Potassium double tungstate waveguides with high ytterbium concentration for optical amplification

Abstract

In this thesis, the research work concerning high ytterbium concentration potassium double tungstate waveguides catered for optical amplification purpose is presented. The scope of the research work includes the investigation of spectroscopic and optical gain properties in epitaxy layers with concentration of trivalent ytterbium (Yb3+) up to 76 at.%, which is equivalent to Yb3+ density of ~5 × 1021 cm-3. Spectroscopic properties of the high ytterbium concentration epitaxy layers, such as luminescence lifetime, transition cross-sections, and their temperature dependence, are examined carefully and in detail. A novel confocal measurement setup is proposed to mitigate the radiation trapping effect which elongates the measured lifetime. It is confirmed that concentration dependent lifetime quenching on high Yb3+ concentration potassium double tungstate epitaxial layers is rather weak. Further analysis on the power dependent luminescence decay curves reveals the presence of energy transfer upconversion (ETU) process. The cross-section spectra of the high ytterbium concentration potassium double tungstate epitaxy layers are similar to those of bulk crystals. It is observed that the cross-section spectra change drastically with the increase of temperature due to two reasons: the fractional population at the starting Stark level and the linewidth of the respective transition at the given temperature. The material gain in thin film configuration are investigated via experimental and numerical approaches. Net gain value of 2.62 dB (817 dB/cm) is achieved in 32 μm thick epitaxial layer without any thermal management using pump wavelength of 932 nm and signal wavelength of 981 nm. In overall, the work described in this thesis provide advances in understanding the characteristics of high Yb3+ concentration potassium double tungstate waveguide layers. The experimental results show that favorable spectroscopic properties are retained in the epitaxial layers. Nevertheless, additional effects such as ETU process, quenched ions, and localized heating within the pumped region have also been discovered and analyzed. Particularly, elevated temperature on the gain medium would severely affect the absorption and emission behavior. The investigation of optical gain and luminescence spectra shows that the thermal effects play a role in high active ion concentration and intensely pumped amplifier.