  A guided-wave chip laser operating in a single longitudinal mode at 2860 nm
is presented. The cavity was set in the Littman-Metcalf configuration to
achieve single-frequency operation with a side-mode suppression ratio above 33
dB. The chip laser's linewidth was found to be limited by mechanical
fluctuations, but its Lorentzian contribution was estimated to be lower than 1
Hz using a heterodyne technique. This demonstration incorporates a high
coherence source with the simplicity provided by the compactness of chip
lasers.
