Fermi–Pasta–Ulam–Tsingou phenomenon in two-core optical fibers for the asymmetric CW state

Abstract Two-core optical fibers support different continuous wave (CW) states. We investigate the Fermi–Pasta–Ulam–Tsingou (FPUT) recurrence for the most general asymmetric CW state in two-core fibers. In the normal dispersion regime, linear coupling triggers modulation instability (MI), thereby giving rise to FPUT recurrence. However, such recurrence is highly constrained, occurring only within specific ranges of linear coupling where the coupling is either sufficiently small or near its maximum. This result contrasts with the case of the symmetric CW state, where FPUT recurrence can be observed over a wide range of linear coupling, and with the antisymmetric CW state, where FPUT recurrence is absent because MI does not occur. Coupling coefficient dispersion (CCD) strongly suppresses FPUT recurrence at weak coupling but enhances it with more recurrence cycles at strong coupling. In the anomalous dispersion regime, FPUT recurrence is observed for all linear coupling, yet its characteristics vary with coupling strength. For linear coupling below (above) a critical value, the energy remains predominantly localized in the initially high-power core with the number of FPUT recurrence increasing with linear coupling monotonically (intense energy exchange between the two cores occurs with only very few FPUT recurrence cycles). CCD significantly modulates the number of FPUT recurrence cycles, exhibiting a non-monotonic dependence. The results differ drastically from the behaviors of the symmetric and antisymmetric CW states.