Long-term stability in continuous wave cavity ringdown spectroscopy experiments.

Area of Science:

• Spectroscopy
• Laser Physics
• Environmental Monitoring

Background:

• Continuous wave cavity ringdown spectroscopy (CW-CRDS) is a powerful technique for gas sensing.
• Understanding and mitigating system drift is crucial for achieving high sensitivity and accurate measurements.
• Near-infrared distributed feedback lasers are often employed in CW-CRDS systems.

Purpose of the Study:

• To characterize the long-term drift of a near-IR DFB laser-based CW-CRDS system using Allan variance analysis.
• To identify the primary sources of drift, such as ambient pressure and temperature.
• To assess the system's methane detection capabilities and stability.

Main Methods:

• Utilized Allan variance to quantify the slow drift of the CW-CRDS system.
• Employed differential measurements of on- and off-peak decay rates to minimize drift effects.
• Investigated the correlation between cavity loss rate and environmental factors like ambient pressure and temperature.

Main Results:

• Observed long-term drift in cavity loss rate, strongly correlated with ambient pressure but not temperature.
• Achieved a sensitivity to bulk cavity loss of 4.4 x 10(-12) cm(-1) with a 30-minute integration time.
• Determined methane detection limits of 0.24 ppbv at 20 Torr and 29 ppbv at 760 Torr.

Conclusions:

• The CW-CRDS system exhibits pressure-dependent drift that can be largely, but not entirely, canceled by differential measurements.
• The system demonstrates excellent stability and sensitivity, capable of detecting sub-ppbv methane concentrations.
• The findings are critical for optimizing CW-CRDS systems for precise environmental gas analysis.