Observation of the universality of nonlinear mode coupling in a fibre laser

Mode coupling is a fundamental aspect of wave propagation and is therefore intrinsic to many branches of physics. We consider the resonant coupling, typically caused by weak perturbations, between solitons—high-intensity nonlinear pulses—and low-amplitude linear waves. These resonances, which are quite common in nature, enable the two modes to exchange energy, contradicting the usual perception of solitons as pulses that propagate without changing shape. The mathematical analysis required to characterize this effect is challenging and was completed only relatively recently, even though its roots date back to the work of G. G. Stokes in the mid-19th century. This analysis predicts that the phenomenon is universal, occurring for many different types of waves irrespective of the nature of the soliton, the linear mode, or the coupling mechanism. However, despite its broad significance, these predictions were never systematically verified experimentally. Here, we validate these predictions in an optics context using a mode-locked fibre laser. We confirm that the coupling is universal and approximately satisfies a general scaling law. By validating long-standing theoretical predictions, we confirm the physical and mathematical relationships of previous experimental observations across a wide variety of perturbed nonlinear waves.