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A laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common and practical type of laser diode is formed from a p-n junction and powered by injected electrical current. These devices are sometimes referred to as injection laser diodes to distinguish them from optically pumped laser diodes, which are more easily produced in the laboratory.
Principle of operationEdit
A laser diode, like many other semiconductor devices, is formed by doping a very thin layer on the surface of a crystal wafer. The crystal is doped to produce an n-type region and a p-type region, one above the other, resulting in a p-n junction, or diode.
As in other diodes, when this structure is forward biased, holes from the p-region are injected into the n-region, where electrons are the dominant carrier. Similarly, electrons from the n-region are injected into the p-region, where holes are common. When an electron and a hole are present in the same region, they may recombine by 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 and hole states involved. These injected electrons and holes represent the injection current of the diode, and spontaneous emission gives the laser diode below lasing threshold similar properties to a LED. Spontaneous emission is necessary to initiate laser oscillation, but it is a source of inefficiency once the laser is oscillating.
Under suitable conditions, the electron and the hole may coexist in the same area for quite some time (on the order of microseconds) before they recombine. Then a nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. This generates another photon of the same frequency, travelling in the same direction, with the same polarization and phase as the first photon. This means that stimulated emission causes 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, thus silicon is not a common material for laser diodes.
As in other lasers, the gain region is surrounded with an optical cavity to form a laser. In the simplest form of laser diode, an optical waveguide is made on that crystal 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-Perot 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 are emitted. 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 "lase".
Some important properties of laser diodes are determined by the geometry of the optical cavity. Generally, in the vertical direction, the light is contained in a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the lateral direction, if the waveguide is wide compared to the wavelength of light, then the waveguide can support multiple lateral optical modes, and the laser is known as "multi-mode". These laterally multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small diffraction-limited beam; for example in printing, activating chemicals, or 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 lateral 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. Note that these lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously.
The wavelength emitted is a function of the band-gap of the semiconductor and the modes of the optical cavity. In general, the maximum gain will occur for photons with energy slightly above the band-gap energy, and the modes nearest the gain peak will lase most strongly. If the diode is driven strongly enough, additional side modes may also lase. Some laser diodes, such as most visible lasers, operate at a single wavelength, but that wavelength is unstable and changes due to fluctuations in current or temperature.
Due to ubiquitous diffraction, the transverse pattern of the laser light leaving the diode from the thin active region will undergo a Fourier transform of intensity very quickly, and will need a collimating lens to make the light a beam. The beam divergence away from the plane of the active region will by far be the highest, and thus for broad area lasers, the lenses most often used are cylindrical. For single spatial mode lasers, using symmetrical lenses, the collimated beam ends up being elliptical in shape, since the divergence is wider in the vertical direction than the lateral direction. This is easily observable with a red laser pointer.
The simple diode described above has been heavily modified in recent years to accommodate modern technology, resulting in a variety of types of laser diodes, as described below.
Laser diode typesEdit
The simple laser diode structure described above is extremely 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 lasersEdit
The first laser diode to achieve continuous wave operation was a double heterostructure demonstrated by Zhores Alferov of the Soviet Union. 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 (AlxGa(1-x)As). Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or 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" 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 from the heterojunction; hence, the light is confined to the region where the amplification takes place.
Quantum well lasersEdit
If the middle layer is made thin enough, it starts acting like a quantum well. This means that in the vertical direction, electron energy is quantised. The difference between quantum well energy levels can be used for the laser action instead of the bandgap. This is very useful since the wavelength of light emitted can be tuned simply by altering the thickness of the layer. The great majority of quantum well lasers operate as inter-band devices however, with the quantum-well energy levels acting as small corrections to the bandgap energy. The efficiency of a quantum well laser is greater than that of a bulk laser due to a tailoring of the distribution of electrons and holes that are involved in the stimulated emission (light producing) process.
Separate confinement heterostructure lasersEdit
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 centre 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.
Distributed feedback lasersEdit
Distributed feedback lasers (DFB) 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 only a single wavelength to be fed back to the gain region and lase. Thus at least one facet of a DFB is 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. Such lasers are the workhorse of demanding optical communications
VCSELsEditVertical cavity surface emitting lasers (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 Fig. 2. The reflectors at the ends of the cavity are dielectric mirrors 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 λ if the thicknesses of alternating layers d1 and d2 with refractive indices n1 and n2 are such that n1d1 + n2d2 = (1/2)λ which then leads to the constructive interference of all partially reflected waves at the interfaces. Because of the high mirror reflectivities, VCSELs have lower output powers when compared to edge emitting lasers.
VECSELs are small, tunable semiconductor lasers similar to VCSELs.
Applications of laser diodesEdit
Laser diodes are important electronic parts. They find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication. They are used in various measuring instruments, eg. rangefinders. Another common use is in barcode readers. Visible lasers, typically red but recently also green, are common as laser pointers. Infrared and red laser diodes are common in CD players, CD-ROMs and DVD technology. Blue lasers will find their use in upcoming HD-DVD and Blu-Ray technology. High-power laser diodes are used in industrial applications such as heat treating, cladding, and seam welding. The use of diode lasers for high speed, low cost, combustion spectroscopy is being explored.
Before the development of reliable laser diodes, small helium-neon lasers were used in CD players and barcode scanners.