An optical fiber preform is a highly pure glass rod, typically 1 to 2 meters long, composed of two main parts: Core (or rod): The center region, responsible for carrying light signals. Cladding: The surrounding layer that keeps the light confined to the core through total internal. Optical fiber preforms are the starting point behind every kilometer of fiber optic cable. Typically, preforms are about 40 cm long with diameters ranging from a few centimeters to as large as 20 cm. What makes fiber optic cables special is their ability to. Heraeus Covantics has been a driving force in the evolution of preform sizes. With the RIC ® process, we can turn your core rod into a full preform. The core rod with the correct b/a is placed into a cylinder and in a consecutive hot forming step the cylinder is collapsed onto the core rod.