The bound state solitons have been observed in the passively mode-locked fiber laser. The spectra are modulated due to interaction of the pulses per round trip. Two bound states have an equal shoulder as seen from the autocorrelation trace and three bound states have the two pairs of shoulder in the autocorrelation trace. The numerical results confirm the experimental results. The unstable soliton pairs can also be observed in the experiments. The pulse pairs with strong strength in the cavity can be potentially applied in the optical communication systems.
"Experimental and theoretical study of soliton pairs in passively mode-locked fiber lasers," Optical Engineering 50(9), 095002 (1 September 2011). https://doi.org/10.1117/1.3619824