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Higher-order harmonic generation of laser radiation due to relativistic plasma resonance at nonrelativistic laser intensity
摘要： The well-known plasma resonance (transformation into plasma waves) at a critical density in an inhomogeneous plasma easily becomes relativistic nonlinear even at nonrelativistic laser intensities because the generated electrostatic ?eld is signi?cantly increased. Therefore, the widely used theory of harmonic generation by plasma resonance in a laser-produced plasma should be re-examined. We formulate the corresponding analytical theory of higher-order harmonic generation by nonrelativistic intense laser radiation propagating in a spatially inhomogeneous plasma. We ?nd the spectral and angular characteristics of the harmonic radiation ?eld and demonstrate the role of relativis- tic nonlinearity at plasma resonance in forming the harmonic spectra. The applicability range of the developed theory is determined by the plasma wave-breaking condition in the vicinity of the critical plasma density, which we analyze explicitly. The proposed theory is compared with the standard perturbation approach. Because the latter corresponds to low laser intensities, this comparison clearly shows the failure of the theory of harmonic generation via linear plasma resonance. The presented relativistic theory, which is applicable up to the laser intensity corresponding to plasma wave breaking, demonstrates the formation of a smooth power-law energy spectrum of higher-order laser harmon- ics in contrast to the standard perturbation theory. A spectral modulation of harmonics is also shown, which is a unique feature of relativistic nonlinearity.
关键词： laser-produced plasma,harmonic generation,wave-breaking condition,relativistic nonlinearity,plasma resonance
更新于2025-09-11 14:15:04
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ACO–OFDM with Improved Bandwidth Efficiency over Long Haul and MIMO Optical Fiber Communication Systems
摘要： Authors propose an improved orthogonal frequency division multiplexing (OFDM) scheme in which the information samples are transmitted in otherwise discarded negative frame of asymmetrically clipped OFDM (ACO–OFDM). ACO–OFDM has two similar positive and negative OFDM envelopes so that clipping does not result in loss of signal information. The flipped polarity negative samples are inserted at the zeros of positive frame or back to back with positive frame, in time-compressed frame such that composite signal has the same frame duration as ACO–OFDM but improved transmission power, bandwidth efficiency. Simulation results for bit error rate with respect to number of subcarriers and optical launch power show that the proposed time-compressed OFDM frames perform better in high-dispersion condition over long distance, high data rate, and high transmission power over single-mode fiber. It has been demonstrated that the proposed time-compressed frame structure also performs well with the 2*2 MIMO setup for multimode fiber link.
关键词： ACO–OFDM,Efficiency,Dispersion,Nonlinearity,MIMO,Bandwidth
更新于2025-09-11 14:15:04
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Generation of terahertz radiation from beating of two intense cosh-Gaussian laser beams in magnetized plasma
摘要： An analytical and numerical study has been carried out for the generation of terahertz (THz) radiation by beating of two intense cosh-Gaussian laser beams (decentered Gaussian beams) in the rippled density magnetized plasma under the relativistic–ponderomotive regime. In this process, both laser beams exert a relativistic–ponderomotive force on plasma electrons at the beat frequency and impart them an oscillatory velocity in the presence of a static magnetic field. Due to coupling between this nonlinear oscillatory velocity with density ripple, nonlinear current is generated that excites the THz radiation at the different frequency. Higher-order paraxial-ray approximation (non-paraxial theory) has been used in this study. The effects of the decentered parameter, magnetic field, and density ripple on the THz radiation generation in ripple density magnetized plasma have been investigated. Further, the effect of beating of laser beams on the THz field amplitude and the efficiency of THz radiation have been studied. The amplitude and efficiency of the emitted radiation are found to be highly sensitive to the decentered parameter, magnetic field, and density ripple. It has been found that the amplitude and efficiency of the generated THz radiation increase significantly with increasing the values of decentered parameter, magnetic field, and density ripple.
关键词： relativistic–ponderomotive nonlinearity,magnetized plasma,cross-focusing,terahertz radiation,Cosh-Gaussian laser beams
更新于2025-09-11 14:15:04
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Analysis on the Influence of the Distributed Photovoltaic on the Line Loss of Distribution System
摘要： In order to analyze synchronization control problems of two non-identical homodromy eccentric rotors (ERs) in a nonlinearly coupled system of vibrating machinery-part (NCS-VMP), a new electromechanical coupling nonlinear dynamic model considering nonlinear acting force of the part and nonlinear support is established, NCS- VMP’s complex control is converted into rotating speed and phase synchronous control of two homodromy non- identical exciters. By considering the dynamic interactions among the vibration body, the part and the ERs, the nonlinear dynamical equation of the NCS-VMP are established. An accurate synchronization control of speed and phase method are proposed for two homodromy ERs in NCS-VMP. The precise speed and phase synchronization control is mainly re?ected as: The cross-coupling control strategy is used which considering the coupling effect between two co-rotating exciters. The radial basis function network adaptive global sliding mode algorithm (RBFN- AGSMA) is used to adaptively approximate the total uncertainty of system including the nonlinear support and the nonlinear force of the parts, which can effectively reduce the estimation error. The radial basis function network method can suppress the jitter of the system and make the in?uence of the system more stable after replacing the sign function. The stability of RBFN-AGSMA controller is proved by Lyapunov theory. The controller’s property is veri?ed through numerical methods and taking the sliding mode control (SMC) algorithm into comparison. Results indicate that the designed control method can reduce chattering clearly compared with the SMC algorithm, and it is capable to improve the control accuracy of two non-identical homodromy exciters. By studying the effects of parameter change in the NCS-VMP on the system, the strong robustness of RBF network global sliding mode controller to parameter perturbations is proved. It is proposed that RBFN-AGSMA controller can control two nonidentical homodromy ERs in NCS-VMP to achieve accurate vibration trajectory in the working direction.
关键词： RBF network adaptive global sliding mode algorithm,stability,nonidentical homodromy coupling exciters,Nonlinearity support,nonlinear coupled vibration system
更新于2025-09-11 14:15:04
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Monitoring of Thermal Aging of Aluminum Alloy via Nonlinear Propagation of Acoustic Pulses Generated and Detected by Lasers
摘要： Nonlinear acoustic techniques are established tools for the characterization of micro-inhomogeneous materials with higher sensitivity, compared to linear ultrasonic techniques. In particular, the evaluation of material elastic quadratic nonlinearity via the detection of the second harmonic generation by acoustic waves is known to provide an assessment of the state variation of heat treated micro-structured materials. We report on the first application for non-destructive diagnostics of material thermal aging of finite-amplitude longitudinal acoustic pulses generated and detected by lasers. Finite-amplitude longitudinal pulses were launched in aluminum alloy samples by deposited liquid-suspended carbon particles layer irradiated by a nanosecond laser source. An out-of-plane displacement at the epicenter of the opposite sample surface was measured by an interferometer. This laser ultrasonic technique provided an opportunity to study the propagation in aluminum alloys of finite-amplitude acoustic pulses with a strain up to 5 × 10?3. The experiments revealed a signature of the hysteretic quadratic nonlinearity of micro-structured material manifested in an increase of the duration of detected acoustic pulses with an increase of their amplitude. The parameter of the hysteretic quadratic nonlinearity of the aluminum alloy (Al6061) was found to be of the order of 100 and to exhibit more than 50% variations in the process of the alloy thermal aging. By comparing the measured parameter of the hysteretic quadratic nonlinearity in aluminum alloys that were subjected to heat-treatment at 220 ?C for different times (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, and 1000 h), with measurements of yield strength in same samples, it was established that the extrema in the dependence of the hysteretic nonlinearity and of the yield strength of this alloy on heat treatment time are correlated. This experimental observation provides the background for future research with the application goal of suggested nonlinear laser ultrasonic techniques for non-destructive evaluation of alloys’ strength and rigidity in the process of their heat treatment.
关键词： laser ultrasonics,thermal aging,hysteretic acoustic nonlinearity,nonlinear acoustics
更新于2025-09-11 14:15:04
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[IEEE 2019 IEEE Photonics Conference (IPC) - San Antonio, TX, USA (2019.9.29-2019.10.3)] 2019 IEEE Photonics Conference (IPC) - Moving Gap Solitons in a Coupled Nonuniform Fiber Bragg Grating with Cubic-Quintic Nonlinearity
摘要： The existence and stability of moving gap solitons in a coupled fiber Bragg gratings with dispersive reflectivity in a cubic-quintic nonlinear medium are investigated. The effect of dispersive reflectivity on the stability of solitons is analyzed.
关键词： dispersive reflectivity,Gap soliton,cubic-quintic nonlinearity
更新于2025-09-11 14:15:04
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Four-Wave Mixing of a Laser and Its Frequency-Doubled Version in a Multimode Optical Fiber
摘要： It is shown that it is possible to couple a laser beam and its frequency-doubled daughter into a multimode optical ?ber through the four-wave mixing nonlinear process and generate a new wavelength. The frequency-doubled daughter can be generated in an external crystal with a large second order nonlinearity. It is argued that while this possibility is within the design parameter range of conventional multimode optical ?bers, it necessitates a lower-bound for the core-cladding refractive index contrast of the multimode optical ?ber.
关键词： Kerr optical ?ber,third harmonic generation,frequency-doubling,Four-wave mixing,new wavelength generation,multimode nonlinearity
更新于2025-09-11 14:15:04
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[IEEE 2018 20th International Conference on Transparent Optical Networks (ICTON) - Bucharest (2018.7.1-2018.7.5)] 2018 20th International Conference on Transparent Optical Networks (ICTON) - Nonlinearity Compensation through Optical Phase Conjugation for Improved Transmission Reach/Rate
摘要： Optical phase conjugation is a well-known technique to provide dispersion and nonlinearity compensation directly in the optical domain. The link properties play an important role in the achievable improvements in transmission reach/rate. Here, we review our recent investigations on the impact of sub-optimum transmission links as well as our effort on the implementation of the optical phase conjugator itself looking beyond using highly nonlinear fibers as nonlinear medium and focusing on integrated silicon waveguides. Preliminary results on combining optical phase conjugation with digital techniques such as probabilistic shaping will also be discussed.
关键词： optical phase conjugation,nonlinearity compensation,four-wave mixing,optical communication
更新于2025-09-10 09:29:36
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[IEEE IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Valencia, Spain (2018.7.22-2018.7.27)] IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Nonlinear Hyperspectral Unmixing Via Modelling Band Dependent Nonlinearity
摘要： Wavelength dependent nonlinearity is an essential issue in hyperspectral unmixing, which was overlooked in the past. In this paper, a band-wise nonlinear unmixing method is presented. An extended multilinear mixing model is adopted for interpreting different degrees of nonlinear contributions per band. Moreover, regularizers including abundances’ sparsity and nonlinear parameters’ smoothness are exploited to formulate the optimization problem and obtain better unmixing results. Finally, unmixing is implemented in the scheme of alternating direction method of multipliers. Experimental results on both simulated and real hyperspectral data validate that the proposed method can improve the unmixing accuracy and reveal the change of nonlinearity at each band as well.
关键词： extended multilinear mixing model,nonlinear spectral unmixing,constrained optimization,Hyperspectral imagery,wavelength-wise nonlinearity
更新于2025-09-10 09:29:36
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[IEEE 2018 41st International Conference on Telecommunications and Signal Processing (TSP) - Athens, Greece (2018.7.4-2018.7.6)] 2018 41st International Conference on Telecommunications and Signal Processing (TSP) - A Novel Discrete Wavelet Transform based Coherent Optical OFDM System
摘要： In optical communication, which is an important part of the Radio over Fiber (RoF) systems, coherent optical OFDM (CO-OFDM) systems allow the use of amplitude and phase belong to the light at the same time for data transmission. However, the data transmission speed and distance of the CO-OFDM system are limited by optical channel impairment effects. One of the major disadvantages of CO-OFDM systems is its sensitivity to fiber nonlinearity effects. For this reason, in order to be used the new signal processing techniques efficiently in the receiver, the optical channel information must be estimated and the received signal must be equalized. In this work, different equalizer constructions are investigated and a frequency domain channel equalizer used in the novel DWT-based CO-OFDM system has been proposed. With the simulation studies made, the equalizers were compared and the results were given with different changes. From the obtained simulation results, it is seen that the proposed method provides about 5.5 OSNR gain for 1E-4 BER value.
关键词： fiber nonlinearity impairment,discrete wavelet transform,Radio over Fiber,CO-OFDM systems,frequency domain equalizer
更新于2025-09-10 09:29:36