Slot Waveguide Research Articles

Compact thin-film lithium niobate modulators using slotted coplanar waveguide electrode suitable for high-volume fabrication

Lithium niobate (LN) is a good candidate for fabricating modulators due to its superior material characteristics. However, the application of traditional LN modulators is limited due to their large footprint and low modulation efficiency resulting from weak optical confinement. In recent years, with the development of the thin-film lithium niobate (TFLN) platform and LN etching technology, the size of the optical mode of the TFLN modulator is 20 times smaller than that of the traditional LN modulator. Furthermore, TFLN modulators have demonstrated a wide bandwidth, low half-wave voltage and small footprint in recent reports. The length of the TFLN modulators can be further reduced by employing a folded design and therefore applicable to compact transceiver package, such as being packaged in the quad small form factor pluggable double density transceiver. In this paper, we report on a folded TFLN modulator fabricated from a 4 inch LN wafer, which is suitable for large-volume fabrication. A fiber-to-fiber insertion loss of 2.5 dB and a voltage–length product of 1.85 V cm have been achieved. The measured electro-optic response curve has a 2.3 dB roll-off at 40 GHz, and the simulated 3 dB bandwidth reaches 65 GHz. Compared to traditional coplanar waveguide traveling wave electrodes, the slotted coplanar waveguide traveling wave electrode (slotted CPW-TWE) design adopted in this work allows adjusting the high-speed characteristics and modulation efficiency with more flexibility. This is the first time a slotted CPW-TWE design has been applied in a folded TFLN modulator.

Design of traveling wave slotted waveguide array antenna with high efficiency

<span lang="EN-US">The slotted waveguide antenna is one of the most important antennas used in high-frequency applications, in radars, navigation systems, remote sensing systems and communications because of its efficiency and high gains. In this paper, the slotted waveguide antenna was designed and simulated with suitable specifications with a working frequency range of 2-2.45 where this antenna was checked by plotting S parameters in the designed frequency band and we got a very good reflection coefficient for the designed antenna (S11) at the operating frequency, draw and illustrate the three-dimensional radiation pattern of the designed antenna that shows the gain and bandwidth at the operating frequency. The performance of a 9-element slotted waveguide array antenna with an operating frequency of up to 12 GHz was also investigated by plotting the S11 parameters and illustrating the designed antenna directivity diagram.We obtained the reflection coefficient of the designed array antenna (S11) below -23 dB at the operating frequency, and the SWG antenna directivity pattern with a maximum value of=13.2 dB and a minimum value of=-23 dB.</span>

Dynamics of a Photochromic-Actuated Slot Microring Photonic Memristor

In electronic circuits, memristors have been defined as resistors whose resistance depends on past signals. Such elements have promise as low-power weighting and self-learning elements in electronic neuromorphic circuits. Photonic memristors, elements whose transmission depends on past optical signals, are experiencing renewed study alongside the rise in interest in neuromorphic photonics. One potential route to creating photonic memristors involves incorporating photochromic materials, whose optical properties continuously change with optical illumination, into photonic integrated circuits (PICs). In this manuscript we lay out a theoretical model to study the transmission dynamics of devices incorporating photochromic compounds into SiN planar microring resonators which utilize slot waveguide structure, and show that such devices fulfill the criteria for memristive behaviour. This represents a practical path towards incorporating photonic memristors into a technologically mature material platform with minimal additional fabrication processes.

Development of a microwave resonant waveguide slot antenna with in-phase slot excitation

The object of research in the work is the process of radiation of electromagnetic waves of a resonant waveguide-slot antenna with in-phase excitation of slots. The subject of research is the wave parameters and directional properties of a resonant slotted waveguide antenna with in-phase slot excitation. The existing problem is that it is necessary to ensure highly directional properties of the antenna with electrical control of its wave parameters at high transmitter power. This problem is due to the fact that to solve the problem of developing equipment for radio control, and radar of aircraft, highly directional antennas of small sizes are required. To solve this problem, the paper proposes the design of a simple and cheap version of a microwave resonant waveguide-slot antenna with in-phase slot excitation.
 As a basis for developing a resonant slotted waveguide antenna, the authors chose a standard R48 rectangular waveguide, which is a classic in the theory of directional systems in the microwave range. This is due to the fact that in order to calculate and study a microwave resonant waveguide-slot antenna with in-phase excitation of slots, the authors used well-known elements of the theory of aperture antennas. The design of a resonant slotted waveguide antenna consists of a rectangular waveguide, an exciter, and a feeder. The radiation surface of the antenna is a wide wall of a standard R48 rectangular waveguide along the central axis, of which slots are symmetrically cut in a checkerboard pattern. The exciter is made in the form of a metal pin inside a rectangular waveguide near the short-circuited wall. This pin acts as an asymmetric vertical vibrator that excites electromagnetic waves in a rectangular waveguide. The antenna is tuned to the maximum radiation power mechanically by moving the short-circuited wall of the rectangular waveguide. The pin feeds a feeder based on a coaxial cable with a characteristic impedance of 75 ohms.
 The developed resonant waveguide-slot antenna with in-phase excitation of slots operates in the frequency range of 4.0–6.0 GHz. In the frequency range of 4.0–5.45 GHz, the value of Voltage Standing Wave Ratio (VSWR) varies from 1.08 to 2.1. In the frequency range of 5.45–6.0 GHz, the Voltage Standing Wave Ratio (VSWR) value varies from 2.1 to 6.55. The directivity of the antenna in the operating frequency range is not less than 90. The width of the main lobe of the antenna pattern in the horizontal plane is not more than 3.1°. The antenna gain in the operating frequency band is at least 100. The efficiency is at least 90 % with a maximum generator signal power of 10 kW.

A compact dual-band dual-linearly polarized waveguide slot array antenna with groove at waveguide bottom

AbstractA dual-band dual-linearly polarized waveguide slot array antenna is proposed in this paper. Two orthogonal slots share a short-circuit wall on broad side. The horizontal and vertical slots, which actualize dual-frequency bands and dual-linear polarization, generate vertically polarized electric fields in 9.39–9.78 GHz and horizontally polarized electric fields in 11.62–12.3 GHz, respectively. The equivalent permittivity of the waveguide is changed by two grooves at the bottom of the waveguide, which improves phase velocity. Two grooves can also reduce the distance between the elements while suppressing the side lobes. Compared to filling the medium in the waveguide, two grooves have less losses. Moreover, the antenna is simple with one input port. The array antenna also overcomes the problems of beam scanning compared with the traditional waveguide slot array antenna. A 1 × 8 antenna array is fabricated and measured to verify the design. The horizontal polarization is formed in 9.39–9.78 GHz, the vertical polarization is generated in 11.62–12.3 GHz, and the antenna gain can reach 15 dBi.

A W‐band dual‐beam low side‐lobe level antenna with shared slotted waveguide arrays

AbstractA W‐band dual‐beam low side‐lobe level antenna with a shared slotted waveguide radiating layer was designed by the authors. The proposed antenna consists of a symmetrical dual ports feed network layer located on two sides of the antenna, a novel shared symmetrical 4 × 32 slotted waveguide array radiation layer, and a bottom layer. The compact slot distance was adjusted to achieve deflection of the radiation pattern and reduce the radiation grating lobes. Good antenna radiation efficiency and port isolation were obtained by optimising the position of the slots at the radiation layer. A low side‐lobe level (SLL) was obtained using Taylor distribution was used in the design of the offset of each slot. The optimised antenna was fabricated and measured. Based on measurement results, the designed antenna has a 24.8 dBi far‐field gain, −17/–25.5 dB E/H plane side lobe level, and 19.8° beam deflection at 94 GHz. The performances of the two ports were in good agreement with each other, and the measured isolation was above 14.5 dB at 94 GHz.

Design of a slotted waveguide antenna to launch lower hybrid fast wave in the KSTAR Tokamak

Lower hybrid fast wave (LHFW) has been proposed for heating and current drive in high density plasmas to overcome the density limit of conventional slow wave current drive (LHCD). The feasibility of the LHFW was tested through coupling experiments in the Versatile Experiment Spherical Torus (VEST) device. We design a 2.45 GHz waveguide antenna for the further investigation of the LHFW in the KSTAR tokamak. RF modelling of a slotted waveguide antenna is conducted in a vacuum and plasma medium using COMSOL in order to launch 2.45 GHz fast waves with n∥0 ∼ 2.87. A slotted waveguide antenna for the LHFW is designed according to simulation results which show field patterns for fast wave polarization, a n∥ spectrum and coupling efficiency between the antenna and plasmas.

Terahertz Waveguides by Coupling Plasmon Polaritons of Cylindrical Metal Wires and a Graphene‐Embedded Slot Waveguide

Transmitting and processing terahertz (THz) signals in a chip‐scale form are tremendously important due to extensive demands for these signals in various applications ranging from sensing, imaging, communication, and spectroscopy. However, developing compact and low‐loss THz devices is still a challenging task. Herein, a novel waveguide structure consisting of a graphene‐embedded slot waveguide sandwiched between two cylindrical metal wires operating in the THz range of f = 1–5 THz is proposed. By appropriately modulating the mode hybridizing from the graphene slot waveguide and cylindrical metal wires, the resultant mode obtains a long propagation distance of 192 μm, while maintaining an ultrasmall mode area of 1.7 × 10−4, thereby achieving an unprecedented high figure of merit (FoM) = 12 985. Compared with other reported structures, the proposed structure outperforms other reported structures in term of FoM, crosstalk, and fabrication difficulty. Furthermore, the degree of integration is evaluated by analyzing crosstalk between adjacent waveguides. With an edge‐to‐edge spacing of 2.7 μm, the power transfer between waveguides can be ignored and the crosstalk is approximately frequency independent. It is believed that the proposed design paves an insightful approach into designing a wider variety of promising THz photonic integrated circuits.

Overview of Optical Biosensors for Early Cancer Detection: Fundamentals, Applications and Future Perspectives

Conventional cancer detection and treatment methodologies are based on surgical, chemical and radiational processes, which are expensive, time consuming and painful. Therefore, great interest has been directed toward developing sensitive, inexpensive and rapid techniques for early cancer detection. Optical biosensors have advantages in terms of high sensitivity and being label free with a compact size. In this review paper, the state of the art of optical biosensors for early cancer detection is presented in detail. The basic idea, sensitivity analysis, advantages and limitations of the optical biosensors are discussed. This includes optical biosensors based on plasmonic waveguides, photonic crystal fibers, slot waveguides and metamaterials. Further, the traditional optical methods, such as the colorimetric technique, optical coherence tomography, surface-enhanced Raman spectroscopy and reflectometric interference spectroscopy, are addressed.

Mid-Wave Infrared Graphene Photodetectors With High Responsivity for On-Chip Gas Sensors

On-chip integration of mid-wave infrared (MWIR) absorption spectroscopy gas sensors is a powerful approach to massproducing compact and inexpensive devices and systems. Moreover, this approach opens new horizons for sophisticated applications in environmental monitoring, quality control, and Internet-of-Things networks. Nonetheless, further development of integrated photonic devices that can operate at the MWIR band is essential. Towards that end, we propose an MWIR graphene photodetector that is integrated into silicon-on-sapphire (SOS) slot waveguides for on-chip absorption spectroscopy applications. This configuration boosts the electric field distribution of the guided mode, which enhances the effective absorption of graphene. In addition, the presence of the guided mode in the slotted air region permits significant interaction with the analyte gas, which is a significant advantage for absorption spectroscopy. The sensing performance of the device is quantified by investigating its minimum detectable concentration of the three most prevalent greenhouse gases in the U.S.: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Numerical results reveal that sub-ppb detection of these gases is achievable at the chip scale. These promising findings pave the way for the emergence of novel designs of portable, inexpensive, and highly sensitive sensors that are integrated into miniaturized electronic–photonic chipsets.

Design of highly sensitive refractive index sensor based on silicon photonic Mach–Zehnder interferometer

A highly sensitive refractive index sensor based on silicon photonic Mach–Zehnder interferometer is proposed. The device consists of a subwavelength-grating slot waveguide in one arm, as a sensing region, to enhance the light-gas interaction. By optimizing the structure parameters, the device operates near the dispersion turning point when the optical mode group velocity difference between the sensing arm and the other reference arm approaches zero. The RI sensitivity is 9.6×105/RIU and gas pressure sensitivity is as high as 232 nm/MPa. The study results show that the proposed design is a potentially robust refractive index sensor and has a good performance in gas pressure sensing.

Grating-assisted hybrid plasmonic slot resonator for on-chip SERS sensor with built-in filter

A grating-assisted hybrid plasmonic slot resonator is proposed and theoretically investigated as an on-chip surface-enhanced Raman scattering (SERS) sensor. A dielectric Bragg grating and a gold baffle are introduced into the hybrid plasmonic slot waveguide as two mirrors to form a plasmonic–dielectric cavity, in which the constructive interference of the input pump light greatly enhances the electric field. The grating reflects the wasted pump light for reverse secondary coupling and acts as a built-in notch filter for filtering out pump light from Raman scattered light simultaneously, improving the coupling efficiency and realizing a compact and miniaturized on-chip SERS sensor. Both the volumetric electric field enhancement and Raman collection efficiency are taken into account. The proposed scheme provides a simple, effective and universal method for improving the performance of the plasmonic-antennas-type waveguide-based SERS sensors.

Silicon Waveguide Sensors for Carbon Dioxide Gas Sensing in the Mid-Infrared Region

Two optical waveguide sensors based on SOS (silicon-on-sapphire) for detecting CO2 are theoretically proposed. The operational wavelength is 4.23 μm, which is the maximum absorption line of CO2. The power confinement factor (η) value is over 40% and 50%, the propagation loss is 0.98 dB/cm and 2.99 dB/cm, respectively, in the slot waveguide and SWGS (subwavelength grating slot) waveguide. An inverted tapered structure is used for the transition from strip waveguide to slot waveguide and constitutes the sensing absorption region, with the coupling efficiency that can reach more than 90%. When the optimal absorption length of the slot waveguide and SWGS waveguide is 1.02 cm and 0.33 cm, respectively, the maximum sensitivity can reach 6.66 × 10−5 (ppm−1) and 2.60 × 10−5 (ppm−1). Furthermore, taking the slot waveguide as an example, spiral and meander structures enable the long-distance sensing path to integrate into a small area.

TE-Pass/TM-Stop On-Chip Mach-Zehnder interferometer with high phase sensitivity

Mach-Zehnder interferometer (MZI) has attracted intense research interest due to the extensive applications in optical integrated systems. This paper presents a TE-Pass/TM-Stop graphene-based on-chip MZI, including splitter, phase shift arm, reference arm and combiner. The splitter and combiner are based on subwavelength gratings (SWGs) coupler. Hybrid slot waveguides with photonic crystals (PhC) are adopted as the phase shift arm and the reference arm, with and without graphene, respectively. The results show that the phase sensitivity is 1010 rad/RIU-cm at 1.55 μm, which is 10 times higher than the strip waveguide-based graphene MZI.

Optimization of bulk sensitivity for strip, slot, and subwavelength grating-based waveguides for dual-polarization operation.

We propose a dual-polarization optimization method for the bulk sensitivity of silicon-on-insulator (SOI) waveguides by defining a multi-objective function that accounts for the substrate leakage losses. The proposed optimization method was used to design micro-ring resonator bulk sensors with strip, slot, subwavelength grating, and subwavelength grating slot waveguides. The subwavelength grating slot waveguide has a bulk sensitivity of 520 nm/RIU and 325 nm/RIU for the TE and TM modes, respectively, both of which are higher than the bulk sensitivities of strip, slot, and subwavelength grating waveguides. Moreover, our Monte Carlo analysis shows that the subwavelength grating slot waveguide has the highest immunity to fabrication errors.

Plasmonic Slot Waveguide Propagation Analysis

Plasmonic slot waveguides provide extreme light confinement with the benefits of having naturally present electrodes for switching and high thermal conductivity of the metal layers to remove excess heat. Past works relied on numerical computation for these structures, which is time-consuming and lacks physical insight. Here, we present an analytical model of plasmonic slot waveguides to determine the modal properties based on single-mode matching to continuum. The model is accurate to within 3% of rigorous numerical simulations. The theory allows for rapid design and provides physical insight into mode propagation in plasmonic slot waveguides for information processing, optical manipulation, and sensing applications.

Lightweight Fan-Beam Microstrip Grid Antenna for Airborne Microwave Interferometric Radiometer Applications.

The microwave interferometric radiometer (MIR) uses aperture synthesis technology to equate multiple small-aperture antennas into a large-aperture antenna to improve spatial resolution. At present, MIR antennas that operate at frequencies above the C-band mostly use horn antennas, waveguide slot antennas, etc., which have the disadvantages of a high profile and large mass. In this paper, a new type of miniaturized, low-profile, and lightweight K-band fan-beam microstrip grid antenna is designed for the airborne campaign of the K-band one-dimensional MIR of a Microwave Imager Combined Active and Passive (MICAP) onboard a Chinese Ocean Salinity Mission (COSM). With a limited size constraint (12.33 mm) on the antenna width, a fan-beam shape antenna pattern was achieved with a 5.34° 3-dB beamwidth in the narrow beam direction and up to a 55° 3-dB beamwidth in the fan-beam direction. A periodic structural unit is proposed in this paper to reduce the design complexity of Taylor weighting, achieving desirable performances on gain (19.1 dB) and sidelobe level (<-20 dB) in the H-plane. Four antenna elements were fabricated and arranged in a non-redundant sparse array. The performance of the four-element sparse array was evaluated with a simulation and real measurement in an anechoic chamber. The coupling between antenna elements was less than -25 dB, and the consistency of phase patterns was better than 3.4°. These results verify the feasibility of the proposed K-band microstrip grid antenna for airborne MIR applications.

Novel Design Methodology for 3D-Printed Lenses for Travelling Wave Antennas

A novel methodology is introduced for designing bespoke homogeneous and graded index lenses for enhancing the gain of travelling wave antenna arrays (TWAs). 3D-printed lenses in the literature are majorly explored with standing wave antennas (SWAs) such as a microstrip or a horn antenna for gain enhancement. As there is progressively less power radiated from each slot in a TWA, as well as the successive phase delay between slots, the existing lens design approaches used for SWAs is not optimal for TWAs. Accordingly, we present a new approach of introducing a curvature to the lens that is derived from studying the power radiation profile of each slot of the TWA. This new methodology is demonstrated on a dielectric filled waveguide (DFW) slot array antenna operating at 26 -30 GHz band. An optimized dielectric graded lens and an optimized homogeneous lens have both been designed, fabricated, and measured with the DFW slot array. The new lens demonstrated a gain enhancement of more than 7 dB compared to less than 4 dB with conventional dielectric lenses. The proposed lens theory has been further verified with a bespoke optimized lens for a periodic stub-loaded microstrip leaky-wave antenna with a beam-scanning of 65∘. Design rules are included that can be applied for any TWA.

3D Printed Hemispherically Radiating Antenna for Broadband Millimeter Wave Applications

Nowadays, additive manufacturing provides far-reaching possibilities for use in radio frequency components. In addition to almost unlimited freedom of design as compared to conventional manufacturing, the absence of further assembly steps is a key aspect of 3D printing. In this paper, a 3D printed monolithic antenna for millimeter wave-sensing applications is presented with a full hemispherical coverage. The antenna is designed as an ensemble of a waveguide horn antenna and a differentially fed dipole antenna. The slotted waveguide approach was utilized to improve the manufacturing quality on the waveguide inside. The influence of two optimized antenna elements, a metal plane, and a cut-out window, on the beam pattern is comprehensively investigated. A huge half power beam width of 142° in both directions, elevation and azimuth, is presented at 79 GHz and a boresight gain of 4.7 dBi was measured. The beam pattern in the frequency range from 76 to 81 GHz is studied in greater detail, where a half power beam width of at least 112° is achieved. Due to the −10 dB matching capability bandwidth of over 28 GHz, the antenna is also suitable for extremely broadband applications with a −5 dB angular width of better than 100°. Furthermore, the system design describes how to integrate the antenna into hybrid circuit designs and the manufacturing tolerances are examined. The antenna offers attractive possibilities for millimeter wave-sensing applications in the area of assisted living and industrial monitoring, especially whenever blind spots have to be avoided.

Low-Loss and High-Confinement Photonic Platform based on Germanium-on-Insulator at Mid-Infrared Range for Optical Sensing

We experimentally demonstrate a low-loss Ge-on-insulator (Ge-OI) passive waveguide with low absorptive yttrium oxide (Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) as an insulator for a photonic platform using a wafer-bonding technology at mid-infrared (MIR) wavelength. The lowest propagation losses of 0.84 dB/cm for a channel waveguide (ChW) and 1.79 dB/cm for a slot waveguide with a slab under the slot (s-SlW) were obtained in fundamental transverse-electric (TE) mode at 4.2 μm wavelength. We also designed additional passive circuits such as grating couplers and mode converters for the implementation of the platform. For an optical gas sensor using the platform, the narrow core width of the ChWs and nanoscale of slot gap of the s-SlWs have been derived based on the numerical design toward high optical confinement, and these were fabricated and systematically analyzed to minimize the propagation loss. From the comparison for loss measurement between a nitrogen (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) ambient and a carbon dioxide (CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) containing normal atmosphere at 4.2 μm wavelength, as a consequence, narrow ChW and s-SlW showed quite high additional optical loss by the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> absorption, revealing high potential as an optical gas sensor. In conclusion, we believe that this Ge-OI platform can be a very promising candidate for the optical sensor in terms of low-loss and high-confinement properties.