According to the helicopter pilot, he dropped off a lineman on top of a power line tower, to install fiber lines. The lineman completed the task and informed the pilot by radio that he was ready to be recovered from the tower. During the recovery, the pilot hovered next to the tower and signaled to the lineman that he was ready for him to transfer from the tower to the left skid of the helicopter. When the lineman reached out to attach his lanyard to the helicopter, the pilot felt the main rotor blades strike the fiber line that was just installed. The helicopter remained controllable and the pilot made a precautionary landing without recovering the lineman. After discussing the event with the lineman, the fiber line was raised to a higher position on the tower prior to the recovery of the lineman. Upon returning to base, a post-accident examination of the helicopter revealed that the helicopter sustained substantial damage to the main rotor blades. The pilot reported that the accident could have been prevented by assessing each approach and landing individually, while looking for any possible new hazards or obstructions. The pilot reported that there were no mechanical malfunctions or failures with the helicopter that would have precluded normal operation.