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Plastic optical fibers that are a typical large-core multimode optical fiber support a great number of modes compared with conventional silica-glass multimode optical fibers. So far the WKB method has been used for most of the modal analyses of these fibers because of a great number of guided modes. We describe the accurate eigenmodal analysis of large-core multimode optical fibers with the finite-element method (FEM) and compute the propagation constants of all LP modes. In addition, the FEM has a strong advantage for arbitrary core profiles whereas the WKB method is not suitable for nonmonotonic profiles. To demonstrate the advantage, we apply the FEM to the fiber having sinusoidal fluctuations. OCIS codes: 060.2310, 060.2400.

The effects of impurities on the second-order nonlinearity of high purity synthetic silica glasses are studied. Trace amounts of sodium ions and water molecules were introduced into the silica glasses. It is shown that the sodium ions increased second harmonic generation (SHG) intensity with nonlinearity localized in the anode surface reason due to the frozen electric field formed by the drift of sodium ions. SHG intensity also rose with increasing water molecules.

In a previous paper [Appl. Opt. 43, 2163 (2004)], there was a mistake regarding Eq. (2). That mistake is corrected here. OCIS codes: 060.2310, 060.2400.