Optical extinction monitor using cw cavity enhanced detection

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 Mm^-1) exhibit a highly linear dependence (r²=0.999 97) when plotted as the cotangent of the phase shift versus the expected extinction. Using heterodyne demodulation techniques, we demonstrat- e a detection limit of 0.04 Mm^-1 (4×10^-10 cm^-1) (2σ) in 10 s integration time and a base line drift of less than ±0.1 Mm^-1 over a 24 h period. Detection limits decrease as the square root of integration time out to ∼150 s.