Experimental observation of spectral diffusion in an optically pumped image crystal

We report on experimental observations of spectral diffusion mechanisms occurring in an optically pumped erbium-doped image crystal. The diffusion is characterized using photon echo spectroscopy on the image transition. Optical pumping creates population inversion on the transition before the echo sequence. Spectral diffusion is then studied under different population inversion conditions. These observations raise new questions on the spectral diffusion mechanisms.