Method and arrangement for measuring the optical absorptions of gaseous mixtures

• US 4,998,017 A
• Filed: 05/01/1989
• Issued: 03/05/1991
• Est. Priority Date: 05/01/1989
• Status: Expired

• Current Assignee: Rosemount Analytical, Inc. (Emerson Electric Company)
• Original Assignee: Rosemount Analytical, Inc. (Emerson Electric Company)
• Inventors: Gottlieb, Milton S., Ryan, Fredrick M.
• Primary Examiner(s): Hannaher, Constantine
• Application Number:
  US07/345,863
• Time in Patent Office: 673 Days
• Field of Search: 250/343, 250/373, 250/338.5, 356/352, 356/351, 356/346
• US Class Current: 250/343
• CPC Class Codes: G01J 2003/2886 Investigating periodic spectrum
• CPC Class Codes: G01J 3/26 using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

A gas analyzer arrangement for detecting the presence or quantity of a gas of interest includes a source of electromagnetic radiation for directing a light beam through a light conditioning device such that the light beam is collimated into a parallel beam of light. The conditioned light beam is then directed through a gas sample cell which contains a sample of the gas of interest. The gas of interest has associated therewith, vibrational rotational absorption lines specific to that gas and which are effective for altering the light beam according to the absorption characteristics thereof. The altered light beam is then directed to a birefringent etalon device which has been sized specifically to match those absorption lines of the gas of interest. An electro-optical modulator device associated with the birefringent etalon modulates the absorption lines between the transmission spectra maxima and a point between the transmission spectra maxima so that an absorption ratio can be utilized to determine the presence or quantity of the gas of interest.

Claims

1. An arrangement for measuring a gas of interest by its optical absorption characteristics, comprising:

a source of electromagnetic radiation;

means for conditioning such electromagnetic radiation such that it passes through such gas of interest;

means for electrically modulating such electromagnetic radiation that has passed through such gas of interest, said modulating means including a birefringent etalon having associated therewith, a periodic spacing equal to the periodicity of the absorption lines associated with such gas of interest;

said modulating means being further effective for varying an electrical field applied to said birefringent etalon between first and second predetermined strengths such that the periodic transmission spectrum is shifted between spectra which coincide substantially with such absorption lines and spectra which fall between such absorption lines; and

means for detecting at least the intensity of such periodic transmission spectra following passage through such gas of interest and determining therefrom at least an amount of such gas of interest present.

2. A gas measurement arrangement as set forth in claim 1 wherein said conditioning means includes a collimating element which collimates such electromagnetic radiation into a parallel light beam which can then be directed through such gas of interest.

3. A gas measurement arrangement as set forth in claim 2 further comprising a sample cell in which such gas of interest is disposed, and an input polarizer disposed following said sample cell such that said light beam, after passing through said sample cell, is polarized thereby prior to such light beam entering said birefringent etalon.

4. A gas measurement arrangement as set forth in claim 3 further comprising a second conditioning means for conditioning such light beam after it has passed through said birefringent etalon, said second conditioning means being effective for removing certain known portions of such light beam prior to such light beam being acted upon by said detecting means.

5. A gas measurement arrangement as set forth in claim 4 wherein said second conditioning means includes an output polarizer followed by a band pass filter which, in conjunction, serve to remove fringe and other portions of such light beam which differ from such periodic transmission spectra that coincide with such absorption spectra associated with such gas of interest and such periodic transmission spectra which fall between such absorption spectra.

6. A gas measurement arrangement as set forth in claim 1 wherein said source of electromagnetic radiation can be one of an ultraviolet and an infrared source and is selected according to the spectral wavelength at which the gas of interest exhibits the optimum detection potential determinable as a function of the periodicity of the absorption lines associated thereby.

7. A gas measurement arrangement as set forth in claim 1 wherein said birefringent etalon is constructed of a material having a high electro-optical coefficient whereby, with such electric field applied thereto, said birefringent etalon is simultaneously effective for operation as a filter specifically responsive to such periodic transmission spectra, and as a modulator which, when such electric field is applied, allows such shifting of such periodic transmission spectra between such spectra which coincides with such absorption lines and such spectra which falls between such absorption lines.

8. A gas measurement arrangement as set forth in claim 1 wherein said birefringent etalon is sized so as to achieve a filtering capability specifically responsive to such periodic transmission spectra associated with such absorption lines indicative of such gas of interest, and wherein said modulating means further includes an electro-optical modulator element constructed of a material having a high electro-optical coefficient.

9. A gas measurement arrangement as set forth in claim 8 wherein said birefringent etalon is constructed separately and distinctly from said electro-optical modulator such that, such periodic transmission spectra can be selectively altered to provide detection of an alternate gas of interest by substitution of an alternate sized birefringent etalon, such substitution of said birefringent etalon being done independent of said electro-optical modulator.

10. A gas measuring arrangement as set forth in claim 1 wherein said modulating means includes first and second electro-optical modulator elements disposed orthogonal to one another so that such electric field can be applied thereto in an orthogonal relationship thereby cancelling the birefrigance properties of said first and second modulator elements.

11. A gas measuring arrangement as set forth in claim 2 wherein said collimating element can be one of a collimating lens and a collimating mirror.

12. An arrangement for measuring a gas of interest by its optical absorption characteristics, said measuring arrangement comprising:

a source of electromagnetic radiation;

means for directing such electromagnetic radiation through such gas of interest;

a first light signal, emerging from passage through such gas of interest, has associated therewith, periodic transmission spectra representative of the absorption lines of such gas of interest;

an interferometric device receptive of said first light signal and having associated therewith, a periodic spacing equal to the periodicity of the absorption lines of such gas of interest;

means for varying an applied electric field of first and second predetermined strengths to said interferometric device so as to modulate such periodic spacing of said interferometric device between transmission spectra which substantially coincide with such absorption lines and transmission spectra which fall between such absorption lines; and

,

means for detecting at least an amount of such gas of interest as a function of the intensity of such periodic transmission spectra following passage through such gas of interest.

13. A measurement arrangement as set forth in claim 12 further comprising a sample cell in which such gas of interest is disposed, and a polarizer element disposed following said sample cell such that said light signal, after passing through such gas of interest, is polarized thereby in advance of said interferometric device.

14. A measurement arrangement as set forth in claim 12 further comprising means for conditioning said first light signal after it has passed through said interferometric device, said conditioning means being effective for removing preselected portions of said first light signal prior to said first light signal being acted upon by said detecting means.

15. A gas measurement arrangement as set forth in claim 12 wherein said interferometric device is a first birefringent etalon having a first path length determinative of such periodic spacing.

16. A gas measurement arrangement as set forth in claim 15 further comprising a second birefringent etalon disposed following said first birefringent etalon;

said second birefringent etalon having a path length essentially twice that of said first birefringent etalon thereby having as a result thereof, an alternate periodic spacing associated therewith that is approximately one half that of said first birefringent etalon.

17. A gas measurement arrangement as set forth in claim 16 wherein said second birefringent etalon is disposed relative to said first birefringent etalon such that the respective periodic spacings of said first and second birefringent etalons are combined in a manner to yield a third periodic spacing which exhibits fewer and narrower absorption lines than such respective periodic spacings taken individually.

18. A method for measuring a gas of interest by its optical absorption characteristics, said measuring method comprising the steps of:

directing a light beam through a quantity of such gas of interest such that a light signal representative of the absorption lines of the gas of interest is generated thereby;

passing said light signal through a birefringent etalon device which has associated therewith, a periodic spacing substantially equivalent to such absorption lines associated with such gas of interest;

applying an electrical field at two distinct field strengths to the birefringent etalon to modulate its transmission spectra from wavelengths which substantially coincide with the absorption lines of the gas of interest and wavelengths which fall between such absorption liens; and

determining at least an amount of such gas of interest as a function of the relationship such spectra which coincide with such absorption lines and such spectra which fall between such absorption lines.

19. A gas measuring method as set forth in claim 18 further comprising the steps of:

polarizing such light beam prior to said passing of such light beam through said birefringent etalon and conditioning such light beam after it has been modulated such that certain known portions of such light beam are removed prior to said amount determining step.

20. An arrangement for measuring a gas of interest by its optical absorption characteristics, said measuring arrangement comprising:

a source of electromagnetic radiation;

means for directing such electromagnetic radiation through such gas of interest;

a first light signal, emerging from passage through such gas of interest, has associated therewith, periodic transmission spectra representative of the absorption lines of such gas of interest;

means for modulating said first light signal between transmission spectra which substantially coincide with such absorption lines and transmission spectra which fall between such absorption lines;

an interferometric device receptive of said first light signal and having associated therewith a periodic spacing equal to the periodicity of the absorption liens of such gas of interest, said interferometric device further having associated therewith, a second periodic spacing substantially equivalent to such transmission spectra which fall between such absorption lines;

said interferometric device being shifted from said periodic spacing to said second periodic spacing by application of an electrical field thereto; and

means for detecting at least an amount of such gas of interest as a function of the intensity of such periodic transmission spectra following passage through such gas of interest.

21. A measurement arrangement as set forth in claim 20 wherein said interferometric device is a compound Fabry-Perot birefringent etalon device having associated therewith, a first index of refraction and a path length which determine, in conjunction, such periodic transmission spectra which substantially coincides with such absorption liens associated with such gas of interest.

22. A measurement arrangement as set forth in claim 21 wherein said birefringent etalon device further has associated therewith, a second index of refraction which determines such periodic transmission spectra which falls between such absorption lines associated with said gas of interest.

23. A measurement arrangement as set forth in claim 22 wherein said second index of refraction can be selected from a range of values such that such periodic transmission spectra which falls between such absorption lines associated with the gas of interest, occur at a wavelength which essentially avoids interference from gases other than such ga of interest.

24. A gas measurement arrangement as set forth in claim 21 wherein said birefringent etalon has applied on opposing ends disposed along the direction of light propagation through said gas measurement arrangement, a partially reflective surface coating, the amount of said surface coating being determinative of such reflectivity and further wherein, such reflectivity can be selectively varied so as to achieve varying degrees of finesse associated with said birefringent etalon.