Waveguide structures in heavy metal oxide glass written with femtosecond laser pulses above the critical self-focusing threshold

We study the feasibility of femtosecond laser writing of optical waveguides in bulk 35PbO∙35Bi₂O₃∙15Ga₂O₃∙15GeO₂ glass, motivated by the extended transparency interval of heavy metal oxide glasses in the mid-infrared regime. Its large linear and nonlinear refractive indices cause critical self-focusing to occur even at low laser energies, leading to filamentary propagation and material damage. However, the vicinity of the laser-damaged region shows a considerable increase in the refractive index, which we attribute to a collateral, stress-induced densification due to the high pressures generated in the focal region. These regions of increased refractive index are strongly birefringent and sufficiently large to support efficient light propagation in transversally written structures. Optical waveguides with a refractive index increase ≥10^-3 and minimal mode ellipticity have been obtained.