  We report on the experimental observation of coherent cavity soliton
frequency combs in silica microspheres. Specifically, we demonstrate that
careful alignment of the microsphere relative to the coupling fiber taper
allows for the suppression of higher-order spatial modes, reducing mode
interactions and enabling soliton formation. Our measurements show that the
temporal cavity solitons have sub-100-fs durations, exhibit considerable Raman
self-frequency shift, and generally come in groups of three or four,
occasionally with equidistant spacing in the time domain. RF amplitude noise
measurements and spectral interferometry confirm the high coherence of the
observed soliton frequency combs, and numerical simulations show good agreement
with experiments.
