Abstract
The wavelength of commercially available semiconductor laser diodes of the AlGaAs (625-700 nm), InGaAsP (735-980 nm) and GaN type (385-420 nm), and the wavelength range obtained by second harmonic and sum frequency generation in non-linear media allow the measurements of almost all elements of the periodic table with good or excellent detection limits in absorption [1,2]. For example, shot noise limited absorption at mW laser power levels has been demonstrated recently [3]. It was shown that it is possible to measure an optical depth ( ) of about 2×l0−7 at a band width of 1 Hz with 2 mW laser power in a two-beam arrangement with logarithmic processing of absorption and reference signal if the diode laser wavelength as well as the absorption are modulated by the frequencies f1 and f2, respectively, and the measurements are performed by lock-in technique at the frequency 2f1±f2. The shot noise limited absorption measurement were made in plasmas which were turned on and off with frequencies in the kHz range.
© 2000 Optical Society of America
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