Transcript zenith program

RECONFIGURATION of the RECEIVER SYSTEM for SODIUM
DOPPLER
WIND/TEMPERATURE LIDAR
Vardan Semerjyan, Undergraduate Researcher
Tao Yuan, Faculty Mentor
Abstract
Redesign of Receiver System
The newly established USU Na Lidar has the capability to measure neutral
temperature and horizontal winds in the mesopause region (80-110 km in altitude) under
clear sky condition in full diurnal cycle. Current system setup allows the observations of
zonal (east-west) and meridional (north-south) winds, but lacks the coverage of the wind
speed in zenith direction, which is essential to estimate the vertical wind perturbations.
Since such perturbations are most likely associated with the atmospheric gravity waves
(bouncy waves) breaking events and the related energy, momentum transfer, this upgrade
of the Na Lidar system will provide further detailed information to the ongoing studies of
such gravity wave dynamics and the induced atmospheric instabilities in the MLT
(mesosphere and lower thermosphere) region. The proposed addition of the fourth channel
and the associated new design of the Lidar receiving system will not only enable the data
acquisition of the zenith channel but, the same time, will produce a more compact and
robust structure than the current design. The new design will accommodate four high
quantum efficiency (40%) Hamamatsu PMTs in the Lidar receiver, therefore, increase the
system signal/noise(S/N) ratio by a factor of two.
Motivation
Although the atmosphere is relatively stable in the vertical direction with average
vertical wind speed in the order of a few cm/s, it can experience large perturbations (tens
of m/s) in the short time scale that are mostly likely induced by wave breaking events.
The proposed addition of the fourth channel, which will be assigned to the fourth zenith
pointing Lidar beam, and the associated new design of the Lidar receiving system, will
allow the Lidar’s measurement of such large vertical wind perturbations.
Introduction
Figure 4. Mechanical design of new receiver system in Solid Edge ST
The Na Lidar is an ideal instrument for
Mesosphere and Lower Thermosphere (MLT)
dynamics research due to its capability to provide
high temporal and spatial resolution measurements
of temperature and winds, etc. The Na Lidar takes
advantage of the naturally occurring atomic
sodium in the upper atmosphere, which can be
used as tracers for Na resonance fluorescence
Lidar to determine temperatures and winds [She
and Krueger, 2007].
Figure 1. Lidar observation
Figure 5. Faraday filter (exploded view )
Figure 2. Running the beams
Figure 2. Lidar Receiver Sytsem
Two 76 cm Newtonian Starsplitter II
telescopes and one 35 cm Schmitt Cassegrain
Celestron C-14 telescope used to collect
backscattered light
Figure 3. Lidar Transmitter System
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•
•
•
•
•
CW Nd:YAG Laser
CW Ring Dye Laser Pulsed
Nd:YAG Laser
Pulsed Dye Amplifier
Doppler-Free Spectroscope
Acoustic Optic Modulator
• Iodinefilter based Chirp Monitor
During night time observations, the
assembly of filters and tubes is moved to
the position where received light passes
through the four optical tubes. During day
time the assembly changes the position
that let the two Faraday filters [Harrell et
al., 2010] slide into the beam path and
block the other two channels the same
time. One of the many challenges is the
positions of the four channels has to be
aligned with great precision so that the
chopper can open and close the four
channels simultaneously. This is critical
for the Lidar operation to block the strong
echo below 5 km altitude but also let the
useful return signal pass through.
The new receiver system consist of the
following units, mounted together on single
mounting plate.
1. Hamamatsu H7421PMTs (four)
2. Faraday filters(two)
3. Mechanical beam chopper (340D4
of SciTech Instrument)
4. Chopper blade
5. Collimating lens mounts(four)
6. Fiber optic cable connecters (four)
7. Lens tubes
8. Lens tubes for nighttime
observation
Lidar Observations
This system operates at three
different frequencies near the
Sodium D2 line, and is
currently using three-beam
setup (West, East and North
Beams) to measure LOS wind
and temperature profiles of
each Lidar beam. The LOS
winds are then converted to
horizontal wind base on the
zenith angles.
Figure 7. Old Chopper System
Currently USU Sodium
Lidar is capable of
simultaneously measuring
temperature, zonal and
meridional winds within
mesopause region over full
diurnal-cycles under clear
sky condition. It started its
regular observation of MLT
region at USU since
September 2010.
Figure 8. Na Lidar temperature measurement
from UT day 220 to day 225 in 2011
Conclusion and Future Work
The new design of the Lidar receiving system is more compact and robust structure than the
current design and enables the data acquisition of the fourth zenith channel. It will
accommodate high efficient Hamamatsu PMTs and will increase the system signal/noise
ratio by a factor of two.
• Integrate new receiver system with the existing USU Lidar System
• Upgrade the signal acquisition program and hardware to allow acquiring signals from
four separate channels
• Convert master control signal (master clock) coming from the optical chopper to 100 Hz
• Substitute electronic control of the USU Lidar with control software implemented in
Labview
References
Harrell, S. D., Chiao-Yao She, Tao Yuan, David A. Krueger, J.M.C. Plane, Tom Slanger (2010) “ The Faraday
Filter-Based Spectrometer for observing sodium nightglow and studying atomic molecular oxygen
associated with the sodium chemistry in the mesopause region. "Journal of Atmospheric and Solar-Terrestrial
Physics
She C.-Y., and D. A. Krueger “Laser-Induced Fluorescence: Spectroscopy in the Sky” Optics &
Photonic News (OPN), 35-41, September, 2007.
Acknowledgement:
Figure 6. Built Receiver System
Special thanks to Terry Zollinger for doing machining work
To learn more about USU Lidar facility visit http://www.usu.edu/crrl/