Large Operating Bandwidth Research Articles

Performance Analysis of Microwave Photonic Spectral Filters based on Optical Microcombs

AbstractMicrowave transversal filters, which are implemented based on the transversal filter structure in digital signal processing, offer a high reconfigurability for achieving a variety of signal processing functions without changing hardware. When implemented using microwave photonic (MWP) technologies, also known as MWP transversal filters, they provide competitive advantages over their electrical counterparts, such as large operation bandwidth, strong immunity to electromagnetic interference, and low loss when processing signals at high frequencies. Recent advances in high‐performance optical microcombs provide compact and powerful multi‐wavelength sources for MWP transversal filters that require a larger number of wavelength channels to achieve high performance, allowing for the demonstration of a diverse range of filter functions with improved performance and new features. Here, a comprehensive performance analysis for microcomb‐based MWP spectral filters based on the transversal filter approach is presented. First, the theoretical limitations are investigated in the filter spectral response induced by finite tap numbers. Next, the distortions are analyzed in the filter spectral response resulting from experimental error sources. Finally, the influence of input signal's bandwidth on the filtering errors is assessed. These results provide a valuable guide for the design and optimization of microcomb‐based MWP transversal filters for a variety of applications.

Reconfigurable optical logic in silicon platform

In this paper, we present a novel, scalable, and reconfigurable optical switch that performs multiple computational logic functions simultaneously. The free-carrier depletion effect is used to perform non-volatile switching operations due to its high speed and low power consumption. We adopt the concept of optical memory using a phase-change material to realize the non-volatile reconfigurability without a constant power supply, in addition to providing a large operating bandwidth required for reconfigurability. The proposed reconfigurable optical logic architecture is realized in a compact microdisk resonator configuration, utilizing both the carrier-depletion-based modulation and phase-change optical memory. This is the first time these two modulation schemes are implemented in the same optical microdisk for the purpose of reconfigurable optical logic.

A High-Density Organic Package Solution to W-band SiGe Flip-Chip Applications

High-density packaging concepts need to be studied to meet the demands from telecommunication and radar applications which require large operational bandwidth and system compactness. In this work, package design of a transmitter SiGe flip-chip with C4 solder ball interface on a multilayer PCB operating around 94 GHz is presented.

Single-channel system for joint unambiguous measurement of DFS and AOA based on serrodyne modulation.

In this paper, a photonic-assisted system for simultaneous and unambiguous measurement of the Doppler frequency shift (DFS) and angle-of-arrival (AOA) using a dual-parallel dual-drive Mach-Zehnder modulator (DP-DDMZM) is proposed and investigated. The echo signals received by two receiving antennas are applied to the radio frequency ports of one sub-DDMZM of the DP-DDMZM. The bias port of the sub-DDMZM is fed by a binary electrical signal that is used to construct two different mapping curves on the relationship between the phase difference and the power of the output intermediate frequency (IF) signal. Therefore, unambiguous AOA measurement with extended range can be realized. The transmitted signal is input into the other sub-DDMZM to implement single-sideband modulation, which is then frequency shifted based on serrodyne modulation. Both the value and direction of DFS can be derived intuitively from the frequency of the output IF signal. Simulation results show that the measurement error of unambiguous DFS measurement is no more than ±0.008H z in the range of -100k H z to 100kHz, and the measurement error of unambiguous AOA is less than ±0.2∘ in the range of -70.8∘ to 70.8°. Moreover, since the scheme does not involve the construction of multi-channels or use of any filter or polarization dependent device, the system has concise structure, high accuracy, large operating bandwidth, and strong robustness, and can be considered as a very promising solution for actual applications.

Highly sensitive terahertz fingerprint sensing based on the quasi-guided modes in a distorted photonic lattice.

Using photonic structures resonating at the characteristic absorption frequency of the target molecules is a widely-adopted approach to enhance the absorption and improve the sensitivity in many spectral regions. Unfortunately, the requirement of accurate spectral matching poses a big challenge for the structure fabrication, while active tuning of the resonance for a given structure using external means like the electric gating significantly complicates the system. In this work, we propose to circumvent the problem by making use of quasi-guided modes which feature both ultra-high Q factors and wavevector-dependent resonances over a large operating bandwidth. These modes are supported in a distorted photonic lattice, whose band structure is formed above the light line due to the band-folding effect. The advantage and flexibility of this scheme in terahertz sensing are elucidated and exemplified by using a compound grating structure on a silicon slab waveguide to achieve the detection of a nanometer scale α-lactose film. The spectral matching between the leaky resonance and the α-lactose absorption frequency at 529.2 GHz by changing the incident angle is demonstrated using a flawed structure which exhibits a detuned resonance at normal incidence. Based on the high dependence of the transmittance at the resonance on the thickness of α-lactose, our results show it is possible to achieve an exclusive detection of α-lactose with the effective sensing of thickness as small as 0.5 nm.

Quasi-guided modes resulting from the band folding effect in a photonic crystal slab for enhanced interactions of matters with free-space radiations.

We elucidate that guided modes supported by a regular photonic crystal slab structure composed of a square lattice of air holes in a silicon slab will transition into quasi-guided (leaky) modes when the radius of every second column of air holes is changed slightly. This intentional geometric perturbation will lead to a doubling of the period in one direction and the corresponding shrinkage of the first Brillouin zone. Because of the translational symmetry in the k-space, leaky waves inheriting the spatial dispersion of the original guided modes, which do not interact with external radiation, will appear with the dispersion curves above the light cone. Our results show that ultrahigh Q-factor resonances with large operating bandwidth can be achieved. Interestingly, the perturbation in only one direction of the photonic lattice will lead to an in-plane wave number-dependent resonance characteristic in both directions. Our numerical results demonstrate a local enhancement of the electric field magnitude by the order of 102, which is even more significant than those in most plasmonic structures. These quasi-guided modes with superior properties will provide a new platform for efficient light-matter interactions.

All-Optical Nonlinear Activation Function Based on Germanium Silicon Hybrid Asymmetric Coupler

Nonlinear activation functions are crucial for optical neural networks (ONNs) to achieve more various functions. However, the current nonlinear functions suffer from some dilemma, including high power consumption, high loss, and limited bandwidth. Here, we propose and demonstrate an all-optical implementation of a nonlinear activation function based on germanium silicon hybrid integration. The principle lies in the intrinsic absorption and the carrier-induced refractive index change of germanium in C -band. It has a large operating bandwidth and a response frequency of 70 MHz, with a loss of 4.28 dB and a threshold power of 5.1 mW. Adopting it to the MNIST handwriting data set classification, it shows an improvement in accuracy from 91.6% to 96.8%. This proves that our scheme has great potential for advanced ONN applications.

Optimization and commissioning of the EPIC commensal radio transient imager for the long wavelength array

ABSTRACT Next-generation aperture arrays are expected to consist of hundreds to thousands of antenna elements with substantial digital signal processing to handle large operating bandwidths of a few tens to hundreds of MHz. Conventionally, FX correlators are used as the primary signal processing unit of the interferometer. These correlators have computational costs that scale as $\mathcal {O}(N^2)$ for large arrays. An alternative imaging approach is implemented in the E-field Parallel Imaging Correlator (EPIC) that was recently deployed on the Long Wavelength Array station at the Sevilleta National Wildlife Refuge (LWA-SV) in New Mexico. EPIC uses a novel architecture that produces electric field or intensity images of the sky at the angular resolution of the array with full or partial polarization and the full spectral resolution of the channelizer. By eliminating the intermediate cross-correlation data products, the computational costs can be significantly lowered in comparison to a conventional FX or XF correlator from $\mathcal {O}(N^2)$ to $\mathcal {O}(N \log N)$ for dense (but otherwise arbitrary) array layouts. EPIC can also lower the output data rates by directly yielding polarimetric image products for science analysis. We have optimized EPIC and have now commissioned it at LWA-SV as a commensal all-sky imaging back-end that can potentially detect and localize sources of impulsive radio emission on millisecond timescales. In this article, we review the architecture of EPIC, describe code optimizations that improve performance, and present initial validations from commissioning observations. Comparisons between EPIC measurements and simultaneous beam-formed observations of bright sources show spectral-temporal structures in good agreement.

Miniaturized Broadband Dual-Polarized Dipole Antenna Based on Multiple Resonances and its Array for Base-Station Applications

A novel bandwidth enhancement method using loading multiple parasitic structures to realize broadband miniaturized antenna is proposed. Based on the proposed strategies, one miniaturized wideband dual-polarized dipole antenna covering LTE/CDMA/GSM bands (690–960 MHz) for base-station application is designed. The antenna consists of two folded bowtie-shaped dipoles fed with a pair of crossed baluns, and is further loaded on a ceramic block with high permittivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon$ </tex-math></inline-formula> ) for miniaturization. A parasitical square loop is loaded for bandwidth enhancement at a lower frequency. In addition, four split rings with a small square loop are firstly adopted for antenna resonance tuning, the impedance bandwidth is thus improved effectively. The antenna size is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.18\,\times \,0.18\,\times \, 0.22\,\,\lambda _{0}$ </tex-math></inline-formula> at the center frequency. The measured result of the fabricated prototype exhibits a relatively large operation bandwidth of 32.73% (VSWR < 1.6) with stable radiation patterns and relatively flat broadside gains. Moreover, to verify the feasibility of the proposed miniaturized antenna element in the base-station application, an array of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\,\times \, 4$ </tex-math></inline-formula> is further designed. The measured results of the array indicate isolation over 20 dB with an impedance bandwidth of 32%. Due to the good performance and compact feature, the proposed miniaturized broadband antenna element can find large application potential in multiband wireless communication base stations.

Dynamic Transmissive Metasurface for Broadband Phase‐Only Modulation Based on Phase‐Change Materials

AbstractDynamic metasurfaces provide exciting opportunities for spatial light modulation with ultra‐compact footprint and high resolution. Designs reported so far typically suffer from efficiency problem due to the strong coupling between amplitude and phase modulation. Huygens metasurface design eliminates the back propagation with interference of dual resonances, offering a potential solution for phase‐only modulation. However, the intrinsic dispersion mismatch between the dual modes restricts this design to a highly limited bandwidth. Here, a dynamic phase‐change metasurface design that enables transmissive phase modulation covering over 240° with high‐level transmisstance maintained over 0.8 and a large operation bandwidth of 80 nm ranging from 1516 to 1596 nm, is presented. To verify the potential usage of this design, a multi‐state phase‐change control scheme that enables quasi‐continuous phase modulation is further provided. The key feature of this design is the leverage of congener Huygens concept. Judicious incorporation of lossless phase‐change material (PCM) with congener Huygens metasurface allows wideband holding of Huygens condition, which provides a perfect platform to simultaneously address the efficiency and bandwidth problems. This novel feature represents a meaningful step forward to integrate spatial light modulation.

Nickel-Based High-Bandwidth Nanostructured Metamaterial Absorber for Visible and Infrared Spectrum.

The efficient control of optical light at the nanoscale level attracts marvelous applications, including thermal imaging, energy harvesting, thermal photovoltaics, etc. These applications demand a high-bandwidth, thermally robust, angularly stable, and miniaturized absorber, which is a key challenge to be addressed. So, in this study, the simple and cost-effective solution to attain a high-bandwidth nanostructured absorber is demonstrated. The designed nanoscale absorber is composed of a simple and plain circular ring of nickel metal, which possesses many interesting features, including a miniaturized geometry, easily fabricable design, large operational bandwidth, and polarization insensitivity, over the previously presented absorbers. The proposed nanoscale absorber manifests an average absorption of 93% over a broad optical window from 400 to 2800 nm. Moreover, the detailed analysis of the absorption characteristics is also performed by exciting the optical light’s various incident and polarization angles. From the examined outcome, it is concluded that the nanostructured absorber maintains its average absorption of 80% at oblique incident angles in a broad wavelength range from 400 to 2800 nm. Owing to its appealing functionalities, such as the large bandwidth, simple geometry, low cost, polarization insensitivity, and thermal robustness of the constituting metal, nickel (Ni), this nano-absorber is made as an alternative for the applications of energy harvesting, thermal photovoltaics, and emission.

Fast method for designing broadband achromatic diffractive optical elements.

Diffractive optical elements play a crucial role in the miniaturization of the optical systems, especially in correcting achromatic aberration. Considering the rapidity and validity of the design method, we propose a fast method for designing broadband achromatic diffractive optical elements. Based on the direct binary search algorithm, some improvements have been made including the selection of the initial height map to mitigate the uncertainty, the reduction of the variations to accelerate the optimization and the increase of sampling rate to deal with the large operation bandwidth. The initial height map is calculated instead of random initial value. Due to different regions of the height map contributing to point spread functions differently, the variations are reduced to speed up the optimization. The large operation bandwidth is solved by increasing the sampling rate at unfitted wavelengths instead of setting weighting coefficients. We demonstrate via simulations that our method is effective through several examples. The design of broadband achromatic diffractive optical elements can be quickly achieved by our method.

Photonic-assisted frequency downconverter with self-interference cancellation and fiber dispersion elimination based on stimulated Brillouin scattering.

A photonic-assisted frequency downconverter with self-interference cancellation and fiber dispersion elimination is proposed for in-band full-duplex (IBFD) radio-over-fiber (ROF) systems based on stimulated Brillouin scattering. In this work, a dual-polarization Mach-Zehnder modulator (DPol-MZM) is employed to cancel the self-interference signal in the optical domain, thus the proposed system has a large operation bandwidth without the limitation of the electrical bottleneck. Meanwhile, a widely tunable microwave photonic filter (MPF) based on stimulated Brillouin scattering (SBS) is constructed to perform carrier-suppressed single-sideband modulation of LO. Therefore, the proposed photonic frequency downconverter is naturally free from fiber dispersion. Furthermore, thanks to the characteristics of the SBS-based MPF, such as large out-of-band rejection ratio, high resolution and wideband tunability, the proposed system has a high flexibility and downconversion efficiency, which is of great significance for IBFD ROF systems.

Wideband Microwave Phase Noise Analyzer Based on All-Optical Microwave Signal Processing

An approach for wideband phase noise measurement of microwave signal sources is proposed based on all-optical microwave signal processing. In the proposed scheme, an optical carrier is sequentially modulated by the signal under test (SUT) in a phase modulator and a dual-polarization modulator to generate two +1st-order sidebands with orthogonal polarizations. A time delay is introduced between two sidebands by a span of low-loss optical fiber. Combined with a polarization controller and a polarizer, photonic microwave downconversion with a desired phase shift and time delay can be realized after photodetection, and the phase noise of the SUT can be calculated thereafter. Since all the microwave signal processing functions in the conventional photonic-delay-line-based phase noise measurement system are implemented in the optical domain, the proposed scheme has a large operational bandwidth and a high measurement sensitivity. In the experimental demonstration, accurate phase noise measurement of SUTs is achieved in a frequency range of 10-35 GHz, and a phase noise floor as low as −132.16 dBc/Hz at 10 kHz is obtained.

Numerical study of terahertz radiations from difference frequency generation with large spectral tunability and significantly enhanced conversion efficiencies boosted by 1D leaky modes

The nonlinear optical process of difference frequency generation (DFG) is a prominent technique to produce continuous-wave terahertz radiations while its low conversion efficiency calls for substantial enhancement using artificial structures. All-dielectric nanostructures supporting the quasi-bound states in the continuum (QBIC) appear as a promising approach to this end. To achieve the utmost of enhancement, both input lightwaves of the DFG should work at the QBIC conditions and in many cases a spectral tunability of the input wavelength is necessary. All these requirements go beyond the capability of conventional QBIC which can only happen within a narrow bandwidth for a given structure. In this work, we numerically demonstrate that these restrictions can be eliminated by using our recently proposed concept of one-dimensional leaky modes with ultrahigh Q factors and large operation bandwidth. Using an elaborately designed structures in the form of binary waveguide gratings (BWGs) made from LiNbO3 thin film, we demonstrate that a conversion efficiency enhanced by the order of 1011 can be achieved using the BWGs made from LiNbO3, compared to the case of a bare LiNbO3 thin film. Furthermore, enhanced THz generations over a large spectral range can be easily achieved by changing the incident angle of one input light beam while tuning its wavelength to match the requirement for the leaky resonance excitation at that angle.

A ScAlN Piezoelectric High-Frequency Acoustic Pressure-Gradient MEMS Vector Hydrophone With Large Bandwidth

In this letter, a scandium-doped aluminum nitride (Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {Al}}_{1-x}\text{N}$ </tex-math></inline-formula> , x = 9.5%) piezoelectric high-frequency MEMS vector hydrophone with large operation bandwidth is presented, which measures acoustic pressure gradient underwater. The hydrophone sensor consists of four groups of sensing cell arrays. Each sensing cell is composed of a hexagonal sensing diaphragm made up of a piezoelectric stack of ScAlN piezoelectric thin-film sandwiched by Mo and highly doped single-crystal silicon, with a top and a bottom oxide film surrounding the piezoelectric stack. The hydrophone sensor can operate in not only omnidirectional receive mode, but also dipole receive mode, depending on the signal processing algorithm used for the four array groups. The vector hydrophone operates in the frequency range of 20 kHz to 160 kHz, which is specially designed to monitor the dolphin calls. The measured maximum receive sensitivity is -175 dB (re: 1 V/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Pa) in the dipole receive mode at 160 kHz and −169±2 dB (re: 1 V/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Pa) in the omnidirectional receive mode in the operation bandwidth, respectively. The reported vector hydrophone also presents a smooth directivity pattern at 100 kHz. The experiments show the reported MEMS vector hydrophone has good potential in conservation of dolphins and other underwater detection at high frequency between 20 kHz to 160 kHz.

1D quasi-bound states in the continuum with large operation bandwidth in the ω∼k space for nonlinear optical applications

The phenomenon of bound state in the continuum (BIC) with an infinite quality factor and lifetime has emerged in recent years in photonics as a new tool to manipulate light–matter interactions. However, most of the investigated structures only support BIC resonances at very few discrete points in the ω ∼ k space. Even when the BIC is switched to a quasi-BIC (QBIC) resonance through perturbation, its frequency will still be located within a narrow spectral band close to that of the original BIC, restricting their applications in many fields where random or multiple input frequencies beyond the narrow band are required. In this work, we demonstrate that a new set of QBIC resonances can be supported by using a special binary grating consisting of two alternatingly aligned ridge arrays with the same period and zero-approaching ridge width difference on a slab waveguide. These QBIC resonances are distributed continuously over a broad band along a line in the ω ∼ k space and can thus be considered as 1D QBICs. With the Q factors generally affected by the ridge difference, it is now possible to arbitrarily choose any frequencies on the dispersion line to achieve significantly enhanced light–matter interactions, facilitating many applications where multiple input wavelengths are required; e.g., sum or difference frequency generations in nonlinear optics.

Ultra-broadband nanostructured metamaterial absorber based on stacked square-layers of TiN/TiO2

Metamaterial-based nano-scale absorbers have been becoming very popular in the modern era due to efficiently absorbing solar radiation to revamp the performance characteristics of thermal emitters and solar thermophotovoltaics (STPV) systems. Here, we explore and implement an ultra-broadband nanostructured metamaterial absorber (NMMA), which comprises a stack of alternating nano-squares of TiN and TiO2 mounted over the dielectric substrate backed by a metallic sheet. The numerical simulations and electromagnetic (EM) characteristics of the proposed NMMA have been investigated by employing the finite difference time domain (FDTD) EM tool. The numerical results indicate that the average absorption of the NMMA reaches 96% in the wavelength range from 200-3000 nm (from ultraviolet to mid-infrared), and the minimal absorption is also above 90% in a continuous large operating spectrum ranging from 200-2800 nm. Surprisingly, the absorption features of the designed nano-absorber remain stable under the influence of oblique incident-angles for both the polarization states (TE &amp; TM). Furthermore, the proposed nano-absorber manifests polarization-insensitive behavior due to the presence of four-fold symmetry of the proposed structure. Large operational bandwidth, miniaturized structure, and the use of thermally stable refractory metal TiN make this NMMA an appealing candidate for the applications of thermal emission, solar thermophotovoltaics, and other opto-electronic devices. In addition, the design of this absorber is also scalable to other operating spectrums through carefully selecting the materials and optimizing the geometry of the proposed structure.

Investigation of signal-to-noise ratio performance of microwave photonic links enhanced by optical injection locking and channelized spectrum stitching.

A signal-to-noise ratio (SNR) improvement method for microwave photonic (MWP) links enhanced by optical injection locking (OIL) and channelized spectrum stitching (CSS) is investigated and experimentally demonstrated. By exploiting the resonant amplification characteristics of OIL, both optical gain and in-band noise suppression of the input radio frequency signal can be achieved. The injection bandwidth is channelized to further suppress noise during OIL, and the input signal can be well reconstructed by spectrum stitching in the digital domain. Experimental results show that the optimal improvement in SNR of 3.6 dB is achieved for linear frequency modulated signals and at least an additional improvement of 7.2 dB can be obtained by adopting CSS. Other broadband signals for radar and communication are used to further verify the ability to improve SNR. The potential for application scenarios with large operating bandwidth and high optical gain is also demonstrated.

Microwave Photonic MIMO Radar for High-Resolution Imaging

A microwave photonic multiple-input and multiple-output (MIMO) radar is proposed and demonstrated to implement high-resolution imaging. In the proposed system, multiple orthogonal linearly frequency modulated (LFM) signals are generated by heterodyning between two optical frequency combs, which enables a MIMO transmitting array with a simple and reconfigurable structure. The receiving array uses photonic frequency mixing to implement multiple channel separation and de-chirp processing simultaneously. This microwave photonic MIMO radar can have a large operation bandwidth and a large equivalent aperture, which helps to achieve high-resolution imaging in both range and azimuth directions. In the experiment, a microwave photonic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{4}\times \text{8}$</tex-math></inline-formula> MIMO radar is established with a 2-GHz bandwidth in each channel. Based on this MIMO radar, high-resolution back-projection (BP) imaging with a theoretical range resolution of 7.5 cm and azimuth resolution of 1.85° is demonstrated. The experimental results can verify the feasibility of the proposed MIMO radar, which is a good solution to high-resolution radar imaging by combining microwave photonic and MIMO technologies.