External folded cavity optical feedback diode laser with megahertz relative linewidth

We present a extended-cavity diode laser (ECDL) with megahertz linewidth by optical feedback from a folded Fabry–Perot cavity, and demonstrate the efficient laser linewidth reduction and frequency stabilization of the optical feedback technique. In our experiments, a folded Fabry–Perot cavity with a finesse of 4750 replaces the reflecting mirror in the traditional ECDL configuration, the folded Fabry–Perot cavity can serve as an optical feedback element, which forces the semiconductor laser automatically to lock its frequency optically to the cavity resonance frequency. The laser’s phase noise is significantly suppressed, and The laser’s linewidth is reduced from about 20GHz to 15MHz.