Fabrication of long period gratings in optical fibers through periodic ablation using a focused CO2 laser beam has been studied. During the thermal ablation process most of the energy is absorbed at the glass surface, due to the high extinction coefficient of silica at the laser wavelength, resulting in rapid increase in temperature. Subsequent heat dissipation occurs through vaporization and ejection of molten material, heat conduction axially along the fiber, radiation and through convection. The high surface temperatures involved during ablation can result in a significant increase in temperature of the fiber itself, causing unwanted off-resonance background losses during grating fabrication. In order to minimize losses the temperature needs to be sufficiently low to avoid micro-bending and core deformation, triggered by the decrease in viscosity, while at the same time enabling strong grating formation through laser induced modification of the glass. In this work we have used short-period fiber Bragg gratings in order to assess the temperature dynamics within the fiber during ablation. Using a single grating written into the core, positioned below the point of ablation, we measure the peak temperature within the core of the fiber. When ablation was performed between two gratings, forming a short Fabry-Perot cavity, a different and faster response was recorded, which we ascribe to thermally induced stress and strain caused by the thermal expansion of the surface during ablation. By identifying suitable processing parameters we successfully fabricate strong long-period gratings with background losses of 0.5 dB to 1 dB when periodically ablating the fiber with up to 50 pulses. Experimental results indicate that the maximum core temperatures during ablation under these conditions are limited to within 600 degrees C to 1000 degrees C.

A detailed study of the dynamics during thermal regeneration of fiber Bragg gratings, written in hydrogen-loaded standard single-mode fibers using a ns pulsed 213 nm UV laser, is reported. Isothermal pre-annealing performed in the range 85 degrees C to 1100 degrees C, with subsequent grating regeneration at 1100 degrees C, resulted in a maximum refractive index modulation, Delta n(m) similar to 1.4.10(-4), for gratings pre-annealed near 900 degrees C while a minimum value of Delta n(m) similar to 2.10(-5) was achieved irrespective of pre-annealing temperature. This optimum denote an inflection point between opposing thermally triggered processes, which we ascribe to the reaction-diffusion mechanism of molecular water and hydroxyl species in silica. The results shed new light on the mechanisms underlying thermal grating regeneration in optical fibers.

High-temperature stable fiber Bragg gratings will be reviewed with an emphasis on recent material studies using Chemical Composition gratings. 

Temperature dynamics in optical fibers during CO2-laser irradiation, at temperatures above the softening point of fused silica, were characterized using chemical composition gratings. The technique offers possibilities for enhanced temperature control during optical fiber processing.

The thermal response of optical fibers during CO2 laser irradiation has been characterized by using thermally stable short-period fiber Bragg gratings, referred to as chemical composition gratings. CO2 laser beam profiling was performed by scanning the beam across a 1 mm long grating, providing a spatial resolution given by the fiber diameter. The thermal dynamics during square pulse irradiation has been recorded for temperatures in excess of 1700°C, with heating and cooling rates as high as 10,500°C  s−1 and 6500°C  s−1, respectively.

The thermal response of optical fibers during CO₂ laser irradiation with a wavelength of 10.6 µm has been characterized by use of thermally stable short period fiber Bragg gratings, referred to as chemical composition gratings.Beam profiling of the collimated beam from a CO₂laser having a 1/e² diameter of ~6.5 mm, was performed with a spatial resolution given by the fiber diameter. Heating- and cooling-dynamics of the fiber irradiated with a square pulse with a 1/e rise time of < 1 ms were performed with a temporal resolution down to~250 µs, for temperatures in excess of 1700 ºC.

A comparison of laser ablation on the front-surface (surface facing the laser) and back-surface of 1.1 mm thick boro-aluminosilicate glass plates has been performed using 1 ps pulses at a wavelength of 800 nm. The resulting structures of front- and back-surface ablation are compared relative to an input design geometry of 40 µm deep and 40 µm wide surface channels. The influence of incubation effects on the final structure is discussed.