Linear absorption spectrometer to optically determine an absolute mole fraction of radiocarbon in a sample
Inventors
FLEISHER, ADAM J. • Long, David A. • Hodges, Joseph T.
Assignees
United States Department of Commerce
Publication Number
US-10067050-B2
Publication Date
2018-09-04
Expiration Date
2037-05-16
Interested in licensing this patent?
MTEC can help explore whether this patent might be available for licensing for your application.
Abstract
A linear absorption spectrometer includes: a laser light source that provides mid-infrared laser light; a high finesse optical resonator that includes: a sample cell operating at a temperature from 220 K to 300 K during linear absorption of mid-infrared laser light by radiocarbon and including: a linear absorption optical path length greater than a kilometer; a first zero-pressure difference mirror mount on which a first supermirror is disposed; a second zero-pressure difference mirror mount on which a second supermirror is disposed; an optical switch interposed between the laser light source and the high finesse optical resonator that modulates and communicates mid-infrared laser light to the high finesse optical resonator; a photoreceiver that receives cavity ring down light and includes a noise equivalent power that is less than a shot noise limit of cavity ring down light.
Core Innovation
The invention relates to a linear absorption spectrometer designed to optically determine an absolute mole fraction of radiocarbon in a sample. The spectrometer includes a laser light source providing mid-infrared laser light and a high finesse optical resonator that is actively stabilized in its resonance frequency. This resonator includes first and second supermirrors with specific radii of curvature and a relative difference of refractive index in a defined range, forming a sample cell between them that operates at controlled temperatures during linear absorption of radiocarbon. The sample cell offers an optical path length greater than a kilometer, and the assembly incorporates zero-pressure difference mirror mounts to maintain mirror integrity. An optical switch modulates the laser light before it enters the resonator, and a photoreceiver with a noise equivalent power less than the shot noise limit detects the cavity ring down light, allowing for determination of radiocarbon mole fractions as low as parts-per-quadrillion.
The problem addressed by this invention is the challenge of sensitive, direct optical detection and quantification of radiocarbon in samples at extremely low concentrations, below modern picomole per mole levels. Conventional methods rely on accelerator mass spectrometry (AMS) which are expensive, complex, and require specialized personnel and infrastructure. This invention proposes a more cost-effective, automated, and sensitive optical detection method, employing linear absorption and cavity ring-down spectroscopy with specialized high-finesse optical resonators, cooling to reduce spectral interferences, and advanced modulation and detection techniques to overcome limitations of previous approaches.
The invention further solves issues related to mirror birefringence and strain by employing zero-pressure difference mirror mounts, enabling stable and reproducible optical resonator performance. The design stabilizes the frequency of the mid-infrared laser light, ensuring high fractional frequency stability. The sample cell can be cooled by a secondary fluid conduit to reduce spectral congestion and improve detection sensitivity. These innovations collectively facilitate high-fidelity measurement of radiocarbon mole fractions ranging from 1 part-per-quadrillion to 2.5 parts-per-trillion in various gaseous samples under controlled pressure and temperature conditions.
Claims Coverage
The patent includes one primary independent claim detailing the components and features of a linear absorption spectrometer for radiocarbon detection, supported by multiple dependent claims that add specific configurations and operational parameters.
High finesse optical resonator with specific supermirror properties
The spectrometer utilizes a high finesse optical resonator actively stabilized in resonance frequency comprising a first and second supermirror with defined radii of curvature and a relative difference of refractive index Δn/n from 1×10⁻⁸ to 6×10⁻⁶; the second supermirror transmits cavity ring down light through a sample cell with an optical path length greater than a kilometer.
Sample cell temperature and design
The sample cell is interposed between the supermirrors, operates at a temperature from 220 K to 300 K during linear absorption, and includes zero-pressure difference mirror mounts mechanically coupled to the cell for strain-free mirror support.
Laser light modulation and detection
An optical switch modulates the mid-infrared laser light before communicating it to the first supermirror; a photoreceiver with noise equivalent power below the shot noise limit detects cavity ring down light, enabling detection of absolute radiocarbon mole fractions from 1 part-per-quadrillion to 2.5 parts-per-trillion.
Incorporation of a semiconductor laser including quantum cascade laser
The laser light source can comprise a semiconductor laser such as a quantum cascade laser, providing mid-infrared laser light with fractional frequency stability greater than 1 in 10⁸.
Use of reference laser and optical frequency comb stabilization
In some embodiments, a reference laser, potentially a second quantum cascade laser stabilized to an optical frequency comb, enhances frequency stability with supermirror reflectivities adapted for reference laser wavelengths.
Sample gas composition and pressure control
The sample can be a gas from petrogenic or biogenic sources contained within the sample cell at pressures from 100 Pascals to 3 kilopascals, optionally cooled via a secondary fluid conduit isolated from the primary fluid conduit.
Mechanical stabilization of the resonator
A spacer member mechanically spaces the zero-pressure difference mirror mounts to provide elongation length stability and maintain optical path length stability.
Photoreceiver positioning and extinction ratio
The photoreceiver is positioned at a distance from the second supermirror that is an integer multiple of the distance between the supermirrors; in combination with the optical switch, this provides an extinction ratio from 20 dB to 100 dB.
The independent claim defines a linear absorption spectrometer with specific optical, mechanical, and operational features enabling ultrasensitive, stable optical detection of radiocarbon at extremely low mole fractions. The dependent claims describe refinements including laser source types, sample and resonator parameters, and detection optimizations to enhance performance and applicability.
Stated Advantages
Significantly reduces the cost and complexity compared to accelerator mass spectrometry for radiocarbon analysis.
Provides automation of spectral analysis, reducing the time required for radiocarbon measurement.
Enables ultrasensitive detection of radiocarbon at mole fractions below 2 parts-per-trillion, well below contemporary ambient levels.
Reduces spectral interferences through sample cooling and optimized optical design.
Minimizes etalon effects and mirror strain to achieve high stability and precision in measurements.
Documented Applications
Detection and quantification of radiocarbon (14C) in gaseous samples such as carbon dioxide from petrogenic or biogenic sources.
Differentiation between biogenic and anthropogenic CO2 sources based on radiocarbon abundances.
Use in atmospheric chemistry, geoscience, biology, archaeology, medical and radioactive gas sample analysis requiring sensitive radiocarbon detection.
Interested in licensing this patent?