Accuracy of zero crossing counting in laser Doppler velocimetry

The statistical properties of photodetector current in laser Doppler velocimeter are studied. The analysis of zero crossing counting shows that it overestimates by certain amount velocity and displacement measurements. The dispersion of zero counts in the presence of additive shot noise is also studied. The simple analytical expression for dispersion of zero counts is obtained in absence of additive shot noise. It shows that the relative accuracy of velocity or displacement measurement is inversely proportional to the square root of measured distance. The relative dispersion of zero counts less than 0.005 % can be achieved while measuring 5 in displacement if certain requirements concerning signal-to-noise ratio are met.