Download Group 5 - Index of

Semiconductor Lasers
Aashwinder Lubana
Brian Urbanczyk
Harpaul Singh Kumar
Kunal Chopra
Introduction
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Light Amplification by Stimulated Emission of Radiation.
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Laser light is monochromatic, coherent, and moves in the same direction.
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A semiconductor laser is a laser in which a semiconductor serves as a photon source.
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The most common semiconductor material that has been used in lasers is gallium
arsenide.
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Einstein’s Photoelectric theory states that light should be understood as discrete lumps of
energy (photons) and it takes only a single photon with high enough energy to knock an
electron loose from the atom it's bound to.
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Stimulated, organized photon emission occurs when two electrons with the same energy
and phase meet. The two photons leave with the same frequency and direction.
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In 1916 Einstein devised an improved fundamental statistical theory of heat, embracing the
quantum of energy. His theory predicted that as light passed through a substance it could
stimulate the emission of more light. This effect is at the heart of the modern laser.
P- and N-type Semiconductors
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In the compound GaAs, each gallium atom has three electrons in its
outermost shell of electrons and each arsenic atom has five. When a trace
of an impurity element with two outer electrons, such as zinc, is added to
the crystal. The result is the shortage of one electron from one of the pairs,
causing an imbalance in which there is a “hole” for an electron but there is
no electron available. This forms a p-type semiconductor.
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When a trace of an impurity element with six outer electrons, such as
selenium, is added to a crystal of GaAs, it provides on additional electron
which is not needed for the bonding. This electron can be free to move
through the crystal. Thus, it provides a mechanism for electrical
conductivity. This type is called an n-type semiconductor.
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Pictorial View
Under forward bias (the p-type side is
made positive) the majority carriers,
electrons in the n-side, holes in the p-side,
are injected across the depletion region in
both directions to create a population
inversion in a narrow active region.
The light produced by radioactive
recombination across the band gap is
confined in this active region
Early Lasers
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The first laser diodes
were developed in the
early 1960s
The device shown is an
early example. It would
require very high current
flow to maintain a
population inversion, and
due to the heat generated
by the steady-state
current, the device would
be destroyed quickly.
Laser Focus World http://lfw.pennnet.com/Articles/Article_Display.cfm?Section=ARCHI&Subsection=Display&ARTICLE_ID=101065
Different types of Lasers are discussed
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Vertical Cavity Surface-Emitting Lasers
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The VCSEL emits its coherent energy perpendicular to the boundaries between the layers.
The vertical in VCSEL arises from the fact that laser diodes are typically diagrammed
showing the boundaries as horizontal planes.
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The divergence of a laser beam is inversely proportional to the beam size at the source—
the smaller the source, the larger the divergence.
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The cavity is along the vertical direction, with a very short length, typically 1-3 wavelengths
of the emitted light.
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The reflectivity required for low threshold currents is greater than 99.9%, Distributed Bragg
Reflectors (DBRs) are needed for this reflectivity.
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DBRs are formed by laying down alternating layers of semiconductor or dielectric materials
with a difference in refractive index.
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The DBR layers also carry the current in the device, therefore, more layers increase the
resistance of the device. As a result, dissipation of heat and growth may become a
problem if the device is poorly designed.
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Materials used include gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs),
and indium gallium arsenide nitride (InGaAsN).
Examples of
VCSELs:
VCSELs have been
constructed that emit
energy at 850 and
1300 nanometers,
which is in the near
infrared portion of the
electromagnetic
spectrum.
http://britneyspears.ac/physics/vcsels/vcsels.htm
Metallic Reflector VCSEL
Etched Well VCSEL
Air Post VCSEL
Buried Regrowth VCSEL
Advantages of VCSEL vs. Edge
Emitting Diode Lasers
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The VCSEL is cheaper to manufacture in quantity
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Easier to test on wafer
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More efficient
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The VCSEL requires less electrical current to produce a given
coherent energy output.
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The VCSEL emits a narrow, more nearly circular beam than traditional
edge emitters (used in optical fiber)
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Wavelength is “tunable”
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Efficiency and speed of data transfer is improved for fiber optic
communications
Quantum Cascade Lasers
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When an electric current flows through a quantum-cascade laser,
electrons cascade down an energy staircase emitting a photon at each
step.
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It is composed of a sliver of semiconductor material. Inside, electrons
are constrained within layers of gallium and aluminum compounds,
called quantum wells, which are a few nanometers thick.
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The electrons jump from one energy level to another, and tunnel from
one layer to the next going through energy barriers separating the
wells. When the electrons jump, they emit photons of light.
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When the lower-energy electron leaves the first well, it enters a region
of material where it is collected and sent to the next well.
Pictorial View
The invisible beam from a highpower quantum cascade laser lights
a match. It emits an optical power in
excess of 200 mW from each facet at
a wavelength of 8.0 µm.
Benefits of QC Lasers
http://www.bell-labs.com/org/physicalsciences/projects/qcl/qcl1.html
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Typically 25 to 75 active wells
are arranged in a QC laser,
each at a slightly lower energy
level than the one before -thus producing the cascade
effect, and allowing 25 to 75
photons to be created per
electron journey.
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By simply changing the
thickness of the
semiconductor layers, the
laser's wavelength can be
changed as well.
The QCL can be regarded as an ‘’electronic
waterfall’’. When a proper bias is applied and
an electric current flows through the laser
structure, electrons cascade down an energy
staircase, and every time they fall down a step
they emit a photon
Quantum Dot Lasers
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Self-organized quantum dot lasers are grown by metal-organic vapor phase epitaxy (MOVPE),
molecular beam epitaxy (MBE), and Stranski-Krastanow method
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Three dimensionally quantum-confined structures, quantum dots, provide atomic-like energy levels
and a delta function density of states.
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Significant milestones in the development of the quantum dot lasers include demonstration of:
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low threshold at room temperature
large differential gain
high output power
wide spectral tunability
better temperature insensitivity of the threshold current than quantum well lasers.
Quantum Dot Lasers
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Used in fields such as fiber-optic
communications and pump sources
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The discrete energy levels in quantum dots
provide for unique laser applications: the
lasing in self assembled quantum dot
devices has been shown to exist for ground
and excited state transitions, which allows for
controlled wavelength switching.
Application of Lasers
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In telecommunications they send signals for thousands of kilometers along
optical fibers.
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In consumer electronics, semiconductor lasers are used to read the data on
compact disks and CD-ROMs.
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The power and tuning range properties of QC lasers makes it ideal for detection
of gases and vapors in a smokestack.
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VCSEL has been proved to be an efficient emitter for fiber data communication
in the speed range of 100Mbps to 1Gbps.
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Medical lasers are used because of their ability to produce thermal, physical,
mechanical and welding effects when exposed to tissues. Some of the
applications of lasers include stone removal (laser lithotripsy), activation of
specific drugs or molecules and denaturizing of tissues and cells in body.
Lasers are also used by law enforcement agencies to determine the speed and
distance of the vehicles.
Lasers are used for guidance purposes in missiles, aircrafts and satellites and
make up for a potential replacement of ballistic missiles.
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Problems of Nanostructured Lasers
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Good laser production above room
temperature is a problem