In fiber optic technology, OS2 refers to single-mode fiber (SMF), which is specifically designed for transmitting a single light ray. OS2 cable offers low signal attenuation and high bandwidth. Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining. The two main types — Single Mode (SM) and Multimode (MM) — differ in construction, performance, and application. This guide explains how to identify them by appearance, labeling, and technical specifications, helping you make the right choice for your installation. What Is Single Mode Fiber? Single. But Fitel's ID-H/R LightFinder is one of the best fiber identifiers.