Widely tunable SG-DBR laser with ultra-narrow linewidth achieved via polarization-controlled feedback

Ultra-narrow linewidth lasers are essential for high-precision applications, including coherent communication, quantum photonics, LiDAR, and spectroscopy. Conventional methods for narrowing laser linewidths using external optical feedback (EOF) are fundamentally constrained by coherence collapse (CC). This instability occurs at feedback levels below 1 % of the output power, causing chaotic laser behavior and significant linewidth broadening. Our research introduces a dual-cavity feedback mechanism within a U-shaped laser configuration. By employing polarization-controlled EOF, we effectively prevent coherence collapse and significantly extend the range of feedback power that can be applied. This method achieves a robust, sub-kilohertz linewidth across a wide 42 nm tuning range in the infrared C-band (1513–1555nm). This advancement offers a promising solution for integrated photonic systems and next-generation coherent communication technologies that require CC-resilient, tunable laser sources with exceptionally narrow linewidths.