CA2183502C - Gas detector - Google Patents

Abstract

A gas detector of gas in a target zone has a transmitter of frequency modulated light and a receiver. In the receiver, a homodyne detection circuit mixes a detected signal with a reference signal at one of the modulation frequencies, where the reference signal has been derived from light that has passed through the target zone. The homodyne detection circuit includes a bandpass filter, amplifier, phase lock loop and phase shifter. Etalon fringes are reduced by moving them to high frequencies and filtering them out. Gas density is measured by comparison of the detected signal from the target zone with a signal from a gas reference cell. A method of detecting gas includes transmitting frequency modulated light through a target zone, receiving the light, and homodyne detection of the detected signal by mixing the detected signal with a reference signal at one of the modulation frequencies, where the reference signal has been derived from light that has passed through the target zone. 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 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 lens system provides for the collection of light from a transmitting fiber optic and receiving fiber optic. The gas detector is provided with means to eliminate phase sensitivity of the detector. Also, a gas detector for detecting a target gas in multiple target zones. A light collector receives light from the first scanning optical element that has returned from the multiple target zones and directs the light towards the receiver. A controller controls the position of the first scanning optical element and thereby controls which of the target zones is traversed by light from the laser transmitter.

Description

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

2~$3 50 2 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, wl, w2. W31 etc is mixed with frequency component wo, which is taken directly from the current modulator for the laser diode. The dc output (wo-wo) 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.

3 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

4 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.

SUMMARY 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 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

6 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

7 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.

8 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

9 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 3 5'02 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 2 1V..JJtJL

11 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 f iberoptic. 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

12 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;

13 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;

14 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;

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.

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

16 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 wl, w2, 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.

17 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 wl 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 wl. However, it is free from other frequencies, which can pass through the bandpass wl 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.

2 i 83502

18 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 wl 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 wl 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 wl signal.
For two tone modulation two PLLs 35a and 35b generating wland w2 are necessary as shown in Fig. 5A.
These are then combined in a secondary mixer 23 to generate a(wl - 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

19 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.

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 2f835Q2

21 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 10 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.

22 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

23 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

24 good quality retroreflector since displacement of the return beam upon reflection tends to make the return beam follow a slightly different path. In addit"ion, 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 2 1U.15VL

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

26 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

27 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

28 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,

29 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 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

31 2183502 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 1 or less. The stepper motor controller 138 is preferably supervised by the computer 135 to coordinate the laser transmitter 131, laser receiver

32 2183502 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 120 would be enough. Stepper motor 142 and mirror 141 are mounted in a frame 148 that is rotatably mounted on bearings 146.

33 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.

34 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 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 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 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.

Claims (24)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A gas detector for detecting the presence of a target gas in a target zone, the target gas having an absorption line, the gas detector comprising:
a laser for producing frequency modulated light output including light having a wavelength that is absorbed by the target gas;
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;
a mixer for mixing the detected signal and the reference signal to produce mixer output; and a signal analyzer connected to the mixer for detecting the presence of the gas from the output of the mixer.

2. The gas detector of claim 1 in which the signal analyzer includes a filter having a pass band that includes the low frequency output of the mixer.

3. The gas detector of claim 1 in which the reference signal generator includes a frequency multiplier for producing a signal having a frequency corresponding to a harmonic of the modulation frequency of the light output from the laser.

4. The gas detector of claim 1 in which the reference signal generator includes a bandpass filter having a pass band that includes the modulation frequency of the light output from the laser.

5. The gas detector of claim 1 in which the reference signal generator is connected to receive output from the photo detector.

6. The gas detector of claim 1 in which:
the laser is mounted in a protective enclosure;
the protective enclosure includes a window for the laser light output to pass through;
means for shifting etalon fringes produced by the window to frequencies that are capable of being filtered out from the detected signal; and the signal analyzer includes a filter for discriminating against the etalon fringes.

7. The gas detector of claim 6 in which the means for shifting the etalon fringes includes the window having a wedge shape.

8. The gas detector of claim 1 in which the laser is a diode laser.

9. The gas detector of claim 1 further comprising:
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;
means to selectively direct light from the gas reference cell or from the target zone to the photodetector;
and a data analyzer including 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.

10. The gas detector of claim 9 further including a light sensor for detecting presence or absence of light returning from the target zone.

11. The gas detector of claim 1 further including a phase shifter for adjusting the phase difference between the detected signal and the reference signal.

12. A gas detector for detecting the presence of a target gas in a target zone, the target gas having an absorption line, the gas detector comprising:
a laser for producing frequency modulated light output including light having a wavelength that is absorbed by the target gas;
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 having a frequency corresponding to a modulation frequency of the light output from the laser;
a mixer for mixing the detected signal and the reference signal to produce mixer output;
the laser being mounted in a protective enclosure;
the protective enclosure including a window for the laser light output to pass through;
means for shifting etalon fringes produced by the window to frequencies that are capable of being filtered out from the detected signal; and a data analyzer connected to the mixer for detecting the presence of the gas from the output of the mixer, the data analyzer having means to discriminate against the etalon fringes.

13. The gas detector of claim 12 in which the means for shifting the etalon fringes includes:

the window having front and back faces that are at an angle to each other.

14. The gas detector of claim 13 in which the front and back faces of the window are at an angle of greater than 1°.

15. The gas detector of claim 12 in which 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.

16. A gas detector for detecting the presence of a target gas in a target zone, the target gas having an absorption line, the gas detector comprising:
a laser for producing frequency modulated light output including light having a wavelength that is absorbed by the target gas;
a gas reference cell for containing a sample of the target gas;
a photo detector for producing a detected signal as output from light from the laser that has passed through the target zone;
means to selectively direct light from the laser to the gas reference cell or the target zone;
means to selectively direct light from the gas reference cell or from the target zone to the photodetector;
a reference signal generator to create a reference signal by detection of light having a frequency corresponding to a modulation frequency of the light output from the laser;
a mixer for mixing the detected signal and the reference signal to produce mixer output;
a data analyzer connected to the mixer for detecting the presence of the gas from the output of the mixer; and the data analyzer including 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.

17. The gas detector of claim 16 further including a light sensor for detecting presence or absence of light returning from the target zone.

18. 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.

19. The method of claim 18 further including the step of:
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.

20. The method of claim 18 further comprising the steps of:
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.

21. The method of claim 18 in which the reference signal is a harmonic of one of the modulation frequencies.

22. The method of claim 21 in which the reference signal is the second harmonic of one of the modulation frequencies.

23. A gas detector for detecting the presence of a target gas selected from the group comprising methane and ammonia in a target zone, the target gas having an absorption line, the gas detector comprising:
a laser having output at about 1.32 µm for producing frequency modulated light output including light having a wavelength that is absorbed by the target gas;
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 having a frequency corresponding to a modulation frequency of the light output from the laser;
a mixer for mixing the detected signal and the reference signal to produce mixer output; and a data analyzer connected to the mixer for detecting the presence of the gas from the output of the mixer.

24. A gas detector for detecting the presence of a target gas in a target zone, the target gas having an absorption line, the gas detector comprising:

a laser for producing frequency modulated light output including light having a wavelength that is absorbed by the target gas;
a gas reference cell for containing a sample of the target gas;
a photo detector for producing a detected signal as output from light from the laser that has passed through the target zone;
means to selectively couple the gas reference cell into a light path between the laser and the photo detector;
a reference signal generator to create a reference signal by detection of light having a frequency corresponding to a modulation frequency of the light output from the laser;
a mixer for mixing the detected signal and the reference signal to produce mixer output;
a data analyzer connected to the mixer for detecting the presence of the gas from the output of the mixer; and the data analyzer including 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.