Characteristics of the Airborne
UV DIAL System
The
airborne UV DIAL (Differential Absorption Lidar) system consists
of two frequency-doubled ND:YAG lasers that are used to pump two
high efficiency dye lasers which are frequency-doubled into the
UV to produce the DIAL on-line (289 nm) and off-line (300 nm) wavelengths. The
residual 1064-nm beams from the frequency-doubling process of the
Nd:YAG lasers are transmitted along with the residual visible beams
from the frequency-doubling process of the dye lasers. In
total there are four wavelengths (288, 300, 600, and 1064 nm) transmitted
simultaneously into the atmosphere below and above the aircraft
for lidar backscatter measurements. The
DIAL on and off wavelengths are produced in sequential laser pulses
with a time separation of 300 ms. This
close spacing ensures that the same atmospheric scattering volume
is sampled at both wavelengths during the DIAL measurement. The
four laser beams are transmitted collinearly with the receiver
telescope through 40-cm diameter fused silica windows in the top
and bottom of the aircraft. The
receiver system consists of two 35-cm diameter Cassegrain telescopes
(one pointed in the zenith and the other in the nadir) with optics
to direct the received signals through narrowband optical filters
and onto detectors. The
detectors include gateable photomultiplier tubes for the UV and
600-nm returns, and avalanche photodiodes (APD) for the 1064-nm
returns. Four different
detectors can be accommodated simultaneously in each direction. The
transmitter and receiver characteristics for the UV DIAL system
are summarized in Table 1, and Figure 1 shows a block diagram of
the system. The measurement
capabilities of the airborne UV DIAL system are given in Table
2.
Table
1. Airborne UV DIAL System Characteristics
Transmitter:
Pump
lasers (2) Continuum
PL-9030
Pulse separation, ms 300
Pulse energy
at 532, mJ 800
Pulse length,
ns 6
Repetition
rate, Hz 30
Transmitted
laser energy at 1064 nm, mJ 400
Dye
lasers (2 Lasers) Continuum
ND-6000
Dye
laser output energy, fundamental, each laser, mJ 220
Doubled
fundamental (UV laser energy), each laser, mJ 30
on /off
UV
laser linewidth, pm <4
Transmitted
UV laser energy, each direction, mJ 15
on & off
Transmitted
fundamental energy, each
direction, mJ 50
Receiver: Wavelength
Regions
289-300
nm 578-600
nm 1064 nm
Efficiency
to detector,3 % 31 40 31
Detector
quantum efficiency, % 21
(PMT) 8
(PMT) 40
(APD)
Total
receiver efficiency, % 6.5 3.2 12.4
Receiver
field-of-view (selectable), mrad <1.5 ≤1.5 ≤1.5
Physical p arameters
Total
weight (lbs) 3825
Dimensions
of lasers and laser support structure (L x W x H) 234” x
40” x 43”
(telescope included in length, but not height)
Power
requirements (kW) 30
1
Near 600 nm; 2 Near 300 nm;
3Includes
filter transmission for daytime operation.
Figure
1. Side-view configuration of the airborne UV DIAL system
on the DC-8 aircraft.
Table 2. Ozone and aerosol measurement parameters for airborne UV DIAL
system.
Vertical resolution
Aerosol vertical averaging interval: 60
m
Ozone
vertical averaging interval: 300
m
Horizontal resolution
Aerosol
horizontal averaging interval: 2
seconds or approximately 470 m
Ozone horizontal
averaging interval: 300
s or approximately 70 km
Measurement accuracy
and precision
Aerosols: Accuracy
and precision: 1%
Ozone: Accuracy:
10% or 2 ppbv, whichever is larger
Precision: better
than 5% or 1 ppbv, whichever is larger
Measurement range
Aerosols: Near
surface to 750 m below aircraft and 750 m above aircraft to
10-15 km above
aircraft
Ozone: 300
m above surface to 900 m below aircraft
and 900 m above
Aircraft
to 10-15 km above aircraft
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