CAN THE MINIMUM ATTENUATION OF FUSED-SILICA BE SIGNIFICANTLY REDUCED BY SMALL COMPOSITIONAL VARIATIONS .1. ALKALI-METAL DOPANTS

The glass transition temperature, T(g), of fused silica is known to be dramatically reduced by small dopant concentrations of alkali metals. Since the dominant loss contribution in pure silica at the minimum attenuation wavelength of 1.55 mum is theoretically proportional to T(g), the question arises as to whether a comparable lowering of total attenuation at 1.55 mum can be achieved in this manner. The primary term opposing loss reduction arises from scattering from compositional concentration fluctuations. Since the binary alkali metal silicates xM2O.(1 - x)SiO2 are known to be prone to actual or incipient immiscibility at low modifier concentrations, losses from concentration scattering in these glasses may also increase dramatically with dopant concentration and possibly annul or even reverse any potential loss reduction accruing from a decrease of T(g). This paper discusses and quantitatively evaluates total 1.55 mum attenuation at small modifier concentration levels, x, for the binary alkali metal silicate glasses. A reduction of total loss below that of pure silica is predicted only for the extremely low concentration range 0 < x less-than-or-equal-to 0.02 and only when M = Na, K or Rb.