Straight Waveguide Research Articles

Ultrafast laser inscription of asymmetric integrated waveguide 3 dB couplers for astronomical K-band interferometry at the CHARA array

We present the fabrication and characterization of 3 dB asymmetric directional couplers for the astronomical K-band at wavelengths between 2.0 and 2.4 µm. The couplers were fabricated in commercial Infrasil silica glass using an ultrafast laser operating at 1030 nm. After optimizing the fabrication parameters, the insertion losses of straight single-mode waveguides were measured to be ∼ 1.2 ± 0.5 d B across the full K-band. We investigate the development of asymmetric 3 dB directional couplers by varying the coupler interaction lengths and by varying the width of one of the waveguide cores to detune the propagation constants of the coupled modes. In this manner, we demonstrate that ultrafast laser inscription is capable of fabricating asymmetric 3 dB directional couplers for future applications in K-band stellar interferometry. Finally, we demonstrate that our couplers exhibit an interferometric fringe contrast of > --> 90 % . This technology paves the path for the development of a two-telescope K-band integrated optic beam combiner for interferometry to replace the existing beam combiner (MONA) in Jouvence of the Fiber Linked Unit for Recombination (JouFLU) at the Center for High Angular Resolution Astronomy (CHARA) telescope array.

Guided Lamb waves in reconfigurable phononic crystal waveguides

We demonstrate experimentally the manipulation of Lamb waves guided along reconfigurable phononic circuits created by defects composed of threaded rods held with nuts in a perforated solid phononic crystal slab. Adjusting the free length of the rod, the resonant frequency of the defect can be tuned, without any change in the supporting phononic crystal slab. Both straight and bent waveguides are fabricated and measured in an aluminum sample with a lattice constant of 20 mm and a complete bandgap extending from 50 to 70 kHz. Guidance of Lamb waves is clearly observed by a Doppler vibrometer, even after 90° bends. The eigenmodes of guided waves are obtained using finite element analysis to explain the tuning of resonances through a bending cantilever model. Numerical and experimental results are generally found to be in fair agreement. They also suggest that the guiding frequency is rather independent of the details of the waveguides. They are of significance for the design of reconfigurable phononic devices.

An ultra-high sensitive plasmonic refractive index sensor using an elliptical resonator and MIM waveguide

In this paper, an elliptical resonator coupled to a straight metal-insulator-metal waveguide through a silicon layer is used for sensing application. The finite-difference time-domain method is used for the numerical investigation of the proposed structure. Such a structure demonstrates a multi resonance mode in the transmission spectrum. Based on the obtained results, the sensitivity can be reached to a value as high as 550 nm per refractive-index unit with a high FOM value of 282.5 RIU −1 around the resonance wavelength of 592 nm. In this structure, in addition to using a high Q-factor resonator, the resonance profile is localized and concentrated on the analyte to achieve an ultra-high sensitive refractive index sensor. Also, the effect of structural parameters on the transmission spectrum is investigated by sweeping them. Taking into account the obtained notable specifications such as ultra-high sensitivity and simplicity of the design and fabrication process, the presented structure can be employed in optical integrated circuits , particularly in high sensitivity sensors. • A novel plasmonic structure for detection of refractive index is proposed. • The designed topology is based on an elliptical resonator coupled to a straight MIM waveguide through a silicon layer. • A high FOM value of 282.5, a high sensitivity value of 550 nm/RIU, and a small FWHM value of 1.947 nm are obtained. • In addition to using a high-Q factor resonator, the resonance profile is localized on the analyte to achieve a high FOM. • The proposed structure benefits from simplicity in the design and fabrication process.

Propagation of fundamental mode in regularly bending multi-mode waveguides

Transmission of the fundamental mode in multi-mode waveguides is an effective scheme for a silicon-based platform to reduce scattering loss. However, the application of the scheme is usually limited to straight waveguides and restricted in multi-mode bending waveguides. This is because the fundamental mode of a straight waveguide is seriously disordered after passing the bend. In this work, we have presented a “matched bending radius” approach, by which an ultra-low loss and negligible modal disorder have been demonstrated in the Si and Si3N4 multi-mode waveguides. The estimated TE0 mode transmission factor is almost 0 dB at the matched bending radius, indicating that the fundamental mode can be re-generated after passing the multi-mode bending waveguide. The proposed approach will contribute to applying the low loss scheme in large-scale photonic integrated circuits.

Micromechanical Fabrication of Resonator Waveguides Integrated Four‐Port Photonic Circuit from Flexible Organic Single Crystals

AbstractBy using a mechanophotonics approach, three flexible organic microcrystals of (E)‐1‐(4‐(dimethylamino)‐phenyl)iminomethyl‐2‐hydroxyl‐naphthalene (DPIN) are micromanipulated, and dual‐waveguides coupled ring resonator is fabricated. The flexibility of the DPIN crystal arises owing to its spring‐like expansion and contraction of π∙∙∙π stacked molecular chains along the (002) plane. The crystals also act as optical waveguides in straight and bent geometries with very low optical loss coefficients. The fluorescence (FL) spectra of the guided light also display Fabry‐Pérot modes. Precise mechanical manipulation of microcrystals with atomic force microscopy cantilever tip bent a straight crystal waveguide to 360° angle forming a ring‐shaped crystal resonator (diameter ≈58.6 µm). The positioning and integration of two straight parallelly oriented waveguiding crystals (bus and drop) along the opposite edge of the ring resonator forms the first of its kind all‐organic crystal‐based dual‐waveguides coupled ring resonator. The fabricated circuit with four ports selectively guides the produced input FL to through and drop ports via active, passive waveguiding and FL reabsorbance mechanisms.

Investigation of long acoustic waveguides for the very low frequency characterization of monolayer and stratified air-saturated poroelastic materials

When sound propagates in a porous medium, it is attenuated via several energy loss mechanisms which are switched on or off as the excitation frequency varies. The classical way of measuring acoustic energy loss in porous materials uses the Kundt impedance tube. However, due to its short length, measurements are made in the steady state harmonic regimes. Its lower cutoff frequency is often limited to a few hundreds of Hertz. Two long acoustic waveguides were assembled from water pipes and mounted to create test-rigs for the low-frequency acoustic characterization of monolayer and stratified air-saturated poroelastic materials. The first waveguide was straight and had a length of 120 m, while the second was coiled to gain space and was 135 m long. The long waveguides appeal to very low frequency measurements using impulsive acoustic waves (with rich spectral content) because the incident waves can be separated in time from echoes off the extremities of the guides. The transmission coefficient of porous materials recovered using the two waveguides compared well with those from the transfer matrix method (TMM) used here in combination with Biot’s 1962 theory to describe propagation in porous dissipative media. This wave-material interaction model permitted the recovery of the properties of poroelastic materials from transmitted acoustic waves propagating in air. The parameters involved are the Young’s moduli, Poisson ratio and microstructural properties such as tortuosity and permeability. Being able to descend to lower frequencies guarantees the correct verification of the magnitude of the measured transmission coefficient which approaches unity towards the static frequency. The coiled and straight waveguides were found to be equivalent and provided data down to frequencies of the order of ≈12 Hz.

Performances characterization of unsaturated polyester resin/polymethylmethacrylate waveguide for refractive index measurement

Investigation of refractive index (RI) and temperature response of unsaturated polyester resin (UPR)/polymethylmethacrylate (PMMA) based waveguides is presented. Three type waveguide structures were fabricated which are straight waveguide (sensor A), curved waveguide (sensor B) and half-filled core waveguide (sensor C). Simple fabrications were done by engraving PMMA sheets using CNC engraving machine to form grooves with cross section of 1×1 mm2. The grooves were then filled with UPR as core material. To investigate the RI response, the waveguides were immersed in glucose solution with various concentration values having RI of 1.33 to 1.3547. Meanwhile, for temperature response characterization, the sensors were placed in distilled water while heated using magnetic stirrer with temperature variation of 35°C-85°C with increment of 5°C. The results showed that the sensors showed good RI sensitivity and linearity with the highest sensitivity of -48.476 dB/RIU. It was also observed that the sensors were temperature dependent. The highest error due to temperature effect is -1.8×10−4 RIU/°C.

Nanocavity mediated directional coupler in plasmonics waveguides

Nanocavity mediated directional coupler in plasmonics waveguides is proposed and numerically investigated. Perfect directionality with enhanced light intensity coupled to the desired direction is obtained through the chiral eigenmode of the nanocavity excited by an elliptically polarized dipole. The directionality is robust to the position and polarization of the dipole. Furthermore, both circular and Huygens types of sources have been demonstrated through the tunable coupling length between the nanocavity and the straight waveguide. This proposal provides a versatile method to unidirectionally control the light flow through the near-field coupling, which might provide opportunities for on-chip optical circuits, either quantum or classical.

A simple strategy for increasing optical waveguide performance using spherical aberration

Femtosecond Laser Writing (FLW) has some advantages over competing techniques; it is a direct, highly flexible, and maskless process that can inscribe waveguides in almost any type of transparent material. FLW has proven to produce photonic integrated circuits with innovative design in a deterministic way for several applications, including quantum photonics and microfluidics. Although direct laser writing of waveguides has been extensively studied, there is still an intense search for methods to produce waveguides with lower losses and higher optical quality, because improvements in this sense will impact photonic research and technology. In this work, straight waveguides were directly inscribed into Eagle XG glass samples in a single process step at fabrication depths up to 370 µm. The propagation loss and the mode profile for single-mode waveguides were obtained. The laser pulse energy threshold that triggers the optical breakdown as a function of the depth was measured and used to study its relationship with the resulting waveguide optical properties. A trend between the waveguide losses and the observed pulse energy threshold for optical breakdown was found, which is connected with the microscope objective spherical aberration. It can be useful for finding the best conditions for producing low-loss waveguides. Compared to the waveguides written with the corrected pulses, propagation losses up to 2.5 times smaller were observed in our experiments when applying the strategy presented here. Therefore, by taking advantage of the inherent spherical aberration, the FLW technique can be made capable of much more at even higher intensities than previously possible.

Micro Metal Additive Manufactured Low-Loss Slotted Rectangular Waveguides Operating at 220-500 GHz

This paper reports the design, fabrication and measurement techniques for a set of low-loss slotted waveguides. The waveguides are fabricated based on a micro metal additive manufacturing technology. They were fabricated layer by layer in one piece without the need of post-fabrication assembly. As examples, straight waveguides in WR-3.4 (220-330°GHz) and WR-2.2 (330-500°GHz) bands were fabricated and tested. Measurement results show the insertion loss per unit length is 0.0615-0.122°dB/mm and 0.116-0.281°dB/mm, respectively.

Inverse asymmetrical ridge taper polarization splitter–rotatorcovering optical fiber communication band from O to U

A novel on-chip ultra-broadband polarization splitter–rotator (PSR) based on an inverse asymmetrical ridge taper coupler is proposed by using evanescent coupling between two straight waveguides with gradual ridge widths. The two ridge-widths decrease inversely to realize mode hybridization between fundamental transverse magnetic (TM0) mode and fundamental transverse electric (TE0) mode, which effectively expands the bandwidth. The structural parameters are optimized by utilizing the frequency-domain finite-difference and eigenmode expansion method. The results show that efficient polarization splitting and rotating are simultaneously achieved with a broad operating bandwidth from 1250 nm to 1750 nm and a high polarization conversion efficiency of more than 95%. The insertion loss is below 0.05 dB and 0.2 dB over the bandwidth for TE0 and TM0, respectively. To the best of our knowledge, the PSR achieves the broadest bandwidth up to 500 nm (covering all the optical fiber communication O-, E-, S-, C-, L-, and U-bands) for the first time.

Design, Simulation, and Analysis of Optical Microring Resonators in Lithium Tantalate on Insulator

In this paper we design, simulate, and analyze single-mode microring resonators in thin films of z-cut lithium tantalate. They operate at wavelengths that are approximately equal to 1.55 μm. The single-mode conditions and transmission losses of lithium tantalate waveguides are simulated for different geometric parameters and silica thicknesses. An analysis is presented on the quality factor and free spectral range of the microring resonators in lithium tantalate at contrasting radii and gap sizes. The electro-optical modulation performance is analyzed for microring resonators with a radius of 20 μm. Since they have important practical applications, the filtering characteristics of the microring resonators that contain two straight waveguides are analyzed. This work enhances the knowledge of lithium tantalate microring structures and offers guidance on the salient parameters for the fabrication of highly efficient multifunctional photonic integrated devices, such as tunable filters and modulators.

Novel high-quality Fano resonance based on metal-insulator-metal waveguide with L-shaped resonators* *Project supported by the National Natural Science Foundation of China (Grant No. 51965007) and the “One thousand Young and Middle-Aged College and University Backbone Teachers Cultivation Program” of Guangxi, China (Grant No. 2019).

Developing a convenient method that can be routinely applied for ascertaining proportions of different vegetable oils employed in commercial blended edible oils remains a significant challenge. We address this issue by proposing a novel method for detecting volume fraction of different oils based on the fact that these oils are optically transparent and have slightly different indices of refraction at a given temperature and wavelength. Accordingly, we develop a highly sensitive sensor for measuring the index of refraction of oil blends based on Fano resonance spectra obtained using a metal-insulator-metal (MIM) waveguide structure comprising a gapped straight waveguide coupled with two L-shaped resonators. The index of refraction sensitivity and figure of merit of the structure are calculated based on modeling using the finite element method, and the waveguide structure is accordingly optimized by adjusting the different geometric parameters to achieve a high-quality Fano resonance spectrum. The optimized structure achieves an ultra-high refractive index sensitivity of 770 nm/RIU in terms of a refractive index unit (RIU) of 1. Moreover, a highly stable linear relationship is obtained between the refractive index of mixed edible oils and the resonance wavelength. Experimental results demonstrate that the proposed structure can detect slight changes in the volume fractions of the components in blended oils.

Inertial Piezoelectric Rotary Motor Based on Low Profile Stator with Trapezoidal Waveguides

Results of numerical and experimental investigations of a novel inertial piezoelectric rotary type motor based on a low profile stator with trapezoidal waveguides. The proposed motor has a simple design and is well scalable. Moreover, the proposed design of the motor allows mount it on a printed circuit board and use it in a small-size mobile positioning and actuating systems. The structure of the stator is based on a square type hollowed steel frame with four straight trapezoidal waveguides that are used to transfer vibrations of the stator to the rotation of the rotor. Piezo ceramic plates are glued on both sides of the stator. The thickness of the assembled stator is 0.9 mm, while the total area needed for stator mounting does not exceed 625 mm2. The driving of the rotor is based on the stick-slip principle, which is induced by excitation of the second in-plane bending mode of the four bimorph plates applying two saw tooth waveform signals with a phase difference by π. The numerical and experimental investigation was carried out to validate the operation principle of the motor and to measure the mechanical and electrical characteristics. The maximum angular rotation speed of 1304 RPM was achieved at a resonance frequency of 44.81 kHz when a preload of a 7.35mN was applied.

Sol–Gel Waveguide-Based Sensor for Structural Health Monitoring on Large Surfaces in Aerospace Domain

The potential of sol–gel-based optical sensors is investigated for applications in the aerospace domain. To this aim, a low-cost and non-intrusive sol–gel sensor based on waveguides, arranged as a 2D matrix structure, is fabricated by UV photolithography for delamination and damage detection. Two different organic–inorganic sol–gels were selected to fabricate the photonic device: TiO2–SiO2 and ZrO2–SiO2, acting as the waveguide core and the cladding, respectively. A systematic study was performed to determine the manufacturing parameters controlling their properties. The results show that large surfaces can be functionalized via sol–gel methods using the direct laser-writing approach. The structures are characterized in terms of refractive index, and the guiding properties were investigated through simulations and experiments, indicating an excellent behavior regarding the light guidance in a straight waveguide or in the 2D matrix structure grid. Additionally, preliminary tests show that the presence of impact can be easily detected after damage through the induced optical losses on large surfaces. This proof of concept sensor is a promising tool for structural health monitoring. To achieve the ultimate goal, the integration of this photonic sensor will be later performed on aircraft wings.

Boundary revision of a beam model for a thin-walled waveguide at bending vibration

The paper discusses the problem of choosing a model type for a straight thin-walled waveguide with a rectangular cross-section during its vibrations. To do this, calculations were made of the first natural frequency of vibration for waveguides of different geometric sizes. The restraints of the waveguide were accepted by a cantilever, hinged, and fixed supports. The difference between the values of the first natural frequency of vibration for the beam and shell waveguide models was estimated. Calculations were carried out analytically on the theory of beam vibration and by the numerical method of finite elements. The results show that the boundary known in static calculations of the applicability of the beam model Rmax/L = 0.1 requires clarification at bending vibrations for the small thickness of the section wall. With small wall thicknesses, the error of calculating the first natural frequency of vibrations will increase sharply due to the effect of a beam section warping.

AlInGaAs Multiple Quantum Well-Integrated Device with Multifunction Light Emission/Detection and Electro-Optic Modulation in the Near-Infrared Range.

A monolithic photonic chip with multifunctional light emission/detection and electro-optic modulation capabilities in the near-infrared range is proposed and realized on an InP-based wafer. Two identical AlInGaAs multiple quantum well (MQW) diodes operating independently as light emission/detection devices are fabricated using a two-step etching process on a single wafer and connected via a straight waveguide. The photocurrent induced in the MQW diode for the detection process is generated by the infrared light emitted by the MQW diode during the emission process and transmitted via the straight waveguide. The MQW diode has an electro-optic modulation characteristic, and its spectral responsivity exhibits a blueshift with an increasingly negative bias voltage under external infrared laser excitation. An on-chip communication test is conducted to study the potential applications of the proposed monolithic photonic chip for transmission of optical signals in the near-infrared range.

Tunable terahertz absorber based on graphene-metal nanostructure as opto-fluid sensor

In this paper, strong and effective absorber is achieved using graphene-metal nano structures. Proposed structures are consisted of multi layers of graphene and metals. By introducing ring shaped graphene layers, the near unity absorption coefficient can be obtained. The effects of different structural parameters like the length, radius and width (h1, t1, a, h2, t2, a1) and chemical potential (Ef) on the absorption spectrum (wavelength and value) are also investigated. The best resonant peak is obtained for the structure with two rings and straight waveguides. The proposed structure can be used as an effective sensor for sensing opto-fluidic materials with different refractive indices (from 1.0006 to 1.6209). The designed structure can be utilized as a strong absorber or sensitive sensor in optical communication or other optical nano systems.

Analytical and Experimental Study of Multilayer Dielectric Rod Waveguides

This article presents a comprehensive study of the design and performance of a multilayer dielectric rod waveguide (MDRW) with a rectangular cross section. The design is comprised of a high permittivity core encased by a low permittivity cladding. A mathematical model is proposed to predict the fundamental mode cutoff frequency in terms of the core dimensions and the core and cladding permittivity. The model is useful for design purposes and it offers an excellent match to full-wave electromagnetic (EM) simulation results. The MDRW performance is measured for the extended Ku-band (10-18 GHz) considering straight and bent waveguides with different radius of curvatures. The particular multilayer configuration shows a 16.7% and 24% extension of the 1 dB low-band cutoff frequency for a 10×10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 20×20 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> cladding area, respectively, when compared to a single layer dielectric rod waveguide (DRW) design of the same core dimensions. MDRW bends also exhibit a significant reduction of the 1 dB low-band cutoff frequency from 20% to 40% when compared to single-layer designs, for the different radius of curvature bends.

Topological phononic crystal plates with locally resonant elastic wave systems

In this paper, we study the topological properties of subwavelength bands in elastic phononic crystal (PC) plate hybridized with extra local resonances and analyze the band-structure evolution of a topological PC plate with local resonator using finite element method (FEM). Our structure can produce two bandgaps in the subwavelength region: one originates from the local resonator of unit cell and the other one stem from breaking the C6v symmetry of an original plate PnC in a honeycomb lattice to introduce valley pseudospins. It is found that the edge states only existed in the Bragg bandgaps and the directional spread of elastic waves can be realized in both straight and bent waveguides, and the numerical simulation in a waveguide plate perfectly agrees with the theoretical results. In addition, experimental test is used to validate theoretical and numerical approach to the design of topological waveguides. This study provides an effective approach of producing robust elastic wave topological edge modes in the subwavelength region for plate-like structure.