Remote sensing of cloud properties traditionally uses "on-beam" lidar, which collects light from a very narrow field-of-view (FOV) centered on the transmitted laser beam. Taking advantage of the "off-beam" scattered light far from the input beam, however, can yield much more information on properties such as the physical thickness and optical depth of clouds, according to researchers at Los Alamos National Laboratory (LANL; Los Alamos, NM).
Anthony B. Davis of LANL reported the results of ground-based observations using a wide-angle imaging-lidar (WAIL) system prototype at an AeroSense (Orlando, FL) conference session on atmospheric sensing on April 18.
"Although it's termed lidar," said Davis of the system, "we're not focusing on the ranging aspect. The interesting part is the time-resolved imagery of the off-beam scattering."
To measure the distribution of multiply-scattered photon paths resulting in clouds, Steven Love, Davis, and colleagues at LANL created the WAIL instrument with a custom imaging detector. The WAIL prototype uses a 532-nm frequency-doubled Nd:YAG laser to produce 0.2 to 0.5 mJ pulses at a typical rate of 12 kHz. Pulse widths ranged from 30 to 50 ns depending on operating conditions. For imaging scattered photons far from the beam at night, a 10-nm-wide bandpass filter was required for background rejection. Because standard interference filters cause a blueshift of the center wavelength with angle of incidence across the FOV, a longer center-wavelength filter provided a more uniform response across the field. This angular sensitivity is actually useful in attenuating the light coming from the bright central spot at normal incidence (see figure).
The custom detector developed at LANL consists of a photocathode-coated vacuum tube intensified by microchannel plates (MCP), read out by a cross-delay line (CDL) anode, and coupled with high-speed pulse absolute-timing electronics. The MCP/CDL technology features photon-counting sensitivity, a 4-cm active diameter, and ultrahigh time resolution of 100 ps. Measuring the arrival times of the pulses determines the position of the original photon event. The MCP/CDL detector gives better spatial and time resolution than CCDs as well as extreme sensitivity, but saturates quickly.
The system combines temporal and spatial information from the off-beam lidar to determine cloud thickness and optical depth. To capture azimuthal dependence of the scattering, as well as radial information on the spatial distribution, the team created a time-resolved two-dimensional image, essentially a high-speed "movie" of the light propagation. In this way, the resulting measurements of cloud thickness are compatible with results from a nonimaging spaceborne system with a wide FOV, such as the shuttle-based Lidar-In-space Technology Experiment (LITE). However, the advantage of a combined space-time scheme with a WAIL-type device is a much less ambiguous measure of optical depth.
Ultimately, reports the team, the off-beam lidar can provide a valuable and cost-effective tool for remote probing of cloud properties, which is of direct interest in meteorological applications such as radiation energetics for climate modeling. Future efforts will explore alternative detector solutions, such as the use of an occulting disk to mask the central region, and possibly the use of a different type of detector. For more details, and viewing of the complete WAIL "movies," point a web browser to nis-www.lanl.gov/~love/clouds.html.
Valerie Coffey-Rosich | Contributing Editor
Valerie Coffey-Rosich is a freelance science and technology writer and editor and a contributing editor for Laser Focus World; she previously served as an Associate Technical Editor (2000-2003) and a Senior Technical Editor (2007-2008) for Laser Focus World.
Valerie holds a BS in physics from the University of Nevada, Reno, and an MA in astronomy from Boston University. She specializes in editing and writing about optics, photonics, astronomy, and physics in academic, reference, and business-to-business publications. In addition to Laser Focus World, her work has appeared online and in print for clients such as the American Institute of Physics, American Heritage Dictionary, BioPhotonics, Encyclopedia Britannica, EuroPhotonics, the Optical Society of America, Photonics Focus, Photonics Spectra, Sky & Telescope, and many others. She is based in Palm Springs, California.