Showing papers on "Bend radius published in 2020"

Investigating the Refractive Index Sensitivity of U-Bent Fiber Optic Sensors Using Ray Optics

Abstract: Geometrically modified fiber optic sensors (FOS), particularly U-bent FOS, have gained significant attention due to their remarkably high refractive index (RI) and evanescent wave absorbance (EWA) sensitivity, as well as their ergonomic design and ease in handling. In this article, we present a theoretical model for the U-bent FOS probes to predict the sensor behavior by numerically simulating the light propagation in an equivalent 2D semi-circular ring using ray tracing approach. In addition to the effects due to the modification of geometry, this article presents a thorough investigation of the influence of the bend-induced material deformation on the nature of light propagation and refractive losses. We introduce “bend ratio” (ratio of bend radius to fiber core radius) to explain the influence of geometry modification and the bend-induced inhomogeneity in RI (BIRI) of the fiber core on RI sensitivity. The bend ratio of bent plastic optical fiber sensors falls under one of the four bending regimes, namely, gentle, geometric, saturation, and plastic, for which the bend ratios are less than ∼35, ∼25, 17, and 7, respectively. The results also show that for bend ratios less than 7, BIRI inhomogeneity is responsible for the high RI sensitivity observed with U-bent probes as opposed to the simple geometric modification. This article also indicates the existence of an optimum bend ratio (for a given value of RI of the surrounding medium) where RI sensitivity is maximum. These findings were validated with previously reported experimental results.

Ultra-thin silicon-on-insulator waveguide bend based on truncated Eaton lens implemented by varying the guiding layer thickness

Abstract: Silicon-on-insulator (SOI) waveguides with different geometries have been employed to design various integrated optical components. Reducing the bending radius of the SOI waveguides with low bending loss is essential in minimizing the footprint of light-wave circuits. The propagating mode is less confined in the core of the ultra-thin SOI waveguide and penetrates to substrate and cladding, leading to higher bending loss compared to conventional SOI waveguides with a thicker guiding layer. Although various bending mechanisms have been utilized to reduce the bending loss of conventional SOI waveguides, the ultra-thin SOI waveguide bends have not been studied in detail. In this paper, we present a 60-nm-thick SOI waveguide bend based on the truncated Eaton lens implemented by varying thickness of the guiding layer. The three-dimensional full-wave simulations reveal that the designed waveguide bend, with a radius of 3.9 μm, reduces the bending loss from 3.3 to 0.42 dB at the wavelength of 1550 nm. Moreover, the bending loss for the wavelength range of 1260–1675 nm is lower than 0.67 dB while the bending loss in the C-band is lower than 0.45 dB.

Design and test of a curved superconducting dipole magnet for proton therapy

Abstract: We report on the design and test of a curved superconducting magnet completed as part of a collaborative project between Lawrence Berkeley National Laboratory, the Paul Scherrer Institute, and Varian Medical Systems focused on developing compact, lightweight gantries for proton therapy. An overview of the magnet design is given first, showing two Nb–Ti Canted-Cosine-Theta (CCT) layers producing a dipole field of 2.4 T in a clear bore of 290 mm diameter, with bend radius of 0.9 m, and magnetic bend angle of 50 degrees. The magnet fabrication process is then presented with a focus on the challenges associated with curved CCT mandrel manufacturing, winding, and assembly. Finally, quench training and magnetic field measurements are reported from a first test in liquid helium during which the magnet reached the design field of 2.4 T.

Design of a compact and broadband terahertz polarization splitter based on gradient dual-core photonic crystal fiber

Abstract: We report a broadband polarization splitter based on polyethylene photonic crystal fiber with microstructured dual refractive index gradient cores. These dual cores consist of a properly optimized arrangement of air holes such that for individual fibers x-polarized modes have large effective indices difference, while this index difference is almost zero for their y-polarized modes, leading to efficient coupling between the y-polarized modes. We have shown that by proper optimization of gradience created in the arrangement of air holes, efficient polarization splitting can be achieved for a broad range of terahertz frequencies. Device length and extinction ratio have been calculated numerically for the proposed configuration. Device length of ∼1.96 to ∼60cm was found to be appropriate for frequencies in the 0.4–1.0 THz range to have high extinction ratios: −38 to −49dB and −15 to −23dB for the x and y polarizations, respectively. The bending loss for the proposed design is quite low: ∼0.05dB/m at 1 THz for the bend radius of 1 cm. These results suggest that a compact, low-loss, and broadband polarization splitter with very high extinction ratios can be achieved by wrapping the fiber around a small mandrel.

Distributed curvature sensing based on a bending loss-resistant ring-core fiber

Abstract: A theoretical and experimental study on curvature sensing using a Brillouin optical time-domain analyzer based on the ring-core fiber (RCF) is reported. The Brillouin gain spectrum of the RCF is investigated, and the Brillouin frequency shift (BFS) dependence on temperature and strain is calibrated. We theoretically analyze the fiber bending-induced BFS and peak Brillouin gain variation for the RCF through a numerical simulation method, and the RCF is revealed to have a high curvature sensitivity. Distributed curvature sensing is successfully demonstrated, with the bending radius ranging from 0.5 to 1.5 cm, corresponding to a BFS variation from 32.90 to 7.81 MHz. The RCF takes advantage of great bending loss resistance, and the maximum macrobending loss at the extreme bending radius of 0.5 cm is less than 0.01 dB/turn. Besides, the peak Brillouin gain of the RCF is discovered to vary significantly in response to fiber bending, which is expected to be another parameter for distributed curvature determination. The results imply that the RCF is a promising candidate for highly sensitive distributed curvature measurement, especially in sharp bending circumstances.

Polarization Coupling of $X$ Polarization Coupling of X-Cut Thin Film LN Based Waveguides

Abstract: Thin film lithium niobate (LN) shows great potentials for highly compact passive and active devices. As LN is an anisotropic material, waveguides made on it exhibit different mode properties from those on conventional isotropic materials. We study the effective refractive indices of fundamental modes of two polarizations in etched ridge waveguides on an $X$ -cut LN thin film. Mode hybridization phenomenon, where the effective refractive indices of the two polarizations are close, is analyzed in detail with different structural parameters. Transmission through a 90° bend, which is a typical routing element for a photonic chip, is simulated. Significant polarization coupling related to the mode evaluation through the bend is observed, and becomes the dominant fact limiting the performance of this element. In order to ensure a low bending loss, the required bending radius is much larger than that for waveguides on an in-plane isotropic material, e.g. a $Z$ -cut LN thin film. Mode hybridization also plays an important role in the performance of the 90° bend, which should be avoided. Generally, decreasing the thickness of the LN thin film, working at a longer wavelength, or confining the propagation angle on a chip would help to decrease the polarization coupling.

Ultra-Sharp Multimode Waveguide Bends With Dual Polarizations

Abstract: A silicon-based on-chip ultra-sharp multimode waveguide bend (MWB) is proposed to work for dual polarizations by introducing modified Euler-curves and shallowly-etched non-uniform subwavelength-gratings (SWGs). For the designed 90° MWB with a core width of 1.01 μm and an effective bending radius as small as 10 μm, the excess losses are less than 0.23 dB and the inter-mode crosstalk is lower than <−26.5 dB over a broad band from 1500 nm to 1600 nm for six mode-channels, including three TE-polarization modes and three TM-polarization modes. It shows a great improvement compared to a regular 90° arc-bend.

Interferometric optical gyroscope based on an integrated silica waveguide coil with low loss

Abstract: An interferometric optical gyro (IOG) based on integrated devices are a promising alternative for miniaturized inertial sensors. However, improving their accuracy, which is determined by the sensing coil insertion loss, is crucial. In this work, an IOG is built using an integrated sensing coil produced from a 2.14-m-long SiO2 waveguide, the minimum bend radius and spacing of which are chosen to minimize the sensing coil insertion loss. The coil length is chosen by considering optimal detection limit constraints. Sinusoidal wave biasing modulation improves the system detection sensitivity. Finally, the IOG realizes the best yet reported bias drift of 7.32°/h.

All-Fiber Vector Bending Sensor Based on a Multicore Fiber With Asymmetric Air-Hole Structure

Abstract: In this article we present an all-fiber vector bend sensor by means of a self-fabricated micro-structured multicore optical fiber. The reported solution is based on differential intensity variations of the light transmitted along the cores whose changes are influenced by the bending angle and orientation. The unique asymmetric structure of the air-holes in the optical fiber provides each core with different confinement losses of the fundamental mode depending on the bending radius and orientation, making each of the cores bend-sensitive in a range of at least 80°. It has been experimentally demonstrated that the reported sensor enables the bending angle and orientation to be detected in a full range of 360° without any dead-zones, and the possibility of end point detection with millimeter precision. Additionally, a reconstruction of the bending vector has been carried out theoretically, and a good match can be observed between the experimental and theoretical data.

Fiber semi-film SPR curvature sensor with the function of directional recognition

Abstract: We propose and demonstrate a fiber surface plasmon resonance (SPR) curvature sensor with the function of directional recognition by coating a metal film on one side of the fiber. By mathematical modeling, the relationship between the reflection angle (SPR incident angle) and bending radius is investigated. Functional testing of the sensing probe with a semi-film is performed. Simulating and testing results indicate that with the increases of the curvature, the resonance wavelength can be tested red shifts when the sensing probe is bent with a convex sensing film, and blue shifts when the sensing probe is bent with a concave sensing film, the depth of the resonance valley can be detected increases. Analysis shows that the testing results have a good agreement with the simulating results. The proposed fiber SPR sensor has the advantages of large curvature sensing range, directional recognition and good strength, which has strong application value.

Ultra-thin silicon-on-insulator waveguide bend based on truncated Eaton lens implemented by varying the guiding layer thickness.

Abstract: Silicon-on-insulator (SOI) waveguides with different geometries have been employed to design various integrated optical components. Reducing the bending radius of the SOI waveguides with low bending loss is essential in minimizing the footprint of light-wave circuits. The propagating mode is less confined in the core of the ultra-thin SOI waveguide and penetrates to substrate and cladding, leading to higher bending loss compared with conventional SOI waveguide with the thicker guiding layer. While various bending mechanisms have been utilized to reduce the bending loss of conventional SOI waveguides, the ultra-thin SOI waveguide bends have not been studied in detail. In this paper, we present a 60 nm-thick SOI waveguide bend based on the truncated Eaton lens implemented by varying thickness of the guiding layer. The three-dimensional full-wave simulations reveal that the designed waveguide bend, with a radius of 3.9 $\mu m$, reduces the bending loss from 3.3 to 0.42 dB at the wavelength of 1550 nm. Moreover, the bending loss for the wavelength range of 1260-1675 nm is lower than 0.67 dB while the bending loss in the C-band is lower than 0.45 dB.

Study of Rotation and Bending Effects on a Flexible Hybrid Implanted Power Transfer and Wireless Antenna System

Abstract: We present rotational misalignment and bending effects on a hybrid system to transfer power and data wirelessly for an implantable device. The proposed system consists of a high-frequency coil (13.56 MHz) to transfer power and an ultra-high frequency antenna (905 MHz) for data communication. The system performance and the transmitted power were studied under two misalignment conditions: (1) receiver rotation around itself with reference to the transmitter, and (2) bending of the implanted receiver under three different radii. Implanted receiver was printed on a flexible Kapton substrate and placed inside a layered body tissue model at a 30 mm depth. It is shown that the inductive link is stable under rotational misalignment and three bending conditions, whereas the communication data link is suitable to be used if the rotation angle is less than 75° or larger than 150°. The results show that the resonance frequency varies by 1.6%, 11.05%, and 6.62% for the bending radii of 120 mm, 80 mm, and 40 mm, respectively. Moreover, transmission efficiency varies by 4.3% for the bending radius of 120 mm. Decreasing the bending radius has more effects on antenna transmission efficiency that may cause severe losses in the communication link.

A modified thin-walled tube push-bending process with polyurethane mandrel

Abstract: Thin-walled (D/t > 30; D, initial outer diameter; t, initial wall thickness) bent tubes with large diameter (D > 100 mm) and small bending radius (R/D < 3; R, centerline radius) are difficult to form integrally using traditional bending methods. In this paper, using a novel loading method, a modified push-bending (MPB) process with polyurethane as mandrel was developed. A stainless steel bent tube with extreme geometrical specification (D = 144 mm, D/t = 72, and R/D = 1.94) was formed integrally by MPB. Based on analytical and finite element (FE) method, the internal pressure, i.e., contact pressure between polyurethane and tube (CPPT) were investigated. The results show that the CPPT decreases linearly from the back end to the front end of polyurethane rod and increases with smaller μ1 (the coefficient of friction between polyurethane and tube) and larger L (the center axis length of polyurethane rod), and the increase of CPPT is helpful to decrease the ovality of the bent tube.

Spectral Dependence of Transmission Losses in High-Index Polymer Coated No-Core Fibers

Abstract: A high-index polymer coated no-core fiber (PC-NCF) is effectively a depressed core fiber, where the light is guided by the anti-resonant, inhibited coupling and total internal reflection effects, and the dispersion diagram shows periodic resonant and anti-resonant bands. In this article, the transmission spectra of the straight and bent PC-NCFs (length > 5 cm) are measured and analyzed from a modal dispersion perspective. For the purpose of the study, the PC-NCFs are contained within a fiber hetero-structure using two single-mode fiber (SMF) pigtails forming a SMF-PC-NCF-SMF structure. The anti-resonant spectral characteristics are suppressed by the multimode interference in the PC-NCF with a short fiber length. The increase of the length or fiber bending (bend radius > 28 cm) can make the anti-resonance dominate and result in the periodic transmission loss dips and variations in the depth of these loss dips, due to the different modal intensity distributions in different bands and the material absorption of the polymer. The PC-NCFs are expected to be used in many devices including curvature sensors and tunable loss filters, as the experiments show that the change of loss dip around 1550 nm is over 31 dB and the average sensitivity is up to 14.77 dB/m−1 in the bend radius range from ∞ to 47.48 cm. Our study details the general principles of the effect of high-index layers in the formation of the transmission loss dips in fiber optics.

A digital manufacturing process for three-dimensional electronics

Abstract: Additive manufacturing (AM) offers the ability to produce devices with a degree of three-dimensional complexity and mass customisation previously unachievable with subtractive and formative approaches. These benefits have not transitioned into the production of commercial electronics that still rely on planar, template-driven manufacturing, which prevents them from being tailored to the end user or exploiting conformal circuitry for miniaturisation. Research into the AM fabrication of 3D electronics has been demonstrated; however, because of material restrictions, the durability and electrical conductivity of such devices was often limited. This thesis presents a novel manufacturing approach that hybridises the AM of polyetherimide (PEI) with chemical modification and selective light-based synthesis of silver nanoparticles to produce 3D electronic systems. The resulting nanoparticles act as a seed site for the electroless deposition of copper. The use of high-performance materials for both the conductive and dielectric elements created devices with the performance required for real-world applications. For printing PEI, a low-cost fused filament fabrication (FFF); also known as fused deposition modelling (FDM), printer with a unique inverted design was developed. The orientation of the printer traps hot air within a heated build environment that is open on its underside allowing the print head to deposit the polymer while keeping the sensitive components outside. The maximum achievable temperature was 120 °C and was found to reduce the degree of warping and the ultimate tensile strength of printed parts. The dimensional accuracy was, on average, within 0.05 mm of a benchmark printer and fine control over the layer thickness led to the discovery of flexible substrates that can be directly integrated into rigid parts. Chemical modification of the printed PEI was used to embed ionic silver into the polymer chain, sensitising it to patterning with a 405 nm laser. The rig used for patterning was a re-purposed vat-photopolymerisation printer that uses a galvanometer to guide the beam that is focused to a spot size of 155 µm at the focal plane. The positioning of the laser spot was controlled with an open-sourced version of the printers slicing software. The optimal laser patterning parameters were experimentally validated and a link between area-related energy density and the quality of the copper deposition was found. In tests where samples were exposed to more than 2.55 J/cm^2, degradation of the polymer was experienced which produced blistering and delamination of the copper. Less than 2.34 J/cm^2 also had negative effect and resulted in incomplete coverage of the patterned area. The minimum feature resolution produced by the patterning setup was 301 µm; however, tests with a photomask demonstrated features an order of magnitude smaller. The non-contact approach was also used to produce conformal patterns over sloped and curved surfaces. Characterisation of the copper deposits found an average thickness of 559 nm and a conductivity of 3.81 × 107 S/m. Tape peel and bend fatigue testing showed that the copper was ductile and adhered well to the PEI, with flexible electronic samples demonstrating over 50,000 cycles at a minimum bend radius of 6.59 mm without failure. Additionally, the PEI and copper combination was shown to survive a solder reflow with peak temperatures of 249°C. Using a robotic pick and place system a test board was automatically populated with surface mount components as small as 0201 resistors which were affixed using high-temperature, Type-V Tin-Silver-Copper solder paste. Finally, to prove the process a range of functional demonstrators were built and evaluated. These included a functional timer circuit, inductive wireless power coils compatible with two existing standards, a cylindrical RF antenna capable of operating at several frequencies below 10 GHz, flexible positional sensors, and multi-mode shape memory alloy actuators.

Experimental determination and ray-tracing simulation of bending losses in melt-spun polymer optical fibres.

Abstract: The damping properties and specifically the bend losses of polymer optical fibres (POFs) have so far only been documented by experimental work, investigating bending parameters such as bending radius, length, and distance of the bends. Even though damping mechanisms and causes are well-known, no simple, generally valid formula exists. Here, a simulation technique is shown that allows producing an optical model for any bending geometries of melt-spun polymer optical fibres. The developed model takes all relevant loss mechanisms into account, especially regarding the scattering losses at the interface of core and cladding as well as those of the cladding-air interface. The latter is caused by interfacial roughness for which experimental data have been obtained by atomic force microscopy measurements. To show the validity of the simulation, the model is compared to experimental results for several fibres and a variety of geometries. The variance between model and experimental data is low (S < 4.6%). The model not only contributes to improving the understanding of the optical properties of POFs, but it also has direct applicability to the design of photonic textile sensors for medicine, where the fibres are incorporated with small bending radii.

Manifestation of an ultra-high sensitive fiber optic microbend sensor realized by shining a Bessel-Gauss beam

Abstract: Bend-induced loss in microbending fiber-optic sensor has proved to be an effective one for the direct and indirect measurement of various physical parameters. In this research, a novel and highly sensitive microbend sensor has been explored by launching a zero order Bessel-Gauss beam inside a waveguide arrangement having a No-core fiber bonded amidst two special higher-order mode supporting fibers. By harnessing the special characteristics of the Bessel-Gauss beam, pairing of manifold high-order modes has been affirmed inside the sensor structure. The captivating feature of such sensor is that it defies the conventional wisdom and significantly improves the sensitivity without any intricate fabrication techniques like in tapering, bending etc. To our knowledge, such realization of Bessel-Gauss beam-shined microbend sensor has not been reported earlier in any of the contemporary literature. In support of our theoretical analysis; a Beam propagation method is employed in OptiBPM software (Optiwave Systems Inc.) to envisage the full transmission spectrum of the waveguide. For different bend radii, the sensor response has been numerically investigated and it is anticipated that the sensitivity is expected to be enhanced by a gentle reduction in the bend radius. With the presence of six microbends, the proposed sensor manifests an average bend sensitivity of 2.8 dB/mm which is 3.2 times superior to the classical microbend sensing configuration. Due to such superior sensing performance, the present paradigm paves the way for many potential applications, like damage detection of various engineering structures, and measurement of different physical parameters like temperature and pressure.

Dependence of macro-bending loss on bending configuration of multimode optical fibers studied by ray-tracing simulation

Abstract: The macro-bending loss of multimode step-index helical, s-shaped, and figure-of-eight-shaped optical fibers is investigated by ray-tracing simulation. In particular, fibers with the same radius of curvature having the three different configurations are compared. It is found that macro-bending loss strongly depends on the configuration as well as the bending curvature of the fibers. For the same curvature, the macro-bending loss of fibers in different configurations differs markedly. Thus, when comparing or reporting the macro-bending loss of multimode optical fibers, it is necessary to specify the bending radius as well as the fiber configuration.

X-ray diffraction from strongly bent crystals and spectroscopy of X-ray free-electron laser pulses.

Abstract: The use of strongly bent crystals in spectrometers for pulses of a hard X-ray free-electron laser is explored theoretically. Diffraction is calculated in both dynamical and kinematical theories. It is shown that diffraction can be treated kinematically when the bending radius is small compared with the critical radius given by the ratio of the Bragg-case extinction length for the actual reflection to the Darwin width of this reflection. As a result, the spectral resolution is limited by the crystal thickness, rather than the extinction length, and can become better than the resolution of a planar dynamically diffracting crystal. As an example, it is demonstrated that spectra of the 12 keV pulses can be resolved in the 440 reflection from a 20 µm-thick diamond crystal bent to a radius of 10 cm.