Optical path switching based differential absorption radiometry for substance detection

• US 20040156050A1
• Filed: 06/19/2003
• Published: 08/12/2004
• Est. Priority Date: 04/20/1998
• Status: Active Grant

• Current Assignee: U.S.A. as represented by the Administrator of the National Aeronautics and Space Administration
• Original Assignee: U.S.A. as represented by the Administrator of the National Aeronautics and Space Administration
• Inventors: Sachse, Glen W.
• Granted Patent: US 6,922,242 B2
• Time in Patent Office: Days
• US Class Current: 356/364
• CPC Class Codes: G01J 3/08 Beam switching arrangements
• CPC Class Codes: G01N 21/314 with comparison of measurements at specific and non-specific wavelengths dual wavelength spectrometry G01J3/427
• CPC Class Codes: G01N 21/3518 Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques

Abstract

An optical path switch divides sample path radiation into a time series of alternating first polarized components and second 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 filters the first polarized components to isolate at least a first wavelength band thereby generating first filtered radiation. A second gasless optical filter train filters the second polarized components to isolate at least a second wavelength band thereby generating second filtered radiation. A beam combiner combines the first and second filtered radiation 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.

Claims

1. A system for detecting a substance, comprising:

means for receiving radiation passing along a sample path of interest;

means for dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components;

means for routing said first polarized components along a first optical path and said second polarized components along a second optical path;

gasless means for passing all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;

means for filtering said second polarized components to isolate at least a second wavelength band wherein second filtered radiation is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;

means for combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation; and

means for monitoring magnitude of said combined beam alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.

2. A system according to claim 1 wherein said gasless means for passing comprises a gasless optical filter disposed in said first optical path for reflecting all wavelengths of said first polarized components except at least a first wavelength band.

3. A system according to claim 2 wherein said means for filtering comprises a second gasless optical filter disposed in said second optical path for reflecting all wavelengths of said second polarized components except at least a second wavelength band.

4. A system according to claim 3 wherein said first gasless optical filter reflects all wavelengths of said first polarized components except a plurality of unique first wavelength bands whereby said first filtered radiation excludes said plurality of first wavelength bands, and wherein said second gasless optical filter reflects all wavelengths of said second polarized components except a plurality of unique second wavelength bands wherein second filtered radiation is generated, said system further comprising:

means, disposed to receive said combined beam, for partitioning said combined beam into a plurality of partitioned beams, each of said plurality of partitioned beams comprising first filtered radiation and second filtered radiation, said first filtered radiation having excluded therefrom one of said plurality of unique first wavelength bands, wherein spectral absorption of a substance of interest is different at said one of said plurality of unique first wavelength bands as compared to one of said plurality of unique second wavelength bands,

wherein said means for monitoring comprises a plurality of detectors, each of said plurality of detectors being disposed to receive one of said partitioned beams.

5. A system according to claim 1 further comprising a gas filter correlation radiometer coupled to said means for routing, said gas filter correlation radiometer using one of said first polarized components and said first filtered radiation and using one of said second polarized components and said second filtered radiation to measure the concentration of at least one gas species in said sample path.

6. A system according to claim 1 wherein said first polarized components are vertically polarized and said second polarized components are horizontally polarized.

7. A system according to claim 1 wherein said means for monitoring is harmonically synchronized to a frequency at which said time series alternates between said first polarized components and said second polarized components.

8. A system for detecting a substance, comprising:

means for receiving radiation passing along a sample path of interest;

means for dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components;

means for routing said first polarized components along a first optical path and said second polarized components along a second optical path;

means for reflecting all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;

means for passing all wavelengths of said second polarized components except at least a second wavelength band wherein second filtered radiation excluding said second wavelength band is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;

means for combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation;

means, disposed to receive said combined beam, for filtering the combined beam, said means for filtering having a bandpass characteristic that spans said excluded first and second wavelength bands; and

means for monitoring magnitude of said combined beam received from the filter alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.

9. A system according to claim 8 wherein said means for reflecting comprises a gasless optical filter disposed in said first optical path for reflecting all wavelengths of said first polarized components except at least a first wavelength band.

10. A system according to claim 9 wherein said means for passing comprises a second gasless optical filter disposed in said second optical path for reflecting all wavelengths of said second polarized components except at least a second wavelength band.

11. A system according to claim 10 wherein said first gasless optical filter reflects all wavelengths of said first polarized components except a plurality of unique first wavelength bands whereby said first filtered radiation excludes said plurality of first wavelength bands, wherein said second gasless optical filter reflects all wavelengths of said second polarized components except a plurality of unique second wavelength bands whereby said second filtered radiation excludes said plurality of second wavelength bands wherein second filtered radiation is generated, said system further comprising:

means, disposed to receive said combined beam, for partitioning said combined beam into a plurality of partitioned beams, each of said plurality of partitioned beams comprising first filtered radiation and second filtered radiation, said first filtered radiation having excluded therefrom one of said plurality of unique first wavelength bands, wherein spectral absorption of a substance of interest is different at said one of said plurality of unique first wavelength bands as compared to one of said plurality of unique second wavelength bands, and

wherein said means for monitoring comprises a plurality of detectors, each of said plurality of detectors being disposed to receive one of said partitioned beams.

12. A system according to claim 8 further comprising a gas filter correlation radiometer coupled to said means for routing, said gas filter correlation radiometer using one of said first polarized components and said first filtered radiation and using one of said second polarized components and said second filtered radiation to measure the concentration of at least one gas species in said sample path.

13. A system according to claim 8 wherein said first polarized components are vertically polarized and said second polarized components are horizontally polarized.

14. A system according to claim 8 wherein said means for monitoring is harmonically synchronized to a frequency at which said time series alternates between said first polarized components and said second polarized components.

15. A system for detecting a substance comprising:

an optical path switch for receiving radiation passing along a sample path of interest, said optical path switch dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components, said optical path switch routing said first polarized components along a first optical path and said second polarized components along a second optical path;

a first gasless optical filter train disposed in said first optical path for reflecting all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;

a second gasless optical filter train disposed in said second optical path for reflecting all wavelengths of said second polarized components except at least a second wavelength band wherein second filtered radiation excluding said at least second wavelength band is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;

a beam combiner disposed to receive said first filtered radiation and said second filtered radiation for combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation;

a filter disposed to receive said combined beam of radiation and having a bandpass characteristic that spans said excluded first and second wavelength bands; and

detector means disposed to monitor magnitude of at least a portion of said combined beam received from said filter alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.

16. A system according to claim 15 further comprising a gas filter correlation radiometer coupled to said optical path switch, said gas filter correlation radiometer using one of said first polarized components and said first filtered radiation and using one of said second polarized components and said second filtered radiation to measure the concentration of at least one gas species in said sample path.

17. A system according to claim 15 wherein said first gasless optical filter train includes a first optical filtering means for reflecting all wavelengths of said first polarized components except a plurality of unique first wavelength bands whereby said first filtered radiation excludes said plurality of first wavelength bands, and wherein said second gasless optical filter includes second optical filtering means for reflecting all wavelengths of said second polarized components except a plurality of unique second wavelength bands whereby said second filtered radiation excludes said plurality of second wavelength bands, said system further comprising:

a beam partitioner disposed to receive said combined beam and for partitioning said combined beam into a plurality of partitioned beams, each of said plurality of partitioned beams having excluded therefrom one of said plurality of unique first wavelength bands and one of said plurality of unique second wavelength bands, wherein spectral absorption of a substance of interest is different at one of said plurality of unique first wavelength bands as compared to said one of said plurality of unique second wavelength bands, and wherein said detector means comprises a plurality of detectors, each of said plurality of detectors being disposed to receive one of said plurality of partitioned beams.

18. A system according to claim 15 wherein said first polarized components are vertically polarized and said second polarized components are horizontally polarized.

19. A system according to claim 15 wherein said detector means is harmonically synchronized to a frequency at which said time series alternates between said first polarized components and said second polarized components.

20. A substance detection system comprising:

an optical path switch for receiving radiation passing along a sample path of interest, said optical path switch dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components, said optical path switch routing said first polarized components along a first optical path and said second polarized components along a second optical path;

a first gasless optical filter train disposed in said first optical path for reflecting all wavelengths of said first polarized components except those in a first wavelength band wherein first filtered radiation excluding said first wavelength band is generated;

at least one gas correlation cell disposed in said first optical path for filtering one of said first polarized components and said first filtered radiation at spectral regions different from that of said first wavelength band;

a second gasless optical filter train disposed in said second optical path for reflecting all wavelengths of said second polarized components except those in a second wavelength band wherein second filtered radiation excluding said second wavelength band is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;

a vacuum cell disposed in said second optical path;

a beam combiner disposed to receive said first filtered radiation and said second filtered radiation for combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation;

a filter disposed to receive said combined beam of radiation and having a bandpass characteristic that spans said excluded first and second wavelength bands;

first detecting means disposed to monitor magnitude of at least a portion of said combined beam received from the filter alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path; and

second detecting means disposed to sense a difference in intensity between said first filtered radiation and said second filtered radiation at said spectral regions to measure the concentration of at least one gas species in said sample path.

21. A system according to claim 20 wherein said first polarized components are vertically polarized and said second polarized components are horizontally polarized.

22. A system according to claim 20 wherein said first detecting means is harmonically synchronized to a frequency at which said time series alternates between said first polarized components and said second polarized components.