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

Abstract

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.

Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications

“…The nonlinear nature of the process makes it difficult to control and thus the waveguide inscription challenging [24,99]. It can distort the waveguide shape [26] or even entirely inhibit waveguide formation [100,101].…”

“…Waveguides have been demonstrated in a multitude of vitreous media, covering the silica, phosphate, heavy metal oxide, chalcogenide and halide glass families [2,[22][23][24][25][26]. These waveguides are mainly based on a single track of smooth, positive refractive index change induced at low peak intensities as shown in Figure 2A.…”

Control of waveguide properties by tuning femtosecond laser induced compositional changes

“…Femtosecond (fs) laser writing 1 has been proven a versatile method for producing optical waveguides inside both glassy 2 and crystalline dielectrics, 3 including highly nonlinear ones. 4 Yet, one of its present limitations is imposed by the maximum refractive index contrast accessible, which is given by the refractive index modification mechanism involved. Refractive index changes upon fs-laser writing have been ascribed to a variety of mechanisms including lattice densification/expansion, 5,6 point defects, 7 photochemical modifications of the glass network, 8 and changes in the polarizability of the glass.…”

Stressed waveguides with tubular depressed-cladding inscribed in phosphate glasses by femtosecond hollow laser beams

“…In such a way, optical waveguides could be generated directly along single tracks. However, in some materials including a large range of glasses, crystals, and polymers, ultrafast laser exposure produces merely a negative refractive index variation due to matter rarefaction or volume damage in the laser induced tracks [2][3][4][5]. Therefore, light guidance within a single track becomes impossible, triggering a search of alternative ways to overcome the situation.…”