Optical heterodyne spectroscopy with frequency- and amplitude-modulated semiconductor lasers

Optical heterodyne spectroscopy has been performed with GaAlAs diode lasers by modulating the injection current at frequencies up to 2.6 GHz. This extremely convenient modulation scheme results in simultaneous frequency and amplitude modulation of the laser output. When the modulated laser output probes a narrow absorption line, a characteristic heterodyne beat signal occurs. We show that the observed spectra are similar to those obtained in pure frequency-modulation spectroscopy. The technique offers high sensitivity and fast detection speed and is applied to the detection of water-vapor absorption lines.