Soft glass optical fibres for telecommunications applications

Abstract

As the appetite for data use across telecommunications networks is predicted to continue to grow rapidly in the coming years, there is an increasing need to address the bandwidth gap that exists between the optical links that underpin high speed networks and the electronic layer typically used for processing signals at the endpoints. Nonlinear fibre optics is a potential avenue to addressing this bandwidth bottleneck, where nonlinear optical phenomena can be exploited to perform signal processing tasks, thereby allowing the broad bandwidth of optical media to be used for signal processing as well as transmission. Indeed the development of such optical signal processing devices is crucial to moving towards the next generation of communications technology - where ultra fast telecommunication networks with speeds approaching 1 Tb/s are required. This work explored the use of the enhanced optical nonlinearity and dispersion engineering possible in soft glass microstructured fibres as a basis for developing devices for broadband telecommunications applications at 1.55 μm. Two applications were considered in this research, namely multicasting and phase sensitive amplification - both of which are signal processing applications that are important to the realisation of all optical networks. A number of soft glass materials were studied in this research, primarily those with high nonlinear refractive indices such as chalcogenides, tellurites, bismuth oxide based glasses and germanates. During the course of this work a novel lead germanate glass was also developed. This glass was shown to have a high nonlinear index and relatively high mechanical strength when compared to tellurite glasses of similar refractive indices. Dispersion tailored, soft glass fibre designs were developed for both multicasting and phase sensitive amplification. The design geometry, referred to as a ‘hexagonal wagon wheel design’, was a hybrid model combining a hexagonal array geometry for dispersion engineering with a suspended core or ‘wagon wheel’ geometry for high nonlinearity. The fibre designs were optimised for each application by using a genetic algorithm based optimisation technique to achieve high and broad gain suitable for efficient signal processing at extremely high bit rates. Each fibre design was modelled for its intended application to demonstrate, numerically, that the designs were indeed capable of performing their intended application over a broad band. The modelling work used a numerical beam propagation model and demonstrated that the designs were capable of operating at the extremely high bit rate of 640 Gb/s. Advances were made to fabrication techniques during the fabrication trials of these novel designs due to the complex nature of the designs and, in some cases, the use of novel materials. A first generation, simplified hexagonal wagon wheel fibre was fabricated in the novel germanate glass developed earlier. A number of characterisation experiments were also performed on fabricated microstructured fibres, including a measurement of the dispersion profile for a tellurite fibre (that was shown to be in good agreement with modelling results) and the measurement of the nonlinear index for a fibre fabricated with the novel germanate glass - one of the few such measurements in the literature for this family of glasses. In addition to these fabrication advances and characterisation experiments, a study of dispersive waves was performed on previously fabricated hexagonal wagon wheel fibres in collaboration with colleagues at the University of California, Merced. These experiments were used to study soliton propagation in these fibres at near infrared wavelengths. Comparison of experimental data to theoretical models is shown to have good agreement - an important validation of the modelling technique.