New Characterization Technique for Femtosecond Laser Written Waveguides in Yb/Er-Codoped Glass

We have developed a procedure for the complete characterization of femtosecond-laser written Er/Yb-codoped glass waveguides. The procedure is based on precise measurements of both pump and 1.5 ¿m -band output powers when the waveguide is diode-laser pumped at 980 nm. The dependence of these optical powers on the input pump power is fitted to the results derived from a detailed numerical model of the propagation of the optical powers inside the waveguide. As a result of the fitting procedure the 980-nm absorption cross-section of the Yb³⁺ ion and the coefficients for both Yb³⁺ ¿ Er³⁺ non-radiative energy-transfer and Er³⁺ ion upconversion are obtained. Moreover, the transmission losses for both pump and signal wavelengths and the coupling losses at each waveguide end are also determined. The model takes into account that the fs-laser writing process may induce transmission losses with a spectral dependence (¿(¿)) departing from the Rayleigh scattering scaling law ( ¿^-4) and is valid both for single mode and multimode propagation. It has been tested in fs-laser written waveguides produced under non-optimal conditions in order to test the robustness of the method, providing excellent results.