19-04-2013, 04:47 PM
The Use of Lasers in Pollution Monitoring
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Abstract
Optical techniques have opened up new possibilities
in air pollution monitoring because of their remote-sensing capability,
very high specificity, and short observation time. Techniques
involving the use of lasers include Raman scattering, emission either
from resonantly excited or from hot gases, and resonant absorption.
Unique advantages in these applications are provided by the recently
developed tunable lasers, including organic dye lasers, parametric
oscillators, spin-flip Raman lasers, and semiconductor lasers.
The absorption technique which promises to have the widest
range of application has been tested in the laboratory by using
tunable diode lasers. High-resolution absorption spectra have been
measured for SF6, NH3, and CM4 at 10.6 pm, SO2 at 8,6 pm, CO at
4.7 pm, and NO at 5.2 pm, and the C2H4 (ethylene) content of automobile
exhausts has been measured by means of a derivative technique.
A sensitivity sufficient to detect one ppm of NH1 in air has
been achieved in a 10-cm-long gas cell.
INTRODUCTION
O PTICAL techniques for the monitoring of atmospheric
pollution have advantages over the
presently used wet chemical methods, since they
can be used for remote sensing and also because they
are instantaneous by comparison. Of greatest interest
is the infrared spectral region from about 2 to 20,m,
because nearly all of the gaseous pollutants have vibration-
rotation resonances in this region which can be
useful for both absorption and emission measurements
[1]. Due to strong absorption of the principal polyatomic
constituents of clean atmosphere C02 and H20,
certain bands in this region (2.5 to 2.9 Am, 4.2 to 4.4
Am, 5.5 to 7.5 ,um.
RAMAN SCATTERING
The Raman technique was proposed [4] and experimentally
tested by Inaba and Kobayasi using a Qswitched
ruby laser at 6943 A [5 ], and also a Q-switched
doubled Nd-YAG laser at 5300 A [6]. For the doubled
Nd-YAG laser with a pulsed energy of 10 J, SO2 concentrations
of 20 to 30 ppt have been detected at a
range of 100 m. The method has also been used for
measuring the major atmospheric constituent concentrations,
including N2 by Cooney [7], H20 by Melfi [8],
and CO2 by Kobayasi and Inaba [5].
One advantage of this technique is the possibility to
detect the presence of different gases by using a single
wavelength laser. Also, the laser and the receiver can be
at the same location, such as the single mobile unit used
in the measurements of SO2 [6]. The absolute concentration
of each pollutant can be easily determined by
comparing the backscattered intensity with the N2 and
02 Raman lines, and spatial resolution can be achieved
by range-gating the receiver.
ABSORPTION
The absorption technique has perhaps the widest
range of application because it can be used both for
point sampling and remote sensing. For long distance
measurements one possible disadvantage is the necessity
for a remote detector or a retroreflector or at least a
scatterer. Wherever this requirement can be met, the
technique is advantageous because it is the most sensitive
and requires the least amount of laser power; consequently,
it promises to be the simplest and least expensive
system. For point sampling, powers as low as
1 ,uW can be used, whereas for remote sensing 1 mW
should be quite sufficient. Semiconductor diode lasers
which can readily provide powers in this range have
proven to be particularly convenient for the absorption
technique [11], [13].
Spin-Flip Raman Lasers
In spin-flip Raman lasers the output wavelength
differs from the pump wavelength by the energy required
to make a transition between magnetic spin
states of electrons in a semiconductor. At present this
has been achieved in InSb pumped with either a CO2
laser [28], [29] at 10.6 u or a CO laser [30] near 5 ,.
The tuning is accomplished by changing the magnetic
field and the two ranges listed in Table II correspond to
the two pump sources. A CW output power of 1 W with
a power efficiency of 70 percent has been achieved [31].
For CW operation the linewidth is expected to be much
smaller than the 1 GHz upper limit determined by a
grating spectrometer. Since these lasers operate in the
wavelength range for vibrational transitions, they
should be particularly useful for resonance fluorescence.
Semiconductor Lasers
Semiconductor lasers at present are rather low power
devices. However, they are very promising for use in
absorption spectroscopy and also as local oscillators in
detecting thermal radiation from hot gases, because of
their relative simplicity, spectral purity, and convenient
means of tuning-by changing the current in the diode,
for example. As shown in Fig. 3, semiconductor lasers
can be tailored to emit at the desired wavelength by
choosing the appropriate composition of one of the different
semiconductor alloys. Diode lasers now cover
continuously the entire wavelength range between 0.63
and 34 Mm.