Optical Amplifiers in BBC.

An opticalamplifier is a device that amplifies an optical signal directly, without the needto first convert it to an electrical signal. An optical amplifier may bethought of as a laser without an optical cavity, or one in which feedback from the cavity issuppressed. Optical amplifiers are important in opticalcommunicationand laserphysics.

There are severaldifferent physical mechanisms that can be used to amplify a light signal, whichcorrespond to the major types of optical amplifiers. In doped fiber amplifiersand bulk lasers, stimulatedemission inthe amplifier's gainmediumcauses amplification of incoming light. In semiconductor optical amplfiers(SOAs), electron-hole recombination occurs. In Raman amplifiers, Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. Parametricamplifiersuse parametric amplification.

Laseramplifiers: Almostany laser active gain medium can be pumped toproduce gain forlight at the wavelength of a laser made with the same material as its gainmedium. Such amplifiers are commonly used to produce high power laser systems.Special types such as regenerative amplifiers and chirped-pulse amplifiers are usedto amplify ultrashort pulses.

2.   Dopedfiber amplifiers

Dopedfiber amplifiers:

(DFAs) are optical amplifiers that use a doped opticalfiber as again medium to amplify an optical signal. They are related to fiberlasers. Thesignal to be amplified and a pump laser are multiplexed into thedoped fiber, and the signal is amplified through interaction with the doping ions. The most common example is the Erbium Doped FiberAmplifier (EDFA), where the core of a silica fiber is doped with trivalent Erbium ions and can be efficiently pumped with a laser ata wavelength of 980 nm or1,480 nm, and exhibits gain in the 1,550 nm region. 

Erbium-doped fiber amplifiers:

The erbium-dopedfiber amplifier (EDFA) is the most deployed fiber amplifier as itsamplification window coincides with the third transmission window ofsilica-based optical fiber.

Two bands havedeveloped in the third transmission window – the Conventional, orC-band, from approximately 1525 nm – 1565 nm, and the Long, orL-band, from approximately 1570 nm to 1610 nm. Both of these bandscan be amplified by EDFAs, but it is normal to use two different amplifiers,each optimized for one of the bands.

EDFAs have two commonly-used pumping bands – 980 nm and 1480 nm.The 980 nm band has a higher absorption cross-section and is generallyused where low-noise performance is required.

Semiconductor optical amplifier:

Semiconductor optical amplifiers (SOAs) are amplifiers which use asemiconductor to provide the gain medium.These amplifiers have a similar structure to Fabry–Pérotlaserdiodes but with anti-reflection design elements at the end faces. Recentdesigns include anti-reflective coatings and tilted waveguideand window regions which can reduce end face reflection to less than 0.001%.Since this creates a loss of power from the cavity which is greater than thegain it prevents the amplifier from acting as a laser.