Nasa Langley Airborne High Spectral Resolution Lidar Instrument Description

NASA Langley Research Center (LaRC) recently developed the LaRC Airborne High Spectral Resolution Lidar (HSRL) to make measurements of aerosol and cloud distribution and optical properties. The Airborne HSRL has undergone as series of test flights and was successfully deployed on the Megacity Initiative: Local and Global Research Observations (MILAGRO) field mission in March 2006 (see Hair et al. in these proceedings). This paper provides an overview of the design of the Airborne HSRL and descriptions of some key subsystems unique to this instrument. 1 INSTRUMENT OVERVIEW A functional block diagram of the HSRL instrument is shown in Fig.1. The Airborne HSRL operates as a high spectral resolution lidar at 532 nm using the iodine vapor filter technique [1-2] and a backscatter lidar at 1064 nm. Depolarization is measured at both wavelengths. The laser transmitter consists of four basic components: an Nd:YAG CW seed laser, an electrooptic feedback loop that locks the seed laser to an iodine absorption line, and an injection-seeded pulsed Nd:YAG laser slaved to the seed [3] and an output conditioning module that provides control over beam steering, output power, and polarization orientation. The seed laser and pulsed slave laser were developed by Innolight GmbH and Fibertek, Inc., respectively. The electroptic control loop and the output conditioning modules were designed and implemented by LaRC. The receiver employs a 16-inch Newtonian telescope to which an aft-optics module is kinematically mounted. The aft optics module separates wavelengths, filters the return to reject out-of-band background light, and separates polarization components. The 532 nm parallel-polarized backscatter is further split in the aft optics module: approximately 4% of the 532 parallel return is sent to the boresight subsystem which automatically maintains alignment between the transmit and receive optical axes; approximately 10% of the parallel backscatter is transferred directly to the detector subsystem; and the remaining 86% of the parallel 532 nm backscatter is directed to the iodine vapor filter module which provides rejection of the Mie component of the backscatter. The 10% channel measures total (molecular plus aerosol) parallel backscatter and the 86% channel is sensitive to only the Fig. 1. HSRL Functional Diagram Boresight Detector 1064nm Perpendicular Polarization Channel I2 Vapor Filter 532nm Molecular Backscatter Channel 532nm Parallel Polarization Channel 532nm Perpendicular Polarization Channel 532nm Etalon (40 pm) APD PMT APD 1064nm Parallel Polarization Channel Lser T rnsm iter PMT