A matched pair of tilted gratings may be formed in a section of optical fiber (or waveguide) and used as a “tap” to measure the power of an optical signal passing through the fiber. By using a pair of highly-tilted gratings (e.g., tilted at an angle of 45° with respect to the optical axis) and orienting the gratings to be orthogonal with respect to one another (i.e., azimuthal orthogonality around the fiber axis), a pair of orthogonally polarized beams will be out-coupled from the propagating signal. Since the pair of beams are orthogonal, their sum can be made insensitive to variations in polarization of the optical signal propagating along the fiber.