Single-mode Waveguides Research Articles

Fabrication of single-mode hybrid crystal-glass waveguides by direct femtosecond laser writing and thermal annealing

Ultrafast lasers pave the way for an effective technique for 3D direct space-selective modification of glasses. This work shows that the combination of laser-induced crystallization, isotropic refractive index modification and thermal post-annealing allows the fabrication of single-mode hybrid crystal-glass waveguides. The laser-written waveguides exhibit single-mode behavior and possess the frequency conversion ability due to second harmonic generation by the crystalline waveguide part. The measured level of optical propagation losses of the 9 mm long hybrid optical waveguides is found to be 1.45±0.16 dB at a wavelength of 1550 nm, confirming their improved characteristics. The presented results contribute to the future development of all-optical crystal-in-glass waveguide-based devices and technologies.

Design of Silicon Nitride Edge Coupler for Monolithically Integrated Laser on Silicon Photonic Circuits With Relaxed Alignment Tolerance and High Efficiency

We propose a bi-layer 5-tip edge coupler in a multilayer silicon nitride-on-silicon (SiN-on-Si) waveguide platform. The coupler is used for the integration between a monolithic 1550 nm laser and a single-mode SiN waveguide. The simulated coupling efficiency is 92.8%. The vertical 1-dB-loss misalignment tolerance is as large as 0.5 μm. Broad 1-dB-drop bandwidth (1338 nm to 1700 nm) and small footprint (total length: 38.2 μm) are achieved simultaneously. A broadband bi-layer SiN-Si adiabatic transition cascaded to the edge coupler is designed to couple the laser power into a single-mode Si waveguide at an efficiency of 90.6%. Low-computation-cost electromagnetic numerical simulation and optimization strategies are applied to improve the reverse design of the complex couplers.

Design of High-Speed Mid-Infrared Electro-Optic Modulator Based on Thin Film Lithium Niobate

It is particularly important to improve the performance of mid-infrared (mid-IR) electro-optic (EO) modulator, which is one of the key devices that will expand the channels of optic communication, and will be utilized in optical free space communication, optical detection for gas molecules, etc. We present and theoretically design the performance of the high-speed mid-IR EO modulator based on lithium niobate on insulator (LNOI) at λ = 3 μm. We have designed, calculated, and optimized single-mode ridge waveguide, 1 × 2 multimode interference (MMI) couplers and coplanar waveguide (CPW) electrodes structures for x-cut lithium niobate. Our device with an extinction ratio greater than 40 dB, a half-wave voltage-length product is about 12 V·cm, and a EO bandwidth up to 12 GHz with interaction length of 15 mm. The performance of interaction length of 5 mm and 10 mm mid-IR EO modulator are also proposed and simulated for a compromise consideration between the EO bandwidth and driving voltage.

Ultralow 0.034 dB/m loss wafer-scale integrated photonics realizing 720 million Q and 380 μW threshold Brillouin lasing.

We demonstrate 0.034 dB/m loss waveguides in a 200-mm wafer-scale, silicon nitride (Si3N4) CMOS-foundry-compatible integration platform. We fabricate resonators that measure up to a 720 million intrinsic Q resonator at 1615 nm wavelength with a 258 kHz intrinsic linewidth. This resonator is used to realize a Brillouin laser with an energy-efficient 380 µW threshold power. The performance is achieved by reducing scattering losses through a combination of single-mode TM waveguide design and an etched blanket-layer low-pressure chemical vapor deposition (LPCVD) 80 nm Si3N4 waveguide core combined with thermal oxide lower and tetraethoxysilane plasma-enhanced chemical vapor deposition (TEOS-PECVD) upper oxide cladding. This level of performance will enable photon preservation and energy-efficient generation of the spectrally pure light needed for photonic integration of a wide range of future precision scientific applications, including quantum, precision metrology, and optical atomic clocks.

Combining one and two photon polymerization for accelerated high performance (3 + 1)D photonic integration

Abstract Dense and efficient circuits with component sizes approaching the physical limit is the hallmark of high performance integration. Ultimately, these features and their pursuit enabled the multi-decade lasting exponential increase of components on integrated electronic chips according to Moore’s law, which culminated with the high performance electronics we know today. However, current fabrication technology is mostly constrained to 2D lithography, and thermal energy dissipation induced by switching electronic signal lines presents a fundamental challenge for truly 3D electronic integration. Photonics reduces this problem, and 3D photonic integration is therefore a highly sought after technology that strongly gains in relevance due to the need for scalable application-specific integrated circuits for neural networks. Direct laser writing of a photoresin is a promising high-resolution and complementary metal-oxide-semiconductor (CMOS) compatible tool for 3D photonic integration. Here, we combine one and two-photon polymerization (TPP) for waveguide integration for the first time, dramatically accelerating the fabrication process and increasing optical confinement. 3D additive printing is based on femtosecond TPP, while blanket irradiation with a UV lamp induces one-photon polymerization (OPP) throughout the entire 3D chip. We locally and dynamically adjust writing conditions to implement (3 + 1)D flash-TPP: waveguide cores are printed with a small distance between neighboring writing voxels to ensure smooth interfaces, mechanical support structures are printed at maximal distance between the voxels to speed up the process. Finally, the entire chip’s passive volume not part of waveguide cores or mechanical support is polymerized in a single instance by UV blanket irradiation. This decouples fabrication time from the passive volume’s size. We succeed in printing vertical single-mode waveguides of 6 mm length that reach numerical apertures up to NA = 0.16. Noteworthy, we achieve exceptionally low −0.26 dB injection losses and very low propagation losses of −1.36 dB/mm at λ 0 = 660 nm, which is within one order of magnitude of standard integrated silicon photonics. Finally, the optical performance of our waveguides does not deteriorate for at least ∼3000 h after printing, and remains stable during ∼600 h of continuous operation with 0.25 mW injected light.

Femtosecond-laser fabrication of magneto-optical waveguides in terbium doped CaLiBO glass

Femtosecond laser micromachining has potential application in integrated photonics thanks to its ability to produce micrometer-size three-dimensional structures in different types of materials, including the ones whose properties can be externally altered, which allows for the fabrication of optically active devices. In this work, we demonstrate the fs-laser fabrication of magneto-optical waveguides in CaLiBO (calcium-lithium tetraborate) glasses codoped with Yb+3 and Tb3+ ions. Single-mode type waveguides were produced using fs-pulses with ∼0.1 J/cm2 and 6.5 × 105 pulses/spot, an important feature for integrated photonic devices. The fabricated waveguides display Faraday effect, with a Verdet constant of 560° T−1m−1, for the sample containing 2% of Tb3+, which is equivalent to the one observed for the bulk. Such a result demonstrates that fs-laser micromachining, in the conditions used here, does not negatively affect the magneto-optical properties of the sample. Thus, our results open a new pathway towards the processing of magneto-optical waveguides aimed at applications in magneto-optical photonic microdevices.

A Theoretical Description of Node-Aligned Resonant Waveguide Gratings

Waveguide gratings are used for applications such as guided-mode resonance filters and fiber-to-chip couplers. A waveguide grating typically consists of a stack of a single-mode slab waveguide and a grating. The filling factor of the grating with respect to the mode intensity profile can be altered via changing the waveguide’s refractive index. As a result, the propagation length of the mode is slightly sensitive to refractive index changes. Here, we theoretically investigate whether this sensitivity can be increased by using alternative waveguide grating geometries. Using rigorous coupled-wave analysis (RCWA), the filling factors of the modes of waveguide gratings supporting more than one mode are simulated. It is observed that both long propagation lengths and large sensitivities with respect to refractive index changes can be achieved by using the intensity nodes of higher-order modes.

Simulation of Single-Mode Waveguides of Gallium Nitride/ Aluminum Nitride (GaN/AlN) on Sapphire by using Finite Difference Mode (FDM) solver

This paper presents a model of the optical waveguide of GaN/ AlN on a sapphire based on the different refractive index (n) between the wave's guideline regions and surrounding medium (cladding). The model analysis is done by using finite-difference mode (FDM) solver simulation is performed by FDTD (finite-difference-time-domain). In the infrared area, this study reveals a fully unique and rigorous modal analysis waveguide. The investigation includes both primary and single-mode polarization; the waveguide is reliant on the refractive index of the layers, thickness, and substrate type. The primary novelty here for the micro-waveguide single-mode is to realize optical characteristics such as attenuation, amplitude, and full width half maximum (FWHM),which has not been done previously for specific thickness (given thickness) and for that geometrical design. Despite we investigated how Optical waveguides are structures that confine and direct optical signals in a region of a higher effective index than its surrounding media. Our experiment (simulation) aims to investigate the single-mode waveguide of Gallium Nitride/ Aluminum Nitride GaN/ AlN on sapphire of a specified thickness in order to obtain infrared waveguide mode with minimum attenuation. Attenuation refers to any decrease in the propagated signal power that does not affect its waveform while the input wavelength is1.55μm. Additionally describe the characterization of the mode, in keeping with our modal analysis. The waveguides have many applications in various fields such as Optical fiber communication and Photonic integrated circuits.

High-Efficiency Continuous-Flow Microwave Heating System Based on Asymmetric Propagation Waveguide

Continuous-flow microwave heating is widely used in chemical engineering and other fields. However, during microwave heating, the load’s electrical permittivity change will affect microwave heating efficiency. In this article, a WR430 asymmetric propagation waveguide is proposed based on gradient-index metasurfaces (GIMs), which can achieve high-efficiency continuous-flow heating with a large range of permittivity. First, asymmetric microwave propagation is realized in a WR430 3-D waveguide model, and the influence of metasurfaces index, thickness, and discrete element size is analyzed. Second, the asymmetric propagation waveguide is realized by fluted medium based on the permittivity equivalent method. The results show that the microwave inside the waveguide can only propagate along one direction at 2.4–2.6 GHz. Finally, the proposed waveguide is applied to a continuous-flow heating system. Compared with the single-mode cavity, multimode cavity, and waveguide water load, the proposed method can improve heating robustness and efficiency with different loads. Experiments show that the microwave absorption efficiency is stable and greater than 90% in the proposed system when the permittivity of the load changes drastically. The proposed system has a simple structure and strong applicability, which can replace the three-stub tuner and get rid of the circulator and water load for continuous-flow heating.

Synergistic effect of graphene and ZnO nanorods in enhancing the performance of MEH-PPV based polymer light emitting diode

The present work is an effort to enhance the performance of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) based light emitting diode by incorporating graphene and zinc oxide (ZnO) nanorods together in the device structure. The typical structure utilizes ZnO nanorods as single-mode waveguide for efficient light extraction and graphene nanoflakes as conductive nanofillers. The luminance characteristics depict that incorporation of ZnO nanorods leads to enhanced device emission while J-V curves show that with graphene doping there is a decrease in turn-on voltage of the device. This increase in luminance is ascribed to the light extraction capability of ZnO nanorods and the reduction in turn voltage is due to the development of graphene assisted electron transport pathways in the emissive layer. The analysis shows that at an optimized doping concentration of 0.005 wt% of graphene, the typical device has been found to be ∼ 60 times more efficient at 4 mA/cm2 than the basic MEH-PPV device owing to the cumulative action of these nanostructures.

Influence of non-Hermitian mode topology on refractive index sensing with plasmonic waveguides

We evaluate the sensing properties of plasmonic waveguide sensors by calculating their resonant transmission spectra in different regions of the non-Hermitian eigenmode space. We elucidate the pitfalls of using modal dispersion calculations in isolation to predict plasmonic sensor performance, which we address by using a simple model accounting for eigenmode excitation and propagation. Our transmission calculations show that resonant wavelength and spectral width crucially depend on the length of the sensing region, so that no single criterion obtained from modal dispersion calculations alone can be used as a proxy for sensitivity. Furthermore, we find that the optimal detection limits occur where directional coupling is supported, where the narrowest spectra occur. Such narrow spectral features can only be measured by filtering out all higher-order modes at the output, e.g., via a single-mode waveguide. Our calculations also confirm a characteristic square root dependence of the eigenmode splitting with respect to the permittivity perturbation at the exceptional point, which we show can be identified through the sensor beat length at resonance. This work provides a convenient framework for designing and characterizing plasmonic waveguide sensors when comparing them with experimental measurements.

A Cost-Effective Edge Coupler With High Polarization Selectivity for Thin Film Lithium Niobate Modulators

Thin film lithium niobate on insulator (LNOI), as an emerging photonic material, provides a promising platform to realize compact, high-speed, and high-efficient electro-optic (EO) modulators. However, due to the large coupling loss caused by the large mode field mismatch between standard single mode fibers and LNOI single mode waveguides, edge or grating couplers are indispensable to LNOI modulators. In this paper, we propose a novel edge coupler employing staircase structure. The proposed coupler is feasible with i-line lithography and avoids high precise alignment. Moreover, such coupler can be applied to wafer-scale manufacture with low cost. The simulation results show a low coupling loss less than 1.60 dB can be achieved at 1.55 μm wavelength for transverse electric (TE) polarized light signals. Specifically, benefited from the novel staircase design, the proposed edge coupler shows a unique property of polarization extinction ratio larger than 10 dB, which will further improve the modulation efficiency and extinction ratio of the EO modulators based on X-cut LNOI.

Study on the single-mode condition for x-cut LNOI rib waveguides based on leakage losses.

Lithium niobate-on-insulator (LNOI) has recently emerged as a promising material platform for high-density and advanced photonics integrated circuits (PICs). And single-mode waveguides (SMW) are the most basic building blocks for structuring various PICs. In this paper, single-mode conditions (SMCs) for shallowly etched LNOI rib waveguides in x-cut LNOI wafer are investigated with the finite element method (FEM) in consideration of the lateral leakage and the magic width for the first time, to our best knowledge. Our results indicate that due to the lateral leakage and the magic width these shallowly etched x-cut LNOI rib waveguides have unique and complex SMCs. Our method and results provide a guidance in designing low-loss LNOI SMW and high-performance PICs.

Nanophotonics-inspired all-silicon waveguide platforms for terahertz integrated systems

Abstract Recent advances in silicon (Si) microphotonics have enabled novel devices for the terahertz (THz) range based on dielectric waveguides. In the past couple of years, dielectric waveguides have become commonplace for THz systems to mitigate issues in efficiency, size, and cost of integration and packaging using metal-based waveguides. Therefore, THz systems have progressively evolved from cumbersome collections of discreet components to THz-wave integrated circuits. This gradual transition of THz systems from numerous components to compact integrated circuits has been facilitated at each step by incredible advances in all-Si waveguides allowing low-loss, low dispersion, and single-mode waveguiding operation. As such, all-Si waveguides position themselves as highly efficient interconnects to realize THz integrated circuits and further large-scale integration in the THz range. This review article intends to reevaluate the evolution stages of THz integrated circuits and systems based on all-Si waveguides.

Eliminating Reflections in Waveguide Bends Using a Metagrating-Inspired Semianalytical Methodology

We present a semianalytical method to obtain perfect transmission across abrupt H-plane bends in single-mode rectangular waveguides using a single passive polarizable element (scatterer). The underlying analysis and synthesis schemes are inspired by the rapidly-growing research on metagratings, typically used to manipulate wave trajectories in free-space. These sparse configurations of subwavelength polarizable particles (meta-atoms) are designed by careful tailoring of inter-element near-field and far-field interactions, relying on analytical models to resolve the required meta-atom distribution and geometry to facilitate a desired interference pattern when excited by the incident wave. Utilizing these metagrating design concepts, we develop a modal formalism for obtaining a collection of locations inside the bend junction, in which a passive scatterer may be placed to zero out the return loss. Subsequently, we propose two different shapes for the scatterer and discuss, for each of them, the ways in which their geometrical characteristics may be retrieved. This versatile and efficient methodology, verified via full-wave simulations, can be utilized to eliminate reflection loss in diverse bend configurations, often found in complex wave-guiding systems used for antenna feeding and power transmission. Moreover, these results demonstrate the usefulness and potential of metagrating design concepts for various applications, beyond free-space beam manipulation.

Engineering Chiral Light-Matter Interactions in a Waveguide-Coupled Nanocavity.

Spin-dependent, directional light–matter interactions form the basis of chiral quantum networks. In the solid state, quantum emitters commonly possess circularly polarized optical transitions with spin-dependent handedness. We demonstrate numerically that spin-dependent chiral coupling can be realized by embedding such an emitter in a waveguide-coupled nanocavity, which supports two near-degenerate, orthogonally polarized cavity modes. The chiral behavior arises due to direction-dependent interference between the cavity modes upon coupling to two single-mode output waveguides. Notably, an experimentally realistic cavity design simultaneously supports near-unity chiral contrast, efficient (>95%) cavity-waveguide coupling and enhanced light–matter interaction strength (Purcell factor FP > 70). In combination, these parameters enable the development of highly coherent spin–photon interfaces ready for integration into nanophotonic circuits.

Photonic waveguide bundles using 3D laser writing and deep neural network image reconstruction.

In recent years, three-dimensional (3D) printing with multi-photon laser writing has become an essential tool for the manufacturing of three-dimensional optical elements. Single-mode optical waveguides are one of the fundamental photonic components, and are the building block for compact multicore fiber bundles, where thousands of single-mode elements are closely packed, acting as individual pixels and delivering the local information to a sensor. In this work, we present the fabrication of polymer rectangular step-index (STIN) optical waveguide bundles in the IP-Dip photoresist, using a commercial 3D printer. Moreover, we reduce the core-to-core spacing of the imaging bundles by means of a deep neural network (DNN) which has been trained with a large synthetic dataset, demonstrating that the scrambling of information due to diffraction and cross-talk between fiber cores can be undone. The DNN-based approach can be adopted in applications such as on-chip platforms and microfluidic systems where accurate imaging from in-situ printed fiber bundles suffer cross-talk. In this respect, we provide a design and fabrication guideline for such scenarios by employing the DNN not only as a post-processing technique but also as a design optimization tool.

Ultrasonic Bubble Imaging in Molten Salt Using a Multi-Mode Waveguide and Time Reversal

Direct ultrasound imaging of technical processes in harsh environments, such as high-temperature melts, is not possible beyond the destruction limits of ultrasound transducers. A waveguide between the measured medium and the sensitive ultrasound transducer allows acoustic access to the former while protecting the latter. State-of-the-art single-mode waveguides allow for acoustical coupling between a transducer and the measured medium but are unsuited for real-time imaging since mechanical scanning is necessary. In contrast, multimode waveguides can transmit complex 2-D signals to an imaged region. While multimode waveguides require adaptive wavefront correction to compensate for the complex propagation inside, they allow the reduction of mechanical complexity with the application of the time-reversal virtual array (TRVA) method. This calibration method takes advantage of the time invariance of sound propagation in linear media to identify and compensate for the propagation inside the waveguide. This work proposes a novel <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> calibration procedure for system identification based on a traversing passive point scatterer. We characterize the imaging capacity of such a TRVA inside a molten nitrate salt at about 300 °C, exceeding the array’s operating temperature by 250 °C. By real-time imaging rising gas bubbles in the melt for the first time, we demonstrate the method’s potential for monitoring dynamic processes, such as metallurgical bubble column reactors with hot and opaque melts.

Unidirectional coupling of a quantum emitter to a subwavelength grating waveguide with an engineered stationary inflection point

In this work, we propose an approach for the design of a waveguide structure that allows for efficient and highly asymmetric coupling of the quantum sources with circularly polarized transition dipole moments to the guided mode of the structure. The approach is based on the mixing of the two quasi-degenerate modes of a periodic waveguide with an auxiliary single-mode waveguide leading to the formation of the dispersion with a stationary inflection point and consequently to the high coupling efficiency of this mode with a dipole source. We show that the distribution of the field polarization inside the waveguide is relatively homogeneous maintaining the circular polarization in a large area. Consequently, this leads to a high degree of tolerance of the coupling asymmetry and strength to the position of the quantum emitter. We believe, that our results will extend the variety of designs of the efficient chiral nanophotonic interfaces based on planar semiconductor nanostructures.

The impact of ZnO nanotube on the performance of hybrid inorganic/organic light-emitting diode as a single-mode ring-core UV waveguide

After a systematic survey in hybrid inorganic/organic light-emitting heterostructure devices based on ZnO in the last decay, in this novel work, the impact of the single-mode ring-core waveguide based on ZnO nanotube (NT)/MEH-PPV for ultraviolet organic light-emitting diode (UV-OLED) application has been carefully scrutinized for the first time. The proposed structure has been fabricated, simulated and compared with conventional ZnO nanorod (NR)/MEH-PPV structure. To synthesize ZnO NTs, the as-grown chemical bath deposited ZnO NRs have been etched in KCL solution in various molar (M) concentration, etching time and etching temperature. The optimized etching condition is obtained in 1 M concentration of KCL solution, 4 h etching time and 90 °C temperature. The structural properties (such as strain, stress and texture coefficient), electrical properties (such as band gap energy) and optical properties (such as Urbach energy, absorbance and photoluminescence spectra) of ZnO NTs have been investigated, systematically. In continue, hybrid UV-OLEDs have been fabricated based on ZnO NRs and ZnO NTs. According to the results, ZnO NT-based OLED depicts superior electrical and optical results including lower turn-on voltage (11.2 V < 15 V) and higher UV peak in electroluminescence spectra with respect to ZnO NR-based device. To acquire more enlightenment about UV emission mechanism, the proposed devices have been simulated through Silvaco TCAD and Lumerical FDTD software. The results from simulations illustrate great agreement with experimental results. Higher radiative recombination rate, higher Purcell factor and single-mode waveguiding effect of ring-core ZnO NT lead to major superiority of the ZnO NT-based UV-OLED fabricated and simulated in this work.