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Preparing REAL for its first scan
REAL Overview:
The Atmospheric Technology Division at
NCAR recently developed a scanning eye-safe aerosol lidar at 1.54 microns. Dr.
Shane Mayor manages the meteorological applications of the instrument
and Dr.
Scott Spuler manages the optical and system engineering aspects of
REAL. Associate scientist Bruce
Morley assists with all aspects of REAL including assembly of systems,
data acquisition and control software, and field deployments.
Light at 1.54 microns is generated by stimulated Raman scattering in a high-pressure
Methane cell following the work of Drs. Norman Kurnit and Bob Karl at Los Alamos
National Labs. NCAR built a new and improved Raman cell following design collaboration
with LANL.
REAL Objective:
We achieved our first objective of a working prototype in July of 2003
(FY03). The initial objective was to be transmitting and receiving light
at 1.54 microns (vertically) from the lab by August of 2003. Success
was measured by the ability to make a time versus height image of aerosol
backscatter at 1.54 microns that contains meteorological structures such
as the entrainment zone and
elevated aerosol layers.
Our goal for FY04 is to develop a field deployable system in a seatainer with
beam steering unit. We are also developing a second 1.5-micron channel to detect
depolarization of the backscatter. For more information on the depolarization
lidar technique, click on the following link to the University
of Wisconsin.
Transmitter Design:
The transmitter design follows that of Kurnit et al. (1997). 1064 nm light from a Nd:YAG laser is injected into a multi-pass gas cell containing methane at 225 PSI. The 1064 nm light generates 1543 nm light by stimulated Raman scattering in the methane. The 1064 light is not be focused in the cell (to prevent breakdown) and higher Stokes and anti-Stokes lines are suppressed by the transmissive properties of the mirrors in the cell at those wavelengths. The 1064 nm is removed with a prism before transmitting into the atmosphere.
Receiver Design:
REAL's receiver is entirely ray-traced for exceptional performance. The 200-micron diameter InGaAs photodiodes are relatively small for use in free-space optics. Therefore, careful specification of all optical surfaces and custom design of some components was necessary to assure an optimal field-of-view.
Related Links:
- Live Weather Webcam over FL1
- Continuum Surelite III pictures
- Receiver and data acquisition
- Pics of the Los Alamos National Labs Raman cell
- Pics of the new NCAR Raman cell
- Pics of REAL's development in our lidar lab
- Pics of Pipsqueek safety radar
About Eye-Safety:
The maximum permissible exposure (MPE) for the human eye is much greater at 1.5 microns that at shorter wavelengths because the light is safely absorbed in the cornea and lens before it can reach your retina. The energy of our 1.5 beam is significantly below the MPE based on ANSI standards. We use a safety radar when conducting tests with the 1 micron beam.
About Range-Resolution:
Several factors can limit the range resolution of a lidar system. In REAL, it is presently limited by the slow response (350 ns rise-time, 1 MHz) of our amplifer. This is equivalent to about 50 m. Fortunately, many of the other components presently used are much faster. For example, the pulse duration is 4 ns (1.2 m), the InGaAs response is 1.8 ns (30 cm) and the digitizer is capable of 100 MHz (1.5 m). Therefore, with a sufficiently fast response amplifier the other components are in place to produce 1.5 m range resolution data.
References:
Mayor, S. D., S. M. Spuler, and B. M. Morley, 2004: NCAR's New Raman-shifted Eye-safe Aerosol Lidar (REAL), To be presented at the 22nd International Laser Radar Conference, 12-16 July 2004, Matera, Italy.
Mayor, S. D. and S. M. Spuler, 2004: Raman-shifted Eye-safe Aerosol Lidar (REAL), Accepted to Applied Optics, To appear in 1 July 2004 issue.
Kurnit, N. A., R. F. Harrison, R. R. Karl Jr., J. P. Brucker, J. Busse, W. K. Grace, O. G. Peterson, W. Baird and W. S. Hungate, 1997: Generation of 1.54 micron radiation with application to an eye-safe lidar, Proc. Inter. Conf. on LASERS '97, 608-610.
Contacts:
Science
Shane Mayor Email: shane
at ucar dot edu
Engineering
Scott Spuler Email: spuler at ucar dot edu
Lidar Support
Bruce Morely Email: morely
at ucar dot edu