2D strain map of femtosecond written waveguides in Lithium Niobate crystals

M.R. TEJERINA; D. BIASETTI; G.A. TORCHIA

Sevilla

Congreso; EUROMAT; 2013

Nowadays the optical circuits fabricated by using ultrashort laser pulses have reached the top of the new approaches for technological applications in the field of the integrated photonic [1,2]. As it well known, by this experimental procedure is possible to extent the waveguide fabrication to any optical material such as: BIBO, YAG, YFL, KDP, etc. [3]. In this sense, in order to improve this technique it is necessary deeply know the basic physical behaviour for the laser matter interaction during and after the femtosecond pulses impinge the material so high performance designs of waveguiding structures in different materials can be tried. In this work, we present the study of the residual stress after femtosecond interaction during the process to inscribe type II waveguides in x cut Lithium Niobate crystals. In order to do that, we have developed a method which combines computational simulations based in a Finite Element Model (FEM) and micro Raman spectroscopy (MR) (Fig. 1 (a)) so to join both techniques we introduce the Potential Deformation Theory (PDT)[4]. Fitting MR data with FEM results, the micro Raman shift field shown in Fig. 1 (b) was obtained and the bidimensional strain field was retrieved. Also, during this analysis, the following geometrical parameters were defined: expansion coefficients and initial geometry of laser-matter interaction zone. To achieve this aim, we have considered the MR data, the guided light by the waveguide and several experimental parameters which were previously determined in [5]. Among these parameters we can stand out the piezo spectroscopic constants (e and f) and dW/dP energy shift coefficients of the A1(TOi) Raman modes under hydrostatic pressure.The method presented in this work can be very useful to measure the strain field after ultrashort interaction in different optical materials when written waveguides are generated. To carry out this method, it is necessary the Raman response of the studied crystal. Finally, it is important to remark that one of the  important advantages of the strain mapping using μ-Raman is the unambiguous character from its results.