Evolution of Optical Fiber Temperature during Fiber Bragg Grating Fabrication Using KrF Excimer Laser

Abstract

The temperature distribution in an optical fiber during the fabrication of fiber Bragg gratings (FBGs) using a KrF excimer laser with a phase mask has been analyzed experimentally for typical fabrication conditions. The fluence of UV laser light at the fiber surface has been varied up to 420 mJ/cm2. These experiments show that (1) scanning electron microscope (SEM) images of the fiber surface facing the excimer laser beam reveal partial physical damage apparently owing to partial melting, so that the surface temperature has increased beyond the softening point of silica glass, which is approximately 1200°C, (2) the optical spectrum transmitted through the FBG during laser irradiation contains spikes coincident in time with the laser pulses that correspond to a near instantaneous shift of the FBG spectrum to a higher temperature spectrum representing fiber core heating of approximately 8°C, and (3) analysis of the energy absorbed by the fiber indicates a bulk temperature rise of approximately 3°C in the fiber. The resulting large variation in the temperature rise over the fiber cross section from a few °C to 1200°C, along with partial physical damage on the surface, will certainly induce large internal stresses in the fiber material and reduce the mechanical strength of the FBG.