Editing Laser diode (section)

=== Vertical-cavity surface-emitting laser ===
{{main|Vertical-cavity surface-emitting laser}}
[[File:Simple vcsel.svg|thumb|350px|Diagram of a simple VCSEL structure; not to scale]]

[[Vertical-cavity surface-emitting laser]]s (VCSELs) have the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the surface of the cavity rather than from its edge as shown in the figure. The reflectors at the ends of the cavity are [[dielectric mirror]]s made from alternating high- and low-refractive-index quarter-wave-thick multilayer.

Such dielectric mirrors provide a high degree of wavelength-selective reflectance at the required free surface wavelength {{Mvar|&lambda;}} if the thicknesses of alternating layers {{Math|''d''<sub>1</sub>}} and {{Math|''d''<sub>2</sub>}} with refractive indices {{Math|''n''<sub>1</sub>}} and {{Math|''n''<sub>2</sub>}} are such that {{Math|1=''n''<sub>1</sub>''d''<sub>1</sub> + ''n''<sub>2</sub>''d''<sub>2</sub> = ''&lambda;''/2}}, which then leads to the constructive interference of all partially reflected waves at the interfaces. But there is a disadvantage: because of the high mirror reflectivities, VCSELs have lower output powers when compared to edge-emitting lasers.

There are several advantages to producing VCSELs when compared with the production process of edge-emitting lasers. Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not work, whether due to bad contacts or poor material growth quality, then the production time and the processing materials have been wasted.

Additionally, because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three-inch gallium arsenide wafer. Furthermore, even though the VCSEL production process is more labor- and material-intensive, the yield can be controlled to a more predictable outcome. However, they normally show a lower power output level.