Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION:
Gas Detector
NAME(S) OF INVENTOR(S):
John Tulip
FIELD OF THE INVENTION
This invention relates to methods and
apparatus used in laser absorption spectroscopy.
BACKGROUND OF THE INVENTION
In laser absorption spectroscopy, absorption
of light emitted from a laser transmitter during
passage through a target zone is detected by reception
of the light at a laser receiver and analysis of the
received signal in a signal analyzer. Numerous methods
of laser absorption spectroscopy are known in the art.
One such method achieves very high
sensitivity, namely laser absorption spectroscopy
using modulation detection, in which a laser diode is
current modulated at a high frequency. This results
in the optical frequency of the laser being modulated
at the same frequency as the current. It also causes
light amplitude modulation at the same frequency. The
frequency modulated light is emitted from the laser
diode, passed through a target zone, which may or may
not contain a gas or gases of interest and received at
a detector, which contains a photo detector. The gas
or gases of interest will have an absorption spectrum
containing one or more lines or frequency bands in
which light of that frequency is absorbed.
As the laser light frequency scans across
the gas absorption lines, the absorption varies. The
challenge in the art is to see the small amplitude
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change in light level caused by gas absorption as the
laser wavelength is scanned across the gas line above
the amplitude variations caused by the laser diode.
The method depends upon the nonlinear
absorption change as the laser line scans across the
Lorentzian absorption line. In one conventional
method, harmonics of the modulation frequency are
measured. The photo detector circuit will see second
third, fourth, etc. harmonics of the laser modulation
frequency caused by the nonlinear gas absorption.
Laser amplitude modulation is dominated by the
fundamental modulation frequency so it does not swamp
out the relatively weak harmonics. In another
conventional method, the laser is modulated at two
frequencies, which is referred to as the "two tone
method." Nonlinear absorption will mix these
frequencies so the photodetector sees a frequency
component, which is the difference between the two
frequency components.
Common to all of these techniques is that
the detecting circuit must select a particular
frequency component and reject the rest. This is known
as homodyne detection. In the art, this is done by
taking a local oscillator at the required frequency
and mixing it with the detected signal. The mixer
will generate a d.c. or low frequency output, which is
easy to isolate using a low pass filter. A detected
signal containing frequency components wo, w1, w2, w3,
etc is mixed with frequency component w0, which is
taken directly from the current modulator for the
laser diode. The dc output (w0-w0) from the mixer is
isolated with a low pass filter and the level of this
signal provides an indication of the presence of a
target gas in the target zone.
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It is also known to simultaneously modulate
the diode current at a relatively low frequency using
a ramp. This ramp has a relatively large amplitude so
it will scan the laser frequency through the
absorption line. In this way it is not necessary to
control the laser frequency so that it exactly
coincides with the gas absorption line, which is
difficult. The detected high frequency signal under
these conditions is not at a d.c. frequency, but is
modulated as the laser scans across the absorption
line. This results in the well known "W" shaped
detected waveforms.
In the art the required local oscillator is
generated by taking the laser modulation signal and
modifying it to give the desired local oscillator, as
for example shown in Koch, United States patent no.
5,301,014, in which the second harmonic signal is
detected. In this case the local oscillator is formed
by taking the diode/laser modulator signal and passing
it through a frequency doubling circuit. As a result
the local oscillator has fixed amplitude and phase.
The use of a mixer to detect a chosen
frequency is sensitive to phase. The mixed output is
maximum when the signal and local oscillator are in
phase and zero when they are 900 out of phase. This
is referred to as phase sensitive detection. This
method is preferred because it results in high signal
to noise ratio. The electrical random noise passing
through a filter is proportional to the square root of
the bandwidth so that a small bandwidth filter results
in a low noise level. If the filter is tuned to the
signal, it will have minimal effect upon the signal so
that a narrow bandwidth filter will provide a high
signal to noise ratio. It is, however, difficult to
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construct electrical filters with a high Q-value,
which is the ratio of the signal frequency and
bandwidth. However the mixing circuit used in phase
sensitive detection shifts the signal frequency to a
low value close to d.c. In this case it is possible
to use a relatively low Q low pass filter and obtain
a small bandwidth and random noise throughput.
Since phase sensitive detection depends upon
the relative phases of the signal and local
oscillator, these phases must be adjusted and then
maintained. For fixed path length applications the
phase of the signal is constant so that adjustment is
usually performed using a phase shifting circuit in
the local oscillator.
Such methods of laser absorption
spectroscopy have achieved high sensitivity but have
yet to achieve widespread practical application.
SU?O ARY OF THE INVENTION
For remote applications of the gas detector,
it is desirable to make operation of the gas detector
as simple as possible. The inventor has identified
that this can be achieved by ensuring that the local
oscillator used for homodyne detection of the detected
signal is always in phase with the detected signal. In
one aspect of the invention, therefore, the detected
signal is used as a source for the local oscillator.
In addition, a frequently occurring problem
in the use of frequency modulated diode lasers for gas
detection is the occurrence of interference fringes,
or etalon fringes, resulting from passage of the laser
light through a window, which acts as a Fabry-Perot
resonator. Various methods have been proposed to
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reduce etalon fringes, but they tend to be complex. In
a further aspect of the invention, the inventor
proposes to reduce etalon fringes by the novel and
surprisingly simple expedient of making the window
5 wedge shaped.
In addition, in prior art detectors a signal
is usually obtained in which the presence or absence
of a target gas is determined but not its density. In
a further aspect of the invention, the inventor
proposes to estimate the density of gas detected, by
passing the laser light through a gas reference cell
and comparing the detected signal from the target zone
with the detected signal from the gas reference cell.
There is therefore provided in accordance
with one aspect of the invention, a gas detector for
detecting the presence of a target gas in a target
zone that includes a laser transmitter having
frequency modulated light output including light
having a wavelength that is absorbed by the target gas
and a laser receiver in which detected light is
detected by mixing the detected signal with a
reference signal derived from light output from the
laser that has passed through the target zone.
In one aspect of the invention, the laser
receiver includes a photo detector for producing a
detected signal as output from light from the laser
that has passed through the target zone, a reference
signal generator to create a reference signal by
detection of light that has passed through the target
zone, the reference signal having a frequency
corresponding to a modulation frequency of the light
output from the laser, and a mixer for mixing the
detected signal and the reference signal to produce
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mixer output. Presence of the target gas is determined
by a signal analyzer connected to the mixer.
In a further aspect of the invention, the
signal analyzer includes a filter having a pass band
that includes the low frequency output of the mixer.
In a further aspect of the invention, the
reference signal generator includes a frequency
multiplier for producing a signal having a frequency
corresponding to a harmonic of a modulation frequency
of the light output from the laser.
In a further aspect of the invention, the
laser is adapted to produce light at one or more
modulation frequencies and the reference signal
generator includes a bandpass filter having a pass
band that includes one of the modulation frequencies
of the light output from the laser.
Preferably, the reference signal generator
is connected to receive output from the photo
detector.
In a further aspect of the invention, there
is provided a gas detector with a tunable gas diode
laser transmitter and a laser receiver, in which the
laser is mounted in a protective enclosure with a
window for the laser light output to pass through, and
there is provided means for shifting etalon fringes
produced by the window to frequencies that may be
filtered from the detected signal. Such a means may be
provided by providing the window with a wedge shape.
In a further aspect of the invention, there
may also be provided a gas reference cell for
containing a sample of the target gas, means to
selectively direct light from the laser to the gas
reference cell or the target zone, and means to
selectively direct light from the gas reference cell
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or from the target zone to the photodetector. In this
aspect, the data analyzer includes means to compare
output of the mixer when the light from the laser has
passed through the gas reference cell and when the
light from the laser has passed through the target
zone.
In a further aspect of the invention, the
gas detector includes a light sensor for detecting
presence or absence of light returning from the target
zone, so as to avoid false negative signals.
In a further aspect of the invention, the
gas detector further includes a phase shifter for
adjusting the phase difference between the detected
signal and the reference signal so as to allow noise
reduction.
In a further aspect of the invention, there
is provided a method for the remote detection of a
target gas in a target zone, the method comprising the
steps of:
transmitting frequency modulated light from
a laser through the target zone, the light being
frequency modulated at one or more frequencies, the
frequency of light transmitted from the laser
including a frequency component that is absorbed by
the target gas;
receiving frequency modulated light from the
laser that has passed through the target zone and
producing a detected signal from the received light;
and
detecting the frequency modulated light by
mixing the detected signal with a reference signal
derived from the frequency modulated light that has
passed through the target zone.
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In a further aspect of the invention, the
method further includes reducing etalon fringes in the
received frequency modulated light by shifting the
etalon fringes to frequencies that may be filtered out
from the detected signal, such as using a wedge shaped
window in the enclosure, and filtering out the etalon
fringes.
The method of the invention may also include
measuring the density of the target gas by comparing
the intensity of detected light that has passed
through the target zone with the intensity of light
that has passed through a gas reference cell
containing a sample of the target gas.
Noise reduction may also be effected by
tuning the laser away from frequencies that are
absorbed by the target gas, adjusting the phase
difference between the reference signal and the
detected signal until noise is reduced to a minimum,
and tuning the laser to transmit light having a
frequency that is absorbed by the target gas.
In a further aspect of the invention,
detection of methane is carried out at about the
1.3165 pm absorption line of methane.
In addition, the laser construction is
complicated by the fact that it is likely to be used
in hazardous environments, with the result that the
package becomes quite expensive. The inventor has
therefore proposed a system in which light from a
laser transmitter propagates along several optical
light guides from a laser transmitter to a laser
receiver. The optical light guides form a guided light
path traversing each of several target zones where
unwanted gas may be present. An optical switch permits
selection of one of the paths and hence one of the
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target zones for the detection of gas. A preferred
light guide uses optical fibers with optical switches,
or a combination of a splitter and a switch. -
A particular arrangement for the collection
of light from a transmitting fiber optic and receiving
fiber optic is also provided.
According to further aspect of the
invention, a remote laser head is coupled in each
guide light path between the laser transmitter and the
laser receiver. The remote laser head, in use, is
installed at a target zone remote from the laser
transmitter and laser receiver.
A gas reference cell is also preferably
provided on a guided light path between the laser
transmitter and laser receiver. Sequential switching
between the remote laser heads and the gas reference
cell permits automatic calibration of each of the
multiple guided light paths.
Many of the prior art methods are sensitive
to the phase of the detected light, and since phase of
the incoming light is altered by the distance from the
laser receiver to the target, the methods have limited
applicability where the distance from the laser
receiver to the target zone is not known with some
certainty or where variations in the phase of the
received light cannot easily be accommodated.
The inventor has provided a phase
insensitive laser receiver described herein. In
addition, there is known a phase insensitive gas
detector described in "Ultrasensitive dual-beam
absorption and gain spectroscopy: applications for
near-infrared and visible diode laser sensors", Mark
G. Allen, Karen L. Carleton, Steven J. Davis, William
J. Kessier, Charles E. Otis, Daniel A. Palombo, and
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David M. Sonnenfroh, Applied Optics, Vol. 34, No. 18,
June 1995, p. 3240 - 3248. The recent development
of phase insensitive techniques of laser absorption
spectroscopy using modulation detection permits the
5 development of portable highly sensitive gas
detectors, and also, the inventor now proposes, the
detection of multiple target zones in, for example, a
room with a single laser and a scanner.
In many gas facilities, there are many
10 potential leaks. A fixed light path through a leaky
region will indicate a leak but not the location along
the path. One solution is to use many paths and
attempt, by geography, to isolate one potential
leaking area; for example, a compressor or a valve.
15 Fibre coupled fixed light paths each require one or
two fibres communicating back to a central laser
system. Governing complex facilities in order to pin
point leaks remotely would be very expensive.
However, especially in toxic gas facilities, remotely
20 spotting leaks would be very desirable. Conventional
electrochemical gas detectors are also too expensive
to extensively locate leaks and usually are placed,
for example, in places where gas accumulates such as
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ceilings for light weight gases and gutters for heavy
gases.
The inventor proposes a system where light
is brought to reflectors either with a fixed laser or
through a fiberoptic. The light path to the reflector
will be changed by placing a mirror system adjacent to
the laser source. The mirrors will steer the light
path by changing the image seen by the laser
transmitter and receiver. If many reflectors are
placed within view of the laser, the mirror may be
adjusted so that any mirror and its associated light
path can become the image seen by the laser source and
detector. In this way a light path may be selected
simply by placing a mirror at the desired location and
adjusting the moving mirror. Many moving light paths
are possible and the gas distribution throughout, for
example, a building may consequently be mapped. Each
reflector may be retroreflecting tape such as is used
on roadsides or plastic reflectors such as those used
for bicycles and car reflectors. Detection of gas in
many paths will hence be inexpensive and possible.
There is therefore provided in accordance
with one aspect of the invention a gas detector for
detecting a target gas in multiple target zones. The
gas detector includes a laser transmitter of light,
the transmission of which is affected by the target
gas, a laser receiver of light emitted by the laser
transmitter, a signal analyzer for analyzing signals
produced by the laser receiver to give an indication
of whether target gas is present in a target zone, a
first scanning optical element separate from the laser
transmitter and disposed to receive light from the
laser transmitter and direct the light towards
multiple target zones, a light collector to receive
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light from the first scanning optical element that has
returned from the multiple target zones and direct the
light towards the receiver, and control means to
control the position of the first scanning optical
element and thereby control which of the target zones
is traversed by light from the laser transmitter.
In a further aspect of the invention, a
monitor is connected to the signal analyzer for
displaying an image indicative of the presence of the
target gas in the target zones.
In a further aspect of the invention, the
laser transmitter includes a laser having frequency
modulated output, the laser light having a phase, and
the signal analyzer is phase insensitive.
In a further aspect of the invention, the
first scanning optical element is a mirror mounted on
a gimbal; and the control means includes a stepper
motor connected to incrementally rotate the mirror and
a controller for the stepper motor.
In a further aspect of the invention, the
mirror has a reflecting surface and the gimbal has
first and second axes of rotation intersecting at a
point of intersection on the reflecting surface of the
mirror, and the gas detector further includes means,
such as a fiber optic, to direct light from the laser
transmitter at the point of intersection of the first
and second axes of rotation of the gimbal.
There is also provided a method of detecting
gas in a room of a gas facility, the method comprising
the steps of:
directing laser light from a laser
transmitter at a scanner;
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controllably rotating the scanner to direct
light sequentially at plural target zones and receive
light reflected back from the plural target zones;
detecting light from the scanner that has
passed through the plural target zones; and
analyzing the detected light for the
presence of gas in the plural target zones.
A second scanning optical element may be
used to scan difficult to reach parts of a room.
These and other aspects of the invention are
described in the detailed description and claimed in
the claims that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
There will now be described preferred
embodiments of the invention, with reference to the
drawings, by way of illustration, in which like
numerals denote like elements and in which:
Fig. 1 shows an overall schematic of a gas
detector, target zone and reflector for use in a gas
detector according to the invention;
Fig. 2 is a schematic showing a first
detection circuit for use in a gas detector according
to the invention;
Fig. 3 is a schematic showing a second
detection circuit for use in a gas detector according
to the invention;
Fig. 4 is a schematic showing a third
detection circuit for use in a gas detector according
to the invention;
Fig. 5 is a schematic showing a fourth
detection circuit for use in a gas detector according
to the invention;
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Fig. 5A is a schematic of part of an
alternative reference signal generator for the
embodiment of Fig. 5 for use in a gas detector
according to the invention;
Fig. 6 is a schematic showing a gas detector
used for the resolution of gas density for use in a
gas detector according to the invention;
Fig. 7 is a schematic showing a section
through a window for use in reducing etalon fringes in
the operation of the invention for use in a gas
detector according to the invention;
Fig. 8 is a schematic showing plural optical
light guides traversing several distinct target zones
and a gas reference cell;
Fig. 9 is a schematic of a gas reference
cell for use in the system shown in Fig. 8;
Fig. 10 is a schematic plural optical light
guides traversing several distinct target zones and a
gas reference cell;
Fig. 11 is a schematic of a light guide
configuration at a target zone;
Fig. 12 is a schematic of an embodiment of
the invention using a single fiber optic for outgoing
and return light;
Fig. 13 is a schematic showing the overall
layout of a gas detector with scanner according to the
invention;
Fig. 14 is a perspective view of a scanner
according to the invention;
Fig. 15 is a side section of a scanner fed
with a fiber optic according to the invention;
Fig. 16 is a side section of a scanner
mirror and laser light collection system according to
the invention;
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Fig. 17 is a front view of a second
embodiment of a scanner according to the invention;
Fig. 18 is a side view of a third embodiment
of a scanner according to the invention;
5 Fig. 19 is a plan of a room in a gas
facility showing location of plural scanners; and
Fig. 20 is a schematic of a laser receiver
with low level light detector.
10 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to Fig. 1, an exemplary gas
detector 10 includes a laser transmitter 12 and laser
receiver 14. Typically, in use, light from the laser
transmitter 12 is directed towards gas in a target
15 zone 16, reflected from a reflector 18, and received
back at the laser receiver 14. The distance from laser
transmitter 12 to laser receiver 14 may be more than
200 meters, and may be an oil or gas installation.
Target gases include hydrogen fluoride, hydrogen
sulphide, ammonia, water, hydrogen chloride, hydrogen
bromide, hydrogen cyanide, carbon monoxide, nitric
oxide, nitrogen dioxide, oxygen and acetylene,
although a major expected use of the invention is for
the detection of methane.
The laser transmitter 12 preferably uses a
tunable diode laser to produce frequency modulated
light output including light having a wavelength that
is absorbed by the target gas. Such tunable diode
lasers are well known in the art in which an injection
current is modulated to produce frequency modulated
output. Since it is difficult to ensure that the
carrier frequency of light from the laser is at an
absorption line of the target gas, the carrier
frequency is preferably tuned through the absorption
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line with a ramp. Typically, therefore the light from
the laser is modulated with a first modulation
frequency corresponding to the frequency of the
modulating current and a second modulation frequency
corresponding to the ramp frequency. In two-tone
modulation, the light from the laser will be modulated
with a third modulation frequency. The light absorbed
by the gas may be the carrier frequency or one of the
sidebands caused by the modulation.
While the laser transmitter 12, modulation
technique and frequency selection are all known in the
art, the laser receiver 14 is new. An exemplary laser
receiver 14 is shown in Fig. 2. Light from the laser
that has passed through the target zone is detected by
photo detector 20, converted to an electrical signal
and passed to mixer 22. The detected signal will
contain many frequencies w1, w2r corresponding to the
modulation frequencies of the light emitted from the
laser and their harmonics. The signal from the photo
detector 20 is also passed to reference signal
generator 24, where the signal is bandpass filtered in
filter 26 to isolate one of the frequencies, for
example wl, and then amplified in amplifier 28 to
produce a reference signal. The reference signal is
supplied as one of the inputs to the mixer 22 where it
is mixed with the detected signal coming direct from
the photo detector 20. Output from the mixer 22 is low
pass filtered in filter 30 and then analyzed, for
example using data analyzer 32 shown in Fig. 6. The
output from low pass filter 30 will show gas absorption
if the target gas is present. Analyzer 32 performs such
functions as signal averaging and also preferably
includes some conventional means of displaying the
detected signal.
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Reference signal generator. 24 may also
develop its reference signal from a second photo
detector (not shown) although it is preferred to use
one photo detector. The reference signal generator 24
generates a local oscillator wl, which is independent
of the laser modulation circuitry. The local
oscillator always has a fixed phase relationship with
the photodetector signal so that this circuit is
independent of the absorption path length.
A further exemplary phase insensitive laser
receiver is shown in Fig. 3. The reference signal
generator 24 of Fig. 3 differs from the reference
signal generator 24 of Fig. 2 by including a phase
lock loop 34. The signal w1 from the amplifier 28 is
used to activate the phase locked loop 34 (PLL) and
the output of this PLL is used as in Fig. 2 as a local
oscillator. The output of the PLL 34 has the same
frequency and phase as the input signal w1. However,
it is free from other frequencies, which can pass
through the bandpass w1 filter, such as the electrical
noise over the filter bandwidth. The use of a PLL for
a local oscillator consequently results in better
signal-to-noise ratio in the mixer output.
A further exemplary and improved phase
insensitive laser receiver is shown in Fig. 4. In
this embodiment a phase shifting circuit 36 is added
to the reference signal generator 24 of Fig. 2. The
phase shifting circuit 36 permits changes in the phase
relationship between the signal wl and the local
oscillator. The noise level in a laser absorption
spectrometer may be reduced by careful phase
adjustment of the reference signal generator 24 in
accordance with known techniques.
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A further exemplary and improved phase
insensitive laser receiver is shown in Fig. 5. In the
embodiment of Fig. 4, the signal at the desired
frequency is used to activate a phase locked loop 34.
For remote application it is common for this
photodetector signal to be too weak to activate the
PLL 34. In the embodiment of Fig. 4, the PLL 34 is
activated by a signal with a fundamental frequency,
which is always stronger than the detected signal.
For the absorption measurement technique in which the
laser is modulated at a single frequency, the detected
signal is commonly the second harmonic 2w1. For the
technique referred to as two tone modulation the laser
is modulated at two frequencies w1 and w2 (besides the
ramp frequency) and the difference signal at frequency
(w1 - w2) is detected. In Fig. 5, the PLL 35 is
activated at frequency w1 and generates an output at
the harmonic frequency 2w1. This harmonic signal then
acts as the local oscillator in the mixer 22 and the
2w1 signal is detected as desired. In this way the
PLL 35, is activated by the much stronger w1 signal.
For two tone modulation two PLLs 35a and 35b
generating w1and w2 are necessary as shown in Fig. 5A.
These are then combined in a secondary mixer 23 to
generate a (w1 - w2) signal, which is then used as the
local oscillator in the detecting circuit and input to
the mixer 22.
In the method of laser absorption
spectroscopy the detected signal is proportional to
the quantity of gas in the absorption path length.
The detected signal can hence be used as a measure of
gas concentration if the path length of absorption is
known. For example, light from a laser absorption
spectrometer may be reflected from a distant object or
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reflector, as for example reflector 18 shown in Fig.
1. Light returning to the spectrometer will sense the
presence of gas if the laser line coincides with the
gas absorption wavelength. An estimation of the path
length of light through the gas cloud will then permit
an estimation of the gas concentration. To be useful
the spectrometer must be calibrated so that readings
of gas concentration do not change because of
instrument or environmental changes. In practice,-
this is very difficult to achieve. Small changes in
laser temperature will cause the laser wavelength to
move away from the gas absorption line because laser
diode wavelength is very sensitive to temperature.
Environmental changes of temperature between -40 C to.
+50 C, as required by industrial equipment, can also
cause changes in the electronic sensitivity. It is
known to use a methane cell together with feedback
circuitry to regulate the laser wavelength onto the
methane absorption line, in which the main limitation
to sensitivity is temperature induced changes. In this
invention the effects of temperature changes are
minimized using a gas reference cell in a manner quite
different from that previously known.
The gas cell is not used to stabilize the
laser wavelength as in the prior art. In the present
invention, the laser wavelength is preferably scanned
using a low frequency ramp diode laser current
modulation. In this way small wavelength changes
caused by environmental changes to the thermo-electric
temperature controlling circuit are not important. If
the laser line scans through the absorption line,
small offsets in the average laser wavelength are not
important. This method of ramping itself is well
known in the art.
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A novel application of a gas reference cell
is shown in Fig. 6. Part of the outgoing beam from
laser transmitter 12 is reflected from beam splitters
40 and 42 into the laser receiver 14 through a small
5 cell 44 containing the gas of interest. The main beam
A is transmitted to the remote reflector 18 and the
reflected beam B is also collated by the laser
receiver 14 as is normal.
A first shutter system 46 is disposed on the
10 light path from the beam splitter 40 to beam splitter
42 through the reference cell 44. A second shutter
system 48 is disposed on the light path from the beam
splitter 40 the beam splitter 42 that passes through
a target zone to the reflector 18. Operation of the
15 shutters 46 and 48 will expose the receiver to light
in an alternating way from either the remote reflector
18 or from the gas reference cell 44. The data
analyzer 32 attached to the receiver 14 output records
and compares the signal from both sources for example
20 using a Kalman filter. The use of Kalman filters and
like digital processing methods for the comparison of
one reference signal with a noisy signal is well known
and need not be further described. Since the gas
density within the reference cell 44 is known, it is
possible to calculate the gas density in the path to
the remote reflector 18 from a comparison of the
intensity of detected light that has passed through
the target zone with the intensity of light that has
passed through the gas reference cell. Detecting the
reference signal and then the signal from the target
sequentially may be carried out several times per
second or as low as several times per hour, but the
duration of transmission of laser light is preferably
kept to a minimum, to fractions of a second, to avoid
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potential damage to the eyes of those who may be
nearby. This technique has several advantages.
Effects of instrument changes and
environmental changes are cancelled because the
changes apply equally to the remote signal and the
reference signal. This system is in effect an
automatic calibration. Further, for detection of
hazardous gases, it is important that equipment
failure is not interpreted as the absence of gas.
This is referred to as a false negative signal. The
sharing of the reference and remote signals within the
system avoids this problem to the extent that the
remote laser beam is not obstructed. For fail safe
operation with this system, it is hence necessary to
make use of a light level sensor 50 to ensure the
presence of a return laser beam. The use of the gas
reference cell requires a known phase relationship for
both the reference and remote signals. It is hence
not possible to simply adjust the phase of the local
oscillator. Hence it is preferable to use the method
shown in Figs. 2 to 5 to avoid phase adjustment for
both reference and remote signals.
In practice, the return signal to the laser
receiver is not in phase with the local oscillator,
which would provide the highest output signal. The
phase of the signal and local oscillator are typically
100 to 40 different. This is necessary to null the
noise caused by laser diode amplitude modulation.
Drift in this phase difference caused by instrument
and environment changes can cause significant increase
in laser noise and degradation of the spectrometer
sensitivity. However in the presence of a signal,
adjustment of the phase to minimize noise is not
possible because the signal also depends upon phase.
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This problem may be solved by tuning the laser
wavelength away from the gas absorption line so that
the signal is reduced to zero. The noise may -then be
reduced to a minimum by phase adjustment and then the
laser line is returned onto the absorption line. Phase
adjustment may be achieved with a phase shifter 36 in
the reference signal generator 24. However, since it
is necessary only to change the phase difference
between detected and reference signals, the phase
shifter 36 may be on the line carrying the detected
signal. These steps may all be undertaken with the
spectrometer control circuits. Consequently, the gas
reference cell may be used to calibrate the remote
signal automatically and also to adjust the phase of
the local oscillator for minimum noise ensuring
reliable fail safe operation of the device over time
and in different environmental conditions.
Laser absorption spectrometers are suitable
for detecting explosive gases such as methane.
However the use of electrical devices in hazardous
environments is highly regulated and usually requires
that the equipment is mounted in explosive proof
enclosures, such as enclosure 52 shown in Fig. 1. The
design of explosion proof enclosures 52 is well known
in the art and requires a thick window 54 through
which the outgoing and return laser beam may pass. As
is well known in the art, windows will behave like
Fabry-Perot resonators and cause interference fringes
known as etalon fringes. This effect causes
wavelength dependent transmission variation, which
competes with the gas absorption and causes serious
reductions in signal to noise ratio and hence system
sensitivity to gas. In particular, thick windows will
cause fringes, which are particularly detrimental. The
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inventor has found that the use of a thick window with
low fringe noise on an explosive proof chamber is
possible if the front and back faces 56 and 58
respectively of the window 54 are at a sufficient
angle to each other to move the fringes to a frequency
that can be discriminated from the detected signal.
The etalon fringes may then be filtered out from the
detected signal using the low pass filter 30. If the
window 54 is made of a laminate, the wedge shape of
the window 54 may be accomplished by introducing a
small wedge 60 between the two laminates 62 and 64
forming the window 54. The wedge causes the frequency
difference between fringe maxima to be reduced.
When the laser wavelength scans the wedged
window, it will pass through several fringes and the
fringe noise recorded by the laser receiver circuit
will be of relatively high frequency. The low pass
filter used after the mixing circuit will hence remove
this source of noise and the fringe noise will not
degrade the spectrometer sensitivity.
The inventor has also discovered that
operation of the invention over distances greater than
200 meters is possible if the light transmitted to a
reflector on the opposite side of the target zone
follows the same return path. In this manner,
deviation of the light path is the same on the
outgoing and returning light path and the return beam
ends up back at the laser receiver, which is
conveniently housed with the laser transmitter.
The reflector should be large enough to
efficiently reflect the thermally deflected and
refracted laser beam, and the light collector on the
laser spectrometer should be large enough to collect
the refracted laser beam. The reflector should be a
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good quality retroreflector since displacement of the
return beam upon reflection tends to make the return
beam follow a slightly different path. In addition, it
is preferable to use as wide a laser beam as is
practical.
Although detection of methane may be carried
out at the commonly used 1.66 pm methane absorption
band, where the absorption is fairly strong and the
signal is not affected by water vapour absorption, it
is preferred to carry out transmission and reception
at the 1.3165 pm absorption line for methane, within
the water vapour window between 1.3162 to 1.3169 pm.
Since there is also an ammonia absorption line at
about 1.3165 pm, if ammonia may be present, detection
should also take place at about 1.3177 pm within the
1.3173 pm to 1.3184 pm water vapour band since ammonia
also has an absorption line at about 1.3177 pm while
methane does not. Hence, during processing of the
detected signal reflected back from a reflector,
detection of absorption at 1.3177 pm distinguishes
ammonia from methane, and absence of detection of
absorption at 1.3177 pm distinguishes methane from
ammonia. The methane absorption line at 1.3165 pm is
an unlikely candidate for practical measurement of
methane presence since the absorption at this line is
about 20 times weaker than at the conventional 1.66 pm
line. However, adoption of this line for detection
allows communication band lasers at about 1.32 pm to
be used for both the detection of methane and ammonia.
Referring now to Fig. 8, a laser transmitter
80 and laser receiver 82 are shown with plural optical
light guides 84 extending between them. The laser
transmitter 80 is preferably but not necessarily a
laser transmitter of the tunable diode type described
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above, and the laser receiver 82 is preferably but not
necessarily made in accordance with the description of
the laser receiver shown in Figs. 2-5, including the
above described system for eliminating phase
5 sensitivity of the receiver. Each optical light guide
84 preferably is formed of a transmitting optical
fiber 84a and a receiving optical fiber 84b. The
transmitting optical fibers 84a are positioned to
receive light from the laser transmitter 80, as for-
10 example through optical fiber 86 and terminate at a
remote laser head 90 at a target zone as shown in Fig.
11. The receiving optical fibers 84b are positioned to
output light to the laser receiver 82, as for example
through lens 88 or like optical element, and each has
15 an end 85 terminating at the remote laser head 90 at
a target zone 92 to receive light from one of the
optical fibers 84a, the light having passed across the
target zone 92.
Each laser head 90 includes a collimating
20 lens 94 spaced from the terminus of one of the optical
fiber 84a to receive and collimate light exiting the
optical fiber 84a. The collimated light is directed
onto a corner cube reflector 96 spaced from the
collimating lens 94 at the opposite side of the target
25 zone 92, the target zone thus being between the laser
head 90 and the reflector 96. Light reflecting from
the corner cube reflector 96 is collected and focused
by an offset parabolic reflector 98 onto an end 85 of
one of the optical fibers 84b. Conveniently, the
parabolic reflector 98 includes a central aperture to
permit passage of light from the optical fiber 84a
through the parabolic reflector 98. The lens 94 and
reflector 98 together form an exemplary optical means
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to direct light from optical fiber 84a through the
target zone to the optical fiber 84b.
In an installation, for example at-an oil
industry installation, a laser head will be installed
in each target zone in the installation that is to be
monitored. There may be for example 30 target zones.
An exemplary target zone might be a control room. With
the optical fiber coupled laser head described, the
laser transmitter and laser receiver may be in a
location remote from each target zone, hundreds of
meters away or more.
Each pair of optical fibers 84a and 84b and
the corresponding laser head 90 together form a
distinct guided light path from the laser transmitter
80 to the laser receiver 82 that traverses the target
zone 92. The optical fibers 84a and 84b are preferably
single mode fibers.
As shown in the embodiment of Fig. 8, an
optical switch 100 is provided at the laser
transmitter 80 to select one of the optical light
guides 84. Selection of one of the optical light
guides 84 connects one of the optical fibers 84a to
the optical fiber 86 to complete a guided light path
between the laser transmitter 80 and the laser
receiver 82 for the detection of gas in the target
zone traversed by the selected one of the optical
light guides. The selection may be computer
controlled. Fiber optic switches of this type are well
known in the art and need not be further described.
The optical fibers 84b guide the light from the remote
laser heads 90 to optics at the laser receiver 82.
An alternative switching system is shown in
Fig. 10. In this case, light from the laser
transmitter 80 is guided by optical fiber 102 to a
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beam splitter 104 where it is split into optical
fibers 84a and guided to remote laser heads 90. Light
from the remote laser heads 90 is carried by optical
fibers 84b to a fiber optic switch 106 similar to
switch 100, except that switch 106 may be multi-mode.
Switch 106 is connected to laser receiver 82 through
optical fiber 108. Setting of the switch 106 selects
one of the guided light paths 84 defined by the
optical fibers 84a, 84b and the optics in the laser
head 90, and connects one of the optical fibers 84b to
the optical fiber 108 to complete a guided light path
between the laser transmitter 80 and the laser
receiver 82 for the detection of gas in the target
zone traversed by the selected one of the optical
light guides.
For the remote detection of gas from plural
zones, it is preferable to locate a gas reference cell
110 in a guided light path selectable by the switch
100 or 106. Hence, for measurement of gas density, the
light from the laser transmitter 80 can be selectively
directed through one of the remote laser heads 90 or
the gas reference cell 110. For use with an optical
fiber, it is preferred that a refocusing lens 112 be
provided in the gas reference cell 110, as shown in
Fig. 9, to collimate light from the optical fiber 84a
and focus it onto optical fiber 84b. Other methods of
focusing the light onto the fiber 84b could be used.
A control 114, which may be part of the data
analyzer 32 shown in Fig. 6, may be used to
sequentially select one of the remote laser heads for
gas detection. In an industrial environment, the
sequential switching between laser heads provides
continuous repeated monitoring of several areas or
zones within the environment. In addition, sequential
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switching between the remote laser heads 90 and the
gas reference cell 110 permits automatic calibration
of each of the multiple guided light paths. -
For the detection of more than one gas, a
second laser transmitter 116 may be connected to the
optical light guides 84 through a combiner 118. The
second laser transmitter 116 may operate in a narrow
band separate from the band of the laser transmitter
80 and thus be used to detect a different gas species.
Either one of the laser transmitters 80 and 116 may be
operated sequentially, or alternately as desired.
A further embodiment of a gas detector with
remote laser head is shown in Fig. 12. A laser
transmitter 80 is attached to one end of a guided
light path extending to a target zone 132. The guided
light path includes an optical fiber 121 connected to
a directional coupler 120, an optical switch 122, a
fiber 123 connecting optical coupler 120 and switch
122, a laser head 126, and an optical fiber 124
connecting switch 122 and laser head 126. Laser head
126 includes an end 128 of fiber optic 126 and a
collimating parabolic offset mirror 130 oriented with
the end 128 at the focus of the mirror. Mirror 130
acts both to collimate light exiting the fiber optic
124 and to collect light returning back from the
reflector 134 at the opposite side of target zone 132
from the laser head 126. Various optical arrangements
may be used with like effect. Mirror 130 is similar to
mirror 98 only mirror 130 does not need to have a
central aperture.
In the gas detector shown in Fig. 12, when
switch 122 is closed to connect fibers 123 and 124,
light from the laser transmitter 80 passes along fiber
121 through directional coupler 120 along fiber 123,
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through switch 122, along fiber 124 to laser head 126.
Light from the end 128 of fiber 124 is collimated by
mirror 130 and directed across the target zone to
reflector 134. Light reflected back from the reflector
134 is collected and focused by mirror 130 back into
fiber 124. With the switch 122 still closed, the light
passes along fiber 123 and is directed by directional
coupler 120 into laser receiver 82. In this manner
only a single optic fiber is required for the guided
light path out to the remote laser head. Only a single
directional coupler 120 is required for several
output/input optical fibers 124 if it is located on
the laser transmitter side of switch 122. Numerous
similar single fiber light paths may be connected
through switch 122 in the same manner as with switch
100. The optical components described here are all
conventional and readily commercially available.
with the remote laser head of the present
invention, the laser transmitter and laser receiver
may be located outside of a hazardous environment and
thus do not need to be housed in an explosion proof
housing. Likewise, the laser head may be simply
constructed with no electrical connections in the
hazardous area.
There has thus been described in relation to
Figs. 8 - 12, a gas detector for detecting gas in
remote facilities. In each facility, for example a
room in a gas plant, there may also be plural areas of
interest, for example an area near a valve or
compressor. Referring to Fig. 13, there is shown a gas
detector for detecting a target gas in multiple target
zones. A laser transmitter 131 of light, the
transmission of which is affected by the target gas,
preferably includes a frequency modulated diode laser
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of conventional construction. Laser receiver 133 of
light emitted by the laser transmitter is preferably
of the type shown in Fig. 5. Alternatively, the laser
transmitter 131 and laser receiver 133 may be of the
5 type described in "Ultrasensitive dual-beam absorption
and gain spectroscopy: applications for near-infrared
and visible diode laser sensors", Mark G. Allen, Karen
L. Carleton, Steven J. Davis, William J. Kessier,
Charles E. Otis, Daniel A. Palombo, and David M.
10 Sonnenfroh, Applied Optics, Vol. 34, No. 18, 20 June
1995, p. 3240 - 3248. In either case, the laser is
preferably phase insensitive. If the laser receiver
133 is not phase insensitive, then the length of the
light path from the laser transmitter to the laser
15 receiver must be fairly well known to account for
phase changes of the light received by the laser
receiver.
A signal analyzer 135 for analyzing signals
produced by the laser receiver is coupled to the laser
20 receiver in conventional fashion. Various such
receivers are also known. The analyzer may for example
be a computer or microprocessor, readily commercially
available, programmed for the purpose. The signal
analyzer 135 provides an output signal that is
25 indicative of whether target gas is present in a
target zone. That output signal may be displayed
digitally or output to a monitor 137 for display as an
image or displayed in any other convenient fashion.
The signal may also be stored for subsequent retrieval
30 from a memory in the computer/analyzer 135.
In order to detect gas in various locations
in a room, a scanning optical element 140 is
positioned in the room separate from the laser
transmitter 131 and disposed in the light path from
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the laser transmitter 131 to receive light from the
laser transmitter 131 and direct the light towards
multiple target zones 196 in a room 191 shown
schematically in Fig. 19. Light may be returned after
passage through a target zone by reflection from a
reflector 195 or directly from a wall 193 if the laser
is sensitive enough to detect light reflecting from
the wall 193. The reflector 195 may be a corner cube
reflector, reflective tape or plates or a painted
reflective surface, all of which are commonly
commercially available.
Light returning from the scanning optical
element 140 after passage through one of the multiple
target zones 196 is collected by a light collector 162
disposed between the scanning optical element 140 and
the laser transmitter 131. The light collector 162 may
for example be a section of a parabolic mirror. The
light collector 162 focuses the light onto the laser
receiver 133. Light from the laser transmitter 131 may
be guided to the mirror 140 through optical fibers as
shown in Fig. 12 or may be transmitted through free
space.
A stepper motor 142 with associated stepper
motor controller 138 forming a control means for the
scanning optical element 140 may be used to control
the position of the scanning optical element 140 and
thereby control which of the target zones 196 is
traversed by light from the laser transmitter 131. The
stepper motor 138 and associated controller are
conventional. It is preferred that a stepper motor 138
with a small angular increment be used, for example in
the order of 10 or less. The stepper motor controller
138 is preferably supervised by the computer 135 to
coordinate the laser transmitter 131, laser receiver
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133, and stepper motor controller 138. For example,
the scanning optical element 140 may scan the room in
a raster fashion, as a television image, and the
resulting signal displayed as a two dimensional image
of gas density on the monitor 137. The scanning
optical element 140 may also scan sequential specific
locations in a room. The controller 138 and controller
139 may be instructed by the computer 135 to move the
mirror a pre-programmed number of increments, and the
laser transmitter 131 turned on to transmit a pulse of
modulated light to the mirror, which is then returned
to the laser receiver and the resultant output signals
analyzed in the computer 135. The controllers 138 and
139 may then move the scanning optical element 140 to
a new location and the process continued until a
number of target zones have been tested for the
presence of gas.
The scanning optical element 140 is
preferably a mirror 141 mounted on a gimbal as shown
in Fig. 14. The mirror 141 is supported by a shaft
143 from stepper motor 142. Shaft 143 defines a first,
vertical, axis of rotation of the mirror 141 that
passes through the center 147 of the reflecting
surface of the mirror. Stepper motor 142 incrementally
rotates the mirror 141. The effect of rotation of the
mirror 141 through n is to move the reflected laser
beam 149 through 2n . The stepper motor 142 may be
geared down to produce any desired angular increment
in the rotation of the reflected laser beam. The
mirror 141 may be rotated a full 360 , although for
most scans 1200 would be enough. Stepper motor 142 and
mirror 141 are mounted in a frame 148 that is
rotatably mounted on bearings 146.
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Bearings 146 define a second, horizontal,
axis of rotation of the mirror 141 that passes through
the center 147 of the reflecting surface of the mirror
thereby intersecting the vertical axis on the surface
of the mirror. Movement of the gimbal mounted mirror
141 about the second axis may be accomplished with a
linear actuator 151 coupled through linear actuator
shaft 144 and shaft 145 coupled at bearing 153. Linear
actuator 151 is controlled by a controller 139 under
supervision of computer 135 to incrementally rotate
the mirror 141 about the horizontal axis and thus
rotate the reflected laser beam 149 vertically. Since
rooms or other gas facilities tend to be flat for
their width rather than tall for their width, the
amount of rotation of the mirror 141 about the
horizontal axis need not be great, for example 22.5 ,
to produce a vertical scan of 45 .
The laser transmitter 131 may be mounted
directly above the scanning optical element 140 and
directed so that its output beam is aligned along the
vertical axis of the mirror 141 and strikes the center
of the reflecting surface of the mirror. However, gas
facilities are hazardous environments and the mounting
of the laser transmitter 131 in the hazardous
environment requires the laser transmitter to be
placed in an explosion proof housing. Therefore, it is
preferable to supply the scanning optical element 140
with light through an optical fiber 154 as shown in
Fig. 15. The optical fiber 154 is fed with light from
a laser transmitter at a distant location as for
example shown in Fig. 12. Only the end of the optical
fiber 154 remote from the laser transmitter is shown.
In Fig. 15, scanning optical element 140 includes a
parabolic mirror 150 rotatably mounted on a shaft 155.
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The optical fiber 154 is suspended on support 152 with
light from the optical fiber 154 directed at the
center of the mirror 150 where its axes of rotation
intersect. As the mirror 150 rotates, the end of the
optical fiber 154 is rotated and its orientation
controlled so that light from the optical fiber 154 is
scanned across a room or area being monitored. Light
157 returning from a target zone is collected by the
reflecting surface of the mirror 150 and focused on
the optical fiber 154. By using the fiber optic 154,
the laser transmitter and laser receiver may be
mounted outside of a room to be scanned and thus do
not have to be mounted in explosion proof housings. In
addition, more than one room may be monitored through
the use of plural optical fibers as illustrated in
Figs. 8 and 10.
In a further embodiment of a scanning optical
element 140 shown in Fig. 16, a laser beam 166 output
from a laser transmitter 131 passes through an
aperture 164 in a mirror 162 having a parabolic
reflecting surface and reflects from rotating mirror
160 mounted on axis 161 towards plural target zones.
The target zones may be scanned as with the embodiment
of Fig. 14 by incremental rotation of the mirror 160
with a stepper motor. Light returning from the target
zones as indicated at 167 is again reflected from the
mirror 160 to mirror 162 and focused on to detector
165 that forms part of a laser receiver.
A second embodiment of a gimbal mount is
shown in Fig. 17 in which mirror 170 is mounted on a
horizontal shaft 171, with vertical movement of an
incident laser beam being controlled by stepper motor
175. Shaft 171 is mounted in frame 176 which itself is
mounted on shaft 172 in frame 173. Rotation of the
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mirror 170 about shaft 172 is controlled by stepper
motor 174 and its associated controller. The gimbal
mount of Fig. 17 works in like fashion to the-gimbal
mount of Fig. 14 in that the mirror may be rotated
5 about each of two mutually perpendicular axes.
In a further embodiment shown in Fig. 18,
two mirrors are used for the scanning optical element.
Mirror 180 is mounted for rotation about a vertical
shaft from a stepper motor 182. Light 186 output from
10 a laser transmitter passes through aperture 183 in
collecting mirror 181 and is directed by mirror 180 to
a second mirror 184 adjacent to mirror 180 mounted on
a horizontal shaft 188 from a stepper motor 185.
Mirror 180 controls the sweep of the laser beam around
15 the vertical axis, and mirror 184 controls the
vertical positioning of the sweeping laser beam. Light
returning from the target zones reflects off both
mirrors 185 and 180 and is focused by collecting
mirror 181 onto detector 187. This embodiment may be
20 used where faster scanning is required, since only the
mirrors and not one of the stepper motors need be
rotated.
A scanning optical element 194 may be
mounted in a corner of a room 191 as illustrated in
25 plan view in Fig. 19. As the scanning optical element
194 rotates the laser light beam is moved sequentially
between positions 197, which are at least each 2n
apart, where n is the angular increment of the stepper
motor, accounting for any gearing down of the stepper
30 motor. By control of the x and y positioning of the
scanning optical element 194, the output laser beam
can be directed sequentially through target zones 196
to reflect off reflectors 195, 198 or 199 in
accordance with a programmed sequence. Each position
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of the outgoing light beam 197 may be selected by
appropriate rotation of the scanning optical element
194. For example, if the horizontal and vertical
position of the mirror when the light beam is directed
at reflector 198 is defined as 2700, 00, then a laser
beam directed at reflector 199 might be at 300 , 0 .
The stepper motors and linear actuator may thus be
programmed to move a set number of increments to the
position of each reflector 198, 199 and 195 in turn.-
If the room to be scanned includes a corner
that cannot be sampled by the scanning optical element
194, then a second scanning optical element 192 may be
mounted in the line of sight from the first scanning
optical element 194. The scanning optical element 194
may be fixed to direct light to the second scanning
optical element 192 while the scanning optical element
192 is rotated to scan area 200 with target zones 201
and reflectors 202.
The reflectors 195, 198, 199 and 202 are set
up in an area containing the target zones so that each
target zone is on a light path between one of the
light reflectors and one of the scanning optical
elements 192 and 194.
As shown in Fig. 20, the laser receiver 204,
which may otherwise constructed in accordance with any
one of Figs. 2-5, or other phase insensitive
detectors, may include a light level detector 206
connected to the output from photodetector 20. This
light level detector 206 detects the amount of laser
light returning in the return beam from the laser
transmitter. If the amount of light in the return beam
is below a given threshold then this is interpreted as
a laser-off condition rather than the presence of
absorbing gas. In addition, an image may be formed of
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the room using the return laser light beam. Stepper
motor controller 138 may be programmed to make an
scanning optical element 140 scan across a room. As
the scanning optical element 140 scans across the
room, the detector 206 outputs a signal that may be
conditioned in conventional manner and output to
monitor 137, where an image of the room may be
displayed. The image need not be refreshed as often as
a television image, as the equipment in the room will
generally not be moving. However, the output from the
laser receiver that is indicative of the presence of
gas may be superimposed on the image produced by the
light level detector 206 so that the location of a gas
leak may be determined quickly.
If the gas detector is to be used in a
hazardous environment, precautions should be taken in
accordance with local regulations governing hazardous
areas. For example, both the linear actuator 151 and
stepper motor 142 should be equipped with zener
barrier circuits 136 to limit the maximum currents to
a safe level. The scanning mirror 141 should be large
enough to produce an image which fills the field of
view of the detector. This can be satisfied by
ensuring that the aperture of the mirror, even when it
is tilted at its maximum angle is bigger that the
detector collecting mirror aperture as shown by the
length G in Fig. 16. Instead of stepping motors,
galvanometers may be used for moving the mirrors.
A person skilled in the art could make
immaterial modifications to the invention described in
this patent document without departing from the
essence of the invention.
