Doctoral Degrees (Physics)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/3640

Browse

Browsing Doctoral Degrees (Physics) by browse.metadata.advisor "Bollig, Christoph"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Thermo-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.