sample end face 24 collects a portion of the beam refracted out of the waveguide 22 under test the detected signal is inversely proportional to the changes in the index of refraction encountered at the waveguide under test end face 24 during a scan across the focus of the beam l from the known refractive index values of the two reference blocks 30 a and 30 b a linear interpolation in the module 42 provides a sample refractive index profile in one embodiment the electrooptic modulator 38 working at 1 to 10 megahertz introduces te tm polarization alternately into the waveguide 22 under test the waveguide 22 under test stress birefringence introduces light intensity modulation at the photodetector 10 at 1 to 10 megahertz an ac signal at 1 to 10 megahertz for example corresponds to the difference between the intensity detected by the photodiode 10 at each polarization the capacitor 46 isolates the modulator 42 and receives the ac component at each x y point on the waveguide 22 from the photodetector 10 the photodetector 10 in one embodiment may have a response time that is the inverse of 10 megahertz and may have an impedance of 1 2 megaohms in one embodiment the difference signal can be scaled to a dc signal which is inversely proportional to the refractive index of the waveguide 22 under test and may be analyzed by the module 44 since the waveguide 22 under test is translated in.