Tuesday, May 22, 2012
VCSEL - ACRONYM
VCSEL - Vertical Cavity Surface-Emitting Laser, a type of semiconductor laser in which the resonant cavity is perpendicular to the junction layer and light is emitted from the surface of the chip.
All early diode lasers oscillated in the plane of the junction or active layer and emitted from the edge of the semiconductor chip. This design is logical because keeps laser oscillation in the plane of the active layer where recombination of current carriers produces a population inversion, so the round-trip gain in the cavity is high. However, because the active layer is very thin, the beams from edge-emitting diode lasers diverge rapidly, particularly in the direction perpendicular to the active layer.
VCSELs oscillate vertically, in a cavity formed by reflective layers on the top and bottom of the chip. Single-pass gain much lower than in edge emitters because only a very thin layer of gain material is between the cavity mirrors but the emitting aperture typically is much wider than the active layer is thick, producing a higher-quality, circular beam. First demonstrated in 1979, VCSELs went through a series of structural refinements to improve their performance and fabrication processes. In current designs, one or often both of the reflectors are multilayer Bragg reflectors containing many the tens of pairs of layers needed to produce the very high reflectivity needed to sustain oscillation with only a thin gain medium.
The short length of VCSEL cavities brings some advantages, including allowing direct current modulation at speeds to 40 gigabits per second (Gbit/s) and without the mode hopping possible in edge emitters. Yet ironically, a commercial attraction of the more complex VCSEL design is that it makes them more economical. All the hard parts of VCSEL production are done by highly automated semiconductor manufacturing techniques. The resulting VCSELs can be tested on the wafer, unlike edge emitters, which can't be tested until the wafer is diced into chips. That combined with their larger emitting area greatly reduces packaging expenses, which account for more of finished product costs than the laser chips. So more complex winds up being cheaper, as well as better for many applications.
And VCSEL types and applications keep growing. Recently, a VCSEL-type cavity was used in optically pumped colloidal quantum dot lasers emitting red, green and blue light.
Complexity in a VCSEL: Beam Express (Lausanne, Switzerland) makes 1310 nm VCSELs by bonding AlGaAs/GaAs distributed Bragg reflectors on top and bottom of an InAlGaAs/InP gain layer containing strained quantum wells and a tunnel junction because high-contrast Bragg reflectors are not practical in InP-based materials.