Amplitude correlation and direct spectral analysis of low photon rate LDV signals.

Photon resolved signals may be converted to signals that are suitable for classical correlation analysis or spectral analysis of the signal amplitudes by low pass filtering the train of photon pulses. The correlations and spectra obtained from the amplitudes contain the same information that photon correlations contain. Amplitude analysis is compared with photon rate analysis in the context of laser Doppler velocimetry. It is shown that modern amplitude correlation techniques permit correlation rates comparable to those of photon correlation. These techniques are advantageous in many situations because they are immune to photon pile up, so the maximum available signal power can be used. Direct spectral analysis of the photon rate is also compared to direct spectral analysis of the amplitudes. In this case amplitude analysis avoids the photon pile up limitation, and it also permits analog anti-aliasing filtering. (A)