A high-power semiconductor diode-laser array operating at 797 nm has been used to pump an Er:YLiF4 upconversion laser with output at 551 nm operating at 48 K. By carefully controlling the beam geometry and oscillating linewidth of the diode-pumped laser, it has been possible to obtain 100 mW of visible output power with a conversion efficiency from the IR to the visible of greater than 5%.