Harmonic Beam Research Articles

Sideband power control in Time-Modulated antenna arrays for bidirectional harmonic beamforming and beam scanning

Beamforming and beam scanning in a specific angular region for establishing a secure communication channel has become the dire need of modern wireless communication system. This paper aims to propose an unconventional yet cost effective solution for beamforming and scanning by controlling the inherent sideband patterns of time-modulated antenna arrays. First of all, beam scanning with different scan angles is targeted by proposing asymmetric sequential time schemes. To achieve that, the phase center of the array is unidirectionally shifted by controlling a cluster of array elements. The idea is to model suitable pulse-shifted time schemes so that a good scanning coverage can be achieved without affecting the shapes of the radiation beam patterns. In this way, simultaneous bidirectional sideband patterns can be controlled for beam scanning. Furthermore, beamforming with enhanced efficiency for single channel secure communication is also addressed in this work by reducing the sideband levels (SBLs) of the array. The same concept of shifting the phase center unidirectionally is used for reducing the SBL, but with a higher number of clusters of array elements. The mathematical validation behind the proposed idea is thoroughly discussed and the closed-form sideband power expression is derived after investigating all possible combinations of pulse-shifted time schemes. 16-element linear timed arrays have been chosen to investigate the proposed idea and the supportive outcomes are presented in detail with comparisons.

Enhanced second harmonic generation in laser-induced air plasma.

We report a systematic investigation into the processes behind a near hundred-fold enhanced second harmonic wave generated from a laser-induced air plasma, by examining the temporal dynamics of the frequency conversion processes, and the polarization of the emitted second harmonic beam. Contrary to typical nonlinear optical processes, the enhanced second harmonic generation efficiency is only observed within a sub-picosecond time window and found to be nearly constant across fundamental pulse durations spanning from 0.1 ps to over 2 ps. We further demonstrate that with the adopted orthogonal pump-probe configuration, the polarization of the second harmonic field exhibits a complex dependence on the polarization of both input fundamental beams, contrasting with most of the previous experiments with a single-beam geometry.

CH3 imaging via photo-fragmentation combined with CH planar laser-induced fluorescence employing C–X (0,0) band excitation and detection

In this paper we demonstrate single-shot imaging of the methyl (CH3) radical using a single laser system. The diagnostic approach combines CH3 photo-fragmentation followed by methylidyne (CH) detection. Specifically, the 5th-harmonic beam of a Nd:YAG laser (λPF = 212.8 nm) facilitates CH3 photo-fragmentation, while the frequency-doubled output of the same laser pumps a dye laser, the output of which is frequency-doubled and tuned to excite CH R-branch transitions within the C–X (0,0) band (λLIF ≈ 311 nm). We detect the resulting fluorescence from the C–X band (primarily the Q and P branches), suppressing laser scattering using a custom long-wave-pass filter. Transitions in the C–X (0,0) band are exceptionally strong, which is an enabling feature for the proposed diagnostic. Indeed, the PLIF excitation pulse energy (at 311 nm) was limited to ∼2 mJ, which is sufficient for high-quality measurements of the CH produced via photo-fragmentation. We illustrate the utility the approach in premixed laminar and turbulent flames (over a range of stoichiometry) and in a methane lifted jet diffusion flame. A noteworthy feature of the proposed diagnostic is that one can detect nascent CH (by blocking the 5th harmonic beam) and hydroxyl (OH) in its A–X (0,0) band by tuning the dye laser to a nearby OH transition. We also propose a simple methodology for acquiring simultaneous OH and CH3 images employing a single camera via the use of structured illumination and demonstrate joint OH + CH3 imaging in the lifted jet diffusion flame.

Programming nonreciprocity and harmonic beam steering via a digitally space-time-coded metamaterial antenna

Recent advancement in digital coding metasurfaces incorporating spatial and temporal modulation has enabled simultaneous control of electromagnetic (EM) waves in both space and frequency domains by manipulating incident EM waves in a transmissive or reflective fashion, resulting in time-reversal asymmetry. Here we show in theory and experiment that a digitally space-time-coded metamaterial (MTM) antenna with spatiotemporal modulation at its unit cell level can be regarded as a radiating counterpart of such digital metasurface, which will enable nonreciprocal EM wave transmission and reception via surface-to-leaky-wave transformation and harmonic frequency generation. Operating in the fast wave (radiation) region, the space-time-coded MTM antenna is tailored in a way such that the propagation constant of each programmable unit cell embedded with varactor diodes can toggle between positive and negative phases, which is done through providing digital sequences by using a field-programmable gate array (FPGA). Owing to the time-varying coding sequence, harmonic frequencies are generated with different main beam directions. Furthermore, the space time modulation of the digitally coded MTM antenna allows for nonreciprocal transmission and reception of EM waves by breaking the time-reversal symmetry, which may enable many applications, such as simultaneous transmitting and receiving, unidirectional transmission, radar sensing, and multiple-input and multiple-output (MIMO) beamformer.

Highly Efficient 3D Nonlinear Photonic Crystals in Ferroelectrics

AbstractNonlinear photonic crystals (NPCs) with spatially modulated second‐order nonlinear coefficients are indispensable in nonlinear optics and modern photonics. To access full degrees of freedom in the generation and control of coherent light at new frequencies, three dimensional (3D) NPCs have been recently fabricated employing the femtosecond laser writing technique in ferroelectric crystals. However, the nonlinear interaction efficiencies in 3D NPCs so far are rather low, which has been a major barrier to further development. Here, fabrication challenges of large‐scale 3D NPCs have been overcome to realize more than four orders of magnitude efficiency improvement over previously reported crystals. With the generation of second harmonic vortex beam as an example, the measured conversion efficiencies as high as 2.9 × 10−6 W−1 (femtosecond pulse‐pumped) and 1.2 × 10−5 W−1 (continuous‐wave‐pumped) are comparable with those in 1D or 2D NPCs. A general approach of structural simplification to relax fabrication challenge of truly complex 3D structures without sacrificing the predesigned functions is also presented. This realization of efficient nonlinear interactions in 3D NPCs paves the way for their novel applications in nonlinear photonics and quantum optics including nonlinear orbital angular momentum multiplexing and high‐dimensional entangled source.

An Interpolation-Based Time-Modulated Antenna Array for Beam Scanning at Carrier Frequency

The formula of the Fourier series for a periodic function indicates that traditional periodic time modulation only provides amplitude weighting at the carrier frequency of a time-modulated array (TMA). Therefore, beam scanning through time modulation technique could only be realized at harmonic frequencies. In order to achieve low sideband levels (SBLs) in harmonic beam scanning, a large number of modulation statuses are required in TMAs, which significantly increases the cost and complexity of the entire system. The intent of this work is to develop a novel TMA with array-interpolation aperture and ON–OFF time modulation to achieve beam scanning at the carrier frequency. By performing interpolation computation at the antenna array level, high-precision beam scanning with low SBLs can be realized with pseudorandom ON–OFF time modulation and a static 1-bit phase control. As compared with the state-of-the-art designs, the proposed TMA requires much fewer modulation statuses, and it significantly facilitates beam-scanning control. A forward design strategy is presented to analytically determine the time modulation sequences and the static 1-bit phase statuses for specified beam scanning angles and sidelobe levels (SLLs). Numerical and experimental results well demonstrate the effectiveness of the proposed TMA.

Phase center motion based time modulated arrays with preprocessed time schemes for selective harmonic beamforming in B5G communication systems

AbstractThe possibilities of achieving beamforming antenna arrays for beyond fifth generation (B5G) communication systems by controlling the inherently generated harmonic beam patterns in “time modulation” based antenna arrays, are proposed in this article. The idea of “time modulation” is to assimilate an additional dimension “time” by using high‐speed switches for synthesizing the array radiation patterns. This in turn generates an infinite number of harmonic frequency patterns apart from the main beam at the fundamental frequency. In general, harmonic frequency patterns may be regarded as wastage of power, but with proper controlling, these spatially independent patterns created at multiple harmonics can be selectively utilized for specific purposes. In this article, the lower‐order harmonic frequency patterns of “time modulated” linear arrays (TMLA) are exploited to produce simultaneous multi‐beamforming for B5G multipoint communication. The capabilities of selective harmonic frequency patterns for multi‐beamforming are mathematically verified by proposing different beamforming switching network combinations. In this regard, phase centered motion (PCM) based TMLA is examined for simultaneous, as well as, monopulse beam scanning. PCM‐based unidirectional and bidirectional beamforming patterns with optimal time schemes are proposed. The mathematical validation of preprocessed symmetric and asymmetric time schemes, as well as, the PCM‐based optimized selective harmonic multi‐beamforming radiation patterns are discussed in detail to show the effectiveness of the proposed method.

A new sixth-order Eigenmike® spherical microphone array for spatial sound field recording

A new spherical microphone array capable of up to 6-th order spatial sound field recording is described. Spherical microphone array beamforming is normally accomplished by judicially combining multiple acoustic pressure microphones appropriately mounted on the surface of a rigid sphere to form a set of orthonormal spherical harmonic modal beams or “Eigenbeams” (also referred to as Higher-Order-Ambisonics). Eigenbeamformer processing exploits the diffraction and scattering of the incident acoustic waves onto the rigid sphere. General beamformers are then created by properly combining the orthonormal Eigenbeams to form and steer a desired set of beampatterns up to the decomposition order limit. One advantage of using the modal decomposition approach to beamforming is that it is computationally compact, and results in an elegant scalable beamformer architecture. Steering beams formed by the spherical array is accomplished by a computationally simple matrix multiply. Another benefit is that beampatterns formed using spherical array processing maintain their design spatial responses independent of the steering direction. Beampatterns can also be frequency independent. In this talk, we will describe a new 64-element spherical microphone array and associated software capable of up to 6-th order performance, as well as present some real-world measurements.

Highly efficient coherent detection of terahertz pulses based on ethanol

Water-based terahertz (THz) coherent detection scheme has been recently proposed, which overcomes the bandwidth limitation or high probe laser power demand in solid-/gas-based schemes. Here, we report a highly efficient THz coherent detection method using ethanol with superior performances under the mechanism of four-wave mixing. We systematically investigate the energy ratios and relative polarizations between the THz-induced second harmonic and control second harmonic (CSH) beams and reveal that ethanol always exhibits significantly higher detection sensitivity than water. The coherent, incoherent, or hybrid detection mode can be flexibly switched by changing the CSH beam polarization. The enhanced sensitivity derives from the much larger third-order nonlinear coefficient and lower ionization potential of ethanol. In addition, for the ethanol–water mixtures with various concentrations, the THz coherent detection signals can always be decomposed into the linear superposition of those from pure ethanol and neat water at the sub-picosecond timescale, indicating the synergistic contribution of ethanol and water molecules in the mixture during the detection process. This work provides a valid method to significantly improve the sensitivity of the liquid-based coherent detection scheme and a research perspective for exploring the solute–solvent molecular interactions.

Measurement of 2D density profiles using a second-harmonic, dispersion interferometer.

A second-harmonic, dispersion interferometer is used to image large-area (≃5cm2) plasma-jet and gas-jet density profiles. Achromatic telescopes magnify the diameters of the primary-laser beam (1064nm) and its second-harmonic (532nm) before probing the sample and de-magnify the beam diameters after the sample, where the primary beam transfers its phase change to a second, second-harmonic beam, allowing the sample's dispersive-phase change to be measured between two, orthogonally polarized second harmonic beams. The telescopes produce an azimuthally symmetric, dispersive-phase shift in the sample + background phase-change image and in the background phase-change image, which is removed by digital subtraction. The interferometer's performance was verified using standard-optical components as dispersive elements (BK7 lenses and wedge plates), resolving a minimum, phase-change sensitivity of Δϕmin ≳ 15 mrad and spatial resolution of Δxres ≃ 100μm. The phase change produced by unknown-density objects (a pulsed-plasma-jet and a pulsed-gas-jet) was measured, and their data were used to recover the original, 2D density profiles using an inverse Abel transform: peak-number density, Ngas ≃ 6 × 1020cm-3 and Ne ≃ 5 × 1016cm-3; line-integrated density, ∫Ngasdl ∼ 2 × 1019cm-2 and ∫Nedl ∼ 1 × 1016cm-2. The techniques and methods developed here are scalable to even larger probe-beam diameters and frame-capture rates, leading to a diagnostic capability that is well-suited for applications involving the real-time measurement of density.

Chromatic aberrations correction of attosecond high-order harmonic beams by flat-top spatial shaping of the fundamental beam

Attosecond pulses created by high-order harmonic generation in gases often exhibit strong chromatic aberrations, arising from the broad bandwidth and wavelength-dependent nonlinear light–matter interaction. When the driving laser intensity varies spatially, as for Gaussian driving beams, the apparent source position of the harmonics differs significantly from one order to the next, thus affecting the achievable intensity and duration of the attosecond pulses when they are focused on a target. We show that these chromatic aberrations can be reduced by spatially shaping the fundamental beam to generate high-order harmonics with a driver having a flat-top profile inside the gas medium. By measuring both the intensity profile and wavefront for each harmonic in a plane, we access the extreme ultra-violet (XUV) beam properties and investigate these properties near focus. We observe that controlling chromatic aberrations by flat-top spatial shaping strongly reduces the variation of the XUV spectrum on the beam axis during propagation and, in return, the longitudinal sensitivity of both the temporal profiles and the temporal shifts of the focused attosecond pulses.

High-Efficiency Modulation and Harmonic Beam Scanning in Time-Modulated Array

A general high-efficiency time-modulated module and harmonic beam scanning time-modulated array (TMA) are proposed and investigated in this paper. By introducing the periodically controlled phased-shifters, a significant improvement on theoretical overall efficiency and transmitted bandwidth can be realized in harmonic beam scanning. Then equivalent cascade structure of the general modulation module is proposed for RF circuit simplification and practicability in actual applications. Theoretical derivation and simulation results show that increased efficiency, 100% feeding network efficiency and theoretical overall efficiency tending to 100% without consideration of insertion loss, can be achieved with the increase the phase-shifter states when only a single sideband is utilized. Compared to the existing state-of-the-art harmonic beam scanning TMAs, the proposed TMA exhibits a much higher overall efficiency while keeps a similar hardware complexity. Additionally, with acceptable increase of circuit complexity less than existing works, similar level of the wide transmission bandwidth can also be achieved. To experimentally verify the feasibility of the proposed general design, a 4-bit time-modulated module with reconfigurable states and the corresponding 8-element harmonic beam scanning TMA are fabricated and tested. The experiments on signal transmission and harmonic beam scanning demonstrate the effectiveness of power loss reduction and improvement in transmitted bandwidth with the increase of modulation phase states.

Observation of strong terahertz field-induced second harmonic generation in plasma filaments.

We report the observation of terahertz field-induced second harmonic (TFISH) generation produced by directly mixing an optical probe beam onto femtosecond plasma filaments. The produced TFISH signal is spatially separated from the laser-induced supercontinuum by impinging on the plasma at a noncollinear angle. The conversion efficiency of the fundamental probe beam to its second harmonic (SH) beam is greater than 0.02%, which represents a record in optical probe to TFISH conversion efficiency that is nearly five orders of magnitude larger than previous experiments. We also present the terahertz (THz) spectral buildup of the source along the plasma filament and retrieve coherent terahertz signal measurements. This method of analysis has the potential to provide local electric field strength measurements inside of the filament.

A Direct Antenna Modulator With Beam Steering Capability Based on Space-Time-Coding Arrays

We propose an RF front-end architecture with simultaneous direct antenna modulation and beam steering capability based on space-time-coding (STC) arrays. In an STC array, digital codes control the state of each array element. The element state determines the amplitude and phase of the radiated wave, and in a time scale over which the state of the elements is changed by a few successive codes, the frequency harmonics are generated. We first introduce a vector representation of bits in a code and their effects on the resultant harmonic wavefront. We show that by choosing proper codes with a minimum of two repetitions, one can modulate data on the phase and amplitude of a desired far-field harmonic where proof-of-concept simulations are given for a 16-APSK digital modulation scheme. We investigate the data rate, spectral efficiency, and the limitation of switching speed in our proposed scheme. Moreover, it is shown that the harmonic beam patterns of an STC array can be easily steered by a consecutive shifting of bits in the code without affecting the transmitted data symbols. Through cosimulation of digital design and coded antenna array, we calculate the constellation of the QPSK digital modulation scheme for a steered beam and study its angular dependency. Finally, we provide an upper bound for the efficiency of STC arrays.

Power leakage minimization in time‐modulated linear antenna array with optimized switching strategies employing modified harmony search algorithm

Time-modulated linear antenna array (TMLAA) with periodic time modulation of antenna elements radiates infinite inevitable harmonic beams. The infinite harmonic sideband radiation is a wastage of power unless the power is exploited. This paper aims to minimize inevitable power wastage in the harmonic beams radiated from TMLAA by employing an optimized periodic timing sequence. The optimization of the timing sequence is conducted employing harmony search (HS) to suppress the power wastage, However, conventional HS achieves an acceptable sidelobe level (SLL) below – 30 dB. Nevertheless, a modified harmony search algorithm (MHSA) is proposed to attain more power in the desired fundamental beam. The algorithm is validated using an array of 4-, 10-, and 16-element uniform half-wavelength spacing TMLAA. The optimization is conducted on the ON and OFF time instants and the real current of the elements. The simulation results exhibit that HS and MHSA perform potentially alike to suppress the SLL of fundamental below –30 dB. However, the further suppression of the sideband levels employing MHSA justifies the modification proposed in this work. The MHSA performance in terms of radiation parameters is compared with the HS and also with previous works to show the potentiality of the modification.

Intense high-harmonic optical vortices generated from a microplasma waveguide irradiated by a circularly polarized laser pulse

A scheme for generating intense high-harmonic optical vortices is proposed. It relies on spin-orbit interaction of light when a relativistically strong circularly polarized laser pulse irradiates a microplasma waveguide. The intense laser field drives a strong surface wave at the inner boundary of the waveguide, which leads to high-order-harmonic generation as the laser is traveling inside. For a circularly polarized drive laser, the optical chirality is imprinted to the surface wave, which facilitates conversion of the spin angular momentum of the fundamental light into orbital angular momenta of the harmonics. A ``shaken waveguide'' model is developed showing that the aforementioned phenomenon arises due to a nonlinear plasma response that modifies the electromagnetic mode at high intensities. We further show that the phase velocities of all the harmonic beams are automatically matched to the driving laser, so that the harmonic intensities increase with propagation distance. The efficiency of harmonic production is related to the surface wave breaking effect, which can be significantly enhanced using a tightly focused laser. Our simulation suggests that an overall conversion efficiency of $\ensuremath{\sim}5%$ can be achieved.

ROMANTIC GUITAR RESTORATION WORK

Bu makalede; Fransa’da üretilen ve yaklaşık 130 yıllık olan bir romantik gitarın restorasyon çalışmasının aşamaları anlatılmaktadır. Söz konusu çalgı elimize geçtiğinde; sapı ve ses kutusu birbirinden ayrılmış, klavyesi ikiye kırılmış, üst ve alt ses tahtalarında çatlaklar oluşmuş, harmonik kirişlerin bir kısmı yapışma yüzeylerinden atmış ve rozetlerindeki süslemelerin ise bazıları düşmüş vaziyetteydi. İcra edilemeyecek halde olan bu çalgı hem tarihi geçmişi, hem de malzemeleri ve işçiliği bakımından detaylı olarak ele alındığında, kıymetli ve dönemini yansıtan iyi bir müzik aleti olduğu anlaşılmış ve restorasyonunun yapılmasına karar verilmiştir. Yapılan değerlendirmenin ardından öncelikle çalışmanın planı hazırlanmış ve bu planlama doğrultusunda çalgının tüm onarımları adım adım gerçekleştirilmiştir. Yürütülen onarım çalışmasında mümkün olduğunca çalgının orijinal yapısı korunmaya özen gösterilmiştir. Bununla beraber onun günümüzde icra edilebilmesi ve uzun yıllar rahatlıkla kullanılabilmesi amacıyla gerekli olan yapısal değişiklikler gerçekleştirilmiştir. Titizlikle yürütülen çalışmanın bütün aşamaları fotoğraflanarak belgelenmiştir. Bu restorasyon sayesinde romantik gitar tekrar işlevsel hale getirilmiş ve icracıların kullanımına kazandırılmıştır.

Quantum vacuum processes in the extremely intense light of relativistic plasma mirror sources

The advent of petawatt-class laser systems allows generating electromagnetic fields of unprecedented strength in a controlled environment, driving increasingly more efforts to probe yet unobserved processes through their interaction with the quantum vacuum. Still, the lowest intensity scale governing these effects lies orders of magnitude beyond foreseen capabilities, so that such endeavor is expected to remain extremely challenging. In recent years, however, plasma mirrors have emerged as a promising bridge across this gap, by enabling the conversion of intense infrared laser pulses into coherently focused Doppler harmonic beams lying in the X-UV range. In this work, we present predictions on the quantum vacuum signatures produced when such beams are focused to intensities between 1024 and 1028 W cm−2, specifically photon–photon scattering and electron–positron pair creation. These signatures are computed via the stimulated vacuum formalism, combined with a model of perfectly focused beam built from PIC-generated harmonics spectra, and implemented on state-of-the-art massively parallel numerical tools. In view of identifying experimentally favorable configurations, we also consider the coupling of the focused harmonic beam with an auxiliary optical beam, and provide comparison with other established schemes. Our results show that a single coherently focused harmonic beam can produce as much scattered photons as two infrared pulses in head-on collision, and confirm that the coupling of the harmonic beam to an auxiliary beam gives rise to significant levels of inelastic scattering, and hence holds the potential to strongly improve the attainable signal to noise ratios in experiments.

A new digital beam position and phase measurement implementation based on a field programmable gate array for the high intensity heavy-ion accelerator iLinac.

A new digital beam position and phase measurement (BPM) system was designed for the ion-Linac accelerator at the high intensity heavy ion accelerator facility. The fundamental and second harmonic signals are retrieved from the BPM electrodes to simultaneously calculate their respective beam positions and phases. All data acquisition and digital signal processing algorithm routines are performed in a field programmable gate array (FPGA). The position and phase information are obtained by using the in-phase and quadrature demodulation method. A practical and straightforward method is used to generate the second harmonic reference signal for processing the second harmonic beam signal. The reconfigurable filters are integrated into the FPGA to allow the measurement of short beam pulse length. The laboratory test results show that the achieved phase resolution is better than 0.2° and 0.03° when the input signal is -60 and -45 dBm, respectively. A position resolution better than 30 μm was achieved for an input power level of approximately -60 dBm, and it can reach 7 μm with the input power higher than -45 dBm. The entire execution time of the algorithm is accomplished within 3.4 µs, which provides a sufficient reaction time for the fast beam interlock signal to the machine protection system. The performance of this newly designed prototype BPM electronics was evaluated with the online proton beam.

Water-Based Coherent Detection of Broadband Terahertz Pulses.

Both solids and gases have been demonstrated as the materials for terahertz (THz) coherent detection. The gas-based coherent detection methods require a high-energy probe laser beam and the detection bandwidth is limited in the solid-based methods. Whether liquids can be used for THz detection and relax these problems has not yet been reported, which becomes a timely and interesting topic due to the recent observation of efficient THz wave generation in liquids. Here, we propose a THz coherent detection scheme based on liquid water. When a THz pulse and a fundamental laser beam are mixed on a free-flowing water film, a second harmonic (SH) beam is generated as the plasma is formed. Combining this THz-induced SH beam with a control SH beam, we successfully achieve the time-resolved waveform of the THz field with the frequency range of 0.1-18THz. The required probe laser energy is as low as a few microjoules. The sensitivity of our scheme is 1 order of magnitude higher than that of the air-based method under comparable detection conditions. The scheme is sensitive to the THz polarization and the phase difference between the fundamental and control SH beams, which brings direct routes for optimization and polarization sensitive detection. Energy scaling and polarization properties of the THz-induced beam indicate that its generation can be attributed to a four-wave mixing process. This generation mechanism makes simple relationships among the probe laser, THz-induced SH, and THz field, favorable for robustness and flexibility of the detection device.