New glasses for graded-index optics:: influence of non-linear diffusion in the formation of optical microstructures

We have designed novel glasses for the formation of graded-index diffractive optical structures. In these glasses silver ion exchange and rapid diffusion produce a large refractive-index variation (0.085), We model the non-linearity of the diffusion process by applying the Boltzmann-Matano technique to the optical profiles and use index variation rather than concentration in the second Fick equation. The simulation of graded-index diffraction gratings formed under thermal and electrically assisted diffusion shows that, other conditions being equal, more efficient gratings can be formed when the diffusion coefficient increases with the dopant concentration. According to the simulation, the application of an electric field of about 0.5 V/mu m enables one to obtain a phase shift equal to 1.5 pi rad if a grating with a period of 4 mu m is formed in the novel glass material. For comparison, a phase shift of only 0.9 pi is predicted when a similar grating is formed in the commercial BK-7 glass, (C) 2000 Elsevier Science B.V. All rights reserved.