Vertical-Cavity Surface-Emitting Lasers With Record-High Birefringence

VCSELs are used extensively today as transmitters in high-speed optical interconnects. A first generation of 28Gbit/s devices is currently being deployed. Although digital modulation at about 50Gbit/s has been shown [1], it is very unlikely that 100Gbit/s signals can be generated by direct current modulation. New approaches must thus be explored to satisfy the future demand for higher data throughput. In that sense, the birefringence, i.e., the frequency difference between the two orthogonally polarized components of a VCSEL mode, may play an important role. It has been demonstrated that by optical spin injection a birefringent VCSEL can be excited to oscillations in the degree of circular polarization [2]. The oscillation frequency is very close to the birefringence. The generation of extremely fast polarization bursts is also possible [2]. It is thus of interest to tailor VCSELs to exhibit a maximum amount of birefringence. Contributions to the birefringence are geometrical anisotropies, the electro-optic effect in the cavity, and incorporated strain [3], where the latter is the strongest.