A device for optically detecting and distinguishing airborne liquid water
droplets and ice crystals includes an illumination portion and a
detection portion. The illumination portion outputs a circularly
polarized illuminating beam. The detection portion receives circularly
polarized backscattered light from moisture in the cloud, in response to
the illuminating beam. The circularly polarized backscattered light is
passed through a circular polarizer to convert it into linearly polarized
backscattered light, which is split into two components. Each of the two
components is optionally subject to further linear polarization to filter
out any leakage-type orthogonal polarization. The two components are then
optically detected and the resulting detection signals are used to
calculate one or more parameters reflective of the presence or absence of
airborne ice crystals and/or water droplets.