Internal and external field fluctuations of a laser oscillator: Part I--Quantum mechanical Langevin treatment

The amplitude and phase noise spectra of the internal and external fields of a laser oscillator are calculated using the quantum mechanical Langevin equations. The outgoing field spectra are demonstrated to be different from the internal field spectra. The outgoing photon fluxN̂from a laser oscillator operating at far above the threshold has a spectral density equal to the classical shot noise level,2̂. On the other hand, the internal photon number spectrum is Lorentzian, and the variance is equal to the average photon number,̂. The difference stems from the quantum mechanical interference between the transmitted internal field and the reflected zero-point fluctuation at the output coupling mirror. The relation between the present analysis and the previous noise theory of a laser is also discussed.