Editing Laser diode (section)

== Theory ==
{{More citations needed section|date=July 2011}}
[[File:Lasers.JPG|thumb|250px|Semi-conductor lasers (Bottom to Top: 660 nm, 635 nm, 532 nm, 520 nm, 445 nm, 405 nm)]]

A laser diode is electrically a [[PIN diode]]. The active region of the laser diode is in the intrinsic (I) region, and the carriers (electrons and holes) are pumped into that region from the N and P regions respectively. While initial diode laser research was conducted on simple P–N diodes, all modern lasers use the double-hetero-structure implementation, where the carriers and the photons are confined in order to maximize their chances for recombination and light generation. Unlike a regular diode, the goal for a laser diode is to recombine all carriers in the I region, and produce light. Thus, laser diodes are fabricated using [[direct and indirect band gaps|direct band-gap]] semiconductors. The laser diode epitaxial structure is grown using one of the [[crystal growth]] techniques, usually starting from an N-[[semiconductor doping|doped]] substrate, and growing the I (undoped) active layer, followed by the P-doped [[cladding (fiber optics)|cladding]], and a contact layer. The active layer most often consists of [[quantum well]]s, which provide lower threshold current and higher efficiency.<ref name="ColdrenCorzine2012" />

=== Electrical and optical pumping ===
Laser diodes form a subset of the larger classification of semiconductor ''p''–''n'' junction diodes. Forward electrical bias across the laser diode causes the two species of [[charge carrier]] – [[electron hole|holes]] and [[electron]]s – to be ''injected'' from opposite sides of the PIN junction into the depletion region. Holes are injected from the ''p''-doped into the undoped (i) semiconductor, and electrons vice versa. (A [[depletion zone|depletion region]], devoid of any charge carriers, forms as a result of the difference in electrical potential between ''n''- and ''p''-type semiconductors wherever they are in physical contact.) Due to the use of charge injection in powering most diode lasers, this class of lasers is sometimes termed ''injection lasers'', or ''injection laser diodes'' (ILD). As diode lasers are semiconductor devices, they may also be classified as semiconductor lasers. Either designation distinguishes diode lasers from [[solid-state laser]]s.

Another method of powering some diode lasers is the use of [[optical pumping]]. Optically pumped semiconductor lasers (OPSL) use a III-V semiconductor chip as the gain medium, and another laser (often another diode laser) as the pump source. OPSLs offer several advantages over ILDs, particularly in wavelength selection and lack of interference from internal electrode structures.<ref>Arrigoni, M. et al. (2009-09-28) [http://www.laserfocusworld.com/articles/2009/09/optically-pumped-semiconductor-lasers-green-opsls-poised-to-enter-scientific-pump-laser-market.html "Optically Pumped Semiconductor Lasers: Green OPSLs poised to enter scientific pump-laser market"], ''[[Laser Focus World]]''</ref><ref>[http://www.repairfaq.org/sam/laserdio.htm#dioopsl "Optically Pumped Semiconductor Laser (OPSL)"], Sam's Laser FAQs.</ref> A further advantage of OPSLs is invariance of the beam parameters – divergence, shape, and pointing – as pump power (and hence output power) is varied, even over a 10:1 output power ratio.<ref>[https://www.photonics.com/WhitePaper.aspx?WPID=1695 Coherent white paper (2018-05). "Advantages of Optically Pumped Semiconductor Lasers – Invariant Beam Properties"]</ref>

=== Generation of spontaneous emission ===
When an electron and a hole are present in the same region, they may [[recombination (physics)|recombine]] or ''annihilate'' producing a [[spontaneous emission]] — that is, the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron's original state and hole's state. (In a conventional semiconductor junction diode, the energy released from the recombination of electrons and holes is carried away as [[phonon]]s (lattice vibrations) rather than as photons.) Spontaneous emission below the [[lasing threshold]] produces similar properties to an [[light-emitting diode|LED]]. Spontaneous emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating.

=== Direct and indirect bandgap semiconductors ===
The difference between the photon-emitting semiconductor laser and a conventional phonon-emitting (non-light-emitting) semiconductor junction diode lies in the type of semiconductor used, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "[[Direct and indirect band gaps|direct bandgap]]" semiconductors. The properties of [[silicon]] and [[germanium]], which are single-element semiconductors, have bandgaps that do not align in the way needed to allow photon emission and are not considered ''direct''. Other materials, the so-called compound semiconductors, have virtually identical crystalline structures as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern to break the symmetry. The transition between the materials in the alternating pattern creates the critical [[direct bandgap]] property. [[Gallium arsenide]], [[indium phosphide]], [[gallium antimonide]], and [[gallium nitride]] are all examples of compound semiconductor materials that can be used to create junction diodes that emit light.

[[File:simple laser diode.svg|frame|right|Diagram of a simple laser diode, such as shown above; not to scale]]
[[File:Metal covered Laser diode switched on.jpg|thumb|left|A simple and low-power metal-enclosed laser diode]]

=== Generation of stimulated emission ===
In the absence of stimulated emission (e.g., lasing) conditions, electrons and holes may coexist in proximity to one another, without recombining, for a certain time, termed the ''upper-state lifetime'' or ''recombination time'' (about a nanosecond for typical diode laser materials), before they recombine. A nearby photon with energy equal to the recombination energy can cause recombination by [[stimulated emission]]. This generates another photon of the same frequency, [[polarization (waves)|polarization]], and [[phase (waves)|phase]], travelling in the same direction as the first photon. This means that stimulated emission will cause gain in an optical wave (of the correct wavelength) in the injection region, and the gain increases as the number of electrons and holes injected across the junction increases. The spontaneous and stimulated-emission processes are vastly more efficient in [[direct bandgap]] semiconductors than in [[indirect bandgap]] semiconductors; therefore, [[silicon]] is not a common material for laser diodes.

=== Optical cavity and laser modes ===
As in other lasers, the gain region is surrounded by an [[optical cavity]] to form a laser. In the simplest form of laser diode, an optical waveguide is made on that crystal's surface, such that the light is confined to a relatively narrow line. The two ends of the crystal are cleaved to form perfectly smooth, parallel edges, forming a [[Fabry–Pérot]] resonator. Photons emitted into a mode of the waveguide will travel along the waveguide and be reflected several times from each end face before they exit. As a light wave passes through the cavity, it is amplified by [[stimulated emission]], but light is also lost due to absorption and by incomplete reflection from the end facets. Finally, if there is more amplification than loss, the diode begins to ''[[lasing threshold|lase]]''.

Some important properties of laser diodes are determined by the geometry of the optical cavity. Generally, the light is contained within a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the transverse direction, if the waveguide is wide compared to the wavelength of the light, then the waveguide can support multiple [[transverse mode|transverse optical modes]], and the laser is known as ''multi-mode''. These transversely multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small [[Diffraction limited beam|diffraction-limited]] TEM00 beam, such as in printing, activating chemicals, microscopy, or [[laser pumping|pumping]] other types of lasers.

In applications where a small, focused beam is needed, the waveguide must be made narrow, on the order of the optical wavelength. This way, only a single transverse mode is supported and one ends up with a diffraction-limited beam. Such single-spatial-mode devices are used for optical storage, laser pointers, and fiber optics. These lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously. The wavelength emitted is a function of the bandgap of the semiconductor material and the modes of the optical cavity. In general, the maximum gain will occur for photons with energy slightly above the bandgap energy, and the modes nearest the peak of the gain curve will lase most strongly. The width of the gain curve will determine the number of additional ''side modes'' that may also lase, depending on the operating conditions. Single-spatial-mode lasers that can support multiple longitudinal modes are called Fabry-Pérot (FP) lasers. An FP laser will lase at multiple cavity modes within the gain bandwidth of the lasing medium. The number of lasing modes in an FP laser is usually unstable and can fluctuate due to changes in current or temperature.

Single-spatial-mode diode lasers can be designed so as to operate on a single longitudinal mode. These single-frequency diode lasers exhibit a high degree of stability, and are used in spectroscopy and metrology and as frequency references. Single-frequency diode lasers are classed as either distributed-feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers.

=== Formation of laser beam ===
Due to [[diffraction]], the beam diverges (expands) rapidly after leaving the chip, typically at 30 degrees vertically by 10 degrees laterally. A [[lens (optics)|lens]] must be used in order to form a [[collimated beam]] like that produced by a laser pointer. If a circular beam is required, then cylindrical lenses and other optics are used. For single-spatial-mode lasers, using symmetrical lenses, the collimated beam ends up being elliptical in shape, due to the difference in the vertical and lateral divergences. This is easily observable with a red [[laser pointer]]. The long axis of the ellipse is at right-angles to the plane of the chip.

The simple diode described above has been heavily modified in recent years{{When|date=November 2024}} to accommodate modern technology, resulting in a variety of types of laser diodes, as described below.