“…Thermal annealing at higher temperatures densifies the sol-gel material, increases the refractive index, and activates optical emission from Er 3+ [13]. Since N and Cl are among the dopant species known to increase the refractive index of silica [27], the sol-gel formulation based on erbium oxide and acetic acid, introduced in [13], is perhaps desirable for analysis compared to alternatives containing nitrate and chloride ions, although the colloids formed by hydrolysis of SiC 8 H 20 O 4 (TEOS) could contain residual C. A 700 • C anneal optimally activates Er 3+ in Er-Ge codoped silica [21] and is used for optically active Er +3 in solgel germania-silica films [42], while 670 • C activates solgel silica-titania films [15]. Annealing at temperatures much above 700 • C, however, may cause short-range redistribution and clustering of the Er impurities, although higher temperatures are used for depleting the trace OH impurities capable of degrading performance at 1.54 µm [12,13] and for bringing residual porosity to very low levels [15].…”