Measurement and analysis of the variation of sidemode noise with cavity length for a semiconductor laser in an external grating cavity

We report on an experiment to study the properties of nonlasing subthreshold sidemodes in a semiconductor laser operating in an external grating cavity configuration. We measure optical spectra consisting of the lasing mode and subthreshold nonlasing sidemodes, as well as radio frequency spectra at low frequency (0-100MHz) and at high frequency near the external grating cavity mode frequency. As the laser frequency is varied, the features of the rf spectra and the optical sidemode spectra all vary systematically. We present results on how these variations depend upon the external grating cavity mode frequency. Our results are compared with previous experiments and with theoretical predictions based upon four-wave mixing between the lasing mode and the adjacent sidemodes.