Fabrication of long-period fiber gratings using end-coupled ultrafast laser pulses.

We propose a novel technique for the fabrication of long-period fiber gratings (LPFGs) using ultrafast laser pulses coupled into opposite ends of an optical fiber. The pulses counter-propagate in the fiber and induce a permanent refractive index change where they overlap. Based on absorption rate measurements, it was confirmed that the pulses were partially absorbed at the overlapping position, with a maximum absorption rate of 6%. Furthermore, a LPFG with a smooth transmission spectrum and low background loss was successfully fabricated using this technique.