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Tunable laser
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===Widely tunable lasers=== [[File:Diode laser.jpg|thumb|right|250px|A typical laser diode. When mounted with external optics, these lasers can be tuned mainly in the red and near-infrared.]] Sample Grating Distributed Bragg Reflector lasers (SG-DBR) have a much larger tunable range; by the use of vernier-tunable [[Bragg mirror]]s and a phase section, a single-mode output range of > 50 nm can be selected. Other technologies to achieve wide tuning ranges for [[DWDM]]-systems<ref>[http://www.lightreading.com/document.asp?doc_id=26332 Tunable Lasers at Lightreading]</ref> are: *External cavity lasers using a MEMS structure for tuning the cavity length, such as devices commercialized by [[Iolon]]. *External cavity lasers using multiple-prism grating arrangements for wide-range tunability.<ref>P. Zorabedian, Tunable external-cavity semiconductor lasers, in ''Tunable Lasers Handbook'', F. J. Duarte, Ed. (Academic, New York, 1995) Chapter 8.</ref> *DFB laser arrays based on several thermal tuned DFB lasers, in which coarse tuning is achieved by selecting the correct laser bar. Fine tuning is then done thermally, such as in devices commercialized by [[Santur Corporation]]. *Tunable VCSELs, in which one of the two mirror stacks is movable. To achieve sufficient output power out of a VCSEL structure, lasers in the {{formatnum:1550}} nm domain are usually either optically pumped or have an additional optical amplifier built into the device. Rather than placing the resonator mirrors at the edges of the device, the mirrors in a VCSEL are located on the top and bottom of the semiconductor material. Somewhat confusingly, these mirrors are typically DBR devices. This arrangement causes light to "bounce" vertically in a laser chip, so that the light emerges through the top of the device, rather than through the edge. As a result, VCSELs produce beams of a more circular nature than their cousins and beams that do not diverge as rapidly.<ref>{{Cite web |title=Optoelectronics, Frequency Changing |url=https://studedu.org/s1266t1.html |access-date=2024-03-07 |website=studedu.org}}</ref> {{As of|2008|12}}{{Update inline|date=November 2024}}, there is no widely tunable VCSEL commercially available any more for [[DWDM]]-system application.{{Citation needed|date=September 2009}} It is claimed that the first infrared laser with a tunability of more than one octave was a germanium crystal laser.<ref>[http://spie.org/x39922.xml See photograph 3 at http://spie.org/x39922.xml]</ref>
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