The vertical-cavity surface-emitting laser, or VCSEL , is a type of
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
laser diode
The laser diode chip removed and placed on the eye of a needle for scale
A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...
with
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
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
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
. VCSELs are used in various laser products, including
computer mice
A computer mouse (plural mice, sometimes mouses) is a hand-held pointing device that detects two-dimensional space, two-dimensional motion relative to a surface. This motion is typically translated into the motion of a pointer (user interface ...
,
fiber optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pre ...
s,
laser printer
Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics (and moderate-quality photographs) by repeatedly passing a laser beam back and forth over a negatively-charged cylinder called a "drum" to ...
s,
Face ID
Face ID is a facial recognition system designed and developed by Apple Inc. for the iPhone and iPad Pro. The system allows biometric authentication for unlocking a device, making payments, accessing sensitive data, providing detailed facial ex ...
, and
smartglasses
Smartglasses or smart glasses are eye or head-worn wearable computers that offer useful capabilities to the user. Many smartglasses include displays that add information alongside or to what the wearer sees. Alternatively, smartglasses are som ...
.
Production advantages
There are several advantages to producing VCSELs, in contrast to 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 function properly, 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
The Vias GmbH (stylized VIAS) is a rail service company based in Frankfurt (Germany). The name of the company was taken from the Latin word via for ''way'' and the letter ''S'' for service. It operates rail services in the states of Hesse, Rhine ...
, the electrical connections between layers of a circuit, have not been completely cleared of
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
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.
Structure
The laser resonator consists of two
distributed Bragg reflector
A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic ...
(DBR) mirrors parallel to the wafer surface with an
active region
An active region is a temporary region in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and solar ...
consisting of one or more
quantum well
A quantum well is a potential well with only discrete energy values.
The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy ...
s 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 embedded 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
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
, 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
Aluminium gallium arsenide (also gallium aluminium arsenide) ( Alx Ga1−x As) is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger bandgap. The ''x'' in the formula above is a number between 0 and 1 - this ...
(Al
''x''Ga
(1-''x'')As). The GaAs–AlGaAs system is favored for constructing VCSELs because the
lattice constant
A lattice constant or lattice parameter is one of the physical dimensions and angles that determine the geometry of the unit cells in a crystal lattice, and is proportional to the distance between atoms in the crystal. A simple cubic crystal has o ...
of the material does not vary strongly as the composition is changed, permitting multiple "lattice-matched"
epitaxial
Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...
layers to be grown on a GaAs substrate. However, the
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
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 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.
The main methods of restricting the current in a VCSEL are characterized by two types: 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."
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
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...
is minimal in this wavelength range.
Special forms
;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)).
;Tunable VCSELs with micromechanically movable mirrors (
MEMS
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
): (either optically or electrically pumped )
;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 (VECSELs):
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:
VCSOAs 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.
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 small active region, compared to edge-emitting lasers, reduces 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.
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.
High-power VCSELs
High-power vertical-cavity surface-emitting lasers can also be fabricated, either by increasing the emitting aperture size of a single device or by combining several elements into large two-dimensional (2D) arrays. There have been relatively few reported studies on high-power VCSELs. Large-aperture single devices operating around 100 mW were first reported in 1993.
Improvements in the epitaxial growth, processing, device design, and packaging led to individual large-aperture VCSELs emitting several hundreds of milliwatts by 1998.
More than 2 W continuous-wave (CW) operation at -10 degrees Celsius heat-sink temperature was also reported in 1998 from a VCSEL array consisting of 1,000 elements, corresponding to a power density of 30 W/cm
2.
In 2001, more than 1 W CW power and 10 W pulsed power at room temperature were reported from a 19-element array.
The VCSEL array chip was mounted on a
diamond
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...
heat spreader, taking advantage of diamond’s very high
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
. A record 3 W CW output power was reported in 2005 from large diameter single devices emitting around 980 nm.
In 2007, more than 200 W of CW output power was reported from a large (5 × 5mm) 2D VCSEL array emitting around the 976 nm wavelength,
representing a substantial breakthrough in the field of high-power VCSELs. The high power level achieved was mostly due to improvements in
wall-plug efficiency
In optics, wall-plug efficiency or radiant efficiency is the energy conversion efficiency with which the system converts electrical power into optical power. It is defined as the ratio of the radiant flux (i.e., the total optical output power) to ...
and packaging. In 2009, >100 W power levels were reported for VCSEL arrays emitting around 808 nm.
At that point, the VCSEL technology became useful for a variety of medical, industrial, and military applications requiring high power or high energy. Examples of such applications are:
* Medical/cosmetics:
laser hair removal, laser wrinkle removal
* Infrared illuminators for military/surveillance
* Pumping of
solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class ...
s and
fiber lasers
* High-power/high-energy
second harmonic generation (blue/green light)
* Laser machining:
laser cutting,
laser drilling,
laser ablation,
laser engraving
Applications
*
Optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...
data transmission
* Analog broadband signal transmission
* Absorption spectroscopy (
TDLAS)
*
Laser printer
Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics (and moderate-quality photographs) by repeatedly passing a laser beam back and forth over a negatively-charged cylinder called a "drum" to ...
s
* Computer mouse
* Biological tissue analysis
* Chip scale
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
*
Lidar for cellphone cameras
*
Structured light (e.g. the “dot projector” for iPhone X)
* Lidar for automobile collision avoidance
History
The surface emission from a bulk semiconductor at ultra-low temperature and magnetic carrier confinement was reported by Ivars Melngailis in 1965. The first proposal of short
cavity VCSEL was done by Kenichi Iga of Tokyo Institute of Technology in 1977. A simple drawing of his idea is shown in his research note. Contrary to the conventional Fabry-Perot edge-emitting semiconductor lasers, his invention comprises a short laser cavity less than 1/10 of the edge-emitting lasers vertical to a wafer surface. In 1979 a first demonstration on a short cavity VCSEL was done by Soda, Iga, Kitahara and
Suematsu,
but devices for
CW operation at room temperature were not reported until 1988.
The term VCSEL was coined in a publication of the
Optical Society of America
Optica (formerly known as The Optical Society (OSA) and before that as the Optical Society of America) is a professional society of individuals and companies with an interest in optics and photonics. It publishes journals and organizes conference ...
in 1987. In 1989, Jack Jewell led a Bell Labs / Bellcore collaboration (including
Axel Scherer, Sam McCall, Yong Hee Lee and James Harbison) that demonstrated over 1 million VCSELs on a small chip.
These first all-semiconductor VCSELs introduced other design features still used in all commercial VCSELs. "This demonstration marked a turning point in the development of the surface-emitting laser. Several more research groups entered the field, and many important innovations were soon being reported from all over the world".
Andrew Yang of the Defense Advanced Research Projects Agency (DARPA) quickly initiated significant funding toward VCSEL R&D, followed by other government and industrial funding efforts.
[ VCSELs replaced edge-emitting lasers in applications for short-range fiberoptic communication such as ]Gigabit Ethernet
In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...
and Fibre Channel, and are now used for link bandwidths from 1 Gigabit/sec to > 400 Gigabit/sec.
See also
*Optical interconnect
In integrated circuits, optical interconnects refers to any system of transmitting signals from one part of an integrated circuit to another using light. Optical interconnects have been the topic of study due to the high latency and power consumpt ...
*Interconnect bottleneck
The interconnect bottleneck comprises limits on integrated circuit (IC) performance due to connections between components instead of their internal speed.
In 2006 it was predicted to be a "looming crisis" by 2010.
Improved performance of compute ...
*Optical fiber cable
A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with ...
* Optical communication
* Parallel optical interface
* Optical cavity
*Optical microcavity
An optical microcavity or microresonator is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium, or by wrapping a waveguide in a circular fashion to form a ring. The former type is a standing wave cavity, a ...
References
External links
Long Wavelength Surface Emitting Lasers: Introduction
{{DEFAULTSORT:Vertical-Cavity Surface-Emitting Laser
Semiconductor lasers