Modal birefringence in weakly guiding fibers

We examine wave propagation in few-mode and multimode fibers with a small index difference between core and cladding, where linearly polarized (LP) “modes” serve as a highly useful simplified solution. However, for a nonzero index difference, each LP “mode” decomposes into two true fiber waveguide modes, typically an HE and an EH mode. These two constituent modes have different group delays, which results in an effect termed modal birefringence. This effect needs to be understood in the design of mode-multiplexed transmission systems. We report an analysis of modal birefringence including scaling rules for fiber design, and provide numerical results for about 50 of the lower order modes.