The effects of the zeroth-order diffraction of a phase mask on the creation of Bragg gratings using the mask technique were theoretically and experimentally investigated. Experimental results showed that the zeroth-order diffraction of 1% total power, while in interference with the first-order diffraction of 44% each, dramatically affected the structure of the gratings in a polymer optical preform, including their period. Theoretical analysis by taking the zeroth-order diffraction into account predicted that a very small part of the zeroth-order component (0.1%) would have substantial effects on the gratings, which will be no longer uniform along both the directions of the incident laser beam propagation and the groove array. Theoretical calculation agrees well with the experimental results.
Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, "Effects of the Zeroth-Order Diffraction of a Phase Mask on Bragg Gratings," J. Lightwave Technol. 17, 2361- (1999)