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Basic Concept
A proper understanding of emission
and absorption is the key to understanding laser technology.
Commonly this knowledge is transfered in an almost theoretical
way due to the lack of simple experiments clearly demonstrating
the effect of absorption and spontaneous and stimulated
emission. The availibilty of simple pump lasers as well as
absorbing and laser active material make it now possible to
design an experiment where, in a practical way, the complex
appearing relations can be clearly demonstrated.
This experiment uses the absorption of diode laser radiation by
the Nd:YAG. The ability to tune the wavelength by varying the
temperature of the diode is exploited to examine the absorption
spectrum. The resulting fluorescence as well as the four - level
lasing system of Nd:YAG is investigated. The most common
technique of generating a population inversion in solids,
excluding semiconductor materials, is the process of optical
pumping. The resulting pump efficiency depends on the choice of
proper absorption and emission transitions. As an important
example; the following set-up for optical pumping of Nd:YAG with
a laser diode is introduced. In the experiment; the relevant
pump and laser transitions are investigated and characterised.
In the second step, the emission generated by the pumping
process will be investigated and the life-time of the initial
energy level of the laser cycle will be measured.
Experimental Set-up
As a pump light source, a 20 mW fibre pig-tailed laser diode
with a thermoelectric cooler is used. The controller and the
laser diode is accommodated inside a slot-in module. The diode
laser radiation is transferred by means of a fibre patch cable
to the fibre telescope (TF-1) which produces an almost parallel
beam. The lens (C) focuses the radiation into the Nd:YAG rod.
The generated emission passes the filter (F1) to remove residual
pump power and is transferred via the fibre telescope (F2)
either to the photodetector slot-in module for measurements in
the time domain or to the OMA for the spectral analysis. A two
channel oscilloscope for displaying the time resolved signals is
necessary. The temperature and current stabilising of the laser
diode to vary the parameters of the laser diode is done by the
slot-in module. By means of a frequency generator, the laser
diode can be modulated. The measurements and settings can be
performed either in local mode or through computer control via
the USB connection of the base housing. |
