Several areas of current interest in fiber optics require knowledge of residual stress in glass optical fiber preforms and fibers [72,85,88,244,249]. First, the residual stress distribution determines the intrinsic strength of the fiber. If the stress at the surface is compressive, any resident microcrack would not tend to grow. Consequently, fibers with compressive stresses in the cladding are more resistant to attack by water. Second, stresses introduce anisotropy into single-mode fiber. Single-mode fiber usually has two propagation modes each with different polarization. For applications in optical sensors, it is desirable to separate the two polarizations. This can be achieved by introduction of unsymmetrical stresses in the fiber. It has also been found that, for multimode fibers, the residual stresses can cause substantial refractive index profile distortions and anisotropy, and influence transmission parameters of the fiber.