Real-time detection of ambient aerosols using photothermal interferometry: Folded Jamin interferometer

Work in our laboratory has been directed at the development of a new class of instrumentation that can directly measure ambient aerosol absorption through photothermal interferometry. The hallmark of this approach is its ability to directly measure aerosol absorption without interference from aerosol scattering since the signal originates from the thermal dissipation of the spectrally absorbed energy. While the principle of the photothermal technique for the detection of aerosols was demonstrated in the mid-1980s, this interferometric technique remains a laboratory technique largely due to sensitivity to mechanical vibrations and other environmental factors that result in unwanted signal interference and commensurate reduction in detection sensitivity. In order to realize its application outside the laboratory, a folded Jamin interferometer design has replaced both the traditional Mach-Zehnder and unfolded Jamin configurations. The folded Jamin affords many advantages, which include high degree of common mode noise rejection, insensitivity to rotation and translation of optical components, inherent double pass configuration, and compact size. In this article, we report on the performance of this optical configuration and present representative data for both absorbing and nonabsorbing aerosols.