VCSEL—what’s behind the abbreviation?

Semiconductor laser diodes convert voltage into photons. VCSELs (vertical-cavity surface-emitting laser, pronounced [vixls]) are one of its “subspecies”. As opposed to edge-emitting laser diodes, light is emitted from the surface vertically. This reduces installation space and allows arrays with dozens, hundreds, or even thousands of emitters to be placed on a single chip. Thanks to their circular beam shape, laser beams from VCSELs also enable highly efficient fiber coupling. Other benefits include inexpensive production on wafers, high modulation speeds, beam quality and energy efficiency.

VCSEL are constructed from bottom to top from

• a highly reflective (99.9%) Bragg reflector (DBR)
• an oxide layer
• the active laser layer (quantum well with quantum film and barriers)
• another oxide layer with a circular opening for beam shaping
• a partially reflective (99%) DBR on top.

When voltage is applied, the laser layer acts as a quantum well. Here—between the two DBR layers—semiconductors based on gallium nitride, gallium arsenide, or indium phosphide are layered, with quantum films and barriers made from different materials alternating. If free electrons are introduced with voltage applied, they are “caught” in the quantum films. They are so thin that excited electrons can only move horizontally in them. On the other hand, they can circulate freely in the barrier layers. If they pass through the high energetic quantum film, this promotes lasing, in other words, energetic excitement and release of photons during relaxation.

An instructive video tutorial can be found here.