Free-space Propagation Research Articles

Ptychographical iterative engine based on pixel binning for improving the computational efficiency

Ptychographical iterative engine (PIE) is an attractive modality of phase retrieval that can provide the quantitative phase of the sample and extend the field of view. For a large sample, a great deal of images with a large dataset are recorded, which will result in high requirements for the computing power and increase the calculation burden. Here, we propose a pixel binning strategy to improve the computational efficiency and reduce the calculation time of PIE. In this method, the recorded image chosen as the amplitude constraint in reconstruction algorithm is compressed by merging the amplitude values of the adjacent positions into one value by the linear superposition, and the compressed pattern is set as a new amplitude constraint to reconstruct the compressed object and probe with fewer pixel number and larger grid size compared to the uncompressed results. Then, the compressed values are extended by the interpolation method. The improvement in the computational efficiency at the point lies in the fact that it takes less time to do the free-space diffraction propagation calculation for images with fewer pixel number. Experiments demonstrate that the proposed method behaves good performance with high computational efficiency. The proposed approach would be helpful for large-scale imaging with high computational efficiency.

Pulse fluence noise dynamics at free-space propagation.

The dynamics of fluence noise of an optical pulse at free-space (e.g.,vacuum) propagation has been studied. It has been shown that the fluence noise with high spatial frequency is effectively cleaned out from the primary smooth pulse either by spatial walk-off or by temporal delay at a relatively small propagation distance. This effect can be referred to as spatial and temporal self-filtering and is of major interest in ultra-high-power and ultra-short-pulse applications. The study comprises a rigorous theory and a few relevant numerical simulation examples.

Planar Sensor for Noise Cancellation During Partial Discharge Detection in Open Substation

In compliance with next-generation high-voltage substations, ultra-high frequency (UHF) planar sensors are becoming more popular than ever for their online features in partial discharge (PD) diagnostics. However, PD signals mixing with different noises still exists as a major challenge for UHF sensors. Especially in open substations, telecommunication interferences become predominant noise sources to an external UHF sensor placed outside of the equipment chamber. Despite band-stop filters are additionally connected to UHF sensors for de-noising, the inherent noise cancellation mechanism is not yet found in any UHF sensor. With the best of our knowledge and survey, we are the first to introduce here a planar sensor that inherently eliminates telecommunication interferences during PD detection for open substations even without using any post-processing filters. To design our sensor, we have applied techniques of truncated partial ground plane for wideband characteristics and modified radiating patch for enhanced gain performance. A novel intermingled coupling resonator is designed to remove the GSM-900 band from within the sensor bandwidth. We have tested our prototype both for the free-space propagation and PD detection performances. The prototype has a compact physical size of 16×8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and an operational bandwidth within 792-1414 MHz. For a surface discharge source, the prototype successfully detects the PD signals at the earliest stage. Our prototype minimizes the ambient noise-floor to 2.5 mV and improves the signal-to-noise ratio by more than 27.9 dB which is relatively higher than conventional UHF sensors. The proposed sensor also has potentials of classifying PD sources.

Improving performance prediction of diode end-pumped solid-state Nd:YAG rod amplifiers by incorporating pump mode evolution.

Master Oscillator Power Amplifier (MOPA) systems find extensive use in laser development to increase the optical power of laser emissions from a Master Oscillator (MO). Commonly used are the cylindrical rod MOPAs that are optically excited using a multimode fiber-coupled (FC) diode laser emission in an end-pumped configuration. Current analytical 3D models that incorporate thermal effects, gain saturation, and iterative Fourier beam propagation methods, collectively, rely on static approximations of the evolution of the FC pump beam profile over the longitudinal volume of the amplifier crystal. Furthermore, in general, the spectral behavior of the FC diode emission is assumed to be static, and the thermal wavelength shift is not accounted for in the simulation. In this work, we demonstrate a novel approach for accurate modeling of the multimode FC pump beam emission as a complex field using a phase-only Gaussian to flat-top (FT) diffractive optical element, thus allowing for the inclusion of the pump beam into the iterative propagation method. Additionally, we present a method for precise calibration of the model using simple experimental measurements of the diode emission spectrum. The theoretical model is experimentally validated using an end-pumped Nd:YAG crystal rod to perform single-pass amplification of a Gaussian beam, showing excellent agreement with predicted output powers over the calibrated range of pump powers. Furthermore, we provide experimental data that exhibits a strong correlation between the Gaussian to FT phase-only transformation and the multimode FC diode evolution in free-space propagation.

Radio Map Assisted Multi-UAV Target Searching

When acting as dynamic receivers, unmanned aerial vehicles (UAVs) have several advantages over the conventional static base stations for received signal strength (RSS) based target searching. It is highly likely that the RSS is becoming stronger as the UAV is moving towards the estimated coordinate, and this leads to a higher estimation precision for each localization update. On the other hand, despite its low cost, low energy consumption and simple hardware requirement, RSS can be severely influenced by land layouts, therefore the assumption of a simple free space propagation model may lead to poor estimation precision and even failure, especially for an area where the topography is highly complex. In this paper, we propose multi-UAV target searching with the assistance of radio maps, which contain abundant information of the topography influence on RSS. We generate radio maps from real topographic data, and derive the minimum mean square error estimator for multi-UAV target searching with the radio map knowledge for memoryless observations. Simulations for areas with complex topography, show that the localization precision and searching efficiency can be considerably improved with the assistance of radio maps.

Scan-free time-of-flight-based three-dimensional imaging through a scattering layer.

Reconstructing an object's three-dimensional shape behind a scattering layer with a single exposure is of great significance in real-life applications. However, due to the little information captured by a single exposure while strongly perturbed by the scattering layer and encoded by free-space propagation, existing methods cannot achieve scan-free three-dimensional reconstruction through the scattering layer in macroscopic scenarios using a short acquisition time of seconds. In this paper, we proposed a scan-free time-of-flight-based three-dimensional reconstruction method based on explicitly modeling and inverting the time-of-flight-based scattering light propagation in a non-confocal imaging system. The non-confocal time-of-flight-based scattering imaging model is developed to map the three-dimensional object shape information to the time-resolved measurements, by encoding the three-dimensional object shape into the free-space propagation result and then convolving with the scattering blur kernel derived from the diffusion equation. To solve the inverse problem, a three-dimensional shape reconstruction algorithm consisting of the deconvolution and diffractive wave propagation is developed to invert the effects caused by the scattering diffusion and the free-space propagation, which reshapes the temporal and spatial distribution of scattered signal photons and recovers the object shape information. Experiments on a real scattering imaging system are conducted to demonstrate the effectiveness of the proposed method. The single exposure used in the experiment only takes 3.5s, which is more than 200 times faster than confocal scanning methods. Experimental results show that the proposed method outperforms existing methods in terms of three-dimensional reconstruction accuracy and imaging limit subjectively and objectively. Even though the signal photons captured by a single exposure are too highly scattered and attenuated to present any valid information in time gating, the proposed method can reconstruct three-dimensional objects located behind the scattering layer of 9.6 transport mean free paths (TMFPs), corresponding to the round-trip scattering length of 19.2 TMFPs.

Long-range frequency-modulated continuous-wave LiDAR employing wavelength-swept optical frequency comb

We propose a FMCW LiDAR using a wavelength-swept optical frequency comb, which overcomes the ranging limitation caused by the receiver bandwidth. We have established the fundamental part of our proposal and conducted proof-of-concept ranging experiments. With optical fiber transmission for emulating long distance free-space propagation, we have succeeded in ranging corresponding to 1605-m free-space propagation with a 10-MHz FMCW receiver.

All-optical plasma modulation by the plasma channel induced in the molybdenum disulfide dispersion solution

The molybdenum disulfide (MoS2) nanosheet is successfully synthesized using a hydrothermal Li-intercalation exfoliation method. We experimentally show that the MoS2 dispersion solution can be used for optical modulation and all-optical switching based on the plasma channel induced by control pulses. The modulated signal light has a good transmission property in free space. The spatial modulation of the signal laser can be easily adjusted through the power of control lasers and the suspension concentration. The influence of suspension concentration and type on the free-space propagation of modulated signal light is also investigated. Results reveal a potential application of MoS2 as an all-optical modulator for diversified photoelectric applications.

Estimation of Probabilistic Characteristics of Reception of Frequency-Effective Signals during Propagation along a Radio Line with Fog

A model of a radio line with fog, which is used to represent distortions of complex envelopes of digital signals because of the absorbing and dispersive properties of the propagation medium, has been considered. It is shown that with an increase in the frequency band of digital signals and with an increase in their order of manipulation (when using frequency-efficient signals), the influence of these distortions leads to energy losses in relation to propagation in free space. The quantitative estimation of energy losses for the digital signals used in applications with multi-position phase, quadrature-amplitude, and amplitude-phase manipulations and for a radio line with fog with a variation of its parameters has been carried out. It has been shown that for signals with multiposition phase and amplitude-phase manipulations (the frequency efficiency coefficient of the signals is 4 bits/s/Hz), the energy losses reach 3.5 and 1.0 dB, respectively.

Performance analysis for a suitable propagation model in outdoor with 2.5 GHz band

As demand for mobile wireless network services continues to rise, network planning and optimization significantly affect development. One of the critical elements in network planning is predicting pathloss. Thus, propagation models predict pathloss in indoor and outdoor environments. Choosing the appropriate propagation model for the area out of existing models is essential for network planning. Selected propagation models suitable with 2.5GHz, such as Friis Free Space Propagation Model (FSPL), Sandford University Interim (SUI), Ericsson, Okumura, and COST-231 HATA models, are utilized for evaluation and compared with empirical data collected from long-term evolution (LTE) networks in urban areas. The best acceptable model is chosen based on statistical results such as mean, standard deviation, and root mean square errors (RMSE). The analytical results show Cost-231 Hata model fits the empirical pathloss with a minimum RMSE of 5.27 dB.

Near-Field Channel Estimation in Mixed LoS/NLoS Environments for Extremely Large-Scale MIMO Systems

Accurate channel model and channel estimation are essential to empower extremely large-scale MIMO (XL-MIMO) in 6G networks with ultra-high spectral efficiency. With the sharp increase in the antenna array aperture of the XL-MIMO scenario, the electromagnetic propagation field will change from far-field to near-field. Unfortunately, due to the near-field effect, most of the existing XL-MIMO channel models fail to describe mixed line-of-sight (LoS) and non-line-of-sight (NLoS) path components simultaneously. In this paper, a mixed LoS/NLoS near-field XL-MIMO channel model is proposed to match the practical near-field XL-MIMO scenario, where the LoS path component is modeled by the geometric free space propagation assumption while NLoS path components are modeled by the near-field array response vectors. Then, to define the range of near-field for XL-MIMO, the MIMO Rayleigh distance (MIMO-RD) and MIMO advanced RD (MIMO-ARD) is derived. Next, a two stage channel estimation algorithm is proposed, where the LoS path component and NLoS path components are estimated separately. Moreover, the Cramér-Rao lower bound (CRLB) of the proposed algorithm is derived in this paper. Numerical simulation results demonstrate that, the proposed two stage scheme is able to outperform the existing methods in both the theoretical channel model and the QuaDRiGa channel emulation platform.

Wireless Mesh Networking Tests and Evaluation in the Karst Natural Caves of Southwest China

Currently, the exploration and development of the Karst natural caves (KNCs) have become hot research topics, and flexible and reliable communication in the KNCs is very important. However, because of the inherent concavity and irregular limestone formations in the KNC, wireless signal propagation is sparse and unpredictable. In this paper, we explore the application of wireless mesh networking (WMN) in the KNC. First, by measuring the relative permittivity of the KNC using the transmission line method at 530 MHz, 670 MHz and 1.4 GHz, we segment the channel characteristics using classical free space propagation modeling, lossy waveguide modeling and bend modeling based on the geographical features of the KNC for reducing the complexity of channel modeling. The measured results are in good agreement with the segment model calculation. Second, we test and evaluate the network performance at 530 MHz, including multihop routings and multidata flows in static scenarios and mobile scenarios. The test results show that audio and video can be transmitted clearly below three hops, it is feasible to deploy WMN in an irregular KNC.

Double and Square Bessel-Gaussian Beams.

We obtain a transform that relates the standard Bessel-Gaussian (BG) beams with BG beams described by the Bessel function of a half-integer order and quadratic radial dependence in the argument. We also study square vortex BG beams, described by the square of the Bessel function, and the products of two vortex BG beams (double-BG beams), described by a product of two different integer-order Bessel functions. To describe the propagation of these beams in free space, we derive expressions as series of products of three Bessel functions. In addition, a vortex-free power-function BG beam of the mth order is obtained, which upon propagation in free space becomes a finite superposition of similar vortex-free power-function BG beams of the orders from 0 to m. Extending the set of finite-energy vortex beams with an orbital angular momentum is useful in searching for stable light beams for probing the turbulent atmosphere and for wireless optical communications. Such beams can be used in micromachines for controlling the movements of particles simultaneously along several light rings.

Spoof surface plasmon polariton supported by square ring metasurface for wearable body area network

A metasurface consisting of a 2D periodic square lattice of metallic square ring resonators is considered for supporting spoof surface plasmon polariton (SSPP) modes. First, SSPP modes are highlighted by simulated dispersion curves. The dispersion curves are analyzed by considering hybridization of modes in either a patch or an aperture array with frequency shifts. Hybridization of modes is also supported by means of surface current and electric field mappings showing consistent symmetric and anti-symmetric fields. Second, it is shown that a dipole antenna located near the metasurface is able to generate SSPP modes. The transmission spectra between two antennas located at each extremity of the metasurface show transmission enhancements compared with propagation in free space. The transmission enhancements are shown for different locations of the antennas either facing each other or positioned in diagonal. Finally, the transmission enhancement is connected to SSPP modes by comparing the transmission spectra and the measured dispersion curves with each other. Specifically, two transmission windows measured for an embroidered metasurface are related to the cutoff frequency of the two SSPP modes supported by the metasurface as evidenced by the measured dispersion curves.

Self-filtering of beam fluence fluctuations at free space propagation

It is shown theoretically and experimentally that free space propagation in vacuum provides self-filtering of beam fluence fluctuations (spatial small-scale noise). The linear interference of the noise and the primary smooth beam during the propagation results in temporal filtering invoked by the noise pulse delay with respect to the primary beam pulse and in spatial filtering caused by the noise spatial walk-off from the aperture of the primary beam.

Properties of circular cosh-gaussian beams focused by a thin lens system under a turbulent atmosphere

The propagation of a Circular cosh-Gaussian beam (CiChGB) through a paraxial ABCD optical system in a turbulent atmosphere is investigated based on the extended Collins formula and Rytov theory. In particular, the analytical expression for the on-axis average intensity of a CiChGB focused by a thin lens in a turbulent atmosphere is derived in detail. From the obtained formula, the on-axis intensity distribution of the output beam is discussed numerically with a comparison to direct propagation in free space. The influences of the turbulence strength and initial beam parameters on the on-axis intensity are examined. It is found that the on-axis intensity near the focus region is less affected by atmospheric turbulence compared to the free space.

Ultra-compact synthesis of space-time wave packets.

Space-time wave packets (STWPs) are pulsed fields in which a strictly prescribed association between the spatial and temporal frequencies yields surprising and useful behavior. However, STWPs to date have been synthesized using bulky free-space optical systems that require precise alignment. We describe a compact system that makes use of a novel optical component: a chirped volume Bragg grating that is rotated by 45° with respect to the plane-parallel device facets. By virtue of this grating's unique structure, cascaded gratings resolve and recombine the spectrum without free-space propagation or collimation. We produce STWPs by placing a phase plate that spatially modulates the resolved spectrum between such cascaded gratings, with a device volume of 25 × 25 × 8 mm3, which is orders-of-magnitude smaller than previous arrangements.

Joint Direct Estimation of Emitter Location and Antenna Beam Direction Based on Baseband Reconstruction

This paper addresses the problem of locating a stationary noncooperative emitter using multiple moving receivers. A signal baseband reconstruction based algorithm is proposed to locate the emitter directly from the received signals, without assuming static delay and Doppler shift in any observing interval. Besides the information of time-differences-of arrival (TDOA) and frequency-differences-of-arrival (FDOA), the received signal strength (RSS) information is further utilized when the source emits signals in free space with an omni antenna. In a more complex scenario where the emitter adopts a directional antenna, an iterative algorithm is proposed to jointly estimate the emitter location and its antenna beam direction, where a traditional direct position determination (DPD) method is used to initiate the iteration. The Cramér-Rao lower bounds (CRLBs) of the emitter position and the beam direction are derived. Three numerical cases are included to investigate the performance of the proposed algorithms. The signal baseband reconstruction based DPD algorithm shows improved applicability when the assumption of static time delay and Doppler shift is inappropriate and performs similarly with the classical DPD methods when the assumption holds. In case of free space propagation, it shows that the localization accuracy can be improved when RSS is further utilized. When the transmitting beam is directional, however, simply ignoring the directivity characteristics would lead to nonnegligible localization biases. By jointly estimating the emitter location and antenna beam direction, the RSS information can be effectively utilized to improve the localization accuracy, and a much more accurate estimation of beam direction can be obtained simultaneously.

Connectivity Analysis with Co-Channel Interference for Urban Vehicular Ad Hoc Networks

In urban vehicular ad hoc networks (VANETs), the complex channel environment and co-channel interference resulted in the uncertain delay of inter-vehicle packet transmission, which causes serious delay jitter. Connectivity is proposed as a key metric to describe this uncertainty. However, existing works lack a discussion of inter-vehicle connectivity in urban VANETs, particularly with regards to the process of transmitting packets between vehicles. In this paper, we analyze the connectivity probability of urban VANETs under co-channel interference with both complete and incomplete information. When the information is complete, we model the time-varying nature of co-channel interference and channel fading as delay jitter and analyze inter-vehicle connectivity in a time-varying environment. Then, when complete information is unavailable, we estimate the probability distribution of co-channel interference by combining the distribution of multiple parameters with the free space propagation model (Friss) model and Nakagami-m fading model. The expression for the connectivity performance of vehicle-to-vehicle (V2V) links is derived from the signal interference plus noise ratio (SINR) of the destination V2V link. Finally, we analyze the implications of various factors on connectivity, such as the transmit power of the signal, the arrival rate of packets, the number of channels and vehicles, and the distance between the transmitting vehicle and the receiving vehicle. The numerical analysis shows that co-channel interference and signal fading significantly affect inter-vehicle connectivity.

Study on the orthogonality property of Zernike modes in light beams undergoing free space propagation.

This paper presents a study to investigate how the orthogonality property of Zernike modes gets modified as a light beam carrying the mode in its phase propagates through free space. We use a numerical simulation using scalar diffraction theory to generate propagated light beams carrying the commonly encountered Zernike modes. We present our results in terms of the inner product and orthogonality contrast matrix at propagation distances ranging from near field to far field regions. Our study will help in the understanding of how far the Zernike modes describing the phase profile of a light beam in a given plane remain approximately orthogonal to one another on propagation.