Editing Laser diode (section)

== Types ==
The simple laser diode structure described above is inefficient. Such devices require so much power that they can only achieve pulsed operation without damage. Although historically important and easy to explain, such devices are not practical.

=== Double heterostructure lasers ===
[[File:Simple dh laser diode.svg|thumb|350px|Diagram of front view of a double heterostructure laser diode; not to scale]]

In these devices, a layer of low-[[bandgap]] material is sandwiched between two high-bandgap layers. One commonly-used pair of materials is [[gallium arsenide]] (GaAs) with [[aluminium gallium arsenide]] (Al<sub>x</sub>Ga<sub>(1-x)</sub>As). Each of the junctions between different bandgap materials is called a ''[[heterostructure]]'', hence the name ''double heterostructure'' (DH) laser. The kind of laser diode described in the first part of the article may be referred to as a ''homojunction'' laser, for contrast with these more popular devices.

The advantage of a DH laser is that the region where free electrons and holes exist simultaneously—the [[active laser medium|active region]]—is confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification—not so many are left out in the poorly amplifying periphery. In addition, light is reflected within the heterojunction; hence, the light is confined to the region where the amplification takes place.

=== Quantum well lasers ===
{{main|Quantum well laser}}
[[File:Simple qw laser diode.svg|thumb|350px|Diagram of front view of a simple quantum well laser diode; not to scale]]

If the middle layer is made thin enough, it acts as a [[quantum well]]. This means that the vertical variation of the electron's [[wavefunction]], and thus a component of its energy, is quantized. The efficiency of a [[quantum well laser]] is greater than that of a bulk laser because the [[density of states]] function of electrons in the quantum well system has an abrupt edge that concentrates electrons in energy states that contribute to laser action.

Lasers containing more than one quantum well layer are known as ''multiple quantum well'' lasers. Multiple quantum wells improve the overlap of the gain region with the optical [[waveguide]] [[normal mode|mode]].

Further improvements in laser efficiency have also been demonstrated by reducing the quantum well layer to a [[quantum wire]] or to a ''sea'' of [[quantum dot]]s.

=== Quantum cascade lasers ===
{{main|Quantum cascade laser}}

In a [[quantum cascade laser]], the difference between quantum well energy levels is used for the laser transition instead of the bandgap. This enables laser action at relatively long [[wavelength]]s, which can be tuned simply by altering the thickness of the layer. They are heterojunction lasers.

=== Interband cascade lasers ===
{{main|Interband cascade laser}}

An [[interband cascade laser]] (ICL) is a type of laser diode that can produce coherent radiation over a large part of the mid-infrared region of the electromagnetic spectrum.

=== Separate confinement heterostructure lasers ===
[[File:Simple sch laser diode.svg|thumb|350px|Diagram of front view of a separate confinement heterostructure quantum well laser diode; not to scale]]

The problem with the simple quantum well diode described above is that the thin layer is simply too small to effectively confine the light. To compensate, another two layers are added on, outside the first three. These layers have a lower [[refractive index]] than the center layers, and hence confine the light effectively. Such a design is called a separate confinement heterostructure (SCH) laser diode.

Almost all commercial laser diodes since the 1990s have been SCH quantum well diodes. {{citation needed|date=April 2018}}

=== Distributed Bragg reflector lasers ===
A [[distributed Bragg reflector laser]] (DBR) is a type of single-frequency laser diode.<ref name="hecht">{{cite book|last1=Hecht|first1=Jeff|title=The Laser Guidebook|date=1992|publisher=McGraw-Hill, Inc.|location=New York|isbn=0-07-027738-9|page=317|edition=Second}}</ref> It is characterized by an [[optical cavity]] consisting of an electrically or optically pumped gain region between two mirrors to provide feedback. One of the mirrors is a broadband reflector and the other mirror is wavelength selective so that gain is favored on a single longitudinal mode, resulting in lasing at a single resonant frequency. The broadband mirror is usually coated with a low-reflectivity coating to allow emission. The wavelength-selective mirror is a periodically structured [[diffraction grating]] with high reflectivity. The diffraction grating is within a non-pumped, or passive, region of the cavity. A DBR laser is a monolithic single-chip device with the grating etched into the semiconductor. DBR lasers can be edge-emitting lasers or [[vertical-cavity surface-emitting laser|VCSELs]]. Alternative hybrid architectures that share the same topology include extended-cavity diode lasers and volume Bragg grating lasers, but these are not properly called DBR lasers.

=== Distributed-feedback lasers ===
{{Main|Distributed-feedback laser}}

A [[distributed-feedback laser]] (DFB) is a type of single-frequency laser diode.<ref name="hecht"/> DFBs are the most common transmitter type in [[DWDM]] systems. To stabilize the lasing wavelength, a diffraction grating is etched close to the ''p''–''n'' junction of the diode. This grating acts like an optical filter, causing a single wavelength to be fed back to the gain region and lase. Since the grating provides the feedback that is required for lasing, reflection from the facets is not required. Thus, at least one facet of a DFB is [[anti-reflection coating|anti-reflection coated]]. The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, and can only be tuned slightly with temperature. DFB lasers are widely used in optical communication applications where a precise and stable wavelength is critical.

The threshold current of this DFB laser, based on its static characteristic, is around 11 mA. The appropriate bias current in a linear regime could be taken in the middle of the static characteristic (50 mA). Several techniques have been proposed in order to enhance the single-mode operation in these kinds of lasers by inserting a one-phase-shift (1PS) or multiple-phase-shift (MPS) in the uniform Bragg grating.<ref>{{cite journal |last1=Bouchene |first1=M.M. |first2=R. |last2=Hamdi |first3=Q. |last3=Zou |title=Theorical analysis of a monolithic all-active three-section semiconductor laser |journal=Photonics Letters of Poland |volume=9 |issue=4 |pages=131–3 |date=2017 |doi= 10.4302/plp.v9i4.785|url=http://www.photonics.pl/PLP/index.php/letters/article/download/9-47/516/0|doi-access=free }}</ref> However, multiple-phase-shift DFB lasers represent the optimal solution because they have the combination of higher side-mode suppression ratio and reduced spatial hole-burning.

=== Vertical-cavity surface-emitting laser ===
{{main|Vertical-cavity surface-emitting laser}}
[[File:Simple vcsel.svg|thumb|350px|Diagram of a simple VCSEL structure; not to scale]]

[[Vertical-cavity surface-emitting laser]]s (VCSELs) have the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the surface of the cavity rather than from its edge as shown in the figure. The reflectors at the ends of the cavity are [[dielectric mirror]]s made from alternating high- and low-refractive-index quarter-wave-thick multilayer.

Such dielectric mirrors provide a high degree of wavelength-selective reflectance at the required free surface wavelength {{Mvar|&lambda;}} if the thicknesses of alternating layers {{Math|''d''<sub>1</sub>}} and {{Math|''d''<sub>2</sub>}} with refractive indices {{Math|''n''<sub>1</sub>}} and {{Math|''n''<sub>2</sub>}} are such that {{Math|1=''n''<sub>1</sub>''d''<sub>1</sub> + ''n''<sub>2</sub>''d''<sub>2</sub> = ''&lambda;''/2}}, which then leads to the constructive interference of all partially reflected waves at the interfaces. But there is a disadvantage: because of the high mirror reflectivities, VCSELs have lower output powers when compared to edge-emitting lasers.

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, then the production time and the processing materials have been wasted.

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. However, they normally show a lower power output level.

===Vertical-external-cavity surface-emitting-laser===
{{main|Vertical-external-cavity surface-emitting-laser}}

Vertical-external-cavity surface-emitting lasers, or [[VECSEL]]s, are similar to VCSELs. In VCSELs, the mirrors are typically grown [[Epitaxy|epitaxially]] as part of the diode structure, or grown separately and bonded directly to the semiconductor containing the active region. VECSELs are distinguished by a construction in which one of the two mirrors is external to the diode structure. As a result, the cavity includes a free-space region. A typical distance from the diode to the external mirror would be 1&nbsp;cm.

One of the most interesting features of any VECSEL is the small thickness of the semiconductor gain region in the direction of propagation, less than 100&nbsp;nm. In contrast, a conventional in-plane semiconductor laser entails light propagation over distances of from 250&nbsp;μm upward to 2&nbsp;mm or longer. The significance of the short propagation distance is that it causes the effect of ''antiguiding'' nonlinearities in the diode laser gain region to be minimized. The result is a large-cross-section single-mode optical beam that is not attainable from in-plane ("edge-emitting") diode lasers.

Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications, including high-power sources for use in industrial machining (cutting, punching, etc.) because of their unusually high power and efficiency when pumped by multi-mode diode laser bars. However, because of their lack of ''p''–''n'' junctions, optically pumped VECSELs are not considered ''diode lasers'', and are classified as semiconductor lasers.{{citation needed|date=December 2012}}

Electrically pumped VECSELs have also been demonstrated. Applications for electrically pumped VECSELs include projection displays, served by [[frequency doubling]] of near-IR VECSEL emitters to produce blue and green light.

=== External-cavity diode lasers ===
External-cavity diode lasers are [[tunable laser]]s which use mainly double heterostructures diodes of the Al<sub>{{Mvar|x}}</sub>Ga<sub>{{Math|1&minus;''x''}}</sub>As type. The first external-cavity diode lasers used intracavity etalons<ref>{{cite journal|doi=10.1364/OL.1.000061|title=External-cavity-controlled 32-MHz narrow-band cw GaA1As-diode lasers|year=1977|last1=Voumard|first1=C.|journal=Optics Letters|volume=1|pages=61–3|pmid=19680331|issue=2|bibcode = 1977OptL....1...61V }}</ref> and simple tuning Littrow gratings.<ref>{{cite journal|author1=Fleming, M. W. |author2=Mooradian, A. |title=Spectral characteristics of external-cavity controlled semiconductor lasers |journal=IEEE J. Quantum Electron. |volume=17 |pages=44–59 |year=1981 |doi=10.1109/JQE.1981.1070634 |bibcode=1981IJQE...17...44F }}</ref> Other designs include gratings in grazing-incidence configuration, multiple-prism grating configurations, and piezo-transduced diode laser configuration.<ref>{{cite book|author=Zorabedian, P. |title=Tunable Lasers Handbook|url=https://books.google.com/books?id=PPMC5BbSN0QC|editor=F. J. Duarte|editor-link=F. J. Duarte|publisher=Academic Press|year=1995|chapter=8|isbn=0-12-222695-X}}</ref><ref>{{Cite journal |last1=Duca |first1=Lucia |last2=Perego |first2=Elia |last3=Berto |first3=Federico |last4=Sias |first4=Carlo |date=2021-06-15 |title=Design of a Littrow-type diode laser with independent control of cavity length and grating rotation |url=https://doi.org/10.1364/OL.423813 |journal=Optics Letters |language=EN |volume=46 |issue=12 |pages=2840–2843 |doi=10.1364/OL.423813 |pmid=34129554 |arxiv=2202.07762 |bibcode=2021OptL...46.2840D |issn=1539-4794|hdl=11696/78722 |hdl-access=free }}</ref>