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Optically Pumped Far Infrared Lasers in the Atmosphere: A New Approach to Chemical Remote Sensing

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Title: Optically Pumped Far Infrared Lasers in the Atmosphere: A New Approach to Chemical Remote Sensing


1
Optically Pumped Far Infrared Lasers in the
Atmosphere A New Approach to Chemical Remote
Sensing The remote sensing of gases in complex
mixtures at atmospheric pressure is a challenging
problem and much attention has been paid to it.
In this presentation, we discuss quantitatively a
new approach that would use a short pulse
infrared laser to modulate the submillimeter/terah
ertz (SMM/THz) spectral absorptions on the time
scale of atmospheric relaxation. We show that
such a scheme has three important attributes
(1) A gain of as much as a factor of 106 in
sensitivity (2) Orders of magnitude greater
specificity (3) A favorable scaling to large
molecules Frank C. De Lucia Ohio State
University Naval Research Laboratory March 20,
2007
2
The Foundations of Molecular Remote Sensing Most
successful molecular remote sensing at great
distance Upper atmosphere and
interstellar medium Detection at
atmospheric pressure, orders of magnitude more
challenging 1. Number of available channels
104 fewer. 2. Linewidths are there
resolvable features? 3. Separation of target
absorption from atmospheric
fluctuations a 106 effect?
Linewidth ratio 10 000 / 1
3
How Hard is Atmospheric Remote Sensing? A Recent
Experimental Result from the Literature
4
A New Approach Double Resonance Modulation for
Remote Sensing
Backscatter for active THz Probe
Pulsed CO2 TEA Laser
1 km
TEA laser modulates target molrcules
Atmospheric fluctuations
THz probes plume
Active/Passive THz Probe
Problem 1 Specificity Dimension 1 Choose
IR pump frequency Dimension 2 Choose SMM/THz
probe frequency Dimension 3 Match pump pulse
sequence to relaxation of atmosphere (100
ps) gt 3-D to increase specificity Problem 2
Separation of target signature from baseline and
clutter Lock on to IR pulse sequence to reject
of atmospheric clutter - The 106 factor
5
3-D SPECIFICITY MATRIX
Time signature
IR Pump Frequency
THz Probe Frequency
6
Concepts are Easy! What about the Details? What
about the Numbers?
7
CH3F - KNOWN ENERGY TRANSFER PARAMETERS
THz Modulation
Pump and Probes


J
tr 10-10 s





































5-6







6
  • 1 modulation
  • 0 modulation
  • Net modulation























































n 1
























4-5


















































Unpopulated
5





















































4









































3
3-
4
Infrared Pump

5




5 GHz







n 0
Populated
4










3








K3
GHz

































300

















250
200
While this is an especially favorable gas for
OPFIR lasers, it is not especially favorable for
remote chemical detection
8
PUMP LASER REQUIREMENTS The Rabi frequency is
To pump 1 meter diameter cross section for full
Rabi excitation requires 1010 W for 10-10
seconds (1 J/micropulse) Laser Energy
relatively small laboratory systems of energy
0.1 J/macropulse to more complex, but compact
systems which produce 100 J/macropulse .
Available Pulse Structures from Literature
native macropulse duration of 100 ns
multigigawatt pulse trains with micropulse widths
of the order of 1 ns passively mode-locked
micropulses of duration 150 psec terawatt
micropulses of duration 160 psec
amplification and generation of micropulses with
duration lt 1 psec An example pulse train An
appropriate modelock might be used to convert a
100 ns macropulse into a train of 10 micropulses,
each of 100 psec duration and separated by 10
ns. For this pulse sequence, the peak power
of each micropulse will range from 108 W for the
small 0.1 J laboratory system to 1011 W for the
100 J system. The continuous tunability of high
pressure TEA lasers is very attractive
9
Modulated Absorption Due to Gas
Ambient gas a 10-2 cm-1 Pressure
broadening in oxygen or nitrogen 1/5 of its
self broadening gtpeak absorption
coefficient in the atmosphere is 5 times
larger. Pump saturation increases
interaction by hn/kT in the microwave
Thus, gt a sample dilution of 10-6 (1 ppm) over a
100 m (104 cm) path yields a modulated signal
absorption
At long wavelength Nl - Nu Nl (hn/kT) gt for
pumped upper state, with empty lower state
interaction is increased by kT/hn
Nu Nl
At thermal equilibrium Nu Nl (exp(-hn/kT))
10
Receiver Strategies For a receiver noise
temperature TN 3000 K and b B 1010 Hz (the
bandwidth required for the mode-locked case), PN
5 x 10-10 W. If after transmission and
scattering loss, we have 10-4 W of probe power
and a 1 modulation/absorption on this power, we
must also consider the noise associated with the
mixing of the blackbody noise with the carrier of
the returned signal . For this case Thus,
the absorbed power (10-6 W) will be 5 times the
noise for each micropulse In our example
macropulse, there are 10 micropulses/macropulse. I
n the example MiniTEA laser, 100 macropulses/sec.
11
How Special is CH3F? What about Larger
Molecules? There are 2J 1 K levels for each J
and, on average, the value of J in a spectrum at
a chosen frequency is inversely proportional to
the rotational constants. Quantitatively B
and C are inversely proportional to the molecular
moments of inertia. Additionally, the density of
these lines and their atmospherically pressure
broadened linewidths essentially eliminate the
need for the unlikely coincidences between laser
pump frequency and molecular absorption frequency
that severely limited the number of efficient
OPFIRS. For smaller molecules, the tunability of
the TEA lasers will be important. Because of the
spectral overlap at high pressure, many of the
rotational partition function problems associated
with large molecules are either reduced or in
some cases even turned into advantages.
12
What are the Scientific and Technical Unknowns?
1. The studies of OPFIR and rotational/vibratio
nal energy transfer have been done at pressure
lower by 104 to 105 (and correspondingly slower
speeds and lower pump intensities) This regime
is unexplored experimentally. 2. A potential
obfuscating effect? Non-linearities or other
higher order effects might cause the TEA laser to
modulate the IR and SMM/THz transmission of the
atmospheres ambient constituents. For example,
although the water absorption bands in the
atmosphere are far removed in frequency from the
CO2 pump, water is abundant and the bands are
strong. 3. At atmospheric pressure, do most
gases of interest have pump overlaps? To
some extent this will be a function of laser
choice 4. How does energy transfer in
overlapping spectra of large molecules work?
This is an unknown spectroscopic frontier. 5.
How specific are the relaxation times?
13
A Proposed Experiment
1 m
Mini CO2 TEA Laser
THz Detector
1 atmosphere air 10-3 10-6 dilution
IF Processing
THz cw source
50 ns
Computer
Time slice for shorter pulses
Recent advances in broad band solid state
technology have made this much more practical
14
An Infrared Alternative? 1. The fundamental
power of the proposed scheme lies primarily in
the use of the TEA laser pump to modulate the
atmosphere on a time scale related to its
relaxation. 2. The advantageous role of
spectral overlap is independent of the probe.
3. At long range, an infrared probe would allow
a much smaller probe beam diameter and
significantly reduce the TEA laser power
requirements. 4. On the other hand, the
SMM/THz is quieter, and it is possible to build
room temperature receivers whose sensitivity are
within an order of magnitude of even the limits
set by these low noise levels. It will also be
much easier to separate the pump and probe
radiation at the detector. 5. Radar signal
processing is very sophisticated.
15
Summary (1) The time resolved pump makes it
efficient to separate signal from atmospheric and
system clutter, thereby gaining a very large
factor in sensitivity. (2) The 3-D
information matrix (pump laser frequency, probe
frequency, and time resolved molecular
relaxation) can provide orders of magnitude
greater specificity than a sensor that uses only
one of these three. (3) The congested and
relatively weak spectra associated with large
molecules can actually be a positive because the
usually negative impact of overlapping spectra
can be used to increase signal strength. (4)
Laboratory scale experiments can address the most
significant scientific uncertainties a)
Possible cross coupling clutter at high pressure
b) The nature of molecular relaxation at high
pressure c) The spectroscopy of large
molecules at high pressure A draft of a paper
submitted to JQE that describes this in more
detail is available.
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