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.

laser tryps and importance

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