Strong-feedback regime of self-injection locking and external cavity laser

Realization of the coupling of the laser diode to an external reflector may provide efficient suppression of the phase noises and significant stabilization of the laser source. Locking a semiconductor laser to high-quality-factor microresonator was shown to result in a laser linewidth narrowing to sub-Hz level. The straightforward way to get better stabilization and wider locking band is to increase the feedback level. However, most of the theories used to describe the self-injection locking effect assume the weak feedback from the external reflector. Here we develop the more complete theory of the laser -- resonant reflector interaction that allows to describe this effect for the high feedback level as well. We define different possible regimes taking place at different feedback levels (including the so-called external cavity laser regime) and study applicability domains of the previous and proposed models. We show that existing model of the self-injection locking to whispering-gallery mode resonator is a consequence of the considered model in the low-feedback regime. Finally, we check the model in high-feedback limit experimentally and show a good correspondence with the theory.