Characterization of erbium- or iron-doped lithium niobate crystals and waveguides

Lithium niobate is one of the most popular materials for integrated optics, mainly due to its electrooptical and nonlinear properties. In the recent years there has been an increasing interest for massive or local doping of lithium niobate in order to extend its functionality. Integrated optical lasers and amplifiers were made possible by doping with rare earths (e.g. with Nd and Er) or with chromium. Diffractive components produced by direct writing were demonstrated in iron-doped niobate. Combining these properties, higher component integration may be achieved; this, of course, is imposing increasingly demanding conditions to the materials and fabrication techniques. Here we present the optical and spectroscopic characterization of optical waveguides produced by Titanium indiffusion in erbium-doped experimentally grown and commercial lithium niobate crystals; both planar and channel waveguides were fabricated. Absorption, photoluminescence and Raman spectroscopies were used, considering also different crystals cuts; a comparison between spectroscopic properties in the bulk and in the waveguide was performed. All the spectra were obtained by using the 514.5 nm line of an Argon laser as excitation source. The power of the laser focused on the sample was measured to be of around 170 mW in the case of the Z-cut bulk and 280 mW for the X-cut sample (bulk crystal and waveguide). Commercial iron-doped niobate crystals were studied as well, and feasibility of laser writing of optical structures was assessed.