  Solitons occur in many physical systems when a nonlinearity compensates wave
dispersion. Their recent formation in microresonators opens a new research
direction for nonlinear optical physics and provides a platform 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 round-trip time. In this
work counter-propagating solitons that simultaneously orbit in an opposing
sense (clockwise/counter-clockwise) are studied. Despite sharing the same
spatial mode family, their round-trip 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 having different repetition rates, but with high relative coherence
useful in both spectroscopy and laser ranging systems.
