Counter-propagating solitons in microresonators

Solitons occur in many physical systems when a nonlinearity compensates wave dispersion. Their recently demonstrated formation in microresonators has opened a new research direction for nonlinear optical physics(1-5). Soliton mode locking also endows frequency microcombs with the enhanced stability necessary for miniaturization of spectroscopy and frequency metrology systems(6). These microresonator solitons orbit around a closed waveguide path and produce a repetitive output pulse stream at a rate set by the roundtrip time. Here, counter-propagating solitons that simultaneously orbit in an opposing sense (clockwise/counter-clockwise) are studied. Despite sharing the same spatial mode family, their roundtrip times can be precisely and independently controlled. Furthermore, a state is possible in which both the relative optical phase and relative repetition rates of the distinct soliton streams are locked. This state allows a single resonator to produce dual-soliton frequency-comb streams with different repetition rates, but with a high relative coherence that is useful in both spectroscopy(7-9) and laser ranging systems(10).