Spherical Ball Lenses are commonly used for laser collimating and focusing, laser-to-fiber coupling, fiber-to-fiber coupling, and fiber-to-detector coupling. Larger spheres are easier to handle and ease the sensitivity of translational alignment. However, smaller spheres has the benefit of fitting into smaller packages. 
Ball lenses are highly polished spheres made of optically transparent homogeneous materials. Carefully controlled manufacturing process produces spheres with precise diameters and unsurpassed surface quality.