An optical communications repeater is used in a fiber-optic communication

Fiber-optic communication

Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information...

s system to regenerate an optical signal by converting it to an electrical signal, processing that electrical signal and then retransmitting an optical signal. Such repeaters are used to extend the reach of optical communications links by overcoming loss due to attenuation

Attenuation

In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

 of the optical fibre and distortion

Modal dispersion

Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes...

 of the optical signal. Such repeaters are known as 'optical-electrical-optical' (OEO) due to the conversion of the signal. Repeaters are also called regenerators for the same reason.

Classification of regenerators

Optical regenerations are classified into 3 categories by the 3 R's scheme.

1. R : reamplification of the data pulse alone is carried out.
2. 2R : in addition to reamplification, pulse reshaping is carried out. E.g.: Mamyshev 2R regenerator
   Mamyshev 2R regenerator
   The Mamyshev 2R regenerator is an all-optical regenerator used in optical communications.In 1998, Pavel V. Mamyshev of Bell Labs proposed and patented the use of the self-phase modulation forsingle channel optical pulse reshaping and re-amplification....
   
   .
3. 3R : in addition to reamplification and reshaping, retiming of data pulse is done.

All-optical regenerators

An alternative method of regeneration is through all-optical without requirement to convert back and forth.
Using Non-linear optical fiber allow designers to use nonlinear effects of frequency shifting and frequency generation effects
for regeneration. The key advantage of all-optical regeneration is power efficiency provided by the device, and simple integration into
an optical network.

Optical amplifiers

Cost efficiency has led to repeaters being largely replaced in long-haul systems by optical amplifier

Optical amplifier

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed...

s since one (broadband) amplifier can be used for many wavelengths in a Wavelength Division Multiplexing (WDM) system. Note that this class of device is sometimes called "Optical Amplifier Repeater".

Electronic vs optical regeneration

Due to the high data rates that can be achieved with optical systems, OEO repeaters are expensive to implement as electronics to handle those high data rates are expensive and difficult to construct. Also, since one repeater is required for each wavelength, and many tens of wavelengths may be transmitted down a single fibre, a lot of equipment is required for each fibre. In contrast, an optical amplifier can amplify all of the wavelengths in a single device. An amplifier does not provide the regeneration ability of a repeater, but loss, rather than distortion is generally the limiting factor in the design of communications system.