Sol-Gel Method for Making Radial Gradient-Index Glass in the SiO2-TiO2-R2O System

The use of sol-gel methods to make titanium silicate gradient-index (GRIN) glass is now well established [1-9]. The basic technique consists of the following steps: 1) make a titanium silicate gel from Si and Ti alkoxides, 2) soak the gel in an acid bath to partly leach and thus create a concentration gradient of Ti, 3) arrest the leaching process by soaking the gel in an appropriate solvent such as alcohol or water, 4) dry the gel, 5) sinter the gel by controlled heating to a high temperature. This technique can be used to produce transparent monolithic radial gradient-index glass rods up to 8mm or more in diameter with a nearly parabolic index profile. A drawback of this technique is that the concentration of titania in the final glass article is limited by devitrification to less than 10 mole percent. As a result, the largest index change so far achieved in the TiO2-SiO2 system is about 0.027 [8].