Optical dephasing rates ${T_2}^{-1\mathrm{}}$ of a dilute concentration of ${\mathrm{Nd}}^{3+}$ ions in a pure silica fiber have been studied from 0.05 to 1.0 K with use of two-pulse photon echoes. Echoes obtained by resonant excitation of ${\mathrm{Nd}}^{3+}$ into the metastable ${}_{}{}^4{}_{}{}^{}F_{\frac{3}{2}}^{}{}_{}{}^{}$ state from the ${}_{}{}^4{}_{}{}^{}I_{\frac{9}{2}}^{}{}_{}{}^{}$ ground state have yielded ${T_2}^{-1\mathrm{}}=1.3\mathrm{}\times {10}^{-6\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ at 0.1 K which follows a $T^{\frac{4}{3}}$ temperature dependence to 1.0 K. A crossover of ${T_2}^{-1\mathrm{}}$ to a $T^2$ temperature dependence above 1 K is inferred by comparison with earlier high-temperature linewidth measurements on this transition.

• Received 23 August 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.2033