laser tryps and importance
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Types of Lasers
There are many different types of lasers, laser medium can be a solid, gas, liquid
or semiconductor. Lasers are commonly designated by the type of lasing material :-
Solid-state lasers have lasing material distributed in a solid matrix (such asthe ruby or neodymium: yttrium-aluminum garnet "Yag" lasers). Theneodymium-Yag laser emits infrared lightat 1,064 nanometers (nm). A
nanometer is 1x10-9 meters.
Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers)have a primary output of visible red light. CO2 lasers emit energy in the far-
infrared, and are used for cutting hard materials.
Excimer lasers (the name is derived from the terms excited and dimers) usereactive gases, such as chlorine and fluorine, mixed with inert gases such as
argon, krypton or xenon. When electrically stimulated, a pseudo molecule
(dimer) is produced. When lased, the dimer produces light in the ultraviolet
range. Dye lasers use complex organic dyes, such as rhodamine 6G, in liquid solution
or suspension as lasing media. They are tunable over a broad range of
wavelengths.
Semiconductor lasers, sometimes called diode lasers, are not solid-statelasers. These electronic devices are generally very small and use low power.
They may be built into larger arrays, such as the writing source in some laser
printers or CD players.
Lasers are divided into several classes depending upon the power or energy of the
beam and the wavelength of the emitted radiation. Laser classification is based on
the laser's potential for causing immediate injury to the eye or skin and/or potentialfor causing fires from direct exposure to the beam or from reflections from
reflective surfaces.
Commercially produced lasers are classified and identified by labels affixed to the
laser. In cases where the laser has been fabricated on campus or is otherwise not
labeled, the LSO should be consulted on the appropriate laser classification and
labeling. Lasers are classified using the physical parameters of power, wavelength,
and exposure duration. A description of laser classes follows.
LASER HAZARD CLASSIFICATIONSThe most important criterion you will use in applying laser safety control measures
is the hazard Classification designated by manufacturers on the equipment labels.
Certain controls are required for each class (except Class 1) listed below:-
Class 1 (exempt lasers) cannot, under normal operating conditions, emit a
hazardous level of optical radiation. Included in this category is laboratory
equipment using lasers with all beam paths and reflections enclosed.
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Class 2, or low-power visible laser device of 1 milli watt, does not have enough
output power to injure a person accidentally, but may injure the eye when stared at
for a long period. A caution label must be on the device. Many HeNe lasers are
Class 2 but not all. These lasers are use d for alignment procedures and optical
experiments.
Class 3a lasers-rated in power from 1 milli watt to 5 milli wattscannot injure a
normal person when viewed with the unaided eye but may cause injury when the
energy is collected and put into the eye as with binoculars. Most laser pointers fall
into this category. A danger or caution sign must label the device, depending on its
irradiance.
Class 3b lasers from 5 milli watts to 500 milli watts can produce eye injury when
viewed without eye protection. This class of laser requires a danger label and could
have dangerous specular reflections. Eye protection is required.
Class 4 lasers above 500 milli watts in power can injure you if viewed directly or by
viewing both the specular and diffuse reflections of the beam. A danger sign will
label this laser. These lasers can also present a fire hazard. Eye and skin protection
is required.
DFB LASSER SINGLE (LONGITODINAL) MODE
Single-longitudinal-mode operation under a gain-switched condition, generating
20ps optical pulses, has been attained in DFB lasers emitting at 1.3m wavelength. It
has been clarified that , which is the difference between the DFB wavelength andthe gain peak wavelength, plays an important role in stable single-mode operation.
The optimum range of was found to be in the range from 15 nm to 0 nm .
Distributed Feedback (DFB) Diode Lasers are fixed wavelength single mode diode
lasers. Typical geometrical sizes of the laser chip are 1000m x 500m x 200m
(length x width x height). The laser chip is grown by MOVPE of compound
semiconductor material. The optical gain is provided by double hetero structure
which includes several Quantum Wells for electronic confinement. Typical emitter
width range between 3m and 7m. The single mode emission is enforced by a
Bragg grating with the laser chip. The surfaces of the laser chip act as cavity mirrors
due to the difference of the refractive index of the laser material and thesurrounding air. The rear facet of the laser chip is provided with a high reflection
coating. The front facet of the laser chip is provided with a high quality
antireflection coating for avoiding the Fabry Perot modes of the laser chip.
Distributed Feedback (DFB) Diode Lasers are available at almost any
wavelength between 760nm and 2800nm.
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DFB wavelength indicated as manufactured on Request within the Specification"
Section is available from stock and is ready to be assembled into the requested type
of housing. Please contact us in case your required wavelength is not included
within the list. Available wavelength range from 760nm to 2800nm. Due to a special
manufacturing method, customized wavelength can be provided with a selection of
below 2nm even for single units. Please contact us with your special wavelength
requirement.
Wavelength selected DFB Lasers are provided for typical applications. Examples are
water vapor, methane and others. Please contact us with your special requirement.
DFB Lasers either TO3-cans with integrated thermoelectric cooler, or TO5 cans with
integrated thermoelectric cooler or butterfly packages with thermoelectric cooler
and single mode fiber coupling.
Coupled Cavity laser
cavity laser is ideally a device with a cavity volume of about one wavelength cubed,
the energy difference between adjacent modes (longitudinal or transverse) is
sufficiently large to couple the semiconductor emission to a single cavity mode, very
small dielectric cavities support continuum (leaky) modes also, rendering dielectric
devices, even those sub-micron size, non-ideal. The field of cavity lasers has grown
quite impressively the last few years and the field is intimately linked to the vertical
cavity semiconductor laser field since the most promising generic cavity structure
is a vertical dielectric post with integrated Bragg mirrors fabricated using epitaxial
deposition or thin film techniques. It is important to realize that a substantial part of
the work in the cavity field has been directed toward the use the miniature cavities
to conduct experimental physics not necessarily toward practical devices or
applications. The micro-cavity laser was proposed by Koybayashi3 and
demonstrated by DeMartini4 using a photo-pumped dye-solution flowing between
two parallel plane dielectric mirrors spaced about a wavelength apart. The
semiconductor dielectric post cavity was pioneered by Jewell5 who had been
making such structures with the initial intention of demonstrating optical bi-
stability. Micro-cavity lasers with various geometries such as planar,6 micro-disc,7'8
hemi-spherical9 and posts.
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Quantum well laser
A quantum well laser is a laser diode in which the active region of the device is so
narrow thatquantum confinementoccurs. The wavelength of the light emitted by aquantum well laser is determined by the width of the active region rather than just
the band gap of the material from which it is constructed, that much shorter
wavelengths can be obtained from quantum well lasers than from conventional
laser diodes using a particular semiconductor material and its efficiency is also
greater than a conventional laser diode due to the stepwise form of its density of
states function.
Concept of quantum wells in 1972, Charles H. Henry, a physicist and newly-
appointed Head of the Semiconductor Electronics Research Department at Bell
Laboratories, had a keen interest in the subject of integrated optics, the fabrication
of optical circuits in which the light travels in waveguides. The problems associated
with waveguides, he had a realization that a double hetero structure is a waveguide
for electron waves, not just light waves. On further reflection, he saw that there is a
complete analogy between the confinement of light by a slab waveguide and the
confinement of electrons by the potential well that is formed from the difference in
band gaps in a double hetero structure.
He realized that there should be discrete modes (levels) in the potential well, and a
simple estimate showed that if the active layer of the hetero structure is as thin as
several tens of nanometers, the electron levels would be split apart by tens of mill
electron volts, which should be observable. This structure is now called a quantum
well.
Henry then calculated how this quantization would alter the optical absorption edge
of the semiconductor and conclusion was instead of the optical absorption
increasing smoothly, the absorption edge of a thin hetero structure would appear as
a series of steps. Contributions, the quantum well (or double-hetero structure laser,
as it was originally known) was actually first proposed in 1963 by Herbert Kroemer
and simultaneously (in 1963) in the U.S.S.R by Zh. I. Alferov and R.F. Kazarinov.
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Experimental verification is early 1973; Henry proposed to R. Dingle, a physicist in
his department, that he look for these predicted steps. The very thin hetero
structures were made by W. Wiegmann using molecular beam epitaxy. The dramatic
effect of the steps was observed in the ensuing experiment, published in 1974.
Invention of the quantum well laser showed the reality of the predicted quantum
well energy levels; Henry tried to think of an application. He realized that the
quantum well structure would alter the density of states of the semiconductor, and
result in an improved semiconductor laser requiring fewer electrons and electron
holes to reach laser threshold. Also, he realized that the laser wavelength could be
changed merely by changing the thickness of the thin quantum well layers, whereas
in the conventional laser a change in wavelength requires a change in layer
composition. Such a laser, he reasoned, would have superior performance
characteristics compared to the standard double hetero structure lasers being madeat that time.
Dingle and Henry received a patent on this new type of semiconductor laser
comprising a pair of wide band gap layers having an active region sandwiched
between them, in which "the active layers are thin enough (e.g., about 1 to 50 nano
metres) to separate the quantum levels of electrons confined therein. These lasers
exhibit wavelength tune ability by changing the thickness of the active layers. Also
described is the possibility of threshold reductions resulting from modification of
the density of electron states. Quantum well lasers require fewer electrons and
holes to reach threshold than conventional double hetero structure lasers and forwell-designed quantum well laser can have an exceedingly low threshold current,
since quantum efficiency (photons-out per electrons-in) is largely limited by optical
absorption by the electrons and holes, very high quantum efficiencies can be
achieved with the quantum well laser for the reduction in active layer thickness, a
small number of identical quantum wells are often used. This is called a multi-
quantum well laser.
Tunable Laser
Tunable lasers
are lasers whose wavelengths can be tuned or varied haviongimportant part in optical communication networks. Recent improvements in
tunable laser technologies are enabling highly flexible and effective utilization of the
massive increases in optical network capacity brought by large-scale application of
dense wavelength division multiplexing. Several tunable laser technologies have
emerged, each with its own set of tradeoffs with respect to the needs of particular
optical networking applications.
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Tunable lasers are produced mainly in 4 ways:
The distributed feedback laser (DFB) The external cavity diode laser, The vertical cavity diode laser and The micro electro mechanical system (MEMS) technology.
Tunable lasers help network administrators to save a lot of cost, by allowing them
to efficiently manage the network with lesser number of spares and also enable
reliable functioning of the optical network, Changing traffic patterns, customer
requirements, and new revenue opportunities require greater flexibility than static
OADMs can provide, complicating network operations and planning. Incorporating
tunable lasers removes this constraint altogether by allowing any channel to be
added by the OADM at any time. In a wavelength-division multiplexed (WDM)
network carrying 128 wavelengths of information, we have 128 different lasers
giving out these wavelengths of light. Each laser is designed differently in order to
give the exact wavelength needed. Even though the lasers are expensive, in case of a
breakdown, we should be able to replace it at a moment's notice so that we don't
lose any of the capacity that we have invested so much money in. So we keep in
stock 128 spare lasers or maybe even 256, just to be prepared for double failures.
if we have a multifunctional laser for the optical network that could be adapted to
replace one of a number of lasers out of the total 128 wavelengths that could be
saved, as well as the storage space for the spares. What is needed for this is a
tunable laser.
Tunable lasers are still a relatively young technology, but as the number of
wavelengths in networks increases so will their importance. Each different
wavelength in an optical network will be separated by a multiple of 0.8 nanometers
sometimes referred to as 100GHz spacing. Current commercial products can cover
maybe four of these wavelengths at a time. While not the ideal solution, this still cuts
your required number of spare lasers down. More advanced solutions hope to be
able to cover larger number of wavelengths, and should cut the cost of spares even
further; the devices themselves are still semiconductor-based lasers that operate onsimilar principles to the basic non-tunable versions. Most designs incorporate some
form of grating like those in a distributed feedback laser. These gratings can be
altered in order to change the wavelengths they reflect in the laser cavity, usually by
running electric current through them. The tuning range of such devices can be as
high as 40nm, which would cover any of 50 different wavelengths in a 0.8nm
wavelength spaced system. Technologies based on vertical cavity surface emitting
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lasers (VCSELs) incorporate moveable cavity ends that change the length of the
cavity and hence the wavelength emitted. Current designs of tunable VCSELs have
similar tuning ranges.
Lasers are devices giving out intense light at one specific color. The kinds of lasers
used in optical networks are tiny devices - usually about the size of a grain of salt.They are little pieces of semiconductor material, specially engineered to give out
very precise and intense light, within the semiconductor material are lots of
electrons - negatively charged particles.
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