| Vertical-cavity surface-emitting lasers (VCSELs) are currently the most popular light source for short-haul optical data communication primarily due to their low cost and low operating power. To address the needs for improved data rate, power consumption, and device lifetime, various approaches of digital modulation utilizing VCSELs with modified structures for mode control are presented. Direct modulation of VCSELs with separate optical and current apertures enables high modulation bandwidth of greater than 18 GHz operating single mode at low current density of less than 6 kA/cm2 because the current aperture diameter can be increased independent of the optical apertures. Polarization modulation of anisotropic VCSELs gives an extinction ratio of greater than 9 dB and at the same time requires low modulation amplitude of less than 200 mV, due to the orthogonality of the polarization states. To achieve polarization modulation requires the capabilities of polarization control and polarization switching. Finally, electrooptically modulated VCSELs with isolated cavity and modulator sections are shown to have the potential of digital modulation with a high data rate at low bias current. Design and fabrication of these VCSELs along with their characterization results will be presented, and approaches to improve their performance will also be suggested. |
