Design of a computer-generated hologram for obtaining a uniform hardened profile by laser transformation hardening with a high-power diode laser

We designed and fabricated a computer-generated hologram (CGH) as a beam shaping optical element for laser transformation hardening with a high-power diode laser (HPDL). Using three-dimensional thermal analysis based on the finite-element method, the intensity distribution of the shaped laser beam was optimized to obtain a uniform hardened depth with 4 mm width in a steel plate. Simple experiments of laser hardening were made to determine a material parameter for the numerical simulation. A binary CGH comprising small subholograms was designed in the spatially incoherent regime, because the HPDL comprised a stack of diode lasers. In addition the CGH was fabricated on a silica glass substrate by photolithography and reactive-ion etching. The observed intensity distribution of the shaped beam was well agreed with the designed intensity distribution.