Faraday rotation spectra analysis of Bi-substituted mixed rare-earth iron garnet crystals in optical communication band

Through combining two types of Bi-substituted rare-earth iron garnet crystals with opposite Faraday rotation wavelength coefficient (FWC) signs, the Bi-substituted mixed rare-earth iron garnet (Tb3-x-yYbyBixFe5O12) crystals with low FWC are used in optical isolators for broadband optical communication. However, the Faraday rotation (FR) spectra of bismuth-substituted mixed rare-earth iron garnet crystals have not yet been theoretically studied in the optical communication band [wavelength (lambda)=1.5-1.62 mu m]. In order to interpret their FR spectra, a theory based on molecular-orbital levels of Fe3+ in tetrahedral (d) and octahedral (a) sites is used. It is shown that in the case of higher Bi3+ substitution, FR depends mainly on the contributions of Fe3+ in the octahedral sites. Furthermore, two types of rare-earth ions (Tb3+ and Yb3+) moderate the contributions to FR from Fe3+ in octahedral and decahedral sites and eventually have caused that Tb3-x-yYbyBixFe5O12 crystals has a near-zero FWC. (c) 2007 American Institute of Physics.