Effects of hydrogen molecule diffusion on LP0m mode coupling of long-period gratings

Abstract

We analyzed the effects of hydrogen molecule diffusion on LP0m mode coupling of long-period gratings with two assumptions. The fibers were treated in a H-2 chamber at a temperature 90 degrees C and pressure around 107 atm for five days. And then we fabricated long-period gratings with these fibers after holding them at room temperature and one atmosphere pressure for 15 min to 55 h and measured coupling of LP0m mode. We used diffusion coefficient of hydrogen molecule, which considered the fiber as a perfect cylinder, and assumed that the core and the cladding have the same diffusion coefficient. Coming 'Flexcor1060' was used as a photosensitive fiber which has cutoff wavelength, 920 +/- 50 nm, and the cladding/coating diameter, 125 +/- 2.0 mu m/250 +/- 15 mu m. Core diameter is 5.0 mu m and numerical aperture is 0.14. The hydrogen molecule concentration is averaged in the region of the core and the cladding with time and LP0m mode coupling was simulated with coupled mode theory and two assumptions. We were able to predict the coupling peak wavelengths that change with time. These results were useful for designing the transmission spectrum of long-period gratings. (C) 1999 Published by Elsevier Science B.V. All rights reserved.