Alleviating the mechanical tolerances in femtosecond laser micromachining by diffractive focusing

To meet the ever-increasing demands of high resolution and versatility of microprocessing technology, diffractive optical elements (DOEs) are used to get the desired focal pattern [1]. The simplest phase DOE is a diffractive lens (DL). The focal length of a DL is given by f=f_o λ_o/λ being f_o the focal length for the central wavelength of the laser λ_o. Within the paraxial approximation, the focal irradiance distribution of a long pulse focused by a refractive lens (RL) or a DL of the same numerical aperture (NA) and focal length is expected to show similar behavior [2]. However, for sub 100 fs pulses, important differences appear owing to the large spectral bandwidth of ultrashort pulsed radiation [3]. Recently, measurements of the intensity ablation threshold by defocusing a RL have been carried out for 100 fs pulses [4]. In this contribution, we compare the ablation region obtained under defocusing conditions for both RL and DL with sub 100 fs pulses.