By heterodyning together the outputs from two stable He-Ne $3.39-\mu$ lasers, we have measured the power spectrum of the laser field over a wide range of output powers. By combining this high-gain laser transition with a very lossy laser cavity, the quantum phase noise contribution to the laser linewidth could be made dominant at low output powers. The laser spectrum was then found to be nearly Lorentzian with a bandwidth inversely proportional to output power, in good agreement with a modified form of the Schawlow-Townes laser linewidth formula. Weak quantum-noise-excited sidebands were also observed in the beat spectrum of a He-Xe $3.508-\mu$ laser. These were identified with a resonance in the He-Xe laser amplitude-fluctuation spectrum.

• Received 18 September 1970

DOI:https://doi.org/10.1103/PhysRevA.4.373