Characteristics of carbon coatings on optical fibers prepared by radio-frequency plasma enhanced chemical vapor deposition with different H2/C2H2 ratios

Characteristics of carbon coatings on optical fibers prepared by radio-frequency plasma enhanced chemical vapor deposition with different H2/C2H2 ratios are investigated. Five kinds of carbon coatings are prepared with H2/C2H2 ratios of 2, 4, 6, 8, and 10. Experimental results show that the deposition rate and surface roughness of carbon coatings decrease as the H2/C2H2 ratio increases. When the H2/C2H2 ratio changes from 2 to 8, the increase of H2/C2H2 ratios detrimentally yields sp3 carbon atoms and sp3-CH3 bonds in the carbon coatings. However, when the H2/C2H2 ratio exceeds 8, the hydrogen retards the growth of the graphite structure. Moreover, the redundant hydrogen radicals favor bonding with the dangling bonds in the coating surface. Therefore, when the H2/C2H2 ratio increases from 8 to 10, the amounts of sp3 carbon atoms and sp3-CH3 bonds in the carbon coatings increase. At an H2/C2H2 ratio of 8, the carbon coating exhibits excellent water-repellency and thermal-loading resistance, and so this ratio is the best for producing a hermetically sealed optical fiber coating.