Influence of surface effects on the performance of lead-niobium-germanate optical waveguides

Lead-niobium-germanate planar waveguides have been produced by pulsed laser deposition. The composition of the waveguides is found to be relatively weakly dependent on the laser fluence, while their surface morphology is affected dramatically. Smooth surfaces are obtained for a narrow fluence range centered at 2.0 J/cm2, while particulates having typical diameters of <0.5 μm or droplets with typical diameters of <10 μm are observed at lower and higher fluences, respectively. The refractive index of the waveguides increases with fluence up to 2.1 at 2.0 J/cm2, which is close to the value of the bulk glass, and remains constant at higher fluences. Propagation losses show instead a minimum (≈6.5 dB/cm) at 2.0 J/cm2. The characteristics of the ablation process that leads to the ejection of solid particulates or molten droplets as well as the increase of the waveguides density on increasing the fluence are discussed to be responsible for the observed optical behavior.