Abstract: Special-purpose transition and fittings for optical fibers or bundles of optical fibers are produced by selecting a hollow shell having an internal surface defining a desired barrier configuration. The shell is filled with a liquid barrier material under an applied vacuum and then partially drained to provide a liquid cladding of the barrier material which is then cured. A core of fiber optic material is cast against the cured cladding of the barrier material under an applied vacuum after which optical entrance and exit surfaces are formed on the cast core for joining with an optical fiber.