CO₂ monitoring and detection using an integrating sphere as a multipass absorption cell

This paper describes a multipass absorption sensor based on an integrating sphere. The sphere has an internal coating which is highly reflective (over 95%) in the near infrared region and this allows for the detection of CO₂ at two regions in the spectrum: 2 νm and 1.57 νm. CO₂ was detected around the 2 νm region using a light emitting diode as the emitter and a photodiode as the detector, while detection at the 1.57 νm region took place using a broadband ASE source as the emitter and a miniature spectrometer as a detector. A 2 in diameter (50.8 mm) integrating sphere was used as an absorption gas cell. This paper reports the generation of effective optical path lengths of 114 cm at the 1.57 νm region and 40 cm at the 2 νm region. Experimental results demonstrating the detection of CO₂ using a 2 in diameter integrating sphere are presented and simulation results based on larger integrating spheres are also reported. Further simulations conducted in the mid-infrared region suggest the possibility of detecting trace levels of CO ₂ (less than 200 ppm) using a 2 in diameter integrating sphere.