Photo darkening characterization and mechanisms: [invited]

Photo darkening (PD) is a serious performance degradation and lifetime limitation for rare earth (RE) co-doped silica fiber devices. Mitigation of PD is the key to improvement of the performance and reliability of high power fiber lasers and amplifiers. It is generally observed that PD of a given fiber material in unseeded amplifier experiments scale with population inversion. These observations are taken to indicate that the PD formation process is a multi step multi photon absorption process, where several excited RE ions co-operate to supply energy for capturing electron and holes at different structural defects. The nature of these defects is, however, still under debate. The electron supplying defects or PD precursors are often attributed to either oxygen deficiency centers (ODC) or charge-transfer-states (CTS) respectively capture of electrons on Yb3+ pair ODC related to Yb2+ formation. The color center becomes hereby either neutral oxygen vacancies that have received an electron to form E’ type color centers or by capture of a hole on surrounding anions such as (AlO4/20) reported for smoky quartz. In this presentation a phenomenological model for PD is suggested and the mechanism both of color center formation and the physics of the color center is described. Experimentally the induced PD is found to be either partially or fully reversed through temperature annealing or exposure to UV and/or visible light. Even bleach by the 915 nm pump power gives this possibility. Temporal reduction of PD is shown by hydrogen loading and co-doping with phosphorous is found to reduce the amount of PD and cerium appears to improve the durability of Yb-doped fibers even further. These various aspects will be discussed in terms of the phenomenological model. The proposed model is applicable to rare earth doped fiber material for amplifiers and lasers. In terms of PD there is no significant difference between cw and pulsed regime of operation in that the phenomenon of PD is slow. The PD of a pulsed system will, however, be a function of both average power (temperature and signal flux) and peak power (flux through the rare earth co-doped active areas). By means of the proposed model a quick assessment of the quality of a given material in terms of PD can be determined based on unseeded amplifier operation.