Influence of diffusion nonlinearity on the characteristics of gradient-index diffractive structures formed in glasses by ion-exchange technique

The influence of the diffusion nonlinearity on the formation of gradient-index diffractive structures by the low-temperature silver ion-exchange method (including electroinduced ion exchange) has been studied for two glasses (light crown type) with the same glass-forming matrix and different alkali compositions. One glass containing only sodium shows the anomalous nonlinearity, and another glass with mixed lithium-sodium composition is characterized by the normal nonlinearity. The silver-sodium exchange in both glasses leads to the same refractive index increments. The simulation of gradient-index diffraction gratings in these glasses demonstrates that the diffusion nonlinearity strongly affects the maximum phase modulations and diffraction efficiency of gradient-index gratings. It is shown that the most efficient diffractive structures can be obtained for glasses with the anomalous diffusion nonlinearity, all other factors being the same.