Single-mode fiber microlens with controllable spot size

A novel method for fabricating microlenses on tapered single-mode fibers is shown to be able to control the lens spot size. The fiber cladding is first symmetrically tapered by etching it with an evaporating ammonium bifluoride solution. A hemispheric lens is then melted on the taper tip with a CO(2) laser. The lens can reduce the fiber mode radius to 40% of its original value. A theoretical calculation of the focused spot size agrees well with experimental results.