High mechanical resistant fiber Bragg gratings fabricated by CW UV laser irradiation

Abstract

The issue of mechanical resistance of optical fibers ii-radiated by UV lascr sources has been recently reported 11 -41. An important mechanical degradation as expressed by the decrease of median breaking strength and the widening of the Weibull distribution has been observed after homogenous irradiation by pulsed laser sources [ 1-31. In contrary, CW UV lase!-irradiation showed considerably better results exhibiting median breaking strengths and m-values very close to that of pristine fiber 141. The effect has an important impact on the fabrication of all-fiber filters such as Bragg and long period gratings. In this paper we report the fabrication of high mechanical resistant Bragg gratings using a CW frequency doubled Ar+ laser.Bragg gratings were fabricated in a 22% mol GeO, fiber using a phase mask with a period of 1.045 pm and zero order of 2%. The dimension of the beam hitting the fiber perpendicularly to the fiber axis was 8 mm x 140 mm covering its whole diameter and the beam intensity was 1.25 W/cm'. Transmission spectra of the gratings were acquired using a tunable laser and an InGaAs PIN photodiode providing a resolution of 10 pin. Three different groups of I5 identical gratings underwent tension tests: The first group was highly reflective (-99%) fabricated in a hydrogenated fiber with a total irradiation dose of 75 J/cm2. The second one was fabricated using the same irradiation dose in a non hydrogenated fiber resulting in a grating reflectivity of R-20%. The third group consisted of R-50% gratings fabricated in a non hydrogenated fiber using a total dose of 680 J/cm2. The length of all gratings was 8 " .The gratings spectral characteristics are shown in Fig. I . Fig. 2 shows Weibull plots resulting from tension tests of all three grating groups where the pristine fibre is given as a reference. To ensure that any possible mechanical degradation is only due to laser irradiation the acrylate fiber coating was stripped off before irradiation by chemical stripping (H2SO4) [ 11. The Weibull plots exhibit almost no influence due to CW irradiation in terms of median breaking strength compared with the pristine fiber mechanicdl performance. In all cases median breaking strengths higher than 5 GPa and m-values of around 60 have been achieved. Hydrogenation doesn't seem to influence the fiber mechanical resistance as shown by the performance of grating 2. The median breaking strengths and the m-values extracted from Fig. 2 are summarised in table I.Several models have been reported in literature 15-71 predicting lifetime of optical fibers under tension. The basic assumption in all of them is that stress concentration at crack tips created on the fiber surface during the fiber fabrication process cause fracture at stress levels lower than the stress resulted by S i 0 tetrahedral bonds. The stress corrosion of silica glass is described by a power law: -= AK" where -is the crack growth, A is a material parameter for the speed of crack growth and ti dt dr is the corrosion susceptibility. The lifetime equation in te...