Self-quenching of the semiconductor laser linewidth below the Schawlow–Townes limit by using optical feedback

Abstract

We demonstrate theoretically and experimentally self-quenching of the fundamental semiconductor laser frequency fluctuations to a level that is orders of magnitude below the Schawlow–Townes limit for a solitary laser. It is shown that the main operative mechanism is the combined action of a frequency-dependent internal loss and amplitude-to-phase coupling. The internal frequency-dependent loss is introduced by means of spectrally narrow external optical feedback, which provides a strong frequency-dependent dispersion. Linewidth reduction by a factor of 2 × 10³ is demonstrated by using a narrow Doppler-free Faraday resonance in Cs vapor.

© 1992 Optical Society of America