Coupling Length Research Articles

Inverse design of ultra-compact high-efficiency broadband interlayer couplers based on random gratings

Ultra-compact high-efficiency broadband interlayer couplers based on random gratings are inversely designed by particle swarm optimization algorithm. The optimal structure is obtained by adjusting the pixel state to achieve a maximum coupling efficiency in 1550 nm band. The two-layer coupler consists of two vertically overlapping inverse taper waveguides with random gratings. With a short coupling length of 4 μm and interlayer gap of 200 nm, the coupling efficiency reaches 93% at a wavelength of 1550 nm, and exceeds 90% over a broadband wavelength range of 1503∼1600 nm, for both TE and TM mode. The coupling efficiency can maintain no less than 90% at a large waveguide misalignment of >100 nm, exhibiting high process error tolerance. The performance of the 4 μm-long inverse-designed coupler is higher than the conventional waveguide with a much large length of 10 μm. To achieve longer transmission distance, a tri-layer interlayer coupler is proposed by cascading two bi-layer couplers, which exhibits a remarkable coupling efficiency of 85% at a gap of 600 nm, more than 30 times higher than the bi-layer structure. This work may pave the way for the development of ultra-compact high-performance interlayer couplers supporting high-integration Si-based photonic integrated circuits.

Compact THz Polarization Splitter with Large Bandwidth Based on Cascade Hexagonal Porous Two-Core PCF

ABSTRACT A novel cyclic olefin copolymer (COC)-based terahertz (THz) polarization beam splitter (PBS) employing photonic crystal fiber (PCF) with cascade hexagonal porous two-core structure is proposed. The porous two-core is achieved by replacing the original two large air holes with three layers of small air holes. The mode coupling characteristics between the even and odd modes in the x- and y-polarization directions for the proposed THz PBS are explored using the time-domain finite difference (FDTD) method. The influence of PBS structural parameters on its coupling length is investigated. Numerical analysis results indicate that the bandwidths of 227.8 GHz (ranging from 1.8970 to 2.1248 THz) and 147 GHz (ranging from 1.928 to 2.075 THz) are acquired for the x- and y-polarization fundamental modes, respectively. The highest extinction ratios (ERs) are 114.5 dB and 72.3 dB, and the minimum insertion losses (ILs) are 0.0142 dB and 0.0369 dB for the x- and y-polarization fundamental modes, respectively. Moreover, the length of the proposed PBS is merely 11.655 mm. The proposed PBS will find potential applications in polarization-diverse THz systems, including broadband line-of-sight communication, real-time security checks, and high-resolution imaging.

A fixed phase tunable directional coupler based on coupling tuning

The field of photonic integrated circuits has witnessed significant progress in recent years, with a growing demand for devices that offer high-performance reconfigurability. Due to the inability of conventional tunable directional couplers (TDCs) to maintain a fixed phase while tuning the reflectivity, Mach-Zehnder interferometers (MZIs) are employed as the primary building blocks for reflectivity tuning in constructing large-scale circuits. However, MZIs are prone to fabrication errors due to the need for perfect balanced directional couplers to achieve 0-1 reflectivity, which hinders their scalability. In this study, we introduce a design of a TDC based on coupling constant tuning in the thin film Lithium Niobate platform and present an optimized design. Our optimized TDC design enables arbitrary reflectivity tuning while ensuring a consistent phase across a wide range of operating wavelengths. Furthermore, it exhibits fewer bending sections than MZIs and is inherently resilient to fabrication errors in waveguide geometry and coupling length compared to both MZIs and conventional TDCs. Our work contributes to developing high-performance photonic integrated circuits with implications for various fields, including optical communication systems and quantum information processing.

Ultracompact polarization beam splitter based on hybrid plasmonic waveguide

A compact polarization beam splitter (PBS) based on a three-waveguide plasmonic directional coupler was proposed and modeled. The directional coupler comprised two common silicon-on-insulator (SOI) ridge waveguides, with a metal-capped plasmonic SOI waveguide sandwiched between them. The geometrical structure of the three-waveguide directional coupled waveguide was optimized, realizing the phase matching of transverse magnetic (TM) modes but not of transverse electric (TE) modes. The structure and performance of the PBS was optimized using a finite-difference time-domain method. Simulation results showed that the birefringence comparison of TE and TM modes in a hybrid plasmonic waveguide was enhanced using a dielectric loading waveguide to obtain an ultra-compact structure; the final optimized coupling length of the device was 6.1 µm. At the center wavelength of 1.55 µm, the polarization extinction ratio of the TE and TM modes was 34.57 dB and 23.24 dB, and the insertion loss was 0.01 dB and 0.13 dB, respectively.

Enhanced anisotropy of Pr-Nd-Fe-B HDDR magnetic powders by regulating the hydrogen decrepitated process

The evolution of microstructures for different hydrogen decrepitated routes was investigated in hydrogenation-disproportionation-desorption-recombination (HDDR) magnetic powders, with particular focus on the fracture of the SC heat-treated alloy during two-step hydrogen decrepitation (HD) and the influence of the first step temperature on magnetic properties. As the temperature increases, the properties of the magnetic powders increase and then decrease, the optimized performance of HDDR magnetic powders was obtained with Br=1.45 T, (BH)max=393 kJ·m−3, DOA=0.79 when the first HD temperature is 65°C. The microscopic analysis shows that there are two problems in the one-step HD route: one is that the particle size is larger ∼150 μm and there are incomplete fractured cracks, the particle surface is more PrNd-rich phase, which leads to different reaction rates between the surface and center in HDDR particles, and the texture orientation of the fine grains at the edge of surface and crack is disordered. The second issue is that HD grains have significant angular difference with the c-axis orientation in incomplete fractured grain clusters, which is directly inherited by the HDDR magnetic powder, resulting in a misalignment texture. In comparison, the two-step HD process effectively addresses the issues derived from one-step HD process. Not only is the grain size of HDDR magnetic powders uniform and the width of the thin grain boundary phases (GBs) is less than the exchange coupling length, but also (Fe+Co) content exceeding 30 at% in GBs with weak magnetism. This is beneficial for enhancing exchange coupling effect between adjacent grains. Moreover, magnetic powder particle exhibits single-domain characteristics, which improves the grain texture orientation. This research provides a new insight for the development of high-orientation and high-performance HDDR magnetic powders.

Nano-strip Engineering for Coupling Length Enhancement in Pattern-Free Suspended Graphene-based Plasmonic Waveguides

Nano-strip Engineering for Coupling Length Enhancement in Pattern-Free Suspended Graphene-based Plasmonic Waveguides

New method for the investigation of mode coupling in graded-index polymer photonic crystal fibers using the Langevin stochastic differential equation

The mode coupling in a graded-index polymer photonic crystal fiber (GI PPCF) with a solid core has been investigated using the Langevin equation. Based on the computer-simulated Langevin force, the Langevin equation is numerically integrated. The numerical solutions of the Langevin equation align with those of the time-independent power flow equation (TI PFE). We showed that by solving the Langevin equation, which is a stochastic differential equation, one can successfully treat a mode coupling in GI PPCFs, which is an intrinsically stochastic process. We demonstrated that, in terms of effectiveness, the Langevin equation is preferable compared to the TI PFE. The GI PPCF achieves the equilibrium mode distribution (EMD) at a coupling length that is even shorter than the conventional GI plastic optical fiber (POF). The application of multimode GI PCFs in communications and optical fiber sensor systems will benefit from these findings.

Experiment on the vortex-induced vibration of an S-shaped flexible riser coupled with an oscillating platform

This paper reports the experimental results of the vortex-induced vibration (VIV) of an S-shaped flexible riser hanged underneath a two-degree-of-freedom (2-DOF) oscillating cylindrical platform. A buoyancy module of approximately triple the riser diameter was mounted on the one-third of the riser length measured from its base to produce the S-shaped configuration. The VIV response of the flexible riser filled with two typical fluid media, including water and helium, was tested in the depth-averaged reduced velocity (U¯r) range of 9.32–26.41. The experimental results indicate that the change of internal fluid medium alters the spatial distribution of dominant frequency and excited modal order. The mode competition is enhanced when the internal fluid shifts from water to helium, and the critical U¯r for mode transition becomes higher. Based on the coincidence of dominant frequency, four types of coupling response are identified, and two characteristic lengths are quantified. The dominant role of the vibration system is determined by the occurrence time percentages of coupling lengths and dominant frequencies. The vibration of water-filled riser is dominated by itself, and the influence of buoyancy module is negligible. In contrast, the in-plane response of helium-filled riser is significantly affected by the buoyancy module, and the influence is amplified with the increase in U¯r.

Transmission Characteristics Analysis of a Twin-Waveguide Cavity

The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the parameters of the effective refractive indices of the upper and lower waveguides, the coupling length Lc, and the ratio of the cavity length L to the coupling length (L/Lc). Filters with single, double, or triple-notch filtering could be obtained by choosing an appropriate L/Lc value. When the facet reflection is taken into consideration, the traveling-wave transmission spectrum is modified by the Fabry––Perot (FP) resonance, making it a standing-wave transmission spectrum. As a result, resonance splitting has been observed in the transmission spectrum of twin-waveguide resonators with high facet reflectivity. Further analysis shows that such an abnormal resonance phenomenon can be attributed to the destructive interference between the two FP resonance modes of the upper and lower waveguide through coherent coupling. In addition, narrow bandwidth amplification has also been observed through asymmetric facet reflections. Undoubtedly, all these unique spectral characteristics should be beneficial to the twin-waveguide cavity, achieving many more functions and being widely used in photonic integration circuits (PICs).

Fission and fusion of heavy nuclei induced by the passage of a radiation-mediated shock in BNS mergers

ABSTRACT We compute the structure of a Newtonian, multi-ion radiation-mediated shock (RMS) for different compositions anticipated in various stellar explosions. We use a multifluid RMS model that incorporates electrostatic coupling between the different plasma constituents as well as Coulomb friction in a self-consistent manner, and approximates the effect of pair creation and the presence of free neutrons in the shock upstream on the shock structure. We find that under certain conditions a significant velocity separation is developed between different ions in the shock downstream and demonstrate that in fast enough shocks ion–ion collisions may trigger fusion and fission events at a relatively high rate. Our analysis ignores anomalous coupling through plasma microturbulence, which might reduce the velocity spread downstream below the activation energy for nuclear reactions. A rough estimate of the scale separation in RMS suggests that for shocks propagating in binary neutron star (BNS) merger ejecta, the anomalous coupling length may exceed the radiation length, allowing a considerable composition change behind the shock via inelastic collisions of $\alpha$ particles with heavy elements at shock velocities $\beta _\mathrm{ u}\gtrsim 0.25$. A sufficient abundance of free neutrons in the shock upstream, as expected during the first second after the merger, is also expected to alter the ejecta composition through neutron capture downstream. The resultant change in the composition profile may affect the properties of the early kilonova emission. The generation of microturbulence due to velocity separation can also give rise to particle acceleration that might alter the breakout signal in supernovae and other systems.

Approaching 20% Efficiency in Ortho-Xylene Processed Organic Solar Cells by a Benzo[a]phenazine-Core-Based 3D Network Acceptor with Large Electronic Coupling and Long Exciton Diffusion Length.

High-performance organic solar cells often rely on halogen-containing solvents, which restrict the photovoltaic industry. Therefore, it is imperative to develop efficient organic photovoltaic materials compatible with halogen-free solvents. Herein, a series of benzo[a]phenazine (BP)-core-based small-molecule acceptors (SMAs) achieved through an isomerization chlorination strategy is presented, comprising unchlorinated NA1, 10-chlorine substituted NA2, 8-chlorine substituted NA3, and 7-chlorine substituted NA4. Theoretical simulations highlight NA3's superior orbit overlap length and tight molecular packing, attributed to interactions between the end group and BP unit. Furthermore, NA3 demonstrates dense 3D network structures and a record electronic coupling of 104.5 meV. These characteristics empower the ortho-xylene (o-XY) processed PM6:NA3 device with superior power conversion efficiency (PCE) of 18.94%, surpassing PM6:NA1 (15.34%), PM6:NA2 (7.18%), and PM6:NA4 (16.02%). Notably, the significantly lower PCE in the PM6:NA2 device is attributed to excessive self-aggregation characteristics of NA2 in o-XY. Importantly, the incorporation of D18-Cl into the PM6:NA3 binary blend enhances crystallographic ordering and increases the exciton diffusion length of the donor phase, resulting in a ternary device efficiency of 19.75% (certified as 19.39%). These findings underscore the significance of incorporating new electron-deficient units in the design of efficient SMAs tailored for environmentally benign solvent processing of OSCs.

Coupling Dynamics and Linear Polarization Phenomena in Codirectional Polariton Waveguide Couplers

AbstractIn this work, the previous studies investigating the potential of integrated devices using polariton waveguides that form codirectional couplers is extended. For suitable coupler parameters, a transfer of condensates between the arms of the coupler occurs leading to the observation of previously predicted Josephson‐like oscillations. The ability to tune the periodicity of these oscillations opens the way to the design of polaritonic circuits in which the directionality of the signal towards the output terminals can be controlled, for a fixed separation between the arms, varying the coupling length. The response of the devices to linearly polarized excitation is also investigated, delving into the dynamics of linear polarization at the output terminals of long couplers, providing valuable insights into their potential applications, including polariton switches and logic gates with efficient operation. The results are supported by numerical simulations based on the generalized Gross–Pitaevskii equation describing the dynamics of coherent polaritons in spatially non‐uniform systems, reproducing the polariton distribution on the output terminals and an oscillatory dependence of the corresponding emission as a function of the polarization azimuth. How the coupling and the controllable polarization degree of freedom in polariton couplers opens avenues for innovative optical architectures and functionalities is shown here.

Design and simulation of a compact polarization beam splitter based on dual-core photonic crystal fiber with elliptical gold layer

For the polarization multiplexing requirements in all-optical networks, this work presents a compact all-fiber polarization beam splitter (PBS) based on dual-core photonic crystal fiber (PCF) and an elliptical gold layer. Numerical analysis using the finite element method (FEM) demonstrates that the mode modulation effect of the central gold layer effectively reduces the dimensions of the proposed PBS. By determining reasonable structural parameters of the proposed PCF, the coupling length ratio (CLR) between X- and Y-polarized super-modes can approach 2, achieving a minimal device length of 0.122 mm. The PBS exhibits a maximum extinction ratio (ER) of − 65 dB at 1.55 μm, with an operating bandwidth spanning 100 nm (1.5–1.6 μm) and a stable insertion loss (IL) of ~ 1.5 dB at 1.55 μm. Furthermore, the manufacture feasibility and performance verification scheme are also investigated. It is widely anticipated that the designed PBS will play a crucial role in the ongoing development process of miniaturization and integration of photonic devices.

Evanescent field engineering to reduce cross-talk in pattern-free suspended graphene-based plasmonic waveguides using nano-strips

Suppression of the cross-talk between neighboring photonic components has been considered one of the most efficient methods for increasing the packing density of photonic integrated circuits. In this study, an effective cross-talk reduction approach is developed for plasmonic waveguides. Firstly, the mode characteristics of the waveguide are investigated, and then two silicon strips are placed beside the two adjacent waveguides. The mechanism of the cross-talk suppression method highly relies on the evanescent waves. The designed strips in this study manipulate the wave vectors of surface plasmon polaritons. The tangential component of the wave vector is conserved along the interface, while the perpendicular component in the surroundings can be equal to zero or imaginary, leading to evanescent waves. Placing the strips increases the perpendicular component of the wave vector in the surroundings, resulting in less cross-talk. So, the coupling length of the waveguide with the strips at the wavelength of 10 μm increases up to 345 μm which is 25 times longer than that in its non-strip counterpart. Also, the figure-of-merit and loss of the designed structure are up to 1000 and 0.13 dB/μm at a chemical potential of 0.8 eV and a wavelength of 10 μm, respectively. So, high figure-of-merit and low loss along with long coupling length are the interesting features of the designed plasmonic waveguide which propose promising structures with high performance in the near-infrared.

Management of lateral misalignment loss and total insertion loss with beam waist control in high contrast single mode coupling fibers

Abstract This paper has illustrated the management of lateral misalignment loss and total insertion loss with beam waist control in high contrast single mode coupling fibers. The beam waist variations are clarified versus the fiber coupler wavelength and coupling length variations for the silica glass/fluoride glass fiber coupler with the optimum incident beam angle of 60°. Besides, the coupling loss is demonstrated against the fiber coupler wavelength and coupling length variations for the silica glass/fluoride glass fiber coupler with the optimum incident beam angle of 60°. The optimum beam waist and optimum coupling loss are deeply studied against the fiber coupler core radius variations for the silica/fluoride glass fiber coupler with the optimum incident beam angle of 60° and wavelength of 1,550 nm.

Experimental demonstration of a silicon four-mode (de)multiplexer based on cascaded triple-waveguide couplers

Silicon photonics based on-chip mode-division multiplexing (MDM) is an appealing technology for enhancing the interconnect capacity in both short- and long-haul optical communication. Here, a silicon four-mode (de)multiplexer [(De)MUX] is proposed, optimized, fabricated, and characterized for on-chip MDM systems, based on three cascaded triple-waveguide couplers (TWCs), with coupling lengths below 24.25 µm. A four-mode MDM-link consisting of two back-to-back mode-(De)MUXs was fabricated and measured, yielding a crosstalk of less than −15.0dB over a bandwidth wider than 72.6 nm for four modes. In addition, the TWC based mode (De)MUX demonstrates a 1-dB bandwidth greater than 40 nm, and can be considered as a robust component for multimode on-chip networks.

Ultra-broadband and high extinction ratio polarization splitter based on triple-tapered directional couplers

The design and numerically investigation of a silicon polarization splitter (PS) is proposed using triple-tapered directional couplers (DCs).The proposed device consists of a triple-tapered DC, a triple-bent DC at through port, and a tapered waveguide at cross port. The coupling length of the device is 20.5 µm. At the 1550 nm wavelength, the realized PS exhibits the extinction ratio (ER) values of 35.7 dB and 37.35 dB as well as the insertion loss values of < 0.58 dB and < 0.4 dB for the TE and TM polarization modes, respectively. Furthermore, the device’s bandwidth for ER > 20 dB and ER > 25 dB are 170 nm and 136 nm, respectively.

Multimode interference coupler based on general interference for integrated optical and quantum optic devices: a review

Integrated optics play important role in the development for optical network devices, optical processing, instrumentation and quantum information device. Here, multimode interference (MMI) coupler is one of basic component preferred for these integrated optic devices in which MMI coupler based on general interference (GIMMI) has been focused for large scale integrated optic devices because of compact size in comparison to MMI coupler based restricted interference (RIMMI). In this paper, we have reviewed different GIMMI structures reported previously. Different waveguide materials are mentioned and compared for fabrication of GIMMI device. The coupling behaviors and modal analysis of GIMMI couplers and their different tapered structures are estimated by using simple model based on sinusoidal modes showing reduced coupling lengths in down tapered structures in comparison to other structures. The excess loss, polarization dependence loss and crosstalk versus fabrication tolerances are obtained to compare their performances revealing almost same losses and fabrication tolerance. The paper reports the use of the GIMMI coupler in wavelength multiplexing, power splitting, switching, optical processing. Finally, Quantum optic application of GIMMI coupler is shown indicating high quantum fidelity of 50:50 coupling ratio.

Compact silicon photonic-lantern mode (de)multiplexer based on tilt slot waveguide

As the key component in on-chip mode-division multiplexing systems, a compact silicon photonic-lantern mode (de)multiplexer is proposed and demonstrated using the shallow-etched tilt slot waveguide. The proposed six-mode (de)multiplexer is designed as a constant coupling length of 11.7 µm for each mode conversion and eliminates the adiabatic transition tapers for cascaded asymmetric directional couplers, which have an ultra-short total length of 69 µm. The measured peak insertion losses of the fabricated device for all mode channels are less than 1.2 dB, and the crosstalk is below -12.6 dB in a 60 nm waveband. Additionally, the simulation results indicate that the device has a good fabrication tolerance. The proposed mode (de)multiplexer is scalable and could provide a feasible solution for the dense integration of on-chip mode division multiplexing systems.

Dual-Core Photonic Crystal Fiber Polarization Beam Splitter Based on a Nematic Liquid Crystal with an Ultra-Short Length and Ultra-Wide Bandwidth

This paper presents a novel pentagonal structure dual-core photonic crystal fiber polarizing beam splitter (PS-DC-PCF PBS) filled with a nematic liquid crystal (NLC) in the central hole. Unlike previous designs with symmetric arrangements, the upper and lower halves of the structure have different air hole arrangements. The upper half consists of air holes arranged in a regular quadrilateral pattern, while the lower half features a regular hexagonal arrangement of air holes. By filling the central hole with birefringent liquid crystal, the birefringence of the structure is enhanced, reducing the coupling lengths along the x polarization and y polarization directions. The polarization properties, coupling characteristics, and the influence of different structural parameters on the extinction ratio of the polarizing beam splitter are analyzed using the full-vector finite element method. Simulation results demonstrate that the designed PS-DC-PCF PBS achieves a maximum extinction ratio (ER) of 72.94 dB with a splitting length of only 61.9 μm and a wide operating bandwidth of 423 nm (1.324–1.747 μm), covering most of the O, E, S, C, L, and U communication bands. It exhibits not only ultra-short splitting lengths and ultra-wide splitting bandwidth but also good manufacturing tolerances and anti-interference capabilities. The designed PS-DC-PCF PBS could provide crucial device support for future all-optical communication systems and has potential applications in fiber optic communication or fiber laser systems.