A method of treating atrial fibrillation includes the step of positioning a distal portion of an ablation instrument in proximity to one or more pulmonary veins. The instrument has a hollow housing and an independently axially positionable energy emitter within the distal portion The energy emitter is configured and the distal portion is shaped such that a distance a beam of ablative energy travels from the energy emitter to the portion of the distal portion in contact with target tissue changes based on the energy emitter's location along the length of the instrument while the distal portion maintains its shape The energy emitter is a light emitting element and a beam-forming optical waveguide that receives light from the light emitting element. The waveguide projects onto the distal portion a hollow cone of radiation.