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We introduce and investigate the feasibility of a novel iterative blind phase noise inter-carrier interference (ICI) mitigation scheme for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The ICI mitigation scheme is performed through the combination of frequency-domain symbol decision-aided estimation and the ICI phase noise time-average approximation. An additional initial decision process with suitable threshold is introduced in order to suppress the decision error symbols. Our proposed ICI mitigation scheme is proved to be effective in removing the ICI for a simulated CO-OFDM with 16-QAM modulation format. With the slightly high computational complexity, it outperforms the time-domain average blind ICI (Avg-BL-ICI) algorithm at a relatively wide laser line-width and high OSNR. [ABSTRACT FROM AUTHOR]

Abstract: We design a compact ring resonator of surface modes based on photonic crystals (PCs). The structure is formed by sandwiching a surface mode ring waveguide (SMRW) into two parallel surface mode waveguide (SMW) based on two dimensional (2D) PCs. The SMRW is created on the surface of a circular photonic crystal (CPC) structure, where the wave propagates with high transmission efficiency. As a fundamental mode is introduced in the input SMW, at certain frequencies, the SMRW modes are enhanced because of resonance and the light-waves are coupled to the output SMW. It is demonstrated by the simulation results that the surface mode ring resonator has a low radiation loss with a very small size because of the good wave-guiding of surface mode based on PCs, and can be used in the future wavelength division multiplex (WDM) optics communication systems. [Copyright &y& Elsevier]

Abstract: A novel three-port channel add/drop filter consisting of two waveguides and two cavities is proposed. One is used for a resonant tunneling-based channel add/drop operation from the bus waveguide to the add/drop waveguide, while the other is used to realize the wavelength-selective reflection feedback in the bus waveguide. By means of coupled mode theory in time, the conditions to achieve 100% add efficiency are derived thoroughly. Based on these theoretical analysis, the channel add filter and some other multi-channel filters are designed in two-dimensional photonic crystals (2D PCs) with square lattice of dielectric rods in air. The numerical results by using the finite-difference time-domain (FDTD) method demonstrate almost complete channel add/drop tunneling at resonance via the three-port systems. [Copyright &y& Elsevier]

Adaptive orthogonal frequency division multiplexing (OFDM) systems are promising for high-bit-rate short-reach communication by optimizing the allocation of modulation format and power to each subcarrier. Because adaptive OFDM systems involve multiple modulation formats, automatic modulation format identification (AMFI) is significant for subsequent signal processing and symbol decision of OFDM receivers. In this work, we proposed and experimentally demonstrated a blind modulation format identification technique based on constellation diagrams achieved from channel estimation of OFDM system. In the proposed AMFI scheme, the identification feature is the number of signals' constellation clusters. First, we use the peak-density clustering algorithm to track the centers of signals' constellation clusters by plotting the density–distance graph, where the clusters' centers have much larger density and distance values than that of the other clusters' points. Then, we use the K nearest neighbor regression algorithm to automatically calculate the number of signals' constellation clusters. Finally, we evaluated and measured the identification accuracies of QPSK, 8-QAM, 8-PSK, 16-QAM, 64-QAM and 128 QAM signals in simulation and experiment. [ABSTRACT FROM AUTHOR]

We report the existence and stability of 1-charge discrete vortex solitons in linear scaled-space square lattices with photorefractive self-focusing nonlinearity. By scaling the square lattice along the edge and diagonal, we obtained rectangular and diamond lattices. In both settings, it is shown that the vortices can be stable in a moderate power region, and the vortices with high power are unstable, suffering from oscillatory instabilities. The structure of the rectangular potential strongly affects the profile of the vortex solitons, leading to distinct intensity asymmetry, phase dislocation, and exponential instability in the low power region. Correspondingly, the vortex profile could be well maintained in diamond lattices. Fascinating, even the low power unstable vortices suffered from both oscillatory and exponential instability in diamond lattices, they have a much weaker instability than counterparts in rectangular lattices. [ABSTRACT FROM AUTHOR]

We demonstrate the periodic interconversion evolution of the double-charge vortex (DCV) and out-of-phase (OOP) quadruple solitons in two-dimensional (2D) optically induced Lieb photonic lattices by single-site excitation under appropriate self-focusing nonlinearity conditions. The numerical results show that the DCV evolves into a rotating quasi-vortex soliton, utilizing an OOP quadrupole-like beam as a transition state to reverse the topological charge and the direction of rotation periodically; The OOP quadrupole beam whose incident orientation along the crystalline c -axis or the diagonal axes of the lattices self-traps into a stable localized mode nearly invariant during propagation, whose Fourier spectrum reveals that they are localized states close to the high-symmetry points X and M of Brillouin Zone. By imposing an initial orientation slightly twisted relative to the lattices' crystalline c -axis, the localized quadruple state tends to evolve into a dynamical rotating DCV and undergoes periodic charge-flipping when the direction of rotation is reversed. Additionally, analysis of different twisting angle shows that the oscillation period will become longer when the initial orientation approaches the lattices' crystalline c -axis. Our findings could give some quantitative insight into the experimental feasibility of observing such phenomena. • The Double-charge vortex and the quadrupole beam will undergo periodic conversion. • Double-charge vortex occurs a flipping of topological charge. • Periodic oscillations can be obtained after changing the initial orientation. • Oscillation rate becomes slower when the angle approaches the crystalline c -axis. • Provide a possibility for a nontrivial type of all-optical switching. [ABSTRACT FROM AUTHOR]

Abstract: The effects of pump noise on noise characteristics of single-pump and two-pump fiber optical parametric amplifiers (FOPAs) are analyzed theoretically. The results show that the increased noise of FOPAs caused by the pump noise is related to the structural parameters of amplifiers. Therefore, the structural parameters of FOPAs should be optimized to balance the conflicting requirement of high gain and low noise. [Copyright &y& Elsevier]

Abstract: In the paper, based on the pump depleted model of two-pump fiber optical parametric amplifier (FOPA), the simulations are done to indicate the effect of amplifier parameters on gain characteristics, such as the input signal power, the wavelength separation between the two pumps, the fiber length, the nonlinear coefficient and the fourth-order dispersion of fiber. By optimizing the amplifier parameters, when FOPA attains low gain-saturation, the FOPA has a higher gain, a broader flat gain bandwidth and a lower noise figure. However, with the enhancement of gain-saturation the parametric gain decreases, the flat gain bandwidth becomes narrow and the noise figure also increases. [Copyright &y& Elsevier]