A hertz-linewidth semiconductor laser self-injection locked to a microresonator with electromagnetically induced...

We present a method for narrowing the intrinsic linewidth of a semiconductor laser to the hertz level using optical feedback from an all-pass microresonator with electromagnetically induced transparency (EIT). EIT resonance resulting from the interference of resonant modes provides much stronger and narrower optical feedback than Rayleigh-scattering-based optical feedback in the same microresonator. Experimental results show that the white frequency noise can be reduced by more than four orders of magnitude using the optical feedback from a spherical microresonator with EIT resonances and a Q-factor of 2.95 million. Specifically, the intrinsic linewidth is narrowed from the original 157 kHz to 6.3 Hz, which is one order of magnitude narrower than that of the DFB laser diode with Rayleigh-scattering-based optical feedback from the same microresonator. Self-injection locking via the EIT resonance-based narrow and strong optical feedback from a microresonator in a simple all-pass configuration can be replicated by the narrow linewidth chip-scale lasers, which are indispensable for coherent optical systems, such as optical communication, metrology, and sensing.