Yb3+-doped single crystals for laser host
In the latest years, with advances of high performance in GaAs laser diode with wavelength between 0.9 and 1.1 ･・, interest in Yb3+-doped materials has been increasing for application in high efficiency and high power diode pumped laser systems. The ytterbium ion is ideally suited for diode pumping since it has the simplest energy level scheme, among the rare earth active ions, consisting in only two levels: the 2F7/2 ground state and the 2F5/2 excited state. This simple electronic structure results in several advantages, comparing with Nd3+ ion for instance, such us no excited state absorption, no absorption in the visible range, no cross-relaxation process and no more up-conversion or any internal mechanism able to reduce the effective laser cross-section. In addition, the intense and broad Yb3+ absorption lines are well suited for IR InGaAs diode laser pumping, the broad emission band allows the generation of ultra-short pulses and the small quantum defect between absorption and emission wavelengths, leads to a low thermal load (11% relative to 30?40% in Nd3+-doped laser hosts). Our group has recently investigated spectroscopic properties of several Yb3+-doped oxide and fluoride single crystals.
Rare-earth co-doped fluoride single crystals for up-conversion laser application
Among the different hosts possible for laser application, fluorides single crystals are interesting because of their high transparency in a wide wavelength region from the VUV to the IR, lower refractive index limiting non-linear effects under intense laser sources pumping and low phonon energy, which increase the radiative transfer probability of the active ions.
The sensitization of rare-earth (RE) doped solid-state laser materials with trivalent ytterbium ions (Yb3+) is a well-known method for increasing the optical pump efficiency by Yb3+-RE energy transfer. The Yb3+ ions exhibit a strong and broad absorption band centered at about 980 nm and can be easily pumped by commercially available InGaAs infrared (IR) laser diodes. Moreover, the development of up-conversion lasers as an alternative solution for visible light generation has gained interest in searching for an efficient monochrome IR diode-pumping scheme. One promising technique, which has been demonstrated, is co-doping with Yb3+ ions to sensitize up-conversion laser emission at blue, green and various orange-red wavelengths from Pr3+ and Tm3+ in the host materials. Up-conversion lasing in Yb3+ codoped crystals has been reported for a few fluoride systems only, such as Yb:BaY2F8 codoped with Ho3+, Tm3+, Er3+ and Pr3+. Our interest is focused on the trivalent praseodymium ion, which exhibits a variety of laser transitions in the visible range based on 4f–4f transitions from the 3PJ=0,1,2 level. Very recently, we have investigated the (Pr, Yb) system in mixed fluoride crystals with respect to realization and better understanding of Pr and Yb energy transfer and promising results were already reported.