Behavior of the relative variance of the turbulent fluctuations of detector photocurrent power is analyzed for CW and pulsed coherent lidars and different schemes of matching the wave fronts in optical heterodyning. It is shown that fluctuations of the photocurrent power occur for high spatial resolution of a single-mode heterodyne detector. The magnitude of these fluctuations is proportional to the turbulent intensity fluctuations of sounding radiation. The proportionality factor depends on the receiver field of view and decreases with its increase.