Oscillatory motion of a counterpropagating Kerr soliton dimer

Both the group velocity and phase velocity of two solitons can be\nsynchronized by a Kerr-effect mediated interaction, causing what is known as\nsoliton trapping. Trapping can occur when solitons travel through single-pass\noptical fibers or when circulating in optical resonators. Here, we demonstrate\nand theoretically explain a new manifestation of soliton trapping that occurs\nbetween counter-propagating solitons in microresonators. When counter-pumping a\nmicroresonator using slightly detuned pump frequencies and in the presence of\nbackscattering, the group velocities of clockwise and counter-clockwise\nsolitons undergo periodic modulation instead of being locked to a constant\nvelocity. Upon emission from the microcavity, the solitons feature a relative\noscillatory motion having an amplitude that can be larger than the soliton\npulse width. This relative motion introduces a sideband fine structure into the\noptical spectrum of the counter-propagating solitons. Our results highlight the\nsignificance of coherent pumping in determining soliton dynamics within\nmicroresonators and add a new dimension to the physics of soliton trapping.\n