  High coherence lasers are essential in a wide range of applications, however,
such performance is normally associated with large laser cavities, because
increasing energy storage reduces quantum phase noise and also renders the
laser frequency less sensitive to cavity vibration. This basic scaling rule is
at odds with an emerging set of optical systems that place focus on compact
(optimally integrable) sources of high coherence light. These include
phase-coherent optical communication using quadrature-amplitude-modulation, and
also record-low phase noise microwave sources based upon optical comb
techniques. In this work, the first, chip-based Brillouin laser is
demonstrated. It features high-efficiency and single-line operation with the
smallest recorded Schawlow-Townes frequency noise for any chip-based laser.
Because the frequency offset between the laser's emission and the input pump is
relatively small, the device provides a new function: spectral purification of
compact, low coherence sources such as semiconductor lasers.
