Length dependence of effective cutoff wavelength in single-mode fibers

Transmission characteristics of single-mode fiber operated in the two-mode (LP
01
and LP
11
) propagation region is investigated experimentally and theoretically. The effective cutoff wavelength of the LP
11
mode λ
ce
is commonly used to assure practically the single-mode operation of the LP
01
mode above λ
ce
up to certain fiber length. The experimental λ
ce
decreases linearly in a logarithmic scale of fiber length as increasing fiber length, and eventually becomes constant after a few kilometers' propagation. Theoretical λ
ce
is obtained from a coupled power equation by taking into account the mode coupling as well as the losses of the two modes. To calculate λ
ce
, 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 λ
ce
in the logarithmic scale is due to a relatively small mode coupling between the LP
01
and LP
11
modes.