Abstract
Blue upconversion lasers based on thulium- doped fluorozirconate glasses are usually pumped in the 1120-1130 nm spectral range.1 A simplified energy level diagram of Tm3+ and the upconversion pumping scheme is shown in figure 1. However, in this spectral range only few powerful pump sources exist. Diode lasers are hardly available and offer output powers of only 0.5 W. The 1122 nm laser transition in Nd:YAG offers output powers of up to several watts but suffers from strong thermal lensing effects in the host material at this wavelength. Nevertheless hundreds of milliwatts blue output power have been demonstrated with typical efficiencies between 10% and 20%.2 One approach to overcome this problem is codoping the fiber with ytterbium allowing pumping in the wavelength range between 1 and 1.1 μm. This wavelength range is covered by several powerful pump lasers with a suitable beam quality, but the efficiency of such a Tm/Yb-codoped upconversion fiber laser is decreased by about a factor of 3 compared with a Tm-doped fiber pumped by 1120 nm.3
© 2001 Optical Society of America
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