Using paraxial approximation to describe the optical setup of a typical EARLINET lidar system

Abstract

Abstract. The mathematical formulation for the optical setup of a typical EARLINET lidar system is given here. The equations describing a lidar system from the emitted laser beam to the projection of the telescope aperture on the final receiving unit (i.e., photomultiplier or photodiode) are presented, based on paraxial approximation and geometric optics approach. The receiving optical setup includes a telescope, a collimating lens, an interference filter and the ensemble objective eyepiece. The set of the derived equations interconnects major parameters of the optical components (e.g., focal lengths, diameters, angles of incidence), revealing their association with the distance of full overlap of the system. These equations may used complementarily with an optical design software, for the preliminary design of a system or can be used as a quick check up tool of an existing lidar system. The evaluation of the formulation on a real system is performed with ray-tracing simulations, revealing an overall good performance with relative differences of the order of 5 % mainly attributed to the limitations of the thin lens approximation.

EARLINET lidar quality assurance tools

“…The distance of full overlap is not below about 100 m. In this case of a well aligned lidar system with well designed eyepieces, the raw signal differences between the telecover sectors indicate just the different sensitivities of the different areas of the photomultipliers, which are different in the three channels. The measured telecover differences can be compared to paraxial (see also Kokkalis (2017)) and exact ray tracing (ZEMAX) simulations of the system including apertures and optical coatings to narrow down possible causes. Figure 26 shows the paraxial near range simulation of the field of view for the full telescope aperture (left), and for the N (blue) and S (magenta) telecover sector with a circular field stop in the focal plane of the telescope (neglecting the obscuration of the secondary mirror).…”

Retrieval of optical and microphysical properties of transported Saharan dust over Athens and Granada based on multi-wavelength Raman lidar measurements: Study of the mixing processes

“…The geometrical specification of EOLE makes feasible the full overlap of the laser beam with the receiver field of view to be reached at heights of the order of 800 m a.g.l. (Kokkalis, et al, 2012;Kokkalis, 2017). An additional depolarization channel at 355 nm was added in 2016 in order to obtain the linear particle and volume depolarization ratio vertical profiles in the atmosphere.…”

Retrieval and evaluation of tropospheric aerosol extinction profiles using MAX-DOAS measurements over Athens, Greece

“…The six-wavelength Raman-backscatter lidar system (EOLE) operates in Athens since February 2000 as part of the EARLINET network (Pappalardo et al, 2014). The system is designed following the optical set-up of a typical member station (Kokkalis 2017), meeting all the quality assurance requirements of the network. The emission unit is based on a pulsed Nd:YAG laser, emitting high energy pulses at 355, 532 and 1064 nm with a repetition rate of 10 Hz.…”

“…In this study, only daytime measurements are used and therefore no overlap correction is applied on the signals. The geometrical configuration of EOLE results in full overlap distance of 500-800 m above ground (Kokkalis 2017). The aerosol extinction values below the 1000 m a.s.l.…”