Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-12-28
2003-06-03
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S325000, C359S279000, C250S343000, C356S364000
Reexamination Certificate
active
06574031
ABSTRACT:
ORIGIN OF THE INVENTION
The invention described herein was made by an employee of the United States Government and may be manufactured and used by the government for governmental purposes without the payment of any royalties thereon or therefor.
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 relates to such measurement using dual beam spectroscopy, including gas filter correlation radiometry. It further relates to the balancing of optical intensities.
2. Discussion of the Related Art
Optical path switching has many potential applications, particularly in the field of dual beam spectroscopy. In dual beam spectroscopy, light from a radiation source traverses a measurement path and is then divided between 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 measurement in this type of spectroscopy is related to the intensity difference of the radiation that takes these two paths. For illustrative purposes, a gas filter correlation radiometer (GFCR), one example of a dual beam spectrometer, will be discussed in detail.
Gas filter correlation radiometers (GFCRs) may inter the concentration of a gas species along some measurement 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 measurement path. These two optical cells, often called the correlation and vacuum cells, are an example of the media found in the two optical paths of a dual beam spectrometer. The correlation cell contains a high optical depth of 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 spectral “notch filter” to the incoming radiation, the spectral notches being 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. The difference in signal between these two views of the emitting/absorbing gas species i within the spectral region of interest plus, or in combination with, the sum of the signals of these two views can be related to the concentration of this gas along the measurement path.
In one known GFCR for measuring a single gas concentration in a particular quantity 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, a non-mechanical optical path switch comprises a polarizer, polarization modulator and a polarization beam splitter. The polarizer polarizes light 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 beam splitter which transmits light of one orthogonal component, e.g., horizontally polarized, and reflects light of a perpendicular component, e.g., vertically polarized, In a gas filter correlation radiometer application, 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 reflected vertically 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 into a single beam which is read by a conventional detector. This approach has numerous advantages, such as no mechanical means being required to alternate the view of the detector through the correlation and vacuum cells, fast 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 an optical path switch. It further would be desirable to do such measurement with optimal optical and/or electronic balancing of optical intensities.
OBJECTS OF THE INVENTION
It is accordingly an object of the present invention to provide a device to simultaneously, but not independently, measure two or more gases of interest .
It is another object of the present invention to provide a device to simultaneously, but not independently, measure two or more gases of interest with negligible or no spectral interference.
It is another object of the present invention to provide a device to simultaneously and independently measure two or more gases.
It is another object of the present invention to provide a device to simultaneously and independently measure two or more gases with negligible or no spectral interference.
It is another object of the present invention to provide a device using an optical switch to simultaneously measure two or more gases for various applications requiring two optical analysis paths.
It is another object of the present invention to perform dual beam spectroscopy such as gas filter correlation radiometry using a single instrument to measure two or more gases in which the difference and sum signals can be obtained from only one detector for each gas wavelength region of interest.
It is another object of the present invention to accomplish simultaneous and independent measurement of two or more gases using a minimum of optical components.
It is another object of the present invention to accomplish 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 sense the total burden 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 one or a combination of several gases using a single instrument.
It is still another object of the present invention to provide a device to simultaneously measure two or more gases of interest and optimize the balance of optical intensities.
It is still another object of the present invention to provide a device to simultaneously measure two or more gases of interest and optically optimize the balance of optical intensities.
It is a further object of the present invention to provide a device to simultaneously measure two or more gases of interest and electronically optimize the balance of optical intensities.
Additional objects and advantages of the present invention are apparent from the specification and drawings which follow.
SUMMARY OF THE INVENTION
The foregoing and additional objects are obtained by modulating a polarized light beam over a broadband of wavelengths between two alternating orthogonal polarization components. One orthogonal polarization component of the polarization modulated 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 spectral discrimination means, with each spectral discrimination means having spectral absorption features of one or more gases of interest being measured. The two optical paths then intersect, and one orthogonal component of the intersected components is transmitted and the other orthogonal component is reflected. This forms 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
Ben Loha
Edwards Robin W.
The United States of America as represented by the Administrator
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