Wideband Sources Research Articles

On-Chip Mid-Infrared Supercontinuum Generation from 3 to 13 μm Wavelength.

Midinfrared spectroscopy is a universal way to identify chemical and biological substances. Indeed, when interacting with a light beam, most molecules are responsible for absorption at specific wavelengths in the mid-IR spectrum, allowing to detect and quantify small traces of substances. On-chip broadband light sources in the mid-infrared are thus of significant interest for compact sensing devices. In that regard, supercontinuum generation offers a mean to efficiently perform coherent light conversion over an ultrawide spectral range, in a single and compact device. This work reports the experimental demonstration of on-chip two-octave supercontinuum generation in the mid-infrared wavelength, ranging from 3 to 13 μm (that is larger than 2500 cm–1) and covering almost the full transparency window of germanium. Such an ultrawide spectrum is achieved thanks to the unique features of Ge-rich graded SiGe waveguides, which allow second-order dispersion tailoring and low propagation losses over a wide wavelength range. The influence of the pump wavelength and power on the supercontinuum spectra has been studied. A good agreement between the numerical simulations and the experimental results is reported. Furthermore, a very high coherence is predicted in the entire spectrum. These results pave the way for wideband, coherent, and compact mid-infrared light sources by using a single device and compatible with large-scale fabrication processes.

Sparse Array Beamforming Design for Wideband Signal Models

We develop sparse array receive beamformer design methods achieving maximum signal-to-interference plus noise ratio (MaxSINR) for wideband sources and jammers. Both tapped delay line (TDL) filtering and the DFT realizations to wideband array processing are considered. The array sparsity stems from the limited number of available RF transmission chains that switch between the sensors, thereby configuring different arrays at different times. The sparse array configuration design problem is formulated as a quadratically constraint quadratic program (QCQP) and solved by using SDR (semidefinite relaxation). A computationally viable approach through SCA (successive convex relaxation) is also pursued. In order to realize an implementable design, in presence of missing autocorrelation lags, we propose parameter-free block Toeplitz matrix completion to estimate the received data correlation matrix across the entire array aperture. It is shown that the optimum wideband sparse array effectively utilizes the array aperture and provides considerable performance improvement over suboptimal array topologies.

Low‐profile and wideband gain enhanced Fabry–Perot cavity antenna using gradient PRS and AMC

Low-profile and wideband gain enhancement Fabry–Perot cavity antennas (FPCA) is proposed using gradient Partially Reflective Surface (PRS) and gradient Artificial Magnetic Conductor (AMC). A wideband source antenna is designed with a −10 dB impedance bandwidth of 58.1% (6.1–11.1 GHz). The PRS is constructed by a complementary frequency-Selective Surfaces (FSS) with a square hole and four small patches in a unit, which realizes phase increasing with frequency. The PRS and the AMC are gradient alone x-direction to achieve wideband phase compensation. While they are uniform in y-direction to realize better gain enhancement. Compared with the source antenna, the gain enhancement of the proposed FPCA is about 4–7 dBi. Moreover, the simulated and measured 3-dB gain bandwidths are 7.26–10.33 GHz (34.8%) and 6.8–10.8 GHz (46%), respectively. The gain enhanced bandwidth is about 52.3% (6.5–11.1 GHz). The impedance bandwidth is 6–11.1 GHz (59.65%). By using the gradient size AMC, the profile of the FPCA is reduced from 0.5λ0 to 0.2λ0, where λ0 is the wavelength of 8.5 GHz. The proposed FPCA has the characteristics of wide impedance bandwidth, wide 3-dB gain bandwidth, wide gain enhanced bandwidth and low-profile.

Adaptive DOA estimation with low complexity for wideband signals of massive MIMO systems

For massive multiple-input multiple-output (MIMO) communication systems, it is a major challenge to develop direction-of-arrival (DOA) estimation algorithm with low complexity and high accuracy for wideband signals. We propose a low-complexity adaptive multistage Wiener filter (MSWF)-based two-sided correlation transformation (AM-TCT) algorithm for wideband DOA estimation of two-dimensional (2D) massive MIMO systems. Unlike conventional TCT algorithms, the proposed AM-TCT algorithm uses the signal subspace that is obtained by forward recursion of the MSWF to construct the focusing matrix, which can reduce the computational complexity. To further reduce the computational complexity, the eigenvalue decomposition (EVD) of the covariance matrix is also replaced by the MSWF. Furthermore, to improve the precision, the Minimum Description Length (MDL) criterion uses the stage of MSWF to adaptively select the appropriate dimension of the noise subspace, and the backward recursion of the MSWF is employed to accurately estimate the noise subspace. Theoretical analysis demonstrates the complexity superiority of the proposed AM-TCT algorithm. Simulation results indicate that the proposed AM-TCT algorithm can effectively estimate the angle of wideband sources in massive MIMO systems and outperform some existing methods.

Precision Millimeter-Wave-Modulated Wideband Source at 92.4 GHz as a Step Toward an Over-the-Air Reference.

As the next-generation communications technology continues to evolve to utilize millimeter-wave frequencies, calibration methods are needed for the nonidealities related to these frequencies in communications electronics. In this article, we demonstrate a 1-GHz bandwidth, 64-quadrature-amplitude-modulated signal source at 92.4-GHz carrier frequency with relative phase and magnitude that may be made traceable to primary standards. By using predistortion techniques, we are able to repeatably obtain a nominal error vector magnitude (EVM) of 1.4%. In addition, we track correlated and uncorrelated uncertainties using a Monte Carlo method to show the distribution of uncertainty of the EVM measurement with the 5th and 95th percentiles at 1.5% and 3.1%, respectively. We examine the dependence of the EVM in the traceable source on digital-to-analog converters' imbalance and system drift over time. Finally, we use the stable, low-EVM signal to ascertain EVM degradation on- and off-axis in a stationary over-the-air setup.

Wideband modal orthogonality: A New approach for broadband DOA estimation

Wideband direction of arrival (DOA) estimation techniques for sensors array have been studied extensively in the literature. Nevertheless, needing prior information on the number and directions of sources or demanding heavy computational load makes most of these techniques less useful in practice. In this paper, a low complexity subspace-based framework for DOA estimation of broadband signals, named as wideband modal orthogonality (WIMO), is proposed and accordingly two DOA estimators are developed. First, a closed-form approximation of spatial-temporal covariance matrix (STCM) in the uniform spectrum case is presented. The eigenvectors of STCM associated with non-zero eigenvalues are modal components of the wideband source in a given bandwidth and direction. WIMO idea is to extract these eigenvectors at desired DOAs from the approximated STCM and test their orthogonality to estimated noise subspace. In the non-uniform spectrum case, WIMO idea can be applied by approximating STCM through numerical integration. Fortunately, STCM approximation and modal extraction can be performed offline. WIMO provides DOA estimation without the conventional prerequisites, such as spectral decomposition, focusing procedure and, a priori information on the number of sources and their DOAs. Several numerical examples are conducted to compare the WIMO performance with the state-of-the-art methods. Simulations demonstrate that the two proposed DOA estimators achieve superior performance in terms of probability of resolution and estimation error along with orders of magnitude runtime speedup.

Micro-Electro-Mechanical System Fourier Transform Infrared (MEMS FT-IR) Spectrometer Under Modulated-Pulsed Light Source Excitation.

Miniaturized Fourier transform infrared (FT-IR) spectrometers suffer from limited optical throughput due to their tiny aperture size. Therefore, coherent wideband sources with high brightness can provide an advantage over the wideband thermal radiation sources. However, the former ones are available based on pulsed operation. In this work, we present and study a miniaturized FT-IR spectrometer with pulsed light sources including chopped thermal source, semiconductor optical amplifier, Q-switched and femtosecond mode-locked laser sources. A system model for the FT-IR spectrometer system under a modulated input light source is presented. The model accounts for the relatively high scanning speed of the micro-electro-mechanical system (MEMS) interferometer. The signal-to-noise ratio of the spectrometer, due to the light source modulation, is calculated at different values of modulation repetition rate ranging from 20 Hz to 2 MHz, and duty cycle values ranging from 1% to 50%. An analytical expression for the worst-case repetition rate for the spectrometer system is derived. The model results are verified by experimental measurements showing good agreement with the theoretical expectations. Spectroscopic measurements for CO2 gas with pressure ranging from 300 mbar to 700 mbar are also performed using a high-repetition rate source, and the measured spectra agree with the simulation results demonstrating the utility of the spectrometer.

Investigation of Double Tunnel Sine Waveguide Slow-Wave Structure for Terahertz Dual-Beam TWT

Traveling wave tube (TWT) is an attractive solution for wideband and high-power sources or amplifiers in the terahertz (THz) band. In this article, a novel slow-wave structure (SWS) for TWT, named double tunnel sine waveguide (DTSW), which is based on sine waveguide (SW), is proposed. The following key technologies are adopted: 1) the rectangular holes, at which the operating mode is cutoff, are alternately opened on the sine-shaped ridges to form two electron beam tunnels and 2) the lower surface of rectangular holes is flush with the top of sine-shaped ridges. Compared with the SW-SWS, DTSW-SWS has the wider operating band and higher interaction impedance under the condition that the phase velocity of the two SWSs is at the same level. In addition, DTSW-SWS is simple in structure and easy to fabricate. The particle-in-cell simulation results reveal that DTSW-TWT has an output power of more than 202 W in the frequency range of 202–255 GHz with the maximum power of ~405 W and electron efficiency of ~9.8% at 215 GHz, under the operating voltage of 23 kV and beam current of 180 mA. Therefore, the DTSW should be considered as a promising SWS for the high-power and wideband THz traveling wave amplifier.

Multi-spectral interference imaging using laser-induced plasma light source

Interference imaging techniques are widely used for studying surface and internal structure of various objects. By varying the operating wavelength of the interference system, one can also obtain the information about the spectral properties of the inspected specimen. For multi-spectral interference imaging applications, wide-band high power light sources are necessary. Most of the existing sources suffer from speckle noise, low brightness or high price. In this paper, we demonstrate the applicability of a laser-induced plasma light source for interference imaging and quantitative phase measurements. Narrow-band spectral filtration of its illumination within a wavelength range 240-2600 nm allows to design cost-effective schemes for a variety of metrological applications including digital holography and spectral-domain optical coherence tomography.

A Milliwatt-Level 70–110 GHz Frequency Quadrupler With >30 dBc Harmonic Rejection

This article presents a two-channel 70-110-GHz frequency quadrupler in the GF8HP 0.12-μm SiGe BiCMOS process. The ×4 frequency multiplication is based on cascaded frequency doublers with an interstage bandpass filter to reject undesired harmonics. The measured peak output power is -1.5 to 3 dBm at 70-110 GHz, with a worst case harmonic rejection ratio (HRR) of >30 dBc. The chip size is 1.9 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and consumes 240 mW for the two-channel operation. To the best of our knowledge, this article represents state-of-the-art in terms of bandwidth, output power, and excellent HRR. Application areas include mm-wave multi-channel, mm-wave network analyzers, and wideband sources for radar and imaging systems and reconfigurable E-band radios.

DOA estimation of wideband sources by sparse recovery based on uniform circle array

The conventional direction of arrival (DOA) estimation by compressed sensing is based on wideband focusing and sparse recovery in discrete domain. The former needs high signal to noise ratio (SNR), and the latter requires the space division, or their performance will degrade seriously. Therefore, a new algorithm of wideband DOA calculation in frequency domain based on uniform circle array (UCA) is proposed, the optimisation problems are respectively disposed at each frequency, then the sparse support set is acquired by the corresponding semidefinite programming, after that DOA is calculated through fusing the data of every frequency, simultaneously the original sources can also be reconstructed. This algorithm averts the grid partition in discrete domain, and it also performs well in condition of small samples and low SNR.

DOA estimation of wideband sources by sparse recovery based on uniform circle array

DOA estimation of wideband sources by sparse recovery based on uniform circle array

Higher-Order-Statistics-Based Direction-of-Arrival Estimation of Multiple Wideband Sources With Single Acoustic Vector Sensor

Acoustic vector sensor (AVS) measures acoustic pressure as well as the acoustic particle velocity to estimate the acoustic intensity. The acoustic intensity, which is a vector quantity, represents the magnitude and direction of the active or propagating part of an acoustic field, thus indicating the direction-of-arrival (DOA) of the received signal. This article proposes higher order statistics (HOS)-based DOA estimation with a single acoustic vector sensor in the underdetermined case where the number of sources is more than the number of sensors constituting a single vector sensor. The use of HOS allows the increase in the number of degrees of freedom, thereby resulting in increase in uniquely identifiable sources. It has been established that in an ideal condition, a single AVS using fourth-order statistics is capable of uniquely identifying eight sources with not more than four sources in a single plane. The number of sources can be further increased by working with HOS. Additional advantages of the proposed algorithm, such as robustness against the presence of nonidentical Gaussian noise or spatially correlated Gaussian noise at each constituent sensor of a single AVS, improved performance over second-order statistics-based algorithm, and ability to identify multiple wideband sources, have also been demonstrated.

An Efficient Framework for Estimating the Direction of Multiple Sound Sources Using Higher-Order Generalized Singular Value Decomposition.

This paper presents an efficient framework for estimating the direction-of-arrival (DOA) of wideband sound sources. The proposed framework provides an efficient way to construct a wideband cross-correlation matrix from multiple narrowband cross-correlation matrices for all frequency bins. In addition, the proposed framework is inspired by the coherent signal subspace technique with further improvement of linear transformation procedure, and the new procedure no longer requires any process of DOA preliminary estimation by exploiting unique cross-correlation matrices between the received signal and itself on distinct frequencies, along with the higher-order generalized singular value decomposition of the array of this unique matrix. Wideband DOAs are estimated by employing any subspace-based technique for estimating narrowband DOAs, but using the proposed wideband correlation instead of the narrowband correlation matrix. It implies that the proposed framework enables cutting-edge studies in the recent narrowband subspace methods to estimate DOAs of the wideband sources directly, which result in reducing computational complexity and facilitating the estimation algorithm. Practical examples are presented to showcase its applicability and effectiveness, and the results show that the performance of fusion methods perform better than others over a range of signal-to-noise ratios with just a few sensors, which make it suitable for practical use.

Through-the-Multilayered Wall Imaging Using Passive Synthetic Aperture Radar

Most of the existing through-the-wall imaging (TWI) methods using synthetic aperture radar (SAR) tend to apply an active system. In this paper, a novel, passive SAR (PSAR), termed TWI-PSAR, is proposed, to focus the image of multi targets behind a single-/multilayered wall. TWI-PSAR would work in a bistatic configuration using wideband sources of opportunity and a single moving platform or a stationary linear array receiver. Incident angle and frequency are considered the parameters that influence TWI image directly. A stepped frequency transmitter with single incident angle is applied to investigate the incident angle effect. It could show the capability of small angle to suppress wall effects. Zero incident angle PSAR (Z-PSAR) is exploited in TWI for enhanced target identification and feature extraction as well as wall effect mitigation. In scenarios where background measurement might not be available or wall parameters are unknown for compensation, Z-PSAR could be adopted. Compared to other conventional imaging methods such as SAR and time reversal, numerical results show the superiority of the proposed TWI system in urgent situations with unknown wall parameters, employing free-space Green’s function. Moreover, to demonstrate the effectiveness of the proposed PSAR method in a real situation, sources of opportunity that are relatively wideband and aligned in several directions, such as analog TV, Digital Video Broadcasting—Terrestrial, GSM, and WiMAX, are used to image targets behind the wall. Also, Monte Carlo method is used to show the effectiveness of TWI-PSAR in different frequency and incident angle scenarios.

Wavelength-tunable LP11 mode pulse fiber laser based on black phosphorus

A wavelength-tunable LP11 mode Q-switched Yb-doped fiber laser was demonstrated based on black phosphorus. To the best of our knowledge, it’s the first time to achieve wavelength-tunable higher-order mode pulse laser output in the 1 μm region. The output central wavelength of this self-starting pulse laser can be tuned from 1054.4 nm to 1068.2 nm continuously in the LP11 mode state. The linewidths of different wavelengths were less than 0.12 nm. During the long-term operation, the central wavelength and bandwidth had no obvious change and the fluctuation of output power was less than 0.67%. This wavelength-tunable higher-order mode pulse laser showed excellent stability, and could be employed as a wideband source for optical and photonic applications.

Direction-of-Arrival Estimation of Circular and Noncircular Wideband Source Signals via Augmented Envelope Alignment

In practical applications of array system, the incident signals may come from either circular or noncircular sources, or both. The problem of direction-of-arrival (DOA) estimation is herein addressed for both circular and noncircular wideband source signals. Based on the concept of effective rank, the aligned augmented effective noise subspace, spanned by the subordinate eigenvectors of the aligned array output augmented covariance matrix, is constructed for each scanning angle. It is shown that the aligned augmented effective noise subspace corresponding to a true DOA of signal is: 1) approximately orthogonal to a frequency-invariant one-dimensional subspace spanned by the aligned augmented transfer vector of the signal in the presence of multiple strictly noncircular sources; and 2) approximately orthogonal to a frequency-invariant two-dimensional subspace spanned by the aligned augmented transfer matrix of the signal in the presence of multiple circular or partially noncircular sources. This enables the noncircularity exploited wideband DOA estimation based on rank-reduction and norm-nulling. The performance of the proposed method has been evaluated and compared with some existing popular wideband DOA estimators by extensive simulations.

Single stage DOA-frequency representation of the array data with source reconstruction capability

In this paper, a new signal processing framework is proposed, in which the array time samples are represented in DOA-frequency domain through a single stage problem. It is shown that concatenated array data is well represented in a G dictionary atoms space, where columns of G correspond to pixels in the DOA-frequency image. We present two approaches for the G formation and compare the benefits and disadvantages of them. A mutual coherence guaranteed G construction technique is also proposed. Furthermore, unlike most of the existing methods, the proposed problem is reversible into the time domain, therefore, source recovery from the resulted DOA-frequency image is possible. The proposed representation in DOA-frequency domain can be simply transformed into a group sparse problem, in the case of non-multitone sources in a given bandwidth. Therefore, it can also be utilized as an effective wideband DOA estimator. In the simulation part, two scenarios of multitone sources with unknown frequency and DOA locations and non-multitone wideband sources with assumed frequency region are examined. In multitone scenario, sparse solvers yield more accurate DOA-frequency representation compared to some noncoherent approaches. For non-multitone wideband source scenario, the proposed method with group sparse solver outperforms some existing wideband DOA estimators in low SNR regime. In addition, simultaneous sources’ recovery and DOA estimation shows significant improvement compared to the conventional delay and sum beamformer and without prerequisites required in sophisticated wideband beamformers.

Comparative study of core materials and multi-degree-of-freedom sandwich piezoelectric energy harvester with inner cantilevered beams

Conventional energy harvesters using a single metal substrate can hardly collect sufficient energies from wideband and low frequency vibration sources. A sandwich structure composed of two thin face sheets and a soft-core material can be an alternative substrate to replace a single metal layer to drop the resonant frequency of the harvester accompanied by the increment of the output voltage. The resonant frequency of the sandwich piezoelectric energy harvester (SPEH) can be adjusted more flexibly by selecting the appropriate core material and varying the geometric dimensions of the sandwich substrate. In this paper, a comparative study of different core materials for SPEH is firstly investigated. Prototypes of the SPEHs with different core materials (natural rubber, silicon rubber, polycarbonate plastic) and conventional harvesters with the same geometrical dimension are fabricated and tested. The experiment displays that the plastic sandwich energy harvester is the best of all in terms of voltage output. To extend the application of SPEH to harvester broadband vibration energies, this paper presents a novel multi-degree-of-freedom sandwich energy harvester with inner cantilevered beams to harvester energy from wideband, low frequency and low amplitude vibration sources. The proposed energy harvester comprises of a partitioned main sandwich substrate with a tip mass and two patches of piezoelectric layer bonded on it. Multiple branches with tip masses are extended from the inner substrate. The finite element model of the proposed sandwich harvester is presented to achieve close resonant frequencies in the low frequency range of interest. Then a multi-degree-of-freedom sandwich harvester with inner cantilevered beams is designed, fabricated and tested under. The novel harvester proposed in this paper has high design flexibility to work in various vibration environments.

Multi-branch sandwich piezoelectric energy harvester: mathematical modeling and validation

Energy harvesting from ambient environment has attracted intensive attention over the years for powering low-power autonomous electronic devices. A conventional piezoelectric energy harvester (CPEH) can hardly meet the requirements of effective energy harvesting from wideband, low frequency and low amplitude vibration sources due to its simple substrate and single resonant peak response. Using sandwich structure comprised of a soft-core layer and two thin skins can drastically reduce the natural frequency and increase voltage output of the harvester. The stiffness of sandwich harvester and consequently its resonant frequency can be tuned more flexibly than the CPEH by choosing different materials and adjusting the geometric dimensions of the core and skins. In this paper, a novel multi-branch sandwich piezoelectric energy harvester (MSPEH) is proposed to harvest energy from wideband, low frequency and low amplitude vibration sources. The proposed harvester comprises of a main sandwich beam as substrate with a patch of piezoelectric layer bonded over it. Multiple inner single branches with tip masses are connected to the main substrate to generate multiple resonant peaks and tune the range of the interested frequency. Firstly, a mathematical model of the proposed harvester is presented, and the electromechanical coupling equations are obtained. Subsequently, a prototype of MSPEH with two inner single branches is fabricated and tested under harmonic base excitation to study its performance. When tested at a low harmonic excitation of 0.02 g, the harvester generates 2.48 V, 6.21 V and 1.55 V at three resonant frequencies of 18.18 Hz, 24.74 Hz and 28.12 Hz, respectively. Finally, the accuracy of derived mathematical model is verified by comparing with the predictions from finite element simulation and experimental results. The novel MSPEH offers excellent design flexibility to adjust resonant frequencies by choosing inner single branches and tip masses and thus it has potential to generate sufficient power output from wideband, low frequency and low amplitude vibration sources.