Dr. Peter Horak, a co-director of the NGCM from the Optoelectronics Research Centre at the University of Southampton, delivered a talk on the 21 March 2019 on his research in optical fibres. Dr. Horak introduced their uses, modelling them, and the challenges faced.
Optical fibres are thin strands of glass similar in diameter to human hair, along which light is guided through via total internal reflection. These fibres typically consist of a core and a cladding layer which have higher and lower refractive indexes, respectively. To model the propagation of light through the fibre, Maxwell’s equations and the nonlinear Schrodinger equation are solved numerically.
The first application described was optical telecommunications, as shown in Figure 1. Over 3 billion kilometres of fibre is installed across the world; equivalent to 20x the Earth-Sun distance! With a 40% increase in worldwide internet traffic every year, higher capacity fibre is critical. As we can see in Figure 2, the capacity limit for single-mode fibre is 10^14 bit/s. To put this into perspective, BT offer up to 300 Mbit/s for their Ultrafast Fibre. Having reached capacity, we look for other technology with a higher capacity limit.
Another application is the delivery of high power fibre lasers for manufacturing and defence. High power lasers require double-cladded fibres to amplify and guide the light. Large power can cause fibre damage, optical nonlinearity and thermal problems. Thermal management is modelled using multiphysics finite element methods. Dr. Horak explained the solution to this, increasing the fibre core size, leads to multiple fibre modes due to dispersion. Multi-modes cannot be avoided. Instead of focusing on the optics, research is active in improving digital data processing of these multiple electronic modes.
Optical fibres are also useful as sensors for detecting external conditions, for example along train tracks in Figure 3. This is used to monitor the position of a train or any defects in the wheels or breakages in the tracks. Other infrastructure such as bridges, tunnels, buildings and oil wells also benefit from fibre sensing. Fibre sensing works by launching a laser pulse along the fibre. Light is reflected back at points along the fibre and is measured, to determine the location of the light scattering. This scattering occurs for a number of reasons, including impurities or density fluctuations in the material. Brillouin scattering is used for simulating this.
Another area by the Optoelectronics Research Centre involves looking at the physical structure of the glass fibres and how the shapes of the pores can change their performance, using the nonlinear Schrodinger equation. This includes the study of mechanical properties and fibre fabrication, i.e. controlling the microstructure shape using gas pressures, temperature and differing speeds, as in Figure 4.
To conclude, Peter introduced us to optical fibres and their applications in this seminar. It is evident that optical fibres play a significant role in today’s society, being widely used in communication, manufacturing and sensing. As a result, there is considerable research in the study of fibre geometries, materials and simulation methods, for improving this technology.
Figure 2 adapted from: Richardson, D. J., Fini, J. M., and Nelson, L. E., Space-division multiplexing in optical fibres, Nature Photonics, 7, 354-362, 2013.
Posted by Rebecca Clements and Vivienne Nenh