Welcome to the homepage of NCAR's
Raman-shifted Eye-safe Aerosol Lidar (REAL)
Click here to see preparation for the Pentagon.
Click here to see observations from this instrument.
Click here to see a log of project milestones.
PROJECT 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.
PROJECT 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.
PROJECT PICS:
Live Weather Webcam over FL1
Dual-wavelength system diagram.
1543 nm only system diagram.
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 our lidar lab
Pics of Pipsqueek safety radar
Lidar Lab Hall of Shame
A WORD 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.
A WORD 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.