Analysis of the optical power loss arising from a fibre coupled integrating sphere used as a compact gas sensor

An analysis describing the path-length enhancement and the power loss that is encountered by a light beam traversing a fibre optic coupled integrating sphere used as a gas sensor cell is reported. The results presented here show the path-length enhancement that is possible with a CO2 integrating sphere gas cell at an optical wavelength of approximately 2 μm. Building on prior research, the relationship between the power loss encountered by light within the integrating sphere and the coupling optics of the output optical fibre are quantified. The results are of value in the implementation of compact optical fibre gas sensors with the integrating sphere acting as the measurement gas cell. It is shown that with a CO2-containing integrating sphere of 5 cm diameter, a path-length enhancement by a factor of 12.5 can be achieved with the use of a sphere surface coating with a reflectance of ∼95% (the enhancement factor may be significantly increased with the use of a coating with a higher surface reflectance), while at the same time an increased power attenuation of approximately −30 dB will be experienced.