Direct Determination of Absorbed Photons

In the methods discussed in the preceding sections, the attenuation of the transmitted light (or of the laser power in intracavity spectroscopy) is monitored to determine the absorption coefficient a a ) or the number density of absorbing species. For small absorptions this means the measurement of a small difference of two large quantities, which limits, of course, the signal-to-noise ratio. 

Several different techniques have been developed where the absorbed radiation power, i.e., the number of absorbed photons, can be directly monitored. These techniques belong to the most sensitive detection methods in spectroscopy, and it is worthwhile to know about them. 

Because a single-mode cw laser is used, the spectral resolution is generally higher than in case of pulsed lasers. It is only limited by the linewidth of the absorption lines. 

Instead of measuring the differences of decay times t and T2 with and without absorbing sample inside the cavity, the time-integrated transmitted intensity can be also monitored as a function of the wavelength of the incident laser pulse [6.37]. This method is similar to the absorption in external cavities, discussed in Sect. 6.2. More information on CRDS can be found in [6.34-6.36] and in several reviews [6.34,6.38,6.39]. 

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