Sellmeier equation for doubly Er/Mg-doped congruent LiNbO_3 crystals

Refractive indices of congruent LiNbO3 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 mol%):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 Betzler. 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% Er2O3. 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.

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