He Lidar for LALO
He lidar has been a method of investigating the Earth’s thermosphere ever since it
was originally simulated by Gerrard et al., 1997. The method is to resonate with the He
(23S) metastable state which has a significant population between 250 and 750 km for
resonant remote sensing. Ground based spectroscopists have investigated the resonant
emission due to solar scattering in the upper atmosphere (e.g. Tinsley, 1968). The He
(23S) is energetically ~19eV above ground state, and is produced in the Earth’s upper
atmosphere by solar EUV photons as well as photoelectrons, and has a lifetime of several
10’s of hours. Consequently, the He metastable has a persistence in the upper
atmosphere at low and mid latitudes, and particularly in the winter when the winter He
bulge persists in the upper atmosphere.
Waldrop et al. [2005] performed a
chemical analysis study and deduced the
state was quenched significantly below
300 km (Fig 1.) Knowledge of the
production/loss enables further
calculations of the populations of the
metastable for two sites at low latitude,
both likely candidates for fielding such a
lidar for thermospheric investigations.
Figure 2 is a plot of metastable
populations vs. altitude for both Arecibo,
PR and Huancayo, Peru.
Note that the peak population is near 400 km and slowly drops in density with altitude
above that altitude of peak population for the SZA~100o.
The considerations to implement lidar observations in this region are unique and
unprecedented. Narrow-band Doppler lidar capabilities would be able to investigate the
He (23S) line shape and shift for measurements of wind, temperature, and non-Boltzmann
distributions of line shapes where source distributions of the observed volume may be
different. Considerations are also to transport the system to the Auroral regions where
secondary electrons are produced in auroral arcs producing large quantities of the
metastable. Those distributions of the metastable which are time are immersed in ion
flows due to electric fields along arcs would also be observed.
Laser technologies are evolving with solid state technologies to be able to tune to
wavelengths not heretofore feasible for lidar remote sensing. The laser described here is
a MOPA (Master Oscillator Power Amplifier). In this case, the seeder source for this
laser is a DBR (Diode Bragg Diffraction) laser diode. The diode is fabricated so that the
Bragg diffraction is tuned to transmit at roughly 1083 nm, with a few mW of power.
Gain is provided by multi-stages of power diode pumped Yb fiber amplifiers. The seeder
is ‘locked’ to the He (23S) peak resonance. This is accomplished by generating a gas cell
of He, exciting a significant population to the He (23S) with an RF discharge, and then
tuning a small beam of the source emission through the cell and locking on a diode sense
of the resonance peak [Carlson et al. 2008]. The center wavelength of the DBR is then
controlled by both the temperature and the current applied to the DBR. The laser lock
controller developed by Carlson et al. works reliably. The first laser produced ~10 W cw
with a bandwidth of ~150 MHz, and the latest configuration is producing ~40 W
[Mangognia et al., 2011].
Fig 3. A master oscillator, power amplifier configuration with an overall gain of 36 db,
10 W single mode output, with 150 MHz bandwidth.
Fig 4. A scan and lock to the 1083.032 nm resonance.
The Doppler broadened line shape for the thermosphere He metastable is shown
in
5.
Fig.
The
Fig 5. A simulation of the
thermospheric
He metastable line width for expected
methods
used in Doppler
resonance measurements
thermospheric
temperatures.
of Na has routinely performed 3-frequency, and
that methodology would be similarly implemented
to perform He Doppler measurements.
Fig 6. The technology for 3-frequency shifting
an order to deduce the population Doppler
width and shift is accomplished by Acoustic
Modulators.
a proven technology.
Testing for He (23S) currently
uses bistatic methods, in which the laser is
separated from the transmitter by some
distance, d, and the receiving telescope
images the beam, which has range
information by the position of the beam
element in the image (see Fig 7). This
method works reasonably well for zenithoriented beams where vertical winds and
temperatures might be sampled, but it is
more challenging to construct the off-axis
sampling.
Pulsed fiber lasers have power
limits due to the limitations of the fiber to
be able to handle high energy densities
associated with pulsing; however, those
technologies are being worked on by a
number of fiber groups including our own
at the University of Illinois, and likely by
the time we field a LALO facility, this
Fig. 7. A bistatic configuration used with a type of laser will be available with a
CW laser beam.
pulsed capability. The laser may also be
chopped to produce a pulse. A 100 W beam can be chopped with a 20% duty cycle, or
20 W average power. A pulse of 100 km would be 0.66 ms with a period of 3.30 ms.
There is little or no return between 30 and 250 km, with a major interest in sampling at
300-800. With this type of chopped beam, a traditional pulse-gated return lidar can be
implemented for He.
A further
discussion of bistatic lidar
is described. The method
is valuable for testing and
large altitude integrals. A
larger baseline (1.5 km)
will be used in Chile with
the SOAR telescope (see
Fig 10).
Fig 9. A projection of pixels onto the U of I telescope
for a 100 m baseline between the transmitter and
receiver.
Bistatic Configuration
θs=1.45 mrad
θs=0.1 mrad
SO
AR
h = 171m
d = 1595m
AL
O
Fig 10. The He metastable configuration for SOAR
in Chile.
He lidar adds unique capabilities to measure the vertical wind in the
thermosphere. The capabilities are extended to a reference of a few m/s, or
temperatures of a few K with statistical samples of ~5000 counts. For long
exposures, there is an inherent advantage with pulse-gated return lidar. The
comparison is made to the time spent on a range volume for a time-gated return,
versus starring at that volume from the ground for the duration of that integrated
volume. The time-gated return for a 50 Hz, 1 W with a 1 km range gate results in
an integration time of signal, background and sensor dark count, of .33 ms for 1
sec of pulses or 1 W. The background and dc integration time for a bistatic
configuration, for a CW laser that is 1 W average power for 1 s, is 1 s, or 3000
times longer than the traditional pulsed system.
The current technology for 1083 He cw Doppler laser transmitters is stable
in locking, to a few m/s. The laser transmitter has been built and laboratory
demonstrated.
As in all lidar, the Power*Aperture product defines the system, and the
laser is available, as is the technology for the aperture. The LALO will truly
enable quality He lidar measurements to explore, for the first time, the
thermosphere and exosphere with lidar.
It should be asked how else these measurements can be made. Clearly, a
space-based limb interferometer can provide some useful information, but likely
not vertical velocity. Upper thermosphere and exosphere measurements have not
been accomplished with the temporal, spectral, and spatial resolution necessary to
address the aeronomic problems facing the community.
AGU style
Gerrard, A. J., T. J. Kane, D. D. Meisel, J. P. Thayer and R. B. Kerr,
“Investigation of a resonance lidar for measurement of thermospheric
metastable helium,” J. Atmos. Solar-Terr. Phys., vol. 59, pp. 2023-2035, 1997.
Waldrop, L. S., R. B. Kerr, S. A. Gonzalez, M. P. Sulzer, J. Noto and F.
Kamalabadi, “Generation of metastable helium and the 1083 nm
emission in the upper thermosphere,” J. Geophys. Res., vol. 110,
A08304, 2005.
Tinsley, B. A., "Measurements of twilight helium 10,830 Å emission,”
Planet. Space Sci. vol. 16, pp. 1103-1107, 1968.
Mangognia, T., and G. Swenson, Resonance Fluorescence He LIDAR. Poster session
presented at: Joint CEDAR-GEM Workshop. The 26th Annual Summer CEDAR
Workshop. 2011 Jun 26 – Jul 01; Santa Fe, NM.
Carlson, C.G., P. D. Dragic, R. K. Price, J. J. Coleman, and G. R. Swenson, "A
Narrow-Linewidth, Yb Fiber-Amplifier-Based Upper Atmospheric Doppler Temperature
Lidar," Selected Topics in Quantum Electronics, IEEE Journal of , vol.15, no.2, pp.451461, March-April 2009
Carlson, G., P. D. Dragic, G. R. Swenson, L. Waldrop, J. J. Coleman, and R. K.
Price,1083 nm Lidar for Observations of Temperature in the Lower Thermosphere.
Poster session presented at: ???