  Acquisition of laser frequency with high resolution under continuous and
abrupt tuning conditions is important for sensing, spectroscopy and
communications. Here, a single microresonator provides rapid and broad-band
measurement of frequencies across the optical C-band with a relative frequency
precision comparable to conventional dual frequency comb systems. Dual-locked
counter-propagating solitons having slightly different repetition rates are
used to implement a Vernier spectrometer. Laser tuning rates as high as 10
THz/s, broadly step-tuned lasers, multi-line laser spectra and also molecular
absorption lines are characterized using the device. Besides providing a
considerable technical simplification through the dual-locked solitons and
enhanced capability for measurement of arbitrarily tuned sources, this work
reveals possibilities for chip-scale spectrometers that greatly exceed the
performance of table-top grating and interferometer-based devices.
