Compact methane sensor using an integrating sphere and interband cascade laser at 3313 nm

We present a sensor for gas detection utilising an integrating sphere and interband cascade laser for tunable diode laser absorption spectroscopy (TDLAS). Measurements were made of the methane absorption line at 3313 nm using an integrating sphere with an effective pathlength of 70.1 cm. Gas concentrations were estimated from normalised absorption spectra with line fitting to 7 absorption lines within the scan. Testing showed that measurements were linear for methane in synthetic air for concentrations in the range 0-50 ppm. With an averaging time of 20 s, the noise-equivalent methane concentration was 180 ppb (1 & sigma;). The system requires minimal alignment, with preliminary measurements indicating the system is insensitive to vibration and misalignment of the laser input. The lack of alignment optics also allows it to be compact (190x170x120mm) and robust.