VCSEL
From Wikipedia, the free encyclopedia
The VCSEL (Vertical-Cavity Surface-Emitting Laser [v'ɪxl]) is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer.
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[edit] Structure
The laser resonator consists of two distributed Bragg reflector (DBR) mirrors parallel to the wafer surface with an active region consisting of one or more quantum wells for the laser light generation in between. The planar DBR-mirrors consist of layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivities above 99%. High reflectivity mirrors are required in VCSELs to balance the short axial length of the gain region.
In common VCSELs the upper and lower mirrors are doped as p-type and n-type materials, forming a diode junction. In more complex structures, the p-type and n-type regions may be buried between the mirrors, requiring a more complex semiconductor process to make electrical contact to the active region, but eliminating electrical power loss in the DBR structure.
In laboratory investigation of VCSELs using new material systems, the active region may be pumped by an external light source with a shorter wavelength, usually another laser. This allows a VCSEL to be demonstrated without the additional problem of achieving good electrical performance; however such devices are not practical for most applications.
VCSELs for wavelengths from 650 nm to 1300 nm are typically based on gallium arsenide (GaAs) wafers with DBRs formed from GaAs and aluminium gallium arsenide (AlxGa(1-x)As). The GaAs/AlGaAs system is favored for constructing VCSELs because the lattice constant of the material does not vary strongly as the composition is changed, permitting multiple lattice matched epitaxial layers to be grown on a GaAs substrate. However, the refractive index of AlGaAs does vary relatively strongly as the Al fraction is increased, minimizing the number of layers required to form an efficient Bragg mirror compared to other candidate material systems. Furthermore, at high aluminum concentrations, an oxide can be formed from AlGaAs, and this oxide can be used to restrict the flow of current in a VCSEL, enabling very low threshold currents.
Longer wavelength devices, from 1300 nm to 2000 nm, have been demonstrated with at least the active region made of indium phosphide. VCSELs at even higher wavelengths are experimental and usually optically pumped.
[edit] 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.
[edit] Characteristics
Because VCSELs emit from the top surface of the chip, they can be tested on-wafer, before they are cleaved into individual devices. This reduces the fabrication cost of the devices. It also allows VCSELs to be built not only in one-dimensional, but also in two-dimensional arrays.
The larger output aperture of VCSELs, compared to most edge-emitting lasers, produces a lower divergence angle of the output beam, and makes possible high coupling efficiency with optical fibers.
The high reflectivity mirrors, compared to most edge-emitting lasers, reduce the threshold current of VCSELs, resulting in low power consumption. However, as yet, VCSELs have lower emission power compared to edge-emitting lasers. The low threshold current also permits high intrinsic modulation bandwidths in VCSELs (Iga 2000).
The wavelength of VCSELs may be tuned, within the gain band of the active region, by adjusting the thickness of the reflector layers.
While early VCSELs emitted in multiple longitudinal modes or in filament modes, single-mode VCSELs are now common.
[edit] Applications
- Optical fiber data transmission
- Analog broadband signal transmission
- Absorption spectroscopy (TDLAS)
- Laser printers
[edit] History
The first VCSEL was presented in 1979 by Soda, Iga, Kitahara and Suematsu (Soda 1979), but devices for CW operation at room temperature were not reported until 1988 (Koyama 1988). The term VCSEL was coined in a publication of the Optical Society of America in 1987[citation needed]. Today, VCSELs have replaced edge-emitting lasers in applications for short-range fiberoptic communication such as Gigabit Ethernet and Fibre Channel.
[edit] References
- Iga, Kenichi (2000), "Surface-emitting laser—Its birth and generation of new optoelectronics field", IEEE Journal of Selected Topics in Quantum Electronics, vol. 6, no. 6, p. 1201–1215
- Soda et al, Haruhisa (1979), "GaInAsP/InP Surface Emitting Injection Lasers", Japanese Journal of Applied Physics, vol. 18, no. 12, p. 2329–2330. DOI:10.1143/JJAP.18.2329
- Koyama et al, Fumio (1988), "Room temperature cw operation of GaAs vertical cavity surface emitting laser", Trans. IEICE, E71(11), p. 1089-1090
