Bent Waveguides Research Articles

A Compact and Easy to Fabricate $E$ -Plane Waveguide Bend

In this letter, we present a new $E$ -plane rectangular waveguide bend that can be micromachined using split-block fabrication. The 90° bend is created by machining two linear channels with a notch left in the outside corner. This notch significantly improves the return loss and the insertion loss of the bend. The primary advantage of this technique is that it produces a compact waveguide bend, minimizing both the size and the conduction loss of the waveguide circuit. Another advantage is that this technique only requires linear channels to be machined, thus it can be fabricated without requiring a computer numerically controlled (CNC) milling machine. The performance of this waveguide bend was optimized using electromagnetic simulation software, and the optimal reflection coefficient was found to be below −28 dB across the entire operational bandwidth of the waveguide. The simulated design was then validated using two experimental prototypes that were tested at microwave (12–18 GHz) and millimeter frequencies (140–220 GHz).

Utilizing Shift-or-Shrink Method to Optimize Multisegment Structures of Bent Optical Waveguides

The authors utilized a heuristic shift-or-shrink method that is associated with the beam propagation method to design an optimal multisegment bent optical waveguide. By transforming the shape of each segment, the multisegment structure of the bent waveguide can be optimized. In addition to achieving maximal transmission efficiency, only four segments are sufficient to satisfactorily improve the transmission performance. Moreover, the optimal path of each of these multisegment bent waveguides remains similar to each other waveguide's optimal path even though the number of the segments increases.

Polymer multimode waveguide bend based on a multilayered Eaton lens.

Reducing the bending radius of low-index-contrast waveguides is essential in reducing the size of the integrated optical components. A polymeric multimode waveguide bend is presented based on the Eaton lens. Ray-tracing calculations are utilized to truncate the Eaton lens in order to improve the performance of the bend. The truncation of the lens decreases the footprint of the bend as well. A designed waveguide bend with a radius of 18.4μm is implemented by a concentric cylindrical multilayer structure. Average bend losses of 0.69 and 0.87dB are achieved for the TM0 and TM1 modes, respectively, in the C-band of optical communication. The bend loss is lower than 1dB in a bandwidth of 1520-1675nm for both modes.

Extremely anisotropic epsilon-near-zero media in waveguide metamaterials.

Extremely anisotropic media such as anisotropic zero-index media allow wave propagation in extraordinary ways that are absent in isotropic systems. Here, we propose an approach to realize a type of extremely anisotropic effective epsilon-near-zero media by exploiting anisotropic waveguide metamaterials. The metamaterial is composed of two kinds of dielectrics with metal wires in waveguides. Extreme anisotropy is realized by alternating layers of the two kinds of dielectrics, which lead to different cut-off frequencies for transverse electric modes in different directions. Anisotropic effective epsilon-near-zero media with low loss can be obtained around the cutoff frequencies. Based on the extreme anisotropy, the unique phenomena of directive emission, nearly perfect bending waveguides and arbitrary control of energy flux are demonstrated. Interestingly, subwavelength focusing of energy flux with a spotsize <0.2λ is observed (λ is the wavelength in free space). Our work provides a convenient approach for the realization of extremely anisotropic epsilon-near-zero media and unique applications in low-loss waveguide metamaterials.

Radiation in bent asymmetric coupled waveguides.

A numerical study of the radiation in coupled bent waveguides is presented. Such arrangements of curved waveguides in reduced radiation-loss configurations can be used to enhance the quality factor of integrated microresonators. 3D full vector computations of the complex modal fields (and propagation constants) and the transient propagation effects have been performed. The results obtained agree qualitatively with former 2D finite-difference time-domain (FDTD) analyses. At the same time, the calculations presented meet the accuracy requirements for the design of practical implementations that are unattainable with 2D approaches.

Guiding light in bent waveguide superlattices with low crosstalk

Waveguide superlattices with a subwavelength pitch and low crosstalk can significantly increase the waveguide integration density and are beneficial for many chip-scale applications. Bending of such high-density waveguide superlattices is necessary for flexible signal routing. However, tight bending tends to induce high crosstalk between guided modes, as witnessed in multimode waveguide bends. Here we explore the mechanisms of light guiding and coupling in a subwavelength-pitch waveguide superlattice bend and analyze how bending further modifies the already “renormalized” parameters of superlattice modes via various physical effects. Particularly, bending can skew the phase mismatch in a waveguide superlattice, sometimes producing a near phase-matching condition and causing salient crosstalk spikes among non-first-nearest neighbors. Interestingly, a waveguide superlattice with less pristine phase mismatch may be more robust against such skew of phase mismatch and can suppress crosstalk spikes by ∼10 dB. Bending with 5–15 μm radii and subwavelength pitches has been demonstrated with crosstalk lower than −19.5 dB. The scaling of the footprint of waveguide superlattice bending is analyzed, and significant footprint reduction can be achieved for chip-scale applications. The rationale for footprint reduction of superlattice bending under crosstalk constraint differs markedly from that of a single waveguide bend.

Dual-Layer Cross-Coupled Tunable Resonator System in a Three-Dimensional Si3N4 Photonic Integration Platform

With a three-dimensional Si3N4 integration, we demonstrate a dual-layer cross-coupled resonator system where an S -bend waveguide is coupled with a racetrack microring resonator loaded with a sub-microring. The device has a compact structure and provides more design freedom. The transfer matrix method is used to model the device. The resonance spectrum is significantly affected by the coupling coefficient between the waveguide and the resonator, generating versatile resonance features. A group of devices with a varying coupling length are implemented, exhibiting an evolution trend consistent with the simulation results. The resonance wavelength can be thermo-optically tuned by the micro-heater placed above the resonator. The three-dimensional Si3N4 platform enables dense integration of high-performance devices, opening new avenues for future compact, high-density, and cost-effective photonic integrated circuits.

Design and Simulation of the Fourier Transform Beam Propagation Method in Different Optical Waveguides

Fundamentals of optical waveguides is a needed resource for any researcher, concerned in optics and communications engineering. Interested by designing or actively running with optical gadgets have to have a company hold close of the standards of light wave propagation. This paper presents an FFT Beam Propagation Method Models for optical waveguides. The technique is based on the use of the Propagation of a Gaussian pulse by a waveguide with a rectangular, triangular optical waveguides and free space the cases of straight and bent optical waveguides are successively considered.

Type-I hyperbolic metasurfaces for highly-squeezed designer polaritons with negative group velocity

Hyperbolic polaritons in van der Waals materials and metamaterial heterostructures provide unprecedented control over light-matter interaction at extreme nanoscales. Here we propose a concept of type-I hyperbolic metasurface supporting highly-squeezed magnetic designer polaritons, which act as magnetic analogs of hyperbolic polaritons in the hexagonal boron nitride (h-BN) in the first Reststrahlen band. Compared with the natural h-BN, the size and spacing of the metasurface unit cell can be readily engineered, allowing for manipulating designer polaritons in frequency and space with greater flexibility. Microwave experimental measurements display a cone-like dispersion in momentum space, exhibiting an effective refractive index up to 60 and a group velocity down to c/400. Tailoring the metasurface, we demonstrate an ultra-compact integrated designer polariton circuit including high-transmission 90° sharp bending waveguides and waveguide splitters. The present metasurface could serve as a platform for polaritonics, and find applications in waveguiding, terahertz sensing, subdiffraction focusing/imaging, low-threshold terahertz Cherenkov radiation, and wireless energy transfer.

Low-index-contrast waveguide bend based on truncated Eaton lens implemented by graded photonic crystals

Low-index contrast waveguides such as silica waveguides are indispensable part of passive integrated optical components. Reducing the bending radius of silica waveguides with low bend loss is vital in miniaturizing silica planar lightwave circuits. The Eaton lens is a gradient index lens that can bend parallel light rays by 90{\deg}, 180{\deg}, or 360{\deg}. We present a low-loss and compact 90{\deg} waveguide bend with a radius of 9 $\mu$m designed by truncating the Eaton lens. The performance of the designed bend, implemented by graded photonic crystal, was evaluated by full-wave two-dimensional finite element method. The average bend loss in the C-band is 0.9dB while the average bend loss for the wavelength range of 1260-1675nm is 1.05dB. The proposed design strategy can be applied to other low-index contrast waveguides.

Large-capacity and low-loss integrated optical buffer.

Temporarily storing light occupies a pivotal position in all-optical packet switching network and microwave photonics. An integrated optical buffer with large capacity and low loss is demonstrated on a silica wafer. The optical buffer consists of four silica waveguide delay lines connected by five thermo-optic switches. With different switch combinations applied, related delay lines are selected to realize a different storage time in the buffer, and a storage time up to 100 ns with a 10-ns step size is implemented. By optimizing the fabrication process and introducing the offsets between straight and bending waveguides, the propagation loss as low as ~1.08 dB/m is achieved. This large-capacity and low-loss buffer enables broad applications in optical communications and microwave photonics.

Variation of device topology for material simplification as three‐dimensional transformation optics methodology

A three-dimensional (3-D) transformation optics (TO) methodology is presented using polyhedral meshing and homogeneous materials, in which, the topology of the device is varied, in order to achieve material simplification. The device is divided into a minimal number of polyhedral segments, and a linear transformation is applied to each polyhedron. The polyhedrons and transformations are tried to be chosen so that the volume is conserved through the transformation, and if possibly, some transformed materials are equivalent to free space. A few 3-D waveguide coupler and carpet cloak problems are solved to demonstrate and evaluate the proposed methodology. For instance, it is shown that a right-angle 3-D rectangular waveguide bend can be designed using only one two-dimensional (2-D) homogeneous material. Also, resize couplers are designed using only one or two 2-D homogeneous materials. A carpet cloak is demonstrated which uses only two material types, filling five polyhedral meshing segments. Similar problems would have required much larger number of meshing segments and material types, using previous methods. Therefore, using the proposed methodology, 3-D TO-based guiding and cloaking devices will be more feasible. Furthermore, a 3-D finite difference time domain (FDTD) method is developed to verify the results of some examples.

Improving the spectral performance of extended cavity diode lasers using angled-facet laser diode chips

We present and compare theoretical and experimental results on the electro-optical performance of extended cavity diode lasers (ECDLs) that employ two ridge waveguide designs for the single-transverse mode GaAs laser diode chip. One facet of the laser diode chips serves as a partially reflective output coupler for the laser cavity. The other facet constitutes an intra-cavity interface which introduces spurious optical feedback to the laser diode chip. The waveguide designs differ with respect to the suppression of this spurious feedback. The first design employs a straight ridge waveguide intersecting both facets at normal incidence. The intra-cavity facet is anti-reflection coated and features a residual intensity reflectivity of the order $$10^{-4}$$ . The second design employs a bent ridge waveguide intersecting the anti-reflection-coated intra-cavity facet at an appropriate angle. This provides an additional suppression of the spurious intensity reflection to a value estimated to be less than $$10^{-6}$$ . We compare the electro-optical performance of both designs theoretically and experimentally. The utilization of a bent waveguide results in an improved spectral stability and purity, specifically a higher side mode suppression and a small intrinsic spectral linewidth over the whole investigated current range, of the external cavity diode laser without sacrificing other parameters such as the output power. The external cavity diode lasers under study exhibit no degradation of the measured frequency noise power spectra and intrinsic linewidths even if there is a drop of the side mode suppression ratio provided that it is not reduced to a very small value. Thus, the usage of a more readily accessible straight waveguide chip in an ECDL could be sufficient if only a limited tuning range and a particularly compact assembly are needed. For spectroscopic applications requiring a small intrinsic spectral linewidth over a large frequency range a bent waveguide chip could be the better choice.

Analysis of 90° bend photonic crystal waveguide: an application to optical interconnect

In this research, we propose and analyse a two-dimensional photonic crystal having L-shaped bend waveguide to realise a negligible bending loss, nonlinearity and confinement loss at the signal of 1.31 μm and 1.55 μm. The proposed waveguide comprises 9 × 9 circular dielectric rods of silicon with air as background material. Further, finite-difference time-domain method is employed for analysing electric field distribution in order to realise effective area as well as high transmission characteristics of the proposed waveguide. Moreover, numerical formulations are used for simulation of bending loss, nonlinearity and confinement loss in the waveguide with suitable structure parameters (diameter of circular rods, nature of the material, lattice spacing). Again, simulation results revealed that ultra-low bending loss of 0.72 × 10−7 dB/m and 0.93 × 10−7 dB/m is experienced by the waveguide at wavelengths 1.31 μm and 1.55 μm, respectively, for 10 nm diameter of the circular rod, whereas a feeble nonlinearity of 0.085 W−1 km−1 and 0.072 W−1 km−1 is also asserted in the same. Furthermore, simulation outcomes proclaimed a negligible confinement loss of 5.7 × 10−11 and 4.8 × 10−11 for the said wavelengths. Finally, this article discloses that the projected silicon-based photonic crystal waveguide appears to be an apt candidate as optical interconnects for application in photonic integrated circuits.

Design of a TE01-Mode Waveguide Bend Based on an Elliptical Waveguide Structure

A Ka-band broadband TE01-mode overmoded waveguide bend based on an elliptical waveguide structure is proposed for high-power transmission line in this paper. To investigate the mode conversion inside the waveguide bend, the mode coupling coefficients of elliptical waveguide bend are theoretically deduced. The influence of the cross section on the power-handling capacity and the transmission efficiency is studied. Calculation and simulation results present that the proposed overmoded waveguide bend has a propagation bandwidth of 7 GHz (relative bandwidth of 21%) with an efficiency over 95%. A prototype is fabricated and measured. The average transmission efficiency is over 93% in the range of 29–36 GHz according to the S-parameter measurement. The results of near-field test demonstrated that a TE01 mode with high purity is obtained at the output port of waveguide bend. The high-power performance is investigated by connecting it with a Gyrotron traveling wave tube, and the waveguide bend works stable under a maximum output power of 150 kW. The measurement results demonstrate that a novel approach is proposed successfully for the design of high-power waveguide bend.

A General S-bend Approximation by Cascading Multiple Sections of Uniformly Curved Waveguides

Curved waveguides are an important wave guiding structure and have been widely used in building integrated photonics circuits with various functions including directional couplers, modulators, ring resonators, and Mach-Zehnder interferometers etc. However, curved structure always leads to attenuation of the guided mode as it propagates through the bend region. In practice the bending loss may be negligible if the radius of curvature is large, but the loss increases rapidly at small radii. Amongst curved waveguide structures the S-bend has been widely used, because it is relatively easy to design and can provide a low transition loss between parallel offset waveguides. In this paper we present a new general approach to S-bend optical waveguides by cascading multiple sections of uniformly curved waveguides. The aims are to offer maximize structure that can be used in more powerful beam propagation methods to estimate the loss in bends of continuously-varying curvature. We approach the S shaped bend waveguide by multiple sections such as 2, 6, 14 and 30 sections back to back and calculate the total loss. The convergence of resulting curvature variation are very rapid and the overall loss calculation found good agreement with analytical calculation based on low slope approximation.

Enhancing the performance of strip and 180-deg slot waveguide bends for integrated optical waveguide modulator

Waveguide bends are essential structures for designing an integrated optical waveguide modulator (IOWM). However, reducing bending loss for strip and slot waveguide bends and obtaining a small increment of footprint size by bending the radius are challenging issues. We aim to evaluate whether using strip waveguides with an adiabatic bend reduces the bending loss and to obtain a specific length suitable for a V-shaped mode converter with a small increment of footprint size in the IOWM. Furthermore, a new 180-deg-bend structure for slot waveguides, to reduce bending loss, is investigated. The results of reducing bending radius (R) with a large coverage angle (αc) for strip waveguide bends revealed a low bending loss and simultaneously a reduction of the footprint size in the IOWM. For the 180-deg slot waveguide bend, both the bending and coupling losses were reduced by optimizing the taper and distance offset, which enhanced transmission power efficiency. We validated the feasibility and efficiency of the proposed bending strip and slot structures in IOWM, which is important for integrated optical applications.

Integrated photonics chip with InGaN/GaN light-emitting diode and bended waveguide for visible-light communications

We propose an integrated photonics chip having InGaN/GaN micro-LED and free-standing bended waveguide on GaN-on-silicon platform to change the propagation direction of light by 180° and achieve free-space visible-light communication (VLC) with high speed. The light transmission performance and light loss due to bended waveguides with different geometrical parameters are studied by analyzing the light intensity of tip and middle of bended waveguide in FDTD simulation and experiment. Moreover, with the waveguide width decreasing from 8 µm to 4 µm, light intensity ratio between emitting light transmitted to waveguide tip and light leakage from middle of waveguide shows an over linear reduction. Meanwhile, the decreasing of the diameter of waveguide curvature from 500 µm to 250 µm only results in under linear increase of light intensity ratio. Besides, the free-space data transmission test with 100 Mbps rate is conducted by using integrated chip as transmitting terminal in VLC. This study opens a novel way for the application of bended waveguides in connecting the multiple independent optics components in photonics chip and confining optical signal in efficient on-chip optical signal transmission in VLC.

Surface coupling of plasma optical emission spectra with bent metal-clad waveguide

We present the design of compact bent metal-clad waveguides to collect the vertically emitted light and to transmit to the horizontal direction. The intention of the design is to collect the plasma optical emission spectra inside the semiconductor etch process chamber to monitor plasma gas phase chemistry resides in the process chamber. Coupler A has been designed for visible spectra range from 748 to 753 nm at which the maximum coupling efficiency is over 52%, while Coupler B operates at the wavelength range from 775 to 780 nm with coupling strength of 62% and Coupler C covers the visible spectrum from 808 to 813 nm with its maximum efficiency of 60%. The surface couplers reveal good spectral characteristics, considerable alignment tolerance and minimal size of 9 × 5 × 1.6 µm3 each. Relatively simple geometrical structure over conventional grating coupler is advantageous to lower the fabrication cost as well.

Demonstration of resonant phenomena analogous to Autler-Townes splitting, electromagnetically induced transparency and Fano resonances in a deformed waveguide resonator

In this paper, we demonstrate resonant phenomena analogous to Autler-Townes splitting, electromagnetically induced transparency and Fano resonances in a resonator based on a single-mode optical waveguide with metallized end-faces. The phenomena are shown to result from strong coupling between the fundamental mode of the waveguide core and the cladding whispering gallery mode. The results obtained open new possibilities in the field of building functional photonic elements using cladding modes of bent waveguides and, in particular, fiber optic refractometers for bio- and chemosensing applications.