Persistent Spectral Hole-Buring and Absorption Fluctuations in Nd-Doped Silica Optical Fibers

We have observed both persistent spectral hole burning and statistical fluctuations in absorption as a function of energy in the sup 4 F sub 3 over 2 - sup 4 I sub 9 over 2 transition of Nd sup 3+ in silica glass. Shallow (1%) holes in the absorption spectrum were burnt using a narrow band width (500kHz) laser, and both holes and absorption fluctuations were detected with the same laser using an FM modulation technique.