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Dear friend！
It is a mode of transmitting optical signals through optical fibers. Optical signals can be transmitted in two modes: single-mode transmission and multi-mode transmission.
The single-mode transmission features a long transmission distance and large bandwidth capacity.

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Hi dear,

fiber optic cable has a small diametral core that allows only one mode of light to propagate. Because of this, the number of light reflections created as the light passes through the core decreases, lowering attenuation and creating the ability for the signal to travel further. This application is typically used in long distance, higher bandwidth companies .

Single Mode fiber is usually 9/125 in construction. This means that the core to cladding diameter ratio is 9 microns to 125 microns.

But in multimode
Due to its large core, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternate paths cause the different groups of light rays, referred to as modes, to arrive separately at the receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits the amount of information that can be sent. This type of fiber is best suited for transmission over short distances

Source: https://www.multicominc.com/training/technical-resources/single-mode-vs-multi-mode-fiber-optic-cable/