An optical fibre having an NA-value of less than 0.34, which optical fibre is built up of a core and an enveloping protective coating, wherein the protective coating forms the outer layer of the optical fibre, which optical fibre has an NA-value ranging from 0.15 to 0.30, with the overall external diameter of the optical fibre corresponding to the internal diameter of an element into which the end of the optical fibre is inserted.