Lidar ceilometers emit linearly polarized laser pulses. When a portion of this light scatters back towards the instrument, the polarization direction may change. This depolarization depends strongly on scatterer shape, orientation, and laser wavelength. CL61 can detect the level of depolarization in the received signal.
The laser pulses that fall back to the receiver have interacted at a 180° backscatter angle with spherical, homogeneous scatterers, such as liquid cloud droplets or small raindrops. Due to the symmetry of the scattering event, the detected return signal is not depolarized.
Multiple scattering in optically thick media, such as liquid clouds, can cause depolarization even when the scatterers are spherical. A portion of the detected light may have experienced forward and backward scattering in angles that deviate from 180°, still reaching the receiver.
Non-spherical solid particles cause significant depolarization due to multiple internal reflections at solid-air interfaces. Larger raindrops with flattened bottom shapes also produce some depolarization.
The level of depolarization for lidars can be described by the linear depolarization ratio (LDR). LDR is the ratio of the perpendicular or cross-polarized (XPOL) components and the parallel (PPOL) signal components and is equal to or greater than zero. The maximum LDR value is less than one because PPOL is the dominant backscattered signal component from atmospheric particles. Both particles and air molecules affect the degree of depolarization.
Their mixed contribution is known as the volume depolarization ratio (VDR). Accurate depolarization measurement requires relatively high backscatter, for example, from dust, smoke, ash, or boundary layer aerosols. CL61 measures VDR.
CL61 alternates acquisition of XPOL and PPOL signals with a measurement time of 0.2 s for each. Vertical profiles from ground level to a measurement range of 15.4 km (9.6 mi) are time averaged for 5 s before analysis.
CL61 measures depolarization with two filters in front of a single avalanche photodiode detector on the coaxial optical path. As the same receiver module is used for XPOL and PPOL signals, there is no need for receiver sensitivity calibration.