BOREAL LASER GasFinder2, Руководство по эксплуатации, Страница: 47 / 74

8.1
Boreal Laser Inc. GasFinder2 Operation Manual — Tutorial July 17th 2017
Section 8. The Basics of Laser Gas Detection
Tuneable Diode Laser
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 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 size of the laser
and focusing optics is similar to the size of the eraser on the end of a pencil.
A Tuneable Diode Laser (TDL) can have its wavelength altered 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