Recognizing the rate-determining nature of the coating removal and recoating steps, applicants have demonstrated that with proper combination of low absorbing polymer, glass and low intensity radiation, UV-induced gratings can be side-written into polymer coated fibers without removing the polymer, thus permitting the possibility of high speed fabrication of fiber gratings.
FIG.2, which shows typical apparatus for practicing the method, includes a typical fiber (20) comprising a core (21), a cladding (22) surrounding the core, and an outer polymer coating (23).
UV light from laser (24) passes through mask (25) the polymer coating (23), and the cladding (22) to write a pattern of index changes along the photosensitive core (21). Alternatively, the pattern can be defined by two interfering beams of UV radiation. The UV radiation should be at a sufficiently low intensity that it does not seriously damage the polymer coating.
A methylsilsesquioxane-coated fiber treated with D 2 was exposed to UV light from a KrF excimer laser. An amplitude mask was used to produce long period gratings. The fiber was held taut to the amplitude mask, and its side adjacent to the mask was exposed to the UV radiation. At 130mJ/cm 2 1 dB loss developed at the selected wavelength after exposure for approximately 5 mintues. Examination of the fiber surface showed some physical damage to the polymer with periodicity comparable to the amplitude mask, but the coating remained intact and the damage appeared superficial. Decreasing the power to 100 mJ/cm 2 resulted in 0.5 dB loss, and minor damage to the surface of the polymer.