Science Fair Project Encyclopedia
A laser diode is a laser where the active medium is a semiconductor p-n junction similar to that found in a light-emitting diode. Laser diodes are sometimes referred to (somewhat redundantly) as injection laser diodes or by the acronyms LD or ILD.
Principle of operation
When a diode is forward biased, holes from the p-region are injected into the n-region, and electrons from the n-region are injected into the p-region. If electrons and holes are present in the same region, they may radiatively recombine —that is, the electron "falls into" the hole and emits a photon with the energy of the bandgap. This is called spontaneous emission, and is the main source of light in a light-emitting diode.
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. If a photon of exactly the right frequency happens along within this time period, recombination may be stimulated by the photon. This causes another photon of the same frequency to be emitted, with exactly the same direction, polarization and phase as the first photon.
In a laser diode, the semiconductor crystal is fashioned into a shape somewhat like a piece of paper—very thin in one direction and rectangular in the other two. An optical waveguide is made on that piece of paper, such that the light is confined to a relatively narrow line. The top of the crystal is n-doped, and the bottom is p-doped, resulting in a large, flat p-n junction. The two ends of the crystal are cleaved so as to form perfectly smooth, parallel edges; two reflective parallel edges form a resonator called a Fabry-Perot cavity. Photons emitted in precisely the right direction will travel along the waveguide and be reflected several times from each end face before they are emitted. Each time they pass through the cavity, the light is amplified by stimulated emission. Hence, if there is more amplification than loss, the diode begins to "lase".
Lasers come in many types. Generally, in the vertical direction, the layers are very thin, and thus the structure supports only a single optical mode in the direction perpendicular to the layers. In the lateral direction, if the waveguide is really wide compared to the wavelength of light, then there are multiple lateral optical modes that are supported by the waveguide, and the laser is known as "multi-mode". In cases where one needs a very large amount of power and does not need a small diffraction limited beam, for example, where one is not coupling to single mode fiber or writing small pixel for optical storage, these laterally multi-mode lasers are adequate. They are used for printing, activating chemicals, or pumping other types of lasers. In applications where a small focused beam is needed, the waveguide has to be made rather 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 between p and n region energy levels. No photons with higher energy than the band-gap will be emitted.
Due to ubiquitous diffraction, 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. Just like a wave packet spreading, 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.
In a simple device, as described above, there is no mechanism to select a particular wavelength, except the length of the cavity itself. Since there are multiple longitudinal modes in a long cavity and the material has optical gain over a fairly large wavelength range, such lasers often operate at multiple wavelengths. In some cases, such as most visible lasers, they operate at a single wavelength, but that wavelength is unstable and changes due to fluctuations in current or temperature. This most simple diode described above has been heavily modified in recent years to accommodate modern technology.
Laser diode types
The type of laser diode just described is called a homojunction laser diode, for reasons which should soon become clear. Unfortunately, they are extremely inefficient. They require so much power that they can only be operated in short "pulses;" otherwise the semiconductor would melt. 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 GaAs with AlGaAs. 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 is 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 lasers
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 efficiency of a quantum well laser is greater than that of a bulk laser due to a tailoring of the distrubution of electrons and holes that are involved in the stimulated emission (light producing) process.
The problem with these devices 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 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 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
VCSELs are semiconductor lasers that emit light perpendicular to the chip surface, contrary to conventional diode lasers that are edge-emitting (in-plane) devices.
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