560 Poling of Glasses and Optical Fibers Another highly relevant study was carried out with a comparison of the ther- mal stability of different types of glasses under isothermal annealing [167]. The decay of the second-order nonlinearity was fitted with two time constants. Alu- minosilicate and aluminoborosilicate glasses exhibited characteristic decay time constants at least five orders of magnitude longer than a silica reference sample. The enhanced stability was attributed to a strong decrease in the mobility of the alkali ions because of the glass composition. Here, poling of the silica sample was carried out at 280 C, whereas all other samples were poled at higher tem- peratures. The most stable sample was poled at the highest temperature (420 C). The stability of poled fibers and devices is a topic that deserves more atten- tion in the literature. Short w (2) decay times (some days) have been observed in normal [168] and special fiber [169]. This led to the development of a fiber with a borosilicate glass ring around the anode to trap negative charges and improve stability [168]. However, long-term stability was reported in reference [170], in which V p was found to be unaffected at room temperature over a period exceeding 2 years. The discrepancy may arise from the difference in materials, poling conditions, or the presence of air in the internal electrodes of references [169] and [168] compared to a solid metal electrode filling the entire cross-section of the holes in reference [170].