The Preparation of Optical Waveguide Preforms by Plasma Deposition

The modified chemical vapor deposition (MCVD) process1 has achieved acceptance as a means of preparing low-loss optical waveguides. A recent study of the preparation of silica-clad germania borosilicate optical fibers by this process2 indicates that it can provide low-loss optical fibers having properties which surpass minimum requirements for wide-band communications applications. Typical fibers produced by M C V D are characterized by low loss (4 dB/km at 1.06 /im), a reproducible graded index profile (a ~ 2.0) and controlled core/cladding ratio, circularity and concentricity. Although current deposition rates are competitive with other processes, higher rates are desirable. Furthermore, the efficiency, defined as the ratio of glass deposited/reactants in, is generally below 50 percent for most Ge02 containing compositions. These considerations stimulated our investigation into using the ionized environment of a plasma to increase the rate and efficiency of the M C V D process. Initially both microwave and RF plasmas were considered. In the former, ionic species are produced which deposit as a vitreous deposit directly on the tube walls as the result of a heterogeneous reaction similar to conventional C V D . 3 This advantage, however, is purchased by the use of reduced pressures which dictates a low flux of reactants. As a result, even at 100 per cent efficiency, the deposition rates in the microwave method need be no higher than M C V D and frequently are lower. 205