Introduction to VCSELs

Light propagation and emission normal to the semiconductor layer structure is characteristic of a Vertical-Cavity Surface-Emitting Laser (VCSEL). In a VCSEL both laser mirrors usually consist of Distributed Bragg Reflectors (DBRs) that are made of numerous pairs of alternating semiconductor layers with different refractive indexes. The radiation emitted and amplified in the active layer is reflected by both DBRs, generating optical feedback and laser operation. Because of the small optical amplification gained by the light passing vertically through the thin active layer, the mirror's reflectivity must be sufficiently large. Typically, lasing activity in a VCSEL requires a DBR reflectivity exceeding 99 %. This way, the VCSEL resonator cavity is already completely on the wafer, emitting light normal to the wafer surface. With typical dimensions of 300 µm x 300 µm, up to 10000 to 20000 VCSELs are on one 2-inch InP-wafer that can be tested like LEDs by using wafer-probing techniques that are fully compatible with standard IC procedures and equipment.
VCSELs usually operate in a single longitudinal mode due to the short resonator length (typ. 2-5 µm). With an appropriate transverse waveguide structure, the VERTILAS VCSELs also operate in a single transverse mode and are in this case called ‘single frequency lasers.’ In the latter case, the VCSELs may adequately substitute state-of-the-art DFB (Distributed FeedBack) lasers. The small volume of a VCSEL is the main reason for the low current threshold around or even below 1 mA at room temperature.

In a nutshell, the essential VCSEL properties are:

• Low current threshold and power consumption
• Longitudinal single-mode operation
• High efficiency
• Circular optical beam
• Low-cost fabrication and testing capability
• Integratability into 1D- and 2D-arrays