Sellmeier equation for doubly Er/Mg-doped congruent LiNbO₃ crystals

Refractive indices of congruent LiNb03 crystals codoped with 0.5 mol% Er₂O₃ and 0-5.0 mol% MgO were measured at room temperature in a wavelength range from 473 to 1536 nm using critical angle measurement and minimum deviation methods. For reference, measurements by the critical angle method were also performed on a congruent LiNbO₃ and a congruent MgO (5 mo1%):LiNbO₃. The measured data are compared with previously reported theoretical and/or experimental results of pure and MgO-doped LiNbO₃, especially with the results predicted by the MgO content-related Sellmeier equation (SE) reported by Schlarb and Bet-zler. It is shown that the Schlarb and Betzler SE consistently predicts a lower index, by as much as 4 × 10^-3, than what is actually measured by the authors for both congruent LiNbO₃ and for MgO :LiNbO₃, whether it is codoped with Er or not. To check the Er doping effect, measurements by the critical angle method were also performed on some congruent LiNbO₃ crystals singly doped with 0-1.0 mol% Er₂O ₃. The results show that Er doping has no measurable effect. It is concluded that there may be a systematic error as one uses the Schlarb and Betzler SE to predict the index of the undoped, only MgO-doped, and Er/Mg-codoped crystals. The Schlarb and Betzler SE parameters are refined to fit our experimental data. The corrected version of the SE suitable for MgO:LiNbO₃ with or without Er doping is given.