When an erbium-doped fluoride fiber was pumped with a red dye laser be
am that resonantly excites the F-4(9/2) state, we detected efficient v
iolet upconversion emission from the P-2(3/2) State that is much stron
ger than the green upconversion emission from the S-4(3/2) state. Such
unusual violet emission was found to be due to a three-photon excitat
ion process. (C) 1997 Optical Society of America.