The single-mode fiber, also called single-mode fiber, is an optical waveguide with a step- index profile, in which a very small core glass with a diameter of 9 µm (8.3 µm) transmits the light practically only in one mode, which is quasi parallel to the axis.
The monomode fiber is characterized by the fact that it has practically no propagation time differences(mode dispersion 0.1 ns/km), since the light passes through the optical waveguide in only one propagation direction, the pulse behavior is thus true to shape, and it has the lowest attenuation values of all optical waveguides.
This is expressed in an attenuation of 0.1 dB/km( LED 1,300 nm). Distances of up to 100 km can be bridged without repeaters. The cladding diameter of the monomode fiber is typically 125 µm, the core diameter typically 9 µm.
The optical window for monomode fibers
Monomode fibers can operate in the wavelength range between 1,280 nm and 1,650 nm, in the 2nd or 3rd optical window. This results in a theoretical bandwidth of 53 THz. In practice, values of 10 Tbit/s over 100 km have already been achieved.
To meet the increased requirements of optical networks with Wavelength Division Multiplexing(WDM) and Dense Wavelength Division Multiplexing( DWDM), dispersion optimized single mode fibers are available: Non Dispersion Shifted Fiber( NDSF), Dispersion Shifted Fiber( DSF), and Non Zero Dispersion Shifted Fiber( NZDSF), which have been standardized by the International Telecommunication Union( ITU) in Recommendations G.650 and following
ITU-T G.652: This is a Non Dispersion Shifted Fiber (NDSF) with minimum dispersion at 1,310 nm. Attenuation at 1,550 nm is 0.25 dB/km, and dispersion is 17 ps/nm based on one kilometer of fiber. This fiber is suitable for LAN applications at up to 2.5 Gbit/s and for Coarse Wavelength Division Multiplexing( CWDM) at 4 x 2.5 Gbit/s.
ITU-T G.653: The single mode fiber standardized under this recommendation is a Dispersion Shifted Fiber (DSF) with minimum dispersion at 1,310 nm. The attenuation is 0.25 dB/km relative to a light source at 1,550 nm.
ITU G.655: A single mode fiber with Non Zero Dispersion Shifted Fiber (NZDSF) and a minimum dispersion at 1,550 nm. The attenuation is 0.25 dB/km and the dispersion is only 4 ps/nm, based on 1 km. This fiber is for DWDM systems.