Temporal Intensity Profile Research Articles

Systematic comparison of commercial devices for temporal characterization of few-cycle laser pulses in the 500-1000 nm spectral range

We compare multiple temporal pulse characterization techniques in three different pulse duration regimes from 15 fs to sub-5 fs, as there are no available standards yet for measuring such ultrashort pulses. To accomplish this, a versatile post-compression platform was developed, where the 100 fs near infrared pulses were post-compressed to the sub-two-cycle regime in a hybrid, three-stage configuration. After each stage, the duration of the compressed pulse was measured with the d-scan, TIPTOE and SRSI techniques and the retrieved temporal intensity profiles, spectrum and spectral phases were compared. Spectral homogeneity was also measured with an imaging spectrometer to understand the input coupling conditions of the temporal measurements. Our findings suggest that the different devices give similar results in terms of temporal intensity profile, however they are extremely sensitive to alignment and to beam quality, especially in the case of the shortest pulses. We address specific steps of measurement procedures, which paves the way towards the standardization of pulse characterization in the near future.

Frequency-mode-stable regenerative amplification at terahertz burst rates

Generation of high-fidelity amplified pulse bursts with a regular interpulse interval yields, in the spectral domain, an equidistant pattern of narrowband spectral modes, similar to frequency combs produced by cw mode-locked lasers but with greatly increased pulse energy. Despite their great potential for nonlinear spectroscopy, material processing, etc., such long frequency-stable bursts are difficult to generate and amplify because of prominent temporal intensity modulation even after strong dispersive pulse stretching. This study presents a burst generation method based on a master-oscillator regenerative-amplifier system that allows for chirped-pulse amplification (CPA) with high scalability in pulse number. A gradual smoothing of temporal intensity profiles at an increasing number of pulses is discovered, demonstrating an unexpected recovery of the CPA performance at terahertz intraburst repetition rates. In consequence, a self-referenced stable burst spectral peak structure with megahertz peak width is generated without risk of amplifier damage caused by interference of chirped pulses. This result eliminates limitations in burst amplification and paves the way for advancements in ultrashort-pulse burst technology, particularly for its use in nonlinear optical applications.

Experimental observation of linear pulses affected by high-order dispersion.

We experimentally study the linear propagation of optical pulses affected by high-order dispersion. We use a programmable spectral pulse-shaper that applies a phase that equals that which would result from dispersive propagation. The temporal intensity profiles of the pulses are characterized using phase-resolved measurements. Our results are in very good agreement with previous numerical and theoretical results, confirming that for high dispersion orders m the central part of the pulses follow the same evolution, with m only determining the rate of evolution.

Simulation of nonlinear propagation of femtosecond laser pulses in air for quantitative prediction of the ablation crater shape.

The utilization of sub-100 fs pulses has attracted attention as an approach to further improve the quality and precision of femtosecond laser microfabrication. However, when using such lasers at pulse energies typical for laser processing, nonlinear propagation effects in air are known to distort the beam's temporal and spatial intensity profile. Due to this distortion, it has been difficult to quantitatively predict the final processed crater shape of materials ablated by such lasers. In this study, we developed a method to quantitatively predict the ablation crater shape, utilizing nonlinear propagation simulations. Investigations revealed that the ablation crater diameters derived by our method were in excellent quantitative agreement with experimental results for several metals over a two-orders-of-magnitude range in the pulse energy. We also found a good quantitative correlation between the simulated central fluence and the ablation depth. Such methods should improve the controllability of laser processing with sub-100 fs pulses and contribute to furthering their practical application to processes over a wide pulse-energy range, including conditions with nonlinear-propagating pulses.

Characterization and performance of the Apollon short-focal-area facility following its commissioning at 1 PW level

We present the results of the first commissioning phase of the short-focal-length area of the Apollon laser facility (located in Saclay, France), which was performed with the first available laser beam (F2), scaled to a nominal power of 1 PW. Under the conditions that were tested, this beam delivered on-target pulses of 10 J average energy and 24 fs duration. Several diagnostics were fielded to assess the performance of the facility. The on-target focal spot and its spatial stability, the temporal intensity profile prior to the main pulse, and the resulting density gradient formed at the irradiated side of solid targets have been thoroughly characterized, with the goal of helping users design future experiments. Emissions of energetic electrons, ions, and electromagnetic radiation were recorded, showing good laser-to-target coupling efficiency and an overall performance comparable to that of similar international facilities. This will be followed in 2022 by a further commissioning stage at the multi-petawatt level.

Polarization-independent pulse retrieval based on frequency resolved optical switching.

We demonstrate experimentally that the frequency resolved optical switching (FROSt) method is independent of the polarization direction of the pulse to be characterized. In this perspective, it is employed to characterize two or three co-propagating pulses linearly polarized in orthogonal directions, enabling to retrieve simultaneously their temporal intensity and phase profiles together with their group delay. This technique is also applied to track a simple nonlinear process involving different polarization states: type-I second harmonic generation (SHG). We are able to characterize the depleted fundamental pulse along with the generated second-harmonic pulse, thus demonstrating that the FROSt technique is a practical and powerful tool to observe nonlinear processes both in the temporal and spectral domains even if it involves different polarization states.

Generalized self-similar propagation and amplification of optical pulses in nonlinear media with high-order normal dispersion

We investigate theoretically and numerically the self-similar propagation of optical pulses in the presence of gain, positive Kerr nonlinearity, and positive (i.e., normal) dispersion of even order $m$. Starting from a modified nonlinear Schr\"odinger equation, separating the evolution of amplitude and phase, we find that the resulting equations simplify considerably in the asymptotic limit. Exact solutions to the resulting equations indicate that the temporal intensity profile follows a $1\ensuremath{-}{T}^{m/(m\ensuremath{-}1)}$ function with an $m$-dependent scaling relation, with a ${T}^{1/(m\ensuremath{-}1)}$ chirp, where $T$ is the pulse's local time. These correspond to a triangle and a step function, respectively, as $m\ensuremath{\rightarrow}\ensuremath{\infty}$. These results are borne out by numerical simulations, although we do observe indications of nonasymptotic behavior.

Magnetothermoelastic vibrations on a viscoelastic microbeam subjected to a laser heat source

The linear theory of viscoelasticity remains an important field of research like most solids and polymer materials when exposed to a vicious dynamic loading effect. This article introduces a new model for describing the behavior of thermoviscoelastic microbeams considering the effects of temperature change and the longitudinal magnetic field. The governing equations in this model are derived based on the Euler–Bernoulli beam theory, Kelvin–Voigt model of viscosity, the generalized thermoelasticity, and the classical Maxwell equations. The two ends of the microbeam are clamped and subjected to the influence of a laser pulse with a temporal intensity profile. The analytical solutions to the physical fields are evaluated using the Laplace transform and its inversion transforms are performed numerically. The thermo-viscoelastic responses of the microbeam are calculated numerically and investigated graphically. The effect of different parameters such as viscosity, laser intensity, and the magnitude of the magnetic field are studied in detail.

Coherent 2D electronic spectroscopy with complete characterization of excitation pulses during all scanning steps.

Coherent two-dimensional (2D) electronic spectroscopy has become a standard tool in ultrafast science. Thus it is relevant to consider the accuracy of data considering both experimental imperfections and theoretical assumptions about idealized conditions. It is already known that chirped excitation pulses can affect 2D line shapes. In the present work, we demonstrate performance-efficient, automated characterization of the full electric field of each individual multipulse sequence employed during a 2D scanning procedure. Using Fourier-transform spectral interferometry, we analyze how the temporal intensity and phase profile varies from scanning step to scanning step and extract relevant pulse-sequence parameters. This takes into account both random and systematic variations during the scan that may be caused, for example, by femtosecond pulse-shaping artifacts. Using the characterized fields, we simulate and compare 2D spectra obtained with idealized and real shapes obtained from an LCD-based pulse shaper. Exemplarily, we consider fluorescence of a molecular dimer and multiphoton photoemission of a plasmonic nanoslit. The deviations from pulse-shaper artifacts in our specific case do not distort strongly the population-based multidimensional data. The characterization procedure is applicable to other pulses-shaping technologies or excitation geometries, including also pump-probe geometry with multipulse excitation and coherent detection, and allows for accurate consideration of realistic optical excitation fields at all inter-pulse time-delays.

A Peregrine Soliton-Like Structure That Has Nothing to Deal With a Peregrine Breather

We report on experimental results where a temporal intensity profile presenting some of the main signatures of the Peregrine soliton (PS) is observed. However, the emergence of a highly peaked structure over a continuous background in a normally dispersive fiber cannot be linked to any PS dynamics and is mainly ascribed to the impact of Brillouin backscattering.

Towards a deeper understanding of historic cosmic ray modulation during solar cycle 20

Given recent successes in modelling the transport of cosmic rays (CRs) in the heliosphere from first principles, taking into account the behaviour of heliospheric turbulence quantities and their influence on the transport parameters of these particles in a manner as self-consistent as possible, the question naturally arises as to what insights such a modelling approach can yield when applied to the historical study of cosmic ray transport. The temporal relationship between the heliospheric magnetic field (HMF) and observed CR intensities at Earth during solar cycle 20 has long been regarded as anomalous with respect to other solar cycles. The present study presents an analysis of historic HMF observations that gleans information as to the behaviour of turbulence quantities during this period. This information is used to analyse CR transport coefficients, which are incorporated into a stochastic CR modulation code. Galactic CR intensities so computed are compared with observations, and it is demonstrated that this approach yields a temporal intensity profile in qualitative agreement with that displayed by neutron monitor observations, thereby providing new insights as to CR modulation in solar cycle 20.

Artificial neural networks for nonlinear pulse shaping in optical fibers

We use a supervised machine-learning model based on a neural network to predict the temporal and spectral intensity profiles of the pulses that form upon nonlinear propagation in optical fibers with both normal and anomalous second-order dispersion. We also show that the model is able to retrieve the parameters of the nonlinear propagation from the pulses observed at the output of the fiber. Various initial pulse shapes as well as initially chirped pulses are investigated.

Temporal X-ray Reconstruction using Temporal and Spectral Measurements at LCLS

Transverse deflecting structures (TDSs) are widely used in accelerator physics to measure the longitudinal density of particle bunches. When used in combination with a dispersive section, the whole longitudinal phase space density can be imaged. At the Linac Coherent Light Source (LCLS), the installation of such a device downstream of the undulators enables the reconstruction of the X-ray temporal intensity profile by comparing longitudinal phase space distributions with lasing on and lasing off. However, the resolution of this TDS is limited to around 1 fs rms (root mean square), and therefore, it is not possible to resolve single self-amplified spontaneous emission (SASE) spikes within an X-ray photon pulse. By combining the power spectrum from a high resolution photon spectrometer and the temporal structure from the TDS, the overall resolution is enhanced, thus allowing the observation of temporal, single SASE spikes. The combined data from the spectrometer and the TDS is analysed using an iterative algorithm to obtain the actual intensity profile. In this paper, we present some improvements to the reconstruction algorithm as well as real data taken at LCLS.

A Fully Time-dependent Ab Initio Cosmic-Ray Modulation Model Applied to Historical Cosmic-Ray Modulation

Abstract Cosmogenic nuclide records can in principle allow for the estimation of the behavior of the heliospheric magnetic field (HMF) in the distant past. This requires careful modeling of cosmic-ray transport in a manner that is as realistic as possible, taking into account as many of the factors affecting the transport of cosmic-rays (CRs) as possible. The present study presents a 3D time-dependent ab initio CR modulation code that utilizes as inputs simple theoretically and observationally motivated temporal profiles to model large-scale (such as the tilt angle) and small-scale (such as the magnetic variance) parameters relevant to CR transport. Galactic CR proton differential intensities computed using this model for the period 1977–2001 are in reasonable to good agreement with spacecraft observations, reproducing the major salient features of the observed CR intensity temporal profile. To investigate pre-space-age cosmic-ray modulation, and to test conclusions previously drawn regarding the relative importance of drift effects on said modulation, historic estimates of the past HMF presented by McCracken & Beer were used as inputs for the model. The resulting CR temporal intensity profile displays clear evidence of drift effects, with a sharp peak in intensities during the Dalton Minimum.

Attosecond-pulse metrology based on high-order harmonic generation

An all-optical method to retrieve the temporal intensity profile of an extreme ultraviolet (XUV) attosecond pulse is proposed based on XUV-assisted high-order harmonic generation (HHG) by an intense infrared (IR) pulse. For a harmonic located on the XUV-induced high-energy plateau (beyond the IR HHG plateau), the measured harmonic yield as a function of the time delay between the XUV and IR pulses is shown to accurately map the temporal intensity profile of the XUV pulse. Single-color and two-color orthogonal, linearly polarized IR pulses are used to demonstrate the method.

A Bayesian Time-Varying Coefficient Model for Multitype Recurrent Events

This article proposes a Bayesian time-varying coefficient model to evaluate the temporal profile of intensity for multitype recurrent events. The model obtains smooth estimates for both time-varying coefficients and the baseline intensity using Bayesian penalized splines. One major challenge in Bayesian penalized splines is that the smoothness of a spline fit is sensitive to the subjective choice of hyperparameters. We establish a procedure to objectively determine the hyperparameters through robust prior specification. To effectively update the high-dimensional spline parameters, we develop a Markov chain Monte Carlo procedure based on the Metropolis-adjusted Langevin algorithms. A joint sampling scheme is used to achieve better convergence and mixing properties. A simulation study confirms satisfactory model performance in estimating time-varying coefficients under different curvature and event rate scenarios. Application to a commercial truck driver naturalistic driving data reveals that drivers with 7-hours-or-less sleep time have a significantly higher safety-critical event intensity after 8 hr of driving and the intensity remains high after taking a break. The findings provide crucial information for the truck driver hours-of-service regulation and fatigue management. The proposed model provides a flexible and robust tool to evaluate the temporal profile of intensity for multitype recurrent events. Supplemental materials for this article are available online.

Clean optical solitons generation at the 1700 nm window in a short photonic-crystal rod

Tunable optical soliton generation from optical waveguides is especially suitable for multiphoton microscopy (MPM). Among the different waveguides, photonic-crystal (PC) rod has been mainly used for high-energy soliton generation at the 1700 nm window, which is promising for deep-tissue MPM. In order to obtain high-energy soliton, short PC rod is commonly used. This leads to dramatic overlap between the soliton spectrum and the residual pump or a second soliton spectrum and strong modulation of the soliton spectrum, when the pump pulse is relatively long, typical of energetic pump lasers for the PC rod. This spectral overlap and modulation make the spectral filtering of the solitons quite ambiguous and difficult. In this paper, we propose an effective method to generate clean solitons at the 1700 nm window, in a 44 cm short PC rod, by compressing the pump pulse width at 1550 nm. We demonstrate comparison experiments with both 500 and 175 fs pump pulse at 1550 nm. Our results clearly show that a shorter excitation pulse enables cleaner soliton generation at the 1700 nm window, in good agreement with theoretical expectations, which facilitates subsequent spectral filtering. We also demonstrate detailed comparison experiments and simulations of the filtered 1660 nm high-energy solitons. Compared with solitons generated by 500 fs pump pulse, the clean solitons generated by the 175 fs pump pulse also show clean temporal profiles manifested by both measured interferometric autocorrelation trace and simulated temporal intensity profile. We expect this technique will be beneficial for deep-tissue MPM excited at this window.

Temporal X-ray reconstruction using temporal and spectral measurements

Transverse deflecting structures (TDS) are widely used in accelerator physics to measure the longitudinal density of particle bunches. When used in combination with a dispersive section, the whole longitudinal phase space density can be imaged. At the Linac Coherent Light Source (LCLS), the installation of such a device downstream of the undulators enables the reconstruction of the X-ray temporal intensity profile by comparing longitudinal phase space distributions with lasing on and lasing off [1]. However, the resolution of this TDS is limited to around 1 fs rms (root mean square), and therefore, in most cases, it is not possible to resolve single self-amplified spontaneous emission (SASE) spikes within one photon pulse.By combining the intensity spectrum from a high resolution photon spectrometer [2] and the temporal structure from the TDS, the overall resolution is enhanced, thus allowing the observation of temporal, single SASE spikes. The combined data from the spectrometer and the TDS is analyzed using an iterative algorithm to obtain the actual intensity profile.In this paper, we present the reconstruction algorithm as well as analyzed data obtained from simulations which shows the reliability of this method. Real data will be published at a later stage.

Chirp-coefficient bisection iteration method for phase-intensity reconstruction of chirped pulses

A new method is proposed to systematically measure the phase-intensity information of chirped pulses that is based on the chirp-coefficient bisection iteration (CBI) concept. Through the CBI procedure with measured spectrum and temporal intensity profiles (or intensity autocorrelations), spectral and temporal amplitude–phase information can be rapidly retrieved. We experimentally verified that our method has high precision for nanosecond- and picosecond-level pulses and low precision for femtosecond-level pulses. Our proposed method does not require a sophisticated setup and has the advantage of accurate determination of temporal and spectral chirp coefficients with various orders. It also has lower cost, simple operation, in particular covers a wider measurement range than the main current methods. Moreover, the retrieved waveforms can reveal both the pulse shape and the actual intensity with spectral and temporal chirped coefficients of various orders, which can be directly used in various pulse propagation analyses such as chirped pulse amplification.

Fiber‐based measurement of temporal intensity and phase profiles of an optical telecommunication pulse through self‐phase modulation

AbstractWe propose and experimentally validate an all‐fiber based approach to characterize the phase and intensity profiles of optical pulses. Based on three optical spectra affected by different levels of self‐phase modulation, we were able to reconstruct the temporal details of pulses typical of optical telecommunications.