Combined broadband photo-acoustic and absorption spectroscopic techniques of measurement for C2H2 with supercontinuum laser

Abstract

C 2 H 2 is a colourless and highly flammable gas that becomes explosive when liquefied, compressed, heated, or mixed with oxygen. The development of compact optical sensors for acetylene is of interest for many applications like manufacturing plants and industrial process monitoring. There are a wide range of spectroscopic and non-spectroscopic techniques employed widely to detect trace gases. Photo-acoustic spectroscopy (PAS) is one very versatile and efficient technique to detect trace gases of the order of ppb levels [1, 2]. A broadband photoacoustic gas sensor with a supercontinuum laser (SCL) in the near infra red region has been proposed for monitoring the (v 1 +v 3 ) vibrational combinational band. In the study, a supercontinuum laser (Leukos, SM-250-IR) with a wide bandwidth (900–2800 nm) was used. To match with the absorption lines of C 2 H 2 , a bandpass filter centered at 1525 nm and with a bandwidth of 50 nm (FWHM) was used. A cylindrical PA cell with an electret microphone (Ono sokki MI 1235 with a preamplifier MI 3111) was used for measuring the PA signal. The signal from the preamplifier was then fed to a lock-in amplifier (SR 7225) which was set at the modulating frequency of 1250 Hz to match the 1st longitudinal mode of the PA cell. The photo-acoustic spectroscopy technique was combined with the direct absorption spectroscopy technique. Direct absorption spectroscopy (DAS) measurements were performed by feeding the transmitted laser beam through the photo-acoustic cell to an optical spectrum analyzer (Yokogawa, AQ6370B) [3]. The broadband absorption of C 2 H 2 was analyzed with a spectral resolution of 0.1 nm and effective data acquisition speed of 1 μs.