Laser absorption spectroscopy

Light transmitted through some medium (solid, liquid or gaseous) experiences attenuation due to scattering and absorption; the latter being the dominant loss contribution in many cases. Loss measurements can thus be used to deduct the concentration of a gas of interest with known absorption strength in a gas mixture. If local, in-situ values are desired, the gas sample is sucked into a gas cell and measured there. However, if a satellite measures the sunlight reflected from the earth, this also constitutes absorption spectroscopy, with the atmosphere between the earth and satellite being the gas sampled (remote sensing). The light attenuation for sufficiently small absorption is described by the Beer-Lambert-Law:

where I is the intensity measured by the detector, I0 is the original intensity entering the gas cell, σ is the absorption cross section of the absorber (in cm² / molec.) and N is the absorber's number density (in molec. / cm³). Those last two multiplied give the absorption coefficient α (α = σ ∙ N, in 1 / cm) and L is the path length of the light-matter interaction (also sometimes called “absorption path length”).

fig. 1:
fig. 1: Simple absorption spectroscopy setup

When employing absorption spectroscopy it is important to ensure specificity (to be able to relate the measurement signal to only the gas of interest) and sensitivity (to be able to measure small quantities of the gas of interest; these are normally denoted as ppbv (parts-per-billion of volume) or pptv (parts-per-trillion of volume)). The latter is often the reason to use optical cavities with high reflective mirrors inside them (see figure below). These enlarge the path length of the light, increasing the possibility of matter-light interaction and thus the measured signal.

There is a big variety of cavity enhanced absorption methods, called for example ICOS (integrated cavity output spectroscopy), IBBCEAS (incoherent broadband cavity enhanced spectroscopy), OF-CEAS (optical feedback cavity enhanced absorption spectroscopy), TDLAS (tunable diode laser absorption spectroscopy), and also cw-CRDS (continuous wave cavity ring-down spectroscopy). All these methods utilize light reflection by mirrors to enhance the optical path length and increase sensitivity while maintaining a small resonator size.

fig. 2:
fig. 2: Enhanced absorption path length by using a cavity with two high reflective mirrors

There are different methods of calculating the absorption loss from measured signals. Read on about Cavity Ring Down Spectroscopy and Cavity Enhanced Absorption Spectroscopy .