Integrating cavity based gas cells: a multibeam compensation scheme for pathlength variation.

Area of Science:

• Spectroscopy
• Analytical Chemistry
• Optical Engineering

Background:

• Integrating spheres are crucial for gas absorption measurements.
• Sphere wall contamination can alter optical pathlength, leading to inaccurate gas absorption coefficients.
• Recalibration is often required to compensate for contamination, disrupting continuous monitoring.

Purpose of the Study:

• To develop a self-calibrating ratiometric setup for gas absorption measurements using an integrating sphere.
• To correct for pathlength variations caused by sphere wall contamination.
• To enable continuous determination of gas absorption coefficients without frequent recalibration.

Main Methods:

• A four-beam ratiometric optical setup was designed for an integrating sphere gas cell.
• The setup measures gas absorption at 1651nm, specifically demonstrating with methane.
• The ratiometric approach compensates for changes in optical pathlength due to sphere wall contamination.

Main Results:

• Uncompensated contamination of 1.2% of the sphere wall caused a ≈41% error in gas absorption coefficient.
• The ratiometric scheme reduced this error to ≈2%.
• The technique demonstrated effective correction for pathlength variations.

Conclusions:

• The four-beam ratiometric setup provides a robust method for continuous gas absorption coefficient determination.
• This approach minimizes errors from sphere wall contamination, enhancing measurement accuracy and reliability.
• Potential limitations, such as severe sphere window contamination, require further investigation.