Q-switching of neodymium yttrium aluminium garnet laser via active and passive techniques

摘要

Light modulation is important for increasing the laser output, but it needs proper technique and delicate nonlinear material which leads it to be costly. In contrast, the current demand is in favour to have a cheaper and user friendly laser. Therefore the aim of this study is to find the technique and alternative material in laser modulation. In attempt to achieve these goals a Nd:YAG rod was utilized as a gain medium and flashlamp as a pumping source. In an active technique, a Pockels cell containing Deuterated Potassium Dihydrogen Phosphate (DKDP) crystal was electrified at constant 3.28 kV high voltage. Variable pumped energy between 25 – 64 J was absorbed by the crystal to produce a maximum 60 mJ / 50 ns Q-switched pulsed energy. The performance of an electro-optically (EO) Q-switched Nd:YAG laser at transition line of 4F3/2 - 4I11/2 Stark levels based on DKDP crystal was demonstrated at various temporal delays in the range between 100 - 500 µs. In general, all the output energies of an EO Q-switched laser have similar parabolic normalized trend. However, the higher the input energy, the longer the temporal delay was realized to achieve the optimum output energy. The detailed results obtained from this study were 25.00 J / 240 µs, 30.25 J / 240 µs, 36.00 J / 240 µs, 42.25 J / 240 µs, 49.00 J / 250 µs, 56.25 J / 260 µs and 64.00 J / 290 µs. The results were then confirmed via spectroscopic analysis. Passively Q-switched technique was demonstrated by using a saturable absorber made of a multi-walled carbon nanotubes-polyethylene oxide (MWCNTs-PEO) film at two positions in the laser resonator to optimize its performance. With 88 J input energy, the Q-switched laser produced an optical signal pulse of 87 ns at position 1 (P1) and 115 ns at position 2 (P2). The conversion efficiency of the Q-switched laser with saturable absorber at P1 was about 0.43% with maximum output energy of 1.66 mJ and about 0.57% with maximum output energy of 1.60 mJ at P2. It can be summarized that P1 which was located nearer to the output coupler (OC) tends to be a better position for allocating MWCNTs-PEO saturable absorber in the laser resonator. Further exploration had been conducted by moving the OC for eight positions towards the saturable absorber at P1 with an increment of 10 mm at constant input energy of 88.36 J. It was found that the output energy increases between 1.54 - 1.68 mJ. In addition, the shortest pulse duration of 83.64 ns was obtained when the OC was at the closest distance to the saturable absorber. Further increase of the input energy to about 90 J tends to burn off the saturable absorber. In summary, Q-switched Nd:YAG laser modulation has been successfully achieved by using both active and passive techniques. The active technique requires optimization in temporal delay for higher output energy while the passive technique indicates that the MWCNTs-PEO has a high potential to be an effective saturable absorber.