Abstract

Upconversion lasers operate at wavelengths that are shorter than those used for pumping. Efficient upconversion lasing has been achieved in rare earth-doped materials using several different ex citation processes such as sequential two-photon absorption, energy transfer upconversion and a so-called absorption avalanche mechanism; in the latter case, the pump photons are not resonant with absorption from the ground state.¹ All these processes involve the use of metastable, intermediate levels which act as a storage reservoir for pump energy. Following the demonstration of the first cw-pumped upconversion laser in 1986, we have achieved upconversion laser operation at violet, blue, green and red wavelengths in several fluoride host crystals doped with Nd³⁺, Ho³⁺, Er³⁺, and Tm³⁺ ions by pumping with cw dye, Ti:sapphire or diode lasers.¹ These upconversion lasers operate at temperatures up to ~ 140 K using pump powers of a few hundred milliwatts. The use of longitudinal pumping of short, monolithic laser crystals resulted in fairly high pump intensities, which is important for these nonlinear excitation processes. Multistep upconversion excitation has been achieved in Er:YLiF₄ by producing laser oscillation at 470 nm from the ²P_3/2 level whose energy is more than three times higher than that of the 969-nm light used for pumping. At 77 K, green 551-nm laser output was obtained by pumping a 2-mm long Er:YLiF₄ crystal with a 809-nm GaAl As diode laser. Recently, green and blue cw upconversion lasing at room temperature has been reported using fluorozirconate fibers doped with Ho³⁺ and Pr³⁺, respectively.²

© 1991 Optical Society of America