Split of Phase Shifts in a Phase Mask for Fiber Bragg Gratings

Using rigorous finite-difference in time-domain analysis for near field diffraction of a phase-shifted phase mask, we discover that the phase shift is not replicated in the fiber by side-writing through the mask, but is split into two half-magnitude phase shifts with a separation proportional to the fiber-to-phase mask spacing. We show by a physical model for cumulated phase that spectral asymmetry observed in a multichannel fiber Bragg grating (FBG) with a binary phase Dammann sampling function is caused by this split. We propose a new binary phase-only sampling design, which precompensates the phase-shift splitting effect and produces high channel count FBGs without asymmetry in the channel spectrum.