- Vertical-cavity surface-emitting laser
The vertical-cavity surface-emitting laser (VCSEL; [v'ɪxl] ) is a type of semiconductor
laser diodewith laserbeam 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.
There are several advantages to producing VCSELs when compared with the production process of edge-emitting lasers. Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not work, whether due to bad contacts or poor material growth quality, the production time and the processing materials have been wasted. VCSELs however, can be tested at several stages throughout the process to check for material quality and processing issues. For instance, if the vias have not been completely cleared of dielectric material during the etch, an interim testing process will flag that the top metal layer is not making contact to the initial metal layer. Additionally, because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three inch Gallium Arsenide wafer. Furthermore, even though the VCSEL production process is more labor and material intensive, the yield can be controlled to a more predictable outcome.
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
diodejunction. 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(Al"x"Ga(1-"x")As). The GaAs–AlGaAs system is favored for constructing VCSELs because the lattice constantof 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 indexof 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 aluminium concentrations, an oxide can be formed from AlGaAs, and this oxide can be used to restrict the current in a VCSEL, enabling very low threshold currents.
Recently the two main methods of restricting the current in a VCSEL were characterized by two types of VCSELs: ion-implanted VCSELs and Oxide VCSELs.
In the early 1990s, telecommunications companies tended to favor ion-implanted VCSELs. Ions, (often hydrogen ions, H+), were implanted into the VCSEL structure everywhere except the aperture of the VCSEL, destroying the lattice structure around the aperture, thus inhibiting the current. In the mid to late 1990s, companies moved towards the technology of oxide VCSELs. The current is confined in an oxide VCSEL by oxidizing the material around the aperture of the VCSEL. A high content aluminium layer that is grown within the VCSEL structure is the layer that is oxidized. Oxide VCSELs also often employ the ion implant production step. As a result in the oxide VCSEL, the current path is confined by the ion implant and the oxide aperture.
The initial acceptance of oxide VCSELs was plagued with concern about the apertures "popping off" due to the strain and defects of the oxidation layer. However, after much testing, the reliability of the structure has proven to be robust. As stated in one study by Hewlett Packard on oxide VCSELs, "The stress results show that the activation energy and the wearout lifetime of oxide VCSEL are similar to that of implant VCSEL emitting the same amount of output power." [http://www.ieee.org/organizations/pubs/newsletters/leos/aug99/article6.htm]
A production concern also plagued the industry when moving the oxide VCSELs from research and development to production mode. The oxidation rate of the oxide layer was highly dependent on the aluminium content. Any slight variation in aluminium would change the oxidation rate sometimes resulting in apertures that were either too big or too small to meet the specification standards.
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. 1310 nm VCSELs are desirable as the dispersion of silica-based optical fiber is minimal in this wavelength range.
* Multiple active region devices (aka bipolar cascade VCSELs). Allows for differential quantum efficiency values in excess of 100% through carrier recycling
* 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 manufactured 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.
* Vertical-cavity semiconductor optical amplifiers, known as VCSOAs . These devices are optimized as amplifiers as opposed to oscillators. VCSOAs must be operated below threshold and thus require reduced mirror reflectivities for decreased feedback. In order to maximize the signal gain, these devices contain a large number of quantum wells (optically pumped devices have been demonstrated with 21–28 wells) and as a result exhibit single-pass gain values which are significantly larger than that of a typical VCSEL (roughly 5%). These structures operate as narrow linewidth (tens of GHz) amplifiers and may be implemented as amplifying filters.
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 Harvard citation|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.
Optical fiberdata transmission
* Analog broadband signal transmission
* Absorption spectroscopy (
* computer mouse
The first VCSEL was presented in 1979 by Soda, Iga, Kitahara and Suematsu Harvard citation|Soda|1979, but devices for CW operation at room temperature were not reported until 1988 Harvard citation|Koyama|1988. The term VCSEL was coined in a publication of the Optical Society of America in 1987Fact|date=February 2007. Today, VCSELs have replaced edge-emitting lasers in applications for short-range fiberoptic communication such as
Gigabit Ethernetand Fibre Channel.
* [http://www.wsi.tum.de/Research/AmanngroupE26/AreasofResearch/SurfaceEmittingLasers/tabid/110/Default.aspx Long Wavelength Surface Emitting Lasers: Introduction]
* [http://britneyspears.ac/physics/vcsels/vcsels.htm Britney's Guide to Semiconductor Physics: VCSELs]
Wikimedia Foundation. 2010.
Look at other dictionaries:
Vertical-Cavity Surface-Emitting Laser — Der vertical cavity surface emitting laser (VCSEL, [v ɪxl], dt. ‚vertikal emittierende Laserdiode‘) ist ein Halbleiterlaser, bei dem das Licht senkrecht zur Ebene des Halbleiterchips abgestrahlt wird, im Gegensatz zum herkömmlichen Kantenemitter … Deutsch Wikipedia
Vertical-external-cavity surface-emitting-laser — A vertical external cavity surface emitting laser (VECSEL) is a small semiconductor laser similar to a vertical cavity surface emitting laser (VCSEL). VECSELs are used primarily as near infrared devices in laser cooling and spectroscopy, but have … Wikipedia
Resonant-cavity light emitting diode — Die „resonant cavity light emitting diode“ (RCLED oder RC LED, dt. »Leuchtdiode mit optischen Resonator«) ist eine Form von Leuchtdioden. Funktionsweise Das Wirkprinzip funktioniert ähnlich einem VCSEL (Vertical Cavity Surface Emitting Laser),… … Deutsch Wikipedia
resonant-cavity light emitting diode — Die „resonant cavity light emitting diode“ (RCLED oder RC LED, dt. »Leuchtdiode mit optischem Resonator«) ist eine Form von Leuchtdioden. Funktionsweise Das Wirkprinzip funktioniert ähnlich einem VCSEL (Vertical Cavity Surface Emitting… … Deutsch Wikipedia
Laser diode — Top: a packaged laser diode shown with a penny for scale. Bottom: the laser diode chip is removed from the above package and placed on the eye of a needle for scale … Wikipedia
Laser à cavité verticale à émission par la surface — Diode laser à cavité verticale émettant par la surface Schéma d une structure VCSEL simple Une diode laser à cavité verticale émettant par la surface (ou VCSEL [v ɪxl] pour l anglais vertical cavity surface emitting laser) est un type de diode… … Wikipédia en Français
Laser à cavité verticale émettant par la surface — Diode laser à cavité verticale émettant par la surface Schéma d une structure VCSEL simple Une diode laser à cavité verticale émettant par la surface (ou VCSEL [v ɪxl] pour l anglais vertical cavity surface emitting laser) est un type de diode… … Wikipédia en Français
Laser acronyms — Here, is a list of acronyms used in laser physics, applications and technology.A*AOM – acousto optic modulator *APD – avalanche photodiode *APM – additive pulse mode locking *ASE – amplified spontaneous emission *AWG – arrayed waveguide… … Wikipedia
Laser — For other uses, see Laser (disambiguation). United States Air Force laser experiment … Wikipedia
laser — /lay zeuhr/, n. Physics. a device that produces a nearly parallel, nearly monochromatic, and coherent beam of light by exciting atoms to a higher energy level and causing them to radiate their energy in phase. Also called optical maser. [1955 60; … Universalium