Passive stabilization cavity keeps ultra-precise laser phase-locked/ frequency-stable without relying on complex electronic feedback loops

https://actu.epfl.ch/news/a-laser-that-stays-locked-without-active-control-3/

"compact semiconductor lasers suffer from frequency fluctuations, overcome using chip-based endless self-injection locking feedback... overlapping laser's stable operating regions enables smooth transitions between stable states as drive currents change from 154 to 300 mA, reducing frequency noise >5,000X, maintaining intrinsic linewidth <10 Hz... integrated piezoelectric actuators also enable mode-hop-free frequency chirps >1.5 GHz without losing its lock or needing real-time feedback"