Fiber optic patch cord means that both ends of the optical cable are equipped with connector plugs to realize the active connection of the optical path; one end with a plug is called a pigtail. Fiber optic patch cords are similar to coaxial cables, but without the mesh shield. In the center is the glass core for light propagation. In a multimode fiber, the diameter of the core is 50 μm to 65 μm, roughly equivalent to the thickness of a human hair. The single-mode fiber core has a diameter of 9 μm. The core is surrounded by a glass jacket with a lower refractive index than the core to keep the fiber inside the core. On the outside is a thin plastic jacket to protect the envelope.