The linear and nonlinear refractive index of amorphous Al₂O₃ deduced from aluminosilicate optical fibers

The Sellmeier coefficients for the linear refractive index of amorphous alumina (a-Al₂O₃) are deduced from measurements on binary aluminosilicate glass optical fibers. This data is then used to approximate the nonlinear refractive index, n₂, for a-Al₂O_3, which was found to be 1.89 × 10⁻¹³ esu (4.77 × 10⁻²⁰ m²/W at a wavelength of 958 nm) and found to increase slightly sublinearly with increasing alumina concentration in silica glass. Several commonalities with some amorphous alumina thin films are suggested. In addition, a proposal for the extension of a simple additive materials model for the calculation of n₂ as a function of composition is presented. Alumina is an important dopant into silica for high energy optical fiber-based lasers and so the determination and modeling of its optical properties is of significant technological value.