Limiting Sensitivity of a Differential Absorption Spectrometer With Direct Detection in the and Vibration Bands

A differential absorption spectrometer for methane detection using tunable laser diodes in the 1.33- m and 1.66- m bands has been studied. The spectral scanning is carried out by the current modulation of the laser diode. We analyzed the perfor- mances with a multimode Fabry-Perot laser diode in the band of methane and a monomode laser diode in the band. The theoretical results are validated by several experiments. To deter- mine the sensitivity limit of the sensor, we have examined the influ- ence of the noise sources. A sensitivity of 10 ppm m was obtained in the 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.