Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask

Fiber Bragg gratings were fabricated in all-silica core fiber by focusing 125-fs 800-nm pulses with an 80-mm lens through a phase mask with $4.28‐\mathit{\mu }\mathrm{m}$ pitch onto a fiber sample. When the phase-mask–fiber separation was 5 mm the observed structure was clearly the result of two-beam interference between the $\pm 1$ orders. The elimination of the remaining 9 orders is a consequence of the walk-off experienced by the mask orders and the short duration of the femtosecond pulse. This effect is unique to the fabrication of Bragg gratings with femtosecond sources and would not be observed with a longer pulse duration or incoherent UV sources.