Stresses produced in the BK7 glass by the K⁺-Na⁺ ion exchange: Real-Time process control method

The paper presents the results of tests on stresses produced by the K⁺↔Na⁺ ion exchange method in BK7 glass. Diffusion ion exchange processes were carried out in glass plates with a surface area of a few cm². The duration of these processes ranged from several hours to several hundred hours; process temperatures from 370 to 402 degree Celsius were used. The area of the glass in which the ion exchange took place shows refractive changes which are also accompanied by stresses. The planar waveguides produced in this way were tested by optical methods (for wavelength λ = 677 nm) and the refractive index profiles for the Transverse Electric (TE) and (space necessary) Transverse Magnetic (TM) polarization states were determined. On the basis of elasto-optic constants, the resulting stresses were determined. The temperature characteristics of diffusion coefficients of exchanged ions were also determined. Based on them a numerical simulation of real-time diffusion processes was possible, which allowed to predict the stresses arising in the glass. A good agreement between these predictions and the results of measurements was obtained.