Revised theory for the linewidth of diode lasers under external optical feedback

摘要

Achieving ever narrower linewidths in diode lasers has become of paramount importance for coherent communications. Coupling a laser chip to a fiber however introduces external optical feedback (EOF) to the cavity which is notoriously detrimental to the stability of the device, in all but very specific and difficult conditions. A better understanding of the interplay between EOF and the laser dynamics is thus crucial for designing narrow-linewidth diode lasers to be used in field-testing scenarios. The standard formalism of EOF in diode lasers relies on the assumption that feedback is present on one side of the laser cavity. However, given the currently available integration technologies, this assumption is no longer justified. In this work a revision of EOF theory is explored based on the updated assumption that feedback can be introduced from both sides of the laser cavity. This is done by obtaining the dynamic equations of the proposed system, including an expression for the power spectral density and linewidth. Additionally, the stability of the system is discussed. Results show that the proposed revised theory can yield stable laser performance while simultaneously reducing laser linewidth.