A laser diode is electrically a P-i-n diode. The active region of the laser diode is in the
intrinsic (I) region, and the carriers, electrons and holes, are pumped into it
from the N and P regions respectively. While initial diode laser research was
conducted on simple P-N diodes, all modern lasers use the
double-heterostructure implementation, where the carriers and the photons are
confined in order to maximize their chances for recombination and light
generation. Unlike a regular diode used in electronics, the goal for a laser
diode is that all carriers recombine in the I region, and produce light. Thus,
laser diodes are fabricated using direct bandgap semiconductors. The laser
diode epitaxial structure is grown using one of the crystal growth
techniques, usually starting from an N doped substrate, and growing the I doped
active layer, followed by the P doped cladding, and a contact layer. The active layer most often
consists of quantum wells, which provide lower threshold
current and higher efficiency.
Laser diodes form a subset of the larger
classification of semiconductor p-n junction diodes. Forward
electrical bias across the laser diode 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. Holes are injected
from the p-doped, and electrons from the n-doped, semiconductor.
(A depletion region, devoid of any charge carriers,
forms as a result of the difference in electrical potential between n-
and p-type semiconductors wherever they are in physical contact.) Due to
the use of charge injection in powering most diode lasers, this class of lasers
is sometimes termed "injection lasers," or "injection laser
diode" (ILD). As diode lasers are semiconductor devices, they may also be
classified as semiconductor lasers. Either designation distinguishes diode
lasers from solid-state lasers.
Another method of powering some diode lasers is the
use of optical pumping. Optically pumped semiconductor
lasers (OPSL) use a III-V semiconductor chip as the gain medium, and another
laser (often another diode laser) as the pump source. OPSL offer several
advantages over ILDs, particularly in wavelength selection and lack of
interference from internal electrode structures.
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. (In a conventional
semiconductor junction diode, the energy released from the recombination of
electrons and holes is carried away as phonons, i.e.,
lattice vibrations, rather than as photons.) 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 one among several sources of
inefficiency once the laser is oscillating.
The difference between the photon-emitting
semiconductor laser and conventional phonon-emitting (non-light-emitting)
semiconductor junction diodes lies 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."
Other materials, the so-called compound semiconductors, have virtually
identical crystalline structures as silicon or germanium but use alternating
arrangements of two different atomic species in a checkerboard-like pattern to
break the symmetry. The transition between the materials in the alternating
pattern creates the critical "direct bandgap" property. 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, termed the "upper-state
lifetime" or "recombination time" (about a nanosecond for
typical diode laser materials), 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; therefore 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–Pérot 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 transverse direction, if the waveguide is wide compared to the
wavelength of light, then the waveguide can support multiple transverse optical modes, and the laser is known as
"multi-mode". These transversely 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 transverse 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 peak of the gain curve will lase most strongly. The width of
the gain curve will determine the number of additional "side modes"
that may also lase, depending on the operating conditions. Single spatial mode
lasers that can support multiple longitudinal modes are called Fabry Perot (FP)
lasers. An FP laser will lase at multiple cavity modes within the gain
bandwidth of the gain medium. The number of lasing modes in an FP laser is
usually unstable, and can fluctuate due to changes in current or temperature.
Single spatial mode diode lasers can be designed so as
to operate on a single longitudinal mode. These are single frequency diode
lasers exhibit a high degree of stability, and are used in spectroscopy and
metrology, and as frequency references. Single frequency diode lasers are
either distributed feedback (DFB) lasers or distributed Bragg reflector (DBR)
lasers.
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.
SOURCE : http://en.wikipedia.org/wiki/Laser_diode
