1. Field of the Invention
The present invention relates to a glass composition suitable for poling and a glass functional product containing the glass composition. Poling is used for forming nonlinear optical elements that are used, for example, in the field of optical communications.
2. Related Background Art
The generation of the second harmonic induced by a second-order nonlinear optical effect can be achieved by introducing a periodic polarization structure into a dielectric material. This technique has advantages that, for example, when compared to the case of using a dielectric crystal, no operation for precise phase matching is necessary and the material to be used is not required to have a specific crystal structure. The polarization structure can be introduced through a polarization process (a poling process) carried out by, for example, applying high voltage to the dielectric material. The poling process causes semipermanent polarization (a poling effect). Particularly, a thermal poling process in which high voltage is applied while heating glass is easy to operate and also is excellent in maintaining the poling effect.
Various studies have been made about poling of silica glass used for optical fibers for optical communications. The poling effect in silica glass is brought through the migration of sodium ions contained as impurities in an electric field (see, R. A. Myers, Optical Letters, vol. 16, 1991, p1732). It has been understood that a depletion layer in which the concentration of sodium ions has decreased due to the migration of sodium ions is formed in glass and an internal electric field is generated therein to produce the nonlinear optical effect.
U.S. Pat. No. 5,239,407 discloses that a greater second-order nonlinear optical effect is obtained by thermal poling of fused silica glass. Furthermore, WO97/46906 describes that in order to produce a great electro-optic effect in optical fibers made of silica glass, it is effective to carry out thermal poling at a high temperature that is at least 450° C. and to apply a strong electric field that is at least 800 V/μm.
In the silica glass that has been used conventionally, however, sodium ions contained as impurities simply are used for poling.