Integrated optical waveplates fabricated by femtosecond laser micromachining

The application of integrated photonic technologies to quantum optics has recently enabled a wealth of breakthrough experiments in several quantum information areas. In particular, femtosecond laser written optical circuits revealed to be the ideal tool for investigating the features of polarization encoded qubits. However, the difficulty of integrating half and quarter wave plates in such circuits avoids the possibility to perform arbitrary rotations of the polarization state of photons on chip. Femtosecond laser written waveguides intrinsically exhibit a certain degree of birefringence and thus they could be exploited as integrated waveplates. In practice, the direction of the birefringence axes of the waveguides is the same of the propagation direction of the writing femtosecond laser beam, namely perpendicular to the substrate surface. Its fine rotation in a controlled fashion, preserving the accuracy of the positioning of the laser focal spot required by the fabrication process, is extremely challenging. In order to achieve this goal, we combine a high NA (1.4) focusing objective partially filled with a reduced diameter writing beam. In this way, the translation of the beam with respect to the objective center produces a rotation of the focusing direction, without altering the focal spot position. With this method we are able to tilt the birefringence axes of the waveguides up to 45, and thus to use them as integrated light polarization rotators. In order to demonstrate the effectiveness of these components, we developed a fully integrated device capable to perform the quantum tomography of an arbitrary two-photon polarization state.