View source for Vertical-cavity surface-emitting laser

{{Short description|Type of semiconductor laser diode}}
{{Technical|date=February 2017}}

[[Image:Simple vcsel.svg|thumb|350px|right|Diagram of a simple VCSEL structure.]]

The '''vertical-cavity surface-emitting laser''' ('''VCSEL''' {{IPAc-en|ˈ|v|ɪ|k|s|əl}}) is a type of [[semiconductor]] [[laser diode]] with [[laser]] beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also called ''in-plane'' lasers) which emit from surfaces formed by cleaving the individual chip out of a [[wafer (electronics)|wafer]].  VCSELs are used in various laser products, including [[computer mice]], [[fiber-optic communication]]s, [[laser printer]]s, [[Face ID]],<ref>{{cite web
|url=http://spie.org/newsroom/faces-light-up-over-vcsel-prospects
|title=Faces light up over VCSEL prospects
|date=9 April 2018
|first=Andy|last=Extance
|publisher=[[SPIE]]
}}</ref> and [[smartglasses]].<ref>{{cite web
|url=https://www.theverge.com/2018/2/5/16966530/intel-vaunt-smart-glasses-announced-ar-video
|title=Intel made smart glasses that look normal
|date=5 February 2018
|first=Dieter|last=Bohn
|publisher=[[The Verge]]
}}</ref>

== 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 [[Via (electronics)|vias]], which are the electrical connections between layers of a circuit, 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.  Thus, although the VCSEL production process is more labor and material intensive, the yield can be controlled to a more predictable and higher outcome.<ref>{{Cite web |last=SEO_INPHENIX |date=2021-09-24 |title=Introduction of VCSEL: Working Principles And Characteristics |url=https://www.inphenix.com/en/vcsel-introduction/ |access-date=2023-12-21 |website=INPHENIX |language=en-US}}</ref>

== Structure ==
[[Image:Real vcsel.svg|thumb|450px|A realistic VCSEL device structure. This is a ''bottom-emitting'' ''multiple-quantum-well'' VCSEL.]]
The laser resonator consists of two [[distributed Bragg reflector]] (DBR) mirrors parallel to the wafer surface with an [[active laser medium|active region]] consisting of one or more [[quantum well]]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 [[Extrinsic semiconductor#P-type semiconductors|p-type]] and [[Extrinsic semiconductor#N-type semiconductors|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]], 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&nbsp;nm to 1300&nbsp;nm are typically based on gallium arsenide (GaAs) wafers with DBRs formed from GaAs and [[aluminium gallium arsenide]] (Al<sub>''x''</sub>Ga<sub>(1−''x'')</sub>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" [[epitaxy|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 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."<ref>{{cite book |last1=Lei |first1=C. |last2=Deng |first2=H. |last3=Dudley |first3=J.J. |last4=Lim |first4=S.F. |last5=Liang |first5=B. |last6=Tashima |first6=M. |last7=Herrick |first7=R.W. |title=1999 Digest of the LEOS Summer Topical Meetings: Nanostructures and Quantum Dots/WDM Components/VCSELs and Microcavaties/RF Photonics for CATV and HFC Systems (Cat. No.99TH8455) |chapter=Manufacturing of oxide VCSEL at Hewlett Packard |url=https://www.photonicssociety.org/newsletters/aug99/article6.htm |publisher=[[IEEE Photonics Society]] |access-date=3 June 2021 |archive-url=https://web.archive.org/web/20161110092609/https://www.photonicssociety.org/newsletters/aug99/article6.htm |archive-date=10 November 2016 |pages=III11–III12 |doi=10.1109/LEOSST.1999.794691 |date=1999 |isbn=0-7803-5633-0 |s2cid=39634122 |url-status=dead}}</ref>
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&nbsp;nm to 2000&nbsp;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&nbsp;nm VCSELs are desirable as the dispersion of silica-based [[optical fiber]] 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<sup>+</sup>p<sup>+</sup>), an electrically advantageous n-n<sup>+</sup>p<sup>+</sup>-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 ([[Microelectromechanical systems|MEMS]]) : (either optically <ref>V. Jayaraman, J. Jiang, B. Potsaid, G. Cole, J Fujimoto, and Alex Cable "Design and performance of broadly tunable, narrow linewidth, high repetition rate 1310nm VCSELs for swept source optical coherence tomography", SPIE volume 8276 paper 82760D, 2012</ref> or electrically pumped <ref>C. Gierl, T. Gruendl, P. Debernardi, K. Zogal, C. Grasse, H. Davani, G. Böhm, S. Jatta, F. Küppers, P. Meißner, and M. Amann, "Surface micromachined tunable 1.55 μm-VCSEL with 102 nm continuous single-mode tuning," Opt. Express  19, 17336-17343 2011</ref><ref>D. D. John, C. Burgner, B. Potsaid, M. Robertson, B. Lee, W. J. Choi, A. Cable, J. Fujimoto, and V. Jayaraman, "Wideband Electrically-Pumped 1050 nm MEMS-Tunable VCSEL for Ophthalmic Imaging", Jnl. Lightwave Tech., vol. 33, no. 16, pp. 3461 - 3468, Feb. 2015.</ref>)
; Wafer-bonded or wafer-fused VCSEL: Combination of semiconductor materials that can be fabricated using different types of substrate wafers<ref>V. Jayaraman, G. D. Cole, M. Robertson, A. Uddin, and A. Cable, "High-sweep-rate 1310 nm MEMS-VCSEL with 150&nbsp;nm continuous tuning range", Electronics Letters, vol. 48, no. 14, pp. 867–869, 2012.</ref>
; 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) : [[VECSEL]]s 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 30&nbsp;W.  The external cavity also allows intracavity techniques such as frequency doubling, single frequency operation and femtosecond pulse modelocking.

;Vertical-cavity semiconductor optical amplifiers: [[Optical amplifier#Vertical-cavity SOA|VCSOA]]s 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 [[semiconductor fabrication|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.<ref name="Iga200">{{cite journal |last=Iga |first=Kenichi |title=Surface-emitting laser—Its birth and generation of new optoelectronics field |journal=IEEE Journal of Selected Topics in Quantum Electronics |volume=6 |issue=6 |year=2000 |pages=1201–1215 |doi=10.1109/2944.902168|bibcode=2000IJSTQ...6.1201I |s2cid=10550809 }}</ref>

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&nbsp;mW were first reported in 1993.<ref name="Peters1993">{{cite journal |last=Peters |first=F. |author2=M. Peters |author3=D. Young |author4=J. Scott |author5=B. Thibeault |author6=S. Corzine |author7=L. Coldren  |date=January 1993 |title= High-power vertical-cavity surface-emitting lasers |journal=Electronics Letters |volume=29 |issue=2 |pages=200–201
|doi=10.1049/el:19930134|bibcode=1993ElL....29..200P }}</ref> Improvements in the epitaxial growth, processing, device design, and packaging led to individual large-aperture VCSELs emitting several hundreds of milliwatts by 1998.<ref name="Grabherr1998">{{cite journal |last=Grabherr |first=M. |author2=R. Jager |author3=M. Miller |author4=C. Thalmaier |author5=J. Herlein |author6=R. Michalzik |author7=K. Ebeling  |date=August 1998 |title= Bottom-emitting VCSEL's for high-CW optical output power |journal=IEEE Photonics Technology Letters |volume=10 |issue=8 |pages=1061–1063 |doi=10.1109/68.701502|bibcode = 1998IPTL...10.1061G |s2cid=22839700 }}</ref> More than 2&nbsp;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&nbsp;W/cm<sup>2</sup>.<ref name="Francis1998">{{cite journal |last1=Francis |first1=D. |last2=Chen |first2=H.-L. |last3=Yuen | first3=W. |last4= Li | first4=G. |last5=Chang-Hasnain |first5=C. |date=October 1998 |title= Monolithic 2D-VCSEL array with >2 W CW and >5 W pulsed output power |journal=Electronics Letters |volume=34 |issue=22 |pages=2132–2133 |doi=10.1049/el:19981517|bibcode=1998ElL....34.2132F }}</ref> In 2001, more than 1&nbsp;W CW power and 10&nbsp;W pulsed power at room temperature were reported from a 19-element array.<ref name="Miller2001">{{cite journal |last=Miller |first=M. |author2=M. Grabherr |author3=R. Jager |author4=K. Ebeling  |date=March 2001 |title= High-power VCSEL arrays for emission in the watt regime at room temperature |journal= IEEE Photonics Technology Letters |volume=13 |issue=3 |pages=173–175 |doi=10.1109/68.914311|bibcode = 2001IPTL...13..173M |s2cid=22964703 }}</ref>  The VCSEL array chip was mounted on a [[diamond]] heat spreader, taking advantage of diamond’s very high [[thermal conductivity]].  A record 3&nbsp;W CW output power was reported in 2005 from large diameter single devices emitting around 980&nbsp;nm.<ref name="Dasaro2005">{{cite journal |last=D’Asaro |first=L. A. |author2=J. Seurin and J.Wynn  |date=February 2005 |title= High-power, high efficiency VCSELs pursue the goal |journal= Photonics Spectra |volume=39 |issue=2 |pages=62–66 |url= http://www.photonics.com/Article.aspx?AID=21099}}</ref>

In 2007, more than 200&nbsp;W of CW output power was reported from a large (5&nbsp;×&nbsp;5mm) 2D VCSEL array emitting around the 976&nbsp;nm wavelength,<ref name="Seurin2007">{{cite journal |last=Seurin |first=J-F. |author2=L. A. D’Asaro |author3=C. Ghosh |date=July 2007 |title= A New Application for VCSELs: High-Power Pump Lasers |journal= Photonics Spectra |volume=41 |issue=7 |url=http://www.photonics.com/Article.aspx?AID=30143}}</ref> 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]] and packaging.  In 2009, >100&nbsp;W power levels were reported for VCSEL arrays emitting around 808&nbsp;nm.<ref name="Seurin2009">{{cite journal |last=Seurin |first=J-F. |author2=G. Xu |author3=V. Khalfin |author4=A. Miglo |author5=J. D. Wynn |author6=P. Pradhan |author7=C. L. Ghosh |author8=L. A. D'Asaro  |editor1-first=Kent D |editor1-last=Choquette |editor2-first=Chun |editor2-last=Lei |date=February 2009 |title= Progress in high-power high-efficiency VCSEL arrays |journal=Proceedings SPIE, in Vertical-Cavity Surface-Emitting Lasers XIII |volume=7229 |pages= 722903–1–11 |doi= 10.1117/12.808294 |series=Vertical-Cavity Surface-Emitting Lasers XIII |bibcode=2009SPIE.7229E..03S |s2cid=109520958 }}</ref>

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]]s and [[fiber laser]]s
* High-power/high-energy [[second harmonic generation]] (blue/green light)<ref name="VanLeeuwen2009">{{cite journal |last1=Van Leeuwen |first1=R. |last2= Seurin | first2=J-F. |last3=Xu |first3= G. |last4=Ghosh |first4= C.|editor1-first=W. Andrew |editor1-last=Clarkson |editor2-first=Norman |editor2-last=Hodgson |editor3-first=Ramesh K |editor3-last=Shori |date=February 2009 |title= High power pulsed intra-cavity frequency doubled vertical extended cavity blue laser arrays |journal=Proceedings SPIE, in Solid State Lasers XVIII: Technology and Devices |volume=7193 |pages=771931D–1–9 |doi= 10.1117/12.816035|series=Solid State Lasers XVIII: Technology and Devices |bibcode=2009SPIE.7193E..1DV |s2cid=21109187 }}</ref>
* Laser machining: [[laser cutting]], [[laser drilling]], [[laser ablation]], [[laser engraving]]

== Applications ==
* [[Optical fiber]] data transmission
* Analog broadband signal transmission
* Absorption spectroscopy ([[TDLAS]])
* [[Laser printer]]s
* Computer mouse
* Biological tissue analysis
* Chip scale [[atomic clock]]
* [[Lidar]] for cellphone cameras
* [[Structured light]] (e.g. the "dot projector" for iPhone [[Face ID]])
* 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.<ref>{{cite book | author=Eli Kapon | title=Semiconductor Lasers II: Materials and Structures | year=1998 | publisher=Elsevier | isbn=9780080516967}}</ref><ref>{{cite book | author=Shun Lien Chuang | title=Physics of Photonic Devices | year=2009}}</ref><ref>{{cite book | author=J.K. Peterson | title= Fiber Optics Illustrated Dictionary | year=2002 | publisher= Taylor & Francis | isbn=9780849313493}}</ref> The first proposal of short [[optical cavity|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 [[Yasuharu Suematsu|Suematsu]],<ref name="Soda1979">{{cite journal |last=Soda |first=Haruhisa |title=GaInAsP/InP Surface Emitting Injection Lasers |journal=Japanese Journal of Applied Physics |volume=18 |issue=12 |year=1979 |pages=2329–2330 |doi= 10.1143/JJAP.18.2329|bibcode = 1979JaJAP..18.2329S |s2cid=122958383 |display-authors=etal}}</ref> but devices for [[Continuous Wave|CW]] operation at room temperature were not reported until 1988.<ref name="Koyama1988">{{cite journal |last=Koyama |first=Fumio |title=Room temperature cw operation of GaAs vertical cavity surface emitting laser |journal=Trans. IEICE |volume= E71 |issue=11 |year=1988 |pages=1089–1090|display-authors=etal}}</ref>  The term VCSEL was coined in a publication of the [[Optical Society of America]] in 1987.<ref>{{cite journal |last1=Christensen |first1=D. H. |last2=Barnes |first2=F. S. |title=Vertical Cavity Surface Emitting Laser in Molecular Beam Epitaxial GaAs/AlGaAs using a Multilayer Dielectric Mirror |journal=Topical Meeting on Semiconductor Lasers, Technical Digest |publisher=Optical Society of America |date=February 1987 |volume=6 |page=WA7-1 |doi=10.1364/SLA.1987.WA7 | isbn=0-936659-39-4|s2cid=257137192 }}</ref> In 1989, Jack Jewell led a Bell Labs / Bellcore collaboration (including [[Axel Scherer (professor)|Axel Scherer]], Sam McCall, Yong Hee Lee and James Harbison) that demonstrated over 1 million VCSELs on a small chip.<ref>{{Cite journal|last1=Jewell|first1=J.L.|last2=Scherer|first2=A.|last3=McCall|first3=S.L.|last4=Lee|first4=Y.H.|last5=Walker|first5=S.|last6=Harbison|first6=J.P.|last7=Florez|first7=L.T.|date=August 1989|title=Low-threshold electrically pumped vertical-cavity surface-emitting microlasers|journal=Electronics Letters|volume=25|issue=17|pages=1123–1124|doi=10.1049/el:19890754|bibcode=1989ElL....25.1123J|s2cid=111035374 }}</ref><ref>{{Cite journal|last1=Lee|first1=Y.H.|last2=Jewell|first2=J.L.|last3=Scherer|first3=A.|last4=McCall|first4=S.L.|last5=Harbison|first5=J.P.|last6=Florez|first6=L.T.|date=September 1989|title=Room-temperature continuous-wave vertical-cavity single-quantum-well microlaser diodes|url=https://authors.library.caltech.edu/452/1/LEEel89.pdf|journal=Electronics Letters|volume=25|issue=20|pages=1377–1378|doi=10.1049/el:19890921|bibcode=1989ElL....25.1377L }}</ref> 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".<ref name=Towe>{{Cite journal|last1=Towe|first1=Elias|last2=Leheny|first2=Robert F.|last3=Yang|first3=Andrew|date=December 2000|title=A historical perspective of the development of the vertical-cavity surface-emitting laser|journal=IEEE Journal of Selected Topics in Quantum Electronics|volume=6|issue=6|pages=1458–1464|doi=10.1109/2944.902201|bibcode=2000IJSTQ...6.1458T|s2cid=46544782 }}</ref> 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.<ref name=Towe/>  VCSELs replaced edge-emitting lasers in applications for short-range fiberoptic communication such as [[Gigabit Ethernet]] and [[Fibre Channel]], and are now used for link bandwidths from 1 to 400 gigabits per second or greater.

== See also ==
* [[Interconnect bottleneck]]
* [[Optical cavity]]
* [[Optical communication]]
* [[Optical fiber cable]]
* [[Optical interconnect]]
* [[Optical microcavity]]
* [[Parallel optical interface]]

== References ==
{{reflist|30em}}

== External links ==
* [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]

{{Semiconductor laser}}

{{DEFAULTSORT:Vertical-Cavity Surface-Emitting Laser}}
[[Category:Semiconductor lasers]]