Improved Method for Two-dimensional Determination of the Magnitude and Orientation of Weak Birefringence François Busque, Benoit Sévigny, Nicolas Godbout, Raman Kashyap, Suzanne Lacroix, Michel Meunier École Polytechnique de Montréal, Department of Engineering Physics, PO box 6079, station Centre-ville, Montreal, Quebec, H3C 3A7, Canada francois.busque@polymtl.ca Abstract: A method for measuring refractive index anisotropy and orientation is described. Birefringence characterization of laser-written integrated optics devices is presented as an example application of the method. ©2007 Optical Society of America OCIS codes: (260.1440) Birefringence; (230.7370) Waveguides 1. Introduction A standard technique to measure low-level birefringence is to compare the birefringence of the device under test with that of a compensator. We herein present an improved method for measuring birefringence of small magnitude as well as its orientation and apply it to characterize a femtosecond laser written waveguide. Compared to most birefringence measurement methods which minimize the transmitted intensity with limited accuracy, our method uses a wide range of transmitted intensities, allowing the determination of very weak birefringence parameters with a resolution as low as 10 -4 rad for the total phase retardation. 2. The phase retardation measurement setup To retrieve the complete phase retardation parameters of a sample, we measure the light transmitted through a system comprising an adjustable phase modulator and the sample under test between crossed polarizers. The setup is described in Fig. 1. a) α θ Waveplate 1 slow axis Waveplate 1 fast axis Waveplate 2 fast axis Waveplate 2 slow axis b) Collimator Ar + laser Polarizer Half-wave plate Modulator Condensor Sample Rotating diffuser Microscope objective Analyzer CCD camera Figure 1 a) The rotating diffuser is used to attenuate the fringes due to interference of an Ar + laser beam (λ = 488 nm) on the optical surfaces of the system. A half-wave plate is rotated in order to fix the transmitted intensity through the polarizer for each angle. The image of the sample is focused onto a linear CCD camera for many combinations of rotation angles of the modulator and of the crossed polarizer-analyzer assembly. b) The phase modulator is a rotating system made of two slightly misaligned compensated quarter-wave plates. The modulator acts as a linear birefringence plate having adjustable relative phase retardation through the offset angle α. The two quarter-wave plate assembly, used as a modulator, has an equivalent maximum phase retardation of 2α, where α is the angular offset between the quarter-wave plates neutral axes taken from the fully compensated angle. To achieve the best results, we adjust α to obtain the total modulator phase retardation similar to the maximum phase retardation of the sample. To reconstruct the complete phase retardation, we fit the measured intensity with respect to the rotation angle of the polarizers and of the phase modulator for each pixel using the absolute value and orientation of the phase retardation as free parameters. We then obtain the local phase retardation and orientation for each pixel of the sample. We recover the birefringence from the phase retardation and the thickness of the sample at the point of measurement. a2546_1.pdf CTuT5.pdf