MXPA00005601A - Multi-gas sensor - Google Patents

Abstract

A multi-gas sensor is provided which modulates a polarized light beam over a broadband of wavelengths between two alternating orthogonal polarization components. The two orthogonal polarization components of the polarization modulated beam are directed along two distinct optical paths. At least one optical path contains one or more spectral discrimination means, with each spectral discrimination means having spectral absorption features of one or more gases of interest being measured. The combined polarization modulated beam is partitioned into one or more smaller spectral regions of interest where one or more gases of interest have an absorption band. The difference in intensity between the two orthogonal polarization components is then determined in each partitioned spectral region of interest as an indication of the spectral emission/absorption of the light beam by the gases of interest in the measurement path which of the concentration of the gases of interest.

Description

MULTI-GAS DETECTOR Origin of the Invention The invention described herein was made collectively by employees of the Government of the United States, and during the execution of work under NASA contracts and was subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 , as amended, Public Law 85-568 (72 Statutes 435; 42 USC 2457), and 35 USC 202, respectively. According to 35 USC 202, the chosen contractor did not retain the title.

BACKGROUND OF THE INVENTION 1. Technical Field of the Invention The present invention relates to the simultaneous measurement of two or more gases using optical path switching. More specifically, it refers to such measurement using dual-beam spectroscopy, including gas filter correlation radiometry. 2. Discussion of Related Art Optical path switching has many potential applications, particularly in the field of double beam spectroscopy. In the spectroscopy of REF. 120752 double beam, the light from a radiation source traverses a measurement path and then divides into two optical paths. Each optical path generally contains some medium through which the radiation is transmitted and thus partially absorbed and / or reflected. The key measurements in this type of spectroscopy refer to the difference in intensity of the radiation taken by these two trajectories. For illustrative purposes, a gas filter correlation radiometer (GFCR), an example of a double beam spectrometer, will be discussed in detail.

Gas filter correlation radiometers (GFCR) could infer the concentration of a gas species between some measurement path, either external or internal to the GFCR. In many GFCRs, gas detection is performed by looking alternatively through two optical cells at the emission / absorption of the gas molecules along the measurement path. These two optical cells, often called correlation and vacuum cells, are an example of the means found in the two optical paths of a double beam spectrometer. The correlation cell contains a high optical depth of the gas species i and thus strongly absorbs radiation at the molecular transition wavelengths of the particular gas. In effect, the correlation cell acts as a "notch filter" of the spectrum for the radiation entering, the notches of the spectrum are coincident with the band structure of the gas species i. The vacuum cell generally stores a vacuum or a gas or mixture of gases, which have negligible or shallow optical depth, e.g., nitrogen, an inert gas, or even clean dry air. The difference in signal between these two inspections of the emission / absorbance of the gas species i in the spectral region of interest plus, or in combination with, the sum of the signals from these two inspections can be related to the concentration of this gas along the measurement path.

In a known GFCR for measuring a concentration of a gas in a particular amount, described in U.S. Pat. No. 5,128,797, published by Sachse et al. and assigned to the National Aeronautics and Space Administration (NASA), the specification of which is incorporated herein by reference, a non-mechanical optical path switching comprises a polarizer, a polarization modulator and a beam splitter of polarization. The polarizer biases the light of a light source into a single, e.g., vertically polarized component that is then rapidly modulated into vertically and horizontally polarized alternate components by a polarization modulator. The polarization modulator could be used in conjunction with an optical wave plate. The modulated polarization beam is then incident in a polarization beam splitter that transmits light from an orthogonal component, e.g., horizontally polarized, and reflects light from a perpendicular component, e.g., vertically polarized. In an application of the gas filter correlation radiometer, the transmitted horizontally polarized beam is reflected by a mirror, passes through a gas correlation cell, and is transmitted through a second beam splitter. The vertically reflected polarized beam passes through a vacuum cell, is reflected by a mirror and then reflected by the second beam splitter. The beam combiner recombines the horizontal and vertical components in a single beam that is read by a conventional detector. This approach has numerous advantages, such as non-mechanical means that are required to alternate the inspection of the detector through the correlation and vacuum cells, rapid response, etc.

It would be desirable, in numerous applications, to be able to measure two or more gas concentrations simultaneously, either independently or non-independently, with a single device using optical path switching. Furthermore, it would be desirable to make such a measurement with optimum optical balance.

Objectives of the Invention Thus it is an object of the present invention to provide a device for measuring simultaneously, but not independently, two or more gases of interest.

It is another object of the present invention to provide a device for measuring simultaneously, but not independently, two or more gases of interest with negligible spectrum interference or without interference.

It is another object of the present invention to provide a device for simultaneously and independently measuring two or more gases.

It is another object of the present invention to provide a device for simultaneously and independently measuring two or more gases with negligible interference or without spectrum interference.

It is another object of the present invention to provide a device that uses optical switching to simultaneously measure two or more gases for various applications that require two paths of optical analysis.

It is another object of the present invention to perform double beam spectroscopy, such that the gas filter correlation radiometry uses a single instrument to measure two or more gases in which the difference and sum signals can be obtained only from one detector for each region of interest of gas wavelength.

It is another object of the present invention to perform simultaneous and independent measurement of two or more gases using a minimum of optical components.

It is another object of the present invention to perform simultaneous but not independent measurement of two or more gases using a minimum of optical components.

It is another object of the present invention to detect the total charge of a mixture of two or more gases using a single instrument.

It is another object of the present invention to detect some threshold level of the presence of any or a combination of several gases using a single instrument.

It is still another object of the present invention to provide a device for simultaneously measuring two or more gases of interest and optimizing the optical balance.

It is still another object of the present invention to provide a device for simultaneously measuring two or more gases of interest and optically optimizing the optical balance.

It is a further object of the present invention to provide a device for simultaneously measuring two or more gases of interest and electronically optimizing the optical balance.

The additional objectives and disadvantages of the present invention are apparent from the specification and drawings that follow.

Brief Description of the Invention The aforementioned and additional objectives are obtained by modulating a beam of light over a wide band of wavelengths between two alternating orthogonal polarization components. An orthogonal polarization component of the modulated polarization beam is directed along a first optical path, and the other orthogonal polarization component is directed along a second optical path. At least one optical path contains one or more spectrum discrimination means, with each spectrum discrimination means having spectrum absorption characteristics of one or more gases of interest to be measured. Then the two optical paths intersect, and an orthogonal component of the intersected components is transmitted and the other orthogonal component is reflected. This forms a modulated beam of combined polarization, which contains the two orthogonal components in alternating order.

The combined biased beam is divided into one or more smaller spectral regions of interest, where one or more gases of interest has an absorption band. The difference in intensity between the two orthogonal polarization components in each distributed spectral region of interest is determined as an indication of the emission / absorption of the light beam spectrum along the measurement path. The emission / absorption of the spectrum is indicative of the concentration of one or more gases of interest in the measurement path.

More specifically, one embodiment of the present invention is a gas filter correlation radiometer comprising a polarizer, a polarization modulator, a polarization beam separator, a do-combiner, a wavelength distribution means and a detection means. The polarizer biases the light of a light source into a single component, e.g., vertically polarized, which is then rapidly modulated into vertically and horizontally polarized alternating components by the polarization modulator. The polarization modulator could be used in conjunction with an optical wave plate. The polarization of the modulated beam is then incident on the bias beam splitter that transmits light from an orthogonal component, e.g., horizontally polarized, and reflects light from a perpendicular component, e.g., vertically polarized. In a GFCR mode that uses two gas cells to measure the two gases (hereinafter "two gas mode / two gas cells"), the polarized beam reflected vertically passes through a first gas correlation cell that it contains a first gas of interest, is reflected by a mirror and is then transmitted or reflected through the beam combiner. The polarized beam transmitted horizontally passes through a second gas correlation cell, which contains a second gas of interest, is reflected by a mirror, and is reflected or transmitted by the beam combiner. The beam combiner recombines the horizontal and vertical components in a single beam in which the polarization is variant with time. The combined light energy is then divided into regions of wavelengths corresponding to each gas absorption band. A first optical bandpass filter focuses on a gas band. This radiation is then focused on a first detector. The reflected radiation of the first optical bandpass filter is incident on a second optical bandpass filter. The radiation within the bandpass of the second filter, centered in the absorption band of the second gas, is transmitted and focused on a second detector. The repartition could be done in a number of ways that include the use of filters, reticles and optical prisms. As long as the first gas does not have absorption characteristics within the region of the spectrum defined by the bandpass filter of the second gas, the first gas correlation cell acts as a vacuum cell for the second gas, and vice versa. In some cases, the first and second gases, e.g., gases that do not interact chemically, could be contained within the same correlation cell. The measurements of both gases are made simultaneously, independently and without interference. In addition, the optical and electronic means are provided to balance the optical intensities between the two optical paths.

Similar configurations are used to measure three or more gases, including a GFCR mode that measures three gases using two gas cells (in the following "three gas / two gas cell mode") and a GFCR mode that measures after gas using three gas cells (hereinafter "three gases / three gas cells" mode). The presence of several gases can also be detected simultaneously but not independently, eg, to detect the total load of two or more gases without the need to know the concentration of each one individually or detect some threshold level of the presence of one or combination of several gases.

Brief Description of the Drawings FIG. 1 is a schematic representation of a GFCR configuration for measuring two gases, using two gas cells according to the present invention.

FIG. 2 is a graph showing the approximate change in the intensity of radiation at some optical wavelength with time along two optical paths generated by the FIG modality. 1.

FIG. 3 is a graph showing the effects of NO on the measurement of CO with GFCR during the simultaneous measurements of NO and CO.

FIG. 4 is a schematic representation of a GFCR data acquisition configuration and control system.

FIG. 5 is a schematic representation of electronic balancing using digital signal processing.

FIG. 6 is a schematic representation of electronic balancing using a gain modulated amplifier.

FIG. 7 is a schematic representation of el-ectronic balancing using an analog A / B amplifier.

FIG. 8 is a schematic representation of electronic balancing using an analog AXB amplifier.

FIG. 9 is a schematic representation of a GFCR mode of three gases / three gas cells; Y FIG. 10 is a schematic representation of a GFCR mode of three gases / two gas cells.

FIG. 11 is a schematic representation of a GFCR configuration for making a measurement of total hydrocarbons.

Detailed Description of the Preferred Modalities Referring to FIG. 1, a filter correlation radiometer (GFCR) 10 according to the present invention is shown. The optical system 12, eg, a telescope or other lens / mirror system, collects light from a radiation source such as the earth and the atmosphere, when GFCR 10 is mounted on a satellite or aircraft, a black body when GFCR 10 is use as a laboratory or instrument in situ, the sun, a laser, etc. This light beam, in general, comprises both the vertically polarized components V and the horizontally polarized components H. The optical polarizer 14 is provided after the optical system 12 and aligned to polarize the radiation entering the desired directional component, eg, vertically in the embodiment described in FIG. 1. A polarization modulator 18 then receives the polarized beam incident vertically and rapidly modulates the output beam between vertical and horizontal polarization. Depending on the application of the measurement and the type of polarization modulator used, the bias modulation frequency could be in the range of almost DC to radio frequency (RF). The polarization modulator could be used in conjunction with an optical wave plate 16.

The polarization beam separator 20 non-mechanically commutates the biased modulated output beam between the two paths, in the embodiment of FIG. 1, reflecting the beam along the OPi path when it is vertically polarized, and transmitting along the path 0P-¡when it is polarized horizontally. Alternatively, the beam splitter 20 could be oriented to transmit vertically polarized light and reflect horizontally polarized light. The approximate time change in the radiation intensity at a specific optical wavelength, for the two optical paths OPi and OP. ,, is shown in FIG. 2 as the polarization that is switched from vertical to horizontal in the specific embodiment shown in FIG. 1.

The polarization beam separator 20 thus alternately directs the beam along the first and second optical paths. In the specific embodiment shown in FIG. 1, the beam splitter is oriented to reflect the vertically polarized light, so that it passes through a first gas correlation cell i 22 containing a high optical depth of the first gas i of interest. The outgoing light is then reflected by the mirror 24 to intersect a beam combiner 26. The optical path of the first beam splitter 20 through the first gas correlation cell i 22 to the beam combiner 26 is designated the first optical path of gas correlation i OP !.

The first beam splitter 20 transmits horizontally polarized light, which then passes through the second gas correlation cell j 28, which contains a high optical depth of the second gas species j of interest. The light that emerges is then reflected by the mirror 30 so that it intersects the beam combiner 26. In some cases, the correlation cells 22 and 28 could be replaced by optical interference elements whose spectrum emissions have been designed to approximately replicate the absorption characteristics of the gas species i and j of interest. However, such interference elements have disadvantages such as a strong angular dependency and wide spectrum notches that allow the interference of any species of gas that interfere spectrally along the measurement path.

The beam combiner 26 could be a second polarization beam splitter for efficiently combining the two GFCR beams, which represent the two orthogonal polarizations, into a single beam in which the polarization state changes in time at the fundamental frequency and harmonics of this frequency of the polarization modulator 18. Alternatively, the beam combiner 26 could be a single wideband beam splitter, 50/50 as an example. With the 50/50 wide band beams separator, the two beams are still combined; however, substantial optical energy is lost. In applications where the operation of the system is not limited to power, the second approach would suffice and would save component costs. The optical path of the first beam splitter 20 through the second gas correlation cell j 28 to the beam combiner 26 is designated the second gas correlation optical path j 0P3 and should be optically similar, eg, in length, to the first optical path of gas correlation i OPi. This optical similarity is not required but it is good optical practice.

The first gas i in the first gas correlation cell i 22 optically acts as a vacuum for the measurement of the second gas j since it is presumed that the gas i has negligible optical absorption or depth characteristics in the optical band pass of the measurement of gas j. Similarly, the second gas j in the second gas correlation cell j 28 acts as a vacuum for the measurement of the first gas i. The measurements of both gases i and j are made simultaneously, independently and without interference. The two gases i and j should not overlap spectrally in the respective optical band passages of the measurements of the two gases; i.e., the spectral absorption characteristics of gas i must not fall within the optical bandpass of gas measurement j and vice versa. CO and NO are examples of the two gases.

The beam combiner 26 may be selected to have the same or opposite transmission and reflection properties, such as the first beam splitter 20. In the embodiment shown in FIG. 1, has opposite properties, transmitting the vertically polarized light of the first optical gas correlation path i OPi and reflecting the horizontally polarized light of the second optical gas correlation path j OP-. The orientation of the mirrors 24 and 30 and the first beam splitter 20 cause the two optical paths to intersect in the beam combiner 26.

After the combiner 26, a wide band is presented, limited only by the source spectrum and the transmitting and reflecting spectral properties of the optical components, the optical wavelengths and their respective polarization states are varied over time in the fundamental and harmonic frequencies of the polarization modulator 18, ie, the beam has to alternate rapidly the vertical and horizontal components. From this point, the optics are used to spread the wide band of the optical wavelengths in smaller spectral regions, where each of the gases i and j of interest have absorption bands. This reappearance could be done in a number of ways that include the use of filters, reticles and optical prisms.

In the embodiment shown in FIG. 1, the optical bandpass filter 32 transmits the radiation centered in the gas band i. This radiation is then focused by means of a focusing mirror or refractive lenses 36 on the first detector 34. This optical focusing element 36 could be eliminated if the concentration of the radiation in the detector 34 to achieve higher measurement operation is not necessary . The information relating to the concentration of the gas i is contained in the output of the detector 34, in the electronic frequencies corresponding to the fundamental and harmonic frequency of the modulator 18 and to the base band; i.e., the baseband of the "DC" signal gives the total power incidence in the detector 34 and could be used to normalize the difference signal.

The beam combiner 26 could be oriented in the opposite direction to the first beam splitter 20, where the horizontal components pass through and the vertical components are reflected to the right, needing to locate the optical bandpass filter 32 under the beam combiner. you make 26 in FIG. 1.

The optical bandpass filter 32 reflects the radiation of other wavelengths, but also present in this reflected radiation is a small amount of radiation corresponding to the spectral region of the gas i. The optical bandpass filter 38 transmits only wavelengths centered approximately in the gas band j and this radiation could be focused by means of the focusing mirror or refractive lenses 42 in the second detector 40. Again, the electronic output of the detector 40 contains information of the concentration of the gas j in the fundamental frequency and harmonics of the modulator 18 and in the base band.

This distribution of wavelengths could be done in other ways. An alternative is to replace a wideband beam splitter with the optical bandpass filter 32. However, if this is done, a bandpass filter should be placed in front of the focusing optics 36 as for the pass filter. band 38. Other combinations could include the use of long wave and / or short wave pass filters with bandpass filters. A reticle or prism could be used to separate several wavelengths.

The DC (I) output of the detectors 34 and 40 is proportional to the incident optical intensity within the bandpass of the species i and j respectively, while the amplitude of the AC output at frequencies corresponding to the fundamental frequency and / or harmonics of the polarization modulator 18 refers to the difference in intensity (? I) between the horizontally and vertically polarized radiation received within the bandpass of the gas species i and j. The magnitudes of the difference signal and the average incident intensity signal refer to many factors, including: (1) the radiation properties of the radiation source; (2) the concentration and distribution of the gas (s) of interest and any species of gas that interfere spectrally along the measurement path; (3) pressure and temperature distributions along the measurement path; (4) length of the measurement path; (5) amount of gas in the correlation cells and length of the cell, etc. The radioactive transfer algorithms could be used together with information from the I and signals for each gas of interest to infer total column amounts of the gases of interest along the measurement path. In addition, any other conventional method could be used to manipulate the results detected by the detectors 34 and 40. For example, an apparatus could be used to calibrate the? I / I response of the GFCR for known concentrations of the gases of interest throughout the measurement trajectory.

An example of a two gas / two gas cell instrument, which has been implemented using the embodiment of FIG. 1, is a device that measures CO at 4.7 μm and NO at 5.2 μm. The results showed that there is no interference for the measurement of NO caused by CO, and without interference for the measurement of CO caused by NO). FIG. 3 illustrates the lack of NO interference in a CO GFCR measurement.

Therefore the present invention allows a single detector to be used for each gas of interest to obtain the difference? I and the sum of I-signals, thus reducing the balancing requirements and the problems of homogeneity of the detector surface associated with the GFCR that require two detectors to detect a single species of gas. The key components of the invention, such as the embodiment of FIG. 1, are the polarizer 14, the polarization beam splitter 16, the polarization modulator 18, the beam combiner 26, the optical bandpass filter 32 and the bandpass filter 38. All are commercially available and some Basic parameters to use in your selection of various applications are discussed in the following paragraphs. Since many of the characteristics of the components are wavelength dependent, the spectral region for a desired application is important in the selection of the component.

Polarizer 14 can be eliminated if a polarized light source such as a polarized laser is used. If necessary, the polarizer 14 linearly polarizes incoming radiation before it is incident on the polarization modulator 18. Important parameters of the polarizer include extinction ratio, transmission and angular acceptance. Common polarizer types include prism, dichroic and wire grid polarizers. The prism and reflection polarizers have high extinction ratios, but their poor angular acceptance could limit their application. The dichroic and wire grid polarizers, on the other hand, have wide angular acceptance. In addition, dichroic polarizers have high extinction ratios and are commercially available for the visible and near infrared region. Wire mesh polarizers have moderate to good extinction ratios and are available for infrared applications.

The purpose of the polarization beam splitter is to separate the orthogonal polarization components from the radiation after the polarization modulator 18. Thus, the ratio of loss and extinction as well as the angular acceptance should be considered. The same consideration should apply to beam combiner 26, which combines the two orthogonal polarizations in GFCR applications. Dichroic polarizers are not acceptable as beam combiners, since they strongly absorb one of the components of the polarization. The prism and reflection bias beam separators could be used only in applications where angular acceptance is not a primary interest. Wire mesh polarizers with their large acceptance angle and moderate to good extinction ratios, both for transmission and reflection, are good candidates for infrared beam combiners.

Polarization modulator 18, which could also be used in conjunction with a wave plate 16, alternatively modulates the polarization state between two orthogonal linear polarizations, H and V. Important parameters include transmission loss and angular acceptance; and since the modulators are energized devices, the energy consumption and heating effects are also important. The commercially available electro-optical and photo-elastic modulators are those that operate in a wide-spectrum region, including UV, visible and infrared. Both types of modulators generate a polarization change modulating the birefringence of an optical crystal. In the electro-optical modulator a strong electric field is applied to produce the desired birefringence change, while in the photo-elastic modulator, the mechanical stress induced by a transducer attached to the optical crystal generates the birefringence change. The magnitude of the voltage applied to an electro-optical modulator for a given birefringence modulation increases with increasing optical wavelength. For this reason, modulators that use the electro-optical effect are generally more suitable for applications of shorter wavelength; i.e., UV, visible and near infrared. One advantage of electro-optical modulators is their wide electronic bandwidth that allows them to be modulated with a variety of electronic waveforms. Square wave or other polarization waveforms may be useful in GFCR applications to approximate the commutation or "modulation" obtained by mechanical commutation. To reduce the directed power requirements of the photo-elastic modulators, these devices are generally excited at the resonant frequency of the photo-elastic crystal. The photo-elastic modulators must therefore be excited with a sinusoidal electronic waveform. The resulting polarization modulation will have a characteristic quasi-sine wave that actually contains frequencies that correspond to the fundamental and harmonic frequencies of the polarization modulator 18. The photo-elastic modulators are commercially available for UV applications, visible and infrared. The heating of the glass, the mechanical strength of the crystals and the loss of optical transmission are factors that limit applications of greater wavelength. Other potential polarization modulators include magneto-optical devices that possibly employ the effects of Faraday or Kerr, liquid crystal devices (LCD), etc.

In general, only a single frequency of the outputs? I of the detector 34 and 40 is demodulated synchronously and processed further. Depending on the phase delay characteristics of the wave plate 16 and the magnitude of the phase delay of the bias modulator 18, the optimum frequency for demodulation could be either the fundamental of the bias modulator 18 or a specific harmonic of the Polarization modulator 18 ..

The electronics 44 and 46 control the operation of the GFCR. The operation of the GFCR 50 can be controlled by a PC-based data acquisition and control system such as that shown in the scheme of FIG. 4, which illustrates the entry of a single detector. The preamplified output of an optical detector is further amplified by means of two variable gain amplifiers 52 and 54, one for the AC portion of the signal at the fundamental and / or harmonic frequencies of the modulator 18, and the other for the portion of DC of the signal. A synchronous demodulator 56 extracts the magnitude of the signal at the fundamental and / or harmonic frequencies of the bias modulator 18, using a frequency reference signal from the bias modulator 18. The AC and DC signals are passed through two frequency filters. step under evenings 58 and 60 to narrow the electronic bandpass, thus suppressing noise. The signals are converted into their digital representations by means of an A / D converter 62, for processing by means of a personal computer (PC, 64). The controller 66 controls the operating temperature of the thermoelectrically cooled detectors in the GFCR -50. The control 68 excites the bias modulator 18 at some frequency (in the case of a photo-elastic modulator, at its resonance frequency) and at the delay level of the desired optical phase, and provides a reference frequency for the demodulation Synchronous The outputs? I of any detector could be balanced to: (1) zero the output of the instrument (i.e.,? I = 0) independently of each gas, when the gas does not appear in the inspection field of the instrument; or (2) "zero" the output of the instrument for some predecessor value of a specific gas, e.g., the typical predecessor level of 1800 ppbv CH4. This balance function performed in any of the detectors, in effect, equals the transmission of optical paths OPi and OP. in the passage of optical band inspected by the detector. By "balancing" the output of? I, certain instrument noises, eg, noise from the systematic radiation source and noise associated with fluctuations of the instrument's inspection field, could be strongly suppressed, thus increasing the sensitivity of the measurement for the kind of particular gas. In a balanced measurement situation, the same source and misaligned noise is inspected alternately, but rapidly, through both optical paths of GFCR and it is common mode to reject them from the resulting signal I.

This balancing of optical intensities between the two optical paths could be achieved by several means. Examples of such means are: (1) adding a polarization-dependent optic against the detectors, and (2) electronic balancing of the detector output that varies the electronic gain synchronously with the optical ha, alternately between the two optical paths . FIG. 1 shows the addition of polarization-dependent optics 33 and 39 versus detectors 34 and 40, respectively. Each optic 33 and 39 could be a film, e.g., a thick plastic membrane of several microns that transmits in the region of the spectrum of interest. The material of the film, thickness and angle of incidence could be chosen to execute the swing of the optimal trajectory. Other optical components could include a thicker infrared transmission glass or an amorphous window material. The surfaces of these windows could also be covered with thin films that would increase their polarization selectivity. An infrared polarizer, e.g., a wire mesh polarizer, could also be used to perform optical balance. In this case, the polarizer is rotated to favor one polarization over another. Polarization dependent optics, for simplification purposes, are not shown in FIGS. 9 to 11.

An optical device as described in the previous paragraph could be installed and set for a particular balance situation and never reset. However, if near perfect balance is required to obtain maximum sensitivity for a given application, small changes in the angle, e.g., of a film, or in the rotation, e.g., of the wire mesh polarizer are necessary. This could be done manually by the operator or could be controlled by computer through a motorized device.

An alternative technique to achieve balance is through the use of electronic methods. Electronic methods could be used to obtain the full balance or could be used in conjunction with an optical method to obtain the balance. For example, the optical technique could achieve rough preset balance, while the electronic method could be used for fine tone and, through computer control, continuously optimize the balance for the measurement and manual task.

The electronic balance could be implemented digitally in the following way. The output V (t) of the amplifier 52 is digitized by means of a digital signal processor (SP) 82 in FIG. 5. The elements in FIGS. 5 to 8 are consistently listed with similar elements of FIG. 4. This signal V (t) includes the baseband signal as well as the difference signal? I (t) at the fundamental and harmonic frequencies of the polarization modulator 18. For zero the difference signal, i.e. ? I = 0, at a frequency f specific to the polarization modulator 18, ie, the fundamental or one of its harmonics, the digitized signal V (T), in real time, is divided by the balance function ß (t), where ß (t) = 1 + a sin (2p ft + f), where the phase f is chosen to be in phase with the signal? I (t) at the frequency f and is set by the computer 64 from time to time to achieve the desired balance level. Then the DSP 82 synchronously demodulates the function V (t) / ß (t) at frequency f. The demodulated signal is then digitally filtered low on the DSP 82, to reduce the resulting bandwidth with greater sensitivity. The magnitude of this demodulated signal refers to the intensity difference of the beam I (which passes through the optical paths Opi and Opj.) This digital demodulated signal is sent to the computer 64, which in turn could use this information The DSP 82 also averages the signal V (t) / ß (t) using a digital low pass filter identical in characteristics to the aforementioned filter. V (t) / ß (t) of the filter refers to the average incident power in the detector, ie, the I signal. This digital signal is also transmitted to the computer 64. Computer 64 could then calculate the ratio? I / I refers to the emission / absorption of the species of interest along the measurement path.

The DSP 82 could perform the balance function by multiplying the signal V (t) by the function? (T) which is simply the inverse of ß (t). That is,? (T) = l / ß (t).

In this way,? (T) is the geometric progression of l / ß (t). For small,? (T) = 1 - sin (2p f t + f).

FIGS. 6 to 8 indicate different ways in which electronic balancing could be achieved using analogous techniques. For example, in FIG 6, an A / B amplifier 102 is used where the input A is the analogous function V (t) and B is an analogous waveform equivalent to β (t). Alternatively, an AXB amplifier 104 could be used, as shown in FIG. 7, where again A is the analogous signal V (t) but B is the analog equivalent of the signal? (T). In another approach shown in FIG., The gain of the sense amplifier 52 is modulated by modulating the resistance of the feedback resistor 84 of the amplifier. For example, this could be done if the feedback resistor 84 is a photoresistor that is modulated with the analogous function? (T). In previous analogous cases, additional electronics must be added to generate waveforms that resemble ß (t), i.e., waveform generator? (t) 90 shown in FIGS. 7 and 8, and waveform generator ß (t) 100 shown in FIG. 6. It is also assumed in the above analogous cases that the synchronous demodulation of the analogue is used. To change or adjust the balance, computer 64 should control the magnitude of a in the analogous waveform generators 90 and 100.

A disadvantage of using polarization-dependent optics is that they must be tilted or turned to change or perhaps maintain balance. However, if used in conjunction with an electronic balancing scheme, polarization-dependent optics could be presented mechanically for some coarse balance. The electronic balancing circuit could use them to take out the balance in some automatic or pre-programmed way. In this way, the highest measurement sensitivity could be achieved consistently.

One GFCR mode of three gases / three gas cells is shown in FIG. 9. The elements in FIGS. 9 to 11 are consistently listed as the elements of FIG. 1. As the measurement of two gases, the three gases to be measured simultaneously and independently should not overlap spectrally in the various optical band passages of the target gases to ensure that each gas is measured independently with negligible interference. This three gas measurement configuration comprises a second optical bandpass filter 82 and a third set of detection components. Looking more specifically at the three gas mode of FIG. 9, the biased modulation gas is incident on the bias beam splitter 20 which transmits light from an orthogonal component, e.g., horizontally polarized, and reflects light from a perpendicular component, e.g., vertically polarized. The vertically reflected polarized beam passes through the first gas correlation cell i 22 containing a first gas i, reflected by the mirror 24, passes through a third gas correlation cell k 72 containing a third gas k, and is then transmitted through a beam combiner 26. The horizontally transmitted polarized beam passes through the second gas correlation cell j 28 which contains a second gas j, is reflected by the mirror 30, and is reflected by the beam combiner 26. The beam combiner 26 combines the horizontal and vertical components into a single beam. The first gas i in the first gas correlation cell i 22 acts as a vacuum cell for measuring the second gas j and third gas k. Similarly, the second gas j in the second gas correlation cell j 28 acts as a vacuum for the measurement of the first gas i and the third gas k, and the third gas k in the third gas correlation cell k 72 acts as a vacuum for the first and second gases i and j. The measurements of the gases i, j and k are made simultaneously, independently and with negligible interference or without interference.

The optical bandpass filter 32 transmits the radiation centered on a gas absorption band i. This radiation is then focused by the focusing mirror or refractive lenses 36 of the first detector 34. The information regarding the concentration of the gas i is contained in the output of the detector 34 at the frequencies corresponding to the fundamental and harmonic frequency of the modulator. 18 and in the baseband.

The reflected radiation of the optical bandpass filter 32 contains a small amount of radiation centered on the gas band i plus all additional wavelengths. The optical bandpass filter 70 then transmits only radiation centered around the gas band j and this beam is subsequently incident at the detector 40, after which it is focused by means of the focusing mirror or refractive lenses 42. Again, the electrical output of this detector 40 contains information of the concentration of the gas j in the fundamental frequency and harmonics of the polarization modulator 18 and in the base band.

The radiation reflected by the optical bandpass filter 70 contains a small amount of radiation in the band of gas i and gas j, in addition to all wavelengths. The bandpass filter 74 transmits only wavelengths centered on the band of the gas k and this radiation is focused on the detector 78 by means of the focusing mirror or refractive lenses 76. Again, the output of the detector 78 contains information of the concentration of the gas k in the fundamental frequency and harmonics of the polarization modulator 18 and in the baseband.

The above discussion pertaining to alternate configurations of the FIG modality. 1 also applies to this mode of three gases / three gas cells.

In the three gas / two gas cell mode shown in FIG. 10, two gases are contained within the second gas cell 28. The two gases must be such that they do not react with each other. As an example of such an embodiment, a GFCR is described which measures the wavelength regions around the NO band of 5.2 μm, the CO band of 4.7 μm and the C130216 band of 4.4 μm. The bandpass filter 32 transmits radiation centered in the 5.2 μm band.

This radiation is focused on the detector 34. The information regarding the concentration of NO is contained in the output of the detector at the frequencies corresponding to the fundamental and harmonic frequency of the polarization modulator 18 and in the baseband.

The reflected radiation from the optical bandpass filter 32 contains a small amount of radiation centered at 5.2 μm, since the optical bandpass filter is not perfect and thus reflects some of this radiation, in addition to all the other wavelengths additional The optical bandpass filter 70 then transmits only radiation centered around the CO band of 4.7 μm and this beam is subsequently incident on the detector 40. Again, the electrical output of this detector contains information of the CO concentration in the fundamental frequency and harmonics of the polarization modulator 18 and in the baseband.

The reflected radiation from the optical bandpass filter 70 contains some small amount of radiation of 5.2 μm and 4.7 μm, in addition to the other wavelengths. The band pass filter 74 transmits only wavelengths centered on the band of C130216 and this radiation is focused on the detector 78. Again, the output of the detector 40 contains information of the concentration of C130216 on the fundamental frequency and harmonics of the modulator of polarization 18 and in the baseband.

The previous discussion that belongs to modalities one and two also applies to this third modality. Four or more gases can be measured simultaneously in a manner similar to the modalities discussed above.

In some applications, it may be important to measure the presence of several gases simultaneously but not independently. Such applications could be (1) to detect the total charge of a mixture of two or more gases need to know the concentration of each individually or (2) to detect some threshold level of the presence of a combination of several gases. An example of the first application is the practice of making a measurement of "total hydrocarbons" in the emission of vehicular traffic, such as automobiles, trucks, etc. Because all hydrocarbons have characteristics in the 3 μm wavelength region due to the rotational-vibration transitions associated with their similar CH (carbon-hydrogen) bands, conventional measurements, eg, with interference filters seek only absorption changes within this spectrum region and do not discriminate between the individual hydrocarbon species. Thus, with these conventional techniques, a measurement of "total hydrocarbons" results; however, substantial interference in the spectrum could also occur for the measurement of other non-hydrocarbon species, e.g., water vapor, and add uncertainty in the measurement. A GFCR measurement, according to the present invention, could be performed by placing two or more of the prominent hydrocarbons expected in the vehicle exhaust in a single correlation cell or individually placing the hydrocarbons in a series of cells, or any combination thereof. . In such an arrangement, a close measurement of "total hydrocarbons" could be made, but with strong suppression of spectral interference of non-hydrocarbon species that also absorb in this region.

FIG. 7 shows such modality. The gas cell 28 contains two or more hydrocarbons. The vacuum cell 22 maintains either a vacuum or a gas or mixture of gases that have negligible optical depth or do not present it. The optical bandpass filter 32 transmits radiation centered in the wavelength region of 3 μm. This radiation is then focused on the detector 34. The information with respect to the concentration of total hydrocarbons is contained in the output of the detector 34 at the frequencies corresponding to the fundamental and harmonic frequency of the polarization modulator 18 and in the baseband .

An example of the second application is the monitoring of an area to detect some low level amount (threshold) of perhaps one or more toxic gases. Again, it may not be necessary to identify each gas, but, at some detectability limit, the instrument must provide a warning of the presence of any or a combination of the toxic gases. As in the previous application, all toxic gases of interest could be contained within a correlation cell or could be placed individually in a series of cells or some combination thereof.

Many modifications, substitutions and improvements will become apparent to one skilled in the art, without departing from the spirit and scope of the present invention, as described herein and defined in the following claims.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (29)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A method for detecting the concentration of two or more gases of interest in a measurement path, characterized in that it comprises the steps of: modulating a beam of polarized light, the beam of light has passed through the measurement path, over a wide band of wavelengths between two alternating orthogonal polarization components; directing an orthogonal polarization component of the polarized modulation beam along a first optical path and directing the other orthogonal polarization components along a second optical path, at least one optical path contains one or more spectrum discrimination means , each spectrum discrimination means has spectrum absorption characteristics of one or more gases of interest; intersect the two optical paths at the point of intersection; transmitting, at the point of intersection, an orthogonal component of the intersected components and reflecting the other orthogonal components, by means of which a combined polarized bias beam is formed, comprising the two orthogonal components in alternating order; distributing the combined biased beam in one or more regions of the spectrum of interest, where one or more gases of interest have an absorption band; and determining the difference in intensity between the two orthogonal polarization components in each region of interest of the partition spectrum as an indication of the emission / absorption of the light beam spectrum by the gases of interest in the measurement path, the emission / Spectrum absorption is indicative of the concentration of one or more gases of interest in the measurement path.

2. The method of claim 1, characterized in that it further comprises the step of polarizing and not polarizing the light beam before modulation.

3. The method of claim 1, characterized in that one or more spectrum discriminating means are one or more gas cells, each gas cell contains one or more gases of interest.

4. The method of claim 1, characterized in that one or more spectrum discrimination means are one or more optical interference elements, each optical interference element having spectrum transmissions that approximately replicate the absorption characteristics of one or more gases of interest.

5. The method of claim 1, characterized in that it further comprises the step of balancing the optical intensities between the two optical paths of one or more regions of distributed spectrum interest.

6. The method of claim 5, characterized in that the balancing is performed optically within the corresponding distributed spectrum interest region.

7. The method of claim 5, characterized in that the balancing is performed electronically.

8. A multi-gas detector for simultaneously measuring the concentration of two or more gases of interest in a measurement path, characterized in that it comprises: a polarization modulator for modulating a polarized light beam, the light beam has passed through the path of measurement, in a wide band of wavelengths between two alternating orthogonal polarization components; a polarization beam splitter for switching the polarized modulated beam, transmitting an orthogonal polarization component of the polarized beam of light along a first optical path and reflecting the other orthogonal polarization component of the polarized beam of light along a second optical path; at least one discrimination means of spectrum located in at least one of the first and second optical paths, each spectrum discrimination means has spectrum absorption characteristics of one or more gases of interest; means for intersecting the two optical paths at an intersection point; a beam combiner located at the point of intersection of the first and second optical paths, the beam combiner transmits an orthogonal component of the intersected components and reflects the other orthogonal components, by means of which a combined polarized bias beam is formed which comprises the orthogonal components in alternating order; means for distributing the combined biasing modulated beam in a detection path for each region of interest of wavelength corresponding to a band of absorption of the gases of interest; a detector in each detection path to receive the incident beam distributed in the region of interest of wavelength and to detect a difference in intensity between the two orthogonal polarization components of the incident beam distributed as an indication of the emission / absorption of spectrum of the light beam by the gases of interest in the measurement path, the emission / absorption of the spectrum is indicative of the concentration of one or more gases of interest in the measurement path; and a means of control and data acquisition to drive the bias modulator, control the temperature of each detector, and process the output of each detector.

9. The multi-gas detector of claim 8, characterized in that each spectrum discrimination means comprises a gas correlation cell, each gas correlation cell contains at least one gas of interest.

10. The multi-gas detector of claim 8, characterized in that each spectrum discrimination means comprises an interference element, having a spectrum transmission that approximately replicates the absorption characteristic of a region of interest of wavelength.

11. The multi-gas detector according to claim 8, characterized in that it further comprises an optical polarizer, for polarizing a beam of light that enters before the modulation by means of a polarization modulator.

12. The multi-gas detector according to claim 8, characterized in that it further comprises an optical wave plate placed before the polarization modulator.

13. The multi-gas detector according to claim 8, characterized in that the polarization modulator is selected from the group consisting of electro-optical, magneto-optical and photo-elastic modulators and liquid crystal devices.

14. The multi-gas detector according to claim 8, characterized in that the polarization beam separator is selected from the group consisting of prism, reflector and wire grid separators.

15. The multi-gas detector according to claim 8, characterized in that the intersection means comprises a respective reflecting mirror arranged in each of the two optical paths.

16. The multi-gas detector according to claim 8, characterized in that the intersecting means is located in such a way that the two optical paths of the polarization beam separator at the intersection point are optically similar.

17. The multi-gas detector according to claim 8, characterized in that the beam combiner is selected from the group consisting of polarization beam separator and wide band beam separator.

18. The multi-gas detector according to claim 8, characterized in that the distribution means comprises one or more optical filters arranged operatively to pass a single wavelength band to each detector, each band of single wavelength corresponds to a band of absorption of one or more gases of interest.

19. The multi-gas detector radiometer according to claim 8, characterized in that the partition means comprises a beam splitter and one or more optical filters downstream, the beam splitter and the optical filters are operatively arranged to pass a band of length single wave to each detector, each band of single wavelength corresponds to an absorption band of one or more gases of interest.

20. The multi-gas detector according to claim 18, characterized in that it further comprises a focusing means placed in front of each detector to focus the incident beam in each detector, wherein the focusing means is selected from the group consisting of focus mirror and refractive lenses.

21. The multi-gas detector according to claim 19, characterized in that it further comprises a focusing means in each detection path to focus the incident beam- on each detector, wherein the focusing means is selected from the group consisting of mirror focus and refractive lenses.

22. The multi-gas detector according to claim 8, characterized in that two or more gases of interest have absorption characteristics in a common wavelength region and are measured simultaneously but not independently.

23. The multi-gas detector according to claim 8, characterized in that the gases of interest do not overlap spectrally in one or more regions of interest of wavelength and are measured simultaneously and independently.

24. The multi-gas detector according to claim 8, characterized in that the optical polarizer is selected from the group consisting of prism, reflection, dichroic and wire grid polarizers.

25. The multi-gas detector according to claim 8, characterized in that it also comprises a vacuum cell located in one of the first and second optical paths.

26. The multi-gas detector of claim 8, characterized in that the control and data acquisition means comprises: a variable gain amplifier corresponding to each detector for amplifying the AC portion of the detector signal; a variable gain amplifier that corresponds to each detector to amplify the DC portion of the detector signal; a synchronous demodulator corresponding to each detector for receiving the amplified signal AC and extracting its magnitude using a frequency reference signal of the polarization modulator; a low pass filter corresponding to each detector to receive the amplified DC signal and narrow its electronic bandpass; a low pass filter corresponding to each detector to receive the demodulated AC signal and narrow its electronic bandpass; an A / D converter to convert the AC signals and DC in digital representations to process in computer; a controller corresponding to each detector to control the operating temperature of the corresponding detector; and a controller for driving the bias modulator and providing a frequency. reference for the demodulator.

27. The multi-gas detector of. claim 8, characterized in that it further comprises a polarization-dependent optic, placed operatively in front of at least one detector for balancing the corresponding optical intensities between the two optical paths.

28. The multi-gas detector of claim 27, characterized in that the polarization-dependent optic is selected from the group consisting of film, infrared transmitting crystal, amorphous window and infrared polarizer.

29. The multi-gas detector of claim 8, characterized in that the control and data acquisition means balances the optical intensities between the two optical paths for one or more regions of distribution spectrum interest.