Utilization efficiency of cladding pump light in a cladding pumped optical device is improved by converting higher order modes travelling in the cladding to lower order modes that enter the core region and participate more effectively in the energy exchange process. The mode conversion is achieved by asymmetric perturbations in the optical fiber. The perturbations are preferably produced by making the optical fiber in the gain section of the device cylindrically asymmetric. The asymmetric perturbations can be chosen so that they have negligible effect on the lower mode signal light in the core of the optical fiber.