Abstract: A method is disclosed for manufacturing a hairspring (1) for a timepiece movement, the method comprising the following steps: producing a malleable elongated element in the form of a fibre or ribbon from a first heated material capable of guiding light, shaping the malleable elongated element into a spiral, and processing the thus obtained spiral element in order to produce a hairspring providing both a mechanical oscillator function in a hairspring-balance wheel assembly and an optical guidance function. The shaping step comprises a step in which the malleable elongated element is wound around a rotary shaping tool.