The
laser diode is the most used
laser type today.
It is used in
laser printers,
CDROM drives,
laser pointers
(annoying things) and several other
devices.
Build up of a normal
laser diode:
:
Beam :
__:__ end facet
| | |
| | |
O---|P | N|---O
| | | laser
|__|__| chip
beam : end facet
__:__
O---| | |---O
-------
Photodiode
The
end facets is simply the end of the
semiconductor
material that works as the
mirrors in the
laser.
Depending on angle the photons hits the facet will or will
not be reflected(the farther from 90 degrees angle they come
more of them will be
reflected).Some of the reflected
photons
will hit
atoms in the junction and
stimulate the
release of
more
photons.
nonreflected light
|\
V \
\|
| "`:
| :
---------------|----:--------- facet
/ \ | | :
/ \ | | :
/ \ | | :
/ \ | | :
/ \ | | :
/ \ | | :
^ ^
| |
Reflected light
In some
diodes the
surface is
specialy coated to
only let near 90
degree angled light through, to make a
tighter beam. The
photodiode's
output is used by the
driver
electronics to
measure the
laser chip output. This
is to prevent
overloading the
chip. If the
chip is feed a
to
high current, even for only a few miliseconds, the
concentrated beam can
damage the facets and the PN junction.
The
laser chip and
photodiode is usualy
mounted in a
metal package(to conduct away the
generated heat) with a
transparent top. As the lightangle of this type of diode
can be up to about 40
degrees, a so called collimator
lens
is
added if a
thin beam with low
spreading is
wanted.
The
laser diode has a long
lifetime(if
properly regulated),
a high
efficiency and are small
compeared with other
laser types.
But their small
size is also a shortcoming, since their
resonance
cavity is so small(less than a
milimeter where a gas pumped
lasers
would at least be 10cm). This means that the
diode laser's
light is less
coherent and
monochrome. But in many
applications
the
laser diode is more than
adequate.
The laser diode is available from small units, with 1mW-10mW
effect, to
arrays of
laser chips with a
output of 100W. These arrays
are used to
drive(pump) Diode Pumped Solid State(DPSS)
lasers
with several kilowatts
output effect.
The
laser diodes that are
available today is mostly in the
infrared
to
red(670nm) spectrum. But other colours has been
developed
and are likely to be
available soon.