LASER

light amplification by stimulated emission of radiation
by
niels horstmann
january, 2002 ? physics and information technology • d. saller
what is laser?
laser is a acronym for light amplification by stimulated emission of radiation.
but what does that mean? a laser is a generator for lightwaves based on atomar and
quantumphysical effects. the light emitted by a laser is more equal to radio waves than to
„usual heat sources“.
the main differences between those two light sources are the coherence of time and place of
the laser that will be explained in more detail later. the main aspects of laser light are the
hight concentration of energy and the monochromatic light.
the „founders“ of laser
the main principles were first written in 1917 by a. einstein but the first laser was built in
1960 by the russians n.s. basov and a.m. prokhorov. the theoretical base came from c.h.
townes. 1964 those three physicians got the nobel prize for their work.
laser basics
coherence
a laser is a device that creates and amplifies a narrow, intense beam of coherent light.
coherent means that the photons emitted by the source are of the same phase and go into the
same direction. there is no coherence in the light of heat sources because the electrons that
emits the photons are not linked together.
energy levels
as mentioned before electrons are responsible for the emission of photons. but why? let’s
have a look on the different energy levels of a atom.
the energy of a electron depends on its distance from the core and its travelling speed. electrons
that a far away from the atom’s core have more energy and core-near electrons have lower
energy.
a electron from a high energy level looses energy by dropingping down to a lower level. it emits
this energy by a photon. let’s say the high energy level is called ehigh and the low energy elow.
with this definition a falling electron emits a photon with the energy hf=ehigh-elow. vice-verca by
giving this energy to a electron you can push it from elow to ehigh.
the first case is called spontaneously emission and the second case is called absorption.
spontaneously means without any outer influence. but there is also the possibility to force a
electron on a high level to emit a photon by shooting at it with a existing photon. this is called a
stimulated emission. those photons have the same energy the same direction and the same
phase like the controlling photon – they are coherent.
ehigh
hf
elow
a photon a pushing a electron to a
high energy level
ehigh
hf
elow
the electron dropings back to a low
energy level
ehigh
hf
elow
the stimulated emission creates a new
photon
this stimulated emission of coherent photons is the main fact of laser light and of course the
reason for the acronym.
materials and function
the most important materials for building a laser are liquids, crystals, gas or semiconductors.
but the main differences do not depend on the used material but on the method
how the electrons are forced to go to a higher energy level.
fluid or crystal lasers will receive photons from another light source. this action is called optical
pumping. semi-conductor lasers will be messed up with high energetic atoms.
optical resonators
to create a narrow and intense beam of coherent light it is not enough to have a laser
active material and to create a stimulated emission. you can create more intensive light by
putting your laser active material between two mirrors.
with this trick the amplified light will be thrown back and forth by the mirrors and the number
of coherent photons increases. this basic element of a laser is called optical resonator.
to get the most out of the laser the distance between the mirrors has to be a factor of the half
wavelength of the emitted light. only with this parameters you will get a stable wave in the
optical resonator.
looking at the dimension between the mirrors there is a space from 10cm to 1m enough space
for up to 1.000.000 light waves.
the main functionality of this element is not only to create a feedback on the amplified wave
and the active material it has a influence on the type of emission too.
at the moment our light is captured between the mirrors. we will now make one of them semitransparent.
this leads to the fact that only those rays that are exactly parallel reflected by the
mirrors can leave the resonator. all other rays have no effect on the amplification.
more power
the power of a laser can be even more improved.