Fabrication of Uniform Phase Structures in the Bulk of a N-BK7 Glass Using Ultrashort Laser Pulses

Abstract We study the possibility of application of direct laser writing technique for the fabrication of large-size uniform phase structures in the bulk of the borosilicate glass N-BK7. The morphology and the type of modifications induced by the sub-picosecond (500 fs) pulses are studied depending on the energy dose of irradiation. We show that with moderate focusing and at pulse energies below the threshold of self-focusing effects, it is possible to choose writing regimes (the scanning speed, the repetition rate of pulses) which allow the fabrication of extended structures with flat modification of the glass refractive index in the layer-by-layer scanning mode. At higher peak intensities of the laser, the Kerr nonlinearities affect the pulse propagation and the dynamics of the relaxation processes, and the resulting structures are essentially inhomogeneous with the alternating sign of refractive index change. To validate the approach, a buried phase structure with large dimensions was fabricated showing a rather uniform refractive index change over the entire processed range.