All-Optical Photoacoustic Multigas Analyzer Using Digital Fiber-Optic Acoustic Detector

Abstract

We present an all-optical photoacoustic (PA) multigas analyzer by using a digital fiber-optic acoustic detector, which consists of a spectrum demodulation based fiber Fabry-Perot (F-P) pressure sensor and a digital virtual lock-in amplifier. The concentration of multigas is quantified by using an intensity-modulated infrared thermal source and two wavelength-modulated laser diodes. Light beams from the three sources are coupled to a single nonresonant PA cell. The generated PA pressure is detected by the fiber F-P pressure sensor, which is demodulated by a near-infrared spectrometer. A lock-in white-light interferometer is developed for detecting first-harmonic (1f ) and second-harmonic (2f ) signals. The noise equivalent detection limits with a 100-s averaging time are 37, 9, 6, 17, 4, and 60 ppb for CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , CO, and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , respectively.