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Enhancement of Absorption in External Cavities

The first step towards higher sensitivities is the increase of the absorption path length L. This can be for instance achieved when the absorption cell is placed inside a multipass cell. One example is shown in Fig. 1.2. Two spherical mirrors with a high reflectivity R are separated by a distance d which nearly equals the mirror radius r forming a nearly spherical resonator (Vol. 1, Sect. 5.2). [Pg.7]

The laser beam enters the resonator through a small hole in one mirror and leaves the resonator after q reflections through the same hole but under a different angle. The number of transits can be chosen by adjusting the difference (tf — r) d. In Fig. 1.2b the laser spots on one mirror are shown for a given value of d — r. The angle 9 between to adjacent spots is given by the relation [Pg.8]

The spherical mirrors refocus the divergent laser beam at each reflection and prevent the laser spots on the mirrors to become too large. This is essential because the neighbouring spots should not overlap which would result in interference effects which give unwanted strucmres in the transmitted intensity when the laser wavelength is tuned. [Pg.8]

Without absorption cell the intensity of the laser beam decreases after one roundtrip by the attenuation factor exp[—2(1 - / )]. For q roundtrips with the absorption cell the transmitted intensity has decreased to 7oexp[—2 (1 — R) 2qaL.  [Pg.8]


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