Optical EngineeringDetectivity of shot-noise-limited interferometric systems measuring weak gaseous absorption
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Three alternative interferometric-calorimetry systems that can be used to detect extremely small concentrations of trace gases are compared for their sensitivity limits due to both shot noise and vibrational noise. The three systems are (1) a pulsed system based on excitation by a repetitively pulsed heating laser; (2) a continuous-wave (cw) acoustic system, based on excitation by a cw heating laser whose intensity is modulated at an acoustic resonance frequency of the absorption cell; and (3) a cw isobaric (optical homodyne) system, based on a cw heating laser whose intensity is modulated at a frequency that is well below the acoustic cutoff of the absorption cell. It is shown that the shot-noiselimited sensitivities of the three systems are comparable, but that the potential presence of vibrational noise strongly favors the pulsed system.