Comment on "Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation".

We present some strong evidence against the assignment of an ytterbium-related oxygen deficiency center (ODC) near 230 nm in Yb-doped aluminosilicate glass in the Letter by Yoo [Opt. Lett.32, 1626 (2007)]. The strong 230 nm absorption band is actually a charge-transfer band corresponding to the transfer of an electron from nearby oxygen ligands to the ytterbium ion and not related to ODCs.