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.

Description:
FIELD OF THE INVENTION 
     This invention relates to laser absorption spectroscopy with frequency modulation detection, and in particular to the remote detection of gases using laser absorption spectroscopy. 
     BACKGROUND OF THE INVENTION 
     In laser absorption spectroscopy with frequency modulation detection, 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 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 &#34;two tone method.&#34; 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 w 0 , w 1 , w 2 , w 3 , etc is mixed with frequency component w 0 , which is taken directly from the current modulator for the laser diode. The dc output (w 0  --w 0 ) 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. 
     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 &#34;W&#34; 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, U.S. Pat. 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 90° 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 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. 
     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 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 homodyne 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 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 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 
     homodyne 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. 
     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 and/or ammonia is carried out at their 1.3165 μm absorption lines. 
     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; 
     FIG. 2 is a schematic showing a detection circuit for a first embodiment of the invention; 
     FIG. 3 is a schematic showing a detection circuit for a second embodiment of the invention; 
     FIG. 4 is a schematic showing a detection circuit for a third embodiment of the invention; 
     FIG. 5 is a schematic showing a detection circuit for a fourth embodiment of the invention; 
     FIG. 5A is a schematic of part of an alternative reference signal generator for the embodiment of FIG. 5; 
     FIG. 6 is a schematic showing an embodiment of the invention used for the resolution of gas density; and 
     FIG. 7 is a schematic showing a section through a window for use in reducing etalon fringes in the operation of the invention. 
    
    
     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 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 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 receiver 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 w 1 , w 2 , 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 w 1 , 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. 
     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 w 1 , 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 embodiment of this invention 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 w 1  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 w 1 . However, it is free from other frequencies, which can pass through the bandpass w 1  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 embodiment of this invention 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 w 1  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. 
     A further exemplary and improved embodiment 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 2w 1 . For the technique referred to as two tone modulation the laser is modulated at two frequencies w 1  and w 2  (besides the ramp frequency) and the difference signal at frequency (w 1  -w 2 ) is detected. In FIG. 5, the PLL 35 is activated at frequency w 1  and generates an output at the harmonic frequency 2w 1 . This harmonic signal then acts as the local oscillator in the mixer 22 and the 2w 1  signal is detected as desired. In this way the PLL 35, is activated by the much stronger w 1  signal. For two tone modulation two PLLs 35a and 35b generating w 1  and w 2  are necessary as shown in FIG. 5A. These are then combined in a secondary mixer 23 to generate a (w 1  -w 2 ) 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 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 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 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 to the beam splitter 42 that passes through a target zone to the reflector 18. Operation of the 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 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 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. 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 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 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 μm 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 μm absorption line for methane, within the water vapour window between 1.3162 to 1.3169 μm. Since there is also an ammonia absorption line at about 1.3165 μm, if ammonia may be present, detection should also take place at about 1.3177 μm within the 1.3173 μm to 1.3184 μm water vapour band since ammonia also has an absorption line at about 1.3177 μm while methane does not. Hence, during processing of the detected signal reflected back from a reflector, detection of absorption at 1.3177 μm distinguishes ammonia from methane, and absence of detection of absorption at 1.3177 μm distinguishes methane from ammonia. The methane absorption line at 1.3165 μm 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 μm line. However, adoption of this line for detection allows communication band lasers at about 1.32 μm to be used for both the detection of methane and ammonia. 
     A person skilled in the art could make immaterial modifications to the invention described and claimed in this patent without departing from the essence of the invention.