Silica fibers with silicon oxynitride core fabricated by plasmachemical technology

It is known that nitrogen can increase the silica-glass refractive index up to 2.0, which corresponds to silicon nitride. However, at present, light-guiding structures based on silicon nitride or oxynitride exist only as planar structures in integrated-optics devices. Such structures are produced by various low-temperature CVD techniques, including those utilizing glow-discharge plasma. Because the raw materials are hydrogen-containing molecular gases, such as silane and ammonia, a large share of optical loss in such lightguides is due to the absorption peaks at 1.38 and 1.52 μm, which are associated with the presence of OH and NH groups in the glass, respectively. It is possible to slightly reduce the concentration of these groups by thermal treatment of the structures. This results in a loss reduction from 2 to 0.2 dB/cm at 1.52 μm.1 Clearly, 0.2 dB/cm, or 20000 dB/km, is acceptable for several-centimeter-long planar lightguides, but it is not acceptable for optical fibers. Therefore, the fabrication of low-loss silicon oxynitride fibers requires a conceptually different technological process that does not involve hydrogen-containing components.