A method and apparatus for treating a body tissue in situ (e.g., an atrial tissue of a heart to treat atrial fibrillation) include a lesion formation tool is positioned against the heart surface. The apparatus includes a guide member having a tissue-opposing surface for placement against a heart surface. The guide member also has interior surfaces and a longitudinal axis. A guide carriage is sized to be received with the guide member and moveable therein along the longitudinal axis. An optical fiber is positioned within the guide carriage with the carriage retaining the fiber. The carriage receives the fiber with an axis substantially parallel to the longitudinal axis and bends the fiber to a distal tip with an axis of said fiber at said distal tip at least 45 degrees to the longitudinal axis and aligned for discharge of laser energy through the tissue opposing surface.