Devices and methods for quartz enhanced photoacoustic spectroscopy
Inventors
Blanchard, Romain • Vakhshoori, Daryoosh
Assignees
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Publication Number
US-10908129-B2
Publication Date
2021-02-02
Expiration Date
Abstract
In quartz-enhanced photoacoustic spectroscopy (QEPAS), an analyte (typically in gas phase) generates a pressure wave in response to incident laser light. A quartz tuning fork (QTF) resonant at the frequency of the pressure wave transduces the pressure wave into an electrical signal. Pulsing the laser briefly reduces the amount of thermal chirp and increases the fraction of time that the laser emits at the wavelength(s) of interest. This increases the measurement efficiency. Pulsing the incident laser light with bursts of short pulses at the QTF resonant frequency increases signal strength. Exciting the sample with a two pulses at different laser wavelengths, separated by a half QTF period yields signal and background acoustic waves that partially cancel when integrated by the QTF, producing a differential measurement. Pulsing the incident laser light at a frequency faster than the gas response cut off frequency can improve the noise performance of a QEPAS measurement.
Core Innovation
The invention improves spectroscopic measurement by modulating a single-mode laser with a repetitive pulse sequence to cause the laser to emit a periodically pulsed laser beam, where each period comprises a plurality of pulses. The sample is illuminated with the periodically pulsed laser beam, and radiation reflected, scattered, transmitted, and/or emitted by the sample in response to the periodically pulsed laser beam is detected with a resonant detector having a resonance frequency equal to the pulse repetition frequency of the periodically pulsed laser beam.
The invention further uses pulse timing and spectral structure so that the resonant detector is excited at the pulse repetition frequency and can support interference or differential detection. A periodically pulsed laser beam includes first pulses at a first wavelength and second pulses at a different second wavelength, with the second pulses delayed by half a period relative to the first pulses, and radiation from the sample in response to the first pulses and second pulses is detected as interference at the resonant detector.
The disclosed approach also incorporates resonant-detector excitation in response to an acoustic wave emitted by the sample and detecting an oscillation of the resonant detector caused by the acoustic wave. Various refinements include constraining duty cycle and pulse bandwidth, constraining burst duration to less than half a period, and using modulation schemes that enable improved noise performance in the measurement context described in the document.
Claims Coverage
The document includes three independent claims covering a method, a system, and another method focused on interference detection. Across the independent claims, the coverage centers on inventive features that link a pulse-repetition-frequency-matched resonant detector with periodically pulsed single-mode laser illumination of a sample, including dual-wavelength half-period timing and resonant interference detection.
Repetitive pulsed single-mode illumination with pulse repetition frequency-matched resonant detection
Modulating a single-mode laser with a repetitive pulse sequence to cause the laser to emit a periodically pulsed laser beam, illuminating the sample with the periodically pulsed laser beam, and detecting radiation reflected, scattered, transmitted, and/or emitted by the sample with a resonant detector having a resonance frequency equal to a pulse repetition frequency of the periodically pulsed laser beam.
Resonant detector excitation and acoustic-wave/oscillation detection
Detecting radiation characteristics from a sample by detecting an acoustic wave emitted in response to a periodically pulsed laser beam and then detecting an oscillation of a resonant detector caused by that acoustic wave.
System pairing single-mode periodic pulsing with resonance-frequency-equal resonant detector
Providing a single-mode laser to illuminate the sample with a periodically pulsed laser beam in response to a repetitive pulse sequence, and providing a resonant detector to detect radiation reflected, scattered, emitted, and/or transmitted by the sample in response to the periodically pulsed laser beam, where the resonant detector has a resonance frequency equal to a pulse repetition frequency of the periodically pulsed laser beam.
Dual-wavelength half-period delayed pulse structure
Illuminating a sample with a periodically pulsed laser beam where each period comprises at least one first pulse centered at a first wavelength and at least one second pulse centered at a second wavelength different than the first wavelength, with the at least one second pulse delayed with respect to the at least one first pulse by half the period of the periodically pulsed laser beam.
Interference detection between sample responses to first and second wavelengths
Detecting, with a resonant detector having a resonance frequency substantially equal to a pulse repetition frequency of the periodically pulsed laser beam, interference between first radiation reflected, scattered, transmitted, and/or emitted by the sample in response to the at least one first pulse and second radiation reflected, scattered, transmitted, and/or emitted by the sample in response to the at least one second pulse.
Overall, the claim set ties spectroscopy to periodic single-mode laser modulation and to a resonant detector whose resonance frequency is matched to the pulse repetition frequency, and further specifies dual-wavelength, half-period-delayed pulses so that interference between sample responses can be detected at the resonant detector.
Stated Advantages
Increases signal strength by using burst timing synchronized to the quartz tuning-fork resonance.
Improves noise performance in the measurement context described in the document by operating with modulation faster than the analyte vibrational-translational relaxation cut-off.
Enables differential/background-cancelling acoustic signals via two-wavelength excitation with a half-QTF-period delay.
Improves electromagnetic pick-up noise performance using tuning-fork architectures and cancellation via switching and harmonic-domain separation as described in the document.
Enables low-cost scalable sensors via integrated compact QEPAS device architectures (including wafer-level bonding and arrays).
Documented Applications
Compact QEPAS sensors for scalable, low-cost detection using integrated device architectures that include optical imaging to the fork region and integrated laser/tuning-fork structures as described in the document.
Humidity sensing / water vapor concentration measurement as described in the document through humidity integration to correct V-T relaxation effects.
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