Surface-emitting Semiconductor Lasers
Semiconductor lasers can be grouped into two classes:
- Edge-emitting lasers, where the laser light propagates parallel to the wafer surface of the semiconductor chip and is reflected or coupled out at a cleaved edge.
- Surface-emitting lasers, where the light propagates in the direction perpendicular to the semiconductor wafer surface.
Surface-emitting lasers are the newer type of semiconductor lasers. A major challenge is to reach the laser threshold, because the optical gain for the intracavity laser beam occurs only on a very small distance (in one or several quantum wells) and is therefore at most a few percent, often even below 1%. It is therefore necessary to realize a laser resonator with very low losses, i.e., Bragg mirrors with high reflectivity.
Surface-emitting semiconductor lasers are further subdivided into monolithic and external-cavity devices:
- Monolithic means that the laser resonator is realized in the form of two semiconductor Bragg mirrors with the quantum well section in between. Such a device is called VCSEL (vertical cavity surface-emitting laser) and is electrically pumped in most cases. Several milliwatts of output power can be obtained in a diffraction-limited output beam.
- External-cavity devices, called vertical external cavity surface-emitting lasers (VECSELs), have only a single Bragg mirror on the chip, and the resonator is completed with an external mirror. Such lasers allow much higher output powers in diffraction-limited beams, particularly when they are optically pumped.
An advantage particularly of VCSELs is that many lasers can be fabricated on a single chip. After epitaxial growth, the chip only has to be sliced, and the single lasers can be packaged without further processing optical interfaces.
The VCSEL concept also allows the fabrication of two-dimensional laser arrays for much higher total output powers.
Vertical emission of light can also be achieved with a kind of laser diode which is based on a waveguide along the chip surface (as in an edge-emitting laser), but emits in the vertical direction because the light is reflected upward with a 45° mirror (horizontal cavity surface-emitting laser, HCSEL). This approach makes it possible to combine some of the advantages of edge-emitting and surface-emitting lasers.
The RP Photonics Buyer's Guide contains 13 suppliers for surface-emitting semiconductor lasers. Among them:
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