Examples of diode laser (DL) random-modulation continuous wave (RM-CW) lidar measurements are reported. The ability of the measurement of the visibility, vertical aerosol profile, and the cloud ceiling height is demonstrated. Although the data shown here were all measured at night time, the daytime measurement is, of course, possible. For that purpose, accurate control of the laser frequency to the center frequency of a narrow band filter is required. Now a new system with a frequency control is under construction

Baba, H.

Ishikawa, N.

Sakurai, K.

Takeuchi, N.

Ueno, T.

English

NASA Technical Reports Server

N87- 10279 '
Visibility and Aerosol Measurement by Diode-Laser
Random-Modulation CW Lidar
N. TAKEUCHI, H. BABA, K. SAKURAI,***T. UENO
and N. ISHIKAWA
National Institute for Environmental Studies
Yatabe, Tsukuba, Ibaraki 305 Japan
* The University of Tokyo, Komaba, Tokyo 153 Japan
** Chiba University, Yayoi-cho, Chiba 260 Japan
*** Meisei Electric Co. Ltd., Moriya, Ibaraki 302-01
I. Introduction. In recent years, the diode laser (DL) has shown a
large development in the lasing power, the mode quality, and
reliability. Lidar systems using DL were re_rted in the appli-
cation) to the slant visibility measurementS" and the ceilo-
meter .However, the DL power is usually limited by the catastro-
phic optical damage at the cavity surface. Therefore, the peak
power of the pulsed operation is within the range of only a few
orders of magnitude larger than the cw power, and single mode
operation is difficult and the excellent features of the DL are
not sufficiently utilized in pulsed operation. In this situa-
tion, random-modulation cw (RM-CW) lidar " is suitable for the
efficient use of the small DL power. We designed a portable DL
RM-CW lidar with the detection ability of the aerosol profile up
to i. km in distance, reported the construction at the 12th
ILRC 4), and published the preliminary results_'. Here we report
the further development of the DL RM-CW lidar and its application
to the field use.
2. System. The principle of the RM-CW lidar is based on the
_-function-like feature of the autocorrelation of pseudo-random
code (we use M-sequence in this system), and the details are des-
cribed in Ref. 3).
The DL RM-CW lidar was constructed as a compact, portable
and easy-handling system. It consists of three parts: lidar head,
processor, and personal computer. The whole system can be carried
by a small van and operated by one person. The details of the
purposes of construction and the original specifications are
described in Ref. 5). The present specification is shown in Table
I. The items of improvement are
I) the power-up of the DL from 15 to 36 mW
2) improvement of the receiving optics (smaller aberration
with larger aperture)
3) change of photomultiplier to the one with a large dynamic
range
3. Outdoor measurements. Different from the laboratory measu-
rement, outdoor operation has the severe conditions such as the
variation of the power supply voltage and the large changes in
environmental temperature and humidity. Sometimes the temperature
41
https://ntrs.nasa.gov/search.jsp?R=19870000846 2020-03-23T14:42:51+00:00Z
of DL is necessary to be kept in a certain range for stable
operation In single-mode. Otherwise a ghost may appear in
lidar data. After the above improvement, weobtained a nighttime
aerosol profile over a 1 km distance. In the following, we intro-
duce some examples of the outdoor measurements.
(i) Visibility. Visibility is the important meteorological para-
meter for the aviation safety as well as an index of comfortable-
ness. Especially slant visibility is an important parameter for
the landing of the airplanes. Visibility is inversely proportio-
nal to a distance averaged value of the extinction coefficient of
the light passing through the atmosphere. Theref_{e the accurate
profile of aerosol is not required. Werner et al " monitored the
visibility at Munich Airport for a long time using a pulsed DL
lidar system. We derived the visibility based on the slope method
and compared it with the data from a transmissometer (Meisei TZE-
3T) set on the same site. An example for fast visibility change
is shown in Fig. I. The lidar shows smaller value than the tran-
smissometer due to the inhomogeneous distribution of the aerosol.
For spatially varied aerosol distribution, the exact visibility
value should be obtained_ by resolving the lidar equation accor-
ding to the Klett method b)
(2) Aerosol vertical profile. Range-corrected aerosol profile
(linear plot) with the elevation angle of i0 deg is shown in
Fig. 2. This was taken automatically with the time interval of 20
min. The integration time was 80 sec. The cross-over function
becomes 1 at about 200 m. In a case of DL, the beam divergence is
very small so that a small size of the laser beam cross section
is small enough to use a mirror for bending the direction of the
laser. Therefore if we set a mirror along the ground surface so
that the crossover function is unity beyond the mirror, we obtain
the vertical aerosol profile from the ground surface.
(3) Cloud movement. When the thickness of the cloud is not thick
enough, the structure of the cloud is easily measured by the
lidar. In Fig. 3, the rapid change of the cloud is shown. The
data are taken every 30 sec with integration time of i0 sec. A
cloud is shown at the height of i000 m. The signal near the
ground is the aerosol.
4. Summary. We report some examples of the DL RM-CW lidar
measurement. They demonstrate the ability for the measurement of
the visibility, vertical aerosol profile, and the cloud ceiling
height. Although the data shown here were all measured at night-
time, the daytime measurement is of course possible. For that
purpose, accurate control of the laser frequency to the center
frequency of a narrow band filter is required. Now a new system
with a frequency control is under construction.
ReferenceS
I) Ch. Werner etal: Application to an eye-safe laser slant visual range measuring device
at Munich Airport, 12th ILRC, B5 (1984).
2) Ceilometer model TXK-3, Meisei Electric Co., Ltd.
3) N. Takeuchi etal: Random modulation cw lidar, Appl. Optics 22, ]382(1983).
4) N. Takeuchi etal: Construction of compact RM-CW lidar systems, 12th ILRC, E9(1984).
5) N. Takeuch_ etal: Diode-laser random-modulation cw lidar, Appl. Optics 25, 63(1986).
6) J.D. Klett: Stable analytical inversion solution for processing lidar returns, Appl.
Optics 20, 211(1981).
42
Table i. Specifications of a DL RM-CW Lidar
LASER: GaAIAs-DL(Sharp)
Wavelength
Output
Driving current
Operating Temp.
Beam Divergence
780 nm
36 mW
160 mA(bias) + i00 mA(mod)
-5 _ 10°C
<0.I mrad (after collimation)
MODULATION: M-seq. random code
Clock time
Number of elements
Period
Range resolution
RECEIVING OPTICS
Telescope
Aperture
Focal length
FOV
60 ns
4095 (=212-1)
240 _sec
9 m
Schmidt-Cassegrain (reflection)
150 mm
1500 mm (eff.)
0.5-- 3.0 mrad
Opt. filter bandwidth 1.2 nm
DETECTOR: PMT Hamamatsu R928
Quantum efficiency 0.i
Amplific. factor 1X 107
SIGNAL PROCESSOR
ADC 3 bit (_ock time
Accumulation up to 2 £O
60 ns)
1985.12.9-10
A
E
v
>.
I--
.-I
!]3
U3
O° I
I
I
23:00
o--me L|DAR
u.--..= TRANSM_IF.R
i\
!, ,q, Io _, , _
':/\V
d
01:00 03:00 05:00
TIME
Fig. Comparison of visibility data between
a DL RM-CW lidar and a transmissometer.
43
E 1985.11.1344 EI-IO"
•"r t e o o o ! o o o o o
o o o u * o o o o o i o o
o o o o * a o o o e o o o o
i i o i i 0 i o i o I i o o
o I I I I o o O O o I o o o _ _*l I ! I I o o O O I I o I O o
, , , , , , , o I , . , , ] :i o ! l * i o iI o a I i o o I I o I o o I o
, , , . , , o , , , , , , , , , ,
i i o o o i i i i o e i i e i
, 0 0 , 0 , , , , , , 0 , , , ,
° o n I o I e e o ° I o e I
, , , ° , , o , , , , , , , ,
i o o i o i e i
..........
, , , ° , , ° , , , , : : •
Z _ I ° ° ° I I I I O I ° I I
, . o , ° , , ° , , , ,
.¢ o o I I o ' o i o o 0 o
o o ° i °
.... : 1L._ o i I i 0 i i i i i o i o
o , , o o o o , , , , , o , o o
o o o I o o 0 o o , , ° o i I I
o o ° , , o , , , • . , . , ,
' ' ° ...... I I I o I
...... ilt1 }o o ° i o i o o o o o pI Io o ° o o o o i
o ! o o o o o ' o
o o I o o o
I o o i o o I o
2::00 •22:00 " 23:00 24:00 01:00 02:O0
TIME
Fig. 2 Sequential measurement of aerosol
profile. Elevation angle: i0 deg e
integration time 80 sec.
o 1905.12.5
O
O"
oo
o ,..._. ,__...:"_ k___:<
i\ i\ !\
o.
21:/,0 21:41 21,42 21,43
TIME
Fig. 3 Temporal variation of the cloud thickness.
Accumulation time: 5 sec, no range correction.
44


Visibility and aerosol measurement by diode-laser random-modulation CW lidar

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