Wavelength Spacing Research Articles

Wavelength-multiplexed meta-image displays with Malus-assisted metasurface

A wavelength-multiplexed metasurface for four-channel display is proposed in this paper benefiting from the orientation degeneracy implied in Malus’s law. We construct the meta-atom by placing two nanorods with different sizes on a substrate, which can achieve amplitude manipulation at different wavelengths. The wavelength-multiplexed metasurface composed of these meta-atoms can display four independent binary grayscale images at different wavelengths in the reflection space. Numerical simulations validate the proposed wavelength-multiplexed metasurface with the ability to record and project four independent binary grayscale images at two wavelengths. With the advantages of high capacity, the proposed wavelength-multiplexed display metasurface will pave a way for multi-channel applications in nano-display, information storage, optical anticounterfeiting, and other relevant fields.

The design and decoupling of TX/RX antenna with omnidirectional conical beam operating at V band

Abstract In this work, a high isolation transmitter–receiver antenna with omnidirectional conical beam operating at 61.5 GHz is presented. The proposed dual-antenna system consists of a transmitter (TX) antenna and a receiver (RX) antenna with the same structure, composed of a circular metal patch with ring slot and metal shorting vias, fed by a coaxial probe at their respective geometric center. And both antennas are placed on a circular metal platform, positioned symmetrically with each other. To increase the isolation between them, three different decoupling methods are proposed, and the one with an electromagnetic band gap (EBG) structure added in-between shows the most promising decoupling effect, as the surface wave current can be maximally suppressed. The overall size of the antenna system is 2.2552 × π × 0.579 λ 0 3, where λ 0 is the free space wavelength at 61.5 GHz. A prototype of the proposed antenna system with EBG decoupling structure is fabricated and measured, demonstrating that the TX–RX isolation is 57.9 dB at 61.5 GHz, 21.6 dB better than that without decoupling structure, with omnidirectional conical beam radiation achieved at the same time.

Data Center Four-Channel Multimode Interference Multiplexer Using Silicon Nitride Technology.

The operation of a four-channel multiplexer, utilizing multimode interference (MMI) wavelength division multiplexing (WDM) technology, can be designed through the cascading of MMI couplers or by employing angled MMI couplers. However, conventional designs often occupy a larger footprint, spanning a few millimeters, thereby escalating the energy power requirements for the photonic chip. In response to this challenge, we propose an innovative design for a four-channel silicon nitride (Si3N4) MMI coupler with a compact footprint. This design utilizes only a single MMI coupler unit, operating within the O-band spectrum. The resulting multiplexer device can efficiently transmit four channels with a wavelength spacing of 20 nm, covering the O-band spectrum from 1270 to 1330 nm, after a short light propagation of 22.8 µm. Notably, the multiplexer achieves a power efficiency of 70% from the total input energy derived from the four O-band signals. Power losses range from 1.24 to 1.67 dB, and the MMI coupler length and width exhibit a favorable tolerance range. Leveraging Si3N4 material and waveguide inputs and output tapers minimizes light reflection from the MMI coupler at the input channels. Consequently, this Si3N4-based MMI multiplexer proves suitable for deployment in O-band transceiver data centers employing WDM methodology. Its implementation offers the potential for higher data bitrates while maintaining an exemplary energy consumption profile for the chip footprint.

A simple reflective metalens based on reverse design for an ultra-high-efficiency free space wavelength splitter

We propose two kinds of high-efficiency free-space wave splitters in the mid-IR band using reverse design. The wavelength divider based on the abnormal reflection principle realizes a beam-splitting angle of 22.00° and 10.92° by controlling the phase distribution, and the reflection efficiency of both wavelengths exceeds 50%. The wavelength divider designed based on the concept of metalens simultaneously accomplishes the functions of focusing and beam splitting. It has a focal length of 100 μm and a relative focal position of 100 μm. Most importantly, the focusing efficiency for the two wavelengths reaches an impressive 48.59% and 72.51%, respectively.

Wavelength continuously tunable and wavelength spacing variable comb filter based on a High-Birefringence fiber loop mirror

We demonstrated an all-fiber comb filter that can continuously tune the wavelength for four switchable wavelength spacings. The comb filter consists of a fiber loop mirror (FLM) that is inserted with three polarization maintaining fibers (PMFs) and three polarization controllers (PCs). The transmission characteristics of the comb filter was theoretically analyzed. The output characteristics of the comb filters shows a dependence of the orientation angles of the PCs. With the angles being optimally set, the comb filter with four different wavelength spacings can be achieved. The wavelength continuously tunable and wavelength spacing variable mechanisms are elaborated. The three PCs are employed to enable the tunable and switchable functions. The two PCs between the PMFs serve as a polarization rotator to align the birefringent axes of the PMFs for varying the wavelength spacing while the PC adjacent to the optical coupler can be used for wavelength tuning function. When the two quarter-waveplates (QWPs) of the PC are optimally set at π/4 and - π/4, the half-waveplate (HWP) acts as a wavelength tuning element, and when the QWPs are set at zero degrees, the HWP serves as an amplitude tuning element. The center wavelength of the comb filter could be linearly tuned with the rotation angle of the HWP, and a π/2 change in the rotation angle leads to a wavelength change in one free-spectral-range (FSR). A π/4 change in the rotation angle could bring the normalized peak transmission from its maximum at 1 to minimum at 0. The proposed tunable technique has been verified by our experimental results. We experimentally demonstrated the comb filters can switch among FSRs of 0.75 nm, 1.01 nm, 1.51 nm, and 3.1 nm with tunable functions.

High-Order Wideband Band-Pass Miniaturized Frequency-Selective Surface with Enhanced Equivalent Inductance

To extend the wideband performance of high-order band-pass filtering applications, optimized designs with knitted structures based on traditional miniaturized frequency-selective surfaces (FSSs) are proposed in this paper. The presented miniaturized FSSs consist of multiple metallic capacitive layers, knitted inductive layers, and substrates. In contrast to the conventional high-order miniaturized FSSs composed of metallic frames, patches, and substrates, the optimized miniaturized FSSs replace the original metallic wire frames with knitted structures. Both proposed modified miniaturized FSSs achieve a flat pass-band from 5.5 GHz to 10.3 GHz with a 3 dB bandwidth of 71.6% under vertical incidence. The unit cells have dimensions of 0.16 λ0 × 0.16 λ0 × 0.284 λ0 and 0.16 λ0 × 0.16 λ0 × 0.279 λ0, respectively, where λ0 is the free space wavelength at 7.9 GHz, which is the center frequency of the operating band. Numerical simulations and measurements demonstrate that the proposed modified miniaturized FSSs exhibit excellent wideband performance with clean transition bands around the pass-band during oblique incidence and are suitable for applications such as radomes, where wideband filtering is essential for covering multi-band functions of radar or communication instruments.

Front‐to‐back ratio improvement of a compact base‐station antenna for 5G NR applications

AbstractThe front‐to‐back ratio (FBR) of a compact dual‐polarized antenna is improved for 5G new radio (NR) base‐station applications. A rim made of partially reflective surfaces is introduced to improve the FBR. A meta‐surface and a parasitic element are employed to lower the antenna height. The dual‐polarized base‐station antenna with improved FBR features a compact size of , where is the wavelength in free space at the lowest operating frequency. The FBR achieved over the 5G NR n28/n82/n83 bands (0.69–0.96 GHz) for return loss >15 dB is higher than 26 dB. The antenna gain is higher than 7 dBi and the isolation is higher than 30 dB.

Gain-enhanced and Mechanical Reconfigurable Slot Antenna with Metasurface

A novel wideband slot antenna with metasurface is presented. In order to achieve broadband, high-gain, and mechanical reconfigurable performance, a metasurface is adopted and combined with a new-type planar slot antenna The antenna and metasurface are designed on F4B substrate, and the overall dimension of the antenna is 1.48λ0×1.48λ0×0.2λ0 (λ0 is the free space wavelength at center frequency). Different from the traditional square metasurface, a structure with different properties in the x and y directions is utilized in the proposed antenna, which can be reconstructed by adjusting the relative position between the slot antenna and the metasurface. By introducing the metasurface, the maximum gain of the whole antenna is improved. The antenna works in two states with respect to the two locations of the metasurface. The impedance bandwidth of the proposed antenna in state A is from 5.49 to 9.40 GHz (52.5%), and the impedance bandwidth of the proposed antenna in state B is from 5.83 to 6.01 GHz (3%) and 7.05 to 9.62 GHz (30.8%). The gain of the whole antenna in both states is higher than that of the original slot antenna (without metasurface), and the maximum gain is 9.1 dBi.

Design of polycarbonate‐based dual‐band conformal frequency‐selective surface for 4G/5G shielding applications

AbstractIn this work, a compact conformal swastik frequency‐selective surface (FSS) has been proposed for shielding 4G/5G signals. The resonating unit of FSS is realized using swastik geometry. The proposed FSS offers resonance at 1800 MHz for 4G band and 3300 MHz for sub‐6 5G band. The unit‐cell dimension is 0.11λ0 × 0.11λ0, where λ0 is the free space wavelength corresponding to the lowest resonating frequency. The proposed FSS provides a stable response for both the TE and TM modes of polarization up to 60°. To validate the simulated results, the FSS was fabricated and tested. The simulation and measurement results are in good agreement.

Self‐isolated quadruplexing SIW antenna with extremely low frequency ratio for C‐band applications

AbstractIn this letter, an substrate integrated waveguide ‐based self‐quadplexing antenna based on a modified Jerusalem cross‐shaped slot is presented. By introducing different lengths of the slots of Jerusalem cross, the antenna operates at four different closely spaced resonant frequencies 4.08, 4.37, 4.61, and 4.97 GHz. The proposed quadplexer antenna achieved a minimum port isolation of 20 dB between any two ports with an extremely low frequency ratio of 1.2. The quadplexer antenna has unidirectional radiation pattern and independent frequency tunability characteristics with a gain of 6.88, 6.51, 7.03, and 7.12 dBi, at corresponding resonant frequencies and a compact size of 0.142, where 0 is the free space wavelength at the lowest resonant frequency. The proposed antenna is suitable for C‐band multiplexing applications.

Tunable single frequency Hz-magnitude narrow linewidth Brillouin fiber laser based on parity-time symmetry.

An Hz-magnitude ultra-narrow linewidth single-frequency Brillouin fiber laser (BFL) is proposed and experimentally demonstrated. The single frequency of the laser is selected by parity-time (PT) symmetry, which consists of a stimulated Brillouin scatter (SBS) gain path excited by a 24 km single-mode fiber (SMF) and an approximately equal length loss path tuned with a variable optical attenuator (VOA). These paths are coupled through a fiber Bragg grating (FBG) into a wavelength space. Accomplishing single-frequency oscillation involves the precise adjustment of polarization control (PC) and VOA to attain the PT broken phase. In the experiment, the linewidth of the proposed BFL is 9.58 Hz. The optical signal-to-noise ratio (OSNR) reached 78.89 dB, with wavelength and power fluctuations of less than 1pm and 0.02 dB within one hour. Furthermore, the wavelength can be tuned from 1549.9321 nm to 1550.2575 nm, with a linewidth fluctuation of 1.81 Hz. The relative intensity noise (RIN) is below -74 dB/Hz. The proposed ultra-narrow single-frequency BFL offers advantages such as cost-effectiveness, ease of control, high stability and excellent output characteristics, making it highly promising for the applications in the coherent detection.

An all‐metal wideband circularly polarized single‐patch antenna based on TM11mode

AbstractThis paper proposes an all‐metal wideband circularly polarized (CP) single‐patch antenna based on TM11mode. The conventional square patch is divided into four small square patch units by loading metal shorting walls under the two vertical centerlines. The shorting walls are windowed to facilitate coupling between the adjacent units. When the TM11mode is excited, the coupling windows not only distribute energy but also provide the 90° phase difference required for CP radiation. An additional feeding network is not required, and its profile is less than 0.05λ0(λ0is the free space wavelength at the center frequency). Three minima in the axial ratio (AR) response are present. The results show that right‐hand circularly polarized (RHCP) radiation is achieved. The measured 3 dB AR bandwidth is 6.9%, ranging from 1.52 to 1.62 GHz, and the RHCP realized peak gain is 7.72 dBic. An all‐metal construction suitable for satellite communication systems is designed that has superior performance in harsh space environments.

Effective hyperspectral image classification based on segmented PCA and 3D-2D CNN leveraging multibranch feature fusion

ABSTRACT We present an innovative hyperspectral image (HSI) classification method addressing challenges posed by closely spaced wavelength bands. Our approach combines 3D-2D convolutional neural networks (CNNs) with multi-branch feature fusion for improved spectral-spatial feature extraction. Using segmented principal component analysis (Seg-PCA), we reduce HSIs’ spectral dimensions into global and local intrinsic characteristics. The integration of 3D and 2D CNNs captures joint spectral-spatial features, while a multi-branch network extracts and merges diverse local features along the spectral dimension. Our method outperforms existing approaches, achieving remarkable accuracy of 99.27%, 100%, and 99.99% on Indian Pines, Salinas Scene, and University of Pavia datasets, respectively.

Design of anti-temperature interference liquid level sensor based on π-phase-shifted LPFG near phase-matched turning points

A new, to the best of our knowledge, type of liquid level sensor with π-phase-shifted long-period fiber grating operating near the phase-matched turning point is proposed. The sensor introduces a π-phase shift at the center of the grating, which can generate three attenuation peaks due to the mode interference effect in the transmission spectrum of the sensor operating near the phase-matched turning points. According to the different liquid level and temperature sensing characteristics of three attenuation peaks, simultaneous detection of liquid level and temperature can be realized through wavelength modulation and intensity modulation detection. In this paper, the structural parameters of this liquid level sensor, such as grating period and grating length, are designed optimally based on the fiber grating coupled mode theory, and the wavelength spacing of the attenuation peaks on both sides and the intensity of the center attenuation peak are analyzed in relation to the liquid level and temperature. The simulation results show that the liquid level sensitivity is 5.58 nm/mm and −1.316dB/mm for wavelength modulation and intensity modulation, respectively, for the liquid level variation range of 0 to 18 mm; and the temperature sensitivity of wavelength modulation and intensity modulation is −0.139nm/∘C and 0.171 dB/°C, respectively, over the temperature variation range of 20°C to 80°C. Therefore the simultaneous detection of liquid level and temperature can be realized by establishing a measurement inversion matrix. Compared with other liquid level sensors, this sensor is expected to be widely used in the field of liquid level sensing due to its simple structure, high liquid level sensitivity, large liquid level measurement range, and narrow monitoring peak bandwidth.

Dual-band miniaturized composite right left handed transmission line ZOR antenna for microwave communication with machine learning approach

This article presents a dual-band miniaturized Composite Right-Left-Handed Transmission Line (CRLH-TL) in an open-ended terminal, employing the Machine Learning (ML) technique. The CRLH-TL antenna is designed on the FR4 epoxy substrate. The substrate size is 0.31λ0 × 0.09 λ0, where λ0 is the free space wavelength. The proposed antenna offers dual-band functionality with resonant frequencies at 2.49 GHz and 5.33 GHz. The measured dual-band impedance bandwidths are 14.46 % and 14.74 %, with gains of 0.85 dB and 1.67 dB, and radiation efficiencies of 91.78 % and 95.45 % obtained at resonating frequencies of 2.49 GHz and 5.33 GHz, respectively. The proposed antenna also offers bipolar-type radiation patterns in the E-plane, and omnidirectional radiation patterns in the H-plane, along with compactness and constant gain. Several ML methods, including Random Forest (RF), Decision Tree (DT), K-Nearest Neighbour (KNN), Extreme Gradient Boosting (XGB), and Artificial Neural Network (ANN), are used to optimize the antenna. Compared to other ML algorithms, RF ML techniques estimate reflection coefficient S11 with an accuracy of above 98 %. The proposed antenna is utilized in WLAN (5.15–5.35, 5.47–5.725 GHz) and Wi-MAX (5.2–5.8 GHz) microwave applications.

Thermally switchable metal-free THz electromagnetic shield using phase change material

A thermally switchable ultrathin terahertz (THz) metal-free electromagnetic shield can be developed with the help of phase change material, vanadium oxide (VO2). The thermal alterations in the phase change material can modulate its conductivity hence the switchable absorptivity can be achieved. An ultrathin, (height ≈λ/7, λ is free space wavelength) ultra-broadband, metal-free absorber structure containing a VO2 resonator, dielectric spacer and graphite reflector operating within 3.88−9.82 THz band is designed and numerically studied. Multiple reflections and destructive interference occurring in the dielectric delivers the absorber operation which is validated through the theory of multiple reflections. Absorber provides the stable response over the incident angle of more than 45°. Moreover, the shielding effectiveness of the developed thin periodic film remains more than 30 dB in the operating frequency range.

Highly-reflective facet-coated multi-wavelength DFB laser array with exact wavelength spacings.

A distributed feedback (DFB) laser array of twenty wavelengths with highly reflective and anti-reflective (HR-AR) coated facets is both theoretically analyzed and experimentally validated. While the HR facet coating enhances high wall-plug efficiency, it inadvertently introduces a random facet grating phase, thereby compromising the lasing wavelength's predictability and the stability of the single-longitudinal-mode (SLM). In this study, two key advancements are introduced: first, the precisely spaced wavelength is achieved with an error of within ±0.2 nm using the reconstruction-equivalent-chirp (REC) technique; second, the random grating phase on the HR-coated facet is compensated by a controllable distributed phase shift through a two-section laser structure. The SLM stability can be improved while the wavelength can be continuously tuned to the standard wavelength grid. The overall chip size is compact with an area of 4000 × 500 µm2. The proposed laser array has a light power intensity above 13 dBm per wavelength, a high side mode suppression ratio above 50 dB, and low relative intensity noise under -160 dB/Hz. These attributes make it apt for deployment in DWDM-based optical communication systems and as a light source for optical I/O.

A compact high‐gain slot antenna array with a bidirectional broadside radiation pattern

AbstractA novel bidirectional broadside slot antenna array is proposed in this letter. The outstanding merits of this antenna array lie in the minimum width and the highest gain‐to‐area ratio (G/A) compared to previous works. The ingenious combination of the radiation mode and the transmission mode of slots is adopted, eliminating the design of the feed network. Thus, a simple and compact configuration is obtained. Moreover, two metal plates are added vertically at the sides of the slot array, with a more homogeneous radiation aperture and a higher G/A. With the 3D structure, a compact volume of the antenna array is ensured. For performance evaluation, our proof‐of‐concept design is fabricated with a cuboid dimension of 1.94λ0 × 0.12λ0 × 0.12λ0, where λ0 denotes the wavelength in free space at the centre frequency. An impedance bandwidth of 2.38 to 2.49 GHz is achieved, with a maximum gain of 9.7 dBi and a higher G/A of 39.5. The advantages including compact size and high G/A are paving the path for the potential application of communications in long and narrow spaces.

A 3‐D metal printed folded‐shorted patch array with an integrated feeding circuit offering dual‐band circularly polarised radiation for cubesat applications

AbstractThis letter presents a three‐dimensional (3‐D) printed metallic antenna array that combines folded‐shorted patches (FSPs) and meandering for structure compactness. The design provides dual‐band functionality, operating in the L‐band and the S‐band. The compact and broadband feeding network, which is also integrated onto the backside of the antenna ground plane, enables circularly polarised radiation over an operational bandwidth of more than 80%. The dimensions of the array are 0.4 × 0.4 × 0.056 (where λ0 is the free space wavelength at 1.2 GHz), making the design suitable for CubeSats and other space constraint applications. The measured right‐handed circular polarised realised gains are 2.32 dBic and 1.84 dBic at 1.2 GHz and 2.45 GHz, respectively. In addition, the 3‐D metallic printed FSP elements can be flexibly arranged for different link requirements and other applications like beam steering.

Design and performance investigation of metamaterial-inspired dual band antenna for WBAN applications.

This paper presents the design and analysis of a metamaterial-based compact dual-band antenna for WBAN applications. The antenna is designed and fabricated on a 0.254 mm thick semi-flexible substrate, RT/Duroid® 5880, with a relative permittivity of 2.2 and a loss tangent of 0.0009. The total dimensions of the antenna are 0.26λo×0.19λo×0.002λo, where λo corresponds to the free space wavelength at 2.45 GHz. To enhance overall performance and isolate the antenna from adverse effects of the human body, it is backed by a 2×2 artificial magnetic conductor (AMC) plane. The total volume of the AMC integrated design is 0.55λo×0.55λo×0.002λo. The paper investigates the antenna's performance both with and without AMC integration, considering on- and off-body states, as well as various bending conditions in both E and H-planes. Results indicate that the AMC-integrated antenna gives improved measured gains of 6.61 dBi and 8.02 dBi, with bandwidths of 10.12% and 7.43% at 2.45 GHz and 5.80 GHz, respectively. Furthermore, the AMC integrated antenna reduces the specific absorption rate (SAR) to (>96%) and (>93%) at 2.45 GHz and 5.80 GHz, meeting FCC requirements for low SAR at both frequencies when placed in proximity to the human body. CST Microwave Studio (MWS) and Ansys High-Frequency Structure Simulation (HFSS), both full-wave simulation tools, are utilized to evaluate the antenna's performance and to characterize the AMC unit cell. The simulated and tested results are in mutual agreement. Due to its low profile, high gain, adequate bandwidth, low SAR values, and compact size, the AMC integrated antenna is considered suitable for WBAN applications.