Background signals in wavelength-modulation spectrometry with frequency-doubled diode-laser light. I. Theory

Various types of background signals appear when wavelength-modulated (WM) d iode-laser light is frequency doubled. We present a theoretical analysis of such background signals in terms of a previously derived formalism for WM spectrometry that is based on a Fourier series. Explicit expressions for va rious nf harmonics of the background signals are derived. The analysis show s that 2f detection will be plagued by significant background signals when frequency-doubled WM diode-laser light is used. It also demonstrates that 4 f and 6f detection will experience background signals but not, however, to the same extent as 2f detection. The analysis illustrates clearly how the v arious nf harmonics of the background signals depend on entities such as mo dulation amplitude, associated intensity modulation, dispersion of the freq uency-doubling material, laser power, and detuning. The background signals can take both positive and negative values, depending on the relation betwe en these entities. Guidelines for how to minimize these background signals are given. (C) 2001 Optical Society of America OCIS codes: 190.2620, 300.10 30, 300.6260, 300.6380.