METHOD FOR FORMING BRAGG DIFFRACTION GRATINGS BY FEMTOSECOND BESSEL BEAMS

摘要

An object of the present invention is to provide a method for writing transparenthigh efficiency volume Bragg gratings or other modified refraction index patterns insidetransparent dielectric media by using ultrashort pulse and high intensity Gauss-Bessellaser beams.The method includes the following steps of: generating ultrashort light pulses,focusing the light pulses so that the light intensity inside the processed material exceedsthe optical damage threshold; temporally and spatially controlling the exposure in orderto limit the appearance of defects induced by thermal and other effects; forming andprewriting a desired pattern by using a focused Gaussian laser beam by scanning thedielectric medium; forming and focusing a Gauss-Bessel beam into the dielectricmedium; monitoring the near-field distribution of the Gauss-Bessel beam in relation withthe target medium; writing the desired pattern by scanning the dielectric medium inrelation to the incident beam and inspecting the performance of the inscribed opticaldevice.The diffraction gratings can be made of variable size by fabrication in multiple layers.