Measurements and analysis of sidemode noise for a diode laser in an external grating cavity

We measure variations in optical and radio frequency (rf) noise for a diode laser operating single mode in an external grating cavity (EGC). The laser operates at fixed current and temperature; and its frequency is varied within a single EGC mode by changing the length of the EGC using a piezo-electric transducer. The major features of the rf noise spectrum are a doublet occurring near the EGC mode separation, and its multiples, and a low frequency rf continuum in the 0-100 MHz frequency range. The optical spectrum, which is measured using a scanning Fabry-Perot interferometer, consists of the lasing mode and subthreshold nonlasing sidemodes. The origin of these sidemodes is four-wave mixing in the laser crystal. As the laser frequency is varied within a given EGC mode, the features of the rf spectra and the optical sidemode spectra all vary systematically. Some of the observed variations can be explained simply and others cannot.