On The Anomalously Strong Dependence Of The Acoustic Velocity Of Alumina On Temperature In Aluminosilicate Glass Optical Fibres - Part 1: Material Modeling & Experimenatal Validation

The thermo-acoustic coefficient (TAC, or the change of acoustic velocity with a change in temperature) of the alumina (Al2O3) component in aluminosilicate core silica clad glass optical fibres is anomalously large. It has been speculated previously that this anomaly is not due to a large alumina TAC, but instead due to the thermal expansion mismatch between the aluminosilicate core glass and the pure silica cladding glass> In this work, it is shown that the acoustic velocity of the aluminosilicate core glass changes much differently than that for a bulk aluminosilicate glass. In the high alumina content compositional range, it is found that the larger core-reaction thermal expansion results in a net positive pressure imparted on the silica component in the core. As the acoustic velocity of silica has a strong negative dependence on any applied pressure, this offsets the bulk positive-valued thermal response of silica. Utilizing a modified form of the Winkelmann-Schott additivity model, the effect of the thermal expansion mismatch between the core and clad glasses is incorporated, and the resultant TAC determined for alumina is fully consistent with data found in the literature for bulk alumina.