Thermo-optical effects in high-power end-pumped vanadate lasers

Strauss, Hencharl Johan (2010-03)

Thesis (PhD (Physics))--University of Stellenbosch, 2010.

Thesis

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.

AFRIKAANSE OPSOMMING: Die uitset drywing van longitudinale-gepompte vaste-toestand lasers word hoofsaaklik beperk deur termiese effekte in die laser kristal. Die kristal word of gekraak of die laser se bundel kwaliteit en uitset drywing verminder. Dit is veral ’n problem in Nd:YVO4 en Nd:GdVO4 kristalle waarin sterk termiese lense voorkom. Hierdie twee Nd3+ vanadaat kristalle is waardevol vanwee hulle ho¨e emissie deursnitte wat hulle uitstekend maak as versterkings materiale vir modus-gesinkroniseerde, ho¨e repitisie, Q-geskakelde en binne-resonator frekwensie-verdubbelde lasers. Die twee vanadate het baie soortgelyke spektrale eienskappe, maar verskeie publikasies beweer dat die duurder Nd:GdVO4 materiaal termo-opties beter is as Nd:YVO4. Onlangse teoretiese berekeninge asook metings van die termiese en termo-optiese kwaliteite van die twee kristalle toon egter die teenoorgestelde. Sover ons weet is daar nog geen direkte vergelyking van die termiese lens in hierdie twee materiale onder identiese kondisies gedoen nie. Ons het dus so ’n meting aangepak vir kristalle met identiese lae konsentrasie van die Nd3+ ioon deur drie verskillende meet metodes te gebruik. Een van die meet metodes is ’n nuwe, meer reproduseerbare metode wat ons ontwikkel het vir hierdie doel. Dit is meer reproduseerbaar omdat dit slegs die binneste deel van die termiese lens meet wat min sferiese aberrasie het. ’n Ander meeting, wat ’n toets-bundel deur die kristal stuur, het getoon dat daar byna geen verhoging in die die temperatuur van die kristal was toe ossilasie in die resonator onderbreek was nie. Dit is ’n aanduiding dat dat daar byna geen op-omskepping teenwoordig is in die kristalle nie wat te danke is aan hul optimale lae konsentrasie van die Nd3+ ioon. ’n Verdere gevolg van die debat is dat daar nog verwarring in die literatuur bestaan oor die waarde van Nd:YVO4 se termo-optiese dne/dT koeffisi¨ent. Hierdie parameter is van groot belang in berekinge van die termiese lens se fokale lengte vir die ho¨er wins -polarisasie. Deur numeriese modellering te gebruik het ons bevind dat die waarde wat verskaf word deur Sato & Taira (2007) ons eksperimentele data die beste pas. Ons metings het ook aangedui dat die dioptriese krag van die termiese lens nie linie¨er toeneem ten opsigte van die geabsorbeerde pomp krag nie. Dit was o¨enskynlik teenstryding met teorie oor termiese lense. Dit is omdat ons bevestig het dat daar geen op-omskepping in die kristalle teenwoordig was nie, wat die standaard verklaring vir die nie linie¨eriteit is. Ons het dus ons numeriese modellering gebruik om die hoofbron van die nie-lini¨ere toename te identifiseer as die veranderende spektrale uitset van die diode pomp laser. Die bevindings in hierdie tesis bou dus kennis op oor die termo-optiese eienskappe van die vanadate en versterk begrip van die sterk termiese lense binne hulle. Verder stel die bevindings ons nou in staat om die uitset drywing van longitudinale-gepompte vanadaat lasers na ho¨er vlakke te skaal.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/4007
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