Basic Concept

A proper understanding of emission and absorption is the key to understanding laser technology. Usually this knowledge is transfered almost theoretical 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 makes it possible to design an experiment where, in a practical way, the complex 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.