Intracavity Laser Absorption Spectroscopy
Intracavity laser absorption spectroscopy is a technique for highly sensitive spectroscopic measurements. The basic principle is as follows. The substance to be evaluated (e.g. some gas sample) is placed within the resonator of a laser, which is preferably based on a gain medium with broad gain bandwidth and a resonator with low losses. When the laser is turned on, it starts to oscillate on many resonator modes simultaneously; only after many resonator round trips will the optical spectrum of the generated light strongly concentrate to the spectral region with highest gain. During this evolution, weak absorption features of the tested sample can imprint signatures on the spectrum, because they can influence the spectrum during many round trips. A measurement of the spectrum is done some time after switching on the laser; this time should be long enough to allow for strong spectral features to develop, but also short enough to prevent too strong narrowing of the spectrum caused by the finite gain bandwidth.
With a carefully optimized setup, intracavity absorption spectroscopy allows for extremely long effective path lengths of tens of thousands of kilometers, and at the same time very high spectral resolution. Suitable laser gain media for the spectroscopy of gases include neodymium-doped fibers and bulk glasses, titanium-doped sapphire, laser dyes, color center crystals, laser diodes, and vertical external cavity surface-emitting lasers. Important issues are to have a broad gain bandwidth, to minimize resonator losses, and to avoid any parasitic reflections within the laser resonator. A review paper  explains in detail many technical issues and variations of the basic technique.
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