Understanding the sensitivity of cavity-enhanced absorption spectroscopy: pathlength enhancement versus noise suppression

Abstract

Cavity-enhanced absorption spectroscopy is now widely used as an ultrasensitive technique in observing weak spectroscopic absorptions. Photons inside the cavity are reflected back and forth between the mirrors with reflectivities R close to one and thus (on average) exploit an absorption pathlength L that is 1/(1 -R) longer than a single pass measurement. As suggested by the Beer-Lambert law, this increase in L results in enhanced absorbance A (given by aL with a being the absorption coefficient) which in turn favours the detection of weak absorptions. At the same time, however, only (1 -R) of the incident light can enter the cavity [assuming that mirror transmission T is equal to (1 -R)], so that the reduction in transmitted light intensity DI caused by molecular absorption equates to that would be obtained if in fact no cavity were present. The enhancement in A = DI/I, where

Spectral Fingerprinting of Individual Cells Visualized by Cavity-Reflection-Enhanced Light-Absorption Microscopy

“…Compared with the homogenous solutions, cells have heterogeneous structures that cause light scattering and refraction. Eq ( 1 ) is valid for homogeneous, low-scattering samples [ 9 ]. Therefore, we simply calculated the inverse of the transparency, I blank / I sample , where I blank represents the blank signals obtained by the empty region of interest, and I sample represents the sample signals.…”

Cavity-based absorption spectroscopy techniques

“…This off-axis ICOS approach has yielded an NEA sensitivity of 3.1 × 10 − 11 cm − 1 Hz − 1/2 , two orders of magnitude fi ner than that for conventional CEAS/ICOS. Ouyang and Jones (2012) have explained that the high sensitivity of CEAS/ICOS relies on a trade-off between pathlength enhancement and noise suppression, such that the quotient [ Δ I / I ] in Equation [6.2] is much increased because the denominator I is much smaller than in conventional single-pass absorption spectroscopy, with a correspondingly smaller contribution to the noise level.…”

Portable broadband cavity-enhanced spectrometer utilizing Kalman filtering: application to real-time, in situ monitoring of glyoxal and nitrogen dioxide

“…The reported 3σ detection precisions were 0.03 ppbv (60 s) with an L eff of 13 km and 0.045 ppbv (5 s) with an L eff of 17.8 km, respectively. The achievable sensitivity of BBCES depends on the number of photons injected into the cavity (which depends on the source brightness and beam imaging efficiency), L eff (which depends on the cavity mirror quality and mirror separation), and how efficiently the various noise components are suppressed [27,51]. In this work, the light injection efficiency was improved with the cage-based configuration.…”