INVESTIGATION OF OPTICAL DOPPLER SUBCARRIER REFLECTED SIGNAL LOSS ATTRIBUTED TO COHERENCE EFFECTS AT HIGH MODULATION FREQUENCIES.

The theoretical discussion indicates the subcarrier remains coherent after reflection from optically diffuse targets. To verify this analysis, the output of a HeNe cw gas laser is split into two beams of equal intensity each of which is reflected back to a photomultiplier by two diffuse targets. The results indicate that the received subcarrier amplitude varies as the determinent cosine pi path differencesubcarrier wavelength. Since the phase relationship is predictable, it is concluded that the theoretical analysis is correct. The beam spread reflection loss is initially examined using analog computer circuitry to determine the amplitude of the reflected subcarrier for various values of maximum phase spread. By lengthening the far-field laser beam over a diffuse aluminum chaff target, actual reading of relative reflected subcarrier amplitude are taken. Both theoretical and experimental evaluations indicate that an 80 to 100 per cent reduction in signal reflectivity will be experienced when the phase spread exceeds two wavelengths of the modulation frequency.