Impact of fiber loss on two-soliton states: Substantial changes in eigenvalue spectrum

The impact of power loss on fiber-optic solitons and soliton compounds has regained interest recently, as coding schemes employing inverse scattering eigenvalues are being discussed. Loss lifts the integrability of the underlying nonlinear Schrödinger equation and has usually been treated by perturbation analysis. Our approach uses localized loss of arbitrary strength. We investigate two-soliton compounds including the N =2 soliton and show that loss causes severe qualitative modifications of the eigenvalue spectrum. Peculiar features include power redistribution between solitons so that one of them is actually enhanced by loss and conversion of solitons at rest into a pair with outward velocities. Earlier reports of such features are put into context. We argue that frequency splitting of soliton pairs requires a mechanism that renders the spectrum double-lobed (or multiply lobed) and that the bifurcation is defined by a balance between dispersive and nonlinear forces. Implications for eigenvalue-based communication formats are pointed out.