Gas sensing system employing raman scattering
Inventors
Falk, Joel • Chen, Peng Kevin • Buric, Michael Paul • Woodruff, Steven D.
Assignees
US Department of Energy • University of Pittsburgh
Publication Number
US-8674306-B2
Publication Date
2014-03-18
Expiration Date
2031-11-21
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Abstract
A gas detection system includes a light detector, a pump laser with spectral emission between UV and IR wavelengths and structured to generate a laser beam, a hollow waveguide structured to receive a sample gas, the hollow waveguide having a bandwidth sufficient to transmit the laser beam and Stokes Raman photons scattered by the sample gas, and an optical system. The optical system is structured to: (i) direct the laser beam into the hollow waveguide such that it propagates in the hollow waveguide in one or more low-order low-loss waveguide modes, and (ii) direct Raman signals generated within the hollow waveguide in response to the laser beam interacting with the sample gas toward the light detector, the Raman signal including the Stokes Raman photons.
Core Innovation
The invention provides an optical Raman scattering gas detection system that uses a hollow waveguide to contain a sample gas, enabling interaction with a laser beam that generates Raman photons indicative of the gaseous components. The hollow waveguide collects a large number of scattered Raman photons and efficiently transfers them to a spectrometer or other detector where they can be measured to identify the sample and its constituents.
The system includes a special optical train designed to optimize light production and collection using a large multimode metal-lined capillary waveguide. This design enables high signal-to-noise ratios (SNRs) and is composed of lenses that launch the excitation laser beam into low-order, low-loss waveguide modes, while also collecting Raman modes propagating at various angles to maximize signal strength.
The problem being solved is the difficulty and high cost associated with detecting and characterizing molecular gases in a sample, such as natural gas, using existing methods like mass spectrometry and gas chromatography. Specifically, there is no commercially available sensor capable of measuring all natural gas components with the required accuracy and speed (better than 0.1% accuracy and at least once per second) to control turbines efficiently. The invention addresses this need by providing a fast, accurate, and reliable sensor system capable of identifying, characterizing, and determining gas concentrations in real-time.
Claims Coverage
The patent includes multiple independent claims covering various embodiments of a gas detection system and method using hollow multimode waveguides and laser excitation with specific optical configurations. The main inventive features focus on waveguide modes, laser beam coupling, optical elements, and system configurations for improved Raman signal detection.
Coupling laser beam into low order waveguide modes in a hollow multimode reflective-lined waveguide
The system uses a hollow multimode reflective-lined waveguide with one or more low-order waveguide modes, and a laser beam generating component producing a collimated laser beam with a predetermined diameter. A focusing optical element is structured to receive and focus the laser beam into the waveguide such that the beam enters at an angle that excites at least one low order waveguide mode, and also collects Raman signals generated in response to interaction with the sample gas and directs them to a light detector.
Use of optical components for optimal beam focusing and Raman signal collection
The system includes an optical compressor to reduce the pump laser beam diameter, a polarization converting element to convert linear polarization to TE polarization, and positioning of focusing lenses at distances related to their focal lengths to optimize coupling into waveguide modes and Raman signal collection.
Two-pass configuration using a spherical mirror at the waveguide's distal end
A spherical mirror is positioned adjacent to the output end of the hollow multimode reflective-lined waveguide to redirect laser light and Raman light back into the waveguide, increasing interaction length and signal strength.
Spatial filtering for noise reduction via physical apertures and fiber optic aperture device
Physical apertures or fiber optic aperture devices are positioned to pass light from the core region of the waveguide while blocking light from outside the core, thereby reducing silica Raman noise and fluorescence noise from waveguide walls or coatings.
Multichannel detection via dichroic beam splitters and detectors for simultaneous multi-gas measurement
The system includes a plurality of dichroic beam splitters arranged along the optical path, each reflecting a respective Raman spectral peak to dedicated detectors, allowing simultaneous measurement of multiple gas species with high accuracy.
Temperature-invariant waveguide containment cell and support structures
The hollow multimode waveguide is fixed at one end in a high-pressure flange and supported by a support tube with an inner diameter larger than the waveguide outer diameter, allowing expansion and contraction at different rates with temperature changes without misalignment, maintaining optical coupling efficiency.
Method for introducing sample gas, launching laser beam into waveguide modes, and analyzing Raman signals
A method including introducing sample gas into a hollow multimode reflective-lined waveguide, launching a laser beam such that it couples into low order-waveguide modes, receiving Raman signals generated from interaction with the gas, and analyzing these signals to identify gases present.
The claims collectively cover the configuration and arrangement of an advanced hollow multimode waveguide based gas detection system, including laser beam shaping and coupling into low-loss modes, noise reduction techniques, optical detection arrangements for multi-gas sensing, and temperature compensation features, as well as corresponding methods for gas detection.
Stated Advantages
High signal-to-noise ratio (SNR) with short sampling times and high accuracy.
Capability for fast, continuous measurement with sampling rates approximately 1 second and accuracy better than 0.1% for multiple gases.
Simultaneous measurement of multiple gases (eight or more) in real-time.
Ability to operate over a wide pressure range including high-pressure samples up to 800 psia.
Temperature-invariant waveguide cell design maintaining alignment and performance over temperature changes.
Improved Raman signal collection efficiency by coupling the laser into low-order low-loss waveguide modes and collecting a wide range of Raman propagation angles.
Reduction of noise sources such as silica Raman noise and fluorescence from plastics to improve measurement quality.
Flexibility to be used in various configurations including multi-channel detectors or tunable filters for selective spectral measurement.
Documented Applications
Control of large natural gas-fired turbine electricity generators by real-time measurement of natural gas fuel composition for improved turbine efficiency.
Monitoring and controlling industrial processes or reactions involving gaseous inputs or outputs.
Detection and analysis of natural gas composition in pipelines.
Monitoring out-gassing phenomena for diagnostics such as transformer failure.
Use as a detector in gas chromatography systems.
Detection of gaseous residues for homeland security purposes.
Monitoring of gases exhaled during anesthesia and in operating rooms.
Monitoring pollutant gases in smoke stacks.
Detection of flammable gases for mine safety applications.
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