Bent Waveguides Research Articles

Investigation on Ultra-Compact 2DPC Coarse Wavelength Division Demultiplexer

A two dimensional Photonic Crystal (2DPC) based eight-channel wavelength division de-multiplexer is proposed and designed for Coarse Wavelength Division Multiplexing (CWDM) applications. The circular ring resonator, channel selector, circulator rod, L bend waveguide and linear bus waveguide are essential parts of the proposed system. The system’s functional parameters such as Transmission efficiency, resonant wavelength, spectral width, channel spacing, Quality factor and crosstalk investigated this paper. The eight different wavelengths of channels are filtered out by altering the size of channel selector rod, setting the radius of the circle shaped cavity and relative refractive index of circulator rod. Initially the Photonic Band gap (PBG) is manipulated by applying Plane Wave Expansion (PWE) method of the 2DPC structure. The functional parameters are analysed by Finite Difference Time Domain (FDTD) method in periodic and non-periodic structure of the proposed system to arrive normalized transmission spectrum. The resonant wavelengths of designed eight paths of the device are varying from 1420nm to 1460nm with average spectral width and channel spacing are 5.8nm, 5.6nm respectively. The footprint of the device is 286.84μm2. Hence, this small device can be implemented for CWDM systems in Photonic Integrated Circuits (PIC)

Investigation of the curvature distribution for TE01-to-TM11 mode converter

An oversized TE-to-TM mode converter based on waveguide bend structure is designed for high power transmission line. The curvature distribution of the waveguide bend is analyzed and optimized using mode coupling theory. Analysis results show that the curvature near the output port plays a decisive role in the content of the parasitic modes, which indicates that the restriction on curvature far from output port could be relaxed. Therefore, the asymmetric waveguide axis with enhanced curvature near the front end and lessened curvature near the output port will have a high conversion efficiency and a broad bandwidth. Combining the curvature optimization and the phase rematching technology proposed in our preliminary work, the TE-to-TM mode converter realizes a 90 bend with a conversion efficiency over 95% in the range of 26–32.2 GHz (the relative bandwidth over 21%). To verify the designed mode converter, a prototype is manufactured and measured.

Ultra-Compact Mode (De)Multiplexer and Polarization Beam Splitter Based on Tapered Bent Asymmetric Directional Couplers

Wepropose and experimentally demonstrate the ultra-compact mode (de)multiplexers and polarization beam splitter (PBS) based on tapered bent asymmetric directional couplers. The strong coupling strength introduced by tapered bent waveguides significantly shortens the coupling length. As a result, high coupling and conversion efficiencies over a broad bandwidth can be obtained with small footprint. The coupling length of the fabricated PBS and mode (de)multiplexers are less than 4.9 μm and 8.7 μm, respectively. From 1534-1600 nm, the measured insertion loss (IL) is less than 1.4 and 1.55 dB for the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> mode (de)multiplexers, respectively, and the crosstalk (CT) is less than −10 dB. The fabricated PBS shows low insertion loss of 0.55 dB and decent crosstalk of < −14 dB for both polarizations over an 85 nm wavelength range from 1505-1590 nm. Meanwhile, the larger fabrication tolerance ensures that the proposed devices can be readily fabricated with the standard complementary metal-oxide-semiconductor (CMOS) process.

Observation of topological valley waveguide transport of elastic waves in snowflake plates

In elastic systems, the researchers have observed the topological valley edge states. However, the topological valley edge states can only enable elastic waves to propagate along the narrow interfaces. Here, we obtain the topological valley waveguide states (TVWSs) by constructing the sandwich-like plates. By virtue of the TVWSs, the transport of elastic waves in the wide straight and bent waveguides is achieved. In addition, we realize the splitting effect and converging of elastic waves according to the valley-locking property. Our research may promote the design of innovative devices about energy harvesting, sensing, and information processing.

90°-bent graded-index core polymer waveguide for a high-bandwidth-density VCSEL-based optical engine.

In this paper, we present a low-loss optical assembly utilizing a 90°-bent graded-index (GI) core polymer optical waveguide on vertical cavity surface emitting laser (VCSEL) based optical transceivers. The proposed assembly can replace conventional components such as mirrors and lenses for realizing subminiature optical engines applicable to on-board integration. To minimize the total insertion loss of the waveguide when connected to a high-speed VCSEL and a GI-core multimode fiber (MMF) at each end, the characteristics of the beam emitted from VCSELs are measured and taken into consideration for the waveguide design. In order to confirm the effect of insertion loss reduction by the waveguide numerical aperture control, 90°-bent GI-core polymer waveguides are fabricated applying the Mosquito method. The fabricated waveguide exhibits a total insertion loss as low as about 2 dB at 850-nm wavelength, which includes the coupling losses at both ends, bending, and propagation losses. We also investigate a way to reduce the insertion loss when a gap exists between the waveguide and VCSEL chip. We theoretically and experimentally confirm that filling the gap with a high index resin can reduce the coupling loss by 5 dB.

Non-self-adjointness of bent optical waveguide eigenvalue problem

In the literature, the mathematical problem of optical wave propagation in dielectric straight waveguides has been systematically studied as a self-adjoint eigenvalue problem with real eigenvalues. In terms of the underlying physics, such real eigenvalues meant no losses during the wave propagation. However, when the waveguides were bent, experiments showed that the wave propagation became lossy. In this paper, optical wave propagation in dielectric bent waveguides is mathematically analyzed. It is shown that the corresponding eigenvalue problem is a non-self-adjoint eigenvalue problem and has complex-valued eigenvalues. The imaginary part of the eigenvalues is a measure of loss. For large-bend radii, the eigenvalue problem for bent waveguides behaves as an eigenvalue problem for straight waveguides, and the complex-valued eigenvalues approach the real-valued eigenvalues of the straight waveguide problem. By expressing the bent waveguide eigenvalue operator as a sum of the self-adjoint operator and the non-self-adjoint operator, asymptotic behaviour of guided modes and their lossy nature are investigated.

Sound-transparent anisotropic media for backscattering-immune wave manipulation

The scattering behavior of an anisotropic acoustic medium is analyzed to reveal the possibility of routing acoustic signals through the anisotropic layers with no backscattering loss. The sound-transparent effect of such a medium is achieved by independently modulating the anisotropic effective acoustic parameters in a specific order, and is experimentally observed in a bending waveguide by arranging the subwavelength structures in the bending part according to transformation acoustics. With the properly designed filling structures, the original distorted acoustic field in the bending waveguide is restored as if the wave travels along a straight path. The transmitted acoustic signal is maintained nearly the same as the incident modulated Gaussian pulse. The proposed schemes and the supporting results could be instructive for further acoustic manipulations such as wave steering, cloaking and beam splitting.

Radiation Leakage Images For Two Different Dielectric-Loaded Surface Plasmon Polariton Waveguides

We have studied about leakage radiation images obtained for two different dielectric-loaded surface plasmon polariton waveguides based on routing structures; linear couplers and bent waveguides. By simultaneously imaging the conjugated aperture and field planes of the microscope, we unambiguously quantify the degeneracy lift occurring for strongly interacting dielectric loaded surface plasmon polariton waveguides and visualize the symmetry of the coupled modes. We have obtained the wave vector distribution and showed its evolution with the bend radius. We have developed a numerical and an analytical analysis for momentum distribution. We have found that for large radii i.e. for vanishing bending loss, we can link the plasmon in Fourier space with the geometrical and modal properties of the bend structure. It was also found that the radial dependence of the wave vector distribution is governed by the phase difference. The obtained results were found in good agreement with previously obtained results.

Mosaic Based Optimization of NRD Guide Devices Using Binary Evolutionary Approaches and 2D-FVFEM

In this paper, efficient optimal design approaches based on mosaic optimization concept are developed for NRD guide devices to realize the high-performance compact millimeter-wave integrated circuit. Binary representation-based genetic algorithm, differential evolution algorithm, harmony search algorithm, firefly algorithm, and particle swarm optimization are developed to efficiently optimize the pixel pattern in the design region of NRD guide devices. To demonstrate the usefulness of these population-based optimizations, a comparative study based on the problem-solving success rate is conducted first. To carry out this study, four NRD circuit components are designed which include low crosstalk waveguide crossing, T-branch power splitter, bending waveguide, and frequency demultiplexer. The proposed optimal devices achieve high transmission efficiencies greater than 99.9%, 49.9%:49.9%, 99.9% at 60 GHz and 96.4%, 98.5% at 59 GHz and 61 GHz. In addition, the same NRD guide components except frequency demultiplexer are also designed at wideband operation and achieve broad bandwidth around 5 GHz, 4 GHz, and 3 GHz. In order to improve the computational efficiency, the originally developed two-dimensional full vectorial finite element method is employed for the numerical simulations. This paper demonstrates the detailed implementation procedure of developed evolutionary approaches for the material distribution in the design region of NRD guide devices, comparative study of developed optimization approaches, and proposed highly attributed NRD circuit components for the realization of NRD based high-performance compact millimeter-wave circuit.

High-Density Electrical and Optical Assembly for Subminiature VCSEL-Based Optical Engine

In this article, we propose a subminiature (10.9 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times56.6$ </tex-math></inline-formula> mm) vertical cavity surface-emitting laser (VCSEL)-based optical engine with a low crosstalk penalty for onboard applications. When applying optical engines to onboard interconnects, ICs [laser drivers and transimpedance amplifiers (TIAs)] and active optical devices (light sources and photodetectors) must be mounted densely to make the footprint as small as possible. It is a concern that such a high-density integration could increase the crosstalk between transmitter (Tx) and receiver (Rx) devices, which could be caused by the supply current difference between the circuit from laser drivers to light source and the circuit from photodetectors to TIAs. In this article, by inserting a gap in the ground electrode, a compact optical engine (less than half of the footprint of quad small form-factor pluggable-double density (QSFP-DD) compliant engines) enabling a 25.78-Gb/s error-free optical transmission is successfully fabricated. We optimize the gap width to decrease the crosstalk while maintaining efficient heat dissipation via the electrode. We compare the characteristics of the fabricated optical engine to the engine with the gap-less ground electrode structure formed in the same compact size. Then, we both theoretically and experimentally confirm a link power budget savings of about 1.8 dB, which is sustained even under high-temperature ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{c} = 70\,\,^{\circ }\text{C}$ </tex-math></inline-formula> ) operation. In addition, to realize further high-density assembly, we also represent a lens-less optical coupling by inserting a 90 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> -bent graded-index (GI) core polymer waveguide between the optical transmitter and multimode fiber. The transmission performance of the 90 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> -bent GI-core waveguide is preliminarily evaluated, and we successfully transmit 53.125-Gb/s PAM4 optical signals experimentally.

Design of an ultrasharp silicon multimode waveguide bend assisted by multiregional subwavelength grating structures

Sharp multimode waveguide bends are beneficial to improve the integration density of the on-chip mode-division multiplexing (MDM) systems, which can be used to increase the optical information transmission capacity. A method of designing a multimode bending structure based on subwavelength gratings is proposed in this paper, through the mode conversion and matching between the straight and bending waveguide. The proposed bending structure is composed of a 90-deg arc-bend with a radius of 5 μm combined with the shallow-etched grating structures on the top, and two-mode converters on the input and output straight waveguide realized also with the help of nonuniform grating-like structures, achieving a total effective bending radius of 8.25 μm and high transmission efficiencies of the first three TE modes (TE0 − TE2) in a wide wavelength range of 1500 to 1600 nm. At the center wavelength of 1550 nm, the calculated excess losses of the three modes TE0, TE1, and TE2 are 0.21, 0.19, and 0.59 dB, respectively, and the crosstalks for all modes are less than −26.58 dB. More importantly, we add a non-uniform grating converter in the straight waveguide region which pre-deviates the mode fields inside the straight waveguide to help further reduce the size of the bending part to only 5 μm, which will be helpful for large scale integration when a great number of bends are cascaded.

A proposal for wide range wavelength switching process using optical force

Optomechanical wavelength up-conversion based on optical force and core–shell scattering effects are used to control light coupling between two waveguides. This system consists of two parallel optical waveguides with 20 μm lengths suspended on a silica substrate embedded with Ag/Si/SiO2 core–shell nanoparticles. By mid-IR plane wave illumination with different intensities and different wavelengths on nanoparticles, scattering would increase and result in an improvement in attractive gradient optical force exerted on waveguides. Via bending waveguides toward each other, visible light propagating in the first waveguide would couple to another. PDMS as a polymer is used to reduce the required power for bending waveguides. Results reveal that when waveguides’ gap equilibrium is 400 nm and wavelengths of control and probe lights are 4.5 μm and 0.45 μm respectively, about 10.75 mW μm−2 power is needed to bend waveguides for total coupling of light between waveguides. The efficiency of the coupled waveguides system is %43.

Full-Vector Analysis of Bending Waveguides by Using Meshless Finite Cloud Method in a Local Cylindrical Coordinate System

By using a meshless finite cloud method, a full-vector mode solver in terms of transverse-magnetic-field components for optical dielectric waveguide bends is developed in a local cylindrical coordinate system. To construct the interpolation function, both the point collocation and the fixed reproducing kernel technique are utilized, and then with the help of the continuity conditions of the longitudinal field components and the perfectly matched layer absorbing boundary conditions, a standard matrix eigenvalue is obtained. Compared with the conventional meshed numerical techniques, the distributions of solution points of the present meshless method can be applied to the area of complexity in a completely free manner. Consequently, excellent convergence with high accuracy can be realized. Moreover, the present method can be applied to both the uniform and non-uniform nodal distributions, thus the computational procedure is relatively flexible. To validate the proposed method, both fundamental and high-order leaky modes for a typical rib bending waveguide are analyzed and their modal field distributions and corresponding effective refractive indexes are presented. Results are in good agreement with those published earlier, showing the effectiveness of the present method.

Power improvement in ridge bent waveguide superluminescent light-emitting diodes based on GaN quantum dots

In this paper, the performance of ridge-waveguide superluminescent light-emitting diodes (SLED) based on nitride quantum dots has been reported. For this purpose, the three-dimensional Schrodinger equation has been solved numerically by the finite volume method for truncated pyramidal quantum dots as active regions taking into account all spontaneous and piezoelectric effects. To calculate the modal gain in the active region, the traveling-wave rate equations model has been used. We solved the traveling-wave equations coupled with the carrier density rate equation to obtain the output power related to the bias current, numerically. The output power is one of the parameters that describe the performance of a superluminescent light-emitting diode. The output power is studied for the SLEDs with different waveguide structural parameters such as the cavity length and radius of curvature. The temperature-dependent of the SLED output power is also investigated. The maximum output power of the SLED increased up to 0.08 W for the cavity with the length of 1200

Ultra-compact multimode waveguide bend with shallowly etched grooves.

In this work, an ultra-sharp multimode waveguide bend (MWB) based on gradient shallowly etched grooves is proposed and demonstrated. With a bending radius of only 5.6 μm, our shallowly-etched-groove multimode waveguide bend (SMWB) can enable low excess loss and low-crosstalk propagation with the four lowest-order TE mode-channels, simultaneously. In the simulation, the excess losses of the proposed 90°- SMWB for TE0-TE3 are all below 0.46 dB and the inter-mode crosstalks are lower than -18 dB in 1500 nm-1600 nm. Furthermore, the measured results of the fabricated 90°- SMWB show that the excess losses for TE0-TE3 are less than 1 dB and the inter-mode crosstalks are all below -14 dB in 1510 nm-1580 nm. Such a proposed device thus provides a promising solution for ultra-compact MWBs in multimode silicon photonics.

Low-Loss and Compact Bends on Multi-Layer SiN-on-SOI Platform for Photonic Integrated Circuits

Low-loss, compact silicon nitride and silicon waveguide bends are demonstrated using COMS-compatible multi-layer PECVD SiN-on-SOI integration platform over the whole C band. The curvature of advanced waveguide bend is designed with linearly changing to reduce bending loss. The bending loss for PECVD SiN reduces from 0.087 dB/90° for normal bend to 0.037 dB/90° for advanced bend with the radius of 30 μm. It indicates a 60% footprint reduction with same loss level compared with the normal bend with 50-μm radius. Wafer-level distribution and uniformity of the bending loss are characterized, showing the stable performance. Similarly, we demonstrate a 3 μm-radius silicon waveguide bend in the same platform, showing the bending loss of only 0.0065 dB/90° bending loss, which is even smaller comparing to 0.012 dB/90° bend loss of a 5 μm-radius normal bend. Such advanced bend design with significantly reduced loss and footprint paves a way for high-dense large-scale photonic integrated circuits.

Elastic Organic Crystals Based on Barbituric Derivative: Multi-faceted Bending and Flexible Optical Waveguide.

Elastic organic single crystals with light-emitting and multi-faceted bending properties are extremely rare. They have potential application in optical materials and have attracted the extensive attention of researchers. In this paper, we reported a structurally simple barbituric derivative DBDT, which was easily crystallized and gained long needle-like crystals (centimeter-scale) in DCM/CH3 OH (v/v=2/8). Upon applying or removing the mechanical force, both the (100) and (040) faces of the needle-like crystal showed reversible bending behaviour, showing the nature of multi-faceted bending. The average hardness (H) and elastic modulus (E) were 0.28±0.01 GPa and 4.56±0.03 GPa for the (040) plane, respectively. Through the analysis of the single crystal data, it could be seen that the van der waals (C-H⋅⋅⋅π and C-H⋅⋅⋅C), H-bond (C-H⋅⋅⋅O) and π⋅⋅⋅π interactions between molecules were responsible for the generation of the crystal elasticity. Interestingly, elastic crystals exhibited optical waveguide characteristics in straight or bent state. The optical loss coefficients measured at 627 nm were 0.7 dBmm-1 (straight state) and 0.9 dBmm-1 (bent state), while the optical loss coefficient (α) were 1.5 dBmm-1 (straight state) and 1.8 dBmm-1 (bent state) at 567 nm. Notably, the elastic organic molecular crystal based on barbituric derivative could be used as the candidate for flexible optical devices.

Tunable acoustic waveguide based on a magnetorheological fluid filling

A novel phononic crystals (PnCs) structure that comprises periodically arranged hollow pillars filled with magnetorheological fluid (MRF) is fabricated by three-dimensional printing. The dispersion relations and transmission characteristics of the PnCs filled with MRF are numerically calculated using the finite element method and verified by experiment. Additionally, straight and bent type waveguides are fabricated. The results show that the MRF filling can control the frequency range at the bandgap and passband of the PnCs plate. The bandgap frequency range shifts to a lower frequency when an external magnetic field is applied.

Discrete singular convolution method for modelling of waveguide interaction of beam-type structures with impedance boundaries

Discrete Singular Convolution (DSC) is an accurate local/global mathematical method for the vibration and acoustic analysis of individual continuous structures. In recent years, successful applications of the DSC method for handling connected structures have been performed. However, the studies are very limited and applicable only for common basic boundary conditions such as the pinned, clamped or free boundaries. This study extends the applicability of the DSC method to waveguide interactions of connected structures with the mechanical impedance boundaries. For this purpose, vibration analyses of structures composed of beams forming I-, L- and T-type structures are performed via the DSC. Taylor series expansion is utilized to implement the mechanical impedance boundary conditions. The structural models support the longitudinal and bending waveguides and related interactions. Natural frequencies and vibration displacement responses to a point force excitation are predicted via the proposed methodology. The results are validated by finite element and/or analytical solutions. It has been shown that the DSC method can be reliably applied for structures exhibiting waveguide interactions with impedance boundaries.

Azimuthally asymmetric tubular lattice hollow-core optical fiber

A new, to the best of our knowledge, hollow-core optical fiber based on a tube lattice geometry is proposed. The fiber cross section is formed by eight tubes with five different thicknesses, and the guidance mechanism is based on the inhibited coupling phenomenon. As such, its transmittance spectrum displays low-loss windows intercalated with high-loss regions, each of the latter related to specific core-cladding modal couplings. The spectral behavior of the straight and bent waveguide is numerically analyzed. Simulations on different curvature radii and directions (angles) show the core mode displacement toward the outer side of the curvature and its impact on the spectral shift of the high-loss wavelengths. The different response of each tube resonance is investigated and discussed. The proposed structure identifies a new and promising path for the development of directional curvature sensors.