Single-mode fiber This refers to the optical fiber that can transmit only one propagation mode in the operating wavelength. It is commonly referred to as single-mode fiber (SMF: Single ModeFiber). In cable TV and optical communication, it is the most widely used optical fiber. Because the fiber core is very thin (about 10μm) and the refractive index is a step distribution, when the normalized frequency V parameter is less than 2.4, theoretically, only single-mode transmission can be formed. In addition, SMF does not have multi-mode dispersion, so the transmission frequency band is larger than that of the mode fiber, and the composite property of SMF happens to form zero dispersion, which makes the transmission frequency band wider. There are many types of SMF due to different dopants and manufacturing methods. Depr-clad Fiber in which the cladding forms a twofold structure, adjacent to the cladding of the core, and possessing a lower refractive index than the outer inverted cladding.