Low-temperature optical dephasing of rare-earth ions in inorganic glasses

A detailed description is presented of photon echo experiments in ${\mathrm{Nd}}^{3+}$-doped silica optical fibers at temperatures below 1 K. Dephasing rates $T_2^{\mathrm{-}1}$ have been obtained from two-pulse echo sequences at 0.89 μm and are found to follow a $T^{1.3}$ temperature dependence. An interpretation of this result is presented which invokes the interaction of tunneling systems with the optical ions via a dipole-dipole elastic interaction. The dynamics of the interaction originates in a spectral diffusion process and three pulse stimulated echo experiments confirm this interpretation.