Tunable diode laser absorption spectroscopy#Cavity-enhanced absorption spectrometry .28CEAS.29

The cavity- enhanced absorption spectroscopy ( CEAS) heard the cavity ring-down spectroscopy ( CRDS ), to the methods of spectroscopy by means of optical resonators. Usually, only the English term is used, it can be translated with " resonatorverstärkte absorption spectroscopy ". Less common is the alternative name Integrated Cavity Output Spectroscopy ( ICOS ).

  • 2.1 Coherent CEAS
  • 2.2 Incoherent broadband CEAS

Operation

The cavity- enhanced absorption spectroscopy is based on the virtual extension of a light path through an optical resonator. Similar to white or Herriott cells increases the light transmitted through an absorbing medium intensity according to Lambert -Beer 's law, is proportional to the optical path from. With a long light path, the signal -to-noise ratio of a transmission measurement can be improved by a weakly absorbing sample.

A typical assembly consists of a light source, a coupling optical system, the optical resonator, a coupling-out and a detector. The optical resonator consists of two highly reflective concave mirrors at a distance between concentric and confocal geometry.

A good understanding of the functioning of CEAS provides viewing an infinitesimally small light package: If it can be assumed that in the highly reflective layer and the mirror substrate there is no absorption, is the sum of transmission and reflection of just one. Of a light bundle which is emitted from the light source, the fraction penetrating the mirror. Considering a full resonator with two mirrors and a medium with absorption, the detector measures behind the resonator according to a simple passage of light through the resonator package:

The significantly greater proportion of the light packet is reflected at the second mirror ( output mirror ) and passes through a second time the resonator, where the first mirror ( input mirror ) the process is repeated. If one considers an infinite number of packets of light - ie a continuously operating light source - is the measured intensity at the output mirror the sum of the transmitted during each passage Services:

For this series converges to:

Is the empty resonator and in general, and thus:

If the absorption losses strictly go to Lambert- Beer with a filled cavity, the following applies:

Thus can be with a known cavity length, absorption length and mirror reflectivity, the absorption coefficient of the absorbing medium in the resonator determine:

Alternative to the phenomenological derivation can be found, the process of a probability analysis, a photon inside the optical resonator are described.

Sensitivity and detection limit

A discussion of sensitivity and detection limit are found, for example in.

Methods

The various CEAS methods differ in the type of light source and the different approaches to determine the reflectivity of the mirrors.

Coherent CEAS

CEAS, OF- CEAS

Incoherent broadband CEAS

BB- CEAS, CE - DOAS

Application

Trace gases, particles, fluids

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