Spectral narrowing due to linear coupling in two-core fibers

We develop for the first time, to the best of our knowledge, a comprehensive theory to describe the nonlinear propagation of picosecond pulses through symmetric two-core fibers (TCFs). Considering the elementary TCF switching process, we derive an exact analytical expression for the nonlinear phase shift of a picosecond pulse of arbitrary shape and chirp. Applying our results to an unchirped Gaussian input pulse, we demonstrate spectral narrowing in the propagation of the pulse through a TCF. Our results show that the linear coupling induces that spectral narrowing. The amount of narrowing of the pulse spectra depends on the peak power of the input pulse.