Radiation induced losses in Ytterbium doped laser materials

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.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