Up-conversion of multi-site Er in LiNbO3 single-crystal fibers

In this work, the near infrared to visible up-conversion process was studied in Er3+ doped LiNbO3 single-crystal fibers. The absorption spectra of the I-4(9/2) multiplet and site-selective excitation of up-converted emission from the S-4(3/2) multiplet were measured at 10 K. Multiple sites for Er3+ were found to exist in the fiber samples. They fall into two groups which may be related to the substitution of Li and Nb by Er. Near-infrared excitation spectra of the up-conversion and transient dynamics have shown that either excited state absorption (ESA) or energy transfer up-conversion (ETU) processes play dominant roles at certain pump wavelengths. In addition, laser-heating-induced up-conversion or photon avalanche up-conversion may also take place at some pump wavelengths.