Optical path switching based differential absorption radiometry for substance detection

• US 6,057,923 A
• Filed: 04/13/1999
• Issued: 05/02/2000
• Est. Priority Date: 04/20/1998
• Status: Expired due to Fees

• Current Assignee: National Aeronautics and Space Administration (Government of the United States of America)
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Sachse, Glen W.
• Primary Examiner(s): Font, Frank G.
• Assistant Examiner(s): Nguyen, Tu T.
• Application Number:
  US09/290,954
• Time in Patent Office: 385 Days
• Field of Search: 356/364-370, 356/432-444, 359/246, 359/245, 359/238, 359/239, 359/281, 359/885, 359/888, 359/891, 250/338.5, 250/339.13, 250/347, 250/349, 250/330, 250/338.1, 250/339.11
• 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

A system and method are provided for detecting one or more substances. 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. 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 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:

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 transmitting 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 filtering said first polarized components to isolate at least a first wavelength band wherein first filtered radiation is generated;

a second gasless optical filter train disposed in said second optical path 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;

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; and

detector means disposed to monitor magnitude of at least a portion 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 as in claim 1 wherein said first gasless optical filter includes a first optical filtering means for passing only said first wavelength band to form said first filtered radiation, and wherein said second gasless optical filter includes a second optical filtering means for passing only said second wavelength band to form said second filtered radiation.

3. A system as in claim 1 wherein said first gasless optical filter includes a first optical filtering means for passing all wavelengths except said first wavelength band to form said first filtered radiation, and wherein said second gasless optical filter includes a second optical filter means for passing all wavelengths except said second wavelength band to form said second filtered radiation.

4. A system as in claim 3 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.

5. A system as in claim 1 wherein said first gasless optical filter includes first optical filtering means for isolating a plurality of unique first wavelength bands, and wherein said second gasless optical filter includes second optical filtering means for isolating a plurality of unique 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 including 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 one of said plurality of unique first wavelength bands as compared to said one of said plurality of unique second wavelength bands, wherein said detector means comprises a plurality of detectors, each of said plurality of detectors disposed to receive one of said plurality of partitioned beams.

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

7. A system as in claim 1 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.

8. 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 transmitting 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 filtering said first polarized components to pass all wavelengths except those in a first wavelength band wherein first filtered radiation 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 filtering said second polarized components to pass all wavelengths except those in 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;

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;

first detecting means disposed to monitor magnitude of at least a portion 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; 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.

9. A system as in claim 8 wherein said first polarized components are vertically polarized and said second polarized components are horizontally polarized.

10. A system as in claim 8 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.