Direct Observation of the Lorentzian Line Shape as Limited by Quantum Phase Noise in a Laser above Threshold

Abstract

The quantum-phase-noise-limited Lorentzian power spectral densities of ${\mathrm{Pb}}_{0.88}$ ${\mathrm{Sn}}_{0.12}$Te diode lasers were directly measured. Linewidths as narrow as 54 kHz were observed and quantitatively analyzed. The predicted inverse dependence of linewidth on laser power was also demonstrated. These results were obtained from the beat-note spectra produced by heterodyning the diode laser, operating cw at 10.6 μm, with a stable, single-frequency C${\mathrm{O}}_2$ gas laser.