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Abstract
Since the broadband near infrared (NIR) emission of bismuth-doped fibers (BDFs), great attention has been paid to the creation of new BDF based laser media operating in the spectral region from 1100 to 1550 nm. However, the unknown origin of bismuth active centers (BACs) responsible for NIR emission and heavy bismuth doping induced strong background losses have become the two main obstacles in developing efficient lasers/amplifiers. According to the previous research, the study of thermal effects on optical properties of Bi doped material is one of the most efficient approaches to reveal the nature of both BACs and passive background losses.
In this thesis, spectral properties of a new type of bismuth (Bi) and erbium (Er) co-doped multicomponent fiber (BEDF), manufactured by National Fiber Facility at UNSW, are assessed. The existence of several BACs in BEDF is confirmed by spectral absorption and emission measurements. To investigate the variation of BACs at different temperatures, thermal and annealing effects on NIR emission and attenuation of BEDF are studied. This thesis mainly shows two new results: (1) the irreversible luminescence quenching effect on BACs, especially BAC-Si and (2) the irreversible thermal darkening effect in BEDF from 600 to 800 °C.
In the first work aiming at investigating thermal effects and annealing effects on NIR emission of BEDF, NIR emission band of BAC-Si at 1420 nm under 830 nm pumping is found to start to be quenched at 100 °C. It means that BAC-Si in BEDF is relatively instable than other BACs. We ascribe the reduction of BAC-Si concentration to the migration of BAC-Si related defects or redistribution of trapped-electrons in Bi doped materials. At 600 °C and the higher temperatures, emission bands of all BACs and Er start to be irreversibly quenched accompanied by strong absorption at 830 nm.
In the second work, thermal effects on the attenuation of BEDF are investigated. The irreversible thermal darkening effect is observed in BEDF in the temperature range from 600 to 800 °C. We ascribe such a strong darkening effect (passive background losses) in both visible and NIR spectral range to the absorption of metallic bismuth nanoparticles formed and aggregated at high temperature, which can be fitted by the hyperbolic dependence ~ (1/λ) in the spectral range 900 to 1600 nm. These results demonstrate that precipitation process of the metallic bismuth nanoparticles can start to occur at 600 °C only in BDFs. We also find that the BEDF with high Bi doping concentration presents the greater attenuation whereas the BEDF with low Bi doping concentration shows less. Meanwhile, strong thermal darkening effect is hard to be observed in the slowly drawn thicker BEDF which is believed to have the fewer defects. Thus, we assume that high concentrations of Bi and drawing induced defects could make precipitation of metallic bismuth nanoparticles take place at relatively low temperatures. Besides, no obvious variation of BAC-Si related absorption is found when BEDFs are annealed at 600 °C and 650 °C respectively, indicating that formation and degradation of BAC-Si do not take part in precipitation process.
These studies mentioned above illuminate the origin of strong thermal darkening effect observed in the BEDF and discussed the possible causes leading to the reduction of BAC-Si at low temperature. The results demonstrate a new evidence for understanding the reduction of BAC-Si in BEDF and provide a potential reference for controlling the thermal darkening effect of Bi-doped fiber by reducing the concentration of Bi and defects during the fabrication processes.