Reexamination Certificate
2000-06-08
2003-03-04
Rosenberger, Richard A. (Department: 2877)
C356S440000
Reexamination Certificate
active
06527398
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to absorption spectroscopy; and more particularly to a waveguide and apparatus for the detection and measurement of trace gases, preferably absorption spectroscopy—and most preferably by wavelength-modulation absorption spectroscopy.
2. Related Art
Much of the development of today's pollution monitoring equipment was carried out after passage of the Clean Air Act. At that time electrooptical technology was in its infancy in comparison with current capabilities. One of the primary disadvantages of optical techniques for gas detection and measurement is that instruments tend to be large and complex.
Analyzers that meet the EPA designated reference method require specialized techniques for specific gases. For example, N
0
2
is detected by chemiluminescence; CO by gas correlation; O
3
by absorption; and SO
2
by fluorescence. There has been no single effective methodology that could be used to monitor key pollutants of interest.
Current analyzers are typically packaged one to a 48 cm (19 inch) rack-mounted chassis. These analyzers typically weigh nearly 25 kg, including the required sample handling components, power supplies and electronics. Ambient air monitoring stations tend to involve buildings to enclose the analyzers and other sophisticated support equipment, and dramatically increase the cost associated with pollution assessment.
Similar problems exist for continuous emission monitoring systems used to evaluate flue gas from coal- and gas-fired electrical generation facilities. In many cases analyzers are mounted inside large dedicated shelters. Samples of the flue gas are transported over long distances in heated sample lines from the stack to the shelters.
Ultraviolet absorption bands of gases tend to be relatively broad due to the nature of the changes experienced by the molecules. There are significant overlaps in spectral features of different molecules.
Spectral selectivity is required to overcome the inherent problem of interference. Lamp inefficiencies also require high heat dissipation. Several UV instruments have been used for pollution monitoring; however, these systems are relatively large and require complicated spectral deconvolution algorithms to eliminate interference.
Both infrared and ultraviolet techniques require relatively long light/sample interaction paths to achieve sensitive detection capabilities. This has been achieved using multipass cells that reflect the light repeatedly over a folded path inside a container that holds the sample.
To achieve adequate pathlengths these cells are often large (>1000 cc) in volume. Noise and drift associated with the measurements impose strict design criteria to maintain alignment of the cells. Cell temperatures are usually controlled to maintain stability, and large pumps are required to achieve rapid pneumatic response.
A prior-art device that potentially provides a more compact gas analyzer is shown in U.S. Pat. No. 5,341,214 to Wong. This patent discloses an inherently rigid cylindrical waveguide/chamber having an internal blackbody radiation source mounted within one end, and two infrared detectors mounted within the opposite end of the chamber.
The radiation is reflected at the walls of the chamber to increase the light path. Each detector includes an optical filter; one filter defines a spectral band that coincides with the infrared absorption of the gas to be measured, and the other defines a nonabsorption bandpass.
The signals from Wong's detectors are processed to provide a ratio related to the concentration of the gas in the sample. The chamber includes a membrane-covered inlet aperture and an outlet aperture to permit flow through the chamber.
A device utilizing this technology is likely suited to detect only a high concentration of a gas. Short optical pathlength limits sensitivity for low concentrations, and the low gas flow limits the response time.
Another prior-art device is shown in U.S. Pat. No. 5,384,640, also to Wong. This patent discloses an evidently rigid cylindrical waveguide/chamber having an integral laser positioned within one end of the chamber, and an integral detector positioned within the opposite end.
Apertures in the wall of the chamber enable flow of gas through the chamber. The cylinder can be partitioned into successive chambers for the detection of different gases. The chamber is relatively large, having a diameter greater than the diameters of the laser and the detector, and therefore as a practical matter is quite limited in optical length and also optical efficiency. From the smallest commercially available laser and detector packaging, it can be seen that the inside diameter of Wong's chamber is at least 5 mm.
Consequently a device based on this patent too appears limited to measuring only high concentrations of a gas, and only by direct absorption (not by wavelength-modulation absorption). The optical length and resulting sensitivity are further limited by partitioning the chamber for the detection of multiple gases.
U.S. Pat. No. 5,696,379 to Stock discloses a prior-art infrared absorption device incorporating a curved tubular waveguide having a plurality of random gas inlets along the length of the waveguide and having an internal radiation source within the waveguide. A second radiation source is positioned within the waveguide near detectors and utilized to compensate for deterioration drift and temperature drift of the detectors. A device based upon this patent is believed to have a relatively large-diameter, low-efficiency waveguide with a relatively short optical path, and consequently to have limited sensitivity and slow response times between measurements.
There have been significant recent developments in hollow optical waveguides for energy delivery in the infrared regions (0.8 to 12 p) of the spectrum. Such improvements are disclosed in U. S. Pat. No. 5,440,664; 5,567,471; and 5,815,627 to Harrington et al., and are assigned to Rutgers, The State University of New Jersey. This technology provides waveguides for preservation of good transverse coherence of input infrared laser radiation, and that transmit substantial power of such radiation with low attenuation. The waveguide typically comprises a small-diameter, thin-wall silica glass tube, a protective outer coating, a reflective layer on the inner surface of the tube, and a dielectric coating on the exposed surface of the reflective layer.
Heretofore, this new waveguide technology has not been effectively applied and packaged for compact absorption spectroscopy.
There have also been significant recent developments of semiconductor diode lasers and the quantum cascade laser. Such improvements represent new compact tunable near- and midinfrared coherent light sources that can operate at room or thermoelectric control temperatures, and can be customized for specific wavelengths.
The explosive growth and miniaturization of electronics provides high-speed analog-to-digital converters and high-throughput digital signal processing capabilities, that have heretofore not been applied and incorporated into systems for compact, accurate and reliable absorption spectroscopy.
As shown above, the related art remains subject to significant problems and has left room for considerable refinement.
SUMMARY OF THE DISCLOSURE
The present invention introduces such refinement, and provides a novel waveguide and optical gas-sensor apparatus that is compact, lightweight, durable, extremely sensitive, accurate and reliable, and has a rapid response time.
The invention has several independently usable facets or aspects, which will now be introduced. In preferred embodiments of a first of its main aspects or facets, the present invention is a waveguide for holding a gaseous specimen for spectral analysis.
The waveguide includes a hollow elongated tube having an interior with inside diameter not exceeding two millimeters (2 mm), a radiation inlet and a radiation outlet—and also having a wall with a substantially smooth reflective inner surface, ada
Ashen & Lippman
Rosenberger Richard A.
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