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Using a near-field scanning optical microscope (NSOM), we found non-uniformly distributed microstructural defects near the fusion boundary of bicrystal substrates. These defects depict themselves as circular dark spots in the optical transmission images. We attribute these optical features to near-surface voids at the boundary previously found in transmission electron microscopy studies. Our results show a direct link between the presence of these defects and the superconducting properties of the YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) grain boundary Josephson junctions (GBJJs). We find that junctions that are fabricated over highly defected regions of the bicrystal substrates have lower critical temperatures and currents. The positions of defects across a junction also play a role in determine the device characteristics. Strain fields associated with these defects are most likely responsible for affecting YBCO film growth locally and junction performance. We employ an NSOM to survey these microstructural defects in bicrystals of various tilt angles and of different materials that are commonly used for fabrication of YBCO GBJJs. We find that these defects and their strain fields are the result of the bicrystal fusion process and are not intrinsic to the materials.