Cavity enhanced laser based isotopic gas analyzer
Inventors
Koulikov, Serguei • Kachanov, Alexander
Assignees
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Publication Number
US-9759654-B2
Publication Date
2017-09-12
Expiration Date
Abstract
Systems and methods for measuring the isotope ratio of one or more trace gases and/or components of gas mixtures such as different gas species present in a gas mixture. The system includes a resonant optical cavity having two or more mirrors and containing a gas, the cavity having a free spectral range that equals the difference between frequencies of two measured absorption lines of different gas species in the gas, or of two different isotopes, divided onto an integer number. The system also includes a continuous-wave tunable laser optically coupled with the resonant optical cavity, and a detector system for measuring an absorption of laser light by the gas in the cavity. The detector system includes one of a photo-detector configured to measure an intensity of the intra-cavity light or both a photo-acoustic sensor configured to measure photo-acoustic waves generated in the cavity and a photo-detector configured to measure an intensity of the intra-cavity light.
Core Innovation
The document describes a cavity-enhanced laser spectroscopy isotopic gas analyzer that uses a resonant optical cavity containing a gas having a chemical species to be measured. The resonant optical cavity defines a plurality of cavity modes having a free spectral range (FSR), and the FSR is matched to the frequency difference between two selected absorption lines divided by an integer. A continuous-wave tunable laser is optically coupled with the resonant optical cavity, and the laser output is scanned across a range of frequencies including the frequency of a cavity mode.
Absorption is measured by a detector system while the cavity is tuned so that a frequency of one of the cavity modes corresponds to an absorption frequency. The detector can measure absorption using intracavity light intensity via a photo-detector and/or measure absorption using photo-acoustic waves via a photo-acoustic sensor with a photo-detector. The measured absorption signal is used to calculate an absorption coefficient and to determine an isotope ratio of a component in the gas mixture.
The document emphasizes improved isotopic ratio accuracy by selecting isotopic lines with similar temperature dependence and pressure broadening, so peak-intensity measurements can replace integral-intensity measurements. It also describes synchronous scanning of cavity modes to enable integral-intensity measurements when needed, and optionally provides baseline determination by measuring absorption at non-resonant or non-absorbing wavelengths.
Claims Coverage
The document includes two independent claims covering (1) a gas analyzer system and (2) a method of performing an absorption measurement in a cavity. Across the independent claims, the core inventive structure is cavity-mode tuning of a resonant optical cavity matched to an absorption frequency, combined with a continuous-wave tunable laser scanned across frequencies and a detector measuring absorption to obtain an absorption coefficient.
Tuned resonant optical cavity matched to absorption frequency
A resonant optical cavity having two or more mirrors and defining a plurality of cavity modes having a free spectral range (FSR), with control circuitry configured to adjust or tune an optical path length so that a frequency of one cavity mode corresponds to an absorption frequency of a chemical species.
Continuous-wave tunable laser scanned across cavity-mode frequencies
A continuous-wave tunable laser optically coupled with the resonant optical cavity, wherein an output of the laser is scanned across a range of frequencies including the frequency of the cavity mode corresponding to the absorption frequency.
Detector measuring absorption of laser light by the gas in the cavity
A detector system configured to measure an absorption of laser light by the gas in the resonant optical cavity at the cavity-mode absorption frequency.
Cavity absorption measurement by tuning, scanning, and signal-based coefficient calculation
A method of performing an absorption measurement in a cavity having two or more cavity mirrors and a plurality of resonant cavity modes having a free spectral range (FSR), including tuning the cavity so a cavity-mode frequency corresponds to an absorption frequency, coupling continuous-wave tunable laser output, scanning the laser output across a range including the cavity-mode frequency, measuring a signal corresponding to absorption at that cavity-mode frequency, and calculating an absorption coefficient based on the measured signal.
The independent claim set is centered on tuning a resonant optical cavity so a cavity mode frequency corresponds to an absorption frequency of a chemical species, scanning a continuous-wave tunable laser across frequencies including that cavity-mode frequency, and measuring an absorption signal to calculate an absorption coefficient.
Stated Advantages
Improved isotopic ratio accuracy by selecting isotopic lines with similar temperature dependence and pressure broadening so peak-intensity measurements can replace integral-intensity measurements.
Documented Applications
Isotopic gas analyzer for measuring an isotope ratio of a component in a gas mixture using a resonant optical cavity matched to selected absorption lines.
Trace gas detection is referenced in the document context as an intended measurement use case.
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