Reexamination of depolarization in lidar measurements

Abstract

Almost all of the depolarization papers in the lidar literature employ a physically inappropriate notation and they use a definition of the depolarization ratio that is not linear in the quantity of interest. This depolarization lidar legacy is misleading and confusing. In particular, subscripts meaning parallel and perpendicular do not apply to atmospheric parameters, such as the volume backscatter coefficient, because (for linear polarization) the two components of the backscattered light are polarized in the transmitted sense and completely unpolarized; the unpolarized component is not "perpendicular." An analysis of lidar depolarization measurements with a particle scattering matrix recently provided in the literature yields algorithms for retrieving the depolarization parameter from either linear or circular depolarization lidar measurements. The analysis, notation, and definitions recommended here harmonize lidar depolarization analysis with radiative transfer theory, particle scattering theory, and standard polarization measurement techniques.

Triple-wavelength depolarization-ratio profiling of Saharan dust over Barbados during SALTRACE in 2013 and 2014

“…The basic lidar-derived quantity is the volume linear depolarization ratio defined as the ratio of cross-to co-polarized signal components (Gimmestad, 2008). The prefixes "co" and "cross" denote the planes of polarization (for which the receiver channels are sensitive) parallel and orthogonal to the plane of linear polarization of the transmitted laser pulses, respectively.…”

Concept Design of a Multiwavelength Aerosol Lidar System With Mitigated Diattenuation Effects and Depolarization-Measurement Capability

“…If we assume that the light entering the optical setup is a combination of linearly polarized light and completely depolarized light, as would be the case if the polarization of the transmitted light is linear and the depolarizing scatterers do not have a preferred orientation [8]- [10], the Stokes vector describing the intensity and polarization state for that radiation will be [11]   || || cos 2 sin 2 0…”

“…Nevertheless, if i) the different wavelengths transmitted all have linear polarizations parallel or that give the power reaching a photodetector and has the same formal dependence on the polarization of the incident wave as the transmittance. DOI: 10.1051/ perpendicular between themselves, ii) the backscattering medium does not change the polarized part of the returned radiation with respect to the polarization of the transmitted beams [8] - [10], nor does it the part of the optical receiving system before the beam splitters, and iii) all the beam splitters in the receiver setup are arranged in such a way that the incidence plane is the same for all of them, the sensitivity of the receiver to the depolarization produced by the medium can be suppressed by orientating the incidence plane to be at a 45º angle with respect the polarization direction of the polarized part of the return radiation, irrespective of the transmission ratio. In this case the depolarization information after transmission or reflections is found in the 3 rd and 4 th components of the Stokes vector, whence it can be retrieved with a quarter-wave plate and linear polarizer arrangement.…”

Depolarization ratio measurement using single photomultiplier tube in micropulse lidar

“…[3][4][5] Polarization MPL has been used to study various phenomenon in the atmosphere, such as hydrometeors, 6 clouds, 7 volcanic aerosols, 8 and polar stratospheric clouds. 9,10 The space-based lidars such as CALIPSO include a crosspolarization receiver channel because of the valuable information that it provides. 9 The value of the depolarization ratio of spherical particles such as liquid droplets is zero for backscattering, based on the Mie-scattering theory; enhancement of the depolarization ratio can arise from scattering by nonspherical particles such as solid particles or by multiple scattering.…”

“…9,10 The space-based lidars such as CALIPSO include a crosspolarization receiver channel because of the valuable information that it provides. 9 The value of the depolarization ratio of spherical particles such as liquid droplets is zero for backscattering, based on the Mie-scattering theory; enhancement of the depolarization ratio can arise from scattering by nonspherical particles such as solid particles or by multiple scattering. 10 It became apparent that in comparison to microwave depolarization from nonspherical particles ͑typically smaller than the incident wavelength͒ the laser depolarization ͑from particles larger than the wavelength͒ was considerably stronger, suggesting that polarization lidar has a promising future for the study of aerosols and the particles in clouds.…”