. However, sensitive to cavity parameters such as saturation gain, initial condition, etc., soliton molecules can manifest themselves with distinct pulse separation, which can be one to two orders of magnitude as large as pulse width depending on the interaction length of the dominating effects. The phase relationship between solitons mainly locates among in-phase, out-of-phase and \(\pm\pi/2\) phase difference, since second-order dispersion leads dispersive terms. Numerical efforts based on modified nonlinear  
Figure 27: Experimental and numerical observation of the coexisting dissipative solitons. **(a)** Measured fast switch of the spectrum of the coexisting dissipative solitons obtained by selecting spectral slice using a monochromator and recording the power trace; **(b)** Calculated spectral bandwidth of the spectrum; **(c)** Two typical spectra, represented by the dashed line in **(a)** at 20 \(\upmu\)s and 42 \(\upmu\)s of the recorded spectrum, showing a change over 1 THz in the centre frequency; **(d)** Numerical simulated switching of the spectrum of the mode-locked state; **(e)** The corresponding temporal evolution of the simulation result. Associated with the spectral switching, the pulse is advanced and slowed due to the centre-frequency shift induced group-velocity change via dispersion. (Reproduced with permission from [112]. © The American Physical Society (College Park, MD, USA), 2015). Schrodinger equation and lumped description method have been made to unveil key roles of historical status, gain and dispersion in formation of bound solitons and their characteristic parameters. Experimental control for efficient generation of soliton molecules and their stabilities are discussed in detail. The complicated interaction mechanisms are additionally addressed. In comparison to the stationary boundary states which are one of the simplest existence states, the complex dissipation dynamics in fibre lasers can lead to many other kinds of multisoliton states and dissipative phenomena including vibrating soliton pairs, soliton rains, rogue waves, bistable coexisting states and so on. Such soliton complexes involving diverse nonlinear behaviours are not only of fundamental interest but also of practical importance, for instance, in applications related to chaos, optical storage and optical switching. The versatility of fibre laser cavities has enriched physical understanding of multipulse interaction mechanisms and realistic findings on generation and control of multiple solitons in such dissipative systems. However, the numerous experimental variables and the limited characterization measures hamper a general description of all discoveries in experiment to date, making a complete dynamics analysis very challenging. Especially when we study such nonlinear optical phenomena at both macroscopic and microscopic levels, namely, at different time scales addressing pulse trains and individual pulses, a precise description of soliton complexes becomes a very tough task. In the future, we anticipate that more experimental reports and more theoretical framework will be carried out. More optical measurement methods will be developed in order to provide worthy means for detailed recording of the properties of multisoliton ensembles such as phase information.