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Wavelength and
Pulse Energy: REAL's wavelength (1.54 microns) lies within
a band that is the safest in the entire optical spectrum. Photons in the
1.5-1.8 micron band are safely absorbed over several millimeters of depth
in the aqueous and vitreous humor of the eye. At shorter wavelengths
photons can reach the retina causing damage and longer wavelengths are
absorbed in near the eye's surface causing damage to the cornea. The
American National Standard for Safe Use of Lasers reports that this
wavelength band is the highest allowable eye-safe region. Therefore, by
operating at this wavelength, REAL can safely transmit very high energy
laser pulses that generate strong aerosol backscatter.
Backscatter
Depolarization: Starting in May of 2005, REAL also measures
relative depolarization of the backscattered light. This feature provides
scientists with information that can be used to infer particle shape.
Perfectly spherical particles (droplets) return REAL's
plane-polarized light in the same polarization plane. Non-spherical
particles (crystals) backscatter light in other polarization angles. REAL's receiver was upgraded to measure the
backscattered light in both parallel and perpendicular polarization planes
(i.e. a two channel receiver). The ratio of the perpendicular
backscatter to the parallel backscatter is the depolarization ratio.
Direct Analog
Detection Method: Many other lidars
are also eye-safe and excel at specific applications. For example, Coherent
Doppler Lidars utilize heterodyne detection for
extremely accurate measurement of the Doppler shift. Micropulse
lidars transmit extremely high pulse repetition
frequency and utilize photon-counting. REAL employs a standard direct
analog detection receiver. Backscatter is sufficiently strong from each
laser pulse that multi-pulse averaging and digital signal processing are
not needed. The result is a highly efficient and simple design.
Applications: REAL
excels at creating high resolution two-dimensional images of the clear
atmosphere. These images can be linked together to form high resolution time-lapse animations of
the structure and motion of the clear air as revealed by suspended
particulate matter. This is particularly useful in studies of the
atmospheric boundary layer. NCAR intends to develop REAL’s
ability to detect depolarization of the backscatter signal as well as apply
algorithms to extract the wind vector field from the aerosol structure
motion.
Clarification on Terminology: Although
the REAL acronym contains the word "Raman", an inelastic process,
REAL is an elastic backscatter lidar (i.e.
photons are elastically scattered in the atmosphere.) REAL employs
Stimulated Raman Scattering in its transmitter.
History: REAL
was initially conceived by Dr. Shane Mayor in 2001 as his Advanced Studies
Program post-doc project. Dr. Scott Spuler joined
EOL in 2002 and applied industry standard optical engineering practices to
bring the concept to reality. Mr. Bruce Morley harnessed the power of Labview to develop REAL's
data acquisition and beam-steering unit control systems. The trio draws on
a wide variety of software, electrical and mechanical engineering
assistance throughout NCAR’s EOL.
REAL made its first laboratory proof of concept in July of 2003. Success
was determined by the ability to make a time versus height image of aerosol
backscatter at 1.54 microns that contained meteorological structures such
as the entrainment zone and elevated aerosol layers.
In May of 2004, REAL made its debut field-deployment in the heart of
Washington, D. C. as part of NCAR’s Pentagon
Shield Experiment. Since then REAL has also traveled to Dugway Proving Ground in Utah
where it has demonstrated high sensitivity to low concentration of
biological aerosols. In October of 2004, NCAR, DARPA, and ITT Industries
formed a partnership to build an unattended and continuously operating
REAL.
The Technical
Challenge
For
maximum efficiency, a lidar’s laser beam
must fit within the lidar receiver’s
field-of-view (FOV). The angular width of the FOV is controlled by the
active area of the photodetector. Currently,
commercially available Indium Gallium Arsinide (InGaAs) avalanche photodetectors
(APDs) are the most cost-effective and reliable
type of detector to use at this wavelength. For a few reasons, the maximum
active area of InGaAs APDs
is currently a tiny 200 microns in diameter. When coupled with REAL’s 40 cm diameter telescope, the
field-of-view subtended in the atmosphere is very narrow (0.54 milliradians full-angle.). Therefore, the laser beam
has to have equal or lower divergence than the field-of-view.
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Journal
Articles
Spuler, S. M. and S. D. Mayor, 2007: Raman shifter optimized for lidar at 1.5-micron wavelength, Appl. Optics, 46, 2990-2995.
Spuler, S. M. and S. D. Mayor, 2005: Scanning Eye-safe
Elastic Backscatter Lidar at 1.54 microns, J. Atmos. Ocean. Technol.,
22, 696-703.
Mayor, S. D. and S. M. Spuler, 2004: Raman-shifted
Eye-safe Aerosol Lidar, Appl. Optics, 43, 3915-3924.
Conference
Papers and Posters
Mayor, S. D., 2008: Raman-shifted
Eye-safe Aerosol Lidar (REAL) observations at the
Canopy Horizontal Array Turbulence Study (CHATS). Oral presentation
& paper 18A.6 in the American Meteorological Society’s 18th Symposium
on Boundary Layers and Turbulence. 9-13 June 2008, Stockholm, Sweden.
Mayor, S. D., S. M. Spuler, and B. M.
Morley, 2008: Raman-shifted Eye-safe Aerosol Lidar.
Poster presentation P1.1
in the Symposium on Recent Developments in Atmospheric Applications of
Radar and Lidar. 88th Annual Meeting of the
American Meteorological Society, 21-24 Jan. 2007, New
Orleans.
Mayor, S. D., S. M. Spuler, B. M. Morley, T.W. Horst, E. G. Patton, and D.
H. Lenschow, 2007: First comparison of
products from the NCAR REAL and ISFF during the CHATS. Oral presentation A41E-06 at the Fall
Meeting of the American Geophysical Union, 13 Dec. 2007, San Francisco.
Mayor, S. D., S. M. Spuler,
B. M. Morley, S. C. Himmelsbach, R. A. Rilling, T. M. Weckwerth, E.
G. Patton, and D. H. Lenschow, 2007: Elastic backscatter lidar observations of sea-breeze fronts in Dixon,
California. Paper 8.5 in the
Seventh Conference on Coastal Atmospheric and Oceanic Prediction and
Processes, American Meteorological Society, 10-13 Sept. 2007, San Diego.
Mayor, S. D., B. M. Morley, S. M.
Spuler, S. C. Himmelsbach,
D. Flanigan, T. M. Weckwerth,
and T. Warner, 2007: Elastic Backscatter lidar observations of a gust front passage over
Washington D.C. on 7 May 2004.
Paper 9.7 in the Seventh Symposium on the Urban Environment,
American Meteorological Society, 10-13 Sept. 2007, San Diego.
Spuler, S. M. and S. D. Mayor, 2007: Eye-safe aerosol lidar
at 1.5 microns: progress towards a scanning lidar
network, SPIE Lidar Remote Sensing for
Environmental Monitoring VIII, Paper 6681-01, San Diego, CA.
Mayor, S. D., S. M. Spuler, B. M. Morley,
2007: Raman-shifted Eye-safe Aerosol Lidar.
2007 DOE-ARM Science Team Meeting, Monterey
CA. 26-29 March. (Poster)
Mayor, S. D., S. M. Spuler, B. M. Morley, E. Loew,
T. M. Weckwerth, S. DeWekker,
D. J. Kirshbaum, 2006: REAL: 1.5 micron wavelength scanning
polarization lidar, 23rd International Laser
Radar Conference, Nara Japan, 24-28 July. Pages 161-164. Best Poster Award.
Poster 2P-25.
Spuler, S. M. and S. D. Mayor, 2006: High-energy multipass
forward Raman shifter as an eye-safe laser source for lidar,
23rd International Laser Radar Conference, Nara
Japan,
24-28 July. Pages 133-136.
Poster 2P-16.
Mayor, S. D., S. M. Spuler, and B. M. Morley, 2006: Three Generations of Raman-shifted Eye-safe
Aerosol Lidars,7th International Symposium on Tropospheric
Profiling, 11-17 June 2006, Boulder, CO. Pages 8.33-8.34 of extended
abstracts. Poster 8.22-P.
Mayor, S. D., S. M. Spuler, and B. M. Morley, 2005: Scanning eye-safe
depolarization lidar at 1.54 microns and
potential usefulness in bioaerosol plume
detection. SPIE Lidar Remote Sensing for
Environmental Monitoring IV, Paper 5887-23, San Diego,
CA.
Mayor, S. D., S. M. Spuler, and B. M. Morley, 2004: NCAR's New Raman-shifted Eye-safe Aerosol Lidar (REAL). Paper S20-10 in ESA SP-561, Reviewed
and Revised Papers Presented at the 22nd International Laser Radar
Conference, Vol. 1, 12-16 July 2004, Matera, Italy. 53-56.
Mayor, S. D., S. M. Spuler, J. R. Fox, T. D. Rucker, B. M. Morley,
2004: NCAR's New Raman-shifted Eye-safe Aerosol Lidar. Presented at the 16th Symp.
on Boundary Layers and Turbulence, 9-13 August
2004, Portland, ME.
Related
Material
Mayor S. D., P. Benda, C. E. Murata, R. J. Danzig, 2008: Lidars: A Key Component of Urban Biodefense,
Biosecur. Bioterror., 6, 45-56, DOI:
10.1089/bsp.2007.0053.
Refaat, T.
F., S. Ismail, T. L. Mack, M. N. Abedin,
S. D. Mayor, S. M. Spuler,
and U. N. Singh, 2007: Infrared
phototransistor validation for atmospheric remote sensing application using
the Raman-shifted eye-safe aerosol lidar, Opt. Eng., 46, August, 086001,
DOI: 10.1117/12.774553
Warner, T., P. Benda, S. Swerdlin, J. Knievel, E. Argenta, B. Aronian, B. Balsley, J.
Bowers, R. Carter, P. A. Clark, K. Clawson, J. Copeland, A. Crook, R. Frehlich, M. L. Jensen, Y. Liu, S. Mayor, Y. Meillier, B. Morley, R. Sharman, S. Spuler,
D. Storwold, J. Sun, J. Weil, M. Xu, A. Yates, and Y. Zhang, 2007: The Pentagon Shield Field Program – Toward Critical
Infrastructure Protection. Bull. Amer. Met. Soc., 88, 167-176.
(DOI:10.1175/BAMS-88-2-167)
American National Standards Institute, "American National Standard
for the Safe Use of Laser, ANSI Z136.1-2000" (American National
Standards Institute, New York,
2000), p. 163.
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