Multipoint focusing of single ultrafast laser pulses
Résumé
Designed laser foci increase the versatility of ultrafast laser pulses for 3D processing of optical materials. Typically beam engineering is employed to control the geometry of interaction and the processing dimension. Parallel photoinscription approaches were equally tested to increase the process efficiency, using a higher number of simultaneous machining points. We present here a particular photoinscription regime where a one-dimensional axial array of regular dots is generated before the region of main laser focus under single pulse exposure in fused silica and borosilicate crown glass without phase engineering. Although the interaction takes place with powers above the self-focusing level, the specific topology of the dots does not rely on nonlinear propagation but it is mainly determined by beam truncation upon focusing and subsequent diffraction. The process is explained by a linear Fresnel propagation formalism taking into account beam apodization and wavefront distortions at the air/glass interface. The array matrix can be controlled by the truncation factor and the focusing length. This photoinscription regime is employed to generate two-dimensional arrays of dots in fused silica. We show that an additional phase modulation renders flexible the pattern geometry. In addition we indicate that the pulse temporal envelope can efficiently control the form factor of laser generated individual dots.