Browsing by Author "Strauss, Hencharl Johan"
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- ItemCorrelations between the standard and alternative definitions of the beam quality factor(Stellenbosch : University of Stellenbosch, 2005-04) Strauss, Hencharl Johan; Von Bergmann, H. M.; Rohwer, Erich G.; University of Stellenbosch. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: The beam quality factor (M2) of a laser is important because it describes both the quality of a laser beam as well as its propagation. Particular applications require that the beam radii (from which the M2 factor is determined) be defined in a specific way. The International Standards Organisation (ISO) recently standardised the theoretically correct, but practically difficult second moment definition of the beam radius. This thesis aims to establish correlations between the second moment definition and other more practical definitions. Chapter 2 presents a systematic introduction of moment theory for general beams. Beam centre and radius definitions as well as beam classification are derived by means of second and mixed moments. In Chapter 3 simple laser beams are modelled by means of a resonator approach. The infinite number of solutions of this model is called Gaussian solutions, which are found to be different for rectangular and cylindrical symmetric resonators. The zero order solution for both symmetries is identified as the ideal solution to which all others can be compared to for quality determinations. Chapter 4 presents different beam scanning devices, beam radius definitions and correlations between these definitions. The theoretical basis for a new correlation theory is also given. Chapter 5 describes programs and computations used to verify existing correlation methods and to calculate correlation factors for a newly proposed theory. Chapter 6 presents the results of the computations for both circular and rectangular symmetric beams. The ISO’s correlation theories are tested first. Newly proposed correlation functions between the second moment and alternative definitions as well as theoretical error graphs are given for various beams. A novel method to correlate several important beam subclasses is also presented. Chapter 7 describes the experimental setup, automation software and post processing techniques that were used to characterise a modified CO2 TEA laser. It is further shown how the theory developed in previous chapters could be applied to the experimental data. Chapter 8 presents the experimental results of the beam characterisation measurements performed on two different beams that emanated from the CO2 TEA laser. Existing and newly developed correlation theories are applied to the experimental data and the corresponding results are compared. In Chapter 9, conclusions and suggestions with respect to the initial aims identified in the first chapter are made. Several suggestions for future work are also made.
- ItemThermo-optical effects in high-power end-pumped vanadate lasers(Stellenbosch : University of Stellenbosch, 2010-03) Strauss, Hencharl Johan; Von Bergmann, Hubertus; Bollig, Christoph; University of Stellenbosch. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: The output power of end-pumped lasers is mainly limited by thermal effects in the bulk crystal gain material. The thermal effects either fracture the crystal or cause degradation in the laser beam quality and output power. This is especially pronounced in Nd:YVO4 and Nd:GdVO4 which exhibit strong thermal lensing. These two Nd3+ vanadate materials are of great value because of their high emission cross sections which makes them excellent gain materials for mode-locked, high repetition rate Q-switched and intra-cavity frequency doubled lasers. The two Nd3+ vanadates have very similar spectral properties but many publications claim that the more expensive Nd:GdVO4 is thermo-optically superior to Nd:YVO4. However, a debate ensued after theoretical calculations as well as measurements of the thermal conductivity and thermo-optical coefficients indicated that the opposite is true. To our knowledge there has never been a direct comparison of the thermal lensing of these two materials under identical pumping and lasing conditions. In order to contribute to the debate we did such measurements for different crystals of these two materials with equal low doping using three different measurement methods. We subsequently determined that Nd:YVO4 has slightly lower thermal lensing for the stronger gain -polarisation. One of the measurement methods we used is a novel more reproducible one that we developed for this purpose. It is more reproducible because it selectively measures only the focal length of the central, relatively unaberrated part of the thermal lens. Another measurement, utilising a probe beam through the laser crystal, found that there was almost no increase in the temperature when lasing is interrupted. This indicated that there is almost no upconversion present in the crystals which is probably due to their optimally chosen low doping. A further consequence of the vanadate debate is that there is still confusion about the value of the important thermo-optical coefficient for the higher gain -polarisation (dne/dT ) of Nd:YVO4. This parameter is of great importance in thermal calculations since the strength of the thermal lens is largely dependent on it. We therefore numerically modelled the thermal lensing in our crystals using different dne/dT values and found that the value given by Sato & Taira (2007) matches our experimental results the best. Our measurements also indicated that the thermal lens dioptric power increased nonlinearly with pump power. This appeared to contradict theory of thermal lensing since we knew that there was no upconversion in the crystals (which is the standard explanation for the nonlinear increase). We proceeded to use our numerical modelling to identify the main source of the nonlinear increase as the varying spectral output of the diode pump laser. The findings in this thesis therefore extend knowledge of the thermo-optical properties of the vanadates and increase understanding of the strongly aberrated thermal lenses formed inside them. Furthermore, the findings now enable the power-scaling of end-pumped vanadates lasers to higher levels.