Spatial-spectral holography using spectral hole burning materials is a powerful technique for performing real-time, wide-bandwidth information storage and signal processing. For operation in the important 1.5 μm communication band, the material Er3+:Y2SiO5 enables applications such as laser frequency stabilization, all-optical correlators, analog signal processing, and data storage. Site-selective absorption and emission spectroscopy identified spectral hole burning transitions and excited state T1 lifetimes in the 1.5 μm spectral region. The effects of crystal temperature, Er3+-dopant concentration, magnetic field strength, and crystal orientation on spectral diffusion were explored using stimulated photon echo spectroscopy, which is the “prototype” interaction mechanism for device applications. The performance of Er3+:Y2SiO5 and related Er3+ materials has been dramatically enhanced by reducing the effect of spectral diffusion on the coherence lifetime T2 through fundamental material design coupled with the application of an external magnetic field oriented along specific directions. A preferred magnetic field orientation that maximized T2 by minimizing the effects of spectral diffusion was determined using the results of angle-dependent Zeeman spectroscopy. The observed linewidth broadening due to spectral diffusion was successfully modeled by considering the effect of one-phonon (direct) processes on Er3+ - Er3+ interactions. The reported studies improved our understanding of Er3+ materials, explored the range of conditions and material parameters required to optimize performance for specific applications, and enabled measurement of the narrowest optical resonance ever observed in a solid—with a homogeneous linewidth of 73 Hz. With the optimized materials and operating conditions, photon echoes were observed up to temperatures of 5 K, enabling 0.5 GHz bandwidth optical signal processing at 4.2 K and providing the possibility for operation with a closed-cycle cryocooler.
Thomas Böttger ; Yongchen Sun ; Charles W. Thiel and Rufus L. Cone. "Material optimization of Er3+:Y2SiO5 at 1.5 μm for optical processing, memory, and laser frequency stabilization applications", Proc. SPIE 4988, Advanced Optical Data Storage, 51 (June 30, 2003); doi:10.1117/12.474784