2014257036
SELİM
KARAÜZÜM
What is the laser?
Laser beam welding is a welding technique used
to join multiple pieces of metal through the use
of a laser. The beam provides a concentrated
heat source, allowing for narrow, deep welds
and high welding rates. The process is
frequently used in high volume applications,
such as in the automotive industry. It is based
on keyhole or Penetration mode welding.
Working Principle Laser
Optically transparent, fully glazed on one end and reflective, half a tube
will be partially glazed two reflective mirror located at the other end.
This gas, liquid and solid filled. Making external light, by passing
electrical current or energy obtained by a chemical route, reach atom in
the environment. Some of them will absorb this energy. More energy,
makes unstable atoms. He is a photon multiplier, excited and unstable
atoms, photons released, giving more energy.
Photons ensures that other publication of similar photos. Photon that
reach the end, go back and continue the reflection from the mirror events.
Photon excitation in the environment and drive increases. Almost all of
the atoms emit photons of light began to strengthen, flew out of the half
glazed.
Lasers used in industrial
•Soli-State Lasers
•Diode Lasers
•Gas Lasers
•Dye Lasers
DIODE LASERS
Diode Laser what is it?
Diodes by passing current in one direction and blocks the other
direction. With this feature useful in a variety of technical
applications several diodes. Some semiconductor diodes, light hair
when electrically stimulated. (Light emmitting LED diodes). In the
same way as light from the diode laser it can be obtained. The main
characteristics of the laser diode:
1-) Diode lasers, IR wavelengths from 3000 nm to 300 nm UV wave
length range of technological manipulation (including visible light)
rays can produce.
2-) Diode lasers are small, it does not take up much space.
3-) Diode lasers do not generate much heat, usually air cooling
system for cooling is sufficient. Therefore, the heavy work,
overheating result (as in Alexandrite laser) does not detonate
chamber.
4-) Laser diode Lamps on the old radio, in television tubes as
durable and does not easily malfunction. Is the age of 50 inherited
from his grandfather on a radio in your home or in your
neighborhood? Think diode lasers as they can.
GAS LASERS
In these lasers the lasing medium is made-up of one or a mixture of gases
or vapors. Gas lasers can be classified in terms of the type of transitions
that lead to their operation: atomic or molecular. The most common of all
gas lasers is the helium-neon (He-Ne) laser. The presence of two atomic
species (helium and neon) in this gas laser might suggest that the medium
is made of molecules, but these two species of atoms do not form a stable
molecule. In fact, all inert atoms like helium, argon, krypton, etc. (those in
the last column of the Periodic Table) hold tightly to their own electron
clouds and seldom form a molecule or react with other atoms (hence the
name: inert). In the He-Ne laser the transition that produces the output
light is an atomic transition. Gas lasers that employ molecular gas or
vapor for their lasing medium use molecular transitions for their lasing
operation. Molecular transitions tend to be more complex than atomic
ones. As a consequence the laser light produced by molecular lasers tends
to have a wider and more varied collection of properties. Examples of
some common molecular gas lasers are carbon monoxide (CO), carbon
dioxide (CO2), excimer, and nitrogen (N2) lasers.
Helium Neon (He-Ne) Lasers
The He-Ne laser was the first continuous wave (cw) laser invented. A few months after
Maiman announced his invention of the pulsed ruby laser, Ali Javan and his associates
W. R. Bennet and D. R. Herriott announced their creation of a cw He-Ne laser. This
gas laser is a four-level laser that use helium atoms to excite neon atoms. It is the
atomic transitions in the neon that produces the laser light. The most commonly used
neon transition in these lasers produces red light at 632.8 nm. But these lasers can also
produce green and yellow light in the visible as well as UV and IR (Javan's first He-Ne
operated in the IR at 1152.3 nm). By using highly reflective mirrors designed for one
of these many possible lasing transitions, a given He-Ne's output is made to operate at
a single wavelength.
He-Ne lasers typically produce a few to tens of mW (milli-Watt, or 10-3 W) of
power. They are not sources of high power laser light. Probably one of the most
important features of these lasers is that they are highly stable, both in terms of their
wavelength (mode stability) and intensity of their output light (low jitter in power
level). For these reasons, He-Ne lasers are often used to stabilize other lasers. They are
also used in applications, such as holography, where mode stability is important. Until
the mid 1990's, He-Ne lasers were the dominant type of lasers produced for low power
applications - from range finding to scanning to optical transmission, to laser pointers,
etc. Recently, however, other types of lasers, most notably the semiconductor lasers,
seem to have won the competition because of reduced costs.
The above energy level diagram shows the two excited states of
helium atom, the 2 3S and 2 1S, that get populated as a result of the
electromagnetic pumping in the discharge. Both of these states are
metastable and do not allow de-excitations via radiative
transitions. Instead, the helium atoms give off their energy to neon
atoms through collisional excitation. In this way the 4s and 5s levels
in neon get populated. These are the two upper lasing levels, each
for a separate set of lasing transitions. Radiative decay from the 5s
to the 4s levels are forbidden. So, the 4p and 3p levels serve as the
lower lasing levels and rapidly decay into the metastable 3s level. In
this way population inversion is easily achieved in the He-Ne. The
632.8 nm laser transition, for example, involves the 5s and 3p levels,
as shown above.
In most He-Ne lasers the gas, a mixture of 5 parts helium to 1 part
neon, is contained in a sealed glass tube with a narrow (2 to 3 mm
diameter) bore that is connected to a larger size tube called a ballast,
as shown above. Typically the laser's optical cavity mirrors, the
high reflector and the output coupler, form the two sealing caps for
the narrow bore tube. High voltage electrodes create a narrow
electric discharge along the length of this tube, which then leads to
the narrow beam of laser light. The function of the ballast is to
maintain the desired gas mixture. Since some of the atoms may get
imbedded in the glass and/or the electrodes as they accelerate within
the discharge, in the absence of a ballast the tube would not last very
long. To further prolong tube lifetime some of these lasers also use
"getters", often metals such as titanium, that absorb impurities in
the gas.
Above photograph shows a commercial He-Ne tube. The thicker
cylinder closest to the meter-stick (shown for scale) is the
ballast. The thinner tube houses the resonant cavity where the lasing
occurs. Notice the two mirrors that seal the two ends of the
bore. For mode stability reasons, these mirrors are concave; they
serve as the output coupler and the high reflector.
A typical commercially available He-Ne produces about a few mW of
632.8 nm light with a beam width of a few millimeters at an overall
efficiency of near 0.1%. This means that for every 1 Watt of input
power from the power supply, 1 mW of laser light is produced. Still,
because of their long operating lifetime of 20,000 hours or more and
their relatively low manufacturing cost, He-Ne lasers are among the
most popular gas lasers.
Argon-ion Lasers
Another commonly used gas laser is the argon-ion laser. In these
lasers, as in the He-Ne the lasing transition type is atomic. But
instead of a neutral atom, here the lasing is the result of the deexcitations of the ion. It takes more energy to ionize an atom than to
excite it. By the same token, more energy can be obtained from the
de-excitation of the ion. So, doubly (Ar++) and singly ionized
(Ar+) argon atoms can radiate shorter wavelength light than could
the neutral argon atom, Ar. Because of this, argon-ion lasers can
produce uv light with a wavelength as short as 334 nm. In addition,
these lasers can produce much more power than He-Ne
lasers. Argon-ion lasers typically range in output power from one to
as much as 20 W. At the higher power levels their output is multimode, i.e. contains several distinct wavelengths. Some of these
wavelengths are:
334 nm, UV
351 nm, UV
364 nm, UV
458 nm, violet
477 nm , violet
488 nm, (strong) blue
497 nm, blue-green
514 nm, (strongest) green
Because of these two reasons, high power and multicolor output,
argon-ion is one of the most commonly used lasers in laser light
shows, as well as in a variety of applications.
The make-up of a typical argon-ion laser is very similar to a HeNe's, but with a few slight differences. First, these lasers are much
larger in size. A typical Ar++ laser tube is about one meter long, as
compared to just 20 cm for that of a He-Ne. Second, the optical
cavity of these lasers is built external to the tube. This is partly
because of the high power operation of the laser and partly because
such external arrangement allows for the use of optional wavelength
selection optics within the optical cavity. A prism or a diffraction
grating located just before the high reflecting mirror selects only one
of the lasing transitions for amplification within the cavity; other
wavelengths are deflected out of the resonant cavity. In this way
these ion lasers can operate in a so called single mode.
With this arrangement the two mirrors holders on opposite sides of
the laser tube are typically attached together with an invar rod for
thermal stabilization. Invar is a steel alloy that contains nickel. Its
most valued property is that it expands and contracts very little
when its temperature changes. As a result, when the laser's
temperature changes as it heats up due to the large electric current
within the electromagnetic pump discharge, the optical path length,
and therefore the modal character of the laser output, remains
relatively unchanged. Finally, because of their high power argon-ion
lasers require active cooling. This is most commonly accomplished
by circulating water, either directly from tap or from commercially
produced chillers, in closed coils that surround the plasma (a gas of
charged ions) tube and parts of the electric power supply. Some of
the lower powered argon-ion lasers are just air cooled using a fan,
which makes them less cumbersome to use.
The above two photographs show a 5 W argon-ion laser. Notice the
one-meter long laser tube, the large ballast, and the umbilical cord
that connects to the laser power supply. This cord contains not only
the power line that supplies the laser with the electric power to
generate the plasma, but also the water lines that circulate water to
cool the laser.
Another type of ion laser, the krypton laser, operates very much the
same as the argon-ion laser. To take advantage of all the colors
available in both argon and krypton lasers, manufacturers make
argon-krypton ion lasers by using a suitable mixture of these two
gases. The mixed gas lasers are very useful for entertainment
applications because, in addition to many colors, they can also
produce a "white" beam. (Why is the word "white" in quotations?)
Carbon-dioxide (CO2 ) and Carbon-monoxide (CO) Lasers
In both of these lasers the gaseous medium is made-up of molecules,
which in addition to electronic energy levels of atoms also have both
molecular vibrational and rotational energy levels. The vibrational
energy levels are similar to finer spaced ladder rungs that span two
rungs of the electronic energy levels. The rotational levels are still
more finely spaced rungs that span the vibrational rungs! In these
gas lasers the lasing transitions occur among the vibrational levels,
typically belonging to different electronic levels.
Above diagram shows two electronic and several of their associated
vibrational levels for a hypothetical molecule. (Electronic levels are
shown as "bent rungs" because in the molecule atoms can change
their separation distance and therefore their electronic energy. Also,
note that rotational levels are not shown.) A thick arrow depicts a
pump that excites the molecule from its lowest vibrational level
belonging to the lower electronic level to the 5th highest vibrational
level of the next upper electronic level. The excited molecule can
then de-excite out of this upper level into many possible vibrational
levels. Each one of these de-excitations produces a photon whose
energy, and therefore its wavelength, corresponds to that specific deexcitation. As a result, when a collection of these molecules are all
excited by this pump they generate a number (eleven, in this
drawing) of different wavelength photons.
Specifically, CO2 lasers can generate an output wavelength from
about 9 micro-meters ( m, or microns) to about 11 microns. (1
micron is one millionth of a meter, or 1000 nm.) These outputs
generally contain many closely spaced wavelengths, if the laser is
used for high power output. But for more wavelength specific
applications the optical cavity of the laser is designed to amplify just
one or a few of the vibrational radiative decay lines. The wavelength
range for the CO laser is lower, from about 5 to 6 m. Another
feature of these gas lasers that make them one of the most versatile
of all gas lasers is that they can be made to operate over a large
range of power outputs, either in a pulsed or cw
mode. The CO2 laser, in particular, ranges in cw power from few
Watts to kWs, making these lasers ideal for many industrial
applications including welding and drilling.
Molecular vibrations for CO2 , a linear molecule, are shown in the
figure below. Other combinations of these are possible but these
three are fundamental. There are different varieties of CO2 lasers
that flow fresh gases through the resonant cavity area in order both
to remove heat and to provide lots of gas to achieve high laser
powers. For these lasers in the cw mode powers can reach as high as
100 kW. These intense laser beams are essentially tremendous
invisible "heat" beams that can cut through thick pieces of metal
and are used extensively in industrial applications.
We mention two interesting tidbits about these lasers. First, since
glass in not very transparent to IR light, the mirrors are actually
made of special crystalline materials that are transparent to the
IR. Second, recall that IR light is invisible to our eyes and so special
precautions are needed to protect people working around these
lasers. It turns out that although these lasers can easily cut through
metal, they cannot pass through a thin sheet of clear plexiglass, and
so often these systems are housed in a plexiglass shell to block any
stray reflected IR light.
Other types of gas lasers include the nitrogen laser (N2), excimers,
copper-vapor, gold-vapor, and chemical lasers. Of these the excimer
lasers and the chemical lasers are the most different from the ones
we have already discussed above.
Excimer Lasers
Similar to CO2 , CO and N2 lasers, these gas lasers also use
molecular transitions for their lasing operation. What makes them
especially different is that the molecular gas used for these lasers has
no ground state! Typically these molecules include an atom
belonging to the inert gas family (argon, xenon, krypton) and one
from the halide group (chlorine, fluorine, and bromine). The inert
gas atoms (also known as the rare gases) do not want to interact with
any other atoms. On the other hand, the halide gases are highly
reactive. Still, they cannot bond with the inert gases to form a
molecule. But when sufficient energy is provided to these atoms they
bind together in a short-lived excited state that soon (few
nanoseconds) decays back into the original two separate atoms (i.e.
the molecule dissociates). Because of this rapid molecular
dissociation, these lasers obtain population inversion just by
excitation alone! In fact, the word excimer is short for "excited
dimer," although most excimer lasers do not use two identical atoms
as a strict dimer would.
The excimer molecules are created from a mixture of inert gases along with one of the
halides. Typically a few percent of Ar, Kr and Xe are mixed with a few percent of a halide
to form excimer molecules: ArF, KrF , and XeF. The other 90% of the gas mix consists of
other inert gases such as He and Ne which act simply as a buffer and do not take part in the
reaction. A large electric pulse is often used for the excitation and formation of the excimer
molecule. The rapid decay of the short-lived molecule then leads to a very short laser pulse
lasting 10 - 100 ns (10-8 - 10-7 s). So, another unusual feature of the excimers is that they do
not require an optical amplifier. They are very efficiently formed in the reaction with an
efficiency of around 30% so that the gain is extremely high. A high reflector and a glass
(really quartz - why?) window is sufficient for laser light production. This means that only
about 4% of the light is reflected back into the cavity at the front window, but the gain is so
high that only a single pass through the cavity is needed to produce lots of uv
light. Typically about 1 Joule of energy is in a 10 ns pulse, so that the pulse power is 1J/10ns
= 100 MW. If this power were steadily produced it would be equivalent to powers
generated by large power plants. However, only about 1 - 100 pulses are produced per
second, so that the average power produced is about 1 - 100 W. Because of the highly
reactive nature of the halide gas used in these lasers, excimers are not very easy to
operate. The halides tend to be very corrosive and therefore add a great deal to the
operational cost (as well as the danger!) of these lasers. But still these lasers are very much
in use because their output wavelength is in the UV, from 350 nm down to as low as 193 nm;
all with a good deal of power.