Optimizing the efficiency of femtosecond-laser-written holograms

Computer-generated binary holograms are written on a polished copper surface using single 800-nm, 120-fs pulses from a 1-kHz-repetition-rate laser system. The hologram efficiency (i.e. the power in the holographic reconstructed image relative to the incoming laser power) is investigated for different laser-structuring parameters. Theoretical diffraction grating efficiencies for a binary amplitude grating show good agreement with the experimental measurements for diameters of the laser-formed holes below the pitch. Modelling based on straightforward geometrical arguments is used to find the optimal hole size. For a coverage (i.e. relative laser-structured area) of ～43 %, the efficiency reaches ～10 %, which corresponds to a relative power transferred to one reconstructed image of ～20 %. The efficiency as a function of pitch (for fixed coverage) is fairly constant from 2 to 6 μm.