Testing a novel μ-Raman method to retrieve refractive index and density field from femtosecond laser-written optical waveguides

Abstract

In this work, we analysed the refractive index field and other local material properties of femtosecond laser-written waveguides properly combining a novel and direct Raman strategy, waveguides coupling and instrumented nano-indentation. At first, we measured a 2D Raman map within the cross section of femtosecond laser-written waveguides. Then, the Raman Phonon Deformation Constants of lithium niobate were employed to retrieve the strain and density variation from the A₁(TO) phonon shifts in the analysed region. We test the results obtained with different combinations of phonons by computing the numerical guided modes and comparing them with those experimentally measured. As a relevant finding, we found that the combination of the A₁(TO)₁ and A₁(TO)₄ phonon shifting is the most proper one to compute strain, density and refractive index variation, almost in this kind of waveguide. Finally, a linear path across waveguides cross section was explored with instrumented nano-indentation and the expected variation of local density was detected through a softening of the elastic module observed in the region directly modified by the ultra-fast laser.