1. Field of the Invention
This invention relates to substance detection using optical systems. More specifically, the invention is a system for detecting the presence and/or concentration of a substance in a sample path using polarization-modulated optical path switching and the principles of differential absorption radiometry.
2. Description of the Related Art
Gas filter correlation radiometers (GFCRs) infer the concentration of a gas species along some sample path either external or internal to the GFCR. In many GFCRs, gas sensing is accomplished by viewing alternately through two optical cells the emission/absorption of the gas molecules along the sample path. These two optical cells are called the correlation and vacuum cells. The correlation cell contains a high optical depth of gas species i that strongly absorbs radiation at specific molecular transition wavelengths of the particular gas while passing all other wavelengths. In effect, the correlation cell defines a plurality of spectral notches (i.e., strong attenuation) coincident with the band structure of gas species i. The vacuum cell generally encloses a vacuum or a gas or gas mixture exhibiting negligible or no optical depth, e.g., nitrogen, an inert gas, or even clean dry air. An optical filter (e.g., interference filter) placed in front of the instrument or in front of the detector limits the spectral information to a region coinciding with an absorption band of the gas of interest. The difference in signal strength between these two views of the emitting/absorbing gas species i can be related to the concentration of this gas along the sample path.
A known GFCR for measuring concentration of a single gas is disclosed in U.S. Pat. No. 5,128,797, issued to Sachse et al. and assigned to the National Aeronautics and Space Administration (NASA), the specification of which is hereby incorporated by reference. The GFCR includes a non-mechanical optical path switch that comprises a polarizer, polarization modulator, and a polarization beamsplitter. The polarizer polarizes light (that has crossed a sample path after originating from a light source) into a single (e.g., vertically polarized) component which is then rapidly modulated into alternate vertically and horizontally polarized components by a polarization modulator. The polarization modulator may be used in conjunction with an optical waveplate. The polarization modulated beam is then incident on a polarization beamsplitter which, transmits light of one component (e.g., horizontally polarized) and reflects light of a perpendicular component (e.g., vertically polarized). The transmitted horizontally polarized beam is reflected by a mirror, passes through a gas correlation cell and on to a beam combiner. The reflected vertically polarized beam passes through a vacuum cell, is reflected by a mirror, and is passed on to the beam combiner. The beam combiner recombines the horizontal and vertical components into a single beam which passes through an optical interference filter that limits the spectral content of the incoming radiation to an absorption band of the gas species of interest. The single beam is then incident on a conventional detector. However, this system is limited in that it can only measure a single gas concentration.
A GFCR for measuring multiple gases based on the same optical path switching technique is disclosed in U.S. patent application Ser. No. 09/019,473, filed Feb. 5, 1998, now U.S. Pat. No. 6,008,928, issued Dec. 28, 1999, by Sachse et al. and assigned to the National Aeronautics and Space Administration (NASA). In this system, each optical path contains one or more cells with each cell having spectral features of one or more gases of interest. The two optical paths are then intersected to form a combined polarization modulated beam which contains the two orthogonal components in alternate order. The combined polarization modulated beam is partitioned 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 partitioned spectral region of interest is then determined as an indication of the spectral emission/absorption of the light beam along the sample path. The spectral emission/absorption is indicative of the concentration of the one or more gases of interest in the sample path.
Both of the afore-described systems require the use of gas correlation cells. However, there are instances where gas correlation cells are not practical. For example, some gases are too dangerous and/or require a gas correlation cell construction that is too expensive for a particular application. Further, some gases such as ozone are too reactive to contain in a gas cell. Still further, it may also be desirable to detect/measure a broad category of gases, e.g., hydrocarbons. However, to accomplish this measurement with a GFCR system, many gases would have to be contained within one cell or the beam would have to be passed through multiple gas cells.
In accordance with the present invention, a system and method are provided for detecting one or more substances. An optical path switch receives radiation passing along a measurement or sample path of interest. The switch divides the radiation into a time series of alternating first polarized components and second polarized components that are orthogonal to the first polarized components. The first polarized components are transmitted along a first optical path and the second polarized components along a second optical path. A first gasless optical filter train disposed in the first optical path filters the first polarized components to isolate at least a first wavelength band thereby generating first filtered radiation. A second optical filter train disposed in the second optical path filters the second polarized components to isolate at least a second wavelength band thereby generating second filtered radiation. The first wavelength band and second wavelength band are unique. Further, spectral absorption of a substance of interest is different at the first wavelength band as compared to the second wavelength band. A beam combiner disposed to receive the first and second filtered radiation combines same to form a combined beam of radiation. A detector is disposed to monitor magnitude of at least a portion of the combined beam alternately at the first wavelength band and the second wavelength band as an indication of the concentration of the substance in the sample path.
Referring now to the drawings, and more particularly to
System 10 includes an optics system 12, e.g., a telescope or other lens/mirror system, that collects light from a radiation source 11, such as the earth, the atmosphere, or the sun when system 10 is mounted on a satellite or aircraft, or a hot filament, light emitting diode, laser, etc., when system 10 is used as a laboratory or in-situ instrument. Radiation from source 11 generally comprises both vertically polarized components V and horizontally polarized components H. The radiation passes between source 11 and system 10 along a sample path SP. The presence of a substance or substances of interest along path SP may affect the radiation in a way that can be detected, measured, and/or characterized by system 10. An optical path switch provided after optics system 12 includes an optical polarizer 14, an optical waveplate 16, a polarization modulator 18, and a polarization beamsplitter 20. Such an optical path switch is disclosed in detail in the afore-mentioned U.S. Pat. No. 5,128,797 to Sachse et al., and will therefore only be described briefly herein.
Optical polarizer 14 is provided after the optics system 12 and is aligned to polarize the incoming radiation in the desired fashion, e.g., vertically in the embodiment depicted in FIG. 1. Polarization modulator 18 (e.g., a photo-elastic modulator) then receives the incident vertically polarized beam and rapidly modulates the output beam between vertical and horizontal polarization. Depending on the measurement application and the type of polarization modulator utilized, the polarization modulation frequency may range from near direct current (DC) to radio frequencies (RF). The polarization modulator may be used in conjunction with optical waveplate 16. The output of modulator 18 is a time series of alternating vertically polarized components V and horizontally polarized components H as illustrated in FIG. 1. The switching frequency between V and H is determined by the modulation frequency of modulator 18.
Polarization beamsplitter 20 non-mechanically switches the polarization modulated output beam between two paths by, for example, transmitting the beam along path 101 when it is vertically polarized and reflecting it along path 102 when it is horizontally polarized. Alternatively, beamsplitter 20 may be oriented so as to reflect vertically polarized light and to transmit horizontally polarized light. Thus, beamsplitter 20 rapidly diverts or routes the radiation beam alternately between optical paths 101 and 102 depending on the rapidly time-varying state of polarization which is controlled by modulator 18. Note that although paths 101 and 102 are illustrated as being perpendicular to one another, this need not be the case as will be apparent in other embodiments of the present invention described later below.
The radiation beam transmitted along optical path 101 is incident on a gasless optical bandpass filter 22 configured to transmit only a wavelength band of radiation centered at λA while reflecting other wavelengths. The radiation beam transmitted along optical path 102 is incident on a second gasless optical bandpass filter 24 configured to transmit only a wavelength band of radiation centered at λB while reflecting other wavelengths. Filters 22 and 24 are selected/constructed such that the bands centered at λA and λB are unique as illustrated in FIG. 2. Further, the spectral absorption of the substance to be detected, measured, and/or characterized must be different at the two bands. The greater the difference in spectral absorption characteristics between the two bands, the greater the measurement sensitivity of system 10. Accordingly, in an example of the ideal case, spectral absorption occurs only in the band centered at λA (i.e., spectral absorption in the band centered at λB would be zero). However, it is to be understood that the present invention will work as long as there is some difference in spectral absorption (of the substance of interest) between the two bands.
The resulting filtered radiation beams passed along optical paths 101 and 102 are directed/reflected by mirrors 26 and 28, respectively, to a polarization beam combiner 30 (e.g., a polarization beamsplitter). Beam combiner 30 outputs a single beam along path 103 in which the beam's polarization state varies in time at the fundamental frequency (and harmonics thereof) of modulator 18. In other words, the output of beam combiner 30 is essentially a time series that alternates between the vertically polarized components V passed by filter 22 and the horizontally polarized components passed by filter 24. The combined radiation beam passes along optical path 103 and is focused by focusing optics 32 onto a detector 34 which is sensitive to the magnitude of the radiation. Because this radiation is in the form of an alternating time series, detector 34 is essentially viewing an amplitude modulated signal. This phenomenon occurs because a gas (or other substance) present along sample path SP absorbs radiation from radiation source 11 differentially at the bands centered at λA and λB. Thus, the differential absorption experienced by the radiation traversing sample path SP is viewed by detector 34 as an amplitude modulated signal. The magnitude of the amplitude modulated signal at the polarization modulation frequency (or its harmonics) is related to the amount or concentration of the substance of interest in sample path SP. Note that if system 10 is subject to changes in the incident radiation due to variations in strength of radiation source 11, turbulence noise, scattering along the optical paths, etc., it may be desirable to normalize the amplitude modulated signal sensed by detector 34. If this is the case, the amplitude modulated signal can be divided by the DC component sensed by detector 34 as is well known in the art.
By way of illustrative example, the present invention will be described briefly for its use in the measurement of hydrocarbons. In this case, filter 22 is chosen so that the band centered at λA coincides with the carbon-hydrogen bond absorption typical of hydrocarbons (i.e., λA is approximately 3.4 microns). Filter 24 is chosen so that the band centered at λB coincides with a wavelength band that is relatively free from hydrocarbon absorption (i.e., λB is approximately 3.0 microns). By monitoring the magnitude of the amplitude modulated signal sensed by detector 34, the absorption by hydrocarbons present in sample path SP can be detected and measured in a simple fashion, i.e., multiple GFCR devices with multiple gas filter correlation cells (e.g., one for each hydrocarbon of interest) are not required.
Although described relative to the embodiment in
After the radiation beams are combined at beam combiner 30, the combined beam is directed along optical path 103 to a partitioning or edge filter 226 configured, for example, to reflect wavelength bands centered at λA1 and λB1 through focusing optics 232 to detector 234 and transmit wavelength bands centered at λA2 and λB2 through focusing optics 233 to detector 235. Thus, detector 234 is sensitive to the amplitude modulation caused by the differential absorption between the bands centered at λA1 and λB1 (i.e., associated with the first substance) while detector 235 is sensitive to the amplitude modulation caused by the differential absorption between the bands centered at λA2 and λB2 (i.e., associated with the second substance). Note that the
Further, other filter configurations are possible. For example, the band centered at λA1 could coincide with a radiation band at which the first substance is absorbed; the band centered at λB1 could coincide with a radiation band at which the first substance is not absorbed; the band centered at λA2 could coincide with a radiation band at which the second substance is not absorbed; and the band centered at λB2 could coincide with a radiation band at which the second substance is absorbed.
Still another embodiment of the present invention is illustrated in FIG. 5 and referenced generally by numeral 300. Once again, like reference numerals will be used for those elements that are the same as those used in the
The embodiment illustrated in
In
Another way to detect or measure two substances simultaneously using bandpass filters in reflection is shown and referenced generally by numeral 500 in FIG. 8. That is, system 500 is an alternative construction that achieves the results described above with respect to FIG. 3. In optical path 101, a first bandpass filter 520 reflects all wavelengths except those in a first band centered at λA1 towards one side of a two-sided mirror 526. Mirror 526 reflects the radiation to a second bandpass filter 521 that reflects all wavelengths except those in a second band centered at λA2. In a similar fashion, bandpass filters 524/525 and mirror 526 cooperate to remove wavelength bands centered at λB1 and λB2 in optical path 102. The single beam output from beam combiner 30 can then be processed as described in the FIG. 3 and
Still another embodiment of the present invention is shown in FIG. 9 and is referenced generally by numeral 600. System 600 is similar to system 500 except that mirror 526 is replaced with a two-sided bandpass absorber 626. Absorber 626 is configured on side 626A to absorb radiation in a third wavelength band centered on λA3 while reflecting all other wavelengths. This configuration can be accomplished by designing a bandpass filter stack that transmits the band centered at λA3 which is then absorbed internally. For example, the substrate material could strongly absorb this wavelength band. On the other side 626B of absorber 626 is a second filter stack that selectively transmits/absorbs a wavelength band centered at λB3. After being combined at beam combiner 30, a system of partitioning filters/focusing optics/detectors 632, similar to the systems disclosed in the embodiments of
In still another embodiment of the present invention, system 700 illustrated in
The advantages of the present invention are numerous. Substance detection and measurement can be achieved without using gas cells. However, the present invention can be configured to provide for simultaneous differential absorption and GFCR measurements. Further, multiple differential absorption measurements associated with multiple substances can be made simultaneously.
Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function and step-plus-function clauses are intended to cover the structures or acts described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.
This application is a divisional patent application of commonly owned, patent application Ser. No. 09/437,616, filed Nov. 10, 1999 now U.S. Pat. No. 6,611,329, which is a divisional of patent application Ser. No. 09/290,954, filed Apr. 13, 1999, now issued as U.S. Pat. No. 6,057,923 on May 2, 2000. This application is also related to patent application Ser. No. 09/019,473, filed Feb. 5, 1998, now issued as U.S. Pat. No. 6,008,928 on Dec. 18, 1999. Pursuant to 35 U.S.C. § 119, the benefit of priority from provisional application No. 60/082,355, with a filing date of Apr. 20, 1998, is claimed for this non-provisional application.
The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties theron or therfor.
Number | Name | Date | Kind |
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4818705 | Schneider et al. | Apr 1989 | A |
5128797 | Sachse et al. | Jul 1992 | A |
5343043 | Johnson | Aug 1994 | A |
6057923 | Sachse | May 2000 | A |
6611329 | Sachse | Aug 2003 | B2 |
Number | Date | Country | |
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20040156050 A1 | Aug 2004 | US |
Number | Date | Country | |
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60082355 | Apr 1998 | US |
Number | Date | Country | |
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Parent | 09437616 | Nov 1999 | US |
Child | 10465386 | US | |
Parent | 09290954 | Apr 1999 | US |
Child | 09437616 | US |