Encyclopedia
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 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.
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
majority carrier. Similarly,
electrons from the n-region are injected into the p-region, where holes are the majority carrier. 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 an
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 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 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
diffraction, the beam diverges rapidly after leaving the chip, typically at 30 degrees vertically by 10 degrees laterally.
A lens must be used in order to form a collimated beam like that produced by a laser pointer.
If a circular beam is required, 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 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 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
The first laser diode to achieve
continuous wave operation was a double heterostructure demonstrated essentially simultaneously by
Zhores Alferov of the
Soviet Union, and Morton Panish and Izuo Hayashi working in the United States .
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 with
aluminium gallium arsenide . 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 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. The efficiency of a quantum well laser is greater than that of a bulk laser because the density of states function of electrons in the quantum well system has an abrupt edge that concentrates electrons in energy states that contribute to laser action.
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.
Further improvements in the laser efficiency have also been demonstrated by reducing the quantum well layer to a quantum wire or to a "sea" of
quantum dots.
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. As of 2005, quantum cascade lasers have not yet been widely commercialized.
Separate confinement heterostructure lasers
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 laser diode.
Almost all commercial laser diodes since the 1990s have been
SCH quantum well diodes.
Distributed feedback lasers
Distributed feedback lasers 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 communication
VCSELs
Vertical cavity surface emitting lasers 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 ? if the thicknesses of alternating layers d
1 and d
2 with refractive indices n
1 and n
2 are such that n
1d
1 + n
2d
2 = ? 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
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 seperately 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. A typical distance from the diode to the external mirror would be 1 cm. Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications including high power sources for use in industrial machining because of their unusually high power and efficiency when pumped by multi-mode diode laser bars.
Electrically pumped VECSELs have also been demonstrated. Applications for electrically pumped VECSELs include projection displays, served by frequency doubling of near-IR VECSEL emitters to produce blue and green light.
Applications of laser diodes
Laser diodes are numerically the most common type of laser, with 2004 sales of approximately 733 million diode lasers , as compared to 131,000 of other types of lasers .
Laser diodes 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. Both low and high-power diodes are used extensively in the printing industry both as light sources for scanning of images and for very high-speed and high-resolution printing plate manufacturing.
Infrared and red laser diodes are common in
CD players,
CD-ROMs and
DVD technology.
Blue-violet 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, seam welding and for pumping other lasers . The use of diode lasers for high-speed, low-cost, combustion spectroscopy is being explored.
In general, applications of laser diodes can be categorized in various ways. Most applications of diode lasers can be served by larger solid state lasers or optical parametric oscillators but it is the ability to mass-produce diode lasers at low cost that makes them essential for mass-market applications. Diode lasers have application to virtually every field of endeavor that attracts wide attention today. Since light has many different properties it is interesting to classify applications by these basic properties.
Many applications of diode lasers primarily make use of the "directed energy" property of an optical beam. In this category one might include the laser printers, bar-code readers, image scanning, illuminators, designators, optical data recording, combustion ignition, laser surgery, industrial sorting, industrial machining, and directed energy weaponry. Some of these applications are emerging whereas many are familiar to the wider society.
Applications which may today or in the future make use of the "coherent" properties of diode-laser-generated light include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions.
Applications which may make use of "narrow spectral" properties of diode lasers include
telecommunications, infra-red countermeasures, spectroscopic sensing, generation of radio-frequency or terahertz waves, atomic clock state preparation, quantum key cryptography, frequency doubling and conversion, water purification , and photodynamic therapy .
Applications where the ability to "generate ultra-short pulses of light" by the technique known as "mode-locking" include clock distribution for high-performance integrated circuits, high-peak-power sources for laser-induced breakdown spectroscopy sensing, arbitrary waveform generation for radio-frequency waves, photonic sampling for analog-to-digital conversion, and optical code-division-multiple-access systems for secure communication.
Failure modes
Laser diodes have similar
reliability and failure issues as
light emitting diodes. In addition, they are subject to
catastrophic optical damage when operated at higher power.
Many of the advances in reliability of diode lasers in the last 20 years remains proprietary to its developers. The reliability of a laser diode can make or break a product line. Moreover, "reverse engineering" is not always able to uncover 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 [110] crystallographic plane in III-V semiconductor crystals 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 by the termination of the perfectly periodic lattice at that plane. Surface states at the cleaved plane, have energy levels within the bandgap of the semiconductor.
Essentially as a result, when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal, a fraction of that light energy is absorbed by the surface states whence it is converted to heat by
phonon-
electron interactions. This heats the cleaved mirror. In addition, the mirror may heat simply because the edge of the diode laser—which is electrically pumped—is in less-perfect contact with the mount that provides a path for heat removal. The heating of the mirror causes the bandgap of the semiconductor to shrink in the warmer areas. The bandgap shrinkage brings more electronic band-to-band transitions into alignment with the photon energy causing yet more absorption. This is thermal runaway, a form of positive feedback, and the result can be melting of the facet, known as
catastrophic optical damage, or COD.
In the 1970's, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 1 µm and 0.630 µm wavelengths , was identified. Michael Ettenberg, a researcher and later Vice President at
RCA Laboratories'
David Sarnoff Research Center in
Princeton, New Jersey, devised a solution. A thin layer of aluminum oxide was deposited on the facet. If the aluminum oxide thickness is chosen correctly little reflectivity at the wavelength of emission and is referred to as an "AR" coating . This alleviated the heating and COD at the facet.
Since then, various other refinements have been employed. One approach is to create a so-called non-absorbing mirror such that the final 10 µm or so before the light emits from the cleaved facet are rendered non-absorbing at the wavelength of interest.
In the very early 1990s, SDL, Inc. began supplying high power diode lasers with good reliability characteristics. CEO Donald Scifres and CTO David Welch presented new reliability performance data at, e.g., SPIE Photonics West conferences of the era. The methods used by SDL to defeat COD were considered to be highly proprietary and have still not been disclosed publicly as of June, 2006.
In the mid-1990s, IBM Research announced that it had devised its so-called "E2 process" which conferred extraordinary resistance to COD in GaAs-based lasers. This process, too, has never been disclosed as of June, 2006.
Reliability of high-power diode laser pump bars remains a difficult problem in a variety of applications, in spite of these proprietary advances. Indeed, the physics of diode laser failure is still in the process of being worked out and research on this subject remains active, if proprietary.
Extension of the lifetime of laser diodes is critical to their continued adaptation to a wide variety of applications.
See also
- Laser diode rate equations
- Collimator
- Superluminescent diode
References
- Kincade, Kathy and Stephen Anderson "Laser Marketplace 2005: Consumer applications boost laser sales 10%", Laser Focus World, vol. 41, no. 1.
- Steele, Robert V. "Diode-laser market grows at a slower rate", Laser Focus World, vol. 41, no. 2.
External links
- by Samuel M. Goldwasser
- by Power Technology, Inc.
- Edge-emitting lasers
