Modal cutoff properties in germanium-doped photonic crystal fiber

The germanium-doped photonic crystal fiber (PCF) has some characteristics that differentiate it from pure-silica PCF for a germanium element being doped in the core, such as the intensified nonlinearity, the enhanced photosensitivity, and so on. To pave the way for the application of the Ge-doped PCF successfully, it is necessary to study its properties. We investigated the modal cutoff properties of Ge-doped PCF quantitatively by using the beam propagation method. The numerical results show that the effective refractive indices and the normalized frequency V of Ge-doped PCF not only depend on the normalized pitch (lambda)/(lambda) but also depend on the normalized hole size d/(lambda), the modal cutoff boundary for the single mode-multimode of the Ge-doped PCF shift to the low d/(lambda) side in contrast to the pure-silica PCF.