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FISIOLASER IRD/IRD 2
The laser beam has actually been used to measure the distance from the earth to the
moon, with an error margin of less than a centimetre.
4) Brilliance
: is the power emitted per surface unit.
This equipment gives the highest intensity possible per space unit. The space can be
as small as a few microns.
COMPONENTS OF LASER SYSTEMS
In general, lasers comprise four structural units :
1
. an
active laser medium
,
2.
an
excitation mechanism
(source of energy, called "
pump
" source)
3.
an
optical cavity,
comprising two mirrors and the space in between them;
4.
an
output mechanism
;
5.
and obviously
a mechanical support structure.
Semiconductor diode lasers:
Diode lasers are all made of semiconductor material and have the typical electrical
properties of electric diodes
.
Therefore, diode lasers have other names such as :
–
semiconductor
lasers: according to the material that they are made of,
–
junction
lasers: when they are made of a pn junction,
–
injection
lasers: when electrons are injected into the junction by applying
voltage.
Even though all these devices operate close to the infrared spectrum, nowadays laser
diodes are also made with visible light.
One useful feature is that many can be adjusted by changing the current applied, the
temperature, or by applying an external magnetic field.
The semiconductors can be used as a small and highly effective source of photons
which can be pumped using a variety of techniques.
They include pumping with other optical sources (photo-pumping), pumping with an
electron beam, or pumping with a pn junction.
The most common technique is by the p-n junction.
The pn junction refers to a “
p
” type semiconductor attached to an “
n
” type.
This junction conducts electricity in a preferred direction.
The increased directional conductivity is the normal mechanism that all diodes and
transistors in electronics work on.
The distribution of the energy bands in the junction is the basis of the laser diode
action.
The highest level of energy occupied by the electrons is called the Fermi level.
When the positive pole of the generator is connected to the
p
side of the
p-n
junction, and the negative pole of the generator is connected to the
n
side, a current
runs through the
p-n
junction changing the population of the energy band.
The layers of semiconductor material are placed in such a way so as to create an
active region in the pn junction where photons are generated by a recombination
process.
The base structure of a simple laser diode is shown in the following figure .
Figura a
: outline structure of a laser diode .
by A. Vasta: i lasers terapeutici...Marrapese
ed.. Roma 1998.
A laser generator can be connected by a metallic layer to the external surface of a
stack of layers.
The side of the crystalline semiconductor is cut in such a way as to allow it to be
used as an optical cavity mirror.
The voltage is applied to the metal on the external layers of the semiconductor.
Since the laser diode is so small, it has a special covering that means it can be easily
handled.
There are different types of covering, but the standard one is similar to transistor
containers. It incorporates a collimated lens that is essential for the creation of a
usable beam (see figure).
Special types of laser diodes have been developed to get high power laser diodes.
