Laser Waveform Research Articles

Pre-Detection and Pre-Registration Averaging of Full Wave Signals in Airborne LiDAR Bathymetry

A well-known technique to enhance the signal to noise ratio (SNR) of repetitive signals is to average them. The coherent parts of the signal add up constructively while the incoherent parts are averaged out. The prerequisite is that the signals are acquired under conditions of high repeatability, i.e., the signals must be sufficiently similar. In the present technical note, we describe an efficient method for maintaining signal similarity by ensuring spatial and temporal proximity of laser waveform signals obtained by a sensor operated from an airborne platform. The method makes use of a few auxiliary parameters such as laser pulse repetition rate, mirror rotation rate, platform altitude and flight speed. The method can be extended to be operated in real time.

Research on Student's T-Distribution Point Cloud Registration Algorithm Based on Local Features.

LiDAR offers a wide range of uses in autonomous driving, remote sensing, urban planning, and other areas. The laser 3D point cloud acquired by LiDAR typically encounters issues during registration, including laser speckle noise, Gaussian noise, data loss, and data disorder. This work suggests a novel Student's t-distribution point cloud registration algorithm based on the local features of point clouds to address these issues. The approach uses Student's t-distribution mixture model (SMM) to generate the probability distribution of point cloud registration, which can accurately describe the data distribution, in order to tackle the problem of the missing laser 3D point cloud data and data disorder. Owing to the disparity in the point cloud registration task, a full-rank covariance matrix is built based on the local features of the point cloud during the objective function design process. The combined penalty of point-to-point and point-to-plane distance is then added to the objective function adaptively. Simultaneously, by analyzing the imaging characteristics of LiDAR, according to the influence of the laser waveform and detector on the LiDAR imaging, the composite weight coefficient is added to improve the pertinence of the algorithm. Based on the public dataset and the laser 3D point cloud dataset acquired in the laboratory, the experimental findings demonstrate that the proposed algorithm has high practicability and dependability and outperforms the five comparison algorithms in terms of accuracy and robustness.

Examination of optimized ultrashort three-color waveforms for generating short and intense isolated attosecond pulses in soft x rays

Isolated attosecond pulses (IAPs) can be readily generated via high-order harmonic generation driven by an ultrashort laser pulse. Here, it is shown that the best way to obtain the ultrashort waveform for producing a short and intense IAP in the soft x rays is to optimize the three-color (TC) laser pulse consisting of the fundamental field and its second and third harmonic fields. To calibrate it, another way of constructing the ultrashort waveform directly in time using a truncated basis set of B-spline functions is first proposed. The calibration waveform (CW) contains more frequency components up to the eighth harmonic order. It is found that the IAP by the TC waveform has a shorter duration after macroscopic propagation in a nonlinear gas medium compared to that by the CW field. It is uncovered that the CW field is additionally modified by the higher-order frequency components during propagation, dominated by the neutral atom dispersion. The effect of phase jitter in the TC waveform and the extension of the TC scheme into higher photon energies are also discussed. Currently, precise control of TC laser waveform synthesis is already achievable in the labs, thus paving an effective way for generating a useful attosecond light source in the soft x rays.

Single-shot measurement of a laser waveform using plasma fluorescence in ambient air.

The temporal characterization of a laser pulse is an important task in studying ultrafast laser-matter interactions. It is ideal to measure the temporal profile of the laser pulse with a single laser shot when the repetition rate is low or its interaction with matter is unstable. Here we report a new approach for the single-shot temporal characterization of a laser pulse, based on the TIPTOE (tunneling ionization with a perturbation of the time-domain observation of an electric field) method. The waveform of the laser pulse is reconstructed from the intensity modulation of plasma fluorescence emission. The result of the single-shot measurement is compared with the time-delay scanning TIPTOE measurements, supporting the validity of the single-shot measurement.

A Scalable, High-Efficiency, Low-Energy-Spread Laser Wakefield Accelerator Using a Tri-Plateau Plasma Channel.

The emergence of multi-petawatt laser facilities is expected to push forward the maximum energy gain that can be achieved in a single stage of a laser wakefield acceleration (LWFA) to tens of giga-electron volts, which begs the question-is it likely to impact particle physics by providing a truly compact particle collider? Colliders have very stringent requirements on beam energy, acceleration efficiency, and beam quality. In this article, we propose an LWFA scheme that can for the first time simultaneously achieve hitherto unrealized acceleration efficiency from the laser to the electron beam of >20% and a sub-1% energy spread using a stepwise plasma structure and a nonlinearly chirped laser pulse. Three-dimensional high-fidelity simulations show that the nonlinear chirp can effectively mitigate the laser waveform distortion and lengthen the acceleration distance. This, combined with an interstage rephasing process in the stepwise plasma, can triple the beam energy gain compared to that in a uniform plasma for a fixed laser energy, thereby dramatically increasing the efficiency. A dynamic beam loading effect can almost perfectly cancel the energy chirp that arises during the acceleration, leading to the sub-percent energy spread. This scheme is highly scalable and can be applied to petawatt LWFA scenarios. Scaling laws are obtained, which suggest that electron beams with parameters relevant for a Higgs factory could be reached with the proposed high-efficiency, low-energy-spread scheme.

Tailoring quantum trajectories for strong-field imaging

Strong-field imaging techniques such as laser-induced electron diffraction (LIED) provide unprecedented combined picometer spatial and attosecond temporal resolution by “self-imaging” a molecular target with its own rescattering electrons. Accessing the rich information contained in these experiments requires the ability to accurately manipulate the dynamics of these electrons—namely, their ionization amplitudes, and times of ionization and rescattering—with attosecond to femtosecond precision. The primary challenge is imposed by the multitude of quantum pathways of the photoelectron, reducing the effective measurement to a small range of energies and providing very limited spatial resolution. Here, we show how this ambiguity can be virtually eliminated by manipulating the rescattering pathways with a tailored laser field. Through combined experimental and theoretical approaches, a phase-controlled two-color laser waveform is shown to facilitate the selection of a specific quantum pathway, allowing a direct mapping between the electron’s final momentum and the rescattering time. Integrating attosecond control with Ångstrom-scale resolution could advance ultrafast imaging of field-induced quantum phenomena.

Intrinsic Spectrum Analysis of Laser Dynamics Based on Fractional Fourier Transform

The intrinsic spectrum that results from the coupling of spontaneous emission in a laser cavity can determine the energy concentration and coherence of lasers, which is crucial for optical high-precision measurements. To date, it has been difficult to analyze the intrinsic spectrum in high-speed laser dynamics processes, especially under the condition of fast wavelength sweeping. In this work, a new method to analyze the laser intrinsic spectrum is proposed with laser energy decomposition into a series of chirp-frequency signals, which is realized by fractional Fourier transform (FRFT) of the coherently reconstructed laser waveform. The new understanding of the energy distribution of lasers contributes to accurate characterization of laser dynamic parameters in the time-frequency domain. In a proof-of-concept experiment, the time-frequency dynamic process of a commercial wavelength-swept laser is tested for different wavelength-scanning speeds, and the most suitable measurement time window width required for the narrowest FRFT-based spectrum is also explored. The proposed analysis method of laser dynamic parameters will promote the understanding of laser dynamics and benefit optical precision measurement applications.

Enhancement and redshift of vortex harmonic radiation in epsilon-near-zero materials.

The harmonic radiation from a vortex laser field interacting with an epsilon-near-zero (ENZ) material is numerically investigated via solving the Maxwell-paradigmatic-Kerr equations. For a laser field of long duration, the harmonics up to the seventh-order can be generated with a low laser intensity (∼109 W/cm2). Moreover, the intensities of high order vortex harmonics at the ENZ frequency are higher than at other frequency points due to the ENZ field enhancement effects. Interestingly, for a laser field of short duration, the obvious frequency redshift occurs beyond enhancement in high order vortex harmonic radiation. The reason is that the strong change of the laser waveform propagating in the ENZ material and the non-constant field enhancement factor around the ENZ frequency. Because the topological number of harmonic radiation is linearly proportional to its harmonic order, the high order vortex harmonics with redshift still possess the exact harmonic orders indicated by the transverse electric field distribution of each harmonic.

Online optimization for optical readout of a single electron spin in diamond

The nitrogen-vacancy (NV) center in diamond has been developed as a promising platform for quantum sensing, especially for magnetic field measurements in the nano-tesla range with a nano-meter resolution. Optical spin readout performance has a direct effect on the signal-to-noise ratio (SNR) of experiments. In this work, we introduce an online optimization method to customize the laser waveform for readout. Both simulations and experiments reveal that our new scheme optimizes the optically detected magnetic resonance in NV center. The SNR of optical spin readout has been witnessed a 44.1% increase in experiments. In addition, we applied the scheme to the Rabi oscillation experiment, which shows an improvement of 46.0% in contrast and a reduction of 12.1% in mean deviation compared to traditional constant laser power SNR optimization. This scheme is promising to improve sensitivities for a wide range of NV-based applications in the future.

Effects of laser waveform on the generation of fast electrons in laser–solid interactions

In the scheme of fast ignition of inertial confinement fusion, the fuel temperature mainly relies on fast electrons, which act as an energy carrier, transferring the laser energy to the fuel. Both conversion efficiency from the laser to the fast electron and the energy spectrum of the fast electron are essentially important to achieve highly effective heating. In this study, a two-dimensional particle in cell simulation is applied to study the generation of fast electrons from solid-density plasmas with different laser waveforms. The results have shown that the slope of the rising edge has a significant effect on fast electron generation and energy absorption. For the negative skew pulse with a relatively slow rising edge, the J×B mechanism can most effectively accelerate the electrons. The overall absorption efficiency of the laser energy is optimized, and the fast electron yield in the middle- and low-energy range is also improved.

Research on Adjustment Method of Pulsed Laser Waveform Based on Beam Splitting and Delay

Research on Adjustment Method of Pulsed Laser Waveform Based on Beam Splitting and Delay

MVCNN: A Deep Learning-Based Ocean–Land Waveform Classification Network for Single-Wavelength LiDAR Bathymetry

Ocean–land waveform classification (OLWC) is crucial in airborne LiDAR bathymetry (ALB) data processing and can be used for ocean–land discrimination and waterline extraction. However, the accuracy of OLWC for single-wavelength ALB systems is low given the nature of the green laser waveform in complex environments. Thus, in this article, a deep learning-based OLWC method called the multichannel voting convolutional neural network (MVCNN) is proposed based on the comprehensive utilization of multichannel green laser waveforms. First, multiple green laser waveforms collected in deep and shallow channels are input into a multichannel input module. Second, a one-dimensional (1-D) convolutional neural network (CNN) structure is proposed to handle each green channel waveform. Finally, a multichannel voting module is introduced to perform majority voting on the predicted categories derived by each 1-D CNN model and output the final waveform category (i.e., ocean or land waveforms). The proposed MVCNN is evaluated using the raw green laser waveforms collected by Optech coastal zone mapping and imaging LiDAR (CZMIL). Results show that the overall accuracy, kappa coefficient, and standard deviation of the overall accuracy for the OLWC utilizing green laser waveforms based on MVCNN can reach 99.41%, 0.9800, and 0.03%, respectively. Results further show that the classification accuracy of the MVCNN is improved gradually with the increase in the number of laser channels. The multichannel voting module can select the correct waveform category from the deep and shallow channels. The proposed MVCNN is highly accurate and robust, and it is slightly affected by aquaculture rafts and the merging effect of green laser waveform in very shallow waters. Thus, the use of MVCNN in OLWC for single-wavelength ALB systems is recommended. In addition, this article explores the relationships between green deep and shallow channel waveforms based on the analysis of CZMIL waveform data.

All-optical sampling of ultrashort laser pulses based on perturbed transient grating.

We propose and demonstrate an all-optical pulse sampling technique based on the transient grating (TG) procedure with perturbation, which provides a simple and robust manner to characterize an ultrashort laser pulse without employing a retrieval algorithm. In our approach, a two-orders weaker perturbation pulse perturbs the diffracted pulse from the TG, which is generated by another strong fundamental pulse. The modulation of the diffracted pulse energy directly represents the temporal profile of the perturbation pulse. We have successfully characterized few-cycle and multi-cycle pulses, which is consistent with the results verified by the widely employed frequency-resolved optical gating (FROG) method. Our method provides a potential way to characterize ultrashort laser waveform from the deep-UV to far-infrared region.

A Compensation Method of Saturated Waveform for Space-Borne Laser Altimeter

Due to the difference in surface reflectivity, the laser measurement waveform data recorded in full waveform have a saturation phenomenon. When the signal is saturated, the echo waveform produces peak clipping and pulse spreading, which seriously restrict the accuracy of laser measurement results and the usability of data. Therefore, we conducted a ranging investigation on the “peak clipping” phenomenon of the saturated waveform and found a nonlinear time delay in the range, which is between the two extreme cases of saturated “dead time” and Gaussian fitting peak time as pulse signal reception time. Subsequently, based on the consistent relationship between the geometric characteristics of the high- and low-gain channels of the space-borne laser altimeter, we constructed a laser waveform saturation compensation model, namely, the laser pulse flight time delay compensation and the laser waveform peak intensity compensation, and carried out the data saturation compensation and validation with the dual-channel measurement data from the GaoFen-7 (GF-7) satellite. The experimental results showed that the saturation compensation model (SCM) proposed in this paper could restore the features of the saturated waveform signal and effectively improve the accuracy of the laser ranging. The accuracy of the laser waveform fitting result after saturation compensation improved from 0.7 ns (0.11 m) to 0.14 ns (0.02 m), which greatly improved the usability of the saturated laser measurement waveform data.

Analysis of a q-deformed hyperbolic short laser pulse in a multi-level atomic system

A fast laser pulse with a q-deformed hyperbolic function shape is investigated in a Multi-level atomic system. Therefore, we first derive the exact solution of the Bloch equations describing a two-level atom excited by a q-deformed laser pulse with dephasing and time-dependent detuning. Next, we analyze the dynamic of the atomic population inversion at resonance and off-resonance of a Rubidium 87 three-level atom driven by a classical weak field and a strong q-deformed control laser. Finally, in order to get a deeper insight of the probe field’s absorption and dispersion properties, we investigate the coherence’s dependence on the q-deformation. Our work demonstrates that, the dynamic of the atomic system can be fully controlled through the manipulation of the asymmetry scaling parameter q of the q-deformed hyperbolic laser wave-form.

An enhancement method of waveform simulation in dual-color laser field imaging based on Internet of Things

AbstractIn view of the fact that the existing methods are easy to be affected by nonlinear factors in the incident process of the light source, resulting in the received shock wave inconsistent with the reality and large calculation error, a two-color laser field imaging waveform simulation and enhancement method based on the Internet of things is proposed. According to the automatic transformation characteristics of augmented reality technology, the waveform simulation control process of two-color laser field imaging is constructed. The Gaussian distribution characteristics are used to analyze the target, the optical grid and double oscillation device are used to control the results, the spectrum Fourier transform is used to adjust the phase of the waveform, and finally the simulated modulated laser waveform is output. Through the comparison of laser waveform spectrum and peak value, the results show that this method is closest to the actual laser waveform, saves calculation time, and has good practical application effect.

Time and space waveform optimization to extend the harmonic cutoff and to produce the water window single attosecond pulse

Abstract Based on the three-step theory of high-order harmonic generation, the harmonic cutoff is very sensitive to the few-cycle laser waveform in both time and space regions. Therefore, in this paper, we propose the method to control the harmonic cutoff and to produce the water window attosecond pulse through the optimization of time and space waveform. It is found that, in the time region, by properly choosing the delay and phase of the few-cycle two-color pulse, not only the harmonic intensity is enhanced, but also the quantum path of the harmonic emission can be controlled. Further, with the introduction of the 3rd pulse (i.e., the infrared pulse or the unipolar pulse), the harmonic cutoff from the single harmonic emission peak can be extended, showing a water window harmonic plateau. In the space region, by using the positive spatial inhomogeneous effect, the harmonic cutoff from the basic two-color waveform can also be extended, which leads to a water window spectral continuum. Finally, by Fourier transformation of harmonics during the water window region, the ultrashort single 29 as pulses can be obtained.

Chirp form selection to produce intense and broad harmonic spectra and attosecond pulses in the presence of single and superposition initial states

The chirp form selection for producing intense and broad high-order harmonic spectra has been investigated when the initial state is chosen to be the single or superposition states. It is found that, for the case of a single ground initial state, the down-chirp is much better for extending the harmonic cutoff with the stronger emission intensity. Moreover, the multi-color combined field is beneficial to produce the larger harmonic cutoff and higher harmonic intensity. After the control of laser waveform, the combination of 3-color down-chirps with a proper UV pulse is the best condition to obtain the intense X-ray spectral continuum and the isolated attosecond pulse. For the case of superposition initial state, both the up-chirp and down-chirp are beneficial to generate the high-intensity spectral regions. However, with the combination of multi-color field, only the harmonic cutoff can be further extended, and the harmonic intensity presents almost no changes for the superposition initial state case. Finally, by properly choosing the 3-color up-chirps or 3-color down-chirps combined pulses, the stronger intensity harmonic spectra covering the X-ray region can be obtained, which can produce the isolated pulses of 37 as.

Strong-field coherent control of isolated attosecond pulse generation

Attosecond science promises to reveal the most fundamental electronic dynamics occurring in matter and it can develop further by meeting two linked technological goals related to high-order harmonic sources: improved spectral tunability (allowing selectivity in addressing electronic transitions) and higher photon flux (permitting to measure low cross-section processes). New developments come through parametric waveform synthesis, which provides control over the shape of field transients, enabling the creation of highly-tunable isolated attosecond pulses via high-harmonic generation. Here we demonstrate that the first goal is fulfilled since central energy, spectral bandwidth/shape and temporal duration of isolated attosecond pulses can be controlled by shaping the laser waveform via two key parameters: the relative-phase between two halves of the multi-octave spanning spectrum, and the overall carrier-envelope phase. These results not only promise to expand the experimental possibilities in attosecond science, but also demonstrate coherent strong-field control of free-electron trajectories using tailored optical waveforms.

Bayesian approach for the optimal control of high-order harmonics for the generation of ultrashort attosecond laser pulses

We present an efficient and powerful method to optimize the production of high-order harmonic generation (HHG) and the synthesis of an isolated attosecond pulse. The system under investigation is the helium atoms that are exposed to the combination of chirped two-color mid-IR laser field and its 1s–2p transition frequency. An ab initio study is conducted by solving the three dimensional time-dependent Schrödinger equation accurately and efficiently by means of the time-dependent generalized pseudospectral method. We applied Bayesian optimization to obtain an optimal laser waveform to generated a HHG spectrum with enhanced harmonics and extended cutoff frequency. Harmonic phases are analysed by the wavelet transform. We demonstrated that an isolated 10 attosecond pulse can be synthesized using a sequence of consecutive and phase-locked harmonics in the plateau region.