An optical fiber being optically transmissive at a predetermined wavelength of light .lamda. and comprising a plurality of coaxial layers. Each layer having an optical path length that varies radially, the coaxial layers being arranged to give the fiber a refractive index profile which, in use, causes sufficient Fresnel diffraction of the light such that it is guided in the fiber. The refractive index of a cladding region (60) is intermittently suppressed by controlling heating of the preform tube, thus forming a chirped saw-tooth profile (70). The optical fiber may include a lens. In this case, each of the layers has an optical path length that increases gradually outwardly by substantially n.times..lamda./2 (n: integer).