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    Area of Science:

    • Spectroscopy
    • Laser Technology
    • Gas Sensing

    Background:

    • Wavelength modulation spectroscopy (WMS) is a sensitive technique for gas detection.
    • Existing WMS systems face challenges in laser frequency stability and signal intensity fluctuations.
    • Accurate gas concentration measurements require robust methods for frequency locking and intensity normalization.

    Purpose of the Study:

    • To develop and demonstrate a novel method for laser frequency locking and intensity normalization in WMS-based gas sensor systems.
    • To improve the measurement accuracy and signal-to-noise ratio (SNR) of WMS gas sensors.
    • To evaluate the performance of the proposed method using a methane (CH4) gas sample.

    Main Methods:

    • Laser frequency locking using the spacing between second harmonic peaks from a triangular wavelength scan.
    • Intensity normalization using the amplitude of the modulation sine wave in a non-absorption spectral region.
    • Demonstration using a 50 ppm CH4:N2 sample in a multi-pass cell at 1 atm.

    Main Results:

    • Frequency locking significantly enhanced measurement accuracy.
    • The novel intensity normalization method achieved a ~3 times SNR improvement over the 1f normalization method.
    • A minimum measurement precision of ~2.5 ppbv was achieved.
    • Normalized noise equivalent absorption coefficient of 1.8 × 10^-9 cm^-1Hz^-1/2 was obtained.

    Conclusions:

    • The developed method provides effective laser frequency locking and intensity normalization for WMS gas sensors.
    • The approach significantly improves SNR and measurement precision.
    • This technique offers a promising advancement for high-accuracy gas sensing applications.