Diffractive optical elements by nanosecond pulsed laser

Abstract

Laser ablation micromachining represents a reliable fabrication method for diffractive optics with moderate costs. However, the process of laser ablation is far from a “clean process”. In fact, during the interaction of focalized and high-energetic laser beams with matter involves many processes. The explosive character of this interaction makes very difficult to predict the result of the interaction. In this Doctorate Thesis, we have implemented a laser ablation system working with laser pulses with durations in the range of nanoseconds. A complete description of the system is developed, explaining the characteristics of the inscriptions depending on the variation of the fabrication parameters (the energy density and the pulses overlapping, fundamentally). The optical behavior of the fabricated devices is also analyzed. Particularly, the near field diffraction by rough diffractive elements over steel tapes is studied. A model taking into account the roughness of the surface and the roughness of the zones affected by laser interaction describes this optical behavior. In addition, we carry out a study of the effect of the curvature of the substrate over the Talbot effect of diffraction gratings. At the same time, we show the properties of diffractive elements michromachined by focalizing the laser pulses in bulk fused silica. The characteristics of the laser ablation system (concretely, the nanosecond duration of the laser pulses) make impossible to avoid thermal damages in the substrates. However, we prove that it is possible to fabricate binary diffractive devices. The quality/cost ratio of the fabricated elements shows that nanosecond laser ablation can be considered as a good alternative for rapid prototyping of embedded diffractive devices. Finally, a study of the diffraction produced by binary non-Ronchi gratings is developed, taking into account the effect of the amplitude/phase character, the period of the grating and the fill factor of each cycle over the Talbot self-imaging process.