Other diodes: Diode types
There are several different types of laser diode. Each laser diode type has its own characteristics and is used for different applications.
There are two major categories of laser diode that may be used - one generates the light emission itself, while the other utilises an external source.
Laser diode major categories
There are two major categories of semiconductor laser diodes. They operate in quite different ways, although many of the concepts used within them are very similar.
- Injection Laser Diode: The Injection laser diode, ILD, has many factors in common with light emitting diodes. They are manufactured using very similar processes. The main difference is that laser diodes are manufactured having a long narrow channel with reflective ends. This acts as a waveguide for the light.
In operation, current flows through the PN junction and light is generated using the same process that generates light in a light emitting diode. However the light is confined within the waveguide formed in the diode itself. Here the light is reflected and then amplified as a result of stimulated emission before exiting though one end of the laser diode as the external beam.
- Optically Pumped Semiconductor Laser: Optically pumped semiconductor laser, OPSL uses a III-V semiconductor chip as its basis. This acts as an optical gain medium, and another laser which may be an ILD is used as the pump source. The optical gain is provided by stimulated emission. The OPSL approach offers several advantages, particularly in wavelength selection and lack of interference from internal electrode structures.
Main laser diode types
Some of the main types of laser diode include the following types:
- Double heterostructure laser diode : The double heterojunction laser diode is made up by sandwiching a layer of a low bandgap material with a layer on either side of high bandgap layers. This makes the two heterojunctions as the materials themselves are different and not just the same material with different types of doping. Common materials for the double heterojunction laser diode are Gallium Arsenide, GaAs, and aluminium gallium arsenide, AlGaAs.
The advantage of the double heterojunction laser diode over other types is that the holes and electrons are confined to the thin middle layer which acts as the active region. By containing the electrons and holes within this area more effectively, more electron-hole pairs are available for the laser optical amplification process. Additionally the change in material at the heterojunction helps contain the light within the active region providing additional benefit.
- Quantum well laser diode: The quantum well laser diode uses a very thin middle layer - this acts as a quantum well where the vertical component of the electron wave function is quantised. As the quantum well has an abrupt edge, this concentrates electrons in energy states that contribute to laser action, and this increases the efficiency of the system.
In addition to the single quantum well laser diodes, multiple quantum well laser diodes also exist. The presence of multiple quantum wells improves the overlap between the gain region and the optical waveguide mode.
- Quantum cascade laser diode: This is a form of heterojunction laser diode which the difference between well energy levels is used to provide the laser light generation. This allows the laser diode to generate relatively long wavelength light - the actual wavelength can be adjusted during fabrication by altering the laser diode layer thickness.
- Separate confinement heterostructure laser diode: This form of laser diode has been widely used for the majority of laser diodes since the 1990s. The separate confinement laser diode overcomes the problem that in many other forms of laser diode the thin laser layer is too thin to confine the light effectively. This laser diode overcomes the problem by adding another two layers with a lower refractive index on the outside of the existing ones. This effectively confines the light to within the diode.
- Distributed feedback laser diode: Distributed feedback laser diodes, DFB, are used in forms of telecommunications or data transmission using optical systems. Here the laser diode wavelength is important, but laser diodes are not particularly stable in this respect with wavelength varying with temperature, voltage, ageing, etc.. A diffraction grating is etched close to the p-n junction of the diode to assist in stabilizing the wavelength of the generated light signal, This grating acts like an optical filter causing a single wavelength to be fed back to the gain region. The pitch of the grating is set during manufacture, and this only varies slightly with temperature.
- VCSEL: This form of laser diode is a Vertical-Cavity Surface-Emitting Laser diode These are a form of surface emitting laser and they emit the laser radiation in a direction perpendicular to the wafer, delivering a few milliwatts with high beam quality.
- External cavity laser diode: External cavity lasers contain a laser diode as the gain medium only of a longer laser cavity. These lasers are often wavelength-tuneable and they exhibit a small emission spectral line width.
- VECSEL: The VECSEL is a Vertical External Cavity Surface Emitting Laser diode. These are a form of optically pumped surface-emitting external-cavity semiconductor laser, capable of generating multi-watt output powers with excellent beam quality, even in mode-locked operation.
There are many laser diode types. Each type of laser diode has its own features and by choosing the correct type of laser diode for the given application, the right performance can be obtained.
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