Diode laser coupled to a high-Q microcavity via a GRIN lens

A miniature high-coherent diode laser was developed. Optical feedback from a high-Q microsphere resonator was used to narrow the spectrum of the laser, and a nearly half-pitch gradient-index lens served as a coupling element. As estimated from the variation in frequency-tuning range (chirp-reduction factor) the fast line width of the laser was reduced by more than three orders. It is remarkable that the system reveals stable single-mode operation at a relatively high feedback level. A tentative explanation is presented in terms of previously given models.