Advances in Mathematical Physics
Volume 2016 (2016), Article ID 5787508, 19 pages
http://dx.doi.org/10.1155/2016/5787508

Research Article

Vector Solitons of a Coupled Schrödinger System with Variable Coefficients

Juan Carlos Muñoz Grajales

Departamento de Matemáticas, Universidad del Valle, Calle 13, No. 100-00, Cali, Colombia

Received 26 April 2016; Accepted 26 June 2016

Academic Editor: Antonio Scarfone

Copyright © 2016 Juan Carlos Muñoz Grajales. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We show the existence of waveforms of finite-energy (vector solitons) for a coupled nonlinear Schrödinger system with inhomogeneous coefficients. Furthermore, some of these solutions are approximated using a Newton-type iteration, combined with a collocation-spectral strategy to discretize the corresponding soliton equations. Some numerical simulations concerned with analysis of a collision of two oncoming vector solitons of the system are also performed.

1. Introduction

Several physical processes related to wave motion can be described using systems of coupled nonlinear Schrödinger (CNLS) equations. Recently there has been a great interest on the study of CNLS systems with nonlinear terms modulated by coefficients which depend either on space, time, or both. This research is motivated by the potential applications of these models to the fields of Bose-Einstein condensates (BECS) [1–8] and nonlinear optics [9–11]. For instance, in the case of BECS [12, 13], a description of the effect of the Feshbach resonances in the mean field limit can be developed using CNLS systems of Gross-Pitaevskii equations [14, 15]. The study of wave propagation in Bose-Einstein two-component condensates with spatially inhomogeneous interactions has been a field of intense research activity in Physics in the last few years [16–24]. In particular, the investigation of multicomponent solitons (also known as vector solitons) has attracted a great deal of attention, starting with the classical work by Manakov [25]. This type of permanent finite-energy waveform arises in CNLS systems due to the interplay between the second-order dispersion and cubic or high-order nonlinearity.

The CNLS systems may also model beam propagation inside crystals, water wave interactions, biophysics [26], finance [27], and oceanography [28], and in the field of communications such equations have been employed to describe pulse propagation along orthogonal polarization axes in nonlinear optical fibers and in wavelength-division-multiplexing systems [29]. Further physical phenomena in nonlinear optics can be described by this family of equations (see [10] and references therein).

In this paper, we will consider theoretically the CNLS-type system

where and . Here denotes the complex conjugate of . We suppose that the coefficients , , , , are positive and depend only on the variable in order to focus on the influence of -nonlinear modulations on the multicomponent solitons of the system. Furthermore , are positive real constants. As mentioned above, such type of CNLS system can be used to model a variety of physical phenomena. In the case that all coefficients are constants, system (1) is a model to describe one-dimensional light propagation through a linearly birefringent lossless optical fiber, taking into account the Kerr effect (see Menyuk [30, 31], Agrawal [32], and Evangelides Jr. [33]). In applications to optical fibers, the variables , in system (1) denote time and space, respectively, means the normalized strength of the linear birefringence ( is the inverse group velocity difference), and the model’s parameters are usually constant or -varying (i.e., they vary along the fiber axis). However, space-time-dependent parameters can be encountered in CNLS systems applied to BECS. Recently, Cardoso et al. [34] and Han et al. [35] used systems in the form of (1) with and to describe the interaction among the modes in one-dimensional Bose-Einstein condensates modulated in space and time. In this type of application, the variables , denote space and retarded time, respectively, the functions , denote the complex envelopes of the propagating beam of the two modes, and are the external potentials, means the group velocity dispersion coefficient, and the physical parameters , , , and (depending in this case on both and ) describe the strength of the cubic nonlinearities. The motivation of the study of the interaction of propagating waves in Bose-Einstein condensates with spatial inhomogeneities comes from the discovery in the last years of novel experimental ways to control experimentally the interactions through optical manipulation of the Fechbach resonances [36].

The first aim of the present paper is to address the physically relevant question of the existence of vector solitons of system (1) with in the form

and in the form

for the case , where and are positive real functions and , , and are real constants.

It is important to point out that exact solutions of system (1) have been obtained only in particular cases. We mention the work by Belmonte-Beitia et al. [38], where explicit solutions of system (1) for and and some examples of variable coefficients were computed using Lie group theory. However, the integrations involved are very long and can not be evaluated as a closed form expression for general model’s coefficients. In [39], Belmonte-Beitia et al. also proved the existence of vector solitons of the system above, in the case that the inhomogeneous coefficients , , , and have compact support and and . Furthermore, Kartashov et al. [11] computed numerically and analyzed the stability of two-component solitons in a medium with a periodic modulation of the nonlinear coefficients for system (1) with and .

In the present paper, we wish to generalize the previous results by establishing analytically existence of vector solitons in forms (2) and (3), of full system (1), considering the extra cross-mode nonlinear terms preceded by the -dependent coefficient , and a nonzero value of the parameter . In the case of CNLS models for pulse propagation in optical fibers, this class of cross-mode terms account for the coherent nonlinear interaction between two linear polarizations of the electromagnetic waves. As pointed out by Menyuk [30], these modulation terms may play an important role in a fiber with very low birefringence. Recently, Muñoz Grajales and Quiceno [37] also illustrated the effect of these extra nonlinear terms and the parameter on modulation instability of a pulse along an optical fiber modelled by full system (1) but with constant coefficients. On the other hand, in [40], some analytical vector bright solitons were calculated for a generalized CNLS system to model BECS including self-phase modulation, cross-phase modulation coefficients, a time-dependent anti-trapping parabolic potential, and four-wave mixing nonlinear terms in the forms and with a time-dependent coefficient.

To study existence of vector solitons of system (1), we apply the positive operator theory introduced originally by Krasnosel’skii [41, 42], following the ideas by Benjamin et al. [43] in the framework of solitary wave solutions of a family of scalar dispersive models for water wave propagation.

In second place, we compute numerically some vector solitons of system (1) in forms (2) and (3) using a Newton iteration, combined with a collocation-spectral strategy to discretize the corresponding soliton equations. This strategy allows us to compute approximations to new vector solitons of the system for a variety of inhomogeneous model’s coefficients. Some numerical simulations concerned with the collision of two oncoming vector solitons of the CNLS system are also performed.

The rest of this paper is organized as follows. In Section 2, we review some known results on fixed points of positive operators in a Fréchet space, necessary in order to develop the existence theory of vector solitons of system (1). In Section 3 we reduce the problem to find a fixed point of a nonlinear positive operator defined on a cone in an appropriate Fréchet space. In Section 4, we use the theory of fixed point index and positive operators to establish the existence of a family of solitons of system (1). In Section 5, we introduce the numerical solver employed to compute solitons of the system and illustrate the theoretical results. Finally, Section 6 contains the conclusions of our work.

2. Preliminary Results

In this section we include a brief review of some results from the functional analysis of positive operators whose domain constitutes a subset of a Fréchet space, following the papers by Benjamin et al. [43] and Chen et al. [44, 45]. We must recall that a Fréchet space is a metrizable and complete, locally-convex, linear topological space (over the real numbers). On a sequence of seminorms can be defined in such a way that for every and every and that the formula

provides a metric that generates a topology that coincides with the original topology on . In this case, we say that is a Fréchet space with generating family of seminorms . Hereafter, we use the notation

It is clear from (4) that we have that . In general, a set in a topological linear space is said to be bounded if, for any neighborhood of 0 in , there is such that . In the case of a Fréchet space with metric given by (4), a set in is bounded if and only if corresponding to each positive integer there is such that . If , then is usually not bounded (for details see [46]). A closed subset of a Fréchet space is a cone if the following conditions hold true:

From (6), must be convex. On the other hand, we also have a partial ordering on given by

For any , let us denote

An operator defined on is said to be positive, if . On the other hand, we say that a positive operator on is -compact, if the set has a compact closure, for each . A triplet is said to be admissible, if

(1)is a convex subset of ,(2) is open in the relative topology on ,(3) is continuous and -compact,(4)there are no fixed points of on , the boundary of the open set in the relative topology on .

From Granas’ work [47], there is an integer-valued function that satisfies the basic axioms of a fixed point index. Among them, we consider the following ones:

(i)Homotopy Invariant. If and are two admissible triplets and the operator is homotopic to the operator on , then .(ii)The Fixed Point Property. If is admissible and , then has at least one fixed point in .(iii)Index of Constant Maps. If is admissible and is constant (i.e., there is a point such that for all ), then

We refer the reader to [43] (see also [41, 42, 47]) for details in the following results. It is assumed throughout that is a cone in a Fréchet space with generating family of seminorms and the standard metric as in (4) and that is continuous, positive, and -compact.

Lemma 1. Suppose that and either

Then one has that is admissible and .

Lemma 2. Suppose that and either

Then is admissible and

The following theorem is a consequence of the first two lemmas.

Theorem 3. If either (10) or (11) holds for satisfying and either (12) or (13) holds for satisfying , then has at least one fixed point in . Moreover, .

An interesting property of system (1), allowing finding new solutions, is described in the following result.

Theorem 4 (Galilean invariance). Let and let , , , , , , , and be constants. If and are a solution to system (1), then another solution is given by

where the velocity is given by

Furthermore, the solutions of system (1) are indifferent to multiplication by , for any constant .

Proof. It follows directly by substitution into system (1).

3. Problem Setting

In the present paper, we are interested in establishing the existence of vector solitons of system (1) for in the form

and in the form

for the case . Here and are positive real functions, and , , and are real constants. In each case, we see that the functions and must satisfy a system in the form (soliton equations)

where the coefficients , , , , , and are defined in the case that by

On the other hand, when ,

We assume that the coefficients , , , , and in system (1) are bounded, continuous, positive, even, and nonincreasing for , , the parameters , , , , , and are such that the coefficients , , , and are positive and bounded, and and are positive.

Abandoning the tildes, we see that to show the existence of a solution of (18) is equivalent to establish the existence of a solution of the fixed point equation:

where the operator is defined by

Let us denote by the Fourier transform of the function . We define the functions and as

Thus and are positive, even, and monotone decreasing on and belong to . Furthermore for ,

Therefore .

Then we can rewrite the operator as

where denotes the convolution between the functions and .

Hereafter, we consider the space of real valued continuous functions defined on (denoted by ), with the topology of uniform convergence on bounded intervals under the seminorms

In this case, the distance is given by

The open ball of radius centered at zero and its boundary are given, respectively, by

Let be the cone defined as

Note the that, for , we have for all that

and so we have for that

We note that the product space is also a Fréchet space with generating family of seminorms

We also define the cone by .

The metric in the product space is defined by

In particular, for ,

We also set

and its boundary

We observe that if , that is, , then we have that

We denote by the annular section of the cone :

Hereafter, according to the notation introduced in the previous section, we set, for , the convex set

4. Main Results

Before we go further, we establish a general result.

Lemma 5. Let be an even bounded positive function on , which is monotone decreasing on . Then the operator defined by

maps into .

Proof. We first note that is bounded due to the fact that and also from the Young inequality, since

Now, we also have that for . In fact,

since and . On the other hand, is also an even function for . In fact,

We claim now that for is a continuous function on . In fact, first note that for any :

Using and the dominated convergence theorem, we conclude that

meaning that is a continuous function on , as long as . Finally, we need to establish that is a nonincreasing function for , for . So let be fixed and take and . Then, we have for any that

and so we have that

Using in the first formula and in the second one, we get that

Now, we note that

since , and the fact that and are nonincreasing for . Now, for the rest of the integral, we use a similar argument, after noting that and are even functions. In fact, first note for that

since is an even nonincreasing function for . So, from this fact, we have that

since , and the fact that is an even nonincreasing function for . In other words, we have shown that

for any and , which means that is a nonincreasing function for .

Lemma 6. Suppose that the coefficients , , , and in system (18) are continuous bounded positive nonzero functions in and , . Furthermore, assume that these coefficients are even and nonincreasing for . Then the operator defined by (25) maps continuously into . For each , the set is a relative compact subset of .

Proof. As we mention above, are positive functions and also from the Young inequality

If we set , then from Lemma 5, we have that for any . In other words, is an even, continuous, positive, bounded function on . Further, is a nonincreasing function on .
Now, let , then we have that and belong to the cone , which means from Lemma 5 that

and also that for is continuous on . Moreover, we also have that and are nonincreasing for , for all . In other words, we have established that .
Finally, we want to prove that the operator maps continuously to . To see this, we must recall that convergence in is equivalent to uniform convergence on closed bounded intervals . Assume that in , as . Let be a fixed closed bounded interval. For given, we choose sufficiently large such that, for ,

Using this fact, we see easily for ,

To analyze the continuity of the first component of the operator , we decompose the integral on as

Observe that is a bounded set in . Thus there exists a constant such that

Let us denote

Therefore, for and large enough,

On the other hand, using the factorizations

we have that

Due to in as , given , there exists such that if ,

Therefore, it follows that

Substituting the results above into inequality (57), it follows that

for . Analogously,

for . We conclude that the operator maps continuously to .
It remains proving that   () is relative compact subset of , which is equivalent that is relative compact subset of , for . To see this, we use the Arzela-Ascoli Theorem to establish the compactness of the families in ,

Let be such that with and . We first note that, for , we have that is a continuous function such that (see Lemma 5)

as (uniformly in ), meaning that is equicontinuous in due to uniformity of the last estimates in . Moreover, the families are equicontinuous in , since are equicontinuous in and the uniform estimate for in (31). On the other hand, for each , the set

has a compact closure in since for any . Here , , are constants which depend only on . From the Arzela-Ascoli Theorem, the families are normal (see Theorem 1.23, chapter VII in [48]). In other words, we have shown that the set is a relative compact subset of .

Lemma 7. Suppose the same hypothesis in Lemma 6. Let

and let , be constants such that , and let be sufficiently close to 1.
Then

(a) for each , and ,(b) for each , and , where is the constant function given by .

Proof. (a) Suppose that there exist and such that and

Clearly, if , we have that which is a contradiction. Suppose that . Then we have that and belong to the cone and therefore

Thus

Since ,

Due to ,

Since , thus we arrive at

which contradicts the selection of .
(b) Suppose that there are and such that

where . Therefore

As a consequence,

Let us define, for ,

Therefore

Analogously, we obtain that

Substituting these the inequalities above into (79), it follows that

We conclude that and are bounded by a constant for all , satisfying (77). In consequence, and are also bounded by a constant for all , satisfying (77).
On the other hand, from (78)

In order to bound the right-hand side of the inequalities above, observe that

Now, since is monotone decreasing on , the function defined by

is periodic and monotone decreasing on and belongs to . Therefore,

and thus

Analogously,

where is the periodic function defined by

with

Introducing these results into (84), we arrive at

From (83), we get that and . Therefore

Thus if we select sufficiently close to 1, such that

we get that

We conclude that

which is a contradiction.

Theorem 8. Suppose the same hypothesis on the coefficients , , , , , , and and as in Lemma 7. Furthermore, assume that system (18) does not have constant solutions . Then the operator has a nontrivial (i.e., or is not constant) fixed point in the cone . Equivalently, there exists a nontrivial solution of system (18). Moreover, the fixed point index of the operator on is .

Proof. Observe that is a trivial solution of system (18). However does not belong to the annular region . Thus, Lemmas 6 and 7 and Theorem 3 guarantee the existence of a nontrivial fixed point (i.e., or is not constant) of the operator in the annular region .

5. Numerical Results

As mentioned above, explicit solutions of full system (1) are not known for general coefficients , , , , and . In this section we will describe a numerical strategy to compute approximations of vector solitons of this system in the form

or

where and are positive, even, and real solutions of the system

The existence of such solutions was already established in the previous section.

To approximate these solutions, following the same collocation-spectral strategy by Pipicano and Muñoz Grajales in [49], let us introduce truncated -periodic cosine expansions for and :

where

and let us introduce analogous expressions for the coefficients . This strategy can be used for approximating solutions decaying to zero at infinity, provided that the period is taken large enough.

By substituting expressions (100) into (99), evaluating them at the collocation points

we obtain a system of nonlinear equations in the form

where coefficients and are the unknowns. Nonlinear system (103) can be solved by Newton’s iteration. Computation of the cosine series in (100) and the integrals in (101) is performed using the FFT (Fast Fourier Transform) algorithm. The Jacobian of the vector field is approximated by the second-order accurate formula

where and . We stop the iteration procedure when the relative error between two successive approximations and the value of the vector field are smaller than .

5.1. Description of the Numerical Experiments

In all numerical experiments the number of FFT points is and . Thus the computational domain of the variable is .

Experiment Set 1(vanishing inverse group velocity). We will first consider the regime where , , , , and . Therefore and . The coefficients in system (1) are taken as

with and . Note that the parameter regulates the position of the irregular section of the coefficients , , , , and . The initial values for the Newton’s iteration are

The resulting approximations of after 10 iterations are presented in Figure 1. To verify that we have computed really a vector soliton of system (1), we run the numerical solver to approximate the solutions to system (1), introduced by Muñoz Grajales and Quiceno in [37], with stepsize , FFT points, using as initial data

The result of this computer simulation at is presented in Figure 2, superimposed with the expected position of the vector soliton given by

We observe a good accordance between the profiles of the components and , corroborating that, in fact, we have an approximation of a vector soliton to system (1).

Figure 1: Vector soliton of system (1) obtained after 10 Newton’s iterations and , , , , and .

Figure 2: Real and imaginary parts of the vector soliton of system (1) at . Solid line: numerical solution of system (1) using the numerical scheme in [37]. Pointed line: expected soliton computed using (108).

We repeat the numerical experiments above with the same model and numerical parameters, but now for Gaussian-type coefficients,

The results are presented in Figure 3.

Figure 3: Vector soliton of system (1) obtained after 10 Newton’s iterations and , , , , and .

Experiment Set 2 (nonzero inverse group velocity). We now set , , and . Thus . We point out that, in this case, , in contrast to the simulations above. The model’s coefficients in system (18) are taken as

The resulting approximation to the pair obtained from Newton’s procedure is presented in Figure 4 and the verification of the characteristics of a soliton solution is displayed in Figure 5. Note that, within this parameter regime, the expected soliton solution is given by

We repeat the previous numerical experiment with the same model and numerical parameters but instead use the coefficients

The results are presented in Figure 6. In all cases, we performed a verification of the approximate soliton solution computed using the numerical scheme developed in [37] for solving coupled system (1). The error observed at was around .

Figure 4: Vector soliton of system (1) obtained after 10 Newton’s iterations and , , and .

Figure 5: Real and imaginary parts of the vector soliton of system (1) at . Solid line: numerical solution of system (1) using the numerical scheme in [37]. Pointed line: expected soliton computed using (111).

Figure 6: Vector soliton of system (1) obtained after 10 Newton’s iterations and , , and .

Experiment Set 3 (effect of inhomogeneous coefficients). The next step is to analyze the effect of the model’s coefficients on the geometry of the computed vector solitons of (18). In the simulations presented in Figure 7, we set , , , , , , and . The other numerical parameters are left unchanged. Observe that the amplitude of the approximate solution decreases when .

Figure 7: Effect of the model’s coefficients , , , on the shape of the soliton solutions. Here , , , , , , and , with .

On the other hand, in Figure 8, we let and compute the solution of system (18) for different values of the amplitude . Other parameters are the same as in the previous experiment. However, for , we have to start Newton’s iteration with the following data:

in order to achieve convergence. From these results, we have numerical evidence on the fact that the amplitude of the solutions of system (18) decreases as the amplitude of the coefficients , , , and increases.

Figure 8: Effect of the model’s coefficients on the shape of the soliton solutions. Here , , , , , , and , with (solid line), (pointed line), and (dashed line).

Experiment Set 4 (single-mode solitons). When or , the cross-modulation terms in system (1) are zero, obtaining that or , respectively, satisfy a scalar Schrödinger equation. The single-mode solitons computed are displayed in Figure 9 for the same model and numerical parameters as in the Experiment Set 1.

Figure 9: Single-mode solitons for system (18).

Experiment Set 5 (influence of the inhomogeneous coefficient  ). In Figure 10, we illustrate the effect of the variable coefficient on the shape of the vector solitons of system (1). The model’s parameters are , , , , , and

with and . The initial values for the Newton iteration in this case are taken as

In solid line, we display the vector soliton computed using , that is, , superimposed with the profiles obtained for nonzero values of this parameter with and . Observe that the intensity of the extrainhomogeneous nonlinearities in system (1), produced by the coefficient , affect significantly the amplitude of the vector soliton obtained.

Figure 10: Effect of the inhomogeneous coefficient on the shape of the vector soliton. Here , , , , , , , , , and . Solid line: ; that is, . Pointed line: . Dashed line: .

Experiment Set 6 (interaction of vector solitons). We are interested in illustrating the collision of coupled-mode waves in the regime where and all model’s coefficients are constants. By using the Galilean invariance of system (1) (see Theorem 4), we can write the vector solitons in a more general form, by considering a nonzero translational velocity. Thus, we run again the numerical solver in [37] with an initial profile consisting of two well-separated solitons:

where the initial separation between the two coupled-mode solitons with frequencies , , is , and and are the differing velocities of the two solitary waves produced by this initial condition. Furthermore, and are the initial phases of the left-going pulses. Here and are a coupled-mode solution of (18) obtained by using Newton’s iteration with , , , , , , and initial values and . The numerical parameters for computing the evolution of these pulses with respect to the variable are (length of the spatial computational domain), (time step size), and (number of FFT points). In Figures 11 and 12 we see the collision of the coupled-mode solitons given in (116) with , and in Figures 13 and 14 are the corresponding contour plots. We observe that the two solitons propagate with the expected velocities and , and they elastically (lose little energy) pass through one another; the emergent solitons appear to be unaltered (with approximately the same amplitude and velocity) after the collision. In Figures 15, 16, 17, and 18, we repeat the previous numerical simulation to illustrate the collision of the coupled-mode solitons given in (116) when the initial phases are changed to and . As in the previous computer simulation, the solitons cross each other, but now some exchange of energy is present between the two coupled components , . As a consequence, we find that they does not maintain their amplitudes after the collision.

Figure 11: Collision of coupled-mode solitons with .

Figure 12: Collision of coupled-mode solitons with .

Figure 13: Contour plot of the collision of solitons in Figure 11.

Figure 14: Contour plot of the collision of solitons in Figure 12.

Figure 15: Collision of coupled-mode solitons with and .

Figure 16: Collision of coupled-mode solitons with and .

Figure 17: Contour plot of the collision of solitons in Figure 15.

Figure 18: Contour plot of the collision of solitons in Figure 16.

6. Conclusions

In this paper, using the theory of positive operators in a cone in a Fréchet space [41, 42], we established the existence of vector solitons of the system of coupled Schrödinger equations (1). To achieve this, we extended the techniques in [43] for the case of a family of scalar dispersive equations. We further illustrated the geometry of these solutions by approximating them through a numerical solver, which involves a Newton-type iteration together with a spectral discretization for the spatial variable. Our numerical simulations give evidence on the existence of vector solitons for the two parameter regimes analyzed, in accordance with the theory presented. Further research is needed to study other issues of great interest, such as orbital stability under small initial disturbances of vector solitons and relationship between periodic and nonperiodic finite-energy solutions of system (1).

Competing Interests

The author declares that there are no competing interests regarding the publication of this paper.

Acknowledgments

This research was supported by Universidad del Valle, Calle 13, No. 100-00, Cali, Colombia, under the research project C.I. 71020.

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