Method for manufacturing of an optical fiber with a decoupling interface for scattered light to monitor the power of light guided through the optical fiber, where the optical fiber has a core having a first refractive index and a cladding surrounding the core, the cladding having a second refractive index smaller than the first refractive index, and where a portion of the optical fiber is substantially straightly aligned in the region of the decoupling interface, in which method the optical fiber is electro-thermally treated at an intermediate position within the substantially straightly aligned portion such that a partial mixture of core material and cladding material and, thereby, formation of scattering centers occurs in an interface region between the core and said the cladding, thereby forming the decoupling interface for scattered light from the modified intermediate position.