Characterization of femtosecond laser written gratings in PMMA using a phase-retrieval approach

In the past years, several examples of waveguides or gratings manufactured by laser radiation in various glasses or transparent polymers have been demonstrated. As these devices rely on a modification of the refractive index in the vicinity of the focal point of a writing laser, phase retrieval methods are well suited to determine the induced phase change inside the material experimentally. Thus, phase retrieval can assist parameter-studies and reveal information about the internal structure of the modifications which can hardly be obtained using other methods. Here, we demonstrate the application of a modified phase retrieval technique especially suited to determine the optical properties of phase objects such as femtosecond written gratings or waveguide arrays inscribed in transparent materials. Using the presented algorithm, we quantify the correlation between pulse energy and phase deviation as well as the structure width for femtosecond laser induced refractive index modifications in poly(methyl methacrylate) which, in the future, can be used to create tailored micro-optical structures for sensing applications.