Limiting sensitivity of a differential absorption spectrometer with direct detection in the 2ν3 and ν2+2ν3 vibration bands [methane detection applications]

A differential absorption spectrometer, for methane detection, using tunable laser diodes in the 1.33-μm and 1.66-μm bands, has been studied. The spectral scanning is carried out by current modulation of the laser diode. We analyzed the performance with a multimode Fabry-Perot laser diode in the ν₂+2ν₃ band of methane and a monomode laser diode in the 2ν₃ band. The theoretical results are validated by several experiments. To determine the sensitivity limit of the sensor, we have examined the influence of the noise sources. A sensitivity of 10 ppm·m was obtained in the 2ν₃ band. The main limiting factors are the relative intensity noise of the laser, optical interferences, and quantization noise. We also analyzed the influence of the temperature on the laser diode emission spectra and the methane absorption spectra.