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. Soliton mode locking also endows frequency microcombs with the enhanced stability necessary for miniaturization of spectroscopy and frequency metrology systems. 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 and laser ranging systems. [ABSTRACT FROM AUTHOR]