Fig. 2

Experimentally observed spectral evolution in a tapered multimode fiber. a The spectrum recorded at the output of the tapered multimode fiber when a 100 fs, 1550 nm pulse is used at the input with a peak power of 750 kW. The NIR frequency components are formed because of multimode solitons (red trace) while the visible part arises from dispersive waves (blue trace). b Spectral evolution in the visible section as measured using a cutback method. The tapered segment of the multimode fiber is 4 m long (its core radius is reduced from 40 to 10 µm) and is preceded by a 10 m uniform fiber. As the pulses enter the tapered section, dispersive waves experience a continuous blue-drifting of more than 45 nm, indicating a speedup in the intermodal oscillations. The tapered multimode fiber is depicted on the right side, where the darker red color represents an increase in the light intensity in the fiber due to a decrease in core size. c The spectrum in b is depicted for comparison, before and after the tapered section where blue-shifting of the dispersive wave comb is evident