A Sensitive Carbon Monoxide Sensor Based on Photoacoustic Spectroscopy with a 2.3 μm Mid-Infrared High-Power Laser...

Area of Science:

• Gas sensing technologies
• Spectroscopic analysis
• Optical instrumentation

Background:

• Carbon monoxide (CO) is a toxic gas requiring sensitive detection methods.
• Photoacoustic spectroscopy (PAS) offers a promising approach for gas sensing.
• Existing PAS sensors face challenges in sensitivity and noise reduction.

Purpose of the Study:

• To develop and demonstrate a highly sensitive PAS-based CO gas sensor.
• To enhance gas absorption and signal-to-noise ratio for improved detection limits.
• To investigate the impact of water vapor on CO detection performance.

Main Methods:

• Utilized a high-power distributed feedback (DFB) continuous wave (CW) diode laser (~8 mW).
• Implemented a right-angle prism for two-times enhanced optical absorption.
• Employed wavelength modulation spectroscopy (WMS) with second-harmonic (2f) detection.
• Optimized modulation frequency and depth; introduced water vapor to enhance vibrational-translational (V-T) relaxation.

Main Results:

• Achieved an ~8 times signal enhancement with water vapor addition compared to dry gas.
• Observed a 1.52-fold improvement in 2f signal amplitude with enhanced absorption.
• Demonstrated excellent linear response to optical power and CO concentration.
• Attained a minimum detection limit (MDL) of 9.8 ppm for CO at 1s integration time.
• Reached an MDL of 530 ppb with an 1100s integration time, showing long-term stability via Allan deviation analysis.

Conclusions:

• The developed PAS sensor demonstrates significant potential for sensitive and stable CO monitoring.
• Enhanced optical absorption and strategic use of water vapor are key to improving sensor performance.
• Further improvements are possible with higher laser power and increased absorption paths.