8.1
Boreal Laser Inc.
GasFinderAB Operation Manual — Tutorial July 17th 2017
Section 8. The Basics of Laser Gas Detection
L A S E R
Laser is an acronym for:
L
ight
A
mplifi cation by
S
timulated
E
mission of
R
adiation
In principle, the laser is a tube with a mirror
at each end. In the tube is a crystal or a gas
mixture which is induced to emit radiation by
stimulating it with electricity. The radiation in
the tube bounces between the two mirrors
and starts a chain reaction. The chain reaction, or
light amplifi cation
, would cause the
device to burn up if continued indefi nitely. For this reason, a small amount of the refl ective
coating on one mirror is removed or a small hole placed in it. This causes a small stream of
identical photons to be released through the hole. The emitted stream of photons is identical
in both time and space. This is the ‘coherent’ radiation
characteristic of a laser. A crystal or gas is selected which
will emit radiation at a specifi c wavelength. A ruby crystal
emits a visible red beam with a wavelength of 6500
angstroms or 650 nanometers.
A
Diode Laser
works on the same principle except
that the tube, mirrors, and crystal are all combined in
a small package using semiconductor technology. The
diode laser is composed of a pn junction, as in the light emitting diode (LED), and specially
cleaved mirror facets that form the optical cavity. The output wavelength is fi xed. The combined
laser and focusing optics are similar in size to the eraser on the end of a pencil.
A
Tuneable Diode Laser (TDL)
can have its wavelength altered or tuned very slightly while it
is operating. This can be done by changing the operating temperature or the drive current.
Absorption Lines
If a beam of sunlight is sent
through a glass prism, a
spectrum of colours can be seen
leaving the prism, which can be
projected onto a screen. Blue,
which has a shorter wavelength,
is bent more than the red. The
visible spectrum is only a small
part of the emissions. Extending
beyond the blue is the ultra-violet
and beyond the red is the infra-
red.
If the projected spectrum is sharp and defi ned, a number of black lines may be seen at
various locations. These lines are absences of light where atoms and molecules of gas in the
sun or the light beam path have absorbed the light at a particular wavelength. This results in
a black line on the display. These lines are called
absorption lines
.
Gas Laser
Diode Laser
Pencil
Summary of Contents for GasFinderAB
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