PRODUCTION OF OPTICAL ELEMENT

PURPOSE:To obtain a refractive index-distribution type optical element in good yield, by holding the axis direction of a glass rod member in the direction perpendicular to the gravitational direction and carrying out ion exchange while rotating the member on the axis as a center. CONSTITUTION:When the specific gravity of a glass rod (glass rod member) 1 is higher than that of molten salt 2 of NaNO3, the glass rod 1 is placed on rollers (4a) and (4b). When a salt having a higher specific gravity than that of glass, e.g. Tl2SO4, is used, the glass rod 1 is placed under the rollers (4a) and (4b) since upward force is applied to the glass. Ion exchange is carried out at 485 deg.C for about 300hr by using the glass rod 1 and molten salt 2 of NaNO3. In the process, the rollers (4a) and (4b) are rotated in the same direction at about 10r.p,m. No deformation which becomes an optical problem is found in the external shape of the glass rod 1 after the ion exchange and the following refractive index distribution in the direction perpendicular to the optical axis is obtained. For example, the refractive index reduces from the center to the outer periphery in proportion to the square of distance from the optical axis with the maximum refractive index difference of about 0.17.