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.
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.
We report on an experiment to study the properties of subthreshold modes of a multiple quantum well diode laser in an external grating cavity configuration. Measurement of the radio frequency (rf) intensity noise near the external cavity mode spacing provides information about the intensity and phases of the adjacent subthreshold modes. The rf spectrum has a double peaked structure, as has been previously reported; and we also measure optical spectra with a Fabry-Perot etalon to isolate the individual side modes. In addition, we observe rf noise and optical spectra at higher multiples of the external cavity mode spacing. Our data are compared with previous theoretical models.