  We explore the practical challenges which should be addressed when designing
a multi-core fiber coupler for nonlinear switching or mode-locking
applications. The inevitable geometric imperfections formed in these fiber
couplers during the fabrication process affect the performance characteristics
of the nonlinear switching device. Fabrication uncertainties are tolerable as
long as the changes they impose on the propagation constant of the modes are
smaller than the linear coupling between the cores. It is possible to reduce
the effect of the propagation constant variations by bringing the cores closer
to each other, hence, increasing the coupling. However, higher coupling
translates into a higher switching power which may not be desirable in some
practical situations. Therefore, fabrication errors limit the minimum
achievable switching power in nonlinear couplers.
