 Research
 Open Access
Symmetric periodic solutions of delaycoupled optoelectronic oscillators
 Chunrui Zhang^{1}Email author and
 Hongpeng Li^{2}
https://doi.org/10.1186/s1366201607550
© Zhang and Li 2016
 Received: 24 September 2015
 Accepted: 11 January 2016
 Published: 29 February 2016
Abstract
Delaycoupled optoelectronic oscillators are considered. These structures are based on mutually coupled oscillators which oscillate at the same frequency. By taking the time delay as a bifurcation parameter, the stability of the zero equilibrium and the existence of Hopf bifurcations induced by delay are investigated, and then stability switches for the trivial solution are found. Conditions ensuring the stability and direction of the Hopf bifurcation are determined by applying the normal form theory and the center manifold theorem. Using the symmetric functional differential equation theories combined with the representation theory of Lie groups, the multiple Hopf bifurcations of the equilibrium are demonstrated. In particular, we find that the spatiotemporal patterns of bifurcating periodic oscillations will alternate according to the change of the propagation time delay in the coupling. The existence of multiple branches of bifurcating periodic solutions and their spatiotemporal patterns are obtained. Some numerical simulations are used to illustrate the effectiveness of the obtained results.
Keywords
 optoelectronic oscillators
 symmetric bifurcation
 stability
 spatiotemporal patterns
 delay
1 Introduction
A system of coupled nonlinear oscillators is capable of displaying a rich dynamics and has applications in various areas of science and technology, such as physical, chemical, biological, and other systems [1–3]. When oscillators are coupled, nonnegligible coupling delays naturally arise because signals in real systems inevitably propagate from one oscillator to the next with a finite propagation speed. These time delays can induce many new phenomena and complex dynamics. Examples include neuronal networks, biological oscillators, and physical models [4–6].
The coupled optoelectronic oscillator is a novel and unique device which simultaneously produces spectrally pure microwave reference signals as in a microwave oscillator [7], and short optical pulses, in a mode locked laser [8–10]. In [8], the authors response of two delaycoupled optoelectronic oscillators. Each oscillator operates under its own delayed feedback. They show that the system can display squarewave periodic solutions that can be synchronized in phase or out of phase depending on the ratio between self and crossdelay times. Furthermore, they show that multiple periodic synchronized solutions can coexist for the same values of the fixed parameters. As a consequence, it is possible to generate squarewave oscillations with different periods by just changing the initial conditions. Furthermore, in [9], the positive delayed feedback is considered, and they show that the scenario arising for positive feedback is much richer than with negative feedback. In [10], the authors model two nonidentical delayline optoelectronic oscillators mutually coupled through delayed crossfeedback. The system can generate multistable nanosecond periodic squarewave solutions which arise through a Hopf instability. They show that for suitable ratios between self and crossdelay times, the two oscillators generate square waves with different amplitude but synchronized in phase, out of phase or with a dephasing of a quarter of the period.
In this paper, considering the special case with \(\gamma=\gamma_{12}=\gamma_{21} \), we attempt to analytically investigate how the time delay can affect its stability, the bifurcations of new solutions when stability is lost, and also spatiotemporal patterns of the bifurcating periodic oscillations. Spatiotemporal patterns reflect the relationship of the evolution of the dynamic behaviors of two subnetworks such as inphase or antiphase oscillation. The results show that the emerging oscillations can exhibit different spatiotemporal patterns sensitive to the delay. It has been shown that even small, compared to the oscillation period, delays may have a large impact on the dynamics of delaycoupled optoelectronic oscillators.
The plan for the article is as follows. In Section 2, we consider the linear stability of equation (1.1) and present some theorems about the region of stability of the trivial solution as a function of the physical parameters in the model. We find some new phenomena such as the stability switch for equation (1.1) which is not mentioned in [8–11]. Coupling can lead synchronization, phase trapping, phase locking, amplitude death, chaos, bifurcation of oscillators, and so on [12, 13]. Since two identical oscillators are coupled symmetrically, the most typical patterns of behavior are perfect synchrony or perfect antisynchrony (in which the oscillators are half a period out of phase with each other); see [14, 15]. In Section 3, we give the \(Z_{2}\)equivariant property of equation (1.1) and the existence of multiple periodic solutions (synchronous (respectively, antiphased). Then we investigate the direction and stability of the Hopf bifurcating periodic solutions in Section 4. In the final section, we present some numerical simulation to support our analytical results. The last section concludes the paper.
2 Stability and bifurcation analysis
 (H_{1}):

either \(1a^{2}>2\varepsilon\) or \(0<1a^{2}<4\varepsilon\);
 (H_{2}):

\(a>1\).
Lemma 2.1
 (1)
If (H_{1}) holds, then equation (2.2) has no purely imaginary roots.
 (2)If (H_{2}) holds, then equation (2.2) has a pair of purely imaginary roots \(\pm i\omega^{+}\) (\(\pm i\omega^{}\), respectively) at \(\tau=\tau^{+}_{j}\) (\(\tau=\tau^{}_{j}\), respectively). Furthermore, the transversality condition is satisfied at \(\tau_{j}^{\pm}\) (\(j=0,1,2,\ldots\)):$$\textstyle\begin{cases} \frac{d\operatorname{Re}(\lambda(\tau))}{d\tau}_{\tau=\tau _{j}^{+},\,\omega=\omega ^{+}}>0, \\\frac{d\operatorname{Re}(\lambda(\tau))}{d\tau}_{\tau=\tau _{j}^{},\,\omega=\omega^{}}< 0. \end{cases} $$
When \(\tau=0\), equation (2.2) has at least one root with positive real part. Since we are concerned about the stability of equation (1.1), we prefer to investigate the relations of \(\tau_{0}^{+}\) and \(\tau_{0}^{}\). From equation (2.6), we have \(\tau_{0}^{+}> \tau_{0}^{}\) if and only if \(\frac{\omega^{}\pi}{\omega^{+}+\omega^{}}>\arccos\frac{1}{a}\).
Applying the above conclusions and bifurcation theorems for functional differential equations, we have the following results presenting the stability and existence bifurcations to the symmetric system (1.1).
Theorem 2.1
 (1)
If either \(1a^{2}>2\varepsilon\) or \(0<1a^{2}<4\varepsilon\), then the zero solution of equation (1.1) is unstable for all \(\tau\geq0\).
 (2)
If \(a>1\) and \(\frac{\omega^{}\pi}{\omega^{+}+\omega^{}}>\arccos\frac{1}{a}\), then \(\tau_{0}^{}<\tau_{0}^{+}\), and there exists a positive integer j such that \(\tau_{j1}^{}<\tau_{j1}^{+}<\tau_{j}^{+}<\tau_{j}^{}\). Then the zero solution of equation (1.1) is asymptotically stable for \(\tau\in (\tau_{0}^{}, \tau_{0}^{+}) \cup(\tau_{1}^{}, \tau_{1}^{+})\cup\cdots\cup(\tau_{j1}^{}, \tau_{j1}^{+})\) and unstable for \(\tau\in[0, \tau_{0}^{})\cup(\tau_{j1}^{}, +\infty)\).
 (3)
If \(a>1\), then for any \(k,l \in\{0,1,2,\ldots\}\), the equation (1.1) undergoes a Hopf bifurcation at \(\tau_{k}^{}\) or \(\tau_{l}^{+}\) for \(\tau_{k}^{} \neq\tau_{l}^{+}\).
Remark 2.1
Theorems 2.1 shows that under the conditions \(a>1\), and \(\frac{\omega^{}\pi}{\omega^{+}+\omega^{}}>\arccos\frac{1}{a}\), there are j switches from stability to instability to stability.
3 Existence of multiple periodic solutions
Theorem 3.1
The trivial solution of system (3.1) undergoes a Hopf bifurcation giving rise to one branch of synchronous (respectively, antiphased) periodic solutions.
Proof
4 Direction and stability of the Hopf bifurcation
Theorem 4.1
Assume that \(\Delta_{1}\) crosses critical values 0. In (4.5), the sign of \(v_{2}\) determines the direction of the Hopf bifurcation: if \({v_{2}} > 0\) (\(v_{2} < 0\)), then the Hopf bifurcation is supercritical (subcritical) and the bifurcating periodic solution exists for \(\tau> {\tau_{j}^{1\pm}}\) (\(\tau< {\tau_{j}^{1\pm}}\)). \({\beta_{2}}\) determines the stability of the bifurcating periodic solution: the bifurcating periodic solution is stable (unstable) if \(\beta_{2} < 0\) (\(\beta_{2} > 0\)).
We also can focus on the case \(\Delta_{2}=0\). Since the conclusions are the same, we shall omit it.
5 Numerical simulations
In this section, we use some numerical simulations to illustrate the analytical results we obtained in the previous sections.
Let \(m=\frac{\pi}{4}, d=1\). Then \(f(x)=\cos^{2}(\frac{\pi}{4}+ \tanh(x))\cos^{2}(m)\).
For parameters \(\gamma=1.155\), \(\varepsilon=1\), using the algorithm in Section 3, we obtain \(\omega^{+}=1.33\), \(\omega^{}=0.75\). Then \(\tau_{0}^{}=0.696< \tau_{0}^{+}=1.97 <\tau_{1}^{+}=4.33 <\tau_{1}^{}=4.87\). From Theorem 4.1, the zero solution is asymptotically stable with \(\tau^{}_{0}<\tau<\tau^{+}_{0}\). When τ meets \(\tau_{j}^{1\pm}\) or \(\tau_{j}^{2\pm}\), the antiphased or synchronous periodic solutions will appear.
6 Conclusion
In this paper, a delaycoupled optoelectronic oscillators described by (1.1) is considered. The effect of the time delay on the linear stability of the system is investigated. Based on the standard Hopf bifurcation theory, we find that as the time delay increases and crosses through the critical values, there exist stability switches in a certain region of the plane of the linear coefficient ε and the coupling strength γ and a branch of periodic solutions bifurcate from the zero equilibrium. By means of the general symmetric local Hopf bifurcation theorem, we not only investigated the effect of a delay of the signal transmission on the pattern formation of model (1.1), but we also obtain some important results about the spontaneous bifurcation of multiple branches of periodic solutions and their spatiotemporal patterns. A remarkable finding is that the spatiotemporal patterns between the two output voltage depend not only on the critical value τ of the coupling time delay, but also on the parameter region where the bifurcation occurs. There are two types of spatiotemporal patterns: one is in phase and the other is in antiphase.
Declarations
Acknowledgements
The authors would like to thank the support of Heilongjiang Province Natural Science Foundations (A2015016).
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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