In a previous paper, it was reported that the tensile strength of optical glass fiber spliced by arc fusion method decreased due to the decrease of fracture toughness in a heat affected zone. In this study, tensile tests of heat treated optical glass fibers changed the heating temperature and heating time were carried out. Obtained results were as the follows: The tensile strength of optical glass fiber began to decrease from heating temperature of about 500K and reached the bottom at the heating temperature of about 600K. Moreover, it decreased with the increase in the heating time. In order to clarify the reason of tensile strength reduction, heat flows of optical glass fiber and optical glass fiber spliced by arc fusion method were compared by using the differential scanning calorimetry. It was found that the structure of heated optical glass fiber transformed from 500K to 600K. Therefore, the structure education of optical glass fiber spliced by arc fusion method was performed by using Raman spectroscopy. As a result, the tensile strength reduction of heat affected zone resulted from a thermal residual stress due to a transformation from the three-membered ring to the six-membered ring.
