Optical Waveguide Theory

Curious wave phenomena that occur in optical fibres due to the interplay of instability and nonlinear effects are reviewed.

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Instabilities, breathers and rogue waves in optics

Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.

Highly confined low-loss plasmons in graphene-boron nitride heterostructures

Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Space-division multiplexing: the next frontier in optical communication

Optical fiber gratings have developed into a mature technology with a wide range of applications in various areas, including physical sensing for temperature, strain, acoustic waves and pressure. All of these applications rely on the perturbation of the period or refractive index of a grating inscribed in the fiber core as a transducing mechanism between a quantity to be measured and the optical spectral response of the fiber grating. This paper presents a relatively recent variant of the fiber grating concept, whereby a small tilt of the grating fringes causes coupling of the optical power from the core mode into a multitude of cladding modes, each with its own wavevector and mode field shape. The main consequence of doing so is that the differential response of the modes can then be used to multiply the sensing modalities available for a single fiber grating and also to increase the sensor resolution by taking advantage of the large amount of data available. In particular, the temperature cross-sensitivity and power source fluctuation noise inherent in all fiber grating designs can be completely eliminated by referencing all the spectral measurements to the wavelength and power level of the core mode back-reflection. The mode resonances have a quality factor of 105, and they can be observed in reflection or transmission. A thorough review of experimental and theoretical results will show that tilted fiber Bragg gratings can be used for high resolution refractometry, surface plasmon resonance applications, and multiparameter physical sensing (strain, vibration, curvature, and temperature).

Tilted fiber Bragg grating sensors

We propose in this paper a graphene-coated tapered nanowire probe providing strong field enhancement in the infrared regimes. The analytical field distributions and characteristic equation of the supported surface plasmons mode are derived. Based on the adiabatic approximation, analytic methods are adopted in the investigation of field enhancement along the tapered region and show well consistence with the rigorous numerical simulations. Both the numerical and analytical results have shown that the graphene-coated nanowire probe could achieve an order of magnitude larger field enhancement than the metal-coated probes. The proposed probe may have promising applications for single molecule detection, measurement and nano-manipulation techniques.

Graphene-coated tapered nanowire infrared probe: a comparison with metal-coated probes

We report in this Letter the resonant coupling mechanism in bending trenched bend-insensitive fiber (BIF) It is found that among the trench parameters, the core-trench distance is predominant for optimized BIF design We reveal that resonant coupling is an intrinsic characteristic of bending trenched BIF, and resonant coupling between the fiber core and the innermost cladding would limit the ultimate bending loss of BIF under tight bend Resonant coupling is also present in double-trenched BIF, and would impair its bending performance

Resonant coupling in trenched bend-insensitive optical fiber

We propose optimizing multifunctional multistage erbium-doped fiber amplifiers (EDFAs) with complex structures by use of a genetic algorithm. With this method, we investigated optimum configurations of C- and L-band gain-flattened multistage EDFAs containing gain-flattening filters and high-loss interstage elements for dense wavelength-division multiplexing systems in detail and compared the amplifiers with various kinds of configurations under different design criteria. With the guidance of optimization results, the roles of all the factors such as pumping schemes, pump-power allocation, component position, and insertion loss in the optimization of EDFAs have been studied, and useful guidelines for optimizations have been provided.

Use of a genetic algorithm to optimize multistage erbium-doped fiber-amplifier systems with complex structures

The complete analytical solutions of the mode-coupling dynamics for seven-core multicore fibers (MCFs) with identical cores is proposed. All the coupling coefficients Cmn of adjacent cores and nonadjacent cores as a function of the structural parameters are investigated. It is shown that the coupling coefficients can decrease by adjusting the structural parameters. In addition, the trench-assisted structure could be used for suppressing cross talk in MCF, and the effective area Aeff can be enlarged without degrading the crosstalk properties. Simulations suggest that low cross talk and/or large Aeff could be achieved by adjusting the trench parameters. Large mode area could be obtained by utilizing small trench (small trench width c, small refractive index difference Δtrench) in the trench-assisted MCF (TA-MCF), whereas low cross talk could be achieved by utilizing larger trench (large trench width c, large Δtrench) in the TA-MCF.

Mode-coupling analysis and trench design for large-mode-area low-cross-talk multicore fiber

In this paper, the influence of mode loss on the feasibility of grating-assisted optical fiber surface plasmon resonance (SPR) refractive index (RI) sensors is investigated. The loss of surface plasmon polarition (SPP) mode plays a key role in the design and implementation of such sensors. It is demonstrated through simulation that the grating length should be smaller than or comparable with the propagation length of SPP mode in order to achieve effective coupling. The loss of SPP mode is the severe limiting factor for the implementation of the grating-assisted SPR-RI sensors. More generally, in order to achieve effective mode coupling with the help of waveguide grating, the grating length is bounded by the shortest propagation length of the modes in lossy waveguides.

Shuisheng Jian

Papers

Graphene-coated tapered nanowire infrared probe: a comparison with metal-coated probes

Resonant coupling in trenched bend-insensitive optical fiber

Use of a genetic algorithm to optimize multistage erbium-doped fiber-amplifier systems with complex structures

Mode-coupling analysis and trench design for large-mode-area low-cross-talk multicore fiber

Influence of Mode Loss on the Feasibility of Grating-Assisted Optical Fiber Surface Plasmon Resonance Refractive Index Sensors