Abstract

Er³⁺ lasers emitting at 1.6 μm are useful for communication and eye-safe atmospheric measurement techniques like LIDAR. In contrast to glass, the garnet crystal YAG (Y₃A1₅O₁₂) as a host for rare earth ions provides good thermal conductivity. The crystals used in this paper were grown in our institute by the Czochralski method. Duczynski et al.,¹ reported Er: YAG laser emission at 1.6 μm pumping with a krypton-ion laser at 647 nm with a maximum slope efficiency of 12.7%. Spariosu and Birnbaum² achieved an Er: YAG laser by exciting the upper laser level at 1.535 μm with an Er:glass laser and obtained slope efficiencies as high as 50%. Due to the weak absorption of the ⁴I_11/2-level of Er around 965 nm (α = 0.34 cm^-1 in Er(0.5%):YAG), we investigated the co-doping with Yb to enhance the pump energy absorption (see Fig. 2). Such a pumping scheme has been used in Er glasses and is particularly interesting because high power InGaAs diode lasers emitting around 970 nm are available. Figure 1 shows the relevant energy levels of Er and Yb together with the pumping mechanism and the laser transition. We used the resonant energy transfer between the ²F_5/2-level of Yb and the ⁴In_/2-level of Er by pumping into the Yb-level. The ⁴I_11/2-level is depopulated by phonon relaxations and 2.7 μm fluorescence into the ⁴I_13/2-level, which is the upper laser level with a lifetime of 6.5 ms in Er(0.5%):YAG. The Fluorescence spectrum of Er,Yb:YAG shows a strong fluorescence of Yb around 1 μm indicating energy back transfer from Er to Yb, when the Er-ions are excited by the 488-nm line of an argon-ion laser (see (Fig. 2). The back transfer is pronouced due to the relatively long ⁴I_11/2 lifetime in Er:YAG.

© 1994 IEEE