Filtered laser beat-frequency fluctuations

The statistics of the bandpass filtered beat signal of two semiconductor lasers with Lorentzian field power spectra are theoretically shown to be Gaussian with the variance given by the product of the beat signal linewidth and an effective bandwidth Be. Be is shown to depend on the filter characteristic and the position of the mean beat frequency within the filter. The theoretical predictions have been confirmed in detail by experiments in which the cumulative distribution function for the output frequency process has been measured for three different filters and five different measurement cases.