Disclosed is a method to prepare and coat an optical fiber with Si.sub.3 ub.4. The Si.sub.3 N.sub.4 is synthesized from SiF.sub.4 +NH.sub.3 and/or SiH.sub.4 +NF.sub.3 by laser photochemical reaction (LPR) techniques in a controlled atmosphere chamber. The silicon fiber which is a representative optical fiber is drawn from a preform at about 2000.degree. C. in a standard fiber drawing furnace. Immediately after drawing of fiber, the fiber is passed through a chamber with a controlled atmosphere containing SiX.sub.4 and NX.sub.3 gases, wherein X equals hydrogen or fluorine. A laser is used to irradiate the atmosphere and the Si.sub.3 N.sub.4 is coated on the optical fiber to a thickness of about 0.02-0.20 micrometer which hermetically seals the fiber and enhances and maintains the mechanical strength thereof.