Radiation losses from couplers

The antisymmetric mode of couplers, composed of two parallel fibers, becomes cut off whenVis sufficiently small. We show that the familiar perturbation modes of this coupler give an excellent approximation to the cutoff value ofVpreviously obtained by an exact analysis. More importantly, by using the perturbation modes, we establish that the rate of leakage below cutoff sets a genuine practical limitation to operation. To do this, we develop a scalar theory for radiation losses.