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New Lasers Push Space-Based Lidar

Photonics Spectra
Sep 1998
William B. Grant

The researchers at the 19th International Laser Radar Conference focused their attention on new lasers for space-based lidar to study atmospheric chemistry and physics. Lidar, which can be rugged enough for space travel, can make measurements day and night and under conditions in which other methods fail, such as for subvisible cirrus and multilayer clouds.

Researchers from NASA's Langley Research Center used the Lidar Atmospheric Sensing Experiment system to measure the mixing ratios of water vapor at various altitudes between the NASA Wallops Flight Facility, Va., and Bermuda.

Behind the drive to put more lidar in space lies the continuing development of solid-state lasers. Peter Moulton of Q-Peak Inc. in Bedford, Mass., (formerly the Boston division of Schwartz Electro-Optics) reported on four systems under development: a diode-pumped Nd:YLF laser that provides 700 mJ/pulse at 1.03 µm and weighs 7 kg, a KTA optoelectronic pulse amplifier that generates 33 W at 1530 nm, a frequency-tripled Ti:sapphire laser that generates 40 mJ/pulse, and tandem optical parametric oscillators (CdSe and KTA) that cover the ranges from 2 to 5.3 µm and from 8 to 11 µm.

Upendra Singh, of NASA's Langley Research Center in Hampton, Va., reported on a 600-mJ/pulse, 2-µm Ho:Tm:YLF laser developed for profiling winds, a project that led to the Space Readiness Coherent Lidar Experiment mission. J.J. Ewing of Ewing Technology Associates in Bellevue, Wash., reported achieving 30 percent optical-to-optical efficiency using InGaAs diodes to pump a cooled Yb:YAG laser.

James Barnes, of the NASA Langley Remote Sensing Technology Branch, discussed two new laser sources for space-based lidar: an Nd:garnet operating at 935 to 945 nm for a water vapor differential absorption lidar system, and an Nd:YLF optical parametric oscillator for an ultraviolet differential absorption lidar system to study ozone.

Lidar missions

Some of the upcoming space-based lidar missions that researchers highlighted:

  • Space Readiness Coherent Lidar Experiment, which will study wind and the feasibility of using a 2.1-µm Doppler lidar for a long space mission.
  • Geoscience Laser Altimeter System, which is designed to study polar ice sheets.
  • athfinder Instruments for Cloud and Aerosol Spaceborne Observations.
  • Ozone Research with Advanced Cooperative Lidar Experiment.
  • Experimental Lidar in Space Equipment, which is a Japanese effort to demonstrate technology and study clouds and aerosols in the troposphere and stratosphere.
The plenary session included invited talks that set atmospheric studies as a major focus for lidar applications. Kumar Patel of the University of California at Los Angeles discussed the use of several lasers, including carbon dioxide and free-electron lasers, for atmospheric remote sensing.

Other researchers reported ground-based measurements of aerosols, clouds, molecular density, ozone, temperature and water vapor. In an invited paper, Phillippe Keckhut of the Service d'Aeronomie laboratory in Paris, covered research using lidar to investigate climatic events in the stratosphere and mesosphere.

In addition, Argentina, Indonesia and the Philippines are using lidar systems to probe aerosols caused by forest fires, agricultural fires and slash-and-burn clearing.

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