Axial-mode instability in tunable external-cavity semiconductor lasers

The axial-mode instability of a tunable external-cavity semiconductor laser was studied experimentally and theoretically. Optical and radio-frequency (rf) spectra and time-domain traces show the laser output in progressive stages of instability. The optical sidemode ratio was used to experimentally characterize the wavelength and current dependencies of the instability. Measurements show a strong tendency for the sidemode ratio to degrade when the laser was tuned within wavelength bands occurring at the periodicity of the laser diode mode spacing. The probability of instability also increased with injection current. Analysis of the single-mode strong feedback rate equations shows that stability of an external-cavity mode is determined by the sign of the chirp reduction factor. A model for the physical origin of instability is provided. Calculations show that for facet reflectivities above a threshold value dependent on the external feedback level and the linewidth broadening factor, unstable and stable axial modes coexist in wavelength regions whose locations agree with regions where sidemode ratio degradation is observed. The stability boundary at which unstable axial modes appear is shown as a surface in the parameter space of facet reflectivity, feedback level, and linewidth broadening factor