Generation of higher-order rogue waves from multibreathers by double degeneracy in an optical fiber

In this paper, we construct a special kind of breather solution of the nonlinear Schrodinger (NLS) equation, the so-called breather-positon (b-positon for short), which can be obtained by taking the limit lambda(j) -> lambda(1) of the Lax pair eigenvalues in the order-n periodic solution, which is generated by the n-fold Darboux transformation from a special seed solution-plane wave. Further, an order-n b-positon gives an order-n rogue wave under a limit lambda(1) -> lambda(0). Here lambda(0) is a special eigenvalue in a breather of the NLS equation such that its period goes to infinity. Several analytical plots of order-2 breather confirm visually this double degeneration. The last limit in this double degeneration can be realized approximately in an optical fiber governed by the NLS equation, in which an injected initial ideal pulse is created by a frequency comb system and a programable optical filter (wave shaper) according to the profile of an analytical form of the b-positon at a certain position z(0). We also suggest a new way to observe higher-order rogue waves generation in an optical fiber, namely, measure the patterns at the central region of the higher-order b-positon generated by above ideal initial pulses when lambda(1) is very close to the lambda(0). The excellent agreement between the numerical solutions generated from initial ideal inputs with a low signal-to-noise ratio and analytical solutions of order-2 b-positon supports strongly this way in a realistic optical fiber system. Our results also show the validity of the generating mechanism of a higher-order rogue waves from a multibreathers through the double degeneration.