A packaged laser diode with
penny for scale.
Image of the actual laser diode chip (shown on the eye of a needle for scale) contained within the package shown in the above
image.
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.
Diagram (not to scale) of a simple laser 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. 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 diffraction, the beam diverges (expands) 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
Diagram of front view of a double heterostructure laser diode (not to scale).
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 lasers
Diagram of front view of a simple quantum well laser diode (not to scale).
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
Diagram of front view of a separate confinement heterostructure quantum well laser diode.
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 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. 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
Diagram of a simple VCSEL structure.
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.
There are several advantages to producing VCSELs when compared with the production process of edge-emitting lasers.
Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not work, whether due to bad
contacts or poor material growth quality, the production time and the processing materials have been wasted. Additionally,
because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens
of thousands of VCSELs can be processed simultaneously on a three inch Gallium Arsenide wafer. Furthermore, even though the VCSEL
production process is more labor and material intensive, the yield can be controlled to a more predictable outcome.
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 = ½λ
which then leads to the constructive interference of all partially reflected waves at the interfaces. But there is a disadvantage
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 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. A typical distance from the diode to the
external mirror would be 1 cm.
One of the most interesting features of any VECSEL is the thin-ness of the semiconductor gain region in the direction of
propagation, less than 100 nm. In contrast, a conventional in-plane semiconductor laser entails light propagation over distances
of from 250 µm upward to 2 mm or longer. The significance of the short propagation distance is that it causes the effect of
"antiguiding" nonlinearities in the diode laser gain region to be minimized. The result is a large-cross-section single-mode
optical beam which is not attainable from in-plane ("edge-emitting") diode lasers.
Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications including high
power sources for use in industrial machining (cutting, punching, etc.) 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.
Failure modes
Laser diodes have similar 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 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
(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.
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 (less so for InP based lasers used for long-haul telecommunications which emit between 1.3 µm and 2 µm), 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 it functions as an anti-reflective coating, reducing reflection at the surface. 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 (NAM)
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 (Ruschlikon, Switzerland) 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 (employed to pump solid state lasers) 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.
Applications of laser diodes
Laser diodes can be arrayed to produce very high power (continuous wave or pulsed) outputs. Such arrays may be used to
efficiently pump solid state lasers for
inertial confinement fusion or high
average power drilling or burning applications.
Laser diodes are numerically the most common type of laser, with 2004 sales of approximately 733
million diode lasers,[1] as compared to 131,000 of other
types of lasers.[2]
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 later 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
(input) of images and for very high-speed and high-resolution printing plate (output) manufacturing. Infrared and red laser diodes are common in CD players,
CD-ROMs and DVD technology. Violet lasers are used in 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, such as diode pumped solid
state 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 (power, wavelength & spectral quality, beam quality,
polarization, etc.) 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 range-finding, 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 (in the UV), and photodynamic
therapy (where a particular wavelength of light would cause a substance such as porphyrin to
become chemically active as an anti-cancer agent only where the tissue is illuminated by light).
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.
History
The first to demonstrate coherent light emission from a semiconductor diode (the
first laser diode), included Robert N. Hall and his team at the General Electric
research center in November 1962.[3]
Other teams at IBM, MIT Lincoln Laboratory, Texas Instruments, and RCA Laboratories were also involved in and receive credit
for historic initial demonstrations of efficient light emission and lasing in semiconductor diodes in 1962 and thereafter.
The first visible light laser diode was demonstrated by Nick Holonyak, Jr., later in
1962[4]
The first laser diode to achieve continuous wave operation was a
double heterostructure demonstrated essentially simultaneously by
Zhores Alferov and collaborators (including Dmitri
Z. Garbuzov) of the Soviet Union, and Morton Panish
and Izuo Hayashi working in the United States.
See also
References
- Zheludev, N. (2007). The life and times of the LED - a 100-year history. Nature Photonics 1(4), 189-192 ( For LED )
- B. Van Zeghbroeck's Principles of Semiconductor Devices( for direct and indirect band gaps)
- Saleh, Bahaa E. A. and Teich, Malvin Carl (1991). Fundamentals of Photonics. New York: John Wiley & Sons. ISBN
0-471-83965-5. ( For Stimulated Emission )
- Koyama et al, Fumio (1988), "Room temperature cw operation of GaAs vertical cavity surface emitting laser", Trans. IEICE,
E71(11): 1089-1090( for VCSELS)
- Iga, Kenichi (2000), "Surface-emitting laser—Its birth and generation of new optoelectronics field", IEEE Journal of Selected
Topics in Quantum Electronics 6(6): 1201–1215(for VECSELS)
External links
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