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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 electric current.
Theory Of Operation
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.
The many types of diode lasers known today collectively form a subset of the larger classification of semiconductor p-n junction diodes. Just as in any semiconductor p-n junction diode, forward electrical bias causes the two species of charge carrier - holes and electrons - to be "injected" from opposite sides of the p-n junction into the depletion region, situated at its heart. Holes are injected from the p-doped, and electrons from the n-doped, semiconductor.
As charge injection is a distinguishing feature of diode lasers as compared to all other lasers, diode lasers are traditionally and more formally called "injection lasers." When an electron and a hole are present in the same region, they may recombine or "annihilate" with the result being spontaneous emission — i.e., 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.
Conventional phonon-emitting (non-light-emitting) semiconductor junction diodes lie in the use of a different type of semiconductor, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "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."
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.
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 and termed the "upper-state lifetime" or "recombination time"
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; therefore silicon is not a common material for laser diodes.
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 wave guide 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.
Laser Diode Types
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 Lasers
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.
Quantum Well Lasers
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 quantised..
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 mode.
Quantum Cascade Lasers
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 wavelengths, which can be tuned simply by altering the thickness of the layer. They are heterojunction lasers.
Sepa 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.
Distributed Feedback Lasers
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 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. , at least one facet of a DFB is anti-reflection coated.
Vertical-cavity surface-emitting laser
Vertical-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.
Vertical external-cavity surface-emitting lasers, or VECSELs, are similar to VCSELs. In VCSELs, the mirrors are typically grown 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
Failure Of Laser Diodes
Laser diodes have the same reliability and failure issues as light emitting diodes. In addition they are subject to catastrophic optical damage (COD) when operated at higher power.
Many of the advances in reliability of diode lasers in the last 20 years remain proprietary to their developers. The reliability of a laser diode can make or break a product line. Moreover, "reverse engineering" is not always able to reveal the differences between more-reliable and less-reliable diode laser products.
At the edge of a diode laser, where light is emitted, a mirror is traditionally formed by cleaving the semiconductor wafer to form a specularly reflecting plane. This approach is facilitated by the weakness of the crystallographic plane in III-V semiconductor crystals (such as GaAs, InP, GaSb, etc.) compared to other planes.
A scratch made at the edge of the wafer and a slight bending force causes a nearly atomically perfect mirror-like cleavage plane to form and propagate in a straight line across the wafer.
But it so happens that the atomic states at the cleavage plane are altered (compared to their bulk properties within the crystal) by the termination of the perfectly periodic lattice at that plane. Surface states at the cleaved plane, have energy levels within the (otherwise forbidden) bandgap of the semiconductor.
Applications Of Laser Diode
Laser diodes find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication. Another common use is in barcode readers. Visible lasers, typically red but later also green, are common as laser pointers.
Applications of laser diodes can be categorized in various ways. Most applications could be served by larger solid state lasers or optical parametric oscillators, but the low cost of mass-produced diode lasers makes them essential for mass-market applications. Diode lasers can be used in a great many fields; since light has many different properties (power, wavelength and spectral quality, polarization, etc.) it is interesting to classify applications by these basic properties.
Applications which may today or in the future make use of the coherence of diode-laser-generated light include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions.
- 405 nm - InGaN blue-violet laser, in Blu-ray Disc and HD DVD drives
- 473 nm - Bright blue laser pointers, still very expensive, output of DPSS systems
- 780 nm - Compact Disc drives
- 808 nm - pumps in DPSS Nd:YAG lasers .
- 1310 nm - fiber-optic communication
At Diode Laser Concepts product quality and customer satisfaction are our top priorities. The company continually invests significant energy and resources in the development of a product whose reliability is proven repeatedly by its low field failure rate.
It is this product reliability that provides the driving force behind DLC's comprehensive Quality Management System. DLC Management believes quality assurance is the responsibility of each employee within the company. Therefore each employee is empowered to stop production on any work being processed and ask for an engineering review. This allows DLC, through continuous quality and process improvement, to provide its customers with product and service that consistently meets or exceeds their quality and reliability needs.
Laser Diodes: Separate Confinement Issues
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Laser Diodes: Horizontal Design
Improved wavelength stability and control can be obtained using distributed Bragg reflectors by convention, when the distributed reflectors are within the active laser cavity the laser is called a DFB laser and when they are outside the active region on either end ofthe device the laser is called a DBR laser. Examples of each are