Concentration Determination of Er ³⁺ in LiNbO ₃ Crystal by Absorption Spectroscopy

A number of X ‐, Y ‐, and Z ‐cut congruent or near‐stoichiometric only Er ³⁺ ‐doped or Er ³⁺ /D ^{n +} ‐codoped (D ^{n +} =Mg ²⁺ , Zn ²⁺ , In ³⁺ , Yb ³⁺ ) LiNbO ₃ crystals were grown by the Czochralski method. The Er ³⁺ concentrations in these crystals were determined at first from neutron activation analysis. Then, the integrated unpolarized Er ³⁺ absorption coefficient at the 1.5 μm wavelength region was measured as a function of Er ³⁺ concentration in crystal. The results show that the integrated absorption coefficient (IAC) is linearly proportional to the Er ³⁺ concentration in the crystal ([Er ³⁺ ] _crystal ) whether the crystal is only Er ³⁺ ‐doped or Er ³⁺ /D ^{n +} ‐codoped, and the fitted expression is IAC=(95.0±4.6)·[Er ³⁺ ] _crystal for the X ‐ or Y ‐cut crystals and IAC=(102.0±5.1)·[Er ³⁺ ] _crystal for the Z ‐cut ones. These linear expressions enable to establish an optical method used to accurately determine Er ³⁺ concentration in any only Er ³⁺ ‐doped or Er ³⁺ /D ^{n +} ‐codoped LiNbO ₃ crystal from the simple and rapid unpolarized absorption measurement, and the error is within 10%.