Laser frequency modulation (FM) spectroscopy is a method of optical heterodyne spectroscopy capable of sensitive and rapid detection of absorption or dispersion features /1/. With this technique, quantum (shot noise) limited sensitivity can be readily achieved /2/, rendering FM spectroscopy a very useful tool e.g. for trace gas detection. A particularly attractive version of FM spectroscopy is high-frequency modulation spectroscopy with semiconductor lasers /3,4/: This scheme is implemented by modulating the injection current of the laser diode at radio frequencies, combining the simple and convenient tuning and modulation methods available for diode lasers with the high sensitivity and fast time response of optical heterodyne techniques. Recently, these features have been demonstrated /5/ with a GaAlAs laser diode emitting in the near infra-red.