Background Signals in Wavelength-Modulation Spectrometry by use of Frequency-Doubled Diode-Laser Light. II. Experiment.
In an accompanying paper [Appl. Opt. 40, 783-793 (2001)], we predict the existence of background signals from a frequency-doubled wavelength-modulated diode-laser system. We now demonstrate and characterize various nf harmonics of such background signals from a system producing light in the 422-nm region by use of a single-pass KNbO(3) crystal with respect to the modulation amplitude, the laser center frequency, and the crystal temperature. It is demonstrated that 2f detection is plagued by considerably larger amounts of background signal than is detection at other higher, even harmonics. This result implies that 4f or 6f detection is often to be preferred in comparison with 2f detection when frequency-doubled wavelength-modulation spectrometry (WMS) is to be used. This preference is illustrated by the detection of Ca in an acetylene-air flame. It is also shown that the background signals have a much stronger dependence on the modulation amplitude than do the analytical signals. This difference implies that the optimum detectability for frequency-doubled WMS is often reached for modulation amplitudes lower than those normally used. An analysis of the effect of a finite temperature stability of the doubling crystal on the drift of the background signals as well as on the detectability is included. The results verify the theoretical description given in our accompanying paper.