Cavity ring-down Faraday rotation spectroscopy for oxygen detection

A combination of the path length enhancement provided by cavity ring-down spectroscopy together with the selectivity and noise suppression capabilities of Faraday rotation spectroscopy is utilized for highly sensitive detection of oxygen at ~762.3 nm. The system achieves a noise-equivalent rotation angle of 1.3 × 10⁻⁹ rad/√Hz, and a trace O₂ detection limit of 160 ppb for 100 s of averaging. The technique relies on measurements of the losses in two orthogonal polarization directions simultaneously, whereby an absolute assessment of the magnetically induced polarization rotation can be retrieved, analogous to the absolute absorption measurement provided by stand-alone cavity ring-down spectroscopy. The differential nature of the technique described here eliminates the need for off-resonance decay measurements and thereby allows for efficient shot-to-shot fluctuation suppression. This is especially advantageous when operating the system under measurement conditions that severely affect the non-absorber related losses, such as particulate matter contamination typically present in combustion or open-path applications.