Background signals in wavelength-modulation spectrometry with frequency-doubled diode-laser light. II. Experiment

In an accompanying paper [Appl. Opt. 40, 783-193 (2001)], we predict the ex istence of background signals from a frequency-doubled wavelength-modulated diode-laser system. We now demonstrate and characterize various nf harmoni cs of such background signals from a system producing light in the 422-nm r egion by use of a single-pass KNbO3 crystal with respect to the modulation amplitude, the laser center frequency, and the crystal temperature. It is d emonstrated that 2f detection is plagued by considerably larger amounts of background signal than is detection at other higher, even harmonics. This r esult implies that 4f or 6f detection is often to be preferred in compariso n with 2f detection when frequency-doubled wavelength-modulation spectromet ry (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 ana lytical signals. This difference implies that the optimum detectability for frequency-doubled WMS is often reached for modulation amplitudes lower tha n those normally used. An analysis of the effect of a finite temperature st ability of the doubling crystal on the drift of the background signals as w ell as on the detectability is included. The results verify the theoretical description given in our accompanying paper. (C) 2001 Optical Society of A merica OCIS codes: 190.2620, 300.1030, 300.6260, 300.6380.