Analysis of Random Noise and Long-Term Drift for Tunable Diode Laser Absorption Spectroscopy System at Atmospheric Pressure

The random noise and the slow drift, which respectively limit the signal-to-noise ratio and the long-term stability, of a wavelength modulated tunable diode laser absorption spectroscopy system, are critically analyzed and evaluated experimentally. The noise sources considered in detail include the noise contributions from the lock-in amplifier, detector-preamplifier combination noise, excess noise from the DFB laser, and detection cell instabilities, especially in multiple reflection long-path cells. We also analyze contributions to long-term drift and find that the drift may be significantly compensated by the use of an optical reference path. We find experimentally that a white cell with a path length of 17.5 m introduces excess noise resulting in its having a performance equivalent to 6-m single pass absorption path. The experimental system optimized using the results of this analysis and using the 17.5-m white cell is found to operate at a sensitivity of 1.5-ppmv carbon monoxide in a bandwidth of 4 Hz. The long-term zero offset drift levels measured over 8 h are found to be $\sim 120$ ppmv without the reference optical path and are reduced to 20 ppmv using the balance path technique.