Coupled Mode Theory (CMT)

This article delves into the intricate concept of metaphor, particularly within the context of political cartoons. Guided by Conceptual Metaphor Theory (CMT), developed by Lakoff and Johnson as the theoretical framework, this study aims to examine how metaphors in political cartoons function as essential components in conveying complex socio-political messages. Through a meticulous exploration of the likeness between two seemingly unlike entities—perceiving one through the lens of the other—this study argues that cartoonists leverage visual metaphors, along with a diverse array of humor, symbolism, and irony, to illustrate societal conflicts and critique political realities, all while effectively conveying nuanced messages in a concise manner. A significant achievement of this study is the development of two comprehensive tables that categorize prevalent source and target domains in the metaphoric conceptualization of political cartoons, accompanied by relevant examples that elucidate their application in this context. The findings reveal how cartoonists utilize these domains to critique social realities and evoke cognitive responses from audiences. By categorizing and providing context for these metaphoric structures, this study not only enhances the understanding of visual metaphors but also contributes significantly to the scholarship on visual rhetoric in political communication.

Time domain analysis is performed on the ultra-short pulsed beam propagating through the fiber bragg grating (FBG) structures. Widely known one way Beam Propagation Method (BPM) is formulated in time domain and applied on the uniform and non-uniform FBG structures. Spectral analysis, which is one of the noteworthy benefit of the TD analysis, is performed from pulsed beam at different stages, and consequently, transmission and reflection coefficient is analyzed. Dimensions of the FBG structures are chosen from the FimmProp software tools used for the same kind of structures, and compared thereof. A satisfactory comparative results are found while doing the comparison.

Practical demonstrations of diode laser emission from the broad surface area rather than from the cleaved facet of the wafer are relatively recent. This is so despite the fact that the concepts are many years old. The vertical-cavity approach was demonstrated by Melngailis in 1964, and the grating surface emitting and folded-cavity approach were reported by many authors in the mid to late 1970s. Many, perhaps most, of the concepts discussed in this book were around for many years before they were actively pursued. The advances over the last ten years, mainly in materials, are largely responsible for the capability to implement the ideas presented into working devices. We can look forward to continued progress in materials, processing, and design during the next decade and can expect to see semiconductor laser performance outstripping even our present dreams.
There are now frequent reports on all three principal types of diode laser
and diode laser array surface emitters in the literature. Unlike edge-emitting
semiconductor lasers, the surface emission approach allows the use of mass production techniques throughout the fabrication process. In addition, the surface emission approach allows testing of the completed devices at the wafer level, before dicing and packaging. These same capabilities yielded tremendous reductions in cost and enormous increases in the performance and reliability of transistors and other solid state electronic devices. Surface-emitting approaches also allow the integration of single or numerous lasers to form photonic integrated circuits or high power, monolithic, two-dimensional arrays.
Because of the now extensive literature on surface-emitting diode lasers
and arrays and the proposed and emerging applications of these exciting and
practical new devices in a variety of systems, we feel that this in-depth book
covering the field will be useful to researchers, users, and students interested in the field of lasers, electrooptics, and optical communication. Recent work has been motivated by numerous goals, including low power, integrated sources to replace electrical interconnects with optical interconnects for ultra large-scale integrated circuits; two-dimensional, independently addressable laser arrays for neural networks; steerable output beams for optical computers; high power with large emitting areas for pumping solid-state lasers; and coherent, single frequency, high-power operation with a controlled output beam for space communication and second harmonic generation.
The information in this book is intended to provide the reader with both
knowledge about fundamental concepts and the present state of the art of surface-emitting lasers. There are definitive chapters on vertical-cavity, etched facet-mirror, and grating surface emitters. Additional chapters treat the operation of Bragg grating couplers; edge-emitting diode laser arrays; the theory of phase locking, modes, and beam steering of surface-emitting arrays; external methods of phase locking arrays; coherence and phase control in laser arrays; and thermal considerations in two-dimensional surface-emitting arrays.
We have fortunately been able to enlist some of the leading researchers and
developers of surface-emitting diode lasers to contribute to this book. We wish to thank them for many interesting and productive discussions in connection with the preparation of this work. We also wish· to thank RCA Laboratories (now the David Sarnoff Research Center), Princeton, New Jersey, for providing both of us with talented collaborators, up-to-date equipment, and a pleasant environment in which most of our work described herein was carried out.

Description of integrated mode profile by determine of κ, γ, δ parameters as functions of the propagation constant (β) and effective refractive index (neff). The profile can be seen from E (x) formula for each guide TE (Transverse Electric) modes. Assumptions given in this slab waveguide is used for wavelength (λ) 1.55 μm, the thickness (d) of the core is 0.9 μm with a type of symmetric step-index slab waveguide, refractive index of n1 is 3.5 and refractive index of n2 is 3, also n3= n1. The results of analysis are presented in ...

We analytically and numerically demonstrate the existence of strongly localized frontlike and quasirectangular solitary waves in anharmonic chains described by the discrete nonlinear Schrödinger equation. The stability of discrete fronts is predicted by direct linear analysis and numerically confirmed. The dynamics of interaction between bright pulses and front solutions is investigated. ͓S0163-1829͑99͒01910-4͔ PHYSICAL REVIEW B

In this study, we have theoretically investigated transmission properties of transverse electric fields at visible region frequencies in one dimensional defect Metallic-Dielectric photonic crystals. We examined the effect of photonic crystals thickness, layer numbers, layer refractive indexes and defects on transparency. We use OptiFDTD software for simulations. OptiFDTD software uses finite-different time-domain method.

The eigenvalue equations are obtained for TE and TM modes in an embedded dielectric slab waveguide. The normalized propagation constant is determined by using eigenvalue equations. The effective waveguide structure is determined by refractive indexes obtained by means of an effective index method. The spatial graphs are plotted for the least low order modes E(x) and E(y) in the waveguide.

One-dimensional pattern-forming systems with a quadratic nonlinearity and a long-wave zero mode in the governing equations are considered. The pattern-generating instability, which may be both steady and oscillatory, is assumed to set in at a finite wave number. Physical examples are the instability of a front of the laser-sustained evaporation of a solid and the instability of seismic waves in a viscoelastic medium. A system of generalized Ginzburg-Landau equations for the complex envelope of a fundamental harmonic and for the real slowly relaxing zero mode are derived in a general form [for the case of the steady instability, essentially the same equations have been proposed earlier by Coullet and Fauve, Phys. Rev. Lett. 55, 2857 (1985)]. Using these equations, stability of spatially periodic patterns is investigated. The main result is that the stability band is anomalously narrow in comparison with the classical Eckhaus band. For the case of the steady instability, the evolution of long-wave modulations of the patterns is investigated in the geometric-optics approximation. It is demonstrated that, unlike the usual Ginzburg-Landau equation, the equations derived in the present work allow for modulation profiles of a permanent shape. The profiles may be transient layers moving at a constant velocity, or quiescent periodic "superstructures, " including a "soliton" as a limiting case. At least some of those profiles can be stable.

We demonstrate that nonlinearity plays a constructive role in supporting the robustness of dynamical localization in a system which is discrete in one dimension, and continuous in the orthogonal one. In the linear regime, time-periodic modulation of the gradient strength along the discrete axis leads to the usual rapid spread of an initially confined wave packet. Addition of the cubic nonlinearity makes the dynamics drastically different, inducing robust localization of moving wave packets. Similar nonlinearity-induced effects are also produced in the presence of a combination of static and oscillating linear potentials. The predicted dynamical localization in the nonlinear medium can be realized in photonic lattices and Bose-Einstein condensates.

A recently observed stable regime in the form of periodically colliding counterpropagating wave packets (pulses) in an annular convection channel at very small positive overcriticalities is described analytically in terms of coupled Ginzburg-Landau equations. First, the existence of this regime is demonstrated in the framework of the simplest system including only the group-velocity difference, weak gain, and nonlinear dissipative coupling between two modes. In this approximation, the shape of the counterpropagating waves remains indefinite. It is demonstrated that additional dispersive terms, regarded as a small perturbation, provide shaping of the wave packets and also give rise to the deviation of the phase velocity from that for purely linear waves.

An additional transparent dielectric layer allows to create an antiresonant light-wave field distribution in layered structure, containing absorbing magneto-optic (MO) film. This results in nonreciprocal phase shift enhancement with simultaneous decrease of nonreciprocal losses and low level of total losses. Phase shifts, arising at the reflection of electromagnetic wave on the boundary of magnetic materials in equatorial Kerr effect geometry at oblique incidence for direct and inverse directions, are different. The difference of these shifts is the nonreciprocal phase shift (NPS). The NPS value is proportional to the specific rotation of MO material which is small in the optical frequency range. MO materials with large specific rotation have a large optical absorption. Reflectance losses and amplitude nonreciprocity, which restrict Kerr effect applications, arise due to absorption. MO layered structures allow to increase the NPS [1]. The interference enhancement of the NPS in thin films was discussed both for partial [2,3] and for total internal reflection [4]. It has been found that the NPS could be large when the wave resonant conditions in the MO film take place (analogous to ones in the resonator Fabry-Pete). Resonance increasing the NPS is followed by analogous increasing of losses in MO film, as will be shown below. So, it is impossible to have a gain in this way. We have analyzed a two-layer system, that consists of absorbing MO film and additional transparent dielectric layer. This layer allowed to create different configurations of electromagnetic wave field in the MO film. The idea to use antiresonant interference conditions cf such a system is put forward. The expression for a reflection coefficient of a two-layer system (see for example Ref. [5]) was analyzed analytically and numerically. Fresnel reflection coefficients for MO film boundaries, which appear in it, were calculated in Ref. [6]. It was taken into account that their phases for partial waves, incident from the MO film on its boundaries, must have opposite signs (this was not done properly

The purpose of this paper is to simulate and analyze the spectral characteristics of the fiber Bragg grating (FBG) to obtain narrow bandwidth and minimization side lobes in reflectivity. Fiber Bragg grating has made a big revolution in telecommunication systems. The existence of fiber Bragg grating is needed when an optical fiber amplifier and filter are used. They can be used as band reject filter or band pass filter for optical devices. The model equations of the cascaded uniform fiber Bragg grating and different cascaded apodization functions such as, Hamming apodized fiber Bragg grating, Barthan apodized fiber Bragg grating, Nuttal apodized fiber Bragg grating, Sinc apodized fiber Bragg grating and Proposed apodized fiber Bragg grating are numerically handled and processed via specially cast software to achieve maximum Reflectivity, narrow bandwidth without side lobes.

We present the investigation of an all-optical demultiplexer based on two dimensional photonic crystal structures for communications systems. The device is based on filtering technology which is achieved by introducing a new kind of squared silicon nitride (Si3N4) microcavity designed by altering the shape of the central rod. The optimum coupling conditions are obtained by removing a line of dielectric rods where minimum of channel spacing and narrow bandwidth signal are obtained. Finally, demultiplexing is done by varying the reflection distance of the filter allowing us for the first time to select four wavelengths with channel spacing lower than 0.1 nm in total footprint of 205 um 2 . The device performances are evaluated by using 2D finite-difference time-domain (FDTD) and PWE based simulations. Results indicate that the average output transmission efficiency and quality factor at the wavelength around 1.55 um can get more than 94.56% and 17634, respectively. In addition an extr...

Highly efficient second-harmonic generation can be achieved by harnessing resonance effects in microring resonator structures. We propose an angular quasi-phase-matching scheme based on the position dependence of polarization inside the ring resonator.

We investigate whether, and to what extent, the physical phenomenon of long-lifetime resonant electromagnetic states with localized slowly-evanescent field patterns can be used to transfer energy efficiently over non-negligible distances, even in the presence of extraneous environmental objects. Via detailed theoretical and numerical analyses of typical real-world model-situations and realistic material parameters, we establish that such a non-radiative scheme can lead to "strong coupling" between two medium-range distant such states and thus could indeed be practical for efficient medium-range wireless energy transfer.

The purpose of this paper is to simulate and analyze the spectral characteristics of the fiber Bragg grating (FBG) to obtain narrow bandwidth and minimization side lobes in reflectivity. Fiber Bragg grating has made a big revolution in telecommunication systems. The existence of fiber Bragg grating is needed when an optical fiber amplifier and filter are used. They can be used as band reject filter or band pass filter for optical devices. The model equations of the cascaded uniform fiber Bragg grating and different cascaded apodization functions such as, Hamming apodized fiber Bragg grating, Barthan apodized fiber Bragg grating, Nuttal apodized fiber Bragg grating, Sinc apodized fiber Bragg grating and Proposed apodized fiber Bragg grating are numerically handled and processed via specially cast software to achieve maximum Reflectivity, narrow bandwidth without side lobes.

A planar all-dielectric metamaterial made of a double-periodic lattice whose unit cell consists of a single subwavelength dielectric particle having the form of a disk possessing a penetrating hole is considered. The resonant states in the transmitted spectra of the metamaterial are identified considering modes inherent to the individual cylindrical dielectric resonator. A correlation between the asymmetry in the particle's geometry, which arises from the off-centered displacement of the hole and the formation of the Mie-type and trapped modes, is established. The advantages of using a coaxial-sector notch instead of a round hole for the trapped mode excitation are explained.

Bulk surface Plasmons resonance devices have been researched for several decades. These devices have found a special niche as high-sensitivity refractive index sensor in biomedical applications. Recent advances in guided wave devices are rapidly changing the capabilities of such sensors, not only increasing convenience of use but also opening opportunities due to their versatility. This paper reviews many of these devices and presents some of their salient features.

Numerical simulation of the properties of guided and leaky modes in W type multilayer cylindri cal optical fibers is performed. The dependences of the losses of leaky modes on the parameters of the W type optical fiber are studied. It is shown that, for certain structures of the W fiber, the leaky modes have substan tial effect on the behavior of propagating radiation. It is shown that, when the mode structure of the radiation in W type optical fibers is studied, both guided and leaky modes should be taken into account.

This paper presents the modeling and characterization of an optical fiber grating for maximum reflectivity. Grating length and change in refractive index are the critical parameters in contributing to the performance of fiber Bragg grating. The wavelength chosen for analysis is from the third window to minimize the attenuation. The reflection spectra, bandwidth and side lobes strength were analyzed with different lengths and change in refractive index. The simulations are based on solving coupled mode equations by transfer matrix method that describes the interaction of guided modes.

This study proposed the modeling of fiber Bragg grating as a sensor for the optimum reflectivity with minimum discontinuities at the end of center wavelength. Recent advancement in optical sensor technology, make it necessary to generate the optimum results with minimum error. In this analysis, dependence of grating output spectra on various parameters has been studied for minimum bandwidth, side lobes and maximum reflectivity. In previous studies effect of length and other parameters has been studied. In this paper author has included the effect of modulation depth also. Study shows that grating length and modulation depth of refractive index of core are very critical factors which significantly alter the grating characteristics. Here low absorption third window of optical communication is used for the sake of low attenuation. All the simulation has done using GratingMOd software using transfer matrix method.

The transverse coupling between the optical dielectric waveguides is modeled and analyzed using the Improved Coupled Mode Theory. The coupling between the modes is established via the evanescent part of the modal field propagating in one waveguide which penetrates the adjacent waveguide. The coupling phenomena is analyzed numerically by evaluating the average overlap integrals, coupling coefficients, asynchronism factor, and coupling length using the Improved Coupled Mode Theory at strong and weakly guiding conditions. The performance of the Improved Coupled Mode Theory is tested and compared to the conventional Coupled Mode Theory.

We report the design procedure for a broadband multichannel cavity-less optical isolator composed of a triangular perturbed nonlinear Parity Time (PT) symmetric lattice. The interplay between the nonlinearity and PT-symmetric lattice that results in nonreciprocal transmission enables us to design the isolator outputs and inputs positions as desired. In the transmitting regime, a number of solitons that are simultaneously launched into individual inputs positioned appropriately, on the waveguides' gain sides in the periodic lattice, swing along the waveguide until they are self-trapped along the respective waveguides axes, where the isolator outputs are located. In the isolating regime, however, the solitons that are launched into the inputsi.e., the outputs of the transmitting regime-propagate along the waveguides axes, until they exit from the new outputs. Simulations show that the neighboring soliton trajectories in each regime are completely isolated and so are the forward and reverse trajectories in each waveguide cell. Moreover, operation of this cavity-less PT-symmetric isolator that is designed on an inhomogeneous slab waveguide does not suffer from the narrow frequency band limitation, unlike their cavity-based counterparts.

In this paper, a heterostructure photonic crystal multichannel drop filter based on ring resonators and microcavities is presented. This structure has been made in the form of a two-dimensional square lattice with two regions with refractive indexes of 3.464 and 3.86. The refractive indexes are so chosen as to allow the easy and practical fabrication of the device. The presented heterostructure photonic crystal multichannel drop filter consists of a waveguide, two ring resonators and a microcavity. This microcavity is placed at the end of the bus waveguide. The ring resonators have been installed in two regions with different refractive indexes. These ring resonators act as energy couplers, and at their resonance frequencies, they capture the electromagnetic energy which is transmitted in the bus waveguide. Filter characteristics have been obtained by using the finite difference time domain method. Finally, we will demonstrate that in the optimal structure, at ports B and D (vertica...

A detailed numerical study of low-loss silicon on insulator (SOI) waveguide bend is presented using the fully three-dimensional (3D) finite-difference time-domain (FDTD) method. The geometrical parameters are optimized to minimize the bending loss over a range of frequencies. Transmission results for the conventional single bend and photonic crystal assisted SOI waveguide bend are compared. Calculations are performed for the transmission values of TE-like modes where the electric field is strongly transverse to the direction of propagation. The best obtained transmission is over 95% for TE-like modes.

In this paper, the structure being investigated consists of periodic layers of InGaP containing a defect region in air hole and GaAs. Finite difference time-domain calculations were performed to show the influences of hole radius on the group velocity (V g), quality (Q) factor and transmission of the structure. As well as effect the hole radius deviation on the Q factor. Also, we will investigate property of the defect region on the band structures.

We propose a very simple but general method to construct solutions of Maxwell equations propagating with a group velocity v gr = c. Applications to wave guides and a possible description of the known experimental evidences on photonic tunneling are discussed.

Nano-photonics is an emerging area of optical materials, which would take the optointegrated circuits towards progress. Photonic crystal (PC) based power splitters are useful constituents for the design of photonic integrated circuits (PICs). They are very important devices for connecting different building blocks on an integrated optical chip. In this paper, a two-dimensional PC Y-junction power splitter (21 × 15 µm) based on the resonance effect with circular air holes etched on a hexagonal lattice with a period a is proposed. The plane wave expansion (PWE) and finite difference time domain (FDTD) techniques are used for analyzing the structure. The simulation results show that the optimum resonance occurs when the radius of the defect hole is 0.3a, leading to the maximum and equal power distribution.

Superluminal light group velocity was formerly reported in anomalous dispersion, nonlinear amplification of light pulse, high-gain lasers' cavities, and waveguides. Motivated by a recent observation of light acceleration in optical microfiber [1], the possibility of attaining the light group velocity exceeds its value in vacuum is investigated. The investigation of superluminal velocity is in tapered optical microfiber that has a radius decreases with propagation axis by a factor 10-3. Our results show the possibility of attaining superluminal group velocity in this microfiber at length of about 1080µm. At this length the instantaneous acceleration of light is found to be 13×10 19 m/s 2 which its corresponding Unruh temperature is 0.527K.

An optical fiber sensor for ethanol detection in exhaled breath has been developed. It has been fabricated by functionalizing a Long Period Grating with a metal-organic framework, ZIF-8. The sensor's response was tested by exposure to exhaled breath of a person before and after the ingestion of alcoholic drinks, showing a higher wavelength difference between the resonance bands in the second case. Further work will analyze cross-sensitivity towards temperature, relative humidity and carbon dioxide.

Eigenmodes of simplified square-latticed photonic crystal waveguides with several layers of periodical rods on both sides of the optical channels are obtained by plane wave expansion (PWE) method and supercell method in this paper. In case of one/two layers of rods, the propagating modes have quasi-periodical variations of intensities in the regions outside the optical paths. As the number of layers increases to equal or more than three, the optical power is well confined to the optical channel and the modal profile approaches the same limit.

A new algorithm for waveguide and leaky modes spectrum calculation in multilayer planar waveguides is proposed. The method is of high computational efficiency due to the preliminary search for propagation constants approximation. Numerical efficiency of this algorithm is demonstrated for several waveguide structures. Waveguide and leaky modes propagation constants calculated for these structures using proposed method are in good agreement with previously published results.

In this article, we apply the coupled-mode theory to vertically-coupled micro-disk resonators presenting an asymmetric distribution of refractive index and a multilayer separation region between the two waveguide cores, resulting in an effective propagation constant phase-mismatch in the coupling region. We introduce a criterion which, given the coupler overall permittivity distribution, clarifies how to best choose the individual decomposition index profiles among the various possible solutions. Following our recent experimental demonstration we subsequently exploit the derived decomposition to evaluate the theoretical transmission characteristics of an AlGaAs/AlOx-based structure as function of wavelength and as function of the position of the resonator relative to the access waveguide.We show that the resonant dips of the intensity transmission, spaced by the cavity FSR, are modulated by an envelop which governs the coupling regime of the resonator-waveguide system.

In this study, we have theoretically investigated transmission properties of transverse electric fields at visible region frequencies in one dimensional defect Metallic-Dielectric photonic crystals. We examined the effect of photonic crystals thickness, layer numbers, layer refractive indexes and defects on transparency. We use OptiFDTD software for simulations. OptiFDTD software uses finite-different time-domain method.

Large and periodically corrugated optical waveguide structures are shown to possess specific modal regimes of slow-light propagation that are easily attainable. The very multimode nature of the coupling is studied by employing coupled-mode theory and the plane-wave expansion method. Given a large enough light cone, associated with a surrounding medium with low enough refractive index, we notably identify a critical slowdown regime with an interesting bandwidth-slowdown product. Essential features of these original systems are further explored: the nature of the coupled modes, the role of gain, symmetry effects, polarization, and relation with photonic-crystal systems. Practical systems are introduced using finite-difference time-domain methods, which provides first-order rules for the use of the above phenomena and their implementation in devices.

Coupling of two-dimensional (2D) vertical-cavity surface-emitting lasers (VCSELs) to give a coherent supermode is described. The top metal layer of a stained-layer InGaAs quantum well VCSEL structure was patterned laterally by depositing various metals with different optical reflectivities. This lateral reflectivity patterning defined a 2D laser array sharing the same ‘‘supercavity’’. It is shown that these 2D arrays oscillate in a stable single, coherent 2D supermode. This was achieved with a simple planar process and without significant deterioration of threshold current and efficiency relative to an equivalent broad-area VCSEL.

In this paper, a high efficiency 2-D T-shaped photonic crystal beam splitter is proposed. It consists of a square lattice of GaAs rods (n = 3.4) embedded in air. The photonic crystal structure proposed can be used for 1550 nm wavelength, which is an important wavelength for optical fiber data transmission. Finite difference time domain (FDTD) simulation results demonstrate that a conventional T-junction can only provide 78% transmission coefficient (39% for each branch) for the incident light, while the proposed T-shaped splitter transmits over 90% of the incident light beam (over 45% from each branch) in the single mode region of waveguide. Especially it transmits nearly 98% (49% from each branch) of the input light in the wavelength of 1550 nm. In other words, the proposed devise shows higher beam splitting efficiency and a wider range of flatness of transmission power spectrum in comparison with previous works.

Recently a finite-difference frequency-domain (FDFD) formulation has been reported for the dispersion analysis of uniform waveguides loaded with anisotropic dielectrics characterized by a diagonal tensor [4]. This formulation, which leads to an eigenvalue problem for the propagation constant ␥, involves four field components. This paper shows that this formulation can be improved by taking advantage of the fact that the structures considered are strictly bidirectional waveguides and thus the dispersion analysis problem can be formulated as an eigenvalue problem for ␥ 2 , involving only two field components. This improved formulation is applied to several waveguiding structures and a comparison of the results with those obtained by the commercial simulator Agilent-HFSS 5.6 show good agreement.

It is commonly known that stable bright solitons in periodic potentials, which represent gratings in photonics/plasmonics, or optical lattices in quantum gases, exist either in the spectral semi-infinite gap (SIG) or in finite bandgaps. Using numerical methods, we demonstrate that, under the action of the cubic self-focusing nonlinearity, defects in the form of "holes" in two-dimensional (2D) lattices support continuous families of 2D solitons embedded into the first two Bloch bands of the respective linear spectrum, where solitons normally do not exist. The two families of the embedded defect solitons (EDSs) are found to be continuously linked by the branch of gap defect solitons (GDSs) populating the first finite bandgap. Further, the EDS branch traversing the first band links the GDS family with the branch of regular defect-supported solitons populating the SIG. Thus, we construct a continuous chain of regular, embedded, and gap-mode solitons ("superfamily") threading the entire bandgap structure considered here. The EDSs are stable in the first Bloch band, and partly stable in the second. They exist with the norm exceeding a minimum value, hence they do not originate from linear defect modes. Further, we demonstrate that double, triple and quadruple lattice defects support stable dipole-mode solitons and vortices.

Modulation instability MI effects are analyzed in the context of WDM optical communication systems. We show how the interplay between four-wa¨e mixing and modulation instability results in a modulation of the channel power as a function of the channel spacing as well as the propagated distance. This modulation can cause se¨ere power penalties.

How much time does a wave packet spent in tunneling a barrier? Quantum mechanical calculations result in zero time inside a barrier. In the nineties analogous tunneling experiments with microwaves were carried out confirming quantum mechanics. Electron tunneling time is hard to measure being extremely short. However, quite recently the atomic ionization tunneling time has been measured. Experimental data of photonic, phononic, and electronic tunneling time is available now. It appears that the tunneling time is a universal property in the investigated time range of twelve orders of magnitude.

The title of this article is misleading. The authors have investigated a resonator but not a tunneling barrier see also Refs.\cite{Winful2} The measured superluminal group velocity and discussed is that studied on a Lorentz-Lorenz oscillator by Sommerfeld and Brillouin a hundred years ago \cite{Brillouin}. It is similar to the faster than light experiment by Wang et al. based also on

The excitation of high quality factor asymmetric Fanotype resonances on a double layer metafilm structure is investigated through numerical simulation. The paper demonstrates that it is possible to design simple structures capable to sustain a very high quality factor resonance by reducing their radiation losses. An equivalent circuit formed by two linearly coupled resonant RLC circuits is extracted in an attempt to explain the observed Fano resonance through classical circuit theory.

In this work, the monitoring of the etching process up to a diameter of 30 µm of two LPFG structures has been compared, one of them had initially 125 µm, whereas the second one had 80 µm. By tracking the wavelength shift of the resonance bands during the etching process it is possible to check the quality of etching process (the 80 µm fibre performs better than de 125 µm fibre), and to stop for a specific cladding mode coupling, which permits to obtain an improved sensitivity compared to the initial structure.

Two different realizations of time-reversal experiments of ultrafast waveforms are carried out in real time by use of four-wave mixing arrangements of spectrally decomposed waves. The f irst, conventional, method is based on phase conjugation of the waveform's spectrum and achieves time reversal of real amplitude waveforms. The second arrangement of the spectrally decomposed waves spatially inverts the waveform's spectrum with respect to the optical axis of the processor and achieves true time reversal for complex-amplitude ultrafast waveforms. We compare and contrast these two real-time techniques.