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In this work, we carry out theoretical investigation of the dispersion relationship both with the radius of the microsphere cavity and wavelength for the first two radial-order mode based on the theory of microcavity dispersion. The results show that the size of the microcavity corresponding to the zero dispersion point of the higher order mode is larger than that of lower order mode at the certain wavelength. The microsphere cavities with slight deformations, which have rich modes and high Q values, have been fabricated and optical frequency comb (OFC) in them with small size is realized by means of mode coupling. We have achieved OFC in cavities as small as 105 m in diameter by assistance of the theoretical study. The total dispersions are -7.1 MHz in 1550 nm and -14.2 MHz in 1310 nm, which are both in the normal dispersion region.

Based on the coupling mode theory and numerical simulation, intramode-group coupling in orbital angular momentum (OAM) fibers with elliptical deformation is analyzed in depth. The elliptical deformation not only introduces a time delay difference between even and odd modes of the same high-order vector mode but also destroys the structure of the high-order vector modes; the structural damage depends on the strength of the elliptical deformation. When the elliptical deformation is relatively small, the structural damage of high-order vector modes can be ignored and coupling can be thought as occurring only between two OAM modes composed of the same high-order vector mode. While the elliptical deformation becomes larger, the structural damage cannot be ignored, coupling appears among four OAM modes in the same OAM mode group, and the coupling between two OAM modes composed of the same high-order vector mode is more serious. The results will pave the way for the establishment of accurate propagation models and the optimization of the OAM fiber design.

A Fabry-Perot open resonator (FPOR) is a tool dedicated to broadband accurate microwave and millimeter-wave characterization of dielectric sheets. Despite the various advantages of that method, one of the major challenges limiting its applicability is unintentional coupling of Gaussian modes of interest with a particular family of parasitic higher order modes. The coupling affects the resonance frequency of the resonator loaded with the material under test, thus disturbing the extracted permittivity. It has already been shown that spectral properties of these parasitic modes can be predicted using a recently developed electromagnetic model of a FPOR based on the scattering matrix method. It is shown in this article that the systematic analysis of experimental results supplemented with accurate numerical computations allows us to suppress the impact of mode coupling on the measured permittivity.

The whispering gallery modes (WGMs) are studied theoretically in a proposed magneto-optic (MO) silicon micro-ring resonator with a radial magnetic field. The dispersion relations between both the frequency and polarization of the resonant mode and the gyration coefficient are achieved through Finite-Element Method (FEM). Nonreciprocal polarization coupling is observed between the intrinsic quasi-TM and quasi-TE modes, due to their different frequency shift coefficients originating from the magneto-optic effect. This polarization coupling results in the rapid changes of linear polarization degrees of the supermodes. The nonreciprocal mode coupling is universal and will be of practical value for the MO optical integration.

Through soliton excitation in a fiber integrated nematic liquid crystals cell, we demonstrated an all-optical response and electrical tunable structure for light coupling and mode conversion in few mode fibers. The light is guided and controlled from the transmitting to the receiving end, making use of the soliton induced waveguide channel, the so called self-guided effect. Both a near-infrared laser, i.e., signal light, and a visible laser, i.e., the probe light, are introduced to demonstrate the self-guided effect and mode conversion, which results match well theoretically and experimentally. Our results show that, the transmitting light can be guided and selectively coupled into lower or higher order modes of the receiving fiber by steering its ‘walk-off’ through voltage control. Besides, the proposed mode converting operation can work in both the polarization-mixed and polarization-separated regimes, and is realizable in a remarkably wide wavelength band.

Nonlinear dynamics of mutually injected twin-microring semiconductor lasers are studied numerically and experimentally. For the twin-microring laser, a bridge waveguide directly connecting two identical microrings is utilized to realize mutual optical injection or mode coupling. The coupling coefficient is related to the waveguide parameter and influences the mutual injection process dramatically. By adjusting the frequency offset between two microrings, four-wave mixing, injection locking, periodic, and multi-period oscillation states are observed from the experimental lasing spectra. The plentiful nonlinear dynamical states are induced by the mode interaction between the two microrings and can be well explained by the rate equations with mutual optical injection.

The Langevin equation is used to investigate the state of mode coupling in step-index plastic optical fibers under varied temperature. The numerical integration of the Langevin equation is based on the computer-simulated Langevin force. The solution matches the experimental data reported previously. We find that elevated temperatures of plastic optical fibers are strengthening mode coupling. We show that, solving the Langevin equation (stochastic differential equation), one can treat a mode coupling in heated multimode step-index plastic optical fibers, which is a result of fiber’s intrinsic random perturbations.

We study orbital angular momentum (OAM) mode coupling in ring-core fibers (RCFs) due to elliptical shape deformation. We introduce a coupling model based on numerical mode solver outputs of perturbation. We show improved predictions in calculating coupling strength compared to the classical modeling approach. Our model captures and quantifies the disparate behaviors of coupling in lower and higher order degenerate OAM modes. The ideal orthogonality of modes is undermined by fiber imperfections. Our model predicts the OAM order at which the orthogonality within OAM mode pair is maintained despite elliptical deformation. We use our coupling model to simulate propagation effects and compare the performance of two fibers (thin and thick RCF) designed under the same constraints. Our numerical propagation results show different performance for the two fibers under the same level of elliptical deformation. This model uncovers distinct digital signal processing requirements for these two types of fiber, and predicts their signal-to-noise ratio penalty. For each fiber, we examine the large number of supported modes and find the optimal subset of mode groups, i.e., the groups with the lowest penalty. We show that this optimal subset is different from that predicted during the fiber design optimization.

The polarization dependence of cladding mode coupling in fiber Bragg grating (FBG) plays a significant role in the excitation of surface plasmon resonances (SPR) for sensing applications. In this work, we numerically analyze the polarization dependent cladding mode coupling enabled by localized fiber Bragg gratings (LFBGs), focusing on the properties that generate from the modal structures and the position of the localized refractive index modifications (RIMs). Our analyses reveal that, both centric and eccentric LFBGs allow strong coupling to cladding modes. In centric LFBGs, coupling to EH/ ${\mathrm {HE}}_{1,m}$ modes dominates, whereas in eccentric LFBG, additional coupling to high azimuthal order cladding modes is permitted. Strong polarization dependence is found in eccentric LFBG: for certain cladding modes, coupling from one polarization can be suppressed due to the cancellation of negative and positive mode coupling. More interestingly, we found that cladding modes with even and odd azimuthal orders response differently to input polarizations, which accounts for the unique polarization dependence (i.e., neighboring resonances exhibit opposite polarization preference in the transmission spectra) in eccentric LFBG. Our numerical results can be verified by the good agreement with existed experimental reports.

The rich design landscape of optical fibers offers many opportunities for refractive index optimization. In particular, the refractive index profiles of multimode fibers (MMFs) and multicore fibers (MCFs) govern the behavior of spatial and polarization modes, including their bandwidth, mode count, mode coupling, modal dispersion, chromatic dispersion, and mode-dependent loss. In this article, we obtain update equations to optimize the shapes of fiber refractive index profiles for various applications using gradient descent. Starting with an initial fiber designed according to standard best practices, our methods iteratively modify the refractive index profile to improve upon the initial design. Interestingly, we see that optimization can sometimes yield large improvements in key fiber properties, such as a 20-fold reduction in the root-mean-square modal group delay spread, even though the index profile is changed very little from the initial design. We show that our methods can be used to tailor the mode coupling, modal dispersion and chromatic dispersion properties to desired values, and that optimization can be successfully applied over a large bandwidth. We provide illustrative design examples, including an optimization of a graded-index MMF with low group delay spread for long-haul mode-division-multiplexed transmission. Our algorithms can be used to design novel fibers or optimize existing fibers for next-generation transmission systems.

We proposed a slanted grating suitable for RGB bands which is polarization-independent and has a high diffraction efficiency of −1st order under normal incidence. We used Rigorous Coupled Wave Analysis (RCWA)and simulated annealing (SA) algorithm to optimize grating parameters and the simulation results showed that the -1st order diffraction efficiency of TE and TM polarizations in the RGB band under normal incidence both exceed 83%, which can be used for three-dimensional (3D) display. Then, by using rigorous modal theory, simplified modal method (SMM), and mode coupling theory, we analyzed the reason for the high-efficiency of −1st order diffracted light. Moreover, the numerical simulation of the simplified modal method is consistent with the rigorous coupled wave analysis of the small slanted angle grating. Therefore, the simplified modal method can be used for small slanted angle gratings as its physical analysis proved that small slanted angle gratings and rectangular gratings have equivalence to a certain extent. This work not only presents numerical solutions of slanted grating with high efficiencies over a wide range of RGB wavelength, but also enables us to have a deeper understanding of the diffraction of slanted gratings.

A completely novel electromagnetic model of a Fabry–Perot open resonator is proposed and validated in this article. The idea that led to the invention of this model is to split the analysis of the resonator into a few nonresonant sections, thus, substantially alleviating computational effort involved in the analysis of such a large high- $Q$ structure. Each section of the resonator, including mirrors, is represented as a multimode scattering matrix and the whole resonator is considered as a cascade of two-port networks. Mode analysis applied in the proposed model allows representing fields propagating along the resonator in the form of an orthogonal set of cylindrical waveguide modes. Due to versatility and large computational efficiency, the method can be used to account for electromagnetic phenomena that cannot be solved with commonly used models of the Fabry–Perot open resonator based on a scalar paraxial approximation. The obtained solution is validated against experimental data. It is also shown that the spurious mode coupling occurring in the measurement of electrically thick dielectric sheet materials can be explained using the proposed scattering matrix method.

By employing the Langevin equation, we have examined a mode coupling in graded index plastic optical fiber. The numerical integration of the Langevin equation is based on the computer-simulated Langevin force. The solution matches the experimental data reported previously. We have shown that by solving the Langevin equation (stochastic differential equation) one can treat a mode coupling in graded index plastic optical fibers, which is result of fiber's intrinsic random perturbations.

A novel technique to measure differential group delay and modal birefringence along few mode fibers is presented. The method relies on the analysis of the spectral correlation between two properly-selected sections of the fiber's Rayleigh-backscattered trace, acquired with an optical frequency domain reflectometer. Experiments performed on a two-mode-group fiber confirm the viability of the method.

In this work, the design and optimization of compact taper is presented to enable coupling of infrared light in the C-band with the nano-photonic silicon-on-insulator (SOI) integrated optical waveg...

This paper presents a 2-D vector microdisplacement sensor based on mode coupling between single-mode fiber (SMF) and two-mode fiber (TMF). The offset alignment between SMF and TMF along the radial direction of optical fibers can influence the mode coefficients of LP01 mode, LP11o mode, and LP11e mode in TMF, including mode power proportion coefficients and mode relative phase coefficients. Modal decomposition based on digital holography is used for obtaining these mode coefficients. The variations of mode coefficients of all modes in TMF with the change of the offset between SMF and TMF (i.e., microdisplacement) in two vertical directions are simulated. Experimental results are consistent with simulation results, displaying that the microdisplacement in the range from −5 to $5~\mu \text{m}$ along the positive or negative $x$ -axis or $y$ -axis direction can be judged by using the proposed scheme based on mode demodulation technology.

Photonic bound states in the continuum (BICs) are eigenmodes with an infinite lifetime, which coexist with a continuous spectrum of radiating waves. BICs are not only of great theoretical interest but also have a wide range of practical applications, e.g. in the design of optical resonators. Here, we study this phenomenon in a new integrated nanophotonic element consisting of a single dielectric ridge terminating an abruptly ended slab waveguide. This structure can be considered as an on-chip analog of the Gires-Tournois interferometer (GTI). We demonstrate that the proposed integrated structure supports high-Q phase resonances and robust BICs. We develop a simple but extremely accurate coupled-wave model that clarifies the physics of BIC formation and enables predicting BIC locations. The developed model shows that the studied BICs are topologically protected and describes the strong phase resonance effect that occurs when two BICs with opposite topological charges annihilate.

The multi-channel high-efficiency absorber in the mid-infrared band has broad application prospects. Here, we propose an SiC-photonic crystal (PhC) heterostructure-SiC structure to realize the absorber. The absorption characteristics of the structure are studied theoretically. The results show that the structure can achieve high-efficiency multi-channel absorption in the mid-infrared range. The absorption peaks come from the coupling of the dual Tamm phonon polariton (TPhP) mode formed at the interface between the two SiC layers and the photonic crystal, and the optical Tamm state (OTS) mode formed in the PhC heterostructure. By adjusting the thickness of the air dielectric layer and the period of the PhC in the heterostructure, the mode coupling intensity can be regulated; thereby, the position and intensity of the absorption peak can be adjusted. In addition, the absorption peaks of TE and TM polarized light can be controlled by changing the incident angle. Adjusting the incident angle can also control the excitation and intensity of the epsilon-near-zero (ENZ) phonon polariton mode produced by TM polarized light. This kind of light absorber may have potential applications in sensors, filters, modulators, switches, thermal radiators, and so on.

This chapter illustrates the coupled mode theory, which describes the mutual light wave interactions between two propagation modes. The chapter illustrates the derivation of coupled mode equations based on perturbation theory, which is first presented and then concrete methods calculating the coupling coefficients for several practically important devices are explained in detail. Finally, several waveguide devices using directional couplers such as Mach-Zehnder interferometers, ring resonators, and bistable devices are described. In axially uniform optical waveguides, a number of propagation modes exists. These propagation modes are specific to each waveguide and satisfy the orthogonality conditions between the modes. When the electromagnetic field distributions after mode coupling do not differ substantially from those before coupling, the propagation characteristics of the coupled waveguides can be analyzed by the perturbation method. The mode-coupling coefficient κ is very important for the calculation of the coupling length in the co directional coupler and the reflectivity in the contra directional coupler. The mode-coupling effect in the directional coupler can be analyzed by the interference phenomena between the even and odd modes in the five-layer slab waveguide.

In this work, a high-sensitivity terahertz (THz) biosensor is achieved by using a graphene/Bragg reflector hybrid structure. This high-sensitivity THz biosensor is developed from the sharp Fano resonance transmission peak created by coupling the graphene Tamm plasmons (GTPs) mode to a defect mode. It is found that the proposed THz biosensor is highly sensitive to the Fermi energy of graphene, as well as the thickness and refractive index of the sensing medium. Through specific parameter settings, the composite structure can achieve both a liquid biosensor and a gas biosensor. For the liquid biosensor, the maximum sensitivity of &gt, &nbsp, 1000&nbsp, °/RIU is obtained by selecting appropriate parameters. We believe the proposed layered hybrid structure has the potential to fabricate graphene-based high-sensitivity biosensors.

The simulation and design of an oversized, two-dimensional, periodic surface lattice (2D PSL) interaction cavity used within a 0.395 THz, pulsed radiation source is presented. Powerful, pulsed radiation with good spectral purity is demonstrated. The device is well-suited for use in DNP-NMR spectroscopy and has numerous other applications due to the scalability of the interaction cavity allowing for radiation output at different frequencies. The device exploits the coupling of volume and surface fields in the 2D PSL interaction cavity to excite a single cavity eigenmode. The observed output power is 13.5kW and the efficiency of the device is 27%

Multimode fibres provide a promising platform for boosting the capacity of fibre links and the output power of fibre lasers. The complex spatiotemporal dynamics of multimode beams may be controlled in spatial and temporal domains via the interplay of nonlinear, dispersive and dissipative effects. Raman nonlinearity induces beam cleanup in long graded-index fibres within a laser cavity, even for CW Stokes beams pumped by highly-multimode laser diodes (LDs). This leads to a breakthrough approach for wavelength-agile high-power lasers. However, current understanding of Raman beam cleanup is restricted to a small-signal gain regime, being not applicable to describing realistic laser operation. We solved this challenge by experimentally and theoretically studying pump-to-Stokes beam conversion in a graded-index fibre cavity. We show that random mode coupling, intracavity filtering and Kerr self-cleaning all play a decisive role for the spatio-spectral control of CW Stokes beams. Whereas the depleted LD pump radiation remains insensitive to them.

This article assembles a distributed feedback (DFB) cavity on the sidewalls of the optical fiber by using very simple fabrication techniques including two-beam interference lithography and dip-coating. The DFB laser structure comprises graduated gratings on the optical fiber sidewalls which are covered with a layer of colloidal quantum dots. Directional DFB lasing is observed from the fiber facet due to the coupling effect between the grating and the optical fiber. The directional lasing from the optical fiber facet exhibits a small solid divergence angle as compared to the conventional laser. It can be attributed to the two-dimensional light confinement in the fiber waveguide. An analytical approach based on the Bragg condition and the coupled-wave theory was developed to explain the characteristics of the laser device. The intensity of the output coupled laser is tuned by the coupling coefficient, which is determined by the angle between the grating vector and the fiber axis. These results afford opportunities to integrate different DFB lasers on the same optical fiber sidewall, achieving multi-wavelength self-aligned DFB lasers for a directional emission. The proposed technique may provide an alternative to integrating DFB lasers for applications in networking, optical sensing, and power delivery.

Optical communication continues to be a major component of the Optical Sciences over 50 years. As a result, advancements in light wave science continues at a rapid pace. A commercial driver is the need to communicate at faster speeds, over longer distances and with greater global connectivity. This demand is ever increasing due to societies demand for more rapid global communication Optical fiber communication relies on the propagation of optical pulses in only one single guided mode. The ability to simultaneously propagate pulses in tens of other guided modes is possible enabling a hundred - fold capacity increase. However light scattering among these simultaneously propagating pulses can rapidly deteriorate signal quality to the point that no signal can be extracted. Principal mode propagation provides a means to overcome this signal deterioration by enabling light localization at the output of the fiber. This methodology brings together two important scientific frontiers, optical communication and the light localization. In this paper, we review this methodology as it relates to optical communication capacity, but also as it relates to light localization. We also review the characterization of these modes both theoretically and experimentally.

We report nonlinear dynamics in directly connected twin circular-sided square microcavity semiconductor lasers with mutual optical injection. Rich nonlinear dynamics including injection locking, four-wave fixing, and multi-period oscillation states are observed experimentally by adjusting the mode frequency offset between two circular-side square microcavities. Internal optical injection or mode coupling can be realized directly in the connected twin circular-sided square microcavity lasers, which effectively relaxes device processing techniques of the integrated microcavities for photonic integration.

Photonic processors are pivotal for both quantum and classical information processing tasks using light. In particular, linear optical quantum information processing requires both large-scale and low-loss programmable photonic processors. In this paper, we report the demonstration of the largest universal quantum photonic processor to date: a low-loss 12-mode fully tunable linear interferometer with all-to-all mode coupling based on stoichiometric silicon nitride waveguides.

We investigate the scattering properties of a paired periodic array of silver-dielectric metasurfaces. We report Fano resonances and Rabi splitting in the spectral response of the scattering parameters. We show that tuning the refractive index of the dielectric scatterer defines whether identical or mismatched resonances are excited in the array.

We demonstrate extraordinarily spectrally selective narrowband mid-infrared radiation absorbance and thermal emittance with resonant peak FWHM ~ 124 nm at l = 5.73 mm, corresponding to a Q-factor of ~ 92:3. This was achieved by harnessing mode coupling between a plasmonic metal-insulator-metal (MIM) metasurface and molecular vibrational mode resonances, with coupling constants ranging from h ~ 3.9% to 6.6%. In addition, thermal radiation emissivity is in close accordance to the metamaterial absorbance spectrum, as described by Kirchhoff’s law of thermal radiation, and furthermore, emission was not angle dependent, unlike that exhibited by grating-based emitters. The experimentally investigated MIM structures remained stable up to a 250C heating temperature. MIM metamatrials with strong and spectrally tailored vibrational coupling behaviors represent a new paradigm in photo-thermal energy conversion. The experimentally observed pronounced resonant coupling behaviour was well described by finite-difference time-domain simulations of the plasmonic structure, where molecular vibration contributions were modeled using the Lorenz oscillator approximation.

We discovered that a specific transmission mode in the fiber waveguide will split into two modes at near cut-off conditions when the fiber is coated with lossy dielectric nanofilms with higher refractive index. One of the two splitting modes is high lossy while the other is slightly lossy. We defined the “mode splitting coefficient” to describe the degree of mode splitting. We found that the biggest mode splitting coefficient was obtained at the mode cut-off wavelength using finite element method. Finally, we put forward an explanation towards the mode splitting phenomenon and expounded the relationship between the mode splitting and mode coupling.

By solving the power flow equation, we investigate the influence of mode coupling on space division multiplexing capability of three multimode step-index silica optical fibres with a different stre...

A method for designing fiber Bragg gratings (FBGs) with arbitrary spectra is proposed according to the Fourier mode coupling theory. The index perturbation in a FBG is derived from the inverse Fourier transform of the amplitude and phase spectra, where the phase spectrum is set as odd symmetry, and the amplitude spectrum results from the square-root and inverse hyperbolic tangent of its arbitrarily desired reflectivity. In order to verify the design method, some FBGs with arbitrary triangular-, double-triangular-, Gaussian- and comb-like-reflectivities were designed and calculated on a general computer, which has the complexity of O(Nlog2N) where N is the number of calculation points. The reflectivities of the designed FBGs were analyzed by using the transfer matrix method or the direct Fourier transform (FT) method, agreeing with the desired reflectivities. The proposed design method was confirmed to be highly fast and sufficiently accurate for designing arbitrary FBGs.

The eigenvalue problem is solved for a longitudinally inhomogeneous waveguide with impedance walls. The cross-section method is applied to reduce the problem to an infinite set of ordinary differential equations for amplitudes of basis modes. As a numerical example, a tapered metallic cavity of currently available THz gyrotron is considered. The combined effect of mode coupling (conversion) and ohmic wall losses on electromagnetic properties of the gyrotron cavity is considered and discussed.

Mode Division Multiplexing (MDM) increases aggregate capacity and improves spectral efficiency when used with wavelength division multiplexing (WDM). This paper numerically simulate the modal analysis and bit-error-rate (BER) evaluation of a new MDM architecture using concentric multi-diameter ring vortices in a multimode fibre (MMF) backbone. The study achieved a data rate of 40 Gbit/s for the MDM system on wavelength 1550.12 nm for a MMF length of 1500 m. The performance evaluation metrics, namely mode coupling coefficient, modal delays, and eye diagrams for individual modes and mode groups, are investigated and analysed for different topological charges and radial offsets to determine the effect on the system BER.

We propose a passive photonic design for an optically pumped laser system and tunable dual-wavelength generation application by employing optical nonlinear mode coupling of two coupled III–V semiconductor microring resonators, which is connected to a pump and drop waveguide buses. One of the two rings contains a grating, whereas the other has a planar surface. The mechanism underlying the dual-wavelength generation can be explained via the resonance detuning of the spectra that results in nonlinear mode mixing. The tunability of the wavelengths can be achieved by altering the grating depth of the microring resonator and the power coupling coefficients. In the grating design of the microring resonators, we have selected a trapezoidal-profiled apodized grating to obtain low reflectivity at the sidelobes. A time-domain traveling wave (TDTW) analysis yields an InGaAsP core refractive index of 3.3. This core is surrounded by a grating InP cladding with a refractive index of 3.2. We further confirm that the propagation of a Gaussian pulse input with a power of 10 mW and a bandwidth of 0.76 ps is well confined within the system mode propagation. The results show a 2:1 fan-out of two spectrally separate signals, which can be employed for compact and high functional sources on chips. Keywords: Microring resonators, Apodized grating, InGaAsP/InP semiconductor, Dual-wavelength

The performance of optical mode division multiplexer (MDM) is affected by inter-symbol interference (ISI), which arises from higher-order mode coupling and modal dispersion in multimode fiber (MMF). Existing equalization algorithms in MDM can mitigate linear channel impairments, but cannot tackle nonlinear channel impairments accurately. Therefore, mitigating the noise in the received signal of MDM in the presence of ISI to recover the transmitted signal is important issue. This paper aims at controlling the broadening of the signal from MDM and minimizing the undesirable noise among channels. A dynamic evolving neural fuzzy inference system (DENFIS) equalization scheme has been used to achieve this objective. Results illustrate that nonlinear DENFIS equalization scheme can improve the received distorted signal from an MDM with better accuracy than previous linear equalization schemes such as recursive‐least‐square (RLS) algorithm. Desirably, this effect allows faster data transmission rate in MDM. Additionally, the successful offline implementation of DENFIS equalization in MDM encourages future online implementation of DENFIS equalization in embedded optical systems.

The impact of thermo-optic refractive-index changes arising from the core heat load in a dual-core fiber amplifier is studied through finite-element based numerical simulations. Trends in coupling lengths, effective area, core overlaps of fundamental and higher-order supermodes, effects of asymmetric heat loads, and thermo-optic mode coupling parameters are quantified. It is concluded that the coupling between cores is only moderately altered by the overall heat load, but can be strongly sensitive to asymmetric loads. The influence of the core coupling strength is demonstrated to be very important on the supermode effective area, besides the shrinking due to thermal effects. While the thermo-optic index perturbations can lead to guidance of higher-order supermodes, it seems realistic to maintain single-mode operation through gain suppression of the higher-order modes. On the other hand, thermo-optic couplings between fundamental supermodes are found to be quite strong.

Helical gratings in waveguide systems are usually considered from the point of view of converting fiber modes with zero orbital angular momentum (OAM) to OAM modes or OAM modes to each other [1] . In this case, the fiber is located inside the helical grating and the presence of the grating leads to a resonant coupling between these modes. In this work we combine the optical properties of a helical grating and a waveguide in one microstructure and demonstrate resonant mode coupling between OAM modes in femtosecond laser written helical Bragg waveguide (HBW) inscribed in YAG crystall [2] .

We simulate depolarization mechanisms in optical fiber based on linear and nonlinear effects, including a linear polarization mode coupling term to reproduce experimental observations.

Using the transformation cavity, a gradient index cavity designed by transformation optics, we propose a hybrid resonator system to extract unidirectional narrow-beam emission from high-Q whispering gallery modes by embedding a transformation cavity inside a deformed uniform index cavity that exhibits unidirectional narrow-beam emission. For effective mode coupling between the transformation cavity and enclosing cavity, the embedded transformation cavity is designed to have bidirectional evanescent emission, which enables most of the emission from the transformation cavity to be laterally incident on the rim of the enclosing deformed cavity. Consequently, ultrahigh-Q resonances of this system can provide a sharp free-space light output, which is difficult to achieve by embedding a homogeneous disk cavity instead of the transformation cavity.

Hyperbolic materials offer much wider freedom in designing optical properties of nanostructures than ones with isotropic and elliptical dispersion, both metallic or dielectric. Here, we present a detailed theoretical and numerical study on the unique optical properties of spherical nanoantennas composed of such materials. Hyperbolic nanospheres exhibit a rich modal structure that, depending on the polarization and direction of incident light, can exhibit either a full plasmonic-like response with multiple electric resonances, a single, dominant electric dipole or one with mixed magnetic and electric modes with an atypical reversed modal order. We derive conditions for observing these resonances in the dipolar approximation and offer insight into how the modal response evolves with the size, material composition, and illumination. Specifically, the origin of the magnetic dipole mode lies in the hyperbolic dispersion and its existence is determined by two diagonal permittivity components of different sign. Our analysis shows that the origin of this unusual behavior stems from complex coupling between electric and magnetic multipoles, which leads to very strong scattering or absorbing modes. These observations assert that hyperbolic nanoantennas offer a promising route towards novel light–matter interaction regimes.

Electromagnetically induced transparency-like (EIT-like) effect is a promising research area for applications of slow light, sensing and metamaterials. The EIT-like effect is generally formed by the destructive interference of bright-dark mode coupling and bright-bright mode coupling. There are seldom reports about EIT-like effect realized by the coupling of two dark modes. In this paper, we numerically and theoretically demonstrated that the EIT-like effect is achieved through dark-dark mode coupling of two waveguide resonances in a compound nanosystem with metal grating and multilayer structure. If we introduce |1⟩, |2⟩ and |3⟩ to represent the surface plasmon polaritons (SPPs) resonance, waveguide resonance in layer 2, and waveguide resonance in layer 4, the destructive interference occurs between two pathways of |0⟩→|1⟩→|2⟩ and |0⟩→|1⟩→|2⟩→|3⟩→|2⟩, where |0⟩ is the ground state without excitation. Our work will stimulate more studies on EIT-like effect with dark-dark mode coupling in other systems.

Lasing properties of the two-dimensional (2D) distributed feedback (DFB) lasers can be engineered by replacing either the gain medium or periodic structures necessary for the feedback mechanism. Quasicrystals are the intermediate class between the periodic and random structures. They have high rotational symmetry and more favorable for the generation of photonic bandgap as compared to periodic structures. In our experiment, we designed a pentagonal prism for the holographic lithography to construct a long-range 10-fold rotational symmetry, which exhibits 2D quasiperiodic structures. A solution-processable colloidal quantum dots (CQDs) was spin-coated on the resultant 2D quasicrystals. An analytical model based on the cavity mode coupling effect was developed to predict the output performance of the 2D DFB CQDs photonic quasicrystals laser. The respective optically pumped 2D photonic quasicrystal samples exhibit multi-wavelength lasing emission in different directions due to long-range rotational symmetry. The five DFB lasing spots are symmetrically distributed in the 2D space, the center of the lasing spots is similar to a star shape. The derived analytical model predictions are in line with the experimental results.

A fully integrated efficient surface plasmon coupler composed of a non-tapered waveguide is designed for the infrared through rigorous numerical and theoretical studies. To study such structures, a general methodology to study rigorously discontinuities in open waveguides is used. It relies on a full vector description given by Maxwell’s equations in the framework of the finite element method. In our case, the discontinuities are provided by the graphene patches that are modelled as 2D conductivity pieces. The leaky modes of the invariant structure are first computed and then injected as incident fields in the full structure with the graphene patches and sheet using a scattered field approach. This method we recently published allows to compute all the field and energy quantities needed to characterize the investigated couplers. These couplers are designed to work on the far infrared regime. TAS, a special chalcogenide glass is considered for the guiding layer. The studied waveguide is a rib waveguide. The input part of the studied device is the bare rib waveguide. Following the propagation axis, the device is composed of the coupler region, made of a finite number of graphene patches located just above the core guiding layer. The role of this coupler is to generate electromagnetic fields in the next continuous graphene sheet (also located on the top of the core layer) where the surface plasmon polariton part of the field is propagating. The graphene properties are the ones provided by the Kubo formula. The studied device is fully integrated and no out of plane waves have to be considered, only electromagnetic fields propagating inside the structures are considered. Parametric numerical studies as a function of the waveguide thickness and of the duty cycle parameter describing the coupling patches have been conducted in order to maximize the plasmonic part of the generated field near the graphene layer. Several waveguide and coupler configurations with realistic parameter have been obtained proving a highly efficient and fully integrated coupler. Among these configurations, one where the mode coupling between the main mode and higher orders modes is minimized is of particular interest for practical applications.

Recently, chirped and tilted fiber Bragg gratings (CTFBGs) have received great attention because they can realize suppression of stimulated Raman scattering (SRS) in high-power fiber lasers. In this study, the possible coupling between the core modes and cladding modes in CTFBGs inscribed in large-mode-area double-cladding fibers is investigated for the first time. Theoretical results show that the coupling between the LP11 mode and cladding modes would destroy the transmission spectra envelope only considering the coupling of LP01 for single-mode CTFBGs, which will degenerate the SRS suppression performance. This was confirmed experimentally by measuring the spectral response under different mode excitations. A reliable method is demonstrated to ease the LP11-excitation-induced spectral deterioration by choosing an appropriate chirp rate for the inscription of CTFBGs, which is useful for improving the Raman suppression effect of large-mode-area double-cladding CTFBGs in high-power fiber lasers.

Conveyance of light in air endows hollow-core optical fibers with remarkably low sensitivity of the propagation delay to temperature changes. This sensitivity was demonstrated to be further reduced and even made negative (crossing zero) in photonic bandgap type of hollow core fibers. When operating long lengths of this fiber close to the zero sensitivity wavelength, it was observed experimentally that there is a small residual variation in propagation delay which had no apparent correlation to imposed temperature changes. In this paper, we analyze the polarization effects that give rise to this variation, showing that the highest level of practically achievable thermal stability of the latency is limited by polarization mode dispersion. We show measurements of differential group delay between polarization modes in long lengths of photonic bandgap fiber at various temperatures and focus on spectral regions where thermally stable latency is predicted and measured. Our experimental observations, corroborated by numerical simulations, indicate the presence of strong polarization mode coupling in the fibers in addition to birefringence. The detailed understanding gained through this study allows us to propose practically achievable (i.e., manufacturable) fiber designs with up to three orders of magnitude lower polarization mode dispersion at wavelengths where the latency is insensitive to thermal fluctuations. This paves the way to fibers with polarization independent and thermally stable latency to serve a multitude of applications.

Orbital angular momentum modes have shown promising achievements, as data carriers, in ameliorating the transmission capacity and raising the spectral efficiency. This has leaded to an increasing interest in exploiting appropriate specialty fibers for supporting robust OAM channels. In this work, we propose a novel inner graded ring core fiber referred to as inverse Gaussian fiber (IGF). To the best of our knowledge, IGF has never been used as optical fiber profile before. We optimize IGF key parameters in order to design optimized IG-few mode fibers (FMF) supporting low radial modes with high intermodal separation that outperforms those reported in literature (minimum of $\Delta n_{\text{eff}}=2.74\times 10^{-4}$ ), this limits mode coupling and reduces channels crosstalk. We investigate the impacts of smoothness parameters ( $\alpha$ & $P$ ) on the properties of OAM channels in the IG-FMFs. We evaluate and compare (with literature) their intermodal separation, their chromatic dispersion, and their associated differential group delay over the C+L ITU-T bands. The selected designs are proposed as favorable candidate to guide OAM modes in next generation OAM-MDM multiplexing optical networks.

We investigate (both theoretically and experimentally) a method for fundamental mode spectral filtering in single-mode optical fibers using the resonant mode coupling effect. We demonstrate the possibility of controlling the spectral bandwidth of the fundamental mode suppression band through appropriate choice of fiber parameters and fiber bending. The developed technique can be very useful for the design of fiber-based spectral filters (i.e., active fibers with suppression of laser emission at undesirable wavelengths, suppression of stimulated Raman scattering).

Transverse mode instability (TMI) induces detrimental mode coupling in fiber-laser systems at high average powers and still represents the main limitation for the further power scaling of diffraction-limited systems. In this contribution, we describe a new approach to mitigate TMI in fiber amplifiers by dynamically modifying the inversion profile in the fiber. When periodically changing the excitation of the active fiber (e.g. with the help of an acousto-optic deflector), the intensity distribution along the fiber is also dynamically modified. If this is done with a frequency of a few hundreds of kHz (i.e. so that the inversion cannot completely adapt to the new intensity pattern), the inversion grating will be washed out and a homogeneous inversion profile can develop. Consequently, the resulting heat distribution will also be homogenized and the formation of a thermally-induced refractive index grating, which is responsible for the TMI-induced mode coupling, can be largely suppressed. Hence, the presented mitigation approach tackles TMI at an early stage by acting upon the root cause of the detrimental modal energy transfer. At the conference, simulations will be presented which will illustrate the working principle of the new mitigation approach and show its potential to increase the TMI threshold of high-power fiber amplifiers by washing out the inversion profile.

In this paper, we demonstrate the spectrally selective fundamental core mode suppression in single-mode fiber by mode anti-crossing technique. A unique feature of the method proposed in the current work is that the fundamental core mode can have rectangular-like excess loss spectrum with bandwidth controllable by fiber bending. This property made the proposed fiber design to be promising for suppression of lasing at unwanted wavelengths in different fiber lasers and amplifiers.

Modal interference can lead to intensity modulations in optical fibers, which can produce refractive index gratings under the influence of quantum defect heating in a fiber laser. These gratings are perfectly phased-matched for mode couplings, which can lead to transverse mode instabilities at high average powers in fiber lasers. A detailed understanding of this process is critical for further power scaling of fiber lasers. We have directly observed and characterized this quantum-defect-assisted mode coupling for the first time using polarization modes in a PM fiber amplifier, providing solid experimental evidence for this key mechanism for transverse mode instability in fiber lasers.