Characteristics of carbon coatings on optical fibers prepared by radio-frequency plasma enhanced chemical vapor deposition with different H-2/C2H2 ratios are investigated. Five kinds of carbon coatings are prepared with H-2/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 H-2/C2H2 ratio increases. When the H-2/C2H2 ratio changes from 2 to 8, the increase of H-2/C2H2 ratios detrimentally yields sp(3) carbon atoms and sp(3)-CH3 bonds in the carbon coatings. However, when the H-2/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 H-2/C2H2 ratio increases from 8 to 10, the amounts of sp(3) carbon atoms and sp(3)-CH3 bonds in the carbon coatings increase. At an H-2/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. (C) 2010 Elsevier B.V. All rights reserved.