Laser construction
A laser
generally consists of three important parts:
- An energy source (usually referred to as the pump or pump
source);
- A gain medium or laser medium;
- A mirror, or system of mirrors, forming an optical resonator.
Schematic diagram of a typical laser, showing the three
major parts
The pump source is the part that provides energy to the laser system.
Examples of pump sources include electrical discharges, flashlamps, arc lamps,
light from another laser, chemical reactions and even explosive devices. The
type of pump source used principally depends on the gain medium, and this
also determines how the energy is transmitted to the medium. A helium-neon(HeNe)
laser uses an
electrical discharge in the helium-neon gas mixture, a Nd:YAG laser uses a light
focussed from a Xenon flash lamp, and
excimer lasers use a chemical reaction.
The gain medium is the major determining factor of the wavelength of
operation, and other properties, of the laser. There are hundreds if not
thousands of different gain media in which laser operation has been achieved.
The gain medium is excited by the pump source to produce a population
inversion, and it is in the gain medium that spontaneous and stimulated emission
of photons takes place, leading to the phenomenon of optical gain,
amplification.
Examples of different gain media include
- Liquids, such as dye lasers. These are usually organic chemical solvents, such
as methanol, ethanol or ethylene glycol, to which are added chemical dyes such
as coumarin, rhodamine and fluorescein. The exact
chemical configuration of the dye molecules determines the operation
wavelength of the dye laser.
- Gases, such as carbon dioxide, argon, krypton and
mixtures such as helium-neon. These lasers are often
pumped by electrical discharge.
- Solids, such as crystals and glasses. The solid host
materials are usually doped with an impurity such as chromium, neodymium,
erbium or titanium ions. Typical hosts include YAG
(yttrium aluminium garnet), YLF
(yttrium lithium fluoride), sapphire (aluminium oxide)
and silica glass. Examples of solid-state laser media include Nd:YAG, Ti:sapphire,
Cr:sapphire (usually known as ruby), Cr:LiSAF (chromium-doped
lithium strontium aluminium
fluoride), Er:YLF and Nd:glass. Solid-state lasers are usually pumped by
flashlamps or light from another laser.
- Semiconductors, a type
of solid, in which the movement of electrons between material
with differing dopant levels can cause laser action. Semiconductor lasers are
typically very small, and can be pumped with a simple electric current,
enabling them to be used in consumer devices such as compact disc players. See
laser
diode.
The optical resonator, or optical cavity, in
its simplest form is two parallel mirrors placed around the gain medium. Light
from the medium, produced by spontaneous
emission, is reflected by the mirrors back into the medium, where it may be
amplified by stimulated emission.
The light may reflect from the mirrors and thus pass through the gain medium
many hundreds of times before exiting the cavity. In more complex lasers,
configurations with four or more mirrors forming the cavity are used. The design
and alignment of the mirrors with respect to the medium is crucial to
determining the exact operating wavelength and other attributes of the laser
system.
Other optical devices, such as spinning mirrors, modulators, filters and
absorbers may be placed within the optical resonator, to produce a variety of
effects on the laser output, such as altering the wavelength of operation or the
production of pulses of laser light.
Laser light has the following properties:
- Laser light is monochromatic. It contains one specific wavelength of light, which as described earlier is determined by the amount of energy released when the electron drops to a lower-energy orbital.
- Laser light is coherent. Each proton moves in step with the other (i.e. all protons have wave fronts that move in unison).
- Laser light is highly directional (i.e. a laser beam is very tight and concentrated)
To produce laser light it is necessary to have a pair of mirrors at either end of the lasing medium. These mirrors are often known as an optical oscillator due to the process of oscillating photons between the two mirrored surfaces. The mirror positioned at one end of the optical oscillator is half-silvered, therefore it reflects some light and lets some light through. The light that is allowed to pass through is the light that is emitted from the laser. During this process photons are constantly stimulating other electrons to make the downward energy jump, hence causing the emission of more and more photons and an avalanche effect, leading to a large number of photons being emitted of the same wavelength and phase.
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