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The laser is a light source that exhibits
unique properties. Since the first demonstration of a ruby
laser by Maiman in 1960, there has been a phenomenal development
in the field of lasers. Lasers dominate our modern world
in a variety of forms ranging from tiny diode lasers in
all CD and DVD players to large industrial lasers, used
extensively for cutting and welding, e.g. in automobile
manufacturing. In fact, lasers are used in thousands of
applications in every section of modern society, including
consumer electronics, information and communications technology,
entertainment, science and industry, the medical field and
defence. The laser triggered the photonics revolution and
is the foundation of modern photonics.
Modern laser research involves fundamental laser physics,
creative development of novel laser concepts and advanced
experimental work and diagnosis. All this can lead to the
development of new lasers, which will fulfil the requirements
of current and future demands in science and industry.
Unique properties of lasers
Principles of laser operation
Basic components of a laser
Unique properties of lasers
Lasers range in size from tiny diode lasers to large systems
the size of a football field. All of these have three basic
properties in common, which separate lasers from ordinary
light sources:
Monochromaticity: conventional light
sources emit light consisting of a broad range of wavelengths
(i.e. colours); a laser, on the other hand, emits only
a very narrow range of wavelengths.
Directionality: conventional light sources,
like a light bulb, emit light in all directions, while
lasers can emit light that spreads (‘diverges’)
only very little with distance. However, all laser beams
eventually diverge as they move through space.
Coherence: Some consider coherence to
be the most fundamental property of laser light, i.e.
where all parts of the electromagnetic waves are in phase.
Principles of laser operation
The word ‘laser‘ is an acronym for light amplification
by stimulated emission of radiation. It is worth noting
that the term ‘light’ refers to a wide spectrum
of wavelengths from soft x-ray, through ultraviolet (UV)
and visible to far infrared (IR).
The two most important basic principles for laser science
are the quantum nature of light and a process called stimulated
emission (both accredited to Einstein). The quantum nature
of light refers to light quantised into discrete energy
portions, nowadays called photons.
Basic components of a laser
A laser basically consists of three parts: a resonant optical
cavity called the optical resonator, a laser gain medium
(also called active laser medium) and a pump source to excite
the particles in the gain medium.
The optical resonator consists of at least two mirrors
between which the light bounces up and down resonantly.
In most cases, one or more mirrors are curved, so that a
resonant optical mode forms. This mode defines the laser
beam. Modern dielectric mirrors used in lasers typically
have a reflectivity of up to 99,9%. However, one of the
end mirrors is usually only partially reflective, so that
a portion of the light is transmitted. This mirror is called
the output
coupler. The transmitted part forms the laser output.
In order to operate, the laser requires a gain medium in
the resonator, which amplifies light and thus compensates
for the loss through the output coupler. Lasers are typically
classified by the type of gain medium they employ (gas laser,
solid-state laser, dye laser, semiconductor laser, etc.).
The stimulated emission process takes place in the gain
medium.
The gain medium amplifies light of any direction. However,
only the light that bounces up and down between the resonator
mirrors is amplified many times and therefore reaches a
high intensity. In a continuous wave (CW) laser, the gain
in the laser gain medium and the loss from the output coupler
plus other losses are in equilibrium. The fact that the
photon energy has to match a given energy transition makes
the laser monochromatic. Since the amplification process
maintains the phase and direction of the light, the laser
output is directional and coherent.
The active particles in the laser gain medium need to be
in a state of inversion for the laser to operate. To reach
this state requires some pumping process, which lifts them
into the required energy state. Typical pumping processes
are electrical current in a gas or semiconductor laser or
optical pumping in a solid-state or dye laser. Optical pumping
is
typically achieved either by flash lamps or by another laser.
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