Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources

Abstract

We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b Σg-X Σg transition in gaseous molecular oxygen. Absorptions signals of less than 1 part in 10 6 can be detected.

Cavity-Enhanced Immunoassay Measurements in Microtiter Plates Using BBCEAS

“…The results obtained in the studied concentration range were significant as they showed the ability of the system to measure higher analyte concentrations and thus broaden its applicability. [1][2][3][4][5] in the concentration range 0 ng mL -1 to 61.6 ng mL -1 . The average CEF and the standard deviation of the measurement is also listed.…”

“…These have used high reflectivity dielectric mirrors to form an optical cavity around the sample which allows light entering through the back of one of the mirrors to be essentially multi-passed many times through the sample and thus greatly increase the effective path length of measurement. The first experimental studies were based on cavity ring down spectroscopy (CRDS) [4] , which required powerful laser light sources and complex and expensive fast detection equipment to measure the decay in intensity of light exiting the cavity on a microsecond timescale. This 'ring down time' could be directly related to the absorption coefficient of an analyte.…”

Optomechanical Shutter Modulated Broad-Band Cavity–Enhanced Absorption Spectroscopy of Molecular Transients of Astrophysical Interest

“…The main advantage of the CRD technique is the high effective path length of the light trapped inside the cavity (making 10 4 −10 5 trips through the sample), resulting in an absorbance sensitivity better than 10 −6 per pass. 72 Furthermore, CRD is insensitive to laser power fluctuations as not the absolute absorption is measured but instead a decay rate is determined.…”

Measurement of aerosol optical extinction using diode laser cavity ringdown spectroscopy

“…In an empty cavity, the intensity of the trapped laser pulses decreases by a small percentage in each round trip due to mirror loss, decaying as an exponential function of time (with a decay time constant or ringdown time τ 0 ). When an extinction aerosol sample is present in the cavity, its scattering and absorption increase the loss, which can be determined from comparing the empty cavity ringdown time (τ 0 ) and the one with the analyte (τ) [18,20]:…”

“…CRDS was first developed by O'Keefe and Deacon [18] in 1988 and utilized for spectroscopic measurements of gas species absorption [19]. The CRDS technique measures the rate, rather than the magnitude, of extinction of light in a highly reflective optical cavity that contains the trace sample [20].…”