Velocity Matching Research Articles

High-power picosecond deep-UV source via group velocity matched frequency conversion

Powerful ultrashort pulses in the deep-ultraviolet (deep-UV) are beneficial for diverse applications from fundamental science to industrial materials processing. However, reaching high powers via conventional approaches is challenging due to three central issues: dispersion, multiphoton absorption, and optical damage. Here, we simultaneously overcome these issues with a novel fifth-harmonic generation architecture optimized for group velocity matching. We use tilted pulse fronts, including a noncollinear geometry in the final sum-frequency generation stage. This enables lower intensities and longer crystals, thereby favoring the birefringently phase matched χ ( 2 ) process over higher-order multiphoton absorption processes. Moreover, we demonstrate low-loss cascaded χ ( 2 ) -based spatiotemporal flattening of the input pulses, which enhances the uniformity of the conversion efficiency throughout the beam profile. Through these techniques, we realize a picosecond deep-UV generation source at 206 nm with record-high 2.5 W average output power and a repetition rate of 100 kHz. This result paves the way for a new era of high-power ultrafast deep-UV lasers.

A 50-Gb/s PAM4 Si-Photonic Transmitter With Digital-Assisted Distributed Driver and Integrated CDR in 40-nm CMOS

A 50-Gb/s four-level pulse-amplitude modulation (PAM4) silicon photonic transmitter is presented, which is composed of a 40-nm bulk CMOS driver hybrid integrated with a 180-nm silicon-on-insulator (SOI) CMOS Mach–Zehnder modulator (MZM). To recover and demodulate 50-Gb/s PAM4 data into dual-25-Gb/s nonreturn to zero (NRZ) streams for the co-designed MZM digital-analog converter (DAC), a reference-less clock and data recovery (CDR) is integrated, and to achieve high swing and high speed, the driver is made in a distributed amplifier (DA), employing push–pull cells that are grouped in three segments. A digital-assisted distributed driver (DADD) that replaces the input T-line by segmented 2:1 multiplexers and buffers is, thus, proposed. Precise time gating in each segment allows for velocity match in both the DA and optical DAC, as well as the in-segment pre-emphasis without power-hungry analog delay stages. Measurement results show that the driver and CDR achieve 4-VPP differential swing, 1.19-ps rms jitter, and 1.34-W power consumption at 50 Gb/s. An optical transmitter is demonstrated by wire-bonding the driver to the MZM DAC, achieving 50-Gb/s PAM4 output with 6.35-ps rms jitter and improved linearity.

Stabilization performances and mechanisms of a diffusion‐like vortex‐tube combustor foroxygen‐enrichedcombustion

The stabilization performance and mechanisms in a diffusion-like vortex-tube combustor is investigated for oxygen-enriched combustion. The stability limit, flame configuration, and pressure fluctuation are investigated under various conditions. Results show that a diffusion-like flame structure is established in the combustor and the nonpremixed peculiarity becomes more prominent with the increase of oxygen mole fraction. The steady combustion can be achieved in the range of global equivalence ratio 0.01 to 1.0 with a low-pressure fluctuation amplitude always less than 1300 Pa, indicating a good combustion stability of this combustor. Additionally, the stabilization mechanism is discussed from the time matching and velocity matching. Based on the axial fuel entry method, the Damköhler number (Da) is always less than 1.0 as a whole, which is the principal reason for the tubular flame shape and the steady combustion procedure in this vortex-tube combustor. The intensified combustion under oxygen-enriched combustion can increase the flame speed, and subsequently reduce the mixing quality and make the yellow flame more visible. Besides, the temperature distribution and the flow field structure can explain the corrugation and deformation of the flame front under oxygen-enriched conditions.

Second-harmonic generation from 2D photonic crystal waveguide with simultaneous near-flat dispersions at fundamental frequency and second harmonic

Photonic crystal waveguides (PCW) have shown great potential to generate second harmonic (SH), but the available schemes with cavity or defect suffer from a lack of multiple wavelengths operation and excessive phase-matching requirement. Here we propose a 2D PCW structure comprised of a modulated single line defect, which is capable of independently controlling the fundamental frequency (FF) and SH modes. Therefore, two near-flat group velocity dispersions can be achieved for these modes, and they simultaneously satisfy phase matching conditions. Based on multiply scattering method, we theoretically demonstrate that the SH resonant modes originated from the nonradiative near-flat dispersion have high quality factors (nearly 6.5×104 for the maximum value). And the maximum SH conversion efficiency for these resonant modes are 2 orders of magnitude higher than that of off-resonant modes. In addition, benefitting from the matching of the group velocities of the second harmonic and the fundamental, the central wavelength of SH can be modulated within wavelength range ∼3nm. These results can provide potential applications in on-chip integration of photonics and quantum optics.

Response of second-harmonic generation of Lamb wave propagation to microdamage thickness in a solid plate

The response features of second-harmonic generation (SHG) of primary Lamb wave propagation to the thickness of microdamage layer (MDL) in a solid plate have been theoretically and numerically investigated in this paper. Here the solid plate with a MDL is regarded as a double-layered plate in analysis of nonlinear Lamb wave propagation. On the basis of a second-order perturbation approximation and modal expansion analysis, the physical process of cumulative SHG by primary Lamb wave propagation in a solid plate with a MDL has been investigated. The influence of variation in the thickness of MDL on the effect of SHG of primary S0 mode, which satisfies an approximate phase velocity matching in the low frequency region, has been theoretically analyzed, and then the finite element (FE) simulation has been carried out to validate the results of the theoretical predictions. A close agreement between the theoretical analyses and FE simulations validates the effectiveness of using the effect of SHG of primary S0 mode for characterizing changes in the thickness of MDL. Moreover, change mechanism of nonlinear acoustic parameter with the thickness of MDL is revealed It is expected that the results obtained can provide a convenient means for accurately characterizing nonhomogeneous microdamage (MDL thickness) in layered plates.

Leader-following flocking for unmanned aerial vehicle swarm with distributed topology control

To address the flocking issues of an unmanned aerial vehicle (UAV) swarm operating at a leader-follower mode, distributed control protocols comprising both kinetic controller and topology control algorithm must be implemented. For flocking the UAV swarm, a distributed control-input method is required forboth maintaining a relatively steady state between neighboring vehicles (including velocity matching and distance maintenance) and avoiding vehicle-to-vehicle collision. Furthermore, the stability of control protocols should be analyzed using the potential energy function. In particular, a distributed $\beta~$-angle test (BAT) rule in the proposed topology-control issue may allow each UAV to determine its neighboring set by exploiting the locally sensed information, thereby significantly reducing the communication overhead of the entire swarm. In addition, node-degree bound is derived to demonstrate the feasibility of the proposed algorithm, in which the optimal value in terms of convergence is analyzed. The flocking of the flying ad-hoc network (FANET) can be achieved in a self-organizing way without the use of an external control center via the distributed control protocols. Ultimately, the proposed analysis is verified by numerical results.

Experimental Demonstration of Hadron Beam Cooling Using Radio-Frequency Accelerated Electron Bunches.

Cooling of beams of gold ions using electron bunches accelerated with radio-frequency systems was recently experimentally demonstrated in the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. Such an approach is new and opens the possibility of using this technique at higher energies than possible with electrostatic acceleration of electron beams. The challenges of this approach include generation of electron beams suitable for cooling, delivery of electron bunches of the required quality to the cooling sections without degradation of beam angular divergence and energy spread, achieving the required small angles between electron and ion trajectories in the cooling sections, precise velocity matching between the two beams, high-current operation of the electron accelerator, as well as several physics effects related to bunched-beam cooling. Here we report on the first demonstration of cooling hadron beams using this new approach.

Broadband 200-nm second-harmonic generation in silicon in the telecom band

Silicon is well known for its strong third-order optical nonlinearity, exhibiting efficient supercontinuum and four-wave mixing processes. A strong second-order effect that is naturally inhibited in silicon can also be observed, for example, by electrically breaking the inversion symmetry and quasi-phase matching the pump and the signal. To generate an efficient broadband second-harmonic signal, however, the most promising technique requires matching the group velocities of the pump and the signal. In this work, we utilize dispersion engineering of a silicon waveguide to achieve group velocity matching between the pump and the signal, along with an additional degree of freedom to broaden the second harmonic through the strong third-order nonlinearity. We demonstrate that the strong self-phase modulation and cross-phase modulation in silicon help broaden the second harmonic by 200 nm in the O-band. Furthermore, we show a waveguide design that can be used to generate a second-harmonic signal in the entire near-infrared region. Our work paves the way for various applications, such as efficient and broadband complementary-metal oxide semiconductor based on—chip frequency synthesizers, entangled photon pair generators, and optical parametric oscillators.

Estimation of remaining useful life of fatigued plate specimens using Lamb wave‐based nonlinearity parameters

The present study focuses on estimating material nonlinearity and consequently remaining useful life of fatigued specimens using the amplitude- and physics-based material nonlinearity parameters evaluated for Lamb wave motion in plate specimens. The amplitude-based nonlinearity parameter depends on amplitudes of the Lamb wave harmonics generated due to material nonlinearity. Here, it is employed to estimate the inherent and dislocation induced material nonlinearities for different stages of fatigue. The cumulative effect is obtained from the strict matching of phase and group velocities of the S1 − S2 mode pair. The physics-based nonlinearity parameter is obtained from the higher order elastic coefficients, plastic coefficients, and substructural evolution parameters. It does not depend on wave propagation distance; however, it depends on percent fatigue life. Thus, it is used to construct a theoretical nonlinearity curve. A spectral amplitude normalization technique is given to systematically evaluate the material nonlinearity, once the Lamb wave data over different wave propagation distances are known either from experiments or from simulations. The values of amplitude-based nonlinearity parameter thus estimated through the simulation and experiments for different fatigue stages are then plotted onto the obtained theoretical nonlinearity curve. A reasonably good agreement is seen between the fatigue life estimations given by both the nonlinearity parameters. Thus, the amplitude-based nonlinearity parameter obtained from the Lamb wave response can be effectively used to estimate the remaining useful life of the fatigued plate specimens.

Numerical Study of Reconfigurable Mid‐IR Single Photon Sources Based on Functional Ferroelectrics

AbstractThe future of quantum photonic technology depends on the realization of efficient sources of single photons, the ideal carriers of quantum information. Parametric downconversion (PDC) is a promising route to create highly coherent, spectrally pure single photons for quantum photonics using versatile group velocity matching (GVM) and tailored nonlinearities. However, the functionality to actively control the poling period of nonlinear crystals used in PDC is currently missing, yet would enable to dynamically modify the wavelength of single photons produced in the PDC process. Herein, a detailed GVM study is presented for functional PMN‐0.38PT material which can be dynamically repolled at ambient conditions with fields as low as 0.4 kV mm−1. The study reveals phase‐matching conditions for spectrally pure single photon creation at 5–6 µm. Further, a practical approach is proposed for on‐flight wavelength switching of the created single photons. The reported reconfigurable functionality benefits a wide range of emerging quantum‐enhanced applications in the mid‐IR spectral region where the choice of single photon sources is currently limited.

Efficient Optical Modulation With High Data-Rate in Silicon Based Laterally Split Vertical p-n Junction

Efficient optical modulation along with high data rate is a prerequisite for the realization of high-performance optical modulator. Optical modulation in silicon with high data-rate and high modulation efficiency is proposed by a laterally separated vertical p-n junction. Two independent but synchronized p-n junctions that supports the common optical mode are created by forming a slot waveguide structure. It provides a prominent way to enhance the light-material interaction necessarily to achieve low V π L along with the low RC time constant which is crucial for the high-speed operation. The proposed device shows a high modulation efficiency of 0.74 V-cm for 1.2 mm long device. The calculated intrinsic 3-dB bandwidth reaches up to ≈ 58 GHz at a reverse bias of 6 V. We show high speed operation up to 25 Gbit/s for the device length of 600 μm with a simple lumped electrode configuration. The Traveling-wave electrodes as a coplanar waveguide is employed to further improve the speed performance of the device. By taking the advantage of excellent velocity matching between optical group index and RF effective index, high data rate performance up to 100 Gbit/s is obtained with an extinction ratio of 2.4 dB. The proposed device opens new avenues for high speed optical interconnect on an SOI platform.

In situ disbond detection in adhesive bonded multi-layer metallic joint using time-of-flight variation of guided wave

Adhesive bonded joints are frequently adopted in structural applications. The adhesive aging, low quality of surface preparation, as well as the exposure to external harsh environment and loading, may degrade the quality of adhesive, leading to disbond and decrease of the interfacial strength of the bonded joints. This study addresses both numerical and experimental investigations of ultrasonic guided wave (UGW) propagating in adhesive bonded metallic waveguide, whereby disbond detection is realized based on variation of the wave arrival time of UGW. First the dispersion curves of UGWs in both intact (bonded) and disbonded joints are obtained via the Semi-Analytical Finite Element (SAFE) method, and are grouped into mode pairs of phase velocity match and mis-match, respectively. Then a model combining SAFE and Frequency Domain Finite Element (SAFE-FDFE) is developed to enable excitation of any UGW of desired single mode-frequency combination and analysis of the wave interaction with disbond. The obtained results indicate that the UGW Mode 2 generated at the low frequency range of the mis-matched group shows a good sensitivity to disbond, featuring variation of the wave arrival time induced by mode conversion. Finally, Time Domain Finite Element and a proof-of-concept experiment, with comb transducers to act as both in situ actuators and sensors made of PVDF sheets embedded into the adhesive layer, well validate the results obtained via SAFE-FDFE. The selected mode-frequency combination Mode 2 at 0.52 MHz for wave time-of-arrival-based disbond detection, compared with conventional signal-amplitude-based disbond indicator using high frequency UGWs (~several MHz), merits the advantages of better controllability of wave excitation, less wave attenuation, and higher robustness.

Fully differential study of ionization of H2 by p impact near velocity matching

SynopsisFully differential cross sections (FDCS) for ionization in p + H2 collisions in the region of the electron-projectile velocity matching were measured and calculated. The FDCS were found to be very sensitive to the details of the few-body dynamics underlying the reaction.

Enhancement effect of cumulative second-harmonic generation by closed propagation feature of circumferential guided waves**Project supported by the National Natural Science Foundation of China (Grant Nos. 11834008, 11704410, 11632004, 11474361, and U1930202).

On the basis of second-order perturbation approximate and modal expansion approach, we investigate the enhancement effect of cumulative second-harmonic generation (SHG) of circumferential guided waves (CGWs) in a circular tube, which is inherently induced by the closed propagation feature of CGWs. An appropriate mode pair of primary- and double-frequency CGWs satisfying the phase velocity matching and nonzero energy flux is selected to ensure that the second harmonic generated by primary CGW propagation can accumulate along the circumference. Using a coherent superposition of multi-waves, a model of unidirectional CGW propagation is established for analyzing the enhancement effect of cumulative SHG of primary CGW mode selected. The theoretical analyses and numerical simulations performed directly demonstrate that the second harmonic generated does have a cumulative effect along the circumferential direction and the closed propagation feature of CGWs does enhance the magnitude of cumulative second harmonic generated. Potential applications of the enhancement effect of cumulative SHG of CGWs are considered and discussed. The theoretical analysis and numerical simulation perspective presented here yield an insight previously unavailable into the physical mechanism of the enhancement effect of cumulative SHG by closed propagation feature of CGWs in a circular tube.

Energy decay and global smooth solutions for a free boundary fluid-nonlinear elastic structure interface model with boundary dissipation

<p style='text-indent:20px;'>We consider a nonlinear, free boundary fluid-structure interaction model in a bounded domain. The viscous incompressible fluid interacts with a nonlinear elastic body on the common boundary via the velocity and stress matching conditions. The motion of the fluid is governed by incompressible Navier-Stokes equations while the displacement of elastic structure is determined by a nonlinear elastodynamic system with boundary dissipation. The boundary dissipation is inserted in the velocity matching condition. We prove the global existence of the smooth solutions for small initial data and obtain the exponential decay of the energy of this system as well.

Electro-optically, widely tunable optical parametric oscillator based on periodically poled lithium niobate

We report fast and wide wavelength tuning of a ns-optical parametric oscillator (OPO) based on a periodically poled lithium niobate crystal (PPLN) with a three-segment structure. An ultra-broad parametric gain was accomplished by appropriately choosing the pump wavelength and the poling period for group velocity matching between the signal and the idler. While the first segment of the PPLN generated ultra-broad signal and idler bands, the third segment amplified selected portions of the spectra, which could be controlled electro-optically by applying a dc electric field in the middle single-domain segment. A ns-laser pumped singly resonant OPO was built, whose signal wavelength was tuned from 1.373 to 1.749μm (corresponding idler tuning range from 2.863 to 1.972μm) by varying the electric field from −6.5 to ＋5.0 kV/mm. The electro-optic tuning provided a fast wavelength selection method for the OPO within the wide infrared spectral range.

Pore pressure prediction in orthotropic medium based on rock physics modeling of shale gas

Pore pressure prediction is critical to reducing drilling costs and exploring hydrocarbons. Current seismic approaches for pore pressure prediction mainly initially establish the relation between vertical effective stress and physical parameters (velocity, resistivity or porosity), and then predict pore pressures with the known effective stress theory. The commonly used Eaton's equation requires the estimation of normal compaction trend (NCT), which cannot be measured but only be inferred. However, the conventional NCT is a curve which is able to describe the normal trend but is relatively rough, limiting the accuracy of pore pressure prediction. In this study, the shale rock physics modeling in orthotropic (OA) medium is designed to equate the NCT. The modeled and the measured P-wave velocity match well in normal pore pressure segments, but they differ in abnormal pore pressure segments. The modeled NCT is utilized to predict pore pressures, and the comparison with the measured pore pressures proves that the accuracy can be improved. Then P-wave velocity is inverted from azimuthal seismic data based on the reflectivity approximation of OA medium, and the pore pressure of the research area is predicted using the modeled P-wave velocity (the target play is a shale gas field in Sichuan basin of China). The prediction result is consistent with the characteristics of overpressure mechanism of hydrocarbon generation, indicating the effectiveness of rock physics modeling in pore pressure prediction.

An Error Evaluation Method for the Ship Angular Flexure Measurement Based on the Principle of Relevance

Due to the lack of true ship angular flexure data, it is difficult to evaluate its measurement error of the angular velocity matching method in practice. In this paper, the cause of the measurement error of the ship flexure angle is analyzed in theory, and an evaluation method for the ship angular flexure measurement error based on the principle of relevance is proposed. The proposed method provides a prediction formula to describe the estimation error of the static flexure angle based on the off-diagonal elements of the error covariance matrix P in Kalman filtering. In addition, the optimized coefficient F is introduced to make the prediction error range better describe the real error variation. The optimized coefficient F ensures that the proposed formula has good prediction effects in all three directions. Simulations based on the actual measured ship flexure data are carried out, and the simulation results verify the capability of the prediction formula. The proposed method can be used in the evaluation of the ship flexure measurement error.

EVALUATION OF K-EPSILON MODEL FOR TURBULENT BUOYANT JET

The modelling of a turbulent buoyant jet is challenging due to the complex nature of such flow, which consists of two fluids with different densities, as well as the multi-scale flow phenomena associated in both space and time. In this paper, the k-epsilon turbulence model is applied to model a turbulent buoyant jet at different flow regimes including laminar and turbulent. The velocity field and centerline velocity are in good agreement with the experiments, as well as the expected results based on jet theory. Moreover, the distribution of the radial velocity matches with Gaussian distribution. The k-epsilon model is an appropriate turbulent model that can be applied for larger Reynolds number flow simulation.&#x0D; Keywords: k-epsilon, turbulence model, CFD, turbulent buoyant jet.

Supercontinuum generation in OHQ-N2S organic crystal driven by intense terahertz fields.

A laser supercontinuum is generated by cross-phase modulation (XPM) driven by an intense terahertz (THz) field in organic crystal OHQ-N2S. In this highly nonlinear medium, the THz electric field induces a time-varying optical phase modulation, which causes a spectacular spectral broadening and shifting of a co-propagating near-infrared laser pulse. The effect is enabled by the large electro-optic coefficient, the low absorption, and the good velocity matching between the laser and the THz pulse over the OHQ-N2S crystal thickness. The XPM occurs when the THz field is aligned along the polar axis of the OHQ-N2S. The results display a promising pathway for ultrafast control of the spectral and temporal properties of laser pulses using THz stimuli.