Optimization of the focal volume in programmable spatial beam shaping

Programmable spatial beam shaping using phase modulation has proven to be an efficient tool for direct and flexible femtosecond laser marking at a Fourier plane of an objective but not well adapted to deep machining or to photowriting in the bulk of a material. We propose an optimization of the focal volume by use of an additional wavefront correction located in the Fourier plane that compensates the focus component in particular. Theoretical predictions involving nonparaxial scalar diffraction are compared to experimental results obtained with a 100 kHz mu J-range femtosecond laser amplifier. Improvements in the beam characteristics, especially the Rayleigh range and the laser fluence, provided by such an additional wavefront control are underlined. (c) 2007 Optical Society of America.