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Fiber AmplifiersWhile the low loss of optical fiber allows signals to travel hundreds ofkilometers, extremely long haul lines and submarine cables requireregenerators or repeaters to amplify the signal periodically. In thebeginning, repeaters basically consisted of a receiver followed by atransmitter. The incoming signal was converted from a light signal to anelectrical signal by a receiver, cleaned up to remove as much noise aspossible, then was retransmitted by another laser transmitter.

Figure 1. Electronic RepeaterThese repeaters added noise to the signal, consumed much power andwere complicated, which means they were a source of failure. They alsohad to be made for the specific bit rate of transmission and upgradingrequired replacing all the repeaters, a really difficult task in an underseacable!Since the 1960s, researchers knew how to make fiber lasers. Proper dopingof the fiber (introducing small amounts of active elements into the glass

fiber) allowed it to be pumped with external light sources until stimulatedemission occurred. While making fiber amplifiers was hypothesized early inthe stages of fiber optic development, it was not until 1987 that workingmodels were realized. Major contributors to the development included BellLabs and NTT.The typical fiber amplifier works in the 1550 nm band and consists of alength of fiber doped with Erbium pumped with a laser at 980. The pumplaser supplies the energy for the amplifier, while the incoming signalstimulates emission as the pulse passes through the doped fiber.

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Figure 2. Basic Fiber Amplifier

Figure 3. Fiber Amplification

The stimulated emission stimulates more emission, so there is a rapid,exponential growth of photons in the dopedfiber. Gains of >40 dB (10,000X)are possible with power outputs >+20 dBm (100 mW).To date, the most efficient fiber amplifiers have been Erbium-Doped Fiber

Amplifiers (EDFAs) operating in the 1550 nm range. Since most systemsstill work at 1310 nm, considerable research has been done to findmaterials that would work in this range. Praseodymium-doped fluoride fiberamplifiers (PDFFAs) using fibers made from zirconium fluoride or hafniumfluoride have shown some promise, but have not developed theperformance needed for widespread applications.The basic structure of an EDFA is very simple. The amplifier itself emitslight energy in a signal wavelength (usually about 1540nm) using energysupplied to it by photons in a pump wavelength (usually 980nm) whenstimulated by incoming photons in the signal - the signal which needsamplification. Just like in a laser, the emitted photons then stimulate otheremissions, so there is an exponential growth of photons. Supporting the

amplifier is a pump laser, which supplies the amplifier's energy, a coupler,which combines the pump laser beams and the signal laser beam and putsthem on a single fiber, and an optical filter, which removes the remainingtraces of the pump beam so that it doesn't interfere with reception of thesignal.Why Erbium?Erbium has several important properties that make it an excellent choice foran optical amplifier. Remember that there are several very specific bands

(wavelengths) that fiber optic cables can carry. Erbium ions (Er3+) havequantum levels that allows them to be stimulated to emit in the 1540nm

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band, which is the band that has the least power loss in most silica-basedfiber. That gives them the ability to amplify signals in a band where high-quality amplifiers are most needed.Erbium's quantum levels also allow it to be excited by a signal at either800nm or 980nm, both of which silica-based fiber can carry without greatlosses, but aren't in the middle of the signal wavelengths. Those bands arealso far enough away from the signal bands that it is easy to keep the pumpbeam and the signal beam separated.

Figure 4. Energy States of ErbiumWhen erbium is excited by photons at 800nm or 980nm, it has a non-radiative decay (energy drops without producing light) to a state where itcan stay excited for relatively long periods of time - on the order of 10ms.This property is extremely important, because the quantum efficiency of thedevice is dependent on how long it can stay in that excited state. If itrelaxes too quickly, more photons are needed to keep it excited, meaningmore input power is needed to make the amplifier work.Erbium can also be excited by photons at 1480nm, but this is typicallyundesirable. When excited at that wavelength, both the energy pumpingprocess and the stimulated emission by the signal are happening in the

same wavelength and energy band, which can create interactions thatlower the efficiency of the device and increase the amplifier noise.

Another important property of erbium for use in a fiber amplifier is that it isfairly soluble in silica, making it easy to dope into mixtures for making silica-based fiber. For many applications, reasonable EDFAs can be made bysimply dissolving Er2O3 in a crucible with the SiO2 used to make silicafiber. By using a co-dopant, such as Al2O3, GeO2-Al2O3, or P2O5, theerbium compound's solubility in the silica mixture can be greatly increased,and some of the EDFA's properties can be improved. For example, GeO2-

Al2O3 can be used to almost double the time it takes for excited erbium torelax, which therefore almost doubles the quantum efficiency of the EDFA.

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EDFAs are not perfect. In practice, you need to have many pump beamsalong the length of a fiber to provide the energy for EDFAs and theserequire power and optics (couplers and filters.) EDFAs also have gain thatvaries with a signal's wavelength which creates problems in many WDMapplications. This can be solved by using special optical passive filters thatare designed to compensate for the gain variation of the EDFA.

Alternative DesignsThe simple diagram of an EDFA shown in Figure 2 is not the only wayEDFAs can be made. Pumping can be done in a forward direction asshown, backward from the output end or in both directions. Optical isolatorsare commonly used at both ends of the EDFA to prevent pump energy fromescaping back down the fiber or unwanted reflections that may affect laserstability. Filters, often Bragg gratings (filters fabricated in fibers), are used to

flatten the gain over the broadest wavelength range for use in WDMsystems.Other ApplicationsBesides being used as repeaters, fiber amplifiers are used to increasesignal level for CATV systems, which require high power levels at thereceiver to maintain adequate signal to noise performance, allowing longercable runs or using splitters to "broadcast" a single signal through a couplerto many fibers, saving the cost of additional transmitters. In telephony, theycombine withDWDM(dense wavelength division multiplexers) to overcome

the inefficiencies of DWDMs for long haul transmission.Future DevelopmentsFiber amplifiers continue to be developed to support Dense WavelengthDivision Multiplexing and to expand to the other wavelength bandssupported by fiber optics. Now that fiber manufacturers have all butremoved the water bands from the spectrum, there is now a range of 1260to 1610 nm available for use. Fiber amplifiers and diode lasers will probablybe developed within this band to completely fill it with useable bandwidth.

A new device being worked on is the Semiconductor Optical Amplifier(SOA) which basically duplicates the function of a FOA but in an integratedcircuit fabricated like a diode laser.

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