Photoelastic measurements using a reflection type of infrared polariscope have been done for the first time to investigate birefringence or residual strain induced in as-grown and pulsed-laser-annealed silicon-on-sapphire (SOS) wafers. It was found that the residual strain, arising from mismatchings of the lattice constants and the thermal expansion coefficients between silicon and sapphire, was reduced effectively by pulsed-laser annealing with laser energy density beyond a threshold value. Also found was a mosaic pattern due to local melting at about the threshold energy density, indicating the coexistence of solid and liquid phases.