Design of a fused optical fibre bundle-based receiver for low-altitude profiling of water vapour using Raman lidar

TLDR: In this paper, an optical fiber-based receiver for low altitude water vapour profiling of surface layer, which constitutes the lowest hundred meters from earth's surface, can aid in the understanding of spatial variability of atmospheric turbulence and the dynamics of boundary layer.

Abstract: Water vapour profiling of surface layer, which constitutes the lowest hundred meters from earth’s surface, can aid in the understanding of spatial variability of atmospheric turbulence and the dynamics of boundary layer. In lidars, the effective area of an optical fiber-based receiver, also called the aperture stop diameter, controls the field of view of the telescope which in turn governs the overlap function. We determined overlap function vs altitude for different aperture stop diameters which showed that lower altitude profiling requires fibre receivers of larger effective area positioned at the location of blur disk or the position of maximum capture of back scattered light. We report on the design of a receiver which comprises of a converging lens system in conjunction with a commercially available fibre bundle of fused hexagonal shaped fibres of adequate numerical aperture and enhanced effective light capturing area. For a specified biaxial Raman lidar system with an excitation laser emitting at 532 nm, placing a one inch diameter lens at the plane of blur disk of diameter ~21 mm and the aforesaid fibre bundle of diameter 7.3 mm at the image plane of the blur disk was found to be suitable for relatively efficient light capture to enable profiling from an altitude of ~8m and above. The light capturing efficiency of the system was determined and compared with that of a conventional circular fibre-based bundle of same diameter. The proposed receiver design offers potential solution for low altitude profiling with reduced central obscuration.