MODULATION SPECTROSCOPY WITH A SEMICONDUCTOR DIODE-LASER BY INJECTION-CURRENT MODULATION

An analysis of the harmonic content in frequency-modulation and wavele ngth-modulation spectroscopy with a semiconductor diode laser under in jection-current modulation is presented. It is argued that it is the o ptical power rather than the electric field that is directly modulated by the injection current. A description that is valid for both freque ncy-modulation and wavelength-modulation spectroscopy based on this co ncept is developed, and special attention is given to the residual amp litude modulation and second-harmonic (2 f) detection. Data obtained b y measuring the magnitudes of the first and second harmonic of the mod ulation frequency of the output of a 1.31-mu m distributed-feedback di ode laser under sinusoidal modulation of the current and the 2 f absor ption spectrum of water vapor in wavelength-modulation spectroscopy ar e presented. These data support the concept that it is the optical pow er of a diode laser that is directly modulated by modulation of the in jection current. (C) 1997 Optical Society of America.