Editing Ytterbium (section)

===Ytterbium as dopant of active media===
The Yb<sup>3+</sup> [[ion]] is used as a [[doping (semiconductors)|doping material]] in [[active laser medium|active laser media]], specifically in [[solid state laser]]s and [[double clad fiber]] lasers. Ytterbium lasers are highly efficient, have long lifetimes and can generate short pulses; ytterbium can also easily be incorporated into the material used to make the laser.<ref>{{cite thesis |last=Ostby |first=Eric |date=2009 |title=Photonic Whispering-Gallery Resonations in New Environments |url=https://thesis.library.caltech.edu/2284/4/03_Ch3_Ostby.pdf
|access-date=21 December 2012 |publisher=[[California Institute of Technology]]}}</ref> Ytterbium lasers commonly radiate in the 1.03–1.12&nbsp;[[μm]] band being [[optical pumping|optically pumped]] at wavelength 900&nbsp;nm–1&nbsp;μm, dependently on the host and application. The small [[quantum defect]] makes ytterbium a prospective dopant for efficient lasers and [[power scaling]].<ref>{{cite journal|doi=10.1070/QE2004v034n03ABEH002621|title=Broadband Radiation Source Based on an Ytterbium-Doped Fibre With Fibre-Length-Distributed Pumping|date=2004|author=Grukh, Dmitrii A.|journal=Quantum Electronics|volume=34|page=247|last2=Bogatyrev|first2=V. A.|last3=Sysolyatin|first3=A. A.|last4=Paramonov|first4=Vladimir M.|last5=Kurkov|first5=Andrei S.|last6=Dianov|first6=Evgenii M.|bibcode = 2004QuEle..34..247G|issue=3 |s2cid=250788004 }}</ref>

The kinetic of excitations in ytterbium-doped materials is simple and can be described within the concept of [[McCumber relation|effective cross-section]]s; for most ytterbium-doped laser materials (as for many other optically pumped gain media), the [[McCumber relation]] holds,<ref name="kouz05">{{cite journal|author=Kouznetsov, D.|author2=Bisson, J.-F.|author3=Takaichi, K.|author4=Ueda, K. |title=Single-mode solid-state laser with short wide unstable cavity|journal=[[Journal of the Optical Society of America B]]|volume=22| issue=8| pages=1605–1619|date=2005|doi=10.1364/JOSAB.22.001605|bibcode=2005JOSAB..22.1605K}}</ref><ref name="mc">
{{cite journal|author=McCumber, D.E. |title= Einstein Relations Connecting Broadband Emission and Absorption Spectra|journal= Physical Review B|volume= 136|issue=4A|pages=954–957|date=1964|doi=10.1103/PhysRev.136.A954|bibcode = 1964PhRv..136..954M }}</ref><ref name="B">{{cite book| author = Becker, P.C.| author2 = Olson, N.A.| author3 = Simpson, J.R. |title =Erbium-Doped Fiber Amplifiers: Fundamentals and Theory| publisher = Academic press| date = 1999}}</ref> although the application to the ytterbium-doped [[composite materials]] was under discussion.<ref name="McCumberA">{{cite journal |author=Kouznetsov, D. |title=Comment on Efficient diode-pumped Yb:Gd<sub>2</sub>SiO<sub>5</sub> laser|journal=Applied Physics Letters |volume=90|date=2007|doi=10.1063/1.2435309 |page=066101|bibcode = 2007ApPhL..90f6101K |issue=6 }}</ref><ref name="McCumberB">{{cite journal |author=Zhao, Guangjun |author2=Su, Liangbi |author3=Xu, Jun |author4=Zeng, Heping |title=Response to Comment on Efficient diode-pumped Yb:Gd<sub>2</sub>SiO<sub>5</sub> laser|journal=Applied Physics Letters |volume=90 |page=066103 |date=2007 |doi=10.1063/1.2435314|bibcode = 2007ApPhL..90f6103Z |issue=6 |doi-access=free }}</ref>

Usually, low concentrations of ytterbium are used. At high concentrations, the ytterbium-doped materials show [[photodarkening]]<ref name="photodarkening">{{cite journal |author=Koponen, Joona J. |author2=Söderlund, Mikko J. |author3=Hoffman, Hanna J. |author4=Tammela, Simo K. T. |name-list-style=amp |title= Measuring photodarkening from single-mode ytterbium doped silica fibers|journal=Optics Express|volume=14 |issue=24 |pages=11539–11544 |doi= 10.1364/OE.14.011539 |date= 2006 |pmid=19529573|bibcode = 2006OExpr..1411539K |s2cid=27830683 |doi-access=free }}</ref>
(glass fibers) or even a switch to broadband emission<ref name="avalanche">{{cite journal
|author=Bisson, J.-F.|author2=Kouznetsov, D.|author3=Ueda, K.|author4=Fredrich-Thornton, S. T.|author5=Petermann, K.|author6=Huber, G.|title=Switching of Emissivity and Photoconductivity in Highly Doped Yb<sup>3+</sup>:Y<sub>2</sub>O<sub>3</sub> and Lu<sub>2</sub>O<sub>3</sub> Ceramics |journal=Applied Physics Letters |volume=90 |page= 201901 |date=2007 |doi=10.1063/1.2739318|bibcode = 2007ApPhL..90t1901B|issue=20 }}</ref> (crystals and ceramics) instead of efficient laser action. This effect may be related with not only overheating, but also with conditions of [[charge compensation]] at high concentrations of ytterbium ions.<ref>{{cite journal|author=Sochinskii, N.V.|author2=Abellan, M.|author3=Rodriguez-Fernandez, J.|author4=Saucedo, E.|author5=Ruiz, C.M.|author6=Bermudez, V. |title=Effect of Yb concentration on the resistivity and lifetime of CdTe:Ge:Yb codoped crystals |date=2007 |journal=Applied Physics Letters |volume=91 |issue=20 |page=202112 |doi=10.1063/1.2815644|bibcode = 2007ApPhL..91t2112S |url=https://digital.csic.es/bitstream/10261/46803/1/ApplPhysLett_91_202112.pdf|hdl=10261/46803|hdl-access=free}}</ref>

Much progress has been made in the power scaling lasers and amplifiers produced with ytterbium (Yb) doped optical fibers. Power levels have increased from the 1&nbsp;kW regimes due to the advancements in components as well as the Yb-doped fibers. Fabrication of Low NA, Large Mode Area fibers enable achievement of near perfect beam qualities (M2<1.1) at power levels of 1.5&nbsp;kW to greater than 2&nbsp;kW at ~1064&nbsp;nm in a broadband configuration.<ref>{{cite journal|doi=10.1038/nphoton.2011.170|title=Doped fibres: Rare-earth fibres power up|journal=Nature Photonics|volume=5|issue=8|pages=466|year=2011|last1=Samson|first1=Bryce|last2=Carter|first2=Adrian|last3=Tankala|first3=Kanishka|bibcode=2011NaPho...5..466S}}</ref> Ytterbium-doped LMA fibers also have the advantages of a larger mode field diameter, which negates the impacts of nonlinear effects such as stimulated [[Brillouin scattering]] and stimulated [[Raman scattering]], which limit the achievement of higher power levels, and provide a distinct advantage over single mode ytterbium-doped fibers.

To achieve even higher power levels in ytterbium-based fiber systems, all factors of the fiber must be considered. These can be achieved only through optimization of all ytterbium fiber parameters, ranging from the core background losses to the geometrical properties, to reduce the splice losses within the cavity. Power scaling also requires optimization of matching passive fibers within the optical cavity.<ref>{{cite web |title=Fiber for Fiber Lasers: Matching Active and Passive Fibers Improves Fiber Laser Performance|url=http://www.laserfocusworld.com/articles/print/volume-48/issue-01/features/matching-active-and-passive-fibers-improves-fiber-laser-performance.html/|date=2012-01-01|publisher=[[Laser Focus World]]}}</ref> The optimization of the ytterbium-doped glass itself through host glass modification of various dopants also plays a large part in reducing the background loss of the glass, improvements in slope efficiency of the fiber, and improved photodarkening performance, all of which contribute to increased power levels in 1&nbsp;μm systems.