Extended Data Fig. 2: Shifting of the soliton spectral envelope relative to the primary mode.
Experimental characterization of two NB solitons where the tuning of the bias current results in a shift of the spectral soliton envelope from the red to the blue side of the primary mode (a and b respectively). The shift of the soliton spectral envelope happens as the currents of the ring and the waveguide are changed. The main reason for this likely lies in the large change of the total GVD, as discussed in the main manuscript, and recently observed experimentally in passive microresonators21. Although the soliton envelope may be positioned differently relative to the primary mode, the expected two π jumps of the intermode phases around the primary mode are still present – indicating that this is indeed a salient feature of NB solitons. The temporal profile of the phase exhibits the familiar 2π ramp within the width of the soliton. We can observe that the direction of the ramp depends on whether the soliton spectral envelope is on the red or on the blue side relative to the primary mode. In a hypothetical state where the soliton envelope would be perfectly symmetric relative to the primary mode (if the soliton spectral center of mass coincides with the position of the primary mode), the 2π phase ramp would comprise two separate π ramps with an opposite direction. However, this state does not represent a stable fixed point, and is likely never to occur experimentally.
