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LASER THEORY
LASERS
What is a laser?
Laser is an acronym for 'Light Amplification of Stimulated Emission of
Radiation'. A laser is a device that creates and amplifies a narrow, intense
beam of coherent light. Two scientists, Schawlow and Townes at Bell Labs
in the US, invented the Laser in 1958.
Click here to run the laser animation
Why are lasers important?
Atoms emit radiation. We see it every day in light bulbs and fluorescent
tubes. This light is 'incoherent', i.e. the light is radiated in random
directions at random times. Coherent light is created when radiated light is
generated in the same direction at the same time. The fact that selective
colors (or discrete wavelengths) are available from different lasers,
coupled with the optical power they can generate, make them of interest
as a predictable light source for many high precision applications. Today,
lasers are also used in a wide range of applications in medicine,
manufacturing, the construction industry, surveying, consumer electronics,
scientific instrumentation and military systems. They range from small
semiconductor laser diodes to high power solid state and gas lasers.
Unfortunately, atmospheric conditions and vibrations can easily and
adversely affect laser beams if transmitted over long distances. Now all
that is needed is a satisfactory medium by which to transmit this light to
eliminate these effects.
View Glossary of Terms
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FIBER OPTICS
What is Optical fiber?
Glass seems a good choice for a waveguide, since we've been using it to
see through for centuries. However, turn a piece of glass on end and try to
look through it. You'll be disappointed because the end will be opaque
green, due to the presence of copper, manganese and other minerals.
Indeed, regular window glass becomes opaque within less than an inch of
depth. In the 1970s, researchers at Corning Glass produced a waveguide
of almost mineral-free glass fiber which is the basis for optical fiber based
telecommunications today. However as we have learned, light wants to go
in a straight line, so how is it kept in the fiber optic?
How does it work?
The glass portion of an optical fiber consists of two regions, the core that
runs through the center of the strand, and the cladding that surrounds the
core. The cladding, which has a different refractive index than the core,
acts as a mirror. It causes the light to reflect back into the core during
their transmission through the fiber.
Links:
http://www.howstuffworks.com/fiber-optic.htm
http://www.bell-labs.com/technology/lightwave/
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TYPES OF FIBER
Multimode Fiber
This fiber has a large core diameter typically 50µm. This fiber will
propagate hundreds of different modes at the same time. Functionally this
fiber can thought of as a light pipe and is used for high power delivery and
light gathering applications.
Singlemode Fiber
This fiber has a small core diameter typically 5/125µm core/cladding. The
light follows a path straight down the middle of the fibers core. It is
typically used for long distance telecommunications.
Polarization-maintaining Fiber
This fiber is a type of Singlemode fiber. It has the ability to maintain a
linear polarisation state. PM fiber is used in applications which take
advantage of the properties of polarised light.
Why is fiber important?
It is actually the laser that is important to the scientists making the
measurements with the light generated by lasers. However, the highly
directional nature of the laser beam can be a disadvantage when you try
to design a high precision instrument with a laser which is often bulky,
unreliable, requiring cooling. For the instrument design engineer, it is the
flexibility of fiber that makes it so useful for many scientific/industrial
applications
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FIBER DELIEVERY SYSTEMS
KineFLEX
Lasers are sometimes large, require cooling, and generate undesirable
heat. With a few exceptions they tend to have reliability problems.
Therefore to have the laser located inside the instrument where the
measurement (often very sensitive) takes place, is not practical.
A fiber delivery system enables instrument designers the ability to place
the laser in a practical location and deliver the beam of light via optical
fiber.
A laser scanning microscopes are used in biological research to examine
cell structure and read genetic information in tissue samples. They often
require a stable temperature environment, and in the case of genetics
instruments, the environment is often unsuitable for the lasers. The fiber
delivery system delivers light from, often many lasers, to the area of
interest without upsetting the surrounding environment. The figure
opposite shows an image of human skin, which was taken using a violet
laser and fiber delivery system.
Why is fiber important?
It is actually the laser that is important to the scientists making the
measurements with the light generated by lasers. However, the highly
directional nature of the laser beam can be a disadvantage when you try
to design a high precision instrument with a laser which is often bulky,
unreliable, requiring cooling. For the instrument design engineer, it is the
flexibility of fiber that makes it so useful for many scientific/industrial
applications
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FIBER COUPLED LASER DIODES
IFLEX
With smaller semiconductor lasers it is often practical to combine the laser
and fiber into 1 packaged system, in which the laser and fiber are
permanently aligned during manufacture. This means that end user does
not have to perform a procedure whereby the laser and fiber are
positioned together to maximise the amount of light coupled into the fiber.
Once again this enables the end user to locate the laser away from
sensitive areas where they wish to observations on their experiments. One
such example is Protein Crystallography
Protein crystallography is a powerful tool in revealing the structures and
interactions of protein molecules, thereby providing valuable information
that can be used to rapidly develop effective pharmaceutical compounds.
To conduct this type of study, scientists must first generate crystals that
are large enough and uniform enough to provide useful structural
information upon analysis. Protein crystals grown in microgravity
environments are often significantly larger and of better quality than those
grown on Earth. See figure opposite.
A NASA led program developed a laser light scattering device which can
detect the early formation of crystals, allowing manual adjustment of
temperatures, which will allow crystals that have formed, to grow more
slowly and more perfectly. The intention was then to fly the experiments
into space where the gravity effects are weak.
The first experiments using fiber coupled lasers were conducted onboard
NASA space shuttle mission STS-95. Crew specialist on that flight was
Senator John Glenn, the first American into space. Experiments have since
taken place on the Mir spacestation and the new International Space
Station.
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LINKS
http://www.bell-labs.com/history/laser/
http://www.howstuffworks.com/laser.htm
http://lasers.llnl.gov/about.html
http://home.achilles.net/~jtalbot/glossary/
http://www.rli.com/tutor1.html
Solid state lasers
http://www.misty.com/people/don/laserssl.htm#ssltoc
Helium Neon Lasers
http://www.misty.com/people/don/laserhen.htm#hentoc
Helium Cadmium Lasers
http://www.misty.com/people/don/laserhec.htm#hectoc
Laser Experiments and Projects
http://www.misty.com/people/don/laserexp.htm#exptoc