Abstract: An air bubble (23) in the core (3) of an optical fiber (1, 1') constitutes a broad band reflector to light waves propagated in the fiber. A fiber end (3) is first stripped and cut in the conventional way, after which in the end surface, a pit (13) in the core region is produced by etching. The fiber end is therefor treated with e.g. diluted hydrofluoric acid, that etches the higher doped core (3) more rapidly than the cladding (5). The etched fiber end (1) is then welded to a standard fiber so that a splice (21) is obtained where the desired air bubble (23) is enclosed in the fiber core. The magnitude of the reflectance of the produced reflector can then be changed by repeated heating steps, in the same way as in welding, of the splice (21) and/or by filling the bubble (23) with another medium than air or by coating one of its walls with a suitable material, e.g. a metal.