Abstract

A dispersive filter based on coupling between two waveguide modes having different group velocities is presented. Coupling between two such modes can be achieved by approaching the cores of two dissimilar waveguides or by inducing a periodic perturbation in one waveguide that couples two of its modes together. In the first case, coupling occurs when the propagation constants of the two modes are the same. If one of the waveguides is tapered so that the propagation constant of its mode varies linearly along the coupling region, then shorter wavelengths are coupled at the input of the filter, and longer wavelengths are coupled near the end (or the opposite). The difference in group velocities then introduces a delay between the different wavelengths that can be adjusted to compensate the dispersion of an optical fiber link or the chirp of modulated semiconductor lasers. When coupling is achieved through a periodic perturbation, a linear variation of the perturbation period along the filter results in the same kind of dispersion. Thus, this filter uses modal dispersion to compensate for chromatic dispersion. Numerical calculations show that a nearly constant dispersion can be obtained across the filter bandwidth. Pulse recompression by a factor of 10 and more is possible. All-fiber designs based on photoinduced gratings will be discussed.

© 1990 Optical Society of America