  To increase the spectral efficiency of coherent communication systems, lasers
with ever-narrower linewidths are required as they enable higher-order
modulation formats with lower bit-error rates. In particular, semiconductor
lasers are a key component due to their compactness, low power consumption, and
potential for mass production. In field-testing scenarios their output is
coupled to a fiber, making them susceptible to external optical feedback (EOF).
This has a detrimental effect on its stability, thus it is traditionally
countered by employing, for example, optical isolators and angled output
waveguides. In this work, EOF is explored in a novel way with the aim to reduce
and stabilize the laser linewidth. EOF has been traditionally studied in the
case where it is applied to only one side of the laser cavity. In contrast,
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.
Numerical results shows an improvement in performance of the double-feedback
case with respect to the single-feedback case. In particularly, by
appropriately selecting the phase of the feedback from both sides, a broad
stability regime is discovered. This work paves the way towards low-cost,
integrated and stable narrow-linewidth integrated lasers.
