Water-Vapor Raman Lidar System Reaches Higher Altitude

Signal-to-noise ratios are increased over those of prior such systems.

A Raman lidar system for measuring the vertical distribution of water vapor in the atmosphere is located at the Table Mountain Facility (TMF) in California. Raman lidar systems for obtaining vertical water-vapor profiles in the troposphere have been in use for some time. The TMF system incorporates a number of improvements over prior such systems that enable extension of the altitude range of measurements through the tropopause into the lower stratosphere.

altOne major obstacle to extension of the altitude range is the fact that the mixing ratio of water vapor in the tropopause and the lower stratosphere is so low that Raman lidar measurements in this region are limited by noise. Therefore, the design of the TMF system incorporates several features intended to maximize the signal-to-noise ratio. These features include (1) the use of 355-nm-wavelength laser pulses having an energy (0.9 J per pulse) that is high relative to the laser-pulse energy levels of prior such systems, (2) a telescope having a large aperture (91 cm in diameter) and a narrow field of view (angular width ≈0.6 mrad), and (3) narrow-bandpass (wavelength bandwidth 0.6 nm) filters for the water-vapor Raman spectral channels. In addition to the large aperture telescope, three telescopes having apertures 7.5 cm in diameter are used to collect returns from low altitudes.

The receiver portion of this lidar system has a total of eight channels (see figure). These include three channels for the water-vapor Raman returns at a wavelength of 407 nm, three channels for the nitrogen Raman returns at a wavelength of 387 nm, and two channels for elastics-cattering returns at the laser wavelength of 355 nm. Three of the channels (a 387-, a 407-, and a 355-nm channel), denoted the near channels, process the Raman and elastic returns collected by the three smaller telescopes. The remaining five channels, denoted the far channels, process the Raman and elastic returns collected by the large telescope. The elastic-scattering returns are used primarily for deriving temperature profiles. The light in each channel is measured by use of a photomultiplier tube, the output of which is fed to a commercially available optical-transient recorder operating as a photon-counting multi-channel scaler. The altitude interval of each bin of the scaler is 7.5 m, but typically, bins are summed together in groups of 10, yielding discretization of altitude in increments of 75 m.

The light collected by the large telescope is focused into an optical fiber, which delivers the light to a lens that collimates the light into a series of beam splitters. Among the beam splitters are a 99:1 beam splitter for each of the two Raman wavelength bands. In addition to extending the dynamic range of the photon counting system, this arrangement enables better corrections for pulse pile-up saturation effects than could otherwise be made. The arrangement is such as to make the 387- and 407-mm Raman signals in the large-telescope 1-percent splitter outputs approximately equal in magnitude to the corresponding signals from the smaller telescopes; this makes it possible to use the signals from the small telescopes to correct for effects of overlap of photon pulses in signals from the large telescope collected from low altitudes.

This work was done by Thierry Leblanc and I. Stewart McDermid of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-45007

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