Basic Concept

Second harmonic generation has its equivalent in electronics. The requirement for this process is the presence of a component with a non-linear characteristic curve. In the case of photonics, a component is needed which must be optically transparent for the fundamental and second harmonic wave. Such components are commonly crystals like for instance, the KTP crystal. The understanding of optical non-linear effects is very important for laser technology, since the processes of generation of short pulses are also based on non-linear effects. Within the experiment, the phase matching condition will be presented and analysed. The efficiency of frequency doubling is to be determined and hints for an optimized conversion rate will be evaluated in the experiment. Incidentally, the understanding of birefringent crystals grows by experience of phase matching. The fundamental wave is generated by a diode laser pumped Nd:YAG laser with an open resonator structure. The non-linear crystal is placed into the resonator and intra-cavity SHG is carried out. The reflectivity of the output coupler of the Nd:YAG laser is chosen as high as possible to obtain several watts of power of the fundamental wave inside the cavity.

Experimental Set-up

The light of the pump laser is transferred via a fibre cable to the fibre telescope (FT-1) which transforms the beam to an almost parallel beam. The lens (C) focuses the radiation into the Nd:YAG rod, which has a mirror coating on its back side and forms the cavity with the laser mirror. The generated laser emission at 1064 nm passes the filter (Fi) and the residual pump light is blocked. The passed laser emission can be transferred by means of the fibre telescope (FT-2) either to the optical multi-channel analyser (OMA) or to the photodetector. When the laser mirror is removed, the excitation spectra can be recorded by means of the OMA. By varying the temperature of the laser diode its wavelength will change. This effect can be studied first and then used to obtain the absorption spectrum of the Nd:YAG crystal. By modulating the pump laser, the time resolved emission spectrum allows the measurement of life-time of the excited state.
By inserting the KTP crystal inside the cavity immediately second harmonic generation takes place. Green laser light is emitted. The subsequent measurements can be performed either in local mode or through computer control via the USB connection of the base housing. This set-up is ideally suited to demonstrate the fundamental behaviour of a solid state laser system, its excitation process as well as its spectroscopic characteristics. Furthermore, the non-linear optic is impressively introduced by converting invisible laser radiation into visible green radiation.