Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
LASER GAS ANALYZER
Field of the Invention
[001] This invention in general relates to gas monitors used, for example, for
environmental atmospheric monitoring. In particular this invention relates to
improvements in detection and measurement of gas concentrations and gas
emissions
based on tunable diode lasers.
Background
[002] Accurate monitoring of gaseous species at low concentrations is required
for a
wide range of industrial, regulatory, and academic fields. The most common
include
atmospheric chemistry, pollution monitoring, industrial process monitoring and
control,
safety, breath analysis, and agricultural research. One of the most reliable
principles for
continuous monitoring of gases is the measurement of gas absorption since most
gases
have one or more absorption lines in the ultra violet, visible or the infrared
part of the
spectrum. This technique is known as absorption spectroscopy. With this method
a beam
of light such as a laser beam that is absorbed by the gas of interest, is
directed through the
gas or a mixture of gases. The degree of absorption of the light beam is then
used as an
indicator for the concentration of the gas to be detected. Many different
spectroscopic
techniques exist, but the use of single line spectroscopy utilizing single
mode tuneable
diode lasers is probably the one giving best sensitivity and selectivity due
to its high
spectral resolution involving a low risk of interference from other gases.
[003] There are two popular spectroscopic methods of laser gas detection. In
one the
frequency of the laser is rapidly scanned across the gas absorption line by
modulation of
the laser diode current. Gas absorption results in modulation of the amplitude
of the
transmitted light and this amplitude can be measured using a photodetector and
some
simple electronics. The absorption of the laser beam on- line and off- line
may be
compared and the gas absorption and concentration computed. This is method is
referred to by several names including scanned direct absorption and rapid
scan
absorption. This method has the advantage of simplicity but it can be
difficult to establish
a zero-absorption baseline. The other popular method is called modulation
spectroscopy;
the most commonly used is referred to as wavelength modulation spectroscopy
(WMS).
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[004] In this method the laser frequency and amplitude are modulated using
laser
current as in the case of direct absorption. In addition, the laser current is
also modulated
at a second relatively high frequency. Gas absorption distorts the amplitude
of the
modulated laser light so that harmonics of the high modulation frequency
appear after the
beam has passed through a gas. These harmonics are measured by demodulating
the gas
signal. Sensitive tunable diode laser (TDL) absorption measurements have been
performed for decades with wavelength modulation spectroscopy (WMS) for a wide
variety of practical applications. With its better noise-rejection
characteristics through
laser wavelength modulation strategies, WMS has long been recognized as the
method of
choice for sensitive measurements of small values of absorption, and thus is
favored for
trace species detection.
[005] A common safety or environmental application of TDL spectroscopy is
perimeter
monitoring of industrial sites where line of sight laser beams surround the
site. Releases
of gas into the atmosphere from the site pass through the downwind perimeter
and are
recorded by the laser analyzer. For gases such as methane normally present in
the
atmosphere the instrument can compare the average concentration along the
upwind and
downwind perimeters and calculate the contribution of gas by the site.
Industrial
perimeter monitoring is characterized by long open path lengths, often
hundreds of
meters in length.
[006] It is well known that that the detection sensitivity of TDL WMS
spectroscopy is
limited by interference fringes and not by the theoretical limit given by
detector noise
[Silver]. The interference fringes stem from Fabry-Perot etalons between
reflecting or
scattering surfaces of optical elements, optical fibre end faces, and
components of
multipass cells [ Masiyano] An early quantitative analysis of the effects of
interference
fringes on detection sensitivity was carried out by [Reid]. In their study,
the authors
estimated that they could improve the detection sensitivity by at least a
factor of 5 if they
could eliminate the fringe interference.
[007] The sensitivity of TDL spectroscopy is further reduced over long open
paths by
atmospheric effects. Atmospheric gases can cause absorption that overlap with
the
absorption of the target gas and cause interference that degrades detection
sensitivity.
Moreover, atmospheric turbulence will also degrade detection sensitivity. The
optical
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effects of atmospheric turbulence on laser beams are produced by the
variations of
refractive index along the path of the laser beam because of fluctuations of
the direction,
phase, and intensity of the laser wavefront [Murfy]. These changes in the
refractive
index are caused by temporal and spatial fluctuations of temperature which
arise in
turbulent mixing of various thermal layers. Temporal and spatial variations of
the beam
wavefront cause the beam to wander and spread and scintillate in a manner
familiar to
astronomers.
[008] If the scale size of the inhomogeneity is much larger than the diameter
of the laser
beam, the entire beam is bent away from the line-of-sight and results in beam
wander or
beam steering where the beam executes a two-dimensional random walk in the
receiver
plane. inhomogeneities of the size of the beam diameter act as weak lenses on
the whole
or parts of the beam with a small amount of steering and spreading. When the
inhomogeneities are much smaller than the beam diameter, small portions of the
beam are
independently diffracted and refracted and the phase front becomes corrugated.
The
propagation of the distorted phase front causes constructive interference over
some parts
of the receiver and destructive interference over others, leading to alternate
bright and
dark areas. The locations of these bright and dark regions change temporally
and lead to
scintillation. Since the atmosphere consists of inhomogeneities of all sizes,
the laser beam
experiences fluctuations of beam size, beam position, and intensity
distribution within the
beam simultaneously. The relative importance of these effects depends on the
path
length, strength of turbulence, and the wavelength of the laser radiation.
[009] Line of sight TDL gas analyzers are configured either as transmitter-
receivers,
also referred to as bi-static, or transceivers where the transmitter and
receiver are
combined and retroreflectors [Cerex] are used to reflect the laser beam back
to the
instrument. The transceiver is also referred to as mono-static detection. For
infrared laser
wavelengths hollow retroreflectors are used. Because of practical optical
limitations the
laser path length of a transmitter-receiver is limited to path lengths of
approximately
100m or less. Transceivers with a single retroreflector are also limited to
approximately
100m or less. An array of multiple retroreflectors is required for path
lengths greater than
100m. Both TDL line of sight gas detection and long path Fourier-transform
infrared
spectroscopy (FT1R) gas detection require a retroreflector array [Cerex].
Unlike a FT1R
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analyzer, however, lasers are coherent. Laser light from each element of an
array is
projected back onto the receiver aperture where optical interference between
the collected
laser light beams can reduce the instrument's gas detection signal to noise
ratio.
[010] TDL WMS spectroscopy is well known to be sensitive to interference
fringes of
all types in the laser beam. Over an open atmospheric path these fringes may
be caused
by path optics such as retroreflectors and by atmospheric turbulence. Because
the
fabrication of retroreflectors is imperfect, distortion of the reflected laser
beam wavefront
will result in interference fringes in the laser beam. For long paths where an
array of
retroreflectors is deployed the path between the laser and each element of the
retroreflector array has a randomly fluctuating atmospheric refractive index
and this
results in random fluctuations of phase of light reaching each element. Small
phase
differences in the light reflected by each element of the retroreflector array
cause a
complex interference fringe pattern at the receiver aperture and consequently
atmospherics cause fringe fluctuation in the received light. This is manifest
as a temporal
fluctuation of WMS signal referred to as fringe noise. Fringe noise from a
retroreflector
array usually dominates the total noise in a long atmospheric open path laser
spectrometer and is typically ten times greater than all other noise sources
combined.
Retroreflector fringe noise reflects the frequency spectrum of atmospheric
turbulence and
this depends upon atmospheric conditions. The frequencies of atmospheric
fluctuations,
however, are typically below 1KHz.
[011] Mechanical methods of averaging out the interference fringes by varying
the
beam path length with an external device have also been proposed. [Bomse], for
example, suppressed etalon interference from a Herriott cell by longitudinal
dithering the
mirrors with a piezoelectric transducer. [Webster] used a vibrating Brewster-
plate to
suppress optical fringes by a factor of 30 in a single pass absorption cell.
[Chou] achieved
a 20-fold reduction in fringe amplitude by using a mirror mounted on a speaker
driven at
audio frequencies.
[012] Mechanical movement of optics is commonly used to reduce laser speckle
in laser
projection systems. Light reflected by an optically rough surface is scattered
in random
directions dictated by the topography of the surface. When the rough surface
is
illuminated by a laser, light reflected by surface features interfere and
forms a grainy
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stationary interference pattern known as speckle. Speckle, which reduces the
image
resolution of laser projectors, may be suppressed by reducing the coherence of
the
projected laser beam with an optical diffuser [Jui-Wen]. More effective
speckle
suppression achieved by rotating the optical diffuser is used in commercial
laser
projectors [Bodkin]. [Masiyano] used this method successfully to reduce
speckle in a
TDL gas sensor.
Summary of the Invention
[013] In this invention we disclose a new method of suppressing retroreflector
fringe
noise in TDL spectrometers (TDLS). According to this method the path between
the laser
and each retroreflector element is changed mechanically for example by using a
movable
support for the retroreflector. The phase of light reaching the
retroreflectors is very
sensitive to the pathlength. A pathlength change of a fraction of the light
wavelength will
significantly change the phase of the light. In this method the pathlength to
each
retroreflector is modulated by mechanical means and this modulation is both
stronger and
faster than atmospheric effects on phase. If mechanical modulation occurs at a
sufficiently high frequency retroreflector fringe noise can be averaged by
integration of
the spectrometer output over a reading period of typically one second. Since
this induced
retroreflector noise is stochastic it can be averaged down into
insignificantly small
amplitude. Surprisingly, lower frequency atmospheric fringe noise also
disappears under
these conditions of mechanical modulation.
[014] In one embodiment of this invention each of the retroreflector elements
is
mounted on a piezo-electric element activated by remote voltage source or
other
translating element. The frequency of the piezo voltage is typically several
hundred hertz
and is designed so that neither the frequency or phase of each element is
related.
[015] In another embodiment the laser beam is reflected from an electrical
scanner and
the beam is periodically deflected across the retroreflector array at a
frequency of
hundreds of Hertz.
[016] In the preferred embodiment of this invention the retroreflector array
is rotated on
the axis of the beam path. Each element of the array performs a circular
excursion
around the axis of rotation and the laser beam falling on any location of the
array now
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experiences moving retroreflectors as they perform this excursion. Each
retroreflector
reflects light onto the receiver aperture from different locations in the
illuminating beam
and repeats this every rotation cycle. In this method the beam pathlength is
modulated
randomly with time and this modulation is both stronger and faster than
atmospheric
effects on phase. The required speed of rotation of the array needed to
suppress fringe
noise varies with pathlength and ambient conditions and is typically several
hundred
RPM.
Brief description of drawings
[017] There will now be described embodiments of the invention with reference
to the
drawings by way of example, in which:
[018] Figure I is a functional schematic of an embodiment of the invention;
[019] Figure 2 is a functional schematic of a test gas absorption cell; and
[020] Figure 3 illustrates the use of this invention to reduce analyzer noise.
Description of the preferred embodiment
[021] The Figure 1 is a functional schematic of an embodiment of the invention
that
uses as a source of electromagnetic radiation a laser 102 for example a laser
that is
suitable for tunable diode laser spectroscopy. This includes lasers known in
the art as
distributed feedback (DFB), quantum cascade laser (QCL), and external cavity
laser
(ECL). The wavelength of the laser is chosen to coincide with absorption of
the target
gas. In this example the laser may be a continuous wave QCL that emits an
average of
100mw of light in the vicinity of I Oum for detection of the target gas
Acrolein. The laser
102 may be contained in an HHL (high heat load) package that also contains a
collimating lens and the necessary cooling means. Electrical power and
controls are
provided by an analyzer 160 to the laser 102 via at least one conductor 162 or
other
suitable means. The laser 102 and other optics including focusing optics 112,
window
114, beam splitter 116, and mirror 118 are contained in an externally mounted
enclosure
120. Light 106 from the laser 102 is transmitted out of the enclosure through
window
114. Window 114 should be transparent at the laser wavelength and designed to
minimize etalon fringes as is well known in the art. In this example the
window material
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may be ZnSe, it is coated to be transmissive at I Oum and is angled at 10
degrees from the
laser axis 104 and is 3" in diameter.
[022] Light 106 from the laser 102 propagates along an open path to a remote
retroreflector array 132. The retroreflector array 132 may be any suitable
retro-reflector
array such as is available commercially from suppliers such as the Newport
Corporation
or PLX Inc. The choice of retroreflector element material and the size and
number of
elements in the array depend upon such factors as the laser wavelength and
beam
divergence and the length of the path from the laser to the retroreflector
array. The array
may be made up of two or more retroreflectors. The design of the
retroreflector array
could be made by one skilled in the art.
[023] In this example the retroreflector array 132 may be made up from 27
hollow 2"
corner cubes and the length of the path is 300m. The array may be mounted on
rotator
134 which is typically a DC motor. In this example the array is mounted on a
DC
pancake motor and the rotation speed could be 100rpm to 3000rpm and may be
operated
at 200rpm. Light 108 reflected from the retroreflector array 132 propagates
back
through window 114 and is focused onto a first photodetector 152 by focusing
optics 112.
The design of the focusing optics 112 may be made by anyone skilled in the
art. In this
example the focusing optic is a 2" off axis parabolic mirror with a focal
length of 2" The
first photodetector 112 should be sensitive to the wavelength of the laser
light and could
be chosen by one skilled in the art. In this example the first photodetector
112 is mid
infrared TEC cooled Mercury Cadmium Telluride detector. Photocurrents from
this
detector are coupled to the analyzer 160 through coaxial cable 164 or other
suitable
communications channels such as wireless signals.
[024] A portion of the laser beam 142 from laser 102 is reflected by beam
splitter 116
and mirror 118 through a sealed gas reference cell 156 onto a second
photodetector 154.
The sealed gas reference cell 156 may be of conventional design. In this
example the
reference cell 156 is fabricated from a Silica tube and Calcium Fluoride
windows are
bonded to the two ends of the cell. The target gas in the cell may be Acrolein
and is used
to regulate the laser wavelength. Photocurrent flows from the second detector
154 to the
analyzer 160 through the coaxial cable 166 or other suitable communications
channels
such as wireless signals. The analyzer 160 uses photocurrents from the first
photodetector
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152 and second photodetector 154 to compute gas target concentration in the
open path in
a way well known in the art. In this example the computation algorithm is
wavelength
modulation spectroscopy.
[025] Figure 2 is a schematic of a testing apparatus. An analyzer 240 built
using the
teachings of this invention was used with a 250m path to the retroreflector
array 220. A
gas absorption cell 230 with windows 232 was placed in the beam path 204.
Target gas
flow into the cell 236 and gas flow out of the cell 238 were turned on and off
over a test
period of several hours. Over periods when the target gas was turned off
nitrogen flowed
through the cell. Retroreflector rotation was also activated and deactivated
over the
course of this several hours test.
[026] Figure 3 shows the output of the gas analyzer over the course of this
test.
Retroreflector rotation dramatically reduced analyzer noise both with target
gas and
nitrogen in the absorption cell.
[027] It was found that the embodiment of Fig. I reduced atmospherically
induced noise
in TDLS gas sensors, provided improved sensitivity of a long open path TDLS
gas sensor
reduced degradation of accuracy and precision, while using a simple method to
reduce
atmospherically induced noise.
[028] Immaterial changes may be made to what is disclosed without departing
from
what is claimed. In the claims, the word "comprising" is used in its inclusive
sense and
does not exclude other elements being present. The indefinite articles "a" and
"an" before
a claim feature do not exclude more than one of the feature being present.
Each one of the
individual features described here may be used in one or more embodiments and
is not,
by virtue only of being described
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