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Radiation induced losses in Ytterbium doped laser materials
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this work, two types of ytterbium (Yb) doped laser materials are studied, crys-talline Yb:YAG and amorphous Yb/Al silica in both preform and fiber. Yb is suitableas a laser ion because of its simple energy level structure and low quantum defect.The ground and excited energy levels are separated by 10000 cm−1, but the exis-tence of another transition in the ultra violet (UV) region causes problems in a Ybdoped laser. The UV absorption band represents a charge transfer (CT) transitionwhich involves the transition of an electron from a nearby oxygen ion to the Yb ionand changes the Yb ion from trivalent to divalent with the corresponding forma-tion of a hole center. The color center formation causes a permanent optical loss inthe material in the visible to near infrared (NIR) spectral region, which absorbs thepump and laser wavelengths. The output power of the laser is reduced and this isknown as the photodarkening (PD) phenomenon.It is suggested that the excited state absorption of the Yb3+ ion is involved in thetransfer route of NIR photons to the UV range.The increase of Yb2+ upon UV irradiation is shown in both Yb:YAG and theYb/Al silca preform. The existence of Yb2+ luminescence from a photodarkenedfiber is also shown, which proves that PD occurs through a CT process.

Abstract [sv]

I det h¨ar arbetet studeras de tv°a ytterbium-dopade lasermaterialen Yb:YAG ochYb/Al-dopat glas i form av b°ade preform och fiber. Ytterbium ¨ar l¨amplig som laser-jon p°a grund av sin enkla engerginiv°astruktur och l°aga kvantdefekt. Grundtillst°andetoch det exciterade tillst°andet ¨ar separerade med 10000 cm−1, men det finns ytterli-gare en ¨overg°ang, i UV-omr°adet, som kan skapa problem i en ytterbium-dopad laser.Denna UV-absorption inneb¨ar en laddnings¨overf¨oring (CT) fr°an en omgivande syre-jon till ytterbiumjonen och leder till en valens¨andring i ytterbiumjonen fr°an trev¨ardtill tv°av¨ard och bildandet av ett f¨argcenter. Bildandet av f¨argcenter skapar en per-manent optisk f ¨orlust i materialet, i det synliga till n¨ara infrar ¨oda omr°adet, vilket ab-sorberar pump- och laserstr°alningen. Laserns uteffekt reduceras och detta fenomen¨ar k¨ant som "photodarkening" (PD).Det f ¨oresl°as att absorption fr°an det exciterade tillst°andet i Yb3+-jonen ¨ar en bidra-gande orsak till den ¨overf¨oring av n¨ara infrar ¨oda fotoner till UV-omr°adet som sker.En ¨okning av Yb2+ vid UV-str°alning visas b°ade f ¨or Yb:YAG och f ¨or den Yb/Al-dopade glaspreformen. F¨orekomsten av Yb2+-luminescens fr°an en Yb/Al-dopadfiber som har genomg°att PD visas ocks°a, vilket bevisar att PD sker genom en CT-process.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2013. , 48 p.
Series
Mid Sweden University licentiate thesis, ISSN 1652-8948 ; 96
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:miun:diva-18687Local ID: STCISBN: 978-91-87103-61-2 (print)OAI: oai:DiVA.org:miun-18687DiVA: diva2:614293
Supervisors
Available from: 2013-04-04 Created: 2013-04-04 Last updated: 2016-10-20Bibliographically approved
List of papers
1. Charge transfer processes and UV induced absorption in Yb:YAG single crystal laser materials
Open this publication in new window or tab >>Charge transfer processes and UV induced absorption in Yb:YAG single crystal laser materials
2013 (English)In: Optical Materials Express, ISSN 2159-3930, E-ISSN 2159-3930, Vol. 113, no 22, Art. no. 223510- p.Article in journal (Refereed) Published
Abstract [en]

Charge transfer (CT) transitions and UV induced color centers in Yb:YAG single crystals have been investigated. A simultaneous pair formation of a stable Yb2+ ion and a hole related (O -) color center (hole polaron) are observed through a CT-process. Slightly different types of hole related color centers are formed in Yb:YAG crystals containing small levels of iron impurities. Furthermore, excitation spectroscopy on the UV irradiated Yb:YAG samples could confirm an energy transfer process between Yb3+ and Yb2+ ions. The findings are important for an increased knowledge of the physical loss mechanisms observed in Yb-doped laser materials, such as the nonlinear decay process in Yb:YAG crystals as well as the photodarkening phenomenon in Yb-doped fiber lasers.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-18685 (URN)10.1063/1.4810858 (DOI)000320675300012 ()2-s2.0-84879212765 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Available from: 2013-04-04 Created: 2013-04-04 Last updated: 2017-12-06Bibliographically approved
2. Experimental evidence for the formation of divalent Ytterbium in the photodarkening process of Yb-doped fiber lasers
Open this publication in new window or tab >>Experimental evidence for the formation of divalent Ytterbium in the photodarkening process of Yb-doped fiber lasers
2013 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, no 6, 6681-6688 p.Article in journal (Refereed) Published
Abstract [en]

In this work we present experimental evidence that the valence instability of the ytterbium ion play a key role for the observed photodarkening mechanism in Yb-doped fiber lasers. Luminescence and excitation spectroscopy performed on UV irradiated Yb/Al doped silica glass preforms and near-infrared diode pumped photodarkened fibers show a concentration increase of Yb2+ ions. A concentration decrease in Yb3+ could also be observed for the UV irradiated preform. The findings contribute to an increased understanding of the kinetic processes related to photodarkening in Yb-doped high power fiber lasers.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-18686 (URN)10.1364/OE.21.006681 (DOI)000316796000004 ()2-s2.0-84875757012 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Available from: 2013-04-04 Created: 2013-04-04 Last updated: 2017-12-06Bibliographically approved

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