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Schematic diagram of a fiber amplifier

In optics, an optical amplifier is called a device which amplifies a light signal without the need to convert it first into an electrical signal before amplifying it with conventional electronics techniques.
A doped fiber amplifier works like a laser. A portion of optical fiber is doped and is optically pumped with a laser to place the doping ions in an excited state. When a light signal passes through this piece of optical fiber, it de-excites the ions by laser effect by producing a photon at all points identical to the incident photon. The light signal has therefore been doubled.

By avalanche effect, the amplification of the signal is done until the exit of the doped section of the optical fiber.
Noise in doped fiber amplifiers
The doped fiber amplifiers are subject to noise which is due to the spontaneous de-excitation of the ions. This de-excitation produces photons in random directions, but only the front direction intervenes in the final noise observed at the fiber output.

The de-excitation in the other directions, in particular in the opposite direction of the normal propagation of the signal is also undesirable, because it reduces the efficiency of the amplifier by de-exciting ions which can thus no longer participate in the amplification "useful Of the signal.

To avoid excessive amplification noise, we are working on moderate gains.

Polarization effect
In doped fiber amplifiers, we can highlight an amplification difference (<0.5dB) depending on the polarization of the incident signal.

Erbium-doped fiber amplifier
Erbium-doped fiber amplifiers are the most common.

The working wavelengths are divided into two windows. The Conventional band (hence C-Band) between 1,525 nm and 1,565 nm and the Long band (called L-Band) between 1,570 nm and 1,610 nm. These two bands can be equally amplified by this type of amplifier, but it is often preferred to use amplifiers optimized for each application.

The main difference between amplifiers for C or L band is that for the L band, the length of doped fiber is significantly longer, which requires less optical pumping.

There are two wavelengths for optical pumping of this type of amplifier: 980 nm and 1480 nm. The wavelength of 980 nm is usually used for low noise equipment. However, since the absorption window is relatively narrow, stabilized laser sources must be used. The absorption window of the wavelength of 1480 nm is wider and is usually used for higher power amplifications. Optical pumping at these two wavelengths is usually used in conjunction in systems.

The erbium-doped fiber amplifiers were invented by a team including David Payne of the University of Southampton as well as a group from AT&T Bell Laboratories including a Frenchman, Emmanuel Desurvire.

EDFA [1] is a fiber optic amplifier doped with erbium ions composed of a short length of fiber doped with Er3 + ions. The optical signal to be amplified as well as the pump laser (supplying energy) are coupled in the doped fiber and emit in the same direction.

Amplifiers for other wavelengths
Thulium is used as a dopant to amplify signals between 1450 and 1490 nm and praseodymium for wavelengths of around 1300 nm. However, these wavelengths have not known a significant commercial use and have not allowed a development of these amplifiers like amplifiers doped with erbium.

Raman amplifiers
Raman amplifiers do not use the atomic transitions of rare earth doped ions in the fibers but are based on an energy exchange by Raman scattering. All materials have a characteristic Raman spectrum. A pumping laser beam injected into the fiber (in the direction of propagation of the signal or preferably in the opposite direction) will amplify the light shifted towards the low frequencies by approximately 13.2 THz, value characteristic of the Raman shift in silica. For the usual optical telecommunications at 1,550 nm, it is therefore necessary to use a pumping laser around 1,450 nm.

Raman amplification has various advantages. It does not require special fibers. It can therefore be used directly in the communication fibers and it is distributed naturally along the fiber. This is called distributed Raman amplification. Distributed amplification degrades the signal-to-noise ratio less than localized amplification. Raman amplification also has significant advantages in multiplexed wavelength communications because its gain bandwidth is greater than that offered by other amplification techniques. On the other hand, it requires high optical pumping power.
Parametric fiber amplifiers
Semiconductor amplifiers
Semiconductor amplifiers have the same structure as a Fabry-Perot type laser diode but without the reflection devices at the ends to avoid the laser effect in this type of application.

This amplification device is small and has the advantage that the pumping is electric (no need to produce a laser light to do the optical pumping). The manufacture of this type of amplifier is cheaper, but has the disadvantage of introducing more noise. It has a more modest gain than fiber amplifiers doped with erbium.

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