Abstract
In this work, a novel configuration of a photothermal gas sensor is demonstrated. The measured gas sample is delivered to a gas cell placed inside the linear cavity of a 1.55 μm mode-locked fiber laser. The gas refractive index modulation resulting from the excitation by an auxiliary continuous wave laser is efficiently probed by analyzing the phase shift of the self-heterodyne beatnote signal of the mode-locked laser. IQ demodulation combined with Wavelength Modulation Spectroscopy-based signal analysis was employed to optimize and simplify the spectroscopic data acquisition and analysis. A proof-of-concept experiment with detection of carbon dioxide at 2 μm wavelength yielded a noise equivalent absorption of 2.25·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−7</sup> for 1000 s integration time. The proposed sensor configuration is versatile and can be used to probe any gas molecule, provided an appropriate excitation source is used to induce the photothermal effect.
Highlights
LASER-BASED gas detection techniques are undergoing relentless development, which is fueled by the rapid technology advancement observable in all industry sectors
A custom built, 20 cmlong gas absorption cell was incorporated inside the ML laser resonator and enabled simple gas sample testing
The gas was excited by a pump, which was a custom-built tunable laser that was coupled in the counter-propagating direction into the gas absorption cell (GAC) through an optical component serving as a mirror for the ML laser cavity and coaligned with the resonating probe beam
Summary
LASER-BASED gas detection techniques are undergoing relentless development, which is fueled by the rapid technology advancement observable in all industry sectors. Not all gas molecules have strong fingerprint spectra in the 1 μm to 2 μm wavelength region, clever techniques combining the advantages of using mid-IR lasers as the excitation sources, with the simplicity and cost effectiveness of using near-IR detectors and optics have been developed One of such methods relies on the photothermal effect, which can be induced in solids, liquids and gases [6]. The main and unique feature of the PTS-based gas sensing technique relies on the possibility of separating the pump and the probe parts of the setup, and each can be precisely tailored for its particular function This directly translates to the possibility of registering the spectroscopic signal using cost-effective and available near-IR laser and detectors, while the gas molecules can be excited by sources operating, e.g. in the mid-IR spectral band, taking the full advantage of strong absorption lines. The re-designed and improved original PTS gas sensor is less complex, and an order of magnitude more sensitive when compared to its previous version reported in [13]
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