Blue and UV transmission in optical fibres
The work presented in this thesis shows that the predominant factor limiting high power transmission at blue/green wavelengths in germanosilicate optical fibres is two-photon absorption leading to the creation of colour centres. It is determined that these colour centres are germania-related, and cause a large induced absorption in the fibre at the operating wavelength. It is demonstrated that these centres can be bleached out by single-photon absorption, leading to an intensity-dependent absorption. The long optical path lengths mean that even at very low concentrations these colour centres can seriously degrade the fibre performance. Theoretical analysis of the experimental results indicates that there are three types of defect involved in the dynamics of the system, each with its own characteristic trapping and decay rates. The precursor defects are shown to be caused by an oxygen deficiency in the glass, and the induced absorption resembles the optical absorption associated with Ge(1) centres. Research into the production of improved fibres is also reported. There are a number of ways in which the fabrication of germanosilicate fibres can be modified, either to reduce their photochromaticity or to enhance it, as required for nonlinear applications such as second harmonic generation or photorefractive effects. For applications where high power handling capabilities are required the development of polarisation-maintaining single-mode optical fibres has been investigated. The fabrication of fibres capable of transmitting several Watts of blue/green light without degradation is presented.