Optical extinction monitor using cw cavity enhanced detection

• Published in: Review of scientific instruments 2007-06, Vol.78 (6), p.063102-063102-9
• Main Authors: Kebabian, Paul L., Robinson, Wade A., Freedman, Andrew
• Format: Article
• Language: English
• Subjects: Amplifiers, Electronic; Equipment Design; Equipment Failure Analysis; Lasers; Light; Photometry - instrumentation; Radiation Dosage; Radiometry - instrumentation; Radiometry - methods; Reproducibility of Results; Sensitivity and Specificity; Spectrum Analysis - instrumentation
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/1.2744223

We present details of an apparatus capable of measuring optical extinction (i.e., scattering and/or absorption) with high precision and sensitivity. The apparatus employs one variant of cavity enhanced detection, specifically cavity attenuated phase shift spectroscopy, using a near-confocal arrangement of two high reflectivity ( R ∼ 0.9999 ) mirrors in tandem with an enclosed cell 26 cm in length, a light emitting diode (LED), and a vacuum photodiode detector. The square wave modulated light from the LED passes through the absorption cell and is detected as a distorted wave form which is characterized by a phase shift with respect to the initial modulation. The amount of that phase shift is a function of fixed instrument properties—cell length, mirror reflectivity, and modulation frequency—and of the presence of a scatterer or absorber (air, particles, trace gases, etc.) within the cell. The specific implementation reported here employs a blue LED; the wavelength and spectral bandpass of the measurement are defined by the use of an interference filter centered at 440 nm with a 20 nm wide bandpass. The monitor is enclosed within a standard 19 in. rack-mounted instrumentation box, weighs 10 kg , and uses 70 W of electrical power including a vacuum pump. Measurements of the phase shift induced by Rayleigh scattering from several gases (which range in extinction coefficient from 0.4 – 32 M m − 1 ) exhibit a highly linear dependence ( r 2 = 0.999 97 ) when plotted as the cotangent of the phase shift versus the expected extinction. Using heterodyne demodulation techniques, we demonstrate a detection limit of 0.04 M m − 1 ( 4 × 10 − 10 cm − 1 ) ( 2 σ ) in 10 s integration time and a base line drift of less than ± 0.1 M m − 1 over a 24 h period. Detection limits decrease as the square root of integration time out to ∼ 150 s .