Direct Absorption (DA) in multipass cell

One of the straightforward and simplest methods to do laser spectroscopy is Direct Absorption (DA) spectroscopy. This technique consists of a tunable laser source, sample cell and detector. If a gaseous absorber is present inside the cell, the transmitted throughput power will have a gap at the absorption wavelength, the amplitude of which is directly proportional to the specious concentration. One can enhance the sensitivity by enlarging a path length up to tens or hundreds of meters in a multiple pass cell, by using mirrors to fold the optical path, giving typically in order of 100 passes of 1 m or less base-length cell. There are two basic conventional designs of the cells: White [1] and Herriott [2] cells.

The cell typically has a physical hole in one of the mirrors, through which the laser light can be coupled and exit at a different angles. The most effective way is to use astigmatic mirrors instead of spherical. In this case full surface of the mirrors will be used by creating Lissajous figures on the surface of the mirrors, instead of just circles (or ellips) for the spherical mirrors case.

Although, DA in multipass cell is usually not so sensitive technique in compare with other conventional spectroscopic techniques, such as CRDS, PAS, ICOS; it offers simplicity, high output power and wide range of the operating wavelenghts.

1. J. U. White, J. Opt. Soc. Am. 66, 411 (1976).
2. D. R. Herriott and H. J. Schulte, "Folded optical delay lines," Appl. Opt. 4, 883-889 (1964).