PhD Thesis Defense
Time: Mon 2015-10-12 10.00
Lecturer: Staffan Tjörnhammar
Title: Properties of Volume Bragg Gratings and Nonlinear Crystals for Laser Engineerig
Abstract: This thesis focuses on two topics: thermal limitations of volume Bragg gratings (VBGs) employed as laser-cavity mirrors and formation of color centers in KTiOPO4 and its isomorphs.
To explore the mechanisms of the thermal limitations of VBGs in high power lasers, I designed and constructed a diode-pumped, solid‑state laser with a VBG as cavity mirror that had a significantly higher absorption than what is typical. Thereby I could study the limiting thermal effects by using only moderate intra-cavity power. Additionally, I designed a computer model to numerically investigate the thermal effects in VBGs. Both the experiments and the simulations showed that the laser became successively more unstable when the power was increased. Absorption of the reflected laser beam causes broadening of the grating spectrum accompanied by decreasing diffraction efficiency. The reduced reflectivity leads to a leakage of the radiation through the grating. Moreover, the simulations showed that this increased instability was due to a reshaping of the intensity distribution profile inside the grating, which, in turn, leads to a sharp reduction of the diffraction efficiency.
High-intensity visible radiation induces color centers in KTiOPO4, which can lead to severe decrease in the performance of the crystal and can cause catastrophic breakdown. The formation of color centers was investigated by measuring picosecond, blue-light induced infrared absorption (BLIIRA) in periodically-poled KTiOPO4, Rb:KTiOPO4, RbTiOPO4, KTiOAsO4 and RbTiOAsO4 through thermal lens spectroscopy using a common-path interferometer. This setup is capable of measuring absorption as low as 10-5 cm-1. The dependence of the BLIIRA signal on blue light average power and intensity as well as on the crystal temperature was studied. The results show the presence of at least two different types of color centers. A higher level of remnant absorption was observed in the phosphates compared to that of the arsenates. The largest portion of the induced absorption is attributed to photo-generated electrons and holes being self-trapped in the proximity to the Ti4+ and O2- ions, respectively, forming polaron color centers. Stabilization of these centers is aided by the presence of mobile alkali metal vacancies in the crystal.
Fakultetsopponent: Professor Takunori Taira, Laser Research center for Molecular Sciense, Inst for Molecular Science, National Institutes of Natural Science (NINS).
Ledamöter i betygsnämnden: Professor Mohamed Bourennane, Kvantinformation och Kvantiotik, Fysikum, Stockholms Universitet, Stockholm
Dr. Jonas Hellström, CTO, Cobolt AB, Solna
Dr. Jenni Nordborg, Enhetschef Bioentreprenörskap, Avd Hälsa, VINNOVA, Stockholm.
Handledare: Prof. Fredrik Laurell