Single mode Fiber (SMF) uses a single ray of light to carry light signal transmission over long distances. Single mode fiber (SMF) can transmit data to distances far more than multi mode fiber (MMF).
Single mode optical fiber (smf) cables have a small core size compared with multimode optical fibers. Typically, single mode optical fiber (smf) cable core has less than 10 microns (µm). Single mode optical fiber (smf) cables permits only one ray of light to be transmitted. Hence the light signals in single mode optical fiber (smf) cables can reach far distances, without much attenuation.
Comparatively reduced attenuation is the main advantage of single mode fiber (SMF). Attenuation is the reduction in the intensity of a light signals (thus quality of light signals) when signals are transferred over long distances because of scattering of light.
Please refer below image to understand how signal is transferred in single mode optical fiber (smf)