When does the standard waveguide approximation quietly break down?

The tight-binding model—physics' workhorse for light hopping between waveguides—gets stress-tested against exact analytical solutions in a pair of coupled waveguides where gain and loss are deliberately balanced. It passes for intensity patterns but fails to reproduce the complex phase oscillations that arise when the system is strongly modulated along its length. The finding draws a clear boundary for when the approximation can be trusted, which matters for designing optical switches and sensors that exploit these gain-loss effects.