A lidar facility using a TEA CO2 laser source is being developed at the ENEA Laboratories for Atmospheric Studies. The different subsystems and the proposed experimental activities are described

Barbini, R.

Ghigo, A.

Giorgi, M.

Iyer, K. N.

Palucci, A.

Ribezzo, S.

English

NASA Technical Reports Server

N87-10366
CO_ LIDAR SYSTEM FOR ATMOSPHERIC STUDIES
R.ZBarbini, A. Ghigo*, M. Giorgi, K.N. lyer**,
A. Palucci*, and S. Ribezzo
ENEA, TIB, Divisione Fisica Applicata, P.O. Box
65, 00044 Frascati (Rome), Italy
ABSTRACT
A Lidar facility using a TEA CO2 laser source is being developed at
the ENEA Laboratories for atmospheric studies. The different subsystems and
the proposed experimental activities are described.
INTRODUCTION
The Lidar technique has been well recognized as a potential tool for
atmospheric studies /I/. Among the various types of Lidar measurements, the
DIAL technique is particularly suited for pollution monitoring and measure-
ment of trace constituents because of its high sensitivity and long ranges
/2/. The 9-12 _m IR spectral region is rich with specific absorption
signatures of many atmospheric pollutants, it is relatively eye safe and
the atmospheric trasmittance is high in this region. Coherent (Doppler)
CO_ Lidars also find unique application for precise wind measurements both
z
from ground based and airborne platforms /3/.
LIDAR FACILITY
a) Lidar Transmitter
The Lidar system (Fig. I) /4/ uses a tunable pulsed TEA CO_ laser source
in theZg-12 um
Fig. 1 - Layout of the LIDAR/DIAL facility.
region as its trans-
mitter. The transmit-
ted beam should have
low divergence to
minimize the illumi-
nated area. The beam
should also have nar-
row pulse length to
improve the range
resolution. A laser
source with these
characteristics has
been achieved using
the Self Filtering
Unstable Resonator
(SFUR)/5/ concept.
In this configura-
tion (Fig. 2) ,a
(*) ENEA Guest.
(**) ENEA Guest. Permanent address: Phys.Dept. Saurashtra University,
360005 Rajkot (India).
287
https://ntrs.nasa.gov/search.jsp?R=19870000933 2020-03-23T14:34:19+00:00Z
.-.tt--L
Fig. 2 - The C02 SFUR resonator'. MI,pM_ are
mirrors; D = pyroelectric detector; photon
drag detector; C.W. = CO2 injection laser; PZT =
= piezo transducer; L = Tens; G = grating.
plane mirror with a
suitably chosen hole,
placed at the confo-
cal point of a nega-
tive branch unstable
resonator, limits
the output beam to
single transverse
mode with small
divergence. Diffrac-
tion at the hole ef-
fectively counteracts
the focusing action
of the negative branch
cavity thereby avoiding
the hot spot and asso-
ciated gas breakdown
or damage to the op-
tical components.
The line tunability
of the cavity can be
achieved using a
plane grating with
a lens in front of
it (Fig. 2) which replaces one of the cavity mirrors. Alternatively a
concave grating can also serve the same function. However, a major
problem in this case is the appearance of astigmatism in the output
beam: work is under way to correct this aberration. Single longitudinal
mode locking has been achieved by injection of an external CW CO2 laser.
Insertion of an intra-cavity low pressure section is under way. The
measured parameters of the laser source along with other Lidar system
parameters are listed in Table I /6/.
b) The Receiving System
The signal backscattered by the naturally occurring aerosols in the
atmosphere (for range resolved measurements) or returned by a calibrated
or topographic target (in path averaged measurement) is collected by a
telescope (Fig. 3) and focused on the HgCdTe (SBRC) detector cooled to
IN2 temperature. The detected signal is amplified and fed to the data
processing system.
c) Data Processing System
The analog signal from range resolved measurement is digitized by an 8
bit transient digitizer (Transiac) at a sampling rate of 100 MHz and
stored in the computer (PDP 11/24) via CAMAC data acquisition system.
288
TABLE Z - CO2 T,ZZ:_/DT.AZ, S_S'I'_!
Laser source: TEA CO2 gr_ing _uned
Pulse ene_
Pulse vidth
Pulse repetition rate
Beaa divex_eace
(I_LIP angle)
Receiver: Telemco_e
BewConi,-- d - 33
f#'3
F_V (adjust&hie) m 1 wad
De_ec_or: H_dTe @ 77 oK
2
Area _
BEP (band I0-8.- i0-12 W
in_e_Ced )
-9.-11== /
g - 2.7 ,T T_
oo
"t =80 as
f=l'. 5Hz
e = o.65 m-_
d)
Data AquSsition System
_ansienc digicize_- a) Tek_rou/x rood.7912
Sampling race 1 GHz
b) Traasiac
Sampling race 100 MHc
Resolution 8 bit"
CAMACin_erface with
PDP 11/2_ c_.pu_ar.
In the case of path averaged mea-
surements using targets the return
signal is digitized by Tektronix
7912 transient digitizer having
a sampling rate of up to I GHz.
The CAMAC system also controls
the measurement cycle, telescope
movements and other instrument
settings.
Calibration
Since the Lidar system employs
the backscattering from the aero-
sols to provide the return signal,
precise knowledge of the aerosol
backscattering coefficient B is
necessary at the different CO2
wavelengths. In order to obtaln
LASER
Fig. 3 - Telescope
289
this, the Lidar system has to be calibrated with targets of known reflec-
tivity. We are planning to make measurements of the reflectivity p* and
its spectral and angular dependence with various targets, such as flame
sprayed Aluminium, flowers of sulfur, Silicon Carbide sand paper which
are expected to be Lambertian surfaces,in the 9-12 um region.
The Lidar beam will be directed horizontally through the atmosphere to
obtain backscatter coefficients of aerosols well below the boundary
layer. Such measurements will also be used to study the amplitude and
phase perturbations to the propagating beam introduced possibly by atmo-
spheric turbulence using optical image processing techniques.
e) Absorption Measurements
For the Differential Absorption Lidar (DIAL) it is necessary to know
the different absorption cross sections which depend on the laser line
shape, pressure broadening effect and interference. An experiment to
introduce a cell containing the desired gas at controlled pressure into
the laser beam path and to measure the absorption coefficient at diffe-
rent CO2 laser wavelengths is being set up. Preliminary measurements
for ozone will be reported.
f) Wind Measurements
It is planned to incorporate heterodyne detection in the future whereby
the Lidar can measure wind velocity to a precision of the order of
a m/s. Coherent detection also improves the sensitivity of pollution
measurements and increases the range.
REFERENCES
I) - D.K. Killinger and A. Mooradian: in "Optical and laser remote sensing"
eds. Springer-Verlag (1983)
2) - R. Baumgartner and R.L. Byer: Opt.Lett. 2, 163 (1978)
3) - J. Bilbro et al.: Bull .of the American Meteorogical Soc. 65, 4,
April 1984
4) - R. Barbini et al.: paper presented at X Int.Conference on Infrared
and Millimeter Waves, Lake Buena Vista (USA), December 1985, Conferen-
ce Digest p. 71.
5) - P.G. Gobbi and G.C. Reali: Opt.Comm. 52, 195 (1984)
6) - R. Barbini et al.: paper presented at Lasers'85 Conference, Las Vegas
(USA), December 1985
290


CO2 lidar system for atmospheric studies

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