Stress-Induced Birefringence in Gradient-Index Structures Fabricated by Ion-Exchange

This paper examines an axial gradient-index (GRIN) glass sample produced by ion-exchange in the glass system: 0.704 SiO 2 + 0.044 Al 2 O 3 + ( 0.252 − χ ( z ) ) Na 2 O + χ ( z ) Li 2 O (where χ(z) represents the degree of exchange versus position, z), with the intention of determining χ(z) from a measurement of the index profile, n(z). Due to the ion-exchange conditions, the sample contains “frozen-in” stresses that strain the glass network, preventing it from obtaining its free expansion volume. The resulting volume change causes an additional birefringent index change. Therefore, when the index profile of the sample, n(z), is measured, it is necessary to take into account the sample’s birefringence to accurately determine the exchange profile, χ(z). The gradient’s index change, Δn(z), (where, Δn(z)=n(z)-nb, and nb is the base index), can be determined to accuracies reaching 0.03%. This paper shows that when frozen stresses are not taken into consideration, errors in determining χ(z) can reach 1.8%. However, it is shown that by also measuring the sample’s birefringence, determination of χ(z) may become more accurate.