Highly efficient polarization independent transmission gratings for pulse stretching and compression

Many industrial and scientific applications require high power ultrashort laser pulses, so amplification of pulses is necessary. To avoid optical damage or nonlinear effects in the amplifier setup, the pulses are stretched before amplification and recompressed afterwards. One possibility for the efficient stretching and recompression is to apply highly efficient diffraction gratings, whereby dielectric gratings and especially dielectric transmission gratings feature a high damage threshold. If the incident pulses are not linear polarized, the polarization sensitive diffraction efficiency of the gratings mostly results in a significantly reduced pulse energy. To overcome this problem we developed highly efficient polarization independent gratings and present a theoretical and experimental study on the design and fabrication of highly dispersive transmission gratings in fused silica, that exhibit a high diffraction efficiency for TE- and TM-polarized illumination as well. The dependence of the diffraction efficiency on the grating parameters is discussed for both polarization directions. One of the theoretical designs shows a diffraction efficiency exceeding 97% for unpolarized illumination. The fabrication of those gratings has been done by electron beam lithography and reactive ion beam etching, whereby the diffraction efficiency was maximized by a special trimming process. The theoretical considerations are confirmed by the fabricated samples.