Single-mode fiber optic cable typically has a single core. This means that it consists of a single strand of glass fiber that carries light signals. The core is the central part of the cable through which the light travels, surrounded by a cladding layer that helps guide the light. Single-mode fiber optic cables single-mode fiber optic cables 1 have a small core, typically around 9µm, and are designed to carry signals over long distances at higher bandwidths. They feature low attenuation benchmarks 2 and minimal dispersion. Single mode fibers are. The number of optical cores in an optical fiber is the total number of equipment interfaces multiplied by 2, plus 10% to 20% of the spare quantity, and if the communication mode of the equipment has serial communication and equipment multiplexing, you can reduce the number of cores.