VCSEL

The VCSEL (Vertical-Cavity Surface-Emitting Laser [v'ɪxl]) is a type of semiconductor laser with laser beam emission perpendicular to the chip surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) where laser light is emitted at one or two edges.

Characteristics

• Low fabrication cost
• Low power consumption
• On-wafer testability
• High coupling efficiency with optical fibers
• Array producibility
• Single-mode lasers available
• Wavelength tunable
• Lower emission power compared to edge-emitting lasers
• High intrinsic modulation bandwidths

Applications

• Fiberoptical data transmission
• Analog broadband signal transmission
• Absorption spectroscopy (TDLAS)
• Laser printers

Structure

The laser resonator consists of two DBR-mirrors (Distributed Bragg-Reflector) parallel to the wafer surface with an active region usually containing quantum-wells for the laser light generation in between. These planar DBR-mirrors consist of alternating layers with high or low refractive index, respectively, each of them with a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivities above 99% as they are needed for VCSELs. Because of the high mirror reflectivities, VCSELs have lower output powers when compared to edge emitting lasers.

A distinction is drawn between optically pumped VCSELs, where the active region is excited for light emission using an external light source with a shorter wavelength, and electrically pumped VCSELs, respectively, representing a p-i-n diode.

VCSELs for wavelengths from 650 nm to 1300 nm are practically solely based on GaAs wafers, whereas for higher wavelenghts from 1300 nm to 2000 nm (long-wavelength VCSELs), at least the active region is InP-based. VCSELs at even higher wavelengths are experimental and usually optically pumped.

Special Forms

• Multiple active regions
• VCSELs with tunnel junctions. Using a tunnel junction (n⁺p⁺), an electrically advantageous n-n⁺p⁺-p-i-n configuration can be built that also may beneficially influence other structural elements (e.g. in the form of a Buried Tunnel Junction (BTJ)).
• Widely tunable VCSEL with micromechanically (MEMS) movable mirror
• Wafer-bonded or wafer-fused VCSEL: Combination of semiconductor materials that can be fabricated using different types of substrate wafers
• Monolithically optically pumped VCSELs: Two VCSELs on top of each other. One of them optically pumps the other one.
• VCSEL with longitudinally integrated monitor diode: A photodiode is integrated under the back mirror of the VCSEL.
• VCSEL with transversally integrated monitor diode: With suitable etching of the VCSEL's wafer, a resonant photodiode can be manufacured that may measure the light intensity of a neighboring VCSEL.
• VCSELs with external cavities, knows as VECSELs or semiconductor disk lasers. VECSELs are optically pumped with conventional laser diodes. This arrangement allows a larger area of the device to be pumped and therefore more power can be extracted - as much as 30W. The external cavity also allows intracavity techniques such as frequency doubling, single frequency operation and femtosecond pulse modelocking.

History

The first VCSEL was presented in 1979 by Soda, Iga, Kitahara and Suematsu, but devices with a threshold current below 1 mA have been reported 1989. Today, VCSELs have replaced edge-emitting lasers in applications for short-range fiberoptical communiation such as Gigabit Ethernet and Fibre Channel.