Flexible Waveguide Research Articles

Elastic orange emissive single crystals of 1,3-diamino-2,4,5,6-tetrabromobenzene as flexible optical waveguides

Single crystals of monoaromatic compounds exhibiting both mechanical softness and optical properties have attracted significant scientific interest in recent years, but they are very scarce.

Application of Low-Loss Transmission Lines with an EHF Radiometer in Gas-Dynamic Experiments

The possibility of using oversized rectangular metal waveguides as elements of the transmission line of an EHF radiometer, which is designed to study fast gas-dynamic processes, is analyzed. The results of a numerical simulation and experimental research of a flexible waveguide for coupling metal waveguides of standard and oversized cross sections, which is constructed on the basis of a dielectric waveguide, are presented. A quantitative estimation of the signal attenuation in the transmission line, which is a combination of a flexible waveguide and an oversized metal waveguide, is performed.

Finite amplitude modal interactions in a weakly nonlinear structural-acoustic cylindrical waveguide

Modal interactions are studied in a weakly nonlinear structural-acoustic waveguide with a cylindrical geometry. The infinite cylindrical waveguide is driven by a flat piston at the center. Standard nonlinear equations are used for the shell and the fluid. The perturbation method separates the nonlinear equations into a linear set and a nonlinear set. At the linear order, only propagating modes are included. Due to the flat nature of the piston, these modes are axisymmetric. These linear order modes interact due to the nonlinearity and generate higher order modes. When a certain inverse relation between phase speeds gets satisfied, spatial resonance (weak shock) occurs, i.e., the solution grows in the propagation direction. Along the same lines, when this matching does not occur, the solution acquires a spatial beating nature that oscillates along the propagation direction. The responses, i.e., resonances or beats are further categorized as self-mode and cross-mode interactions. The conditions and the closed form solutions for resonances and beats are derived. Further, the effect of the boundary flexibility on the said resonances and beats is also presented. And it is seen that the nonlinear wave propagation is significantly different between a rigid and a flexible waveguide.

A Multimodal Dielectric Waveguide-Based Monopulse Radar at 160 GHz

For highly integrated imaging systems above 100 GHz, the complexity and chip area increase significantly with an increasing number of channels. In addition, bulky dielectric lenses prevent applications in spatially restricted surroundings. The presented concept of an imaging monopulse radar with a mechanically flexible front end reduces the required chip area and allows the antenna to be placed in any desired position apart from the sensitive electronics. The radar system is based on a two-channel 160-GHz microwave monolithic integrated circuit (MMIC) feeding a flexible dielectric waveguide. Depending on the angle of the incidence signal, a sum mode (HE11 mode) and a difference mode (HE21 mode) are excited in the dielectric waveguide. MMIC and waveguide are connected by a self-aligning transition reducing the requirements for packaging accuracy. The required chip area of the transition is only $0.022\lambda ^{2}$ with a spacing between the on-chip antennas of $\lambda /4$ . The measured ambiguity-free region between −18° and 15° is defined by the modified elliptical lens antenna focusing in the $E$ -plane only. A mechanical bending of the flexible waveguide is possible down to a radius of at least 2 cm without affecting the angle estimation capability.

Fabrication of Sub-Micron Polymer Waveguides through Two-Photon Polymerization in Polydimethylsiloxane.

Flexible ultra-compact low-loss optical waveguides play a vital role in the development of soft photonics. The search for suitable materials and innovative fabrication techniques to achieve low loss long polymer optical waveguides and interconnects has proven to be challenging. In this paper, we demonstrate the fabrication of submicron optical waveguides in polydimethylsiloxane (PDMS) using divinylbenzene (DVB) as the photopolymerizable monomer through two-photon polymerization (2PP). We show that the commercial oxime ester photoinitiator Irgacure OXE02 is suitable for triggering the DVB photopolymerization, resulting in a stable and controllable fabrication process for the fabrication of defect-free, 5-cm long waveguides. We further explore a multi-track fabrication strategy to enlarge the waveguide core size up to ~3 μm for better light confinement and reduced cross-talk. In these waveguides, we measured a refractive index contrast on the order of 0.005 and a transmission loss of 0.1 dB/cm at 710 nm wavelength.

Crystal Engineering of a Hydrazone Molecule toward High Elasticity and Bright Luminescence.

Flexible luminescent crystals have attracted increasing attention on account of the potential application value in optical material fields. How to design crystals with high elasticity and bright luminescence is an urgent duty that material researchers want to effectuate. Crystal engineering could achieve different crystal materials with various functions based on one molecular structure. Therefore, crystal engineering provides a potential strategy to improve the properties of flexible crystals. In this paper, we gave a pioneer work to improve the properties of flexible luminescent crystals by designing on crystal structure. By disclosing the relationship of structure-property, our work extracted the advantages (elasticity or bright emission) of two inferior crystals and obtained three elastic bending crystals with outstanding emission behaviors. Notably, our work not only achieves the fabrication of flexible crystals based on crystal engineering aspect but also provides good candidates for flexible optical waveguiding materials.

A Flexible High-Selectivity Single-Layer Coplanar Waveguide Bandpass Filter Using Interdigital Spoof Surface Plasmon Polaritons of Bow-Tie Cells

In this article, a flexible high-selectivity single-layer coplanar waveguide (CPW) bandpass filter using interdigital spoof surface plasmon polaritons (SSPP) of bow-tie cells is proposed, simulated, and fabricated. The low and high cut-off frequency can be adjusted independently, which is one of the characteristics of the presented bandpass filter. The SSPP of bow-tie cells is used to adjust the high cut-off frequency, while the interdigital coupling structure of the SSPP is designed to adjust the lower one. The proposed filter also has advantages of single-layer structure and high selectivity. Simulated results of the proposed filter show a theoretical 3-dB bandwidth from 2.28 to 5.12 GHz (about 76.8%) with high selectivity ( $\vert S_{11}\vert dB and −0.4 dB $> \vert S_{21}\vert > -1.1$ dB in the passband), and measured results agree well with simulated ones. Since the filter in this article is designed based on the flexible printed circuit (FPC) board, the circuit can be tested in bent, folded, and even twisted states, and these measured results are all basically consistent with the test result of the flat state circuit. This characteristic can greatly reduce the size of the filter and increase the flexibility of the circuit application.

Bend- and Twist-Insensitive Flexible Multimode Polymer Optical Interconnects

Polymer multimode optical waveguides can enable high-speed short-reach optical interconnection at low cost within high performance electronic systems. The formation of such waveguides on flexible substrates can offer important additional advantages such as light weight, ability to be tightly bent, and reconfigurability which are particularly important in environments where space, weight, and shape conformity are critical, for instance in vehicles and aircraft. The ability of such flexible optical interconnects to be tightly bent and twisted with low excess loss is crucial in enabling their use in systems with limited space and with movable parts. As a result, in this work, we present a new design of such flexible polymer multimode waveguides that achieves improved bending loss performance over the conventional waveguide design. It is experimentally shown that the proposed design achieves a very low excess loss of 0.5 dB for a 3 mm radius bend under a 50 μm MMF launch. In comparison, flexible waveguides with the conventional design exhibit a 2 dB excess loss under the same launch and bend conditions. Additionally, useful rules that associate the twisting loss performance of flexible polymer waveguide samples with their geometric characteristics are derived. It is shown that negligible twisting losses (<0.1 dB for a 50 μm MMF input) can be achieved when the dimensions of the waveguide samples are appropriately selected. The results demonstrate the strong potential of such bend- and twist-insensitive flexible polymer waveguides for use in next-generation vehicles and aircraft.

P‐82: Color Flexible Waveguide Display using Polymer Stabilized Liquid Crystal

In this paper, we report an advanced color flexible waveguide display which consists of two flexible plastic substrates and a sandwiched layer of liquid crystal/polymer composite. The display is edgelit by white and color (R, G, and B) LEDs. In the absence of applied voltage, the liquid crystal is in a uniform aligned state and is transparent, and the incident light is waveguided through the display and no light comes out of the front plane of the display. When a voltage is applied, the liquid crystal is switched to a poly‐domain state and becomes scattering, and the incident is scattered out of the front plane of the display. The display does not need polarizers and is thus compatible with plastic substrates with non‐uniform birefringence. It has sub‐ms response time and thus color sequential scheme can be used to display full color images. This flexible display has a big advantage in terms of energy efficiency because it does not need color filters and polarizers, and is suitable for transparent display application.

Attenuation analysis of flexural modes with absorbent lined flanges and different edge conditions.

The analysis of fluid-structure coupled waveforms and the attenuation of these waveforms in a flexible waveguide is carried out. The physical configuration contains an expansion chamber connected by an extended inlet/outlet by means of vertical lined flanges. The governing boundary value problem is solved by using Mode-Matching (MM) technique. The associated eigen expansions of field potentials include non-orthogonal eigenfunctions and the related eigen-sub-systems are classified in the non-Sturm Liouville category, whereby the use of generalized orthogonal characteristics has ensured the point-wise convergence of solution. The low frequency approximation (LFA) which relies on the limited propagating modes is developed and is compared with MM solution. In the low frequency regime, a good agreement between MM and LFA results is found. Furthermore, the numerical experiments are performed to analyze the effects of absorbent linings and edge conditions on the attenuation of flexural modes. The guiding structure is exited with the structure-borne mode incident as well as the fluid-borne mode incident. It is found that the use of edge conditions and the absorbent linings significantly affect the attenuation of structure-borne mode and fluid-borne mode, respectively.

Mechanophotonics: Flexible Single‐Crystal Organic Waveguides and Circuits

AbstractWe present the one‐dimensional optical‐waveguiding crystal dithieno[3,2‐a:2′,3′‐c]phenazine with a high aspect ratio, high mechanical flexibility, and selective self‐absorbance of the blue part of its fluorescence (FL). While macrocrystals exhibit elasticity, microcrystals deposited at a glass surface behave more like plastic crystals due to significant surface adherence, making them suitable for constructing photonic circuits via micromechanical operation with an atomic‐force‐microscopy cantilever tip. The flexible crystalline waveguides display optical‐path‐dependent FL signals at the output termini in both straight and bent configurations, making them appropriate for wavelength‐division multiplexing technologies. A reconfigurable 2×2‐directional coupler fabricated via micromanipulation by combining two arc‐shaped crystals splits the optical signal via evanescent coupling and delivers the signals at two output terminals with different splitting ratios. The presented mechanical micromanipulation technique could also be effectively extended to other flexible crystals.

Mechanophotonics: Flexible Single-Crystal Organic Waveguides and Circuits.

We present the one-dimensional optical-waveguiding crystal dithieno[3,2-a:2',3'-c]phenazine with a high aspect ratio, high mechanical flexibility, and selective self-absorbance of the blue part of its fluorescence (FL). While macrocrystals exhibit elasticity, microcrystals deposited at a glass surface behave more like plastic crystals due to significant surface adherence, making them suitable for constructing photonic circuits via micromechanical operation with an atomic-force-microscopy cantilever tip. The flexible crystalline waveguides display optical-path-dependent FL signals at the output termini in both straight and bent configurations, making them appropriate for wavelength-division multiplexing technologies. A reconfigurable 2×2-directional coupler fabricated via micromanipulation by combining two arc-shaped crystals splits the optical signal via evanescent coupling and delivers the signals at two output terminals with different splitting ratios. The presented mechanical micromanipulation technique could also be effectively extended to other flexible crystals.

Radio-frequency design and commissioning of a flexible waveguide for high-vacuum S-band applications

Flexible waveguides, which allow for greater deformation as compared to traditional hard waveguides, are highly useful for simplifying waveguide system assemblies. However, the peak powers of these waveguides have been limited to several megawatts in non-vacuum environments. Herein, a novel flexible waveguide was developed for the S-band (2856 MHz) with the aim of facilitating the interconnection of two hard waveguides. The flexible waveguide was composed of brass, and its surface was plated with oxygen-free high-conductivity copper for brazing with stainless steel flanges in a vacuum furnace. By connecting the stainless steel flanges together, it can be applied to a high-vacuum environment. The prototype exhibited good measurement results in low-power microwave tests; the results of a 72-h vacuum leak test showed satisfactory vacuum performance as well, and no obvious surface collapse was observed. High-power microwave commissioning was conducted between June and July 2019; the waveguide showed a maximum average power of approximately 2.7 kW. Unfortunately, the prototype was damaged after reaching that power level, which indicates the need for further optimization of the fabrication process. From our experiment, it is clear that the flexible waveguide cannot be used in large accelerators because of its average power limit.

Nonlinear sound and structure interaction in a fluid–filled flexible waveguide

In this study, a 2-D infinite flexible waveguide is considered. The waveguide carries a weakly nonlinear acoustic fluid. It is bounded on one side by a weakly nonlinear flexible membrane and the other side is rigid. The infinite waveguide is driven at the origin by a piston oscillating at a single frequency. However, we focus only on the positive side of the piston. As the coupled waves propagate in the membrane and the fluid, the modal interactions lead to resonances and beats which form the main focus of this work. A regular perturbation method is used to derive the linear and the quasilinear order equations which are then solved. At the linear order, the primary wavenumbers are solved for and the modes are found to be non-orthogonal because of the flexible membrane. Only the propagating waves are included in the analysis. Both self-mode and cross-mode interactions of the planar and the non-planar modes are considered. The novelty of this work lies in obtaining conditions and the closed form solutions for the resonances and beats along the spatial coordinate. It is found that the self-mode interactions lead to beats only. And in the self-mode interactions, the coupled planar mode plays an important role. On the other hand, the cross-mode interactions can lead to either resonances or beats.

Graphene electrodes for electric poling of electro-optic polymer films.

We propose electric poling of electro-optic (EO) polymer films with graphene electrodes. The use of graphene electrodes can waive the use of buffer layers and minimize the poling voltage. To demonstrate the idea, we prepared EO polymer thin-film waveguides for poling with traditional Au/ITO electrodes and graphene electrodes, where the EO polymer is a guest-host system formed by doping 15 wt% of dipolar polyene chromophore AJLZ53 into the random copolymer P(S-co-MMA). Our experiments confirm that the use of graphene electrodes can significantly reduce the poling voltage. For a 3.8-µm-thick EO polymer film, we achieve high EO coefficients of 82 pm/V at 1541 nm and 110 pm/V at 1300 nm with a poling voltage of 420 V. In addition, the use of graphene electrodes allows more flexible waveguide designs and can potentially simplify the fabrication of devices based on EO polymer.

On the attenuation of fluid–structure coupled modes in a non-planar waveguide

In this article, the attenuation of fluid–structure coupled modes of non-planar waveguide involving cavities is discussed. The physical problem is modeled to illustrate the scattering behavior of acoustic waves in a flexible waveguide composed of thin elastic elements having edges or joints and structural discontinuities. The fluid–structure coupled waveforms scatter after interacting with the discontinuities and edges of the underlying structure. An appropriate choice of edges offers scattering or resonance processes, which also guide fluid–structure coupled waves. The mode-matching technique, together with low-frequency approximation, is used to determine velocity potentials. The guiding structure is then analyzed and validated through scattering energy functionals by varying the dimensions of the cavities and the wave frequency. The results are formulated and analyzed by tuning the device using an appropriate choice of edge conditions, dimension of cavities, and wave frequencies, thereby validating the obtained solutions. These descriptions are very useful for the active control measure of structural vibrations.

Self-Aligning and Flexible Dielectric Waveguide Plug for MMICs at G-Band

The packaging technology for transitions to dielectric waveguides in the frequency range above 100 GHz is complex and must be highly precise, and the waveguides are usually permanently connected. This letter presents a transition from a monolithic microwave integrated circuit (MMIC) to a flexible dielectric waveguide at $G$ -Band (140–220 GHz), which is self-aligning and, thus, reduces the requirements for packaging accuracy. Furthermore, the transition is mechanically decoupled to avoid mechanical stress to the MMIC and to reconnect it arbitrarily often. A patch radiator on a quartz-glass carrier is excited by a coupler on the MMIC. It feeds the $\mathrm {HE_{11}}$ mode into a rigid, high-permittivity dielectric dome, which increases the coupling efficiency. The flexible dielectric waveguide is placed above the dome and fixed with Rohacell half shells. The minimum insertion loss measured with a back-end-of-line (BEOL) MMIC is 3.0 dB at 168 GHz.

Low-Loss, Thermally Insulating, and Flexible Rectangular Dielectric Waveguide for Sub-THz—Signal Coupling in Superconducting Receivers

Dielectric waveguides have potential as transmission lines in subterahertz (sub-THz) applications due to their low-loss. A new application of a rectangular dielectric waveguide (RDW) for a THz superconducting receiver system for radio astronomy is investigated and demonstrated for the first time. The RDW can be used to transmit sub-THz local oscillator (LO) signals into the cryostat for driving the superconductor insulator superconductor mixer, with very low heat transfer. A series of RDWs with different lengths are fabricated. The measured average attenuation constants of the RDW are 0.034 and 0.069 dB/mm at 3.4 and 300 K measurement environments over 240–300 GHz, respectively. The double-sideband system noise temperature of the receiver system is also measured. With this LO signal feeding system, there is only 0.69 mW thermal transfer power from the 300 to the 3.4 K cold stage. Compared to traditional pumping signal feeding schemes, such as the horn-to-horn structure and beam splitter method, this approach outperforms the traditional schemes in terms of feeding flexibility, insertion loss, and thermal isolation for the cryostat. The proposed RDW can be widely applied to superconducting receivers for radio astronomy in the future.

Mapping a quantum walk by tuning the coupling coefficient

We present a method to map the evolution of photonic quantum walks that is compatible with nonclassical input light. Our approach leverages a newly developed flexible waveguide platform to reconfigure the jumping rate between spatial modes, allowing the observation of a range of evolution times in a chip of fixed length. In a proof-of-principle demonstration, we reconstruct the evolution of photons through a uniform array of coupled waveguides by monitoring the end-face while tuning the device. This approach enables direct observation of mode occupancy at arbitrary resolution, extending the utility of photonic quantum walks for quantum simulations and related applications.

Multipurpose Polymer Bragg Grating-Based Optomechanical Sensor Pad.

Flexible epoxy waveguide Bragg gratings are fabricated on a low-modulus TPX™ polymethylpentene polyolefin substrate for an easy to manufacture and low-cost optomechanical sensor pad providing exceedingly multipurpose application potentials. Rectangular EpoCore negative resist strip waveguides are formed employing standard UV mask lithography. Highly persistent Bragg gratings are inscribed directly into the channel waveguides by permanently modifying the local refractive indices through a well-defined KrF excimer laser irradiated +1/-1 order phase mask. The reproducible and vastly versatile sensing capabilities of this easy-to-apply optomechanical sensor pad are demonstrated in the form of an optical pickup for acoustic instruments, a broadband optical accelerometer, and a biomedical vital sign sensor monitoring both respiration and pulse at the same time.