What is a vertical cavity surface emitting laser (VCSELs)?

VCSELs are semiconductor lasers, more specifically laser diodes with a monolithic laser resonator, where the emitted light leaves the device in a direction perpendicular to the chip surface. The larger output aperture of VCSELs, compared to most edge-emitting lasers, produces a lower divergence angle of the output beam, and makes possible high coupling efficiency with optical fibers.

Because VCSELs emit from the top surface of the chip, they can be tested on-wafer, before they are cleaved into individual devices. It allows VCSELs to be built not only in one-dimensional but also in two-dimensional arrays.