View source for Optical fiber

{{short description|Light-conducting fiber}}
{{Use American English|date = March 2024}}

[[File:fibreoptic.jpg|thumb|right|upright|A bundle of optical fibers]]
[[File:Fiber optic illuminated.jpg|thumb|A [[TOSLINK]] fiber optic audio cable with red light shone in one end]]

An '''optical fiber''', or '''optical fibre''', is a flexible [[glass]] or plastic [[fiber]] that can transmit [[light]]{{efn| Including [[infrared]] and [[ultraviolet radiation]].}} from one end to the other. Such fibers find wide usage in [[fiber-optic communication]]s, where they permit transmission over longer distances and at higher [[Bandwidth (computing)|bandwidths]] (data transfer rates) than electrical cables. Fibers are used instead of metal [[wire]]s because signals travel along them with less [[Attenuation|loss]] and are immune to [[electromagnetic interference]].<ref>{{cite book |ref=Senior |last1=Senior |first1=John M. |last2=Jamro |first2=M. Yousif |title=Optical fiber communications: principles and practice |date=2009 |publisher=Pearson Education |isbn=978-0130326812 |pages=7–9}}</ref> Fibers are also used for [[illumination (lighting)|illumination]] and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a [[fiberscope]].<ref>{{cite web |title=Birth of Fiberscopes |url=http://www.olympus-global.com/en/corc/history/story/endo/fiber/|website=www.olympus-global.com |publisher=Olympus Corporation |access-date=17 April 2015}}</ref> Specially designed fibers are also used for a variety of other applications, such as [[fiber optic sensor]]s and [[fiber laser]]s.<ref>{{cite journal |last1=Lee |first1=Byoungho |title=Review of the present status of optical fiber sensors |journal=Optical Fiber Technology |date=2003 |volume=9 |issue=2 |pages=57–79 |doi=10.1016/s1068-5200(02)00527-8 |bibcode=2003OptFT...9...57L}}</ref>

Glass optical fibers are typically made by [[Drawing (manufacturing)|drawing]], while plastic fibers can be made either by drawing or by [[extrusion]].<ref>{{cite web|title=Optical Fiber |url=http://www.thefoa.org/tech/ref/basic/fiber.html |website=www.thefoa.org |publisher=[[The Fiber Optic Association]] |access-date=17 April 2015}}</ref><ref>{{cite web| title=Manufacture of Perfluorinated Plastic Optical Fibers |url=http://chromisfiber.com/pdf/ofc2004.pdf|date=2004|accessdate=2023-09-11|publisher=chromisfiber.com}}</ref> Optical fibers typically include a [[Core (optical fiber)|core]] surrounded by a transparent [[Cladding (fiber optics)|cladding]] material with a lower [[index of refraction]]. Light is kept in the core by the phenomenon of [[total internal reflection]] which causes the fiber to act as a [[Waveguide (optics)|waveguide]].<ref>[[#Senior|Senior]], pp. 12–14</ref> Fibers that support many propagation paths or [[transverse mode]]s are called [[multi-mode fiber]]s, while those that support a single mode are called [[single-mode fiber]]s (SMF).<ref name="pears1">{{cite book |last1=Pearsall |first1=Thomas |title=Photonics Essentials |edition=2nd |publisher=McGraw-Hill |date=2010 |url=https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html |isbn=978-0-07-162935-5 |access-date=2021-02-24 |archive-date=2021-08-17 |archive-url=https://web.archive.org/web/20210817005021/https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html |url-status=dead}}</ref> Multi-mode fibers generally have a wider core diameter<ref>{{Cite book |url=https://books.google.com/books?id=HFMiAQAAMAAJ&q=Multi-mode+fibers+generally+have+a+wider+core+diameter+and+are+used+for+short-distance+communication+links |title=The Optical Industry & Systems Purchasing Directory |date=1984 |publisher=Optical Publishing Company |language=en}}</ref> and are used for short-distance communication links and for applications where high power must be transmitted.<ref>{{Cite book |url=https://books.google.com/books?id=Nrs6DwAAQBAJ&q=multimode+fiber+used+for+short+communication+or+high+power+transfer&pg=RA1-PA19 |title=Photonic Devices and Systems |last=Hunsperger |date=2017-10-19 |publisher=Routledge |isbn=9781351424844 |language=en}}</ref> Single-mode fibers are used for most communication links longer than {{convert|1050|sp=us|m|ft}}.<ref>{{cite book |last1=Fennelly |first1=Lawrence J. |title=Effective Physical Security |date=26 November 2012 |publisher=Elsevier Science |isbn=9780124159815 |page=355 |edition=4th |url=https://books.google.com/books?id=XJ5HE-5wAHQC}}</ref>

Being able to join optical fibers with low loss is important in fiber optic communication.<ref>[[#Senior|Senior]], p. 218</ref> This is more complex than joining electrical wire or cable and involves careful [[Cleave (fiber)|cleaving]] of the fibers, precise alignment of the fiber cores, and the coupling of these aligned cores. For applications that demand a permanent connection a [[Fusion splicing|fusion splice]] is common. In this technique, an electric arc is used to melt the ends of the fibers together. Another common technique is a [[mechanical splice]], where the ends of the fibers are held in contact by mechanical force. Temporary or semi-permanent connections are made by means of specialized [[optical fiber connector]]s.<ref>[[#Senior|Senior]], pp. 234–235</ref>

The field of applied science and engineering concerned with the design and application of optical fibers is known as ''fiber optics''. The term was coined by Indian-American physicist [[Narinder Singh Kapany]].<ref name="southasia.ucsc.edu">{{cite web |url=http://southasia.ucsc.edu/endowed-chairs/narinder-singh-kapany.html|title=Narinder Singh Kapany Chair in Opto-electronics|publisher=ucsc.edu |access-date=2016-11-01|archive-date=2017-05-21|archive-url=https://web.archive.org/web/20170521222508/http://southasia.ucsc.edu/endowed-chairs/narinder-singh-kapany.html|url-status=dead}}</ref>
{{toclimit|3}}

== History ==
{{See also|Fiber-optic communication#History}}
[[File:DanielColladon's Lightfountain or Lightpipe,LaNature(magazine),1884.JPG|thumb|upright|Colladon's "light fountain"]]

[[Jean-Daniel Colladon|Daniel Colladon]] and [[Jacques Babinet]] first demonstrated the guiding of light by refraction, the principle that makes fiber optics possible, in [[Paris]] in the early 1840s.<ref>{{cite journal |first=Jean-Daniel |last=Colladon |title=On the reflections of a ray of light inside a parabolic liquid stream |journal=Comptes Rendus |year=1842}}</ref> [[John Tyndall]] included a demonstration of it in his public lectures in [[London]], 12 years later.<ref name=regis>{{cite book|last =Bates|first =Regis J|title =Optical Switching and Networking Handbook|publisher =McGraw-Hill|year= 2001|location =New York|isbn=978-0-07-137356-2|page =10}}</ref> Tyndall also wrote about the property of [[total internal reflection]] in an introductory book about the nature of light in 1870:<ref>{{cite book |first=John |last=Tyndall |year=1870 |title=Notes about Light |chapter=Total Reflexion |chapter-url=https://archive.org/details/notesofcourseofn00tyndrich}}</ref><ref>{{cite book |first=John |last=Tyndall |year=1873 |title=Six Lectures on Light |location=New York |publisher=D. Appleton |url=https://archive.org/details/sixlecturesonlig00tynduoft}}</ref>{{blockquote|When the light passes from air into water, the refracted ray is bent ''towards'' the [[perpendicular]]... When the ray passes from water to air it is bent ''from'' the perpendicular... If the angle which the ray in water encloses with the perpendicular to the surface be greater than 48 degrees, the ray will not quit the water at all: it will be ''totally reflected'' at the surface... The angle which marks the limit where total reflection begins is called the limiting angle of the medium. For water this angle is 48°27′, for flint glass it is 38°41′, while for a diamond it is 23°42′.}}

In the late 19th century, a team of Viennese doctors guided light through bent glass rods to illuminate body cavities.<ref>{{cite web |url=http://inventors.about.com/library/weekly/aa980407.htm |title=How Fiber Optics Was Invented |author=Mary Bellis |access-date=2020-01-20 |archive-date=2012-07-12 |archive-url=https://archive.today/20120712024320/http://inventors.about.com/library/weekly/aa980407.htm |url-status=dead }}</ref> Practical applications such as close internal illumination during dentistry followed, early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter [[Clarence Hansell]] and the television pioneer [[John Logie Baird]] in the 1920s. In the 1930s, [[Heinrich Lamm]] showed that one could transmit images through a bundle of unclad optical fibers and used it for internal medical examinations, but his work was largely forgotten.<ref name=regis/><ref name=Hecht2004/>

In 1953, Dutch scientist [[Bram van Heel]] first demonstrated image transmission through bundles of optical fibers with a transparent cladding.<ref name=Hecht2004>{{cite book |first=Jeff |last=Hecht |title=City of Light: The Story of Fiber Optics |publisher=Oxford University |edition=revised |date=2004 |isbn=9780195162554 |pages=55–70}}</ref> Later that same year, [[Harold Hopkins (physicist)|Harold Hopkins]] and [[Narinder Singh Kapany]] at [[Imperial College]] in London succeeded in making image-transmitting bundles with over 10,000 fibers, and subsequently achieved image transmission through a 75&nbsp;cm long bundle which combined several thousand fibers.<ref name=Hecht2004/><ref>{{cite journal |author1=Hopkins, H. H. |author2=Kapany, N. S. |year=1954 |journal=Nature |doi=10.1038/173039b0 |volume=173 |pages=39–41 |title=A flexible fiberscope, using static scanning |issue=4392 |bibcode=1954Natur.173...39H |s2cid=4275331}}</ref><ref>[https://web.archive.org/web/20110629061117/http://nobelprize.org/nobel_prizes/physics/laureates/2009/sciback_phy_09.pdf Two Revolutionary Optical Technologies]. Scientific Background on the Nobel Prize in Physics 2009. Nobelprize.org. 6 October 2009</ref> The first practical fiber optic semi-flexible [[gastroscope]] was patented by [[Basil Hirschowitz]], C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the [[University of Michigan]], in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material.<ref name=Hecht2004/>

Kapany coined the term ''fiber optics'' after writing a 1960 article in ''Scientific American'' that introduced the topic to a wide audience. He subsequently wrote the first book about the new field.<ref name=Hecht2004/><ref>[http://news.rediff.com/report/2009/oct/08/how-india-missed-another-nobel-prize.htm How India missed another Nobel Prize – Rediff.com India News]. News.rediff.com (2009-10-12). Retrieved on 2017-02-08.</ref>

The first working fiber-optic data transmission system was demonstrated by German physicist [[Manfred Börner]] at [[Telefunken]] Research Labs in Ulm in 1965, followed by the first patent application for this technology in 1966.<ref>{{cite patent |country=DE |number=1254513 |status=patent |title=Mehrstufiges Übertragungssystem für Pulscodemodulation dargestellte Nachrichten. |gdate=1967-11-16 |inventor=Börner, Manfred |assign1=Telefunken Patentverwertungsgesellschaft m.b.H.}}</ref><ref>{{cite patent |country=US |number=3845293 |status=patent |title=Electro-optical transmission system utilizing lasers| inventor=Börner, Manfred}}</ref> In 1968, NASA used fiber optics in the television cameras that were sent to the moon. At the time, the use in the cameras was [[Classified information in the United States|classified]] ''confidential'', and employees handling the cameras had to be supervised by someone with an appropriate security clearance.<ref>[https://history.nasa.gov/alsj/MSC-SESD-28-105.pdf Lunar Television Camera. Pre-installation Acceptance Test Plan]. NASA. 12 March 1968</ref>

[[Charles K. Kao]] and [[George A. Hockham]] of the British company [[Standard Telephones and Cables]] (STC) were the first to promote the idea that the [[attenuation]] in optical fibers could be reduced below 20 [[decibel]]s per kilometer (dB/km), making fibers a practical communication medium, in 1965.<ref name=hecht1999>{{cite book |last=Hecht |first=Jeff |title=City of Light, The Story of Fiber Optics |publisher=[[Oxford University Press]] |location=New York |year=1999 |url=https://books.google.com/books?id=4oMu7RbGpqUC&pg=PA114 |isbn=978-0-19-510818-7 |page=114}}</ref> They proposed that the attenuation in fibers available at the time was caused by impurities that could be removed, rather than by fundamental physical effects such as scattering. They correctly and systematically theorized the light-loss properties for optical fiber and pointed out the right material to use for such fibers—[[silica glass]] with high purity. This discovery earned Kao the [[Nobel Prize in Physics]] in 2009.<ref>{{cite web |url=http://nobelprize.org/nobel_prizes/physics/laureates/2009/press.html |title=Press Release&nbsp;– Nobel Prize in Physics 2009 |publisher=The Nobel Foundation |access-date=2009-10-07}}</ref> The crucial attenuation limit of 20&nbsp;dB/km was first achieved in 1970 by researchers [[Robert D. Maurer]], [[Donald Keck]], [[Peter C. Schultz]], and Frank Zimar working for American glass maker [[Corning Glass Works]].<ref name=hecht1999b>{{cite book |last=Hecht |first=Jeff |title=City of Light, The Story of Fiber Optics |publisher=[[Oxford University Press]] |location=New York |year=1999 |url=https://books.google.com/books?id=4oMu7RbGpqUC&pg=PA114 |isbn=978-0-19-510818-7 |page=271}}</ref> They demonstrated a fiber with 17&nbsp;dB/km attenuation by [[doping (semiconductor)|doping]] silica glass with [[titanium]]. A few years later they produced a fiber with only 4&nbsp;dB/km attenuation using [[germanium dioxide]] as the core dopant. In 1981, [[General Electric]] produced fused [[quartz]] [[ingots]] that could be drawn into strands {{convert|25|mi|km}} long.<ref>{{cite web |title=1971–1985 Continuing the Tradition |work=GE Innovation Timeline |publisher=General Electric Company |url=http://www.ge.com/innovation/timeline/eras/continuing_tradition.html |access-date=2012-09-28}}</ref>

Initially, high-quality optical fibers could only be manufactured at 2 meters per second. Chemical engineer [[Thomas Mensah (engineer)|Thomas Mensah]] joined Corning in 1983 and increased the speed of manufacture to over 50 meters per second, making optical fiber cables cheaper than traditional copper ones.<ref name=RSCiB>{{cite web |title=About the Author – Thomas Mensah |url=http://rightstuffcomesinblack.com/about-the-author-dr-thomas-mensah |publisher=The Right Stuff Comes in Black |access-date=29 March 2015 |archive-date=2 January 2015 |archive-url=https://web.archive.org/web/20150102211229/http://rightstuffcomesinblack.com/about-the-author-dr-thomas-mensah |url-status=dead }}</ref>{{self-published inline|certain=y|date=October 2022}}<ref>{{cite web|title=Executive Profile: Thomas O. Mensah|url=https://www.bloomberg.com/research/stocks/private/person.asp?personId=32455813&privcapId=108950704|publisher=Bloomberg|access-date=31 March 2015|archive-date=10 February 2015|archive-url=https://web.archive.org/web/20150210141514/http://www.bloomberg.com/research/stocks/private/person.asp?personId=32455813&privcapId=108950704|url-status=dead}}</ref><ref>{{cite web |title=Thomas O. Mensah |date=20 November 2015 |url=https://www.aiche.org/giving/supporters/profiles/thomas-o-mensah|publisher=AIChE|access-date=30 July 2024}}</ref> These innovations ushered in the era of optical fiber telecommunication.

The Italian research center [[CSELT]] worked with Corning to develop practical optical fiber cables, resulting in the first metropolitan fiber optic cable being deployed in Turin in 1977.<ref>{{cite conference |last1=Catania |first1=B. |last2=Michetti |first2=L. |last3=Tosco |first3=F. |last4=Occhini |first4=E. |last5=Silvestri |first5=L. |title=First Italian Experiment with a Buried Optical Cable |pages=315–322 |url=http://www.chezbasilio.org/immagini/cos1_tests.pdf |book-title=Proceedings of 2nd European Conference on Optical Communication (II ECOC) |date=September 1976 |access-date=2022-08-18}}</ref><ref>{{Cite web |url=http://archiviostorico.telecomitalia.com/italia-al-telefono-oltre/15-settembre-1977-torino-prima-stesura-al-mondo-di-fibra-ottica-in-esercizi |title=15 settembre 1977, Torino, prima stesura al mondo di una fibra ottica in esercizio.|website=Archivio storico Telecom Italia |access-date=2017-02-15 |archive-date=2017-09-17 |archive-url=https://web.archive.org/web/20170917171214/http://archiviostorico.telecomitalia.com/italia-al-telefono-oltre/15-settembre-1977-torino-prima-stesura-al-mondo-di-fibra-ottica-in-esercizi |url-status=dead}}</ref> CSELT also developed an early technique for splicing optical fibers, called Springroove.<ref>{{cite web|url=http://archiviostorico.telecomitalia.com/italia-al-telefono-oltre/springroove-giunto-per-fibre-ottiche-brevettato-nel-1977|title=Springroove, il giunto per fibre ottiche brevettato nel 1977|archive-url=https://web.archive.org/web/20160816221359/http://archiviostorico.telecomitalia.com/italia-al-telefono-oltre/springroove-giunto-per-fibre-ottiche-brevettato-nel-1977|archive-date=2016-08-16|url-status=dead|website=Archivio storico Telecom Italia|access-date=2017-02-08}}</ref>

Attenuation in modern optical cables is far less than in electrical copper cables, leading to long-haul fiber connections with repeater distances of {{convert|70|–|150|km|mi|sp=us}}. Two teams, led by [[David N. Payne]] of the [[University of Southampton]] and [[Emmanuel Desurvire]] at [[Bell Labs]], developed the [[erbium-doped fiber amplifier]], which reduced the cost of long-distance fiber systems by reducing or eliminating optical-electrical-optical repeaters, in 1986 and 1987 respectively.<ref>{{cite journal |last1=Mears |first1=R.J. |last2=Reekie |first2=L. |last3=Poole |first3=S.B. |last4=Payne |first4=D.N. |title=Low-threshold tunable CW and Q-switched fibre laser operating at 1.55 μm |journal=[[Electronics Letters]] |date=30 January 1986 |volume=22 |issue=3 |pages=159–160 |doi=10.1049/el:19860111 |bibcode=1986ElL....22..159M |url=https://eprints.soton.ac.uk/78562/1/240.pdf}}</ref><ref>{{cite journal |first1=R.J. |last1=Mears |first2=L. |last2=Reekie |first3=I.M. |last3=Jauncey |first4=D.N. |last4=Payne |title=Low-noise erbium-doped fibre amplifier operating at 1.54&nbsp;μm |journal=[[Electronics Letters]] |date=10 September 1987 |volume=23 |issue=19 |pages=1026–1028 |doi=10.1049/el:19870719 |bibcode=1987ElL....23.1026M |url=https://eprints.soton.ac.uk/78511/1/304.pdf}}</ref><ref>{{cite journal |first1=E. |last1=Desurvire |first2=J. |last2=Simpson |first3=P.C. |last3=Becker |title=High-gain erbium-doped traveling-wave fiber amplifier |journal=[[Optics Letters]] |volume=12 |issue=11 |date=1987 |pages=888–890 |pmid=19741905 |doi=10.1364/OL.12.000888 |bibcode=1987OptL...12..888D}}</ref>

The emerging field of [[photonic crystal]]s led to the development in 1991 of [[photonic-crystal fiber]],<ref>{{cite journal |journal=Science |year=2003 |volume=299 |title=Photonic Crystal Fibers |author=Russell, Philip |doi=10.1126/science.1079280 |pages=358–62 |pmid=12532007 |issue=5605 |bibcode=2003Sci...299..358R |s2cid=136470113 }}</ref> which guides light by [[diffraction]] from a periodic structure, rather than by total internal reflection. The first photonic crystal fibers became commercially available in 2000.<ref>{{cite web |url=http://www.crystal-fiber.com/ |access-date=2008-10-22 |title=The History of Crystal fiber A/S |publisher=Crystal Fiber A/S |archive-date=2001-07-23 |archive-url=https://web.archive.org/web/20010723040541/http://www.crystal-fiber.com/ |url-status=dead }}</ref> Photonic crystal fibers can carry higher power than conventional fibers and their wavelength-dependent properties can be manipulated to improve performance. These fibers can have hollow cores.<ref>{{doi|10.1126/science.282.5393.1476}}.</ref>

== Uses ==
=== Communication ===
{{Main|Fiber-optic cable}}
{{Broader|Fiber-optic communication}}
[[File:Optical-fibre-junction-box.jpg|thumb|right|upright|A [[19-inch rack|wall-mount cabinet]] containing optical fiber cables. The yellow cables are [[single-mode optical fiber|single mode fibers]]; the orange and aqua cables are [[multi-mode optical fiber|multi-mode fibers]].]]

Optical fiber is used as a medium for [[telecommunication]] and [[computer network]]ing because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because [[infrared light]] propagates through the fiber with much lower [[attenuation]] compared to electricity in electrical cables. This allows long distances to be spanned with few [[repeater]]s.

10 or 40&nbsp;Gbit/s is typical in deployed systems.<ref>{{cite book|last=Yao|first=S.|date=2003|chapter-url=http://www.generalphotonics.com/downloads/techpubs/Polarization-in-Fiber-Systems-Squeezing-out-More-Bandwidth.pdf|chapter=Polarization in Fiber Systems: Squeezing Out More Bandwidth|archive-url=https://web.archive.org/web/20110711082842/http://www.generalphotonics.com/pdf/PSReprint.pdf|archive-date=July 11, 2011|title=The Photonics Handbook|publisher=Laurin Publishing|page=1}}</ref><ref>{{cite press release|url=http://www.ciena.com/news/news_2007pr_6976.htm|website=Ciena|title=JANET Delivers Europe's First 40 Gbps Wavelength Service|archive-url=https://web.archive.org/web/20100114013532/http://ciena.com/news/news_2007pr_6976.htm|archive-date=2010-01-14|url-status=dead|date=2007-07-09|access-date=29 October 2009}}</ref>

Through the use of [[wavelength-division multiplexing]] (WDM), each fiber can carry many independent channels, each using a different wavelength of light. The net data rate (data rate without overhead bytes) per fiber is the per-channel data rate reduced by the [[forward error correction]] (FEC) overhead, multiplied by the number of channels (usually up to 80 in commercial [[dense WDM]] systems {{As of|2008|lc=on}}).

{| class="wikitable"
|+ Transmission speed milestones
! Date 
! Milestone
|-
| 2006 
| 111 [[gigabit per second|Gbit/s]] by [[Nippon Telegraph and Telephone|NTT]].<ref name=NTT>{{cite press release |author=NTT |publisher=Nippon Telegraph and Telephone |url=http://www.ntt.co.jp/news/news06e/0609/060929a.html |title=14 Tbps over a Single Optical Fiber: Successful Demonstration of World's Largest Capacity |date=September 29, 2006 |access-date=2017-02-08 |archive-date=2017-09-21 |archive-url=https://web.archive.org/web/20170921122920/http://www.ntt.co.jp/news/news06e/0609/060929a.html |url-status=dead }}</ref><ref>{{Cite news |last = Alfiad |first= M. S.|year = 2008|title = 111 Gb/s POLMUX-RZ-DQPSK Transmission over 1140&nbsp;km of SSMF with 10.7 Gb/s NRZ-OOK Neighbours|periodical = Proceedings ECOC 2008|pages = Mo.4.E.2|url = https://w3.tue.nl/fileadmin/ele/TTE/ECO/Files/Pubs_2009/Alfiad_OFC_09_OThR4.pdf |display-authors = etal|access-date = 2013-09-17|archive-url = https://web.archive.org/web/20131204015524/https://w3.tue.nl/fileadmin/ele/TTE/ECO/Files/Pubs_2009/Alfiad_OFC_09_OThR4.pdf |archive-date = 2013-12-04|url-status = dead}}</ref>
|-
|2009
|100&nbsp;Pbit/s·km (15.5&nbsp;Tbit/s over a single 7000&nbsp;km fiber) by Bell Labs.<ref>{{cite press release |url=http://www.physorg.com/news173455192.html/ |title=Bell Labs breaks optical transmission record, 100 Petabit per second kilometer barrier |url-status=dead |archive-url=https://web.archive.org/web/20091009052534/http://www.physorg.com/news173455192.html/ |archive-date=October 9, 2009 |website=Phys.org |author=Alcatel-Lucent |date=September 29, 2009}}</ref>
|-
|2011
|101&nbsp;Tbit/s (370 channels at 273&nbsp;Gbit/s each) on a single core.<ref>
 {{cite magazine
 | title = Ultrafast fibre optics set new speed record
 | magazine=New Scientist
 | volume=210
 | issue=2809
 | page=24
 | date = 2011-04-29
 | url = https://www.newscientist.com/article/mg21028095.500-ultrafast-fibre-optics-set-new-speed-record.html
 | access-date = 2012-02-26| bibcode=2011NewSc.210R..24H
 | last1=Hecht
 | first1=Jeff
 | doi=10.1016/S0262-4079(11)60912-3
 }}</ref>
|-
|January 2013
|1.05&nbsp;Pbit/s transmission through a multi-core ([[Lightpath (optical network)|lightpath]]) fiber cable.<ref>
 {{cite web
 | title = NEC and Corning achieve petabit optical transmission
 | publisher = Optics.org
 | date = 2013-01-22
 | url = http://optics.org/news/4/1/29
 | access-date = 2013-01-23}}</ref>
|-
| June 2013 
| 400&nbsp;Gbit/s over a single channel using 4-mode [[orbital angular momentum multiplexing]].<ref>{{Cite journal | last1 = Bozinovic | first1 = N. | last2 = Yue | first2 = Y. | last3 = Ren | first3 = Y. | last4 = Tur | first4 = M. | last5 = Kristensen | first5 = P. | last6 = Huang | first6 = H. | last7 = Willner | first7 = A. E. | last8 = Ramachandran | first8 = S. | doi = 10.1126/science.1237861 | title = Terabit-Scale Orbital Angular Momentum Mode Division Multiplexing in Fibers | journal = Science | volume = 340 | issue = 6140 | pages = 1545–1548 | year = 2013 | pmid =  23812709| bibcode = 2013Sci...340.1545B | s2cid = 206548907 | url = http://pdfs.semanticscholar.org/24f6/27102b3a39615b9bf4e6af4197ce32d8aad1.pdf | archive-url = https://web.archive.org/web/20190220065344/http://pdfs.semanticscholar.org/24f6/27102b3a39615b9bf4e6af4197ce32d8aad1.pdf | url-status = dead | archive-date = 2019-02-20 }}</ref>
|-
| October 2022
| 1.84&nbsp;Pbit/s using a [[Photonic integrated circuit|photonic chip]]<ref>{{cite press release | url=https://www.fotonik.dtu.dk/nyheder/nyhed?id=4a44c973-224a-49b8-a21c-ef36c372740e | title=Petabit per second data transmission speeds from a single chip-scale light source - DTU Electro |publisher=Technical University of Denmark |date=31 October 2022 |url-status=dead |archive-url=https://web.archive.org/web/20230112184606/https://www.fotonik.dtu.dk/nyheder/nyhed?id=4a44c973-224a-49b8-a21c-ef36c372740e |archive-date=2023-01-12}}</ref><ref>{{cite press release |url=https://www.dtu.dk/english/news/all-news/new-data-transmission-record?id=213f1735-036d-44c9-b229-d25d74dd3f02 |publisher=Technical University of Denmark |title=New data transmission record |date=20 October 2022 |first=Lotte |last=Krull |access-date=2024-06-17}}</ref>
|-
| October 2023
| 22.9&nbsp;Pbit/s by [[National Institute of Information and Communications Technology|NICT]]<ref>{{Cite web|url=https://www.nict.go.jp/en/press/2023/11/30-1.html|title=World Record Optical Fiber Transmission Capacity Doubles to 22.9 Petabits per Second &#124; 2023|website=NICT - National Institute of Information and Communications Technology}}</ref>
|}

For short-distance applications, such as a network in an office building (see [[fiber to the office]]), fiber-optic cabling can save space in cable ducts. This is because a single fiber can carry much more data than electrical cables such as standard [[category 5 cable]], which typically runs at 100&nbsp;Mbit/s or 1&nbsp;Gbit/s speeds.

Fibers are often also used for short-distance connections between devices. For example, most [[high-definition television]]s offer a digital audio optical connection. This allows the streaming of audio over light, using the [[S/PDIF]] protocol over an optical [[TOSLINK]] connection.

=== Sensors ===
{{Main|Fiber-optic sensor}}

Fibers have many uses in [[remote sensing]]. In some applications, the fiber itself is the sensor (the fibers channel optical light to a processing device that analyzes changes in the light's characteristics). In other cases, fiber is used to connect a sensor to a measurement system.

Optical fibers can be used as sensors to measure [[deformation (mechanics)|strain]], [[temperature]], [[pressure]], and other quantities by modifying a fiber so that the property being measured modulates the [[intensity (physics)|intensity]], [[phase (waves)|phase]], [[polarization (waves)|polarization]], [[wavelength]], or transit time of light in the fiber. Sensors that vary the intensity of light are the simplest since only a simple source and detector are required. A particularly useful feature of such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter. [[Distributed acoustic sensing]] is one example of this.

In contrast, highly localized measurements can be provided by integrating miniaturized sensing elements with the tip of the fiber.<ref>
{{cite journal
 | pmid = 24599822
 | year = 2014
 | last1 = Kostovski | first1 = G.
 | title = The optical fiber tip: An inherently light-coupled microscopic platform for micro- and nanotechnologies
 | journal = Advanced Materials
 | volume = 26
 | issue = 23
 | pages = 3798–820
 | last2 = Stoddart | first2 = P. R.
 | last3 = Mitchell | first3 = A
 | doi = 10.1002/adma.201304605
 | bibcode = 2014AdM....26.3798K
 | s2cid = 32093488
}}</ref> These can be implemented by various micro- and [[nanofabrication]] technologies, such that they do not exceed the microscopic boundary of the fiber tip, allowing for such applications as insertion into blood vessels via hypodermic needle.

Extrinsic fiber optic sensors use an [[optical fiber cable]], normally a multi-mode one, to transmit [[modulation|modulated]] light from either a non-fiber optical sensor—or an electronic sensor connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach otherwise inaccessible places. An example is the measurement of temperature inside [[jet engine]]s by using a fiber to transmit [[radiation]] into a [[pyrometer]] outside the engine. Extrinsic sensors can be used in the same way to measure the internal temperature of [[electrical transformer]]s, where the extreme [[electromagnetic field]]s present make other measurement techniques impossible. Extrinsic sensors measure vibration, rotation, displacement, velocity, acceleration, torque, and torsion. A solid-state version of the gyroscope, using the interference of light, has been developed. The [[fiber optic gyroscope]] (FOG) has no moving parts and exploits the [[Sagnac effect]] to detect mechanical rotation.

Common uses for fiber optic sensors include advanced [[Perimeter intrusion detection system|intrusion detection security systems]]. The light is transmitted along a fiber optic sensor cable placed on a fence, pipeline, or communication cabling, and the returned signal is monitored and analyzed for disturbances. This return signal is digitally processed to detect disturbances and trip an alarm if an intrusion has occurred.

Optical fibers are widely used as components of optical chemical sensors and optical [[biosensors]].<ref>{{cite book |first=Florinel-Gabriel |last=Bănică |title=Chemical Sensors and Biosensors: Fundamentals and Applications |publisher=John Wiley and Sons |location=Chichester |year=2012 |at= Ch. 18–20 |isbn=978-0-470-71066-1}}</ref>

=== Power transmission ===
Optical fiber can be used to transmit power using a [[photovoltaic cell]] to convert the light into electricity.<ref>{{cite web |author=Anna Basanskaya |url=https://spectrum.ieee.org/electricity-over-glass |title=Electricity Over Glass |work=IEEE Spectrum |date=1 October 2005}}</ref> While this method of power transmission is not as efficient as conventional ones, it is especially useful in situations where it is desirable not to have a metallic conductor as in the case of use near MRI machines, which produce strong magnetic fields.<ref>{{cite web |title=Photovoltaic feat advances power over optical fiber |website=Electronic Products |url=http://www2.electronicproducts.com/Photovoltaic_feat_advances_power_over_optical_fiber-article-olap01-jun2006-html.aspx |url-status=dead |archive-url=https://web.archive.org/web/20110718095104/http://www2.electronicproducts.com/Photovoltaic_feat_advances_power_over_optical_fiber-article-olap01-jun2006-html.aspx |archive-date=2011-07-18 |date=2006-06-01 |access-date=2020-09-26}}</ref> Other examples are for powering electronics in high-powered antenna elements and measurement devices used in high-voltage transmission equipment.

=== Other uses ===
[[File:Flashflight red.jpg|thumb|A [[frisbee]] illuminated by fiber optics]]
[[File:OpticFiber.jpg|thumb|left|upright|Light reflected from optical fiber illuminates exhibited model]]
[[File:Looking at optical fibres in the dark.jpg|thumb|An optical fiber lamp]]

Optical fibers are used as [[light tube|light guides]] in medical and other applications where bright light needs to be shone on a target without a clear line-of-sight path. Many [[microscopes]] use fiber-optic light sources to provide intense illumination of samples being studied.

Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an [[endoscope]], which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures. Industrial endoscopes (see [[fiberscope]] or [[borescope]]) are used for inspecting anything hard to reach, such as jet engine interiors.

In some buildings, optical fibers route sunlight from the roof to other parts of the building (see [[nonimaging optics]]). [[Optical-fiber lamp]]s are used for illumination in decorative applications, including [[Commercial signage|signs]], [[art]], toys and artificial [[Christmas tree]]s.  Optical fiber is an intrinsic part of the light-transmitting concrete building product [[LiTraCon]].

Optical fiber can also be used in [[structural health monitoring]]. This type of [[sensor]] can detect stresses that may have a lasting impact on [[structures]]. It is based on the principle of measuring analog attenuation.

In [[spectroscopy]], optical fiber bundles transmit light from a spectrometer to a substance that cannot be placed inside the spectrometer itself, in order to analyze its composition. A spectrometer analyzes substances by bouncing light off and through them. By using fibers, a spectrometer can be used to study objects remotely.<ref>{{cite journal |title=In situ real-time monitoring of a fermentation reaction using a fiber-optic FT-IR probe |journal=Spectroscopy |date=June 2001 |first1=Zaid |last1=Al Mosheky |last2=Melling |first2=Peter J.  |last3=Thomson |first3=Mary A.  |url=http://www.remspec.com/pdfs/SP5619.pdf |volume= 16|issue =6|page=15}}</ref><ref>{{cite journal |title=Reaction monitoring in small reactors and tight spaces |first1=Peter |last1=Melling |last2=Thomson |first2=Mary |journal=American Laboratory News |date=October 2002 |url=http://www.remspec.com/pdfs/amlab1002.pdf }}</ref><ref>{{cite book |chapter=Fiber-optic probes for mid-infrared spectrometry |first1=Peter J. |last1=Melling |last2=Thomson |first2=Mary |title=Handbook of Vibrational Spectroscopy |editor1-last=Chalmers |editor1-first=John M.|editor2-last=Griffiths |editor2-first=Peter R.|publisher=Wiley |year=2002 |chapter-url=http://www.remspec.com/pdfs/2703_o.pdf }}</ref>

An optical fiber [[dopant|doped]] with certain [[rare-earth element]]s such as [[erbium]] can be used as the [[gain medium]] of a [[fiber laser]] or [[optical amplifier]]. Rare-earth-doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular (undoped) optical fiber line. The doped fiber is [[optical pumping|optically pumped]] with a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is [[stimulated emission]].

Optical fiber is also widely exploited as a nonlinear medium. The glass medium supports a host of nonlinear optical interactions, and the long interaction lengths possible in fiber facilitate a variety of phenomena, which are harnessed for applications and fundamental investigation.<ref name="Agrawal2012">{{Cite book | isbn = 978-0-12-397023-7 | title = Nonlinear Fiber Optics |edition=5th | last1 = Govind | first1 = Agrawal | date = 10 October 2012 | publisher = Academic Press }}</ref> Conversely, fiber nonlinearity can have deleterious effects on optical signals, and measures are often required to minimize such unwanted effects.

Optical fibers doped with a [[wavelength shifter]] collect [[scintillator|scintillation]] light in [[physics experiment]]s.

[[Iron sights#Fiber optic|Fiber-optic sights]] for handguns, rifles, and shotguns use pieces of optical fiber to improve the visibility of markings on the sight.

== Principle of operation ==
[[File:Fiber-engineerguy.ogv|thumb|An overview of the operating principles of the optical fiber]]
[[File:Optical fiber types.svg|thumb|upright=1.35|Optical fiber types]]
An optical fiber is a cylindrical [[dielectric waveguide]] ([[insulator (electrical)|nonconducting]] waveguide) that transmits light along its axis through the process of total internal reflection. The fiber consists of a ''core'' surrounded by a [[cladding (fiber optics)|cladding]] layer, both of which are made of [[dielectric]] materials.<ref name=RPP /> To confine the optical signal in the core, the [[refractive index]] of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in ''[[step-index profile|step-index fiber]]'', or gradual, in ''[[graded-index fiber]]''. Light can be fed into optical fibers using [[laser]]s or [[LED]]s.

Optical fibers are immune to electrical interference as there is no [[crosstalk|cross-talk]] between signals in different cables and no pickup of [[Noise (electronics)|environmental noise]]. Information traveling inside the optical fiber is even immune to [[electromagnetic pulse]]s generated by nuclear devices.{{efn|This feature is offset by the fiber's susceptibility to the gamma radiation from the weapon. The gamma radiation causes the optical attenuation to increase considerably during the gamma-ray burst due to the darkening of the material, followed by the fiber itself emitting a bright light flash as it anneals. How long the annealing takes and the level of the residual attenuation depends on the fiber material and its temperature. See [[Radiation effects on optical fibers]]}}<ref>{{Cite web |date=January 2024 |title=EFFECTS OF ELECTROMAGNETIC PULSES ON COMMUNICATION INFRASTRUCTURE |url=https://securityandtechnology.org/wp-content/uploads/2024/01/Effects-of-Electromagnetic-Pulses-on-Communication-Infrastructure.pdf |quote=Generally, fiberoptic cables, which make up most longdistance telephone lines in the United States, are more resistant to EMPs than metallic line networks. However, these wired infrastructures depend on repeaters in order to amplify the optical signal; in the case of undersea cables, this means that repeaters are placed every 60 to 70 km on the ocean floor.}}</ref>

Fiber cables do not conduct electricity, which makes them useful for protecting communications equipment in [[high voltage]] environments such as [[power generation]] facilities or applications prone to [[lightning]] strikes. The electrical isolation also prevents problems with [[Ground loop (electricity)|ground loops]]. Because there is no electricity in optical cables that could potentially generate sparks, they can be used in environments where explosive fumes are present. [[Wiretapping]] (in this case, [[fiber tapping]]) is more difficult compared to electrical connections.

Fiber cables are not targeted for [[metal theft]]. In contrast, copper cable systems use large amounts of copper and have been targeted since the [[2000s commodities boom#Copper|2000s commodities boom]].

=== Refractive index ===
The [[refractive index]] is a way of measuring the [[speed of light]] in a material. Light travels fastest in a [[vacuum]], such as in outer space. The speed of light in vacuum is about 300,000 [[kilometers]] (186,000 miles) per second. The refractive index of a medium is calculated by dividing the speed of light in vacuum by the speed of light in that medium. The refractive index of vacuum is therefore 1, by definition. A typical single-mode fiber used for telecommunications has a cladding made of pure silica, with an index of 1.444 at 1500&nbsp;nm, and a core of doped silica with an index around 1.4475.<ref name=RPP>{{cite encyclopedia  |title=Fibers |url=https://www.rp-photonics.com/fibers.html |encyclopedia=[[Encyclopedia of Laser Physics and Technology]] |publisher=RP Photonics |first=Rüdiger |last=Paschotta |access-date=Feb 22, 2015}}</ref> The larger the index of refraction, the slower light travels in that medium. From this information, a simple rule of thumb is that a signal using optical fiber for communication will travel at around 200,000 kilometers per second. Thus a phone call carried by fiber between Sydney and New York, a 16,000-kilometer distance, means that there is a minimum delay of 80 milliseconds (about <math>\tfrac {1}{12}</math> of a second) between when one caller speaks and the other hears.{{efn|The fiber, in this case, will probably travel a longer route, and there will be additional delays due to communication equipment switching and the process of encoding and decoding the voice onto the fiber.}}

=== Total internal reflection ===
When light traveling in an optically dense medium hits a boundary at a steep angle of incidence (larger than the [[critical angle (optics)|critical angle]] for the boundary), the light is completely reflected. This is called [[total internal reflection]]. This effect is used in optical fibers to confine light in the core. Most modern optical fiber is ''weakly guiding'', meaning that the difference in refractive index between the core and the cladding is very small (typically less than 1%).<ref name="Gloge1971">{{cite journal |last1=Gloge |first1=D. |date=1 October 1971 |title=Weakly Guiding Fibers |url=https://opg.optica.org/ao/fulltext.cfm?uri=ao-10-10-2252&id=73014 |journal=Applied Optics |volume=10 |issue=10 |pages=2252–8 |bibcode=1971ApOpt..10.2252G |doi=10.1364/AO.10.002252 |pmid=20111311 |access-date=2023-12-21}}</ref> Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding.

Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the [[acceptance cone]] of the fiber. There is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The [[sine]] of this maximum angle is the [[numerical aperture]] (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding. Single-mode fiber has a small NA.

=== Multi-mode fiber ===
{{Main|Multi-mode optical fiber}}
[[File:Optical-fibre.svg|thumb|The propagation of light through a [[multi-mode optical fiber]].]]
[[File:Laser in fibre.jpg|thumb|A laser bouncing down an [[poly(methyl methacrylate)|acrylic]] rod, illustrating the total internal reflection of light in a multi-mode optical fiber.]]

Optical fibers with a large core diameter (greater than 10&nbsp;micrometers) may be analyzed by [[geometrical optics]]. Such fibers are called ''multi-mode fibers'', from the electromagnetic analysis (see below). In a step-index multi-mode fiber, [[ray (optics)|rays]] of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at an angle (measured relative to a line [[surface normal|normal]] to the boundary) greater than the [[critical angle (optics)|critical angle]] for this boundary, are completely reflected. The critical angle is determined by the difference in the index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the [[core (optical fiber)|core]] into the cladding where they terminate. The critical angle determines the [[guided ray|acceptance angle]] of the fiber, often reported as a [[numerical aperture]]. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of [[dispersion (optics)|dispersion]] as rays at different angles have different [[optical path length|path lengths]] and therefore take different amounts of time to traverse the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high-angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a [[parabola|parabolic]] relationship between the index and the distance from the axis.{{citation needed|date=October 2021}}

=== Single-mode fiber ===
{{Main|Single-mode optical fiber}}
[[File:Singlemode fibre structure.svg|thumb|The structure of a typical [[single-mode fiber]].<br />
1. Core: 8&nbsp;μm diameter<br />
2. Cladding: 125&nbsp;μm dia.<br />
3. Buffer: 250&nbsp;μm dia.<br />
4. Jacket: 400&nbsp;μm dia.]]

Fibers with a core diameter less than about ten times the [[wavelength]] of the propagating light cannot be modeled using geometric optics. Instead, they must be analyzed as an electromagnetic waveguide structure, according to [[Maxwell's equations]] as reduced to the [[electromagnetic wave equation]].{{efn|The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when [[coherence (physics)|coherent]] light propagates in multi-mode fiber.}} As an optical waveguide, the fiber supports one or more confined [[transverse mode]]s by which light can propagate along the fiber. Fiber supporting only one mode is called ''single-mode''.{{efn|The behavior of larger-core multi-mode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.}} The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an [[evanescent wave]]. The most common type of single-mode fiber has a core diameter of 8–10 micrometers and is designed for use in the [[near infrared]]. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometers and as large as hundreds of micrometers.

=== Special-purpose fiber ===
Some special-purpose optical fiber is constructed with a non-cylindrical core or cladding layer, usually with an elliptical or rectangular cross-section. These include [[polarization-maintaining optical fiber|polarization-maintaining fiber]] used in fiber optic sensors and fiber designed to suppress [[whispering gallery mode]] propagation.

[[Photonic-crystal fiber]] is made with a regular pattern of index variation (often in the form of cylindrical holes that run along the length of the fiber). Such fiber uses [[diffraction]] effects instead of or in addition to total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a wide variety of applications.

== Mechanisms of attenuation ==
{{See also|Transparent materials}}
[[File:Si ZBLAN comparison.jpg|thumb|upright=1.35|Experimental attenuation curve of low loss multimode silica and ZBLAN fiber. Black triangle points and gray arrows illustrate a four order of magnitude reduction in the attenuation of silica optical fibers over four decades from ~1000&nbsp;dB/km in 1965 to ~0.17&nbsp;dB/km in 2005.]]
[[File:Silica core fiber minimum attenuation.jpg|thumb|Experimentally measured spectral attenuation of silica core optical fiber.<ref name="record-0.14">{{Cite journal |last1=Khrapko |first1=R. |last2=Logunov |first2=S. L. |last3=Li |first3=M. |last4=Matthews |first4=H. B. |last5=Tandon |first5=P. |last6=Zhou |first6=C. |date=2024-04-15 |title=Quasi Single-Mode Fiber With Record-Low Attenuation of 0.1400 dB/km |journal=IEEE Photonics Technology Letters |volume=36 |issue=8 |pages=539–542 |doi=10.1109/LPT.2024.3372786 |issn=1041-1135|doi-access=free |bibcode=2024IPTL...36..539K }}</ref> Minimum attenuation is 0.1400&nbsp;dB/km at 1560&nbsp;nm wavelength.]]
Attenuation in fiber optics, also known as transmission loss, is the reduction in the intensity of the light signal as it travels through the transmission medium. Attenuation coefficients in fiber optics are usually expressed in units of dB/km. The medium is usually a fiber of silica glass{{efn|For applications requiring spectral wavelengths, especially in the mid-infrared wavelengths (~&nbsp;2–7&nbsp;μm), a better alternative is represented by [[fluoride glass]]es such as [[ZBLAN]] and '''I'''nF<sub>3</sub>.}} that confines the incident light beam within. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. The four orders of magnitude reduction in the attenuation of silica optical fibers over four decades was the result of constant improvement of manufacturing processes, raw material purity, preform, and fiber designs, which allowed for these fibers to approach the theoretical lower limit of attenuation.<ref name="record-0.14"/>

Single-mode optical fibers can be made with extremely low loss. Corning's  Vascade® EX2500 fiber, a low loss single-mode fiber for telecommunications wavelengths, has a nominal attenuation of 0.148&nbsp;dB/km at 1550&nbsp;nm.<ref name="Vascade® EX2500 fiber">{{cite web |title=Corning Submarine Optical Fibers |url=https://www.corning.com/optical-communications/worldwide/en/home/products/fiber/optical-fiber-products/submarine-fibers.html |website=Corning.com |publisher=Corning Incorporated |access-date=28 March 2024}}</ref> A 10&nbsp;km length of such fiber transmits nearly 71% of optical energy at 1550&nbsp;nm.

Attenuation in optical fiber is caused primarily by both [[scattering]] and [[absorption (electromagnetic radiation)|absorption]]. In fibers based on fluoride glasses such as ZBLAN, minimum attenuation is limited by impurity absorption. Vast majority of optical fibers are based on silica glass, where impurity absorption is negligible. In silica fibers attenuation is determined by intrinsic mechanisms: [[Rayleigh scattering]] in the glasses through which the light is propagating, and infrared absorption in the same glasses. Absorption in silica increases steeply at wavelengths above 1570&nbsp;nm. At wavelengths most useful for telecommunications, Rayleigh scattering is the dominant loss mechanism. At 1550&nbsp;nm attenuation components for a record low loss fiber are given as follows: Rayleigh scattering loss: 0.1200&nbsp;dB/km, infrared absorption loss: 0.0150&nbsp;dB/km, impurity absorption loss: 0.0047&nbsp;dB/km, waveguide imperfection loss: 0.0010&nbsp;dB/km.

=== Light scattering ===
[[File:Angular dependence of Rayleigh Scattering.jpg|thumb|Angular dependence of Rayleigh scattering]]
The propagation of light through the core of an optical fiber is based on the total internal reflection of the lightwave, in terms of geometric optics, or guided modes, in terms of electromagnetic waveguide. In a typical single mode optical fiber about 75% of light is propagating through the core material, having higher refractive index, and about 25% of light is propagating through the cladding, having lower refractive index. The interface between the core and cladding glasses is exceptionally smooth and does not give rise to a significant scattering loss or a waveguide imperfection loss. The scattering loss originates primarily from the Rayleigh scattering in the bulk of the glasses composing the fiber core and cladding.

The scattering of light in optical quality glass fiber is caused by molecular level irregularities (compositional fluctuations) in the glass structure. Indeed, one emerging school of thought is that glass is simply the limiting case of a polycrystalline solid. Within this framework, ''domains'' exhibiting various degrees of short-range order become the building blocks of metals as well as glasses and ceramics. Distributed both between and within these domains are micro-structural defects that provide the most ideal locations for light scattering.

Scattering depends on the [[wavelength]] of the light being scattered and on the size of the scattering centers. Angular dependence of the light intensity scattered from an optical fiber matched that of Rayleigh scattering, indicating that the scattering centers are much smaller than the wavelength of propagating light. It originates from the density fluctuations driven by fictive temperature of the glass, and from the concentration fluctuations of dopants in both the core and the cladding. Rayleigh scattering coefficient, ''R'', can be presented as :
<math display="block">R = R_\text{d} + R_\text{c}</math>
where {{math|''R''{{sub|d}}}} represents Rayleigh scattering on density fluctuations and {{math|''R''{{sub|c}}}} represents Rayleigh scattering on dopant concentration fluctuations. Dopants, such as [[germanium dioxide]]  or fluorine, are used to create the refractive index difference between the core and the cladding, to form a waveguide structure.
<math display="block">R_\text{d} = \frac{8 \pi^3}{3 \lambda^4} n^8 p^2 \beta_\text{c} k_\text{B} T_\text{f} </math>
where {{mvar|λ}} is wavelength, {{mvar|n}} is [[refractive index]], {{mvar|p}} is photo-elastic coefficient, {{math|''β''{{sub|c}}}} is isothermal compressibility, {{math|''k''{{sub|B}}}} is the [[Boltzmann constant]], {{math|''T''{{sub|f}}}} is fictive temperature. The only physically significant variable affecting scattering on density fluctuations is the fictive temperature of the glass, lower fictive temperature results in a more homogeneous glass and lower Rayleigh scattering. Fictive temperature may be dramatically reduced by about 100 wt. ppm of alkali oxide dopant in the fiber core, as well as slower cooling of the fiber during the fiber draw process. These approaches are used to produce optical fibers with the lowest attenuation, especially those for submarine telecom cables.

For small dopant concentrations, {{math|''R''{{sub|c}}}} is proportional to {{math|''x''(d''n''/d''x''){{sup|2}}}}, where {{mvar|x}} is the mole fraction of the dopant in SiO<sub>2</sub>-based glass and {{mvar|n}} is the refractive index of the glass. When GeO<sub>2</sub> dopant is used to increase the refractive index of the fiber core, it increases the concentration fluctuation component of Rayleigh scattering, and attenuation of the fiber. This is why the lowest attenuation fibers do not use GeO<sub>2</sub> in the core, and use fluorine in the cladding, to reduce the refractive index of the cladding. {{math|''R''{{sub|c}}}} in pure silica core fiber is proportional to the overlap integral between LP01 mode and fluorine-induced concentration fluctuation component in the cladding.

In the core of potassium-doped pure silica-core (KPSC) fiber only density fluctuations play a significant role, as the concentrations of K<sub>2</sub>O, fluorine and chlorine are very low. The density fluctuations in the core are moderated by lower fictive temperature resulting from potassium doping, and are further reduced by annealing during the fiber draw process. This differs from the cladding, where higher fluorine dopant levels and the resulting concentration fluctuations add to the loss. In such fibers the light travelling through the core experiences lower scattering and lower attenuation compared to the light propagating through the cladding segment of the fiber.

At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber.<ref>{{cite journal |doi=10.1364/AO.11.002489 |title=Optical Power Handling Capacity of Low Loss Optical Fibers as Determined by Stimulated Raman and Brillouin Scattering|year=1972 |last1=Smith|first1=R. G. |journal=Applied Optics|volume=11|issue=11|pages=2489–94 |pmid=20119362|bibcode=1972ApOpt..11.2489S}}</ref><ref>{{cite encyclopedia |title=Brillouin Scattering |url=https://www.rp-photonics.com/brillouin_scattering.html |encyclopedia=Encyclopedia of Laser Physics and Technology |publisher=RP Photonics |first=Rüdiger |last=Paschotta}}</ref>

=== UV-Vis-IR absorption ===
In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths. Primary material considerations include both electrons and molecules as follows:
* At the electronic level, it depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific wavelength or frequency in the ultraviolet (UV) or visible ranges. This is what gives rise to color.
* At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how closely packed its atoms or molecules are, and whether or not the atoms or molecules exhibit long-range order. These factors will determine the capacity of the material to transmit longer wavelengths in the infrared (IR), far IR, radio, and microwave ranges.

The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The [[crystal structure]] [[absorption (electromagnetic radiation)|absorption]] characteristics observed at the lower frequency regions (mid- to far-IR wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive [[coupling]] between the motions of thermally induced vibrations of the constituent [[atom]]s and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching) in the far-infrared (>10&nbsp;μm).

In other words, the selective absorption of IR light by a particular material occurs because the selected frequency of the light wave matches the frequency (or an integer multiple of the frequency, i.e. [[harmonic]]) at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of IR light.

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When IR light of these frequencies strikes an object, the energy is either reflected or transmitted.

=== Loss budget ===
{{main|Optical power budget}}
Attenuation over a cable run is significantly increased by the inclusion of connectors and splices.  When computing the acceptable attenuation (loss budget) between a transmitter and a receiver one includes: 
* dB loss due to the type and length of fiber optic cable,
* dB loss introduced by connectors, and 
* dB loss introduced by splices.

Connectors typically introduce 0.3&nbsp;dB per connector on well-polished connectors.  Splices typically introduce less than 0.2&nbsp;dB per splice.{{citation needed|date=September 2022}}

The total loss can be calculated by:
: Loss = dB loss per connector × number of connectors + dB loss per splice × number of splices + dB loss per kilometer × kilometers of fiber,

where the dB loss per kilometer is a function of the type of fiber and can be found in the manufacturer's specifications.  For example, a typical 1550&nbsp;nm single-mode fiber has a loss of 0.3&nbsp;dB per kilometer.{{citation needed|date=September 2022}}

The calculated loss budget is used when testing to confirm that the measured loss is within the normal operating parameters.

== Manufacturing ==
=== Materials ===
Glass optical fibers are almost always made from [[silica]], but some other materials, such as [[fluoride glass|fluorozirconate]], [[fluoride glass|fluoroaluminate]], and [[chalcogenide glass]]es as well as crystalline materials like [[sapphire]], are used for longer-wavelength infrared or other specialized applications. Silica and fluoride glasses usually have refractive indices of about 1.5, but some materials such as the [[chalcogenide]]s can have indices as high as 3. Typically the index difference between core and cladding is less than one percent.

[[Plastic optical fiber]]s (POF) are commonly step-index multi-mode fibers with a core diameter of 0.5 millimeters or larger. POF typically have higher attenuation coefficients than glass fibers, 1&nbsp;dB/m or higher, and this high attenuation limits the range of POF-based systems.

==== Silica ====
Silica exhibits fairly good optical transmission over a wide range of wavelengths. In the [[near-infrared]] (near IR) portion of the spectrum, particularly around 1.5&nbsp;μm, silica can have extremely low absorption and scattering losses of the order of 0.2&nbsp;dB/km. Such low losses depend on using ultra-pure silica. A high transparency in the 1.4-μm region is achieved by maintaining a low concentration of [[hydroxyl group]]s (OH). Alternatively, a high OH [[concentration]] is better for transmission in the [[ultraviolet]] (UV) region.<ref>{{Cite journal | last1 = Skuja | first1 = L. | last2 = Hirano | first2 = M. | last3 = Hosono | first3 = H. | last4 = Kajihara | first4 = K. | title = Defects in oxide glasses | doi = 10.1002/pssc.200460102 | journal = Physica Status Solidi C | volume = 2 | issue = 1 | pages = 15–24 | year = 2005 | bibcode = 2005PSSCR...2...15S }}</ref>

Silica can be drawn into fibers at reasonably high temperatures and has a fairly broad [[glass transformation range]]. One other advantage is that fusion splicing and [[Cleave (fiber)|cleaving]] of silica fibers is relatively effective. Silica fiber also has high mechanical strength against both pulling and even bending, provided that the fiber is not too thick and that the surfaces have been well prepared during processing. Even simple cleaving of the ends of the fiber can provide nicely flat surfaces with acceptable optical quality. Silica is also relatively [[chemically inert]]. In particular, it is not [[Hygroscopy|hygroscopic]] (does not absorb water).

Silica glass can be doped with various materials. One purpose of doping is to raise the [[refractive index]] (e.g. with [[germanium dioxide]] (GeO<sub>2</sub>) or [[aluminium oxide]] (Al<sub>2</sub>O<sub>3</sub>)) or to lower it (e.g. with [[fluorine]] or [[boron trioxide]] (B<sub>2</sub>O<sub>3</sub>)). Doping is also possible with laser-active ions (for example, rare-earth-doped fibers) in order to obtain active fibers to be used, for example, in fiber amplifiers or [[laser]] applications. Both the fiber core and cladding are typically doped, so that the entire assembly (core and cladding) is effectively the same compound (e.g. an [[aluminosilicate]], germanosilicate, phosphosilicate or [[borosilicate glass]]).

Particularly for active fibers, pure silica is usually not a very suitable host glass, because it exhibits a low solubility for rare-earth ions. This can lead to quenching effects due to the clustering of dopant ions. Aluminosilicates are much more effective in this respect.

Silica fiber also exhibits a high threshold for optical damage. This property ensures a low tendency for laser-induced breakdown. This is important for fiber amplifiers when utilized for the amplification of short pulses.

Because of these properties, silica fibers are the material of choice in many optical applications, such as communications (except for very short distances with plastic optical fiber), fiber lasers, fiber amplifiers, and fiber-optic sensors. Large efforts put forth in the development of various types of silica fibers have further increased the performance of such fibers over other materials.<ref>{{cite journal |author=Glaesemann, G. S. |bibcode=1999SPIE.CR73....3G|title=Advancements in Mechanical Strength and Reliability of Optical Fibers|journal=Proc. SPIE|volume=CR73 |page=1|year=1999}}</ref><ref name=Kurkjian>{{cite journal |doi=10.1111/j.1151-2916.1993.tb03727.x |title=Strength, Degradation, and Coating of Silica Lightguides |year=1993 |last1=Kurkjian |first1=Charles R. |last2=Simpkins |first2=Peter G. |last3=Inniss |first3=Daryl |journal=Journal of the American Ceramic Society |volume=76 |pages=1106–1112 |issue=5}}</ref><ref>{{cite journal|doi=10.1016/0022-3093(88)90114-7 |title=Mechanical stability of oxide glasses |year=1988 |last1=Kurkjian |first1=C |journal=Journal of Non-Crystalline Solids |volume=102 |issue=1–3 |pages=71–81 |bibcode=1988JNCS..102...71K}}</ref><ref>{{cite journal |doi=10.1109/50.50715 |title=Strength and fatigue of silica optical fibers |year=1989 |last1=Kurkjian |first1=C. R. |last2=Krause |first2=J. T. |last3=Matthewson |first3=M. J. |journal=Journal of Lightwave Technology |volume=7 |pages=1360–1370 |issue=9 |bibcode=1989JLwT....7.1360K}}</ref><ref>{{cite conference |doi=10.1117/12.372757 |journal=Proceedings of SPIE |year=1999 |last1=Kurkjian |first1=Charles R.|page=77 |volume=3848 |title=Strength variations in silica fibers |book-title=Optical Fiber Reliability and Testing |series=Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series |conference=Photonics '99 |editor1-last=Matthewson |editor1-first=M. John |last2=Gebizlioglu |first2=Osman S. |last3=Camlibel |first3=Irfan |bibcode=1999SPIE.3848...77K |s2cid=119534094}}</ref><ref>{{cite conference|doi=10.1117/12.396408|journal=Proceedings of SPIE |year=2000 |last1=Skontorp |first1=Arne |page=278 |volume=4073|title=Nonlinear mechanical properties of silica-based optical fibers|conference=Fifth European Conference on Smart Structures and Materials|editor1-last=Gobin|editor1-first=Pierre F|editor2-last=Friend|editor2-first=Clifford M |bibcode=2000SPIE.4073..278S |s2cid=135912790}}</ref><ref>{{cite journal |doi=10.1098/rspa.1967.0085|title=The Strength of Fused Silica|year=1967|last1=Proctor|first1=B. A. |last2=Whitney|first2=I.|last3=Johnson |first3=J. W.|journal=[[Proceedings of the Royal Society A]]|volume=297|pages=534–557|issue=1451 |bibcode=1967RSPSA.297..534P |s2cid=137896322}}</ref><ref>{{cite journal|doi=10.1016/0022-3093(68)90007-0 |title=The structure and strength of glass fibers|year=1968|last1=Bartenev|first1=G |journal=Journal of Non-Crystalline Solids|volume=1|issue=1|pages=69–90 |bibcode=1968JNCS....1...69B}}</ref>

==== Fluoride glass ====
[[Fluoride glass]] is a class of non-oxide optical quality glasses composed of [[fluoride]]s of various [[metal]]s. Because of the low [[viscosity]] of these glasses, it is very difficult to completely avoid [[crystallization]] while processing it through the glass transition (or drawing the fiber from the melt). Thus, although [[heavy metal (chemistry)|heavy metal]] fluoride glasses (HMFG) exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks. Their best attribute is that they lack the absorption band associated with the [[hydroxyl]] (OH) group (3,200–3,600&nbsp;cm<sup>−1</sup>; i.e., 2,777–3,125&nbsp;nm or 2.78–3.13 μm), which is present in nearly all oxide-based glasses. Such low losses were never realized in practice, and the fragility and high cost of fluoride fibers made them less than ideal as primary candidates.

Fluoride fibers are used in mid-[[IR spectroscopy]], [[fiber optic sensor]]s, [[thermometry]], and [[optical imaging|imaging]]. Fluoride fibers can be used for guided lightwave transmission in media such as YAG ([[yttrium aluminium garnet]]) [[laser]]s at 2.9&nbsp;μm, as required for medical applications (e.g. [[ophthalmology]] and [[dentistry]]).<ref>{{cite journal |doi=10.1109/JLT.1984.1073661 |last1=Tran |first1=D. |last2=Sigel |first2=G. |last3=Bendow |first3=B. |title=Heavy metal fluoride glasses and fibers: A review |journal= Journal of Lightwave Technology|volume=2 |pages=566–586 |year=1984 |issue=5 |bibcode = 1984JLwT....2..566T}}</ref><ref>{{cite conference |conference=International Symposium on Optical Science and Technology |doi=10.1117/12.405276 |journal=Proceedings of SPIE |year=2000 |page=122 |volume=4102 |title=Optical and surface properties of oxyfluoride glass|book-title=Inorganic Optical Materials II|last1=Nee |first1=Soe-Mie F. |last2=Johnson |first2=Linda F. |last3=Moran |first3=Mark B. |last4=Pentony |first4=Joni M. |last5=Daigneault |first5=Steven M. |last6=Tran |first6=Danh C. |last7=Billman |first7=Kenneth W. |last8=Siahatgar |first8=Sadegh |bibcode=2000SPIE.4102..122N|s2cid=137381989 |url=https://zenodo.org/record/1235596 }}</ref>

An example of a heavy metal fluoride glass is the [[ZBLAN]] glass group, composed of [[zirconium]], [[barium]], [[lanthanum]], [[aluminium]], and [[sodium]] fluorides. Their main technological application is as optical waveguides in both planar and fiber forms. They are advantageous especially in the [[mid-infrared]] (2,000–5,000&nbsp;nm) range.

==== Phosphate glass ====
[[File:Phosphorus-pentoxide-3D-balls.png|thumb|The P<sub>4</sub>O<sub>10</sub> cagelike structure—the basic building block for phosphate glass]]

[[Phosphate glass]] is a class of optical glasses composed of [[metaphosphate]]s of various metals. Instead of the SiO<sub>4</sub> <!-- I don't think there is an SiO4 --> [[tetrahedra]] observed in silicate glasses, the building block for this glass [[phosphorus pentoxide]] (P<sub>2</sub>O<sub>5</sub>), which crystallizes in at least four different forms. The most familiar [[polymorphism (materials science)|polymorph]] is the cagelike structure of P<sub>4</sub>O<sub>10</sub>.

Phosphate glasses can be advantageous over silica glasses for optical fibers with a high concentration of doping rare-earth ions. A mix of fluoride glass and phosphate glass is fluorophosphate glass.<ref>{{cite journal |doi=10.1016/S0022-3093(01)00615-9 |title=Mechanical and structural properties of phosphate glasses |year=2001 |last1=Karabulut |first1=M. |journal=Journal of Non-Crystalline Solids |volume=288 |issue=1–3 |pages=8–17 |bibcode=2001JNCS..288....8K |last2=Melnik |first2=E. |last3=Stefan |first3=R |last4=Marasinghe |first4=G. K. |last5=Ray |first5=C. S. |last6=Kurkjian |first6=C. R. |last7=Day |first7=D. E.}}</ref><ref>{{cite journal|doi=10.1016/S0022-3093(99)00637-7 |title=Mechanical properties of phosphate glasses |year=2000 |last1=Kurkjian |first1=C. |journal=Journal of Non-Crystalline Solids |volume=263–264 |issue=1–2 |pages=207–212 |bibcode=2000JNCS..263..207K}}</ref>

==== Chalcogenide glass ====
The [[chalcogen]]s—the elements in [[group (periodic table)|group 16 of the periodic table]]—particularly [[sulfur]] (S), [[selenium]] (Se) and [[tellurium]] (Te)—react with more [[electropositive]] elements, such as [[silver]], to form [[chalcogenides]]. These are extremely versatile compounds, in that they can be crystalline or amorphous, metallic or semiconducting, and conductors of [[ion]]s or [[electron]]s. [[chalcogenide glass]] can be used to make fibers for far infrared transmission.<ref>{{cite journal |title=Trends and prospects for development of chalcogenide fibers for mid-infrared transmission |journal=Journal of Non-Crystalline Solids |year=2013 |doi=10.1016/j.jnoncrysol.2012.12.048 |last1=Shiryaev |first1=V.S. |last2=Churbanov |first2=M.F. |volume=377 |pages=225–230 |bibcode=2013JNCS..377..225S }}</ref>

=== Process ===
{{more citations needed section|date=April 2016}}

==== Preform ====
[[File:OF-MCVD.svg|thumb|upright=1.35|Illustration of the modified chemical vapor deposition (inside) process]]

Standard optical fibers are made by first constructing a large-diameter ''preform'' with a carefully controlled refractive index profile, and then ''pulling'' the preform to form the long, thin optical fiber. The preform is commonly made by three [[chemical vapor deposition]] methods: ''inside vapor deposition'', ''outside vapor deposition'', and ''vapor axial deposition''.<ref name=gowar1993>
{{cite book
 |title=Optical communication systems
 |last= Gowar |first= John
 |edition= 2nd
 |publisher= Prentice-Hall
 |location= Hempstead, UK
 |year= 1993
 |isbn= 978-0-13-638727-5
 |page= 209
}}</ref>

With ''inside vapor deposition'', the preform starts as a hollow glass tube approximately {{convert|40|cm|in|sp=us}} long, which is placed horizontally and rotated slowly on a [[lathe]]. Gases such as [[silicon tetrachloride]] (SiCl<sub>4</sub>) or [[germanium tetrachloride]] (GeCl<sub>4</sub>) are injected with [[oxygen]] in the end of the tube. The gases are then heated by means of an external hydrogen burner, bringing the temperature of the gas up to 1,900&nbsp;[[Kelvin|K]] (1,600&nbsp;°C, 3,000&nbsp;°F), where the tetrachlorides react with oxygen to produce [[silica]] or [[germanium dioxide]] particles. When the reaction conditions are chosen to allow this reaction to occur in the gas phase throughout the tube volume, in contrast to earlier techniques where the reaction occurred only on the glass surface, this technique is called ''modified chemical vapor deposition''.

The oxide particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot. The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outward in a process known as [[thermophoresis]]. The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient amount of material has been deposited. For each layer the composition can be modified by varying the gas composition, resulting in precise control of the finished fiber's optical properties.

In outside vapor deposition or vapor axial deposition, the glass is formed by ''flame hydrolysis'', a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water in an [[oxyhydrogen]] flame. In outside vapor deposition, the glass is deposited onto a solid rod, which is removed before further processing. In vapor axial deposition, a short ''seed rod'' is used, and a porous preform, whose length is not limited by the size of the source rod, is built up on its end. The porous preform is consolidated into a transparent, solid preform by heating to about 1,800&nbsp;K (1,500&nbsp;°C, 2,800&nbsp;°F).

[[File:DShaped1.png|right|upright|thumb|Cross-section of a fiber drawn from a D-shaped ''preform''. The preform for this test fiber was not polished well, and cracks are seen with the confocal [[optical microscope]].]]
Typical communications fiber uses a circular preform. For some applications such as [[double-clad fiber]]s another form is preferred.<ref name="Kouznetsov">{{cite journal|title=Highly efficient, high-gain, short-length, and power-scalable incoherent diode slab-pumped fiber amplifier/laser| author= Kouznetsov, D.|author2=Moloney, J.V.| journal=[[IEEE Journal of Quantum Electronics]]| volume=39 | year=2003 | issue=11 | pages=1452–1461 | doi=10.1109/JQE.2003.818311|bibcode = 2003IJQE...39.1452K | citeseerx= 10.1.1.196.6031}}</ref> In [[fiber laser]]s based on double-clad fiber, an asymmetric shape improves the [[filling factor]] for [[laser pumping]].

Because of the surface tension, the shape is smoothed during the drawing process, and the shape of the resulting fiber does not reproduce the sharp edges of the preform. Nevertheless, careful polishing of the preform is important, since any defects of the preform surface affect the optical and mechanical properties of the resulting fiber.

==== Drawing ====
The preform, regardless of construction, is placed in a device known as a [[drawing tower]], where the preform tip is heated and the optical fiber is pulled out as a string. The tension on the fiber can be controlled to maintain the desired fiber thickness.

=== Cladding ===
The light is guided down the core of the fiber by an optical cladding with a lower refractive index that traps light in the core through total internal reflection. For some types of fiber, the cladding is made of glass and is drawn along with the core from a preform with radially varying index of refraction. For other types of fiber, the cladding made of plastic and is applied like a coating (see below).

=== Coatings ===
The cladding is coated by a buffer, (not to be confused with an actual buffer tube), that protects it from moisture and physical damage.<ref name=Kurkjian /> These coatings are UV-cured [[urethane acrylate]] composite or [[polyimide]] materials applied to the outside of the fiber during the drawing process. The coatings protect the very delicate strands of glass fiber—about the size of a human hair—and allow it to survive the rigors of manufacturing, proof testing, cabling, and installation. The buffer coating must be stripped off the fiber for termination or splicing.

Today's glass optical fiber draw processes employ a dual-layer coating approach. An inner primary coating is designed to act as a shock absorber to minimize attenuation caused by [[microbend]]ing. An outer secondary coating protects the primary coating against mechanical damage and acts as a barrier to lateral forces, and may be colored to differentiate strands in bundled cable constructions. These fiber optic coating layers are applied during the fiber draw, at speeds approaching {{convert|100|km/h|mph|-1|sp=us}}. Fiber optic coatings are applied using one of two methods: ''wet-on-dry'' and ''wet-on-wet''. In wet-on-dry, the fiber passes through a primary coating application, which is then UV cured, then through the secondary coating application, which is subsequently cured. In wet-on-wet, the fiber passes through both the primary and secondary coating applications, then goes to UV curing.<ref>{{Cite journal |last1=Khan |first1=Zeeshan |last2=Islam |first2=Saeed |last3=Shah |first3=Rehan Ali |last4=Khan |first4=Muhammad Altaf |last5=Bonyah |first5=Ebenezer |last6=Jan |first6=Bilal |last7=Khan |first7=Aurangzeb |date=2017-01-01 |title=Double-layer optical fiber coating analysis in MHD flow of an elastico-viscous fluid using wet-on-wet coating process |journal=Results in Physics |volume=7 |pages=107–118 |doi=10.1016/j.rinp.2016.11.062 |bibcode=2017ResPh...7..107K |issn=2211-3797|doi-access=free }}</ref>

The thickness of the coating is taken into account when calculating the stress that the fiber experiences under different bend configurations.<ref name="Matthewson">{{Cite conference |last=Matthewson |first=M. |date=1994 |title=Optical Fiber Mechanical Testing Techniques |url=http://www.rci.rutgers.edu/~mjohnm/reliability/SPIE_1993_CR50_testing_techniques.pdf |series=Critical Reviews of Optical Science and Technology |volume=CR50 |pages=32–59 |publisher=Society of Photo-Optical Instrumentation Engineers |conference=Fiber Optics Reliability and Testing, September 8–9, 1993, Boston, Massachusetts |bibcode=1993SPIE10272E..05M |doi=10.1117/12.181373 |book-title=Fiber Optics Reliability and Testing |s2cid=136377895 |access-date=2019-05-02 |archive-date=2019-05-02 |archive-url=https://web.archive.org/web/20190502232228/http://www.rci.rutgers.edu/~mjohnm/reliability/SPIE_1993_CR50_testing_techniques.pdf |url-status=dead}}</ref> When a coated fiber is wrapped around a mandrel, the stress experienced by the fiber is given by<ref name="Matthewson" />{{rp|45}}
<math display="block">\sigma = E {d_\text{f} \over d_\text{m} + d_\text{c}},</math>
where {{mvar|E}} is the fiber's [[Young's modulus]], {{math|''d''{{sub|m}}}} is the diameter of the mandrel, {{math|''d''{{sub|f}}}} is the diameter of the cladding and {{math|''d''{{sub|c}}}} is the diameter of the coating.

In a two-point bend configuration, a coated fiber is bent in a U-shape and placed between the grooves of two faceplates, which are brought together until the fiber breaks. The stress in the fiber in this configuration is given by<ref name="Matthewson" />{{rp|47}}
<math display="block">\sigma = 1.198E {d_\text{f} \over d - d_\text{c}},</math>
where {{mvar|d}} is the distance between the faceplates. The coefficient 1.198 is a geometric constant associated with this configuration.

Fiber optic coatings protect the glass fibers from scratches that could lead to strength degradation. The combination of moisture and scratches accelerates the aging and deterioration of fiber strength. When fiber is subjected to low stresses over a long period, fiber fatigue can occur. Over time or in extreme conditions, these factors combine to cause microscopic flaws in the glass fiber to propagate, which can ultimately result in fiber failure.

Three key characteristics of fiber optic waveguides can be affected by environmental conditions: strength, attenuation, and resistance to losses caused by microbending. External [[optical fiber cable]] jackets and buffer tubes protect glass optical fiber from environmental conditions that can affect the fiber's performance and long-term durability. On the inside, coatings ensure the reliability of the signal being carried and help minimize attenuation due to microbending.

=== Cable construction ===
{{Main|Fiber-optic cable}}
[[File:Optical fiber cable.jpg|thumb|An [[optical fiber cable]]]]
In practical fibers, the cladding is usually coated with a tough [[resin]] and features an additional ''[[buffer (optical fiber)|buffer]]'' layer, which may be further surrounded by a ''jacket'' layer, usually plastic. These layers add strength to the fiber but do not affect its optical properties. Rigid fiber assemblies sometimes put light-absorbing glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces [[crosstalk]] between the fibers, or reduces [[Lens flare|flare]] in fiber bundle imaging applications.<ref>{{cite web|url=http://zone.ni.com/devzone/cda/ph/p/id/129#toc2 |title=Light collection and propagation |work=National Instruments' Developer Zone |publisher=National Instruments Corporation |access-date=2007-03-19 |url-status=dead |archive-url=https://web.archive.org/web/20070125051848/http://zone.ni.com/devzone/cda/ph/p/id/129 |archive-date=January 25, 2007 }}</ref><ref name=hecht2002>{{cite book| first=Jeff| last=Hecht| title=Understanding Fiber Optics| year=2002| edition=4th| isbn=978-0-13-027828-9|publisher= Prentice Hall}}</ref> Multi-fiber cable usually uses colored buffers to identify each strand.

Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, high voltage isolation, dual use as power lines,<ref>{{cite web |url=http://www.dced.state.ak.us/dca/AEIS/PDF_Files/AIDEA_Energy_Screening.pdf |title=Screening report for Alaska rural energy plan |work=Alaska Division of Community and Regional Affairs|access-date=April 11, 2006|archive-url=https://web.archive.org/web/20060508191931/http://www.dced.state.ak.us/dca/AEIS/PDF_Files/AIDEA_Energy_Screening.pdf|archive-date=May 8, 2006}}</ref>{{Failed verification|date=November 2008}} installation in conduit, lashing to aerial telephone poles, [[Submarine communications cable|submarine installation]], and insertion in paved streets.

Some fiber optic cable versions are reinforced with [[aramid]] yarns or glass yarns as an intermediary [[strength member]]. In commercial terms, usage of the glass yarns are more cost-effective with no loss of mechanical durability. Glass yarns also protect the cable core against rodents and termites.

== Practical issues ==
{{more citations needed section|date=April 2016}}

=== Installation ===
Fiber cable can be very flexible, but traditional fiber's loss increases greatly if the fiber is bent with a radius smaller than around 30&nbsp;mm. This creates a problem when the cable is bent around corners. ''Bendable fibers'', targeted toward easier installation in home environments, have been standardized as ITU-T [[G.657]]. This type of fiber can be bent with a radius as low as 7.5&nbsp;mm without adverse impact. Even more bendable fibers have been developed.<ref>{{cite press release|title=Corning announces breakthrough optical fiber technology |publisher=[[Corning Incorporated]] |date=2007-07-23 |url=http://www.corning.com/news_center/news_releases/2007/2007072301.aspx |access-date=2013-09-09 |url-status=dead |archive-url=https://web.archive.org/web/20110613040314/http://www.corning.com/news_center/news_releases/2007/2007072301.aspx |archive-date=June 13, 2011 }}</ref> Bendable fiber may also be resistant to fiber hacking, in which the signal in a fiber is surreptitiously monitored by bending the fiber and detecting the leakage.<ref>{{cite web |url=http://blogs.techrepublic.com.com/security/?p=222 |title=Protect your network against fiber hacks |access-date=2007-12-10 |last=Olzak |first=Tom |date=2007-05-03 |work=Techrepublic |publisher=CNET |archive-url=https://web.archive.org/web/20100217210636/http://blogs.techrepublic.com.com/security/?p=222 |archive-date=2010-02-17}}</ref>

Another important feature of cable is cable's ability to withstand tension which determines how much force can be applied to the cable during installation.

=== Termination and splicing ===
[[File:ST-optical-fiber-connector-hdr-0a.jpg|thumb|upright|[[ST connector]]s on [[multi-mode fiber]]]]
{{main|Fiber cable termination}}
Optical fibers are connected to terminal equipment by [[optical fiber connector]]s. These connectors are usually of a standard type such as ''FC'', ''SC'', ''ST'', ''LC'', ''MTRJ'', ''MPO'' or ''SMA''. Optical fibers may be connected by connectors typically on a [[patch panel]], or permanently by [[optical splice|splicing]], that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is [[fusion splicing]], which melts the fiber ends together. For quicker fastening jobs, a ''mechanical splice'' is used. All splicing techniques involve installing an enclosure that protects the splice.

Fusion splicing is done with a specialized instrument. The fiber ends are first stripped of their protective polymer coating (as well as the more sturdy outer jacket, if present). The ends are [[Cleave (fiber)|cleaved]] with a precision cleaver to make them perpendicular, and are placed into special holders in the fusion splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and fusion after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between [[electrodes]] at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the [[melting point]] of the glass, fusing the ends permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding do not mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1&nbsp;dB is typical. The complexity of this process makes fiber splicing much more difficult than splicing copper wire.
[[File:Optical Splice Enclosure.jpg|thumb|An aerial optical fiber splice enclosure lowered during installation. The individual fibers are fused and stored within the enclosure for protection from damage]]

Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning, and precision cleaving. The fiber ends are aligned and held together by a precision sleeve, often using a clear [[index-matching gel]] that enhances the transmission of light across the joint. Mechanical splices typically have a higher optical loss and are less robust than fusion splices, especially if the gel is used.

Fibers are terminated in connectors that hold the fiber end precisely and securely. An optical fiber connector is a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be ''push and click'', ''turn and latch'' (''[[bayonet mount]]''), or ''screw-in'' (''threaded''). The barrel is typically free to move within the sleeve and may have a key that prevents the barrel and fiber from rotating as the connectors are mated.

A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick-set adhesive is usually used to hold the fiber securely, and a [[strain relief]] is secured to the rear. Once the adhesive sets, the fiber's end is polished. Various polish profiles are used, depending on the type of fiber and the application. The resulting signal strength loss is called ''[[gap loss]]''. For single-mode fiber, fiber ends are typically polished with a slight curvature that makes the mated connectors touch only at their cores. This is called a ''physical contact'' (PC) polish. The curved surface may be polished at an angle, to make an ''angled physical contact (APC)'' connection. Such connections have higher loss than PC connections but greatly reduced back reflection because light that reflects from the angled surface leaks out of the fiber core. APC fiber ends have low back reflection even when disconnected.

In the 1990s, the number of parts per connector, polishing of the fibers, and the need to oven-bake the epoxy in each connector made terminating fiber optic cables difficult. Today, connector types on the market offer easier, less labor-intensive ways of terminating cables. Some of the most popular connectors are pre-polished at the factory and include a gel inside the connector. A [[cleave (fiber)|cleave]] is made at a required length, to get as close to the polished piece already inside the connector. The gel surrounds the point where the two pieces meet inside the connector for very little light loss.<ref>{{cite web |url=https://www.lightwaveonline.com/optical-tech/article/16648500/optical-gels-improve-fiber-optic-connectors-and-splices |title=Optical gels improve fiber-optic connectors and splices |date=October 1999 |access-date=2024-06-25}}</ref> For the most demanding installations, factory pre-polished pigtails of sufficient length to reach the first fusion splice enclosure assures good performance and minimizes on-site labor.

=== Free-space coupling ===
It is often necessary to align an optical fiber with another optical fiber or with an [[optoelectronic device]] such as a [[light-emitting diode]], a [[laser diode]], or a [[modulator]]. This can involve either carefully aligning the fiber and placing it in contact with the device, or can use a [[lens (optics)|lens]] to allow coupling over an air gap. Typically the size of the [[fiber mode]] is much larger than the size of the mode in a laser diode or a [[Silicon photonics|silicon optical chip]]. In this case, a [[Tapered fiber|tapered]] or [[lensed fiber]] is used to match the fiber mode field distribution to that of the other element. The lens on the end of the fiber can be formed using polishing, laser cutting<ref>{{cite web |title=Laser Lensing |url=https://optek.humaneticsgroup.com/applications/optical-fiber-processing-applications/lensedopticalfiber |url-status=live |archive-url=https://web.archive.org/web/20120127133056/http://www.opteksystems.com/laser-lens |archive-date=2012-01-27 |access-date=2012-07-17 |work=OpTek Systems Inc.}}</ref> or fusion splicing.

In a laboratory environment, a bare fiber end is coupled using a fiber launch system, which uses a [[microscope objective lens]] to focus the light down to a fine point. A precision [[translation stage]] (micro-positioning table) is used to move the lens, fiber, or device to allow the coupling efficiency to be optimized. Fibers with a connector on the end make this process much simpler: the connector is simply plugged into a pre-aligned fiber-optic collimator, which contains a lens that is either accurately positioned to the fiber or is adjustable. To achieve the best injection efficiency into a single-mode fiber, the direction, position, size, and divergence of the beam must all be optimized. With good optimization, 70 to 90% coupling efficiency can be achieved.

With properly polished single-mode fibers, the emitted beam has an almost perfect Gaussian shape—even in the far field—if a good lens is used. The lens needs to be large enough to support the full numerical aperture of the fiber, and must not introduce [[optical aberration|aberrations]] in the beam. [[Aspheric lens]]es are typically used.

=== Fiber fuse ===
At optical intensities above 2 [[megawatt]]s per square centimeter, when a fiber is subjected to a shock or is otherwise suddenly damaged, a ''fiber fuse'' can occur. The reflection from the damage vaporizes the fiber immediately before the break, and this new defect remains reflective so that the damage propagates back toward the transmitter at 1–3 meters per second (4–11&nbsp;km/h, 2–8&nbsp;mph).<ref>{{cite journal |first=R. M. |last=Atkins |author2=Simpkins, P. G. |author3=Yablon, A. D. | title=Track of a fiber fuse: a Rayleigh instability in optical waveguides |journal=Optics Letters|year=2003 |volume=28 |issue=12 |pages=974–976 |doi=10.1364/OL.28.000974 |pmid=12836750|bibcode = 2003OptL...28..974A }}</ref><ref>{{cite journal |first=Breck |last=Hitz |title=Origin of 'fiber fuse' is revealed |journal=Photonics Spectra |date=August 2003 |url=http://www.photonics.com/Article.aspx?AID=16745 |access-date=2011-01-23 |archive-date=2012-05-10 |archive-url=https://web.archive.org/web/20120510162148/http://www.photonics.com/Article.aspx?AID=16745 |url-status=dead }}</ref> The [[open fiber control]] system, which ensures [[laser safety|laser eye safety]] in the event of a broken fiber, can also effectively halt propagation of the fiber fuse.<ref>{{cite journal |first=Koji |last=Seo |title=Evaluation of high-power endurance in optical fiber links |journal=Furukawa Review |issue= 24 |date=October 2003 |pages=17–22 |issn=1348-1797|url=http://www.furukawa.co.jp/review/fr024/fr24_04.pdf|access-date=2008-07-05|display-authors=etal}}</ref> In situations, such as undersea cables, where high power levels might be used without the need for open fiber control, a fiber fuse protection device at the transmitter can break the circuit to minimize damage.

=== Chromatic dispersion ===
{{Main|Dispersion (optics)}}
The refractive index of fibers varies slightly with the frequency of light, and light sources are not perfectly monochromatic. Modulation of the light source to transmit a signal also slightly widens the frequency band of the transmitted light. This has the effect that, over long distances and at high modulation speeds, different portions of light can take different times to arrive at the receiver, ultimately making the signal impossible to discern.<ref>G. P. Agrawal, Fiber Optic Communication Systems, Wiley-Interscience, 1997.</ref> This problem can be overcome in several ways, including the use of extra repeaters and the use of a relatively short length of fiber that has the [[Non-zero dispersion-shifted fiber|opposite refractive index gradient]].

== See also ==
{{Portal|Electronics}}
{{div col|colwidth=20em}}
* [[Fiber Bragg grating]]
* [[Fiber management system]]
* [[The Fiber Optic Association]]
* [[Gradient-index optics]]
* [[Leaky mode]]
* [[Modal bandwidth]]
* [[Optical mesh network]]
* [[Optical power meter]]
* [[Passive optical network]]
* [[Return loss]]
* [[Subwavelength-diameter optical fiber]]
{{div col end}}

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

== Further reading ==
* {{cite book|first=Govind|last=Agrawal|title=Fiber-Optic Communication Systems|publisher=Wiley|year=2010|isbn=978-0-470-50511-3|doi=10.1002/9780470918524|url=http://www3.ub.tu-berlin.de/ihv/000364146.pdf |edition=4th}}
* {{cite journal | last1 = Gambling | first1 = W. A. | year = 2000 | title = The Rise and Rise of Optical Fibers | journal = IEEE Journal of Selected Topics in Quantum Electronics | volume = 6 | issue = 6| pages = 1084–1093 | doi = 10.1109/2944.902157 | bibcode = 2000IJSTQ...6.1084G | s2cid = 23158230 }}
* Mirabito, Michael M. A.; and Morgenstern, Barbara L., ''The New Communications Technologies: Applications, Policy, and Impact'', 5th Edition. Focal Press, 2004. ({{ISBN|0-240-80586-0}}).
* Mitschke F., ''Fiber Optics: Physics and Technology'', Springer, 2009 ({{ISBN|978-3-642-03702-3}})
* {{cite journal | last1 = Nagel | first1 = S. R. | last2 = MacChesney | first2 = J. B. | last3 = Walker | first3 = K. L. | year = 1982| title = An Overview of the Modified Chemical Vapor Deposition (MCVD) Process and Performance | journal = IEEE Journal of Quantum Electronics | volume =  30| issue = 4| pages =  305–322| doi = 10.1109/TMTT.1982.1131071 | bibcode = 1982ITMTT..30..305N | s2cid = 33979233 }}
* {{cite book|author1=Rajiv Ramaswami|author2=Kumar Sivarajan|author3=Galen Sasaki|title=Optical Networks: A Practical Perspective|url={{google books|plainurl=yes|id=WpByp4Ip0z8C}}|date=27 November 2009|publisher=Morgan Kaufmann|isbn=978-0-08-092072-6}}
* {{cite book |first=Thomas L. |last=Friedman |title=The World is Flat |publisher=Picador |year=2007 |isbn=978-0-312-42507-4 |url-access=registration |url=https://archive.org/details/worldisflat00thom }} The book discusses how fiber optics has contributed to [[globalization]], and has revolutionized communications, business, and even the distribution of capital among countries.
* [https://telecom-info.njdepot.ericsson.net/site-cgi/ido/docs.cgi?ID=SEARCH&DOCUMENT=GR-771& GR-771, ''Generic Requirements for Fiber Optic Splice Closures''], Telcordia Technologies, Issue 2, July 2008. Discusses fiber optic splice closures and the associated hardware intended to restore the mechanical and environmental integrity of one or more fiber cables entering the enclosure.

== External links ==
{{Commons category|Optical fibers}}
* [https://www.thefoa.org/Lennie/ ''Lennie Lightwave's Guide to Fiber Optics''], The Fiber Optic Association, 2016.
* {{cite web|last=Paschotta|first=Rüdiger| title=Tutorial on Passive Fiber optics| url=https://www.rp-photonics.com/passive_fiber_optics.html| publisher=RP Photonics| access-date=17 October 2013}}
* "[https://www.rp-photonics.com/fibers.html Fibers]", article in RP Photonics' ''Encyclopedia of Laser Physics and Technology''
* "[http://www.gare.co.uk/technology_watch/fibre.htm Fibre optic technologies]", Mercury Communications Ltd, August 1992.
* "[http://www.gare.co.uk/technology_watch/photo.htm Photonics & the future of fibre]", Mercury Communications Ltd, March 1993.
* {{web archive |url=https://web.archive.org/web/20181023040952/https://arcelect.com/fibercable.htm |title=Fiber Optics - The Basics of Fiber Optic Cable}} Educational site from Arc Electronics
* [http://ocw.mit.edu/resources/res-6-005-understanding-lasers-and-fiberoptics-spring-2008/laser-fundamentals-i/ MIT Video Lecture: Understanding Lasers and Fiberoptics]
* {{citation |url=http://spie.org/Documents/Publications/00%20STEP%20Module%2007.pdf |archive-url=https://web.archive.org/web/20130424224449/http://spie.org/Documents/Publications/00%20STEP%20Module%2007.pdf |archive-date=2013-04-24 |work=Fundamentals of Photonics |title=Optical Waveguides and Fibers |author=Ajoy Ghatak and K. Thyagarajan |publisher=Indian Institute of Technology}}
* [http://webdemo.inue.uni-stuttgart.de/webdemos/02_lectures/uebertragungstechnik_2/chromatic_dispersion Webdemo for chromatic dispersion] at the Institute of Telecommunicatons, University of Stuttgart

{{Optical communication}}
{{Glass science}}
{{Telecommunications}}
{{Authority control}}

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