Dr Light's introduction to Optical Communications
An optical signal contains a spread of different wavelength components. In other words, each signal has a frequency spectrum. Each wavelength or frequency will travel down an optical fiber with a slightly different velocity, and therefore arrive slightly different times. Therefore an initially narrow optical pulse launched into a fiber will spread out as it travels down the fiber; eventually the 1's and O's in a digital signal become blurred and indistingishable. Chromatic dispersion is a weaker limites the data rate and distance for optical fiber systems.
Dense Wavelength Division Multiplexing is technique which allows many channels to be carride by the same fiber. Each channel is mapped to a different wavelength. DWDM systems typically use channels which correspond to the wavelengths standardized by the ITU.
Four wave mixing is a non-linear effect which occurs when the optical power levels are sufficient to cause adjacent wavelength channels to interfere. It is caused by the optical signal temporarily changing the properties of the optical fiber as it passes through the optical core. It can lead to degraded system quality in DWDM systems.
The ITU Grid is a set of wavelength channels standardized for use in optical fiber communication systems by the International Telecommunications Union. The standards facilitate interoperability between components and subsystems made by different companies.
An optical signal traveling within a multimode optical fiber can take multiple paths (modes), along the fiber core. Each optical path has a slightly different path length, and there each optical mode will reach the other end of the fiber at slightly different times. This causes the signal to become distorted; an initially narrow pulse will spread out, as it travels further down the fiber. Eventually, the distoration causes the signal to become unrecognizable. This effect is called modal dispersion. It limits the practical length of multimode fiber systems to a few hundred meters. (Less if the data rate is greater than 1Gbit/s).
Multimode fiber is a cheaper type of optical fiber used to carry signals typically less than 1 Gbit/s, over a few tens of meters. The fiber core is typically 50 or 120 microns in diameter. Signals traveling down multimode fiber suffer from modal distortion, limiting the distances they can support. Single mode fiber is used to support higher data rates and longer distances.
An optical add drop multiplexer is a optical system component which allows one or more signals or channels to be introduced or removed from a optical fiber system that contains many channels or signals.