When used as samples cells for optical absorbance measurements, integrating spheres offer increased pathlengths compared to single pass cells combined with tolerance to misalignment. This makes them attractive during alignment of optical instruments and in challenging environments subject to vibration. However, integrating spheres can suffer problems when used in sensitive and / or accurate absorbance measurement. We present our work to date to address these issues in high resolution laser spectroscopy.<p> </p> Firstly, optical interference effects include both random laser speckle and structured interference fringes created by optical feedback to the laser. Secondly, the sphere’s optical pathlength is a combination of multiple paths that take an exponential pathlength distribution. At low values of absorbance, the measured signal is linear with concentration, but at higher absorbances signals follow a nonlinear but predictable function of absorbance. Thirdly, our most recent work concerns calibration of the optical pathlength, which is a sensitive function of its internal reflectivity. In-situ calibration is needed if the sphere is to be used in dirty environments or with condensing samples. Measurements from multiple independent sources and / or detectors are combined to provide compensation from fouling of the sphere walls and windows. <p> </p>Results are presented for an integrating sphere used in the measurement of methane. The emission from a tunable DFB laser at 1651nm was tuned across the gas absorption line to measure its concentration. Reduced sphere reflectivity was simulated by applying small areas of black tape on the inner surface. Finally, we give an example of one application where our results are being put into practice: use of an integrating sphere with a tunable laser at 3.3μm to measure atmospheric methane, installed on a two seater light aircraft.
Detection of methane at 3.3μm using a DFB Interband Cascade Laser and gold coated integrating sphere is performed. A 10cm diameter sphere with effective path length of 54.5cm was adapted for use as a gas cell. A comparison between this system and one using a 25cm path length single-pass gas cell is made using direct TDLS and methane concentrations between 0 and 1000 ppm. Initial investigations suggest a limit of detection of 1.0ppm for the integrating sphere and 2.2ppm for the single pass gas cell. The system has potential applications in challenging or industrial environments subject to high levels of vibration.