Intracavity-absorption spectroscopy is an ultrasensitive method for measuring very small absorptions. Some examples are very low concentrations of analytes, and very weak absorption such as infrared overtone bands. The technique takes advantage of the very high light intensities within a laser resonance cavity, and the sensitivity of the lasing output to losses within the cavity. Small losses within the laser cavity due to absorption results in large changes in the intensity of the laser output.
Schematic of an intracavity-absorption experiment
To record a spectrum the laser output must be tunable, therefore the most common lasers used in intracavity-absorption measurements are dye lasers.
Samples can be solids, liquids, or gases. Liquids and gases are contained in a sample holder that has windows at Brewster's angle to minimize losses due to reflection.
Small absorptions are detected by monitoring the output of the laser beam with a detector. Since the laser light is relatively intense, the main requirement for the detector is that it not be destroyed by the laser beam.