1A7 - The measurement of the frequency fluctuations of a laser field

Many authors have analyzed the fluctuations of the laser oscillator, considering it as a regenerative classical oscillator. According to this model, the spectral line broadening can be calculated as due to the amplitude and to the phase fluctuations of the laser oscillation. There is as yet no good method available for the measurement of the phase or frequency fluctuations and the resultant spectral broadening. The method we propose allows the measurement of the cross-correlation function of the phase or frequency fluctuations of the optical field at two points of a laser beam. The optical signal from a single laser at two points P
1
and P
2
is mixed separately with two other independent lasers in two photo-devices. After detection of the beats, the two signals are passed each through a limiter and a frequency discriminator tuned to the center frequency of the beats. The output signals are then proportional to the fluctuating instantaneous frequency differences between the incoming beam and each of the local oscillator lasers. These two signals are then correlated with an electronic correlator. The details of the calculations are presented. It is shown that the signal-to-noise ratio attainable in such a measurement can be quite satisfactory.