“Single-mode fibres” are indicator species for light waves. Single-mode fibres or mono-mode fibres have a diameter of 10 µm. They are channelled through and may transport data over long distances. The core diameter of a single-mode fibre is that small that only one modus might diffuse. For the use of single-mode fibres, extremely expensive lasers are applied that involve significant costs. However, the requirement should not be underrated as they need to deliver a sufficient bandwith to allow for a flexible network design.