An analysis of the harmonic content in frequency-modulation and wavelength-modulation spectroscopy with a semiconductor diode laser under injection-current modulation is presented. It is argued that it is the optical power rather than the electric field that is directly modulated by the injection current. A description that is valid for both frequency-modulation and wavelength-modulation spectroscopy based on this concept is developed, and special attention is given to the residual amplitude modulation and second-harmonic (2 f) detection. Data obtained by measuring the magnitudes of the first and second harmonic of the modulation frequency of the output of a 1.31-µm distributed-feedback diode laser under sinusoidal modulation of the current and the 2 f absorption spectrum of water vapor in wavelength-modulation spectroscopy are presented. These data support the concept that it is the optical power of a diode laser that is directly modulated by modulation of the injection current.
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Zhu et al. (1997) studied this question.
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