Spectrum of a free running laser diode and spectrum of laser diode coupled to a WGM cavity. (a) Spectrum of a single mode free-running Fabry-Perot laser used in the experiment. (b) Spectra of the self-injection locked Fabry-Perot laser taken using an optical spectrum analyzer (OSA). The laser is locked to several different WGM modes spanning most of the emission bandwidth of the laser diode. The linewidth is limited by the resolution of the OSA. (c) Optical spectrum taken in a broader spectral window for one of the modes used in (b). No significant spurious cavity harmonics are observed.