Editing Germanium (section)

=== Optics ===
[[File:Singlemode fibre structure.svg|thumb|right|upright=.65|alt=A drawing of four concentric cylinders.|A typical single-mode optical fiber. Germanium oxide is a [[dopant]] of the core silica (Item 1). {{olist
    |Core 8&nbsp;µm
    |Cladding 125&nbsp;µm
    |Buffer 250&nbsp;µm
    |Jacket 400&nbsp;µm
}}]]
The notable properties of [[Germanium dioxide|germania]] (GeO<sub>2</sub>) are its high [[refractive index|index of refraction]] and its low [[Dispersion (optics)|optical dispersion]]. These make it especially useful for [[wide-angle camera lens]]es, [[microscopy]], and the core part of [[optical fiber]]s.<ref>{{cite journal |title=Infrared Detector Arrays for Astronomy |journal=Annual Review of Astronomy and Astrophysics |date=2007 |doi=10.1146/annurev.astro.44.051905.092436 |last=Rieke |first=G. H. |s2cid=26285029 |volume=45 |issue=1 |pages=77–115 |bibcode=2007ARA&A..45...77R}}</ref><ref name="Brown">{{cite web |url=http://minerals.usgs.gov/minerals/pubs/commodity/germanium/220400.pdf |title=Germanium |first=Robert D. Jr. |last=Brown |publisher=U.S. Geological Survey |year=2000 |access-date=2008-09-22 |archive-date=2011-06-08 |archive-url=https://web.archive.org/web/20110608071221/http://minerals.usgs.gov/minerals/pubs/commodity/germanium/220400.pdf |url-status=live}}</ref> It has replaced [[titanium dioxide|titania]] as the [[dopant]] for silica fiber, eliminating the subsequent heat treatment that made the fibers brittle.<ref>{{cite web |url=http://ptgmedia.pearsoncmg.com/images/1587051052/samplechapter/1587051052content.pdf |title=Chapter III: Optical Fiber For Communications |publisher=Stanford Research Institute |access-date=2008-08-22 |archive-date=2014-12-05 |archive-url=https://web.archive.org/web/20141205210827/http://ptgmedia.pearsoncmg.com/images/1587051052/samplechapter/1587051052content.pdf |url-status=live}}</ref> At the end of 2002, the fiber optics industry consumed 60% of the annual germanium use in the United States, but this is less than 10% of worldwide consumption.<ref name="Brown" /> [[GeSbTe]] is a [[phase change material]] used for its optic properties, such as that used in [[DVD-RW|rewritable DVDs]].<ref>{{cite web |url=http://www.osta.org/technology/pdf/dvdqa.pdf |archive-url=https://web.archive.org/web/20090419202545/http://www.osta.org/technology/pdf/dvdqa.pdf |archive-date=2009-04-19 |title=Understanding Recordable & Rewritable DVD |edition=First |access-date=2008-09-22 |publisher=Optical Storage Technology Association (OSTA)}}</ref>

Because germanium is transparent in the infrared wavelengths, it is an important [[infrared]] optical material that can be readily cut and polished into lenses and windows. It is especially used as the front optic in [[Thermographic camera|thermal imaging cameras]] working in the 8 to 14&nbsp;[[micrometre|micron]] range for passive thermal imaging and for hot-spot detection in military, mobile [[night vision]], and fire fighting applications.<ref name="Moska" /> It is used in infrared [[spectroscope]]s and other optical equipment that require extremely sensitive [[thermography|infrared detectors]].<ref name="Brown" /> It has a very high [[refractive index]] (4.0) and must be coated with anti-reflection agents. Particularly, a very hard special antireflection coating of [[diamond-like carbon]] (DLC), refractive index 2.0, is a good match and produces a diamond-hard surface that can withstand much environmental abuse.<ref>{{cite journal |first=Alan H. |last=Lettington |doi=10.1016/S0008-6223(98)00062-1 |title=Applications of diamond-like carbon thin films |volume=36 |issue=5–6 |date=1998 |pages=555–560 |journal=Carbon |bibcode=1998Carbo..36..555L}}</ref><ref>{{cite journal |first=Michael N. |last=Gardos |author2=Bonnie L. Soriano |author3=Steven H. Propst |title=Study on correlating rain erosion resistance with sliding abrasion resistance of DLC on germanium |journal=Proc. SPIE |volume=1325 |page=99 |date=1990 |doi=10.1117/12.22449 |issue=Mechanical Properties |series=SPIE Proceedings |editor1-last=Feldman |editor1-first=Albert |editor2-last=Holly |editor2-first=Sandor |bibcode=1990SPIE.1325...99G |s2cid=137425193}}</ref>