Note: Descriptions are shown in the official language in which they were submitted.
CIRCULATING RAMAN-MEDIA LASER ~DAR
MET~O~ AND APPARATUS ~OR REMOTE MEASUREMENT
OF GASES IN THE ATMOSPHERE
ACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the deteetion of
~ases using laser beams. More particularly, this
invention relates to the remote detection of gases
in the atmosphere using the outputs of excimer/dye
lasers which have been frequency-shifte~ in
circulating-media Raman cells.
Z. Description of the Prior Art
The det~ction of gases in the atmosphere
using laser beams in the infrared region is known to
those skilled in the art. Canadian Patent No.
808,760, for example, describes the detection of
hydrocarbon gases using noble gas lasers such as a
helium-neon laser mounted on an aircraft~ The
method comprises the use of two laser beams of
slightly different wavelengths, either from the same
laser or from different la ers. One preselected
wavelength is highly absorbed by the gas to be
detected while the other is not highly absorbed,
thereby providin9 a differential coeficient. The
laser beams pass through the gas in question and are
reflected bac.k ~o a common detection source which
measures the intensities of the two beams. Any
difference ;n the measured intensities determines
the presence and quantity of ~he gas in question.
~ust, water droplets and other li~ht scattering
materials in the atmosphere act in a similar manner
on the two beams and are thus factored out.
~ hile such detection schemes should be
highly satisfactory in determining the presence or
absence of preselected gases, in practice they are
restricted by the limited number of wavelengths
emitted by such lasers and the number of interfering
gases possibly present in the atmosphere either
alone or in combination with other gases. For
example, the most popular and frequently used of
such lasers, the helium-neon laser, only emits 10
possible wavelengths in the frequency band where
detection takes place. While these particular
wavelengths have been found selective with respect
to methane, they are not useful for detection of
ethaner for example.
Other lasers may be substituted for the
helium-neon laser to permit selective detection of
other gases such as ethylene, which cannot be
detected satisfactorily with the helium-neon
laser. ~s described by E.R. Murray and J.E. van der
Laan in an article entitled "Remote Measurement of
Ethylene" in A~lied O~ics, Volume 17 at page ~14
~March 1, 197~) and in U.S. Patent No. 4~450r356y
or example, the detect;on o ethylene or other
gases in the atmosphere may be accomplished by
selective absorption of wavelengths emitted by a C02
laser. Also, as tau~ht by Alden et al. in "Remote
Measurement of Atmospheric Mercury Using
Differential Absorption Lidar", Optics Letters, Yol.
7, No.5, May 1982, sulphur dioxide, nitrogen dioxide
and mercury may ~e remotely measured in the
ultraviolet spectral region using an Nd:YAG laser.
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However, in such systems or the detection
of gases in the atmosphere, there is still a need
for more flexible laser systems which can emit a
sufficient number of different wavelengths in
wavelength regions for remote gas detection so that
spectral matching with the desired gases may be
accomplished. Some gases may interfere ~with the
desired gases by having differential absorption
coefficients at a particular pair of wavelengths
that are sufficiently large so that the gases cannot
be distinguished from each other. Accordingly, more
wavelengths in the desired micron wavelength region
are necessary so that more particularized wavelength
pairs for analysis of desired gases may be
selected. One such technique is disclosed by the
above-mentioned Murray et al U.S. Patent No.
4,450,356, wherein a first CO2 laser beam is passed
through a frequency doubling crystal and summed with
a second CO2 laser beam. Each CO2 laser in the
Murray et al system is capable of being tuned to 80
different frequencies to provide a total of 6400
frequencies for selection. Although this represents
a significant improvement in frequency selection, it
is desirable to provide an unlimited number of such
combinations to improve measurement accuracy.
It is known that the number of wavelengths
emitted by a laser source can be increased and the
frequency range changed by the use of Raman
shifting. For example, Alden et al disclose that
Raman shifting may be used in conjunction with an
Nd:YAG laser to reach the mercury absorption line.
In addition, Paul Rabinowitz, Bruce Perry and N.
Levinos in an artiole entitled "A Continuously
Tunable Sequential Stokes Raman Laser" describe
frequency shifting of excimer/dye laser radiation
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using a high pressure hydrogen cell with a confocal
resonator ~o produce multiple gain paths in a ~aman-
active medium. All things being equal, the system
described by Rabinowitz et al. should allow for an
unlimited number of operating wavelengths, and the
problem of limited target gas selection in the
above-described systems would appear to be solved.
However, in the application of such
technology for the detection of gases in the
atmosphere, ther~ is still a need for a system
capable of making more sensitive differential
measurements, for the present state of the art
considers only stationary Raman-active media. In
such systems with stationary Raman-active media, the
excited medium may not relax before the next firing
of the laser. ~hus, not only is the repetition rate
greatly diminished, resulting in repetition rates
too low for remote measurement by aircraft, for
example, but the beam may not be efEectively shifted
when the Raman-active medium has not had time to
relax to its ground energy level. Accordingly,
lidar systems constructed using such techniques for
wavelength optimizatio~ have been found to yield
se~sitivities lower than that useful for many
applications such as remote gas measurement.
When making measurements from a rapidly
moving airborne platform for gases present in the
near surface atmospherep such as those from chemical
plant spills, the problems of low repetition rate
lidar systems usin~ Raman-shifting in stationary
media become even more apparent. Thus, it is still
necessary to increase the sensitivity and repetition
rate of systems for remotely measurinq gases in the
atmosphere such as the above-mentioned lidar systems
of the prior art in order to achieve more reliable
measurements.
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5VMMARY OF THE INVENTION
.
The purpose of the present invent;on i5 to
overcome the disadvantages in the prior art ~ystems
noted above by increasing the sensitivity and
repetition rate at which gases in the atmosphere may
be measured remotely, as from an aircraft.
This is accomplished in accordance with the
present invention using a circulating Raman-active
medium. More particularly, it has now been
discovered that differential absorption lidar can be
used to detect a number of gases in the atmosphere
by passing excimer/dye laser radiation through a
circulating Raman-active medium so as to
significantly increase the measurement repetition
rate. ~he resultant system is capable of
effectively producing an infinite number of
different wavelengths in the 3 micron region where
numerous gases, including light hydrocarbons,
selectively absorb radiation, and this system mayj
operate at repetition rates in excess of 250 times
per second.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of
the present invention will become more apparent and
more readily appreciated from the following
description of the presently preferred exemplary
embodiment thereof, taken in conjunction with the
accompanying:drawings, of which:
FIGURE 1 is a schematic depiction of the
laser beam ~ransmitter of the preferred embodiment;
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FIGURE 2 is a schematic depiction of the
receiving system of the preferred embodiment; and
FIGURE 3 is a more detailed schematic view
of the dual chambered Raman Cell used in the laser
beam transmitter of FIGURE 1.
DETAILED DESCRIPTION OF THE PRESENTLY P~aEFERRED
EXEMPLARY EMBODIMENT
.
Referring to FI~URE 1, a laser beam
transmitter 2 is schematically shown comprising a
first excimer/dye laser source 10 and a second
excimer/dye laser source 20. Laser sources 10 and
20 comprise a high repetition rate excimer-pumped
dye laser system such as that available from Lambda
Physik in Acton, Massachusetts. One or both sources
should be tunable to a particular wavelength using
suitable tuning means such as a diffraction grating.
A laser beam 12 from source 10 is passed
through a circular polarizer 14 to increase the
Raman-interaction cross-section of the beam 12 for
interaction with the circulating Raman-active medium
31 in dual-chambered Raman cell 30 ~shown in more
detail in FIGURE 3). Laser beam 12 is passed
through calcium fluoride entrance window 32 and then
through a hole 33 in mirror 34. The beam 12 then
passes through circulating Raman-active medium 31 to
produce a beam of Raman-shifted radiation l00, with
the shifted wavelength being characteristic of the
particular Raman-active medium being employed lin
the case of hydrogen gas as the Raman-active medium;
for instance, the wavelength shift is N(41SScm)- lt
where N - ~/- 1, 2, 3,...). Beam 12 then bounces
off mirror 36 and is returned to mirror 34. This
process continues several times until beam 12 and
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Raman-shifted beam 100 both exit through hole 35 in
mirror 36, and then exit through calcium fluoride
exit window 38. Energy from beam 12 is thus
transferred to Raman-~hifted beam 100 on each pass
through the medium, and this multi-pass technique
allows practical packaging of the long interaction
length which is necessary to ~enerate appreciable
energy at the Raman-shifted wavelength.
The Raman cell 30 effects absolute
frequency changes in the inputted laser beam by
shifting the laser light's frequency in the spectral
bandwidth of the laser light. More particularlyr
different gases may be used in the Raman cell to
change the frequencies of the pump laser output by
adding a certain number of angstroms to the inputted
laser light's wavelength. This occurs since a pulse
of the inputted light interacts with a dipole moment
of the Raman-active medium to produce a non-linear
effect. In other words, different molecules of the
light experience differing strobe shifts so that it
is possible to continuously vary the frequency
output of the Raman cell 30 by keeping the Raman
active medium 31 circulating in the Raman cell 30.
Also, it is desirable to use a common gas in the
Raman cell 30 as Raman-active medium 31 to incre~se
the stability of such shifting. By thus passing the
laser beams through such a common circulating Raman-
medium 31, which is kept circulating by fans 301,
the tuning range of the inputted optically pumped
laser beams may be extended to ranges acceptable for
laser prospecting with sufficient energy without the
need for complicated apparatus.
Referring back to FI~URE 1, the Raman~
shifted laser beam 100 and laser beam 12 outputted
by Raman cell 30 are next passed through
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interference filter 40, which allows Raman-shifted
beam 100 to pass, but does not allow beam 12 to
pass. The filtered Raman-shifted radiation 190 then
is reflected by mirror 41 and passed through beam
combiner 42 to beam steering mirror 44 before being
transmitted to the mea3urement region of interest by
projection optics system 50 as shown. Beam steering
mirror 44 directs the laser beam from each laser
down to the target region along the optical axis of
the optics system 50, thereby ensuring that the
laser beam irradiated area is always kept within the
optical field of view of optics system 50.
Since a di~ferential absorption measurement
requires two ~avelengths for measurement, it is then
necessary to generate a second Raman-shifted laser
beam 120 having a slightly different wavelength than
the first beam 100. This beam must be generated
within about 100 microseconds of the first beam 100
in order to minimize the scintillation effects of
the atmosphere. This beam 120 is generated in the
second chamber of the dual-chambered Raman-shifter
30 from laser beam 22 in the same manner described
above for the generation of the first output beam
100. This double-barrelled nature of the Raman-
shiftiny apparatus 30 in the preferred embodiment is
easier to fabricate and requires less space and is
thus preferred, although not ~ecessary.
Referring now to FIGURE 2, after.travelling
to the target region, reflecting off tvpographic or
man-made targets, and returning to the measurement
platform, both beams are sequentially collected by
receiving telescope 200. Receiving telescope 200
may be a wide aperture Newtonian telescope such as
the type manufactured by Coulter Optics in
Idyllwild, California. The beams are then
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sequentially detected by a photovoltaic or
photoconducting detector 210 at the receiver. In
the preferred embodiment, a liquid nitrogen-cooled
InSb detector, available from Santa Barbara
Research, Goleta~ California, may be used for this
purpose.
The signals resulting from detector 210 are
amplified by preamplifier 220. Discriminator 2~0
then detects the presence of these signals and
supplies a gate signal to a gated analog-to-digital
converter 240. The signals representing the
received intensities of both beams are then
sequentially digitized by analog-to-digital
converter 240.
In addition to the received signal pre-
processing, data required by the data processing
system (microcomputer) 250 includes information from
power monitor system 60 and path length counter
260. Power monitor 60, with electronics similar to
the receiver, detects the small fraction of
transmitted power passing through mirror 80, and the
detected power is used for normalizing the
transmitted energy. Path length counter 260, on the
other hand, provides range information.
The signals from analog-to-digital
converter 240, power monitor 60 and path length
counter 260 are then processed by the data
processing system lmicro-computer~ 250 to determine
the amoun~ of a~sorption by the gas in question.
~his measurement is carried out on both beams, the
first beam having a ~avelength which is highly
absorbed by the gas to be detected, while the second
beam has a wavelength which is weakly absorbed. The
backscat~ered signal from the first frequency pulse
is used to calibrate the gain of the system and the
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reflectance oF the topographic target. The ratio of
the backscattered signals between the two
frequencies thus is a direot m~asure of the product
of concentration and path length/ and because the
technique involves the measurement of a differential
rather than an absolute magnitude, it is self-
calibratin~
The data processing system 250 calculates
the path-averaged concentration of the detected gas
by first forming a ratio of the return signal at the
two wavelengths, then taking the logarithm of the
ratio, and finally multiplying this guantity by the
inverse of the differential absorption coefficient
times the range to the target. The target range
varies considerably even for a given application and
is automatically established by the time of flight
of the laser pulses. The data processing system 250
may then output a continuous digital read~ut on
monitor 270, a printed hardcopy on printer 280, or
an audio alarm on alarm 290 for concentrations above
a certain threshold.
As noted above, the Raman-active medium 31
of Raman cell 30 may be kept in oontinuous
circulation by circulator fans 301; therefore, it is
possible to fire lasers lO and 20 a very short time
after the first measurement cycle has been
completed. When using previous Raman-shifter
designs employing stationary Raman-active media,
however, it is not possible to fire lasers lO and 20
so close together in time since it is necessary to
wait for the Raman-active medium to return to its
relaxed state or to replace the old gas with new gas
between laser firings, whlch requires at least 1
ms. Thus, molecules in the optical paths of
unshifted pump beams 12 and 22 in the prior art may
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not have had time to relax to their ground energy
level state before the next pulse is encounteredO
They are therefore not available to act as Raman
"scatterers" and cannot effectively shift the
wavelength of the incoming pump beam.
Since the accuracy of a lidar ~ystem
increases as the square root of the number of
measurement pulse pairs which are used, the
measurement systems which can perform a greater
number of measurements per unit time are more
accurate. Excimer/dye laser systems have been
constructed which have pulse repetition rates of up
to 250 Hz. Using Raman-shifting devices with
stationary media as in the prior art, however, it
would be necessary to limit the pulse repetition
rates of such lasers to the order of 1 - 10 Hz in a
lidar system. By contrast, in accordance with the
present invention, pulse repetition rates of 2S0 Hz
can easily be achieved by using a Raman-shifting
device in which the Raman-active medium is kept
circulating~ Thus, increases in the accuracy of
measur~ment of S to lS ~( ~0) 2 to (2S0) 2) times
are achievable in accordance with the present
invention~
Although only a single exemplary embodiment
of this invention has been described above in
detail, those skilled in the art will readily
appreciate the many modifications are possible in
the exemplary embodiment without materially
departing from the novel teachings and advantages of
the invention. For example, a single laser source
capable of firing laser pulses in rapid succession
may be used in place of the two excimer/dye laser
~3~
12
sources disclosed. Accordingly, this and other such
modifications are intended to be included within the
scope of the invention as def ined in the following
claims .
'~ r,'."
