Spectral and temporal Bloch oscillations in optical fibres

We report a detailed analysis of experimental parameters and fundamental mechanisms contributing to the strong nonlinear scattering of femtosecond and short picosecond laser pulses (50–6000 fs) at focusing in ambient air and protecting gases. The experimental conditions under consideration are typical for a variety of applications: micromachining of metals and dielectrics, high power laser exposure of targets. Such scattering, being the most noticeable manifestation of the complex phenomenon of conical emission in the focusing geometry, puts fundamental limitations to the incident power of precision laser targeting, restricting in this way potential productivity of micromachining in this pulse width range. The phenomena analyzed here are: ionization, the optical Kerr effect, the distortion of spectra via self-phase modulation, and the refractive ability of microplasma, namely the scattering. Characteristics of the last were investigated with respect to the incident wavelength and pulse width, for focusing of Gaussian and flat-top beams of different diameters. The obtained extensive data allowed us to trace the clue trends and relationships of this phenomenon and to find several ways to reduce or even eliminate the scattering in a broad range of the incident energies.