The observation of statistical absorption fluctuations of an inhomogeneously broadened optical transition in a glass is reported. Frequency modulation spectroscopy in a 10-ppm ${\mathrm{Nd}}^{3+}$-doped-silica optical fiber has also revealed spectral holes that persist for days at 4.2 K. Homogeneous linewidths ${\Gamma }_h$ measured using hole burning and fluctuations are in good mutual agreement but are significantly larger than those inferred from photon echo decays in the same system. This difference confirms recent predictions of a time-dependent ${\Gamma }_h$(t) arising from the tunneling dynamics of the glass.

• Received 24 April 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.1833