Length dependence of effective cutoff wavelength in single-mode fibers

Transmission characteristics of single-mode fiber operated in the two-mode (LP01and LP11) propagation region is investigated experimentally and theoretically. The effective cutoff wavelength of the LP11mode λceis commonly used to assure practically the single-mode operation of the LP01mode above λceup to certain fiber length. The experimental λcedecreases linearly in a logarithmic scale of fiber length as increasing fiber length, and eventually becomes constant after a few kilometers' propagation. Theoretical λceis obtained from a coupled power equation by taking into account the mode coupling as well as the losses of the two modes. To calculate λce, the losses of the two modes are individually measured by a new measurement technique. It is clarified from the theory that the nonlinear property of the length dependence of λcein the logarithmic scale is due to a relatively small mode coupling between the LP01and LP11modes.