# Dynamics of rogue waves on a multi-soliton background in a vector nonlinear Schrödinger equation

Abstract. General higher order rogue waves of a vector nonlinear Schrödinger equation (Manakov system) are derived using a Darboux-dressing transformation with an asymptotic expansion method. The th order semi-rational solutions containing free parameters are expressed in separation of variables form. These solutions exhibit rogue waves on a multisoliton background. They demonstrate that the structure of rogue waves in this two-component system is richer than that in a one-component system. The study of our results would be of much importance in understanding and predicting rogue wave phenomena arising in nonlinear and complex systems, including optics, fluid dynamics, Bose-Einstein condensates and finance and so on.

Keywords. Vector nonlinear Schrödinger equations, rogue waves, Darboux-dressing transformation

AMS subject classification. 22E46, 53C35, 57S20

## 1 Introduction

It is well known that many nonlinear wave equations of physical interest support solitons, which are localised waves arising from a balance between dispersion and nonlinearity, and which can propagate steadily for a long time. Recently, over the last two decades, it has been recognised that another class of solutions, namely breathers, are also of fundamental importance. Breathers propagate steadily, and are localised in either time or space, while being periodic in either space or time. Further, due to their localisation properties, breathers have been invoked as models of rogue waves, also called freak waves, which are large amplitude waves which apparently appear without warning, and then disappear without trace. While they have been most often found in the context of water waves [1]-[3], they have also been found in other physical contexts such as optical fibres [4]-[6].

The breather solutions of the focusing nonlinear Schrödinger equation (NLSE) have been widely invoked as models of rogue waves, see the references above and Akhmediev [7] for instance. The NLSE equation is integrable [8] and many kinds of exact solutions have been found. In particular, the Peregrine breather,[9] the Akhmediev breather (AB) [7] and the Kuznetsov-Ma breathers (KM) [10]-[11] have been associated with rogue waves, as the potential outcome of the modulational instability of a plane wave. AB is periodic in space and localized in time, while KM is periodic in time and localised in space. The Peregrine breather is especially considered as a rogue wave prototype because it is localised in both time and space, and so captures the fundamental features of rogue waves. Importantly it has a peak amplitude which is exactly three times the background. Also, it is the generic outcome of a wide class of modulated plane waves [12]. When the spatial or temporal period is taken to be infinite, AB or KM become Peregrine breather in the limit. Peregrine breather of NLSE has been observed experimentally in water wave tanks [13] and also in nonlinear fibre optics [4]-[5].

The Peregrine breather is the lowest order item in a hierarchy of rational solutions for NLSE.
Since these higher-order forms may also be invoked as models for rogue waves, with higher amplitudes,
it is of interest to find explicit expressions for these higher-order rogue wave solutions.
For example, Akhmediev [14] presented some low-order rational solutions
using a Darboux transformation method. Ohta and Yang [15] obtained general high-order rogue waves
in terms of determinants using the Hirota bilinear method. Guo [16]-[17] constructed the -order rogue wave solutions
using a generalized Darboux transformation. Dubard *et al.* [18] discussed the quasi-rational solutions via algebraic-geometry method.
As well as for the NLSE, rational solutions have also been explored for some other nonlinear wave equations in [15], [19],
[20]-[23].

Here we shall extend a recent study of the integrable vector nonlinear Schrödinger equations (VNLSE), or Manakov system, by
Baronio *et al.* [24] This can be expressed in dimensionless form as,

(1) |

Here the subscripts stand for partial differentiation with resect to , an evolution variable, and , a spatial variable.
The dependent variables represent wave envelopes, whose physical meaning
depends on the particular context. In particular the system (1) has applications
in nonlinear optics [25]-[27]
and in Bose-Einstein condensates [28]-[30].
Also note that Eqs. (1) correspond to the self-focusing (or anomalous dispersion) regime.
The fundamental vector rogue wave solutions have been recently reported by several authors [24], [28], [31].
Then the first-, second- and third-order rogue wave solutions of VNLSE were explicitly presented [32].
Here our aim is to find the general -th order rogue wave solutions. We will construct hierarchies of semi-rational solutions of
the VNLSE (1), using an asymptotic expansion method. Our obtained solutions are an extension of the results of
Baronio *et al.* [24] to higher-order rogue wave solutions, and the pure rational solutions of our results
can be identified with the ones obtained by Zhai *et al.* [32] As the representations obtained are a combination of rational and exponential functions, some interesting structures can be observed, such as (1) the coexistence of higher-order rogue waves and
bright-dark multi-soliton solutions, and (2) the coexistence of multi-rogue waves and bright-dark multi-soliton solutions.
In general our obtained solutions indicate the complexity that can arise when rogue waves interact with solitons.

The paper is organised as follows. In section 2, The Lax pair and Darboux-dressing transformation of VNLSE (1) are briefly reviewed. In section 3 the dynamic behaviour of a family of fundamental rogue wave solutions is examined. Then in section 4, we present some novel periodic breathers. In section 5, we use an asymptotic expansion method to obtain -th order rogue wave solutions of VNLSE (1). Then, in section 6, we give some examples to illustrate the a range of dynamic behaviour of our obtained rogue wave solutions. We conclude with a summary in section 7.

## 2 Asymptotic expansion of Darboux-dressing transformation

The VNLSE (1) is integrable, and is a condition for the compatibility of the Lax pair,

(2) |

Here is a matrix variable, is the complex spectral parameter, is a constant diagonal matrix and is the matrix variable

It is straightforward to check the condition of compatibility between the two equation in (2)

(3) |

leads directly to the VNLSE(1).

A suitable Darboux-dressing transformation for the VNLSE (1) is given by [24] [33]for instance,

(4) | |||

Here, and is the fundamental solution for the Lax equations (2) corresponding to and for the seed solutions of the VNLSE (1). The constant parameter is complex while is an arbitrary nonzero complex 3-dimensional constant vector. Next, it is useful to note that the Darboux-dressing transformation (4) can be replaced with the alternative form

(5) |

Since is also a Darboux transformation of NLSE, it follows that

(6) |

The matrices and are obtained by replacing with in and , respectively. Then inserting (5) into (6) we find that

(7) |

(8) |

In general, for a Darboux transformation, the zero seed solution allows for the construction of a hierarchy of multisoliton solutions , while a plane wave seed solution results in a hierarchy of breather-type solutions related to modulation instability. However, here we note that

(9) |

This means that the Darboux-dressing transformation (5) cannot be iterated continuously for the same spectral parameter.
In order to eliminate this limitation, we introduce the following expansion theorem
which can then be used to produce new solutions for the same spectral parameter.

Theorem Let be a solution of the Lax pair system (2) corresponding to the spectral parameter and a seed solution . If has an expansion at

(10) |

(11) |

where | ||||

and |

are solutions of the Lax pair system (2) corresponding to the same spectral parameter and solution

Remark: In the above Theorem, the denotation means that are replaced correspondingly by but are left unchanged in . In addition, we introduce the denotation

(12) |

The meaning of is same as . Concretely, let us illustrate these denotations by two simplest examples: for , . For , .

Generally, employing the iteration relation (11) and with the help of the following expansion

with

Then could be rewritten in a more explicit form

(13) |

It is natural to obtain

(14) |

and

(15) |

Based on these preparations, we turn to the proof of Theorem.

Proof: Substituting (10) into (2) and equation to zero, the coefficients of , we arrive at

(16) |

and

(17) |

with

From the first equation of (16) and (17), we conclude that is a solution of the the linear systems (2) with and . Thus, we can construct a Darboux transformation as in (5) above

(18) | |||

(19) |

Meanwhile, the corresponding solution of the VNLSE is given by

Thus we have shown that the Theorem holds for .

Next we use mathematical induction for integers . Assume the Theorem holds for , that is,

(20) | |||

(21) |

Employing the following properties

(22) | |||

(23) | |||

(24) |

the left side of (20) is given by

Comparing this with the right side of (20), we get that

(25) |

Factoring out the factor in (2), we arrive at

(26) |

Furthermore, from (14), (15) and (16), we have

(27) |

and

(28) |

For an arbitrary nonnegative integer , by a comparison (2) with (2), it indicate that can be obtained directly after replacing by in . As a result, from (26), we immediately get

(29) |

Now, with the help of (26) and (29), it is derived that

(30) |

The assumption (20) allows us to construct the next step of the Darboux dressing transformation which satisfies

(31) |

Therefore, using (30) and (31), it follows that

(32) |

Next, we need to show that

(33) |

Indeed, using (20)-(2), we get that

Similarly, for the temporal flow, we can show that

(34) |

This completes the proof by induction.

In addition, by a slight adjustment of the above proof of our Theorem,
it will provide a justifiable way to prove the Theorem 2 in Guo *et al*. [16].

## 3 Exact breather solutions of the Lax pair system

Before using the expansion Theorem to construct higher-order rogue wave solutions, we use the Darboux-dressing transformation (2) to construct a new class of breather solutions. It is readily shown that the VNLSE (1) admit the plane wave background solution,

(35) |

where and are arbitrary parameters which, without loss of generality, are taken as real, and is a wavenumber.

The corresponding solution of the Lax system (2) is sought in the form

(36) |

(37) |

and is an arbitrary complex vector, and . Here, it is required that the constant matrices and satisfy

(38) |

Inserting (36) into (2) yields

(39) |

Solving the conditions (38) and (39), we obtain

Then the exponential matrices in (36) can be written as

where

Similarly the exponential matrices in (36) can be written as

where