An optical surgical fiber assembly for delivering laser radiation from a laser radiation source to a treatment site has a sealed off capillary enclosing a delivery end of the fiber. The capillary is formed from an outermost layer of fused silica and an adjacent layer of boron-doped fused silica having a higher CTE than that of the fused silica. The capillary is shrink-fitted onto the delivery end of the fiber. A compressive stress is imparted to the outermost layer of the capillary as a result of the shrink-fitting process and the CTE difference between the layers. This provides mechanical hardening of the surface of the outermost layer.
