Polarization switching induced by phase change in extremely short external cavity vertical-cavity surface-emitting lasers

We experimentally investigate the spectral and polarization properties of vertical-cavity surface-emitting lasers (VCSELs) subject to polarization insensitive optical feedback from an extremely short external cavity (few microns) and find that the wavelength and the current at which the light polarization switches between the two linearly polarized fundamental modes of the VCSEL are periodically modulated with the length of the external cavity. High contrast polarization switching is demonstrated for a quarter-wavelength change of the external cavity length. To explain our experimental findings we suggest a two-mode rate equation VCSEL model that considers the feedback optical field as instantaneous and accounts for multiple roundtrips in the external cavity with different coupling efficiencies back into the laser. Our numerical results are in very good agreement with the experiments.