Editing Optical amplifier (section)

=== Erbium-doped optical fiber amplifiers ===

The erbium-doped fiber amplifier (EDFA) is the most deployed fiber amplifier as its amplification window coincides with the third transmission window of silica-based optical fiber. The core of a silica fiber is doped with trivalent [[erbium]] ions (Er<sup>3+</sup>) and can be efficiently pumped with a laser at or near wavelengths of 980&nbsp;[[nanometre|nm]] and 1480&nbsp;nm, and gain is exhibited in the 1550&nbsp;nm region. The EDFA amplification region varies from application to application and can be anywhere from a few nm up to ~80&nbsp;nm. Typical use of EDFA in telecommunications calls for ''Conventional'', or C-band amplifiers (from ~1525&nbsp;nm to ~1565&nbsp;nm) or ''Long'', or L-band amplifiers (from ~1565&nbsp;nm to ~1610&nbsp;nm). Both of these bands can be amplified by EDFAs, but it is normal to use two different amplifiers, each optimized for one of the bands.

The principal difference between C- and L-band amplifiers is that a longer length of doped fiber is used in L-band amplifiers. The longer length of fiber allows a lower inversion level to be used, thereby giving emission at longer wavelengths (due to the band-structure of Erbium in silica) while still providing a useful amount of gain.{{Citation needed|date=August 2020}}

EDFAs have two commonly used pumping bands – 980&nbsp;nm and 1480&nbsp;nm. The 980&nbsp;nm band has a higher absorption cross-section and is generally used where low-noise performance is required. The absorption band is relatively narrow and so wavelength stabilised laser sources are typically needed. The 1480&nbsp;nm band has a lower, but broader, absorption cross-section and is generally used for higher power amplifiers. A combination of 980&nbsp;nm and 1480&nbsp;nm pumping is generally utilised in amplifiers.

Gain and lasing in erbium-doped fibers were first demonstrated in 1986–87 by two groups; one including [[David N. Payne]], [[Robert J. Mears|R. Mears]], I.M Jauncey and L. Reekie, from the [[University of Southampton]]<ref>Mears, R.J. and Reekie, L. and Poole, S.B. and Payne, D.N.: "Low-threshold tunable CW and Q-switched fiber laser operating at 1.55&nbsp;μm", Electron. Lett., 1986, 22, pp.159–160</ref><ref>R.J. Mears, L. Reekie, I.M. Jauncey and D. N. Payne: “Low-noise Erbium-doped fiber amplifier at 1.54&nbsp;μm”, Electron. Lett., 1987, 23, pp.1026–1028</ref> and one from AT&T Bell Laboratories, consisting of E. Desurvire, P. Becker, and J. Simpson.<ref>E. Desurvire, J. Simpson, and P.C. Becker, High-gain erbium-doped traveling-wave fiber amplifier," Optics Letters, vol. 12, No. 11, 1987, pp. 888–890</ref> The dual-stage optical amplifier which enabled dense wave division multiplexing (DWDM) was invented by Stephen B. Alexander at Ciena Corporation.<ref>United States Patent Office #5696615; “Wavelength division multiplexed optical communication systems employing uniform gain optical amplifiers.”</ref><ref>{{cite web|url=http://massis.lcs.mit.edu/archives/george.gilder.essays/coming.of.the.fibersphere|format=TXT|title=Subject: Into the Fibersphere|website=Massis.lcs.mit.edu|access-date=2017-08-10|archive-date=2016-03-05|archive-url=https://web.archive.org/web/20160305035422/http://massis.lcs.mit.edu/archives/george.gilder.essays/coming.of.the.fibersphere|url-status=dead}}</ref>