Narrow-linewidth lasers are building blocks of coherent communication systems, as lower linewidths enable higherorder modulation formats with lower bit-error rates. For this purpose, diode lasers are in high demand due to their low power consumption, compactness, and potential for mass production. In field-testing scenarios, their output is coupled to a fiber, making them susceptible to external optical feedback (EOF), which is notoriously detrimental to their stability. This challenge is traditionally combated by using, for example, angled output waveguides and optical isolators. The approach reported in this work makes use of EOF in a new way, to reduce and stabilize the laser linewidth. Whereas research in this field has focused on EOF applied to only one side of the laser cavity, this work gives a generalization to the case of feedback on both sides. It is implemented using photonic components available via generic foundry platforms, thus creating a path towards devices with high technology-readiness level. It is numerically observed that the double-feedback case can lead to improved performance with respect to the single-feedback case. In particular, by correctly tuning the phase of the feedback from both sides, a broad region of stability is discovered. This work paves the way towards low-cost, integrated and stable narrow-linewidth integrated lasers.
