Nonlinear Fiber Couplers Research Articles

Research on few-mode PAM regenerator based on nonlinear optical fiber loop mirror

In recent years, the demand for network bandwidth has increased significantly, and the capacity of wave division multiplexing (WDM) systems has reached the nonlinear Shannon limit. In order to adapt to the development of future networks, space division multiplexing (SDM) technology attracts more and more attention. In this paper, we put forward a novel structure of pulse amplitude modulation(PAM) regenerator based on few-mode nonlinear optical fiber loop mirror (FM-NOLM) for the first time, and theoretically analyze the working principle for few-mode reshaping. It can regenerate degraded PAM signals and improve transmission performance in SDM system. The detailed design steps of the regenerator are given, in which the sulfide highly nonlinear fiber and multimode coupler are used to build up the FM-NOLM and their mode characteristics are simulated by COMSOL software. The parameters of the regenerator are determined by taking the few-mode optical fiber supporting LP<sub>01</sub>, LP<sub>11</sub>, and LP<sub>21</sub> modes as an example, and then the power transfer function (PTF) curve of each mode for PAM signals is calculated. We simulate and analyze the noise reduction ratio (NRR) performance of the few-mode regenerator for PAM-4 signals, and compare with the case of single mode fiber. Our simulation shows that: (1) for each spatial mode of PAM signal, all regenerative levels have the same consistent power transfer performance; (2) for the input signal-to-noise ratio (SNR) greater than 20 dB, the NRR for each mode can exceed 3 dB, and increase with the input SNR at the slope of about 1.2; (3) the NRR difference between the three modes is less than 1.1 dB for the same input SNR. In order to illustrate the reshaping function of the regenerator, we also present the power distribution histograms for PAM-4 signals before and after regeneration when the input SNR is 25 dB. This scheme proposed here has some advantages over the existing regenerators in the applicability for the long-haul SDM system with high spectral efficiency and regeneration of any level number of PAM signals in theory due to its uniform multi-level regeneration function, but also is capable of being extended to the wavelength domain for higher transmission capacity.

Efficiency and intensity noise of an all-fiber optical parametric oscillator

In this paper, we simulate numerically an all-fiber optical parametric oscillator comprising a highly nonlinear optical fiber and wavelength division multiplexing couplers. The model is based on a set of coupled nonlinear Schrodinger equations that spectrally resolve the gain bandwidth of the pump, signal, and idler waves. We show that such a high wavelength resolution is necessary to obtain the correct threshold pump powers, stationary output powers, and relative intensity noise. Finally, we compare seeded and unseeded configurations and find that the seeded configuration generates low-noise idler output at conversion efficiencies of up to 10% below the threshold power for unseeded operation, but that both seeded and unseeded oscillators produce comparable results above the threshold.

The effect of relative phase on the stability of temporal dark soliton in $${{\mathcal {PT}}}$$ PT -symmetric nonlinear directional fiber coupler

In this paper, we numerically investigate the effect of relative phase on the stability of dark solitons in $${\mathcal {PT}}$$ -Symmetric nonlinear directional coupler (NLDC), by considering gain in the bar and loss in the cross in the range of $$\theta =0^\circ$$ to $$180^\circ$$ . The $${\mathcal {PT}}$$ -Symmetric perturbed eigenfunctions are used to study the soliton stability. The results of simulations are shown that in the first half region of the relative phase the soliton is unstable while in the second one, is stable. In the stable region gain and loss cancel each other and also the perturbed eigenfunctions have no effect on solitons while in the unstable region solitons are amplified in the bar and attenuated in the cross except for some small intervals which the roles are changed. The behavior of such perturbation can be interpreted as self all-optical phase soliton switching and optical logic gates.

Pt-Symmetric Nonlinear Directional Fiber Couplers with Gain and Loss for Ultrashort Optical Pulses

In this paper, we study the higher-order nonlinear Schrdinger (HNLS) equations for a directional fiber optic coupler with the third-order dispersion (TOD), self-steeping (SS) and stimulated Raman scattering (SRS) effects and gain in one waveguide and loss in the other. We have reduced the system equations to an uncoupled HNLS equation which has bright and dark soliton solutions. It is shown that the Hamiltonian for this equation has a PT-symmetric form. Furthermore the equilibrium points are found and the discussion about their stability is presented.

Modulation instabilities in asymmetric nonlinear fiber coupler

We have investigated the modulation instability (MI) in a nonlinear optical coupler using a generalized model describing the pulse propagation of two-core fiber. In particular, we discuss some influence of the MI of asymmetric nonlinear fiber coupler. The results show that input power and nonlinear coefficients of two cores influence the maximum value of the MI gain and MI bandwidth. When nonlinear coefficients of two cores are both negative, better modulation stability can be obtained than other situations. In particular, if the negative nonlinear coefficients are equal, the modulation instabilities will not appear.

Modelling a nonlinear optical switching in a standard photonic crystal fiber infiltrated with carbon disulfide

In this letter, a numerical analysis is developed for the propagation of ultrafast optical pulses through a standard photonic crystal fiber (PCF) consisting of two infiltrated holes using carbon disulfide (CS2). This material is a good choice since it has highly nonlinear properties, what makes it a good candidate for optical switching and broadband source at low power compared to traditional nonlinear fiber coupler. Based on supermodes theory, a set of generalized nonlinear equations is presented in order to study the propagation characteristics. It is shown in this letter that it is possible to get optical switching behavior at low power and how the dispersion, as well as, the two infiltrated holes separation influence this effect. Finally, we see that supercontinuum generation can be induced equally in both infiltrated holes despite no initial excitation at one hole.

Stability of Temporal Dark Soliton in PT-Symmetric Nonlinear Fiber Couplers in Normal Dispersion Regime

In this paper, we present analytical soliton solutions in a nonlinear PT-symmetric coupler with gain in one fiber and loss in the other one in the normal dispersion regime. As usual, we derive a temporal dark soliton solution for the coupler in the normal dispersion regime. We study the stability of the soliton solution by imposing a perturbation in the system by using the eigenvalue method. The results show that there exist two forms of perturbed soliton solutions, bright and dark. The numerical calculations show that the perturbed solutions in the bright form are stable and the dark form are unstable. Previous works in bright solitons show that there exists only one perturbed bright soliton solution which is stable only in particular regions.

Non-linear switching based on dual-core non-linear optical fiber couplers with XPM and Raman intrapulse applied to femtosecond pulse propagation

In this work, we investigated the optical switching process for three shapes of femtosecond pulses (soliton, Gaussian and super-Gaussian) propagating inside a symmetrical dual-core non-linear directional coupler by simulating their propagation via the coupled non-linear Schrödinger equations. In all simulations, we considered the dispersive effects of second and third order, besides the self-phase modulation and self-steepening non-linear effects. We studied three scenarios for each of the three pulse shapes under investigation. In the first scenario, we added only cross-phase modulation (XPM); in the second approach, we added only Raman scattering; in the third one, we combined both. The study was performed for distinct polarization modes and for different values of the Raman factor, with power range varying from 1 to 300 W. We noted that the XPM non-linear effect results in a decrease in the critical power threshold, whereas the Raman scattering causes an increase. For the first scenario (only XPM effect), the critical power threshold reduced from 113.72 to 104.69 W for the soliton pulse, from 111.49 to 100.77 W for the Gaussian and from 92.79 to 80.47 W for the Super-Gaussian pulse shape. For the second scenario (only Raman scattering), the critical power increased for a Raman factor varying from 1 to 10 fs, and the three pulse shapes reached thresholds above 150 W from a 5 fs factor, reaching more than 200 W for the super-Gaussian pulse as the Raman factor increased. For the third scenario (with both effects combined), we highlight that for a fixed XPM factor of 2, the critical power remained unchanged with the variation of the Raman factor. Hence, we observed that the Super-Gaussian pulse reached lower values for critical power when compared to the other pulse shapes.

Numerical investigation of dark soliton switching in asymmetric nonlinear fiber couplers

Switching characteristics of dark soliton in asymmetric nonlinear directional couplers are numerically studied. Every individual effect, responsible for producing asymmetry in nonlinear directional coupler, has been analyzed in detail. It is found that group-velocity mismatch and the difference in group-velocity dispersions between the two cores have no impact on dark soliton, which can be specified as an advancement over the bright solitons. The effects of phase velocity, the difference in effective mode areas and the linear coupling coefficient on the dark soliton switching are also investigated.

Dark soliton switching in nonlinear fiber couplers with gain

Abstract In this paper, we present a study of numerical simulations for all-optical dark soliton switching in nonlinear directional couplers with gain additions in two waveguides whilst considering intermodal dispersion and its higher-order effects. The effects of possible combinations of gain addition in the bar channel as well as in the cross channel of the nonlinear couplers are theoretically presented. In addition, the consequence of second-order coupling coefficient dispersion based on coupled mode theory for dark soliton switching is numerically depicted.

三芯光纤耦合器的开关和传输特性研究

三芯光纤耦合器的开关和传输特性研究

Bifurcation Analysis at a Singular Soliton State in Fiber Couplers

Abstract The system describes pulses in nonlinear fiber couplers. It has the family (U1+λ, -U1+λ), -1 < λ < ∞, of soliton states (that is, homoclinic solutions to the origin), where For λ ≥ 1, the equilibrium (0, 0) is not hyperbolic and therefore the soliton state (U1+λ, −U1+λ) can be qualified as “singular”. In N. Akhmediev and A. Ankiewicz [1], it is observed numerically that a branch of homoclinic solutions bifurcates subcritically at λ = 1 from the family (U1+λ, −U1+λ). The aim of the present paper is to give a rigorous proof of the existence of this bifurcation, as desired in A. Ambrosetti and D. Arcoya [3]. A particular feature of the present problem is that the linearized system at (U2, −U2) has a non-constant bounded solution that does not vanish at infinity. Hence the bifurcating homoclinic solutions have a transient “spatial” region where they are well described with the help of this bounded function. Moreover the decay to 0 is governed by two different scales, the larger one originating from the singular aspect of (U2, −U2). The existence proof developed here relies on the “broken geodesic” technique to match the inside transient region with the outside region.

All-Optical Steering of Light Through Nonlinear Twin-Core Photonic Crystal Fiber Coupler at 850 nm

We intent to investigate the dynamics and steering characteristics of an optical pulse propagation through photonic crystal fiber coupler (PCFC) using the projection operator method (POM) at 850 nm. For the proposed study, we begin with the analysis of silica PCFC design. From the proposed design, we examine the propagation of Gaussian pulse through PCFC, by means of coupled nonlinear Schrodinger equations. By employing POM, we derive the equations of motion describing the dynamics of the pulse parameters, called collective variables (CVs), namely the amplitude, pulse width, phase and chirp. The corresponding results obtained through POM are compared and verified numerically by split step Fourier method (SSFM). Furthermore, we propose a novel chloroform filled PCF structure that operates as a single mode at 850 nm, featuring an enhanced dispersion and nonlinearity for efficient switching, thus enabling to accomplish switching at a low input pulse power and low loss over shorter distances. Efficient optical pulse switching through both the designs are demonstrated through the simulation of the transmission curve. From the transmission curve, we infer that efficient switching can be achieved at relatively low input power in the proposed chloroform filled PCF.

Influence of the retarded response on an ultrafast nonlinear optofluidic fiber coupler

We demonstrate by numerical simulations of coupled generalized nonlinear Schrödinger equations that the input pulse duration as well as the retarded material response have a crucial impact on the properties of an ultrafast nonlinear optofluidic fiber coupler. This device is composed of two waveguides in close vicinity embedded in a photonic crystal fiber which are filled with a highly nonlinear liquid. We show that in particular the critical peak power above which the coupling between the waveguides is suppressed increases dramatically for short input pulses and long characteristic response times of the liquid. We establish a simple model which describes these effects with high accuracy.

Study of the Performance of an All-Optical Half-Adder Based on Three-Core Non-Linear Directional Fiber Coupler Under Delayed and Instantaneous Non-Linear Kerr Responses

Recently, much attention has been given to the influence of the relaxation process of the non-linear response, because the usual assumption of instantaneous non-linear response fails for ultra-short pulses, and additional contributions coming from non-linear dispersion and delayed non-linearity have to be taken into account. This article presents a numerical analysis of the symmetric planar and asymmetric planar three-core non-linear directional fiber couplers operating with a soliton pulse, where effects of both delayed and instantaneous non-linear Kerr responses are analyzed for implementation of an all-optical half-adder. To implement this all-optical half-adder, eight configurations were analyzed for the non-linear directional fiber coupler, with two symmetric and six asymmetric configurations. The half-adder is the key building block for many digital processing functions, such as shift register, binary counter, and serial parallel data converters. The optical coupler is an important component for applications in optical-fiber telecommunication systems and all integrated optical circuit because of its very high switching speeds. In this numerical simulation, the symmetric/asymmetric planar presents a structure with three cores in a parallel equidistant arrangement, three logical inputs, and two output energy. To prove the effectiveness of the theoretical model for generation of the all-optical half-adder, the best phase to be applied to the control pulse was sought, and a study was done of the extinction ratio level as a function of the Δ > parameter, the normalized time duration, and the Sum and Carry outputs of the (symmetric planar/asymmetric planar) non-linear directional fiber coupler. In this article, the interest is in transmission characteristics, extinction ratio level, normalized time duration, and pulse evolution along the non-linear directional fiber coupler. To compare the performance of the all-optical half-adders, the figure of merit of the logic gates was used. All results were obtained numerically, considering a simple model for generation of an all-optical half-adder.

All optical method of developing OR and NAND logic system based on nonlinear optical fiber couplers

Logic gates are the fundamental building blocks in any digital data processing system. Several all optical logic operations are already proposed by scientists and technologists. Here the authors proposed a new method of all optical logic operation based on nonlinear directional coupler. Effective optical switching is achieved by modifying the coupling length between the coupler waveguides by means of an optical signal.

Optical solitons switching in asymmetric dual-core nonlinear fiber couplers

With study on the switching characteristics in a asymmetric dual-core nonlinear fiber coupler, it is found the switch efficiencies when the dispersion values of two fibers have opposite signs are much higher than those when they have same signs. Meanwhile, when the dispersion values of two fibers have same signs, the switch efficiencies become much higher if the dispersion value of one core is decreased. In addition, the switch threshold power becomes lower than the one with the same sign if the nonlinearity coefficient of two fibers is opposite. The result shows that asymmetric coupler has much higher switch efficiencies and lower switch threshold power than symmetric coupler.

Effects of intrapulse stimulated Raman scattering and intermodal dispersion on short pulse propagation in a nonlinear two-core PCF coupler

The switching dynamics of ultrashort pulse under the effects of intrapulse stimulated Raman scattering (ISRS) and intermodal dispersion in a nonlinear two-core photonic crystal fiber coupler is investigated theoretically. A general model is proposed that includes all the significant linear and nonlinear effects. In particular, we demonstrate with numerical examples how the interaction of ISRS and intermodal dispersion affects the pulse shape.

Effects of Second-Order Coupling Coefficient Dispersion on Short-Pulse Propagation and Switching in an Active Two-Core Nonlinear Fiber Coupler

The propagation and switching of short pulses in an active two-core fiber nonlinear directional coupler have been investigated theoretically by using the split-step Fourier method. The analysis highlights the effects of the second-order coupling coefficient dispersion, the linear gain coefficient, and the finite-gain bandwidth on the switching and propagation of short pulses. The research indicates that the linear gain can sharpen the switching characteristic and reduce considerably the switching threshold power, as well as significantly increase the switching efficiency with the influences of the second-order coupling coefficient dispersion. However, both the second-order coupling coefficient dispersion and the finite-gain bandwidth degrade the switching characteristics. In addition, the finite-gain bandwidth of linear gain not only suppresses significantly the pulse compression and amplification caused by the linear gain coefficient, but also suppresses effectively the frequent pulse fluctuation on pulse propagation induced by the second-order coupling coefficient dispersion; consequently, as in the case of the passive fiber coupler, optical pulses tend to restore periodical coupling propagation in active two-core fiber coupler.

超短光脉冲在有源三芯非线性光纤耦合器中开关和传输特性的数值分析

超短光脉冲在有源三芯非线性光纤耦合器中开关和传输特性的数值分析