Flat-top Profile Research Articles

Coherence characterization of nanosecond rectangular pulses in passively mode-locked fiber lasers

Dissipative soliton resonance (DSR) phenomenon is a recent concept of pulse formation proposed to achieve a high-energy passively mode-locked fiber laser. Such pulses have general features, e.g., flat-top temporal profile, energy scalability, and temporal coherence. To identify the nature of various rectangular pulses which share most of DSR features, it is important to verify their coherence. When a pulse is in the nanosecond range, the experimental study of the coherence is a complex task, which is overlooked in the literature. In this work, a novel approach is proposed for the first time to study the coherence of nanosecond rectangular pulses. It is based on two different methods: Mach-Zehnder interferometer (MZI) and dispersive Fourier transformation (DFT).

A FEM Enhanced Transfer Matrix Method for Optical Grating Design

A method to design gratings in integrated photonics, is presented. The method is based on a transfer matrix formalism enhanced by Finite Element Method (FEM) parameter calculations. The main advantages of the proposed technique are the easy of use, the fast optimization time and the versatility of the approach. Few examples of optimized gratings to obtain various scattered light field profiles for different applications are presented: a double-Gaussian profile, a flat top square profile, a spot profile on a chip surface, profiles suited to get efficient and selective coupling to single mode and multimode fibers. A discussion of the limits of the method and some insights on how to improve it are also discussed.

Computational modelling of light propagation in a turbid medium illuminated by a LED array–diaphragm system

Light sources generated by LASER and LED are progressively used in the medical field. For many applications there is the need to use light sources with a well-defined size. This can be achieved when a large LED array is associated to a thin plane mask pierced with a circular hole. In this paper three tissue illumination modes were studied. The one (reference mode) allows to illuminate the tissue with a collimated light beam of finite radius, while the two others generate an illumination that depends on the relative position of the diaphragm and LED array with respect to the tissue boundary. Finite Element Method (FEM) and Monte Carlo (MC) were used to compute the fluence rate (axial and radial profiles) inside the tissue as function of source radius, realistic source shapes, tissue optical properties, and boundary conditions. Different types of boundary conditions (i.e. Robin boundary conditions (RBC), and altered with the consideration of the optical features of the interface tissue-diaphragm: -reflecting,-absorbing,-diffusing,-and partially absorbing-diffusing) were evaluated in order to compare the three above mentioned tissue illumination modes. FEM provided accurate results in cases of flat top source profiles and for simple tissue-diaphragm interfaces (absorbing and reflecting) as well as for tissue-air boundary conditions (RBC). MC simulations showed good agreements with experiments when a LED array–diaphragm system (partially absorbing-diffusing) is positioned near the tissue (2.5 mm), while FEM underestimated the fluence rate by a large error within tissue-depths of 5–10 mean-free-paths. These investigations emphasize the need to accurately model all optical events occurring in complex tissue-illumination modes. Monte Carlo simulations further confirmed that a critical beam radius Rc (Rc≥4δ) should be used to retrieve the light penetration depth δ within the tissue, in the same conditions as those of the 1-D geometry.

Dynamics of Sunspot Shock Waves in the Chromosphere and Transition Region

Abstract We study the dynamics of shock waves observed in the umbra of a sunspot using the spectroscopic observations from the Interface Region Imaging Spectrograph. The presence of a shock significantly deforms the shape of the spectral lines of Mg ii, C ii, and Si iv. We found that C ii 1335.71 Å and Si iv 1393.75 Å show double-peaked profiles that change to a single peak later on. However, the Mg ii h 2803.53 Å line first shows flat-top profiles that change into double peaks followed by the single peak. To study the shock dynamics, we isolate the shock component from the spectra by fitting two Gaussians. We find that the lifetime of the shock is largest in the Mg ii h 2803.53 Å line. Moreover, the plasma motion shows both the acceleration and deceleration phases of the shock. Yet, in C ii 1335.71 Å and Si iv 1393.75 Å, only the deceleration phase is observed. We observe a strong correlation between the largest blueshift of the shock and deceleration for all three spectral lines. We find a positive (negative) correlation between intensities contributed by the shocks in Mg ii and C ii (Si iv). This suggests that the shocks are first amplified in C ii, followed by a decline in the height range corresponding to Si iv. These results may indicate the dissipation of shocks above the formation height of C ii, and the shocks may have important roles in the dynamics of the upper chromosphere and transition region above sunspots.

Intensity and Coherence Characteristics of a Radial Phase-Locked Multi-Gaussian Schell-Model Vortex Beam Array in Atmospheric Turbulence

The theoretical descriptions for a radial phase-locked multi-Gaussian Schell-model vortex (RPLMGSMV) beam array is first given. The normalized intensity and coherence distributions of a RPLMGSMV beam array propagating in free space and atmospheric turbulence are illustrated and analyzed. The results show that a RPLMGSMV beam array with larger total number N or smaller coherence length σ can evolve into a beam with better flatness when the beam array translating into the flat-topped profile at longer distance z and the flatness of the flat-topped intensity distribution can be destroyed by the atmospheric turbulence at longer distance z. The coherence distribution of a RPLMGSMV beam array in atmospheric turbulence at the longer distance will have Gaussian distribution. The research results will be useful in free space optical communication using a RPLMGSMV beam array.

Rotor segment split and its optimization of axial-field dual-rotor segmented switched reluctance machine

Switched reluctance machine (SRM) produces vibration and acoustic noise due to the significant torque ripple. The widespread use of SRM is thus limited. For the discussed axial-field dual-rotor segmented switched reluctance machine (ADS-SRM), this paper describes a proposal for a new rotor profile featured by the rotor segment splits. These splits minimize the undesired torque ripple. The effects of each geometric parameter, such as split width and split offset, are investigated using finite-element method (FEM). After optimizing the rotor-segment geometry, the approximately flat-top torque profile is achieved. The motion-coupled analysis further validates the effectiveness of this novel technique. It is found that, by incorporating the optimization of the turn-on and turn-off angles, the torque-ripple coefficient decreases by about 73%, from 0.995 to 0.27. However, the average torque decreases by only 4.2%.

The effects of 808-nm near-infrared laser light irradiation on actin cytoskeleton reorganization in bone marrow mesenchymal stem cells.

Tailoring the cell organelles and thus changing cell homeostatic behavior has permitted the discovery of fascinating metabolic features enabling enhanced viability, differentiation, or quenching inflammation. Recently, photobiomodulation (PBM) has been accredited as an effective cell manipulation technique with promising therapeutic potential. In this prospective, in vitro results revealed that 808-nm laser light emitted by a hand-piece with a flat-top profile at an irradiation set up of 60J/cm2 (1W, 1W/cm2; 60s, continuous wave) regulates bone marrow stromal cell (BMSC) differentiation toward osteogenesis. Considering the importance ofactin cytoskeleton reorganization, which controls a range of cell metabolic activities, comprising shape change, proliferation and differentiation, the aim of the current work is to assess whether PBM therapy, using a flat-top hand-piece at higher-fluence irradiation on BMSCs, is able to switch photon signals into the stimulation of biochemical/differentiating pathways involving key activators that regulate de novo actin polymerization. Namely, for the first time, we unearthed the role of the flat-top hand-piece at higher-fluence irradiation on cytoskeletal characteristics of BMSCs. These novel findings meet the needs of novel therapeutically protocols provided by laser treatment and the manipulation of BMSCs as anti-inflammatory, osteo-inductive platforms.

Longitudinal phase space manipulation with planar corrugated wakefield structures

Wakefield structures have found a broad range of applications in beam longitudinal phase space manipulation in recent years. In this paper, we report on several electron beam manipulation experiments with planar corrugated wakefield structures following a photocathode RF gun, in which the bunch length of the electron beams are much longer, comparable, and much shorter than the characteristic wavelength of the corrugated structures, respectively. (1) A 15.6-ps beam forms a 0.17-THz bunch train via ballistic bunching, after gaining periodic energy modulation through these structures via self-induced wakefield. (2) For a 6-ps beam, the combination of wake structures and a drift converts its flat-top longitudinal profile into a ramped shape, remarkably improving its capability of efficiently exciting electromagnetic fields in plasma. (3) A 0.2-pC, 250-fs electron pulse subject to a drive beam’s wake is manipulated for different purposes, such as acceleration or deceleration with a ±200 keV gain in energy; or compression to ∼70 fs via ballistic compression. The results illustrate the unique and versatile capability of wakefield structures in beam manipulation for various applications. The simplicity, robustness, and inherent synchronization of such devices may open up many new possibilities in accelerator facilities.

Quantum limited source localization and pair superresolution in two dimensions under finite-emission bandwidth

Optically localizing a single quasi-monochromatic source to sub-diffractive precisions entails, in the photon-counting limit, a minimum photon cost that scales as the squared ratio of the width, $w$, of the optical system's point-spread function (PSF) and the sought localization precision, $d$, i.e., as $\alpha(w/d)^2$. For sources with a finite emission-frequency spectrum, while the inverse quadratic scaling is expected to remain unchanged, the coefficient $\alpha$ must increase due to a degrading fidelity of localization as the imaging bandwidth increases and PSF undergoes a frequency-dependent widening. We specifically address how rapidly $\alpha$ must increase with increasing width of a flat-top spectral profile of emission of a point source being localized in two dimensions by an imager with a clear circular aperture by calculating quantum Fisher information (QFI), whose inverse yields the lowest possible unbiased-estimation variance of source-localization error. The novel use of prolate spheroidal wave functions as a basis for obtaining a solution of the eigenvalue problem of the single-photon density operator needed for the QFI calculation helps us develop the notion of an effective dimensionality of the continuous-state problem in terms of the associated space-bandwidth parameter. We subsequently extend our considerations of QFI to treat the finite-bandwidth pair superresolution problem in two dimensions, obtaining similar results. We also consider generalizations to emission power spectra of arbitrary profiles.

Laser gain module small-signal gain increase due to suppression of parasitic generation with pump geometry optimization

A method for optical pump scheme optimization for a laser gain module with pulsed diode arrays for Nd:YAG rods (Ø10 × 130 mm & Ø10 × 160 mm) is presented. The goal is to maximize the small-signal gain of the laser gain module while maintaining a flat-top amplification profile. A possible way of the overall small-signal gain reduction due to amplified spontaneous emission is discussed and a method to deal with this phenomenon is given. As a result of optical pump scheme optimization, the gain module with the maximum pump power of 40 kW for 250 µs pulse duration at repetition rates of up to 25 Hz is created. The maximum small-signal gain G = 100 at 1.064 is achieved.

Discovery of optical outflows and inflows in the black hole candidate GRS 1716−249

ABSTRACT We present optical spectroscopy obtained with the GTC, VLT, and SALT telescopes during the decline of the 2016–2017 outburst of the black hole candidate GRS 1716−249 (Nova Oph 1993). Our 18-epoch data set spans 6 months and reveals that the observational properties of the main emission lines are very variable, even on time-scales of a few hours. Several epochs are characterized by P-Cyg (as well as flat-top and asymmetric) profiles in the Hα, Hβ, and He ii (λ4686) emission lines, implying the presence of an accretion disc wind, which is likely hot and dense. The wind’s terminal velocity (∼2000 km s−1) is similar to that observed in other black hole X-ray transients. These lines also show transient and sharp red-shifted absorptions, taking the form of inverted P-Cyg profiles. We argue that these profiles can be explained by the presence of infalling material at ∼1300 km s−1. We propose a failed wind scenario to explain this inflow and discuss other alternatives, such as obscuration produced by an accretion-related structure (e.g. the gas stream) in a high inclination system.

Two-color flat-top solitons in microresonator-based optical parametric oscillators

We studied numerically the generation of the two-color flat-top solitonic pulses, platicons, in the microresonator based doubly resonant optical parametric oscillator. We revealed that if the signs of the group velocity dispersion (GVD) coefficients at interacting harmonics are opposite, platicon excitation is possible via pump amplitude modulation or controllable mode interaction approach. Upon pump frequency scan generation of platicons was observed at positive pump frequency detunings for the normal GVD at pump frequency and at negative detunings in the opposite case. Interestingly, we found the effect of the transformation of the flat-top platicon profile at half-frequency into the bell-shaped bright soliton profile upon frequency scan. For platicon excitation one needs simultaneous accurate matching of the microresonator free spectral ranges at interacting harmonics and resonant eigenfrequencies. Excitation conditions and platicon generation domains were found for the different generation methods, and properties of the generated platicons were studied for various combinations of the medium parameters.

Subterahertz refractive flat-top beam shaping via 3D printed aspheric lens combination

The viability of 3D printed aspheric lenses for the purpose of frequency-scalable subterahertz Gaussian to flat-top beam shaping is evaluated. A cylindrical one-dimensional Fresnel–Kirchhoff diffraction equation was implemented in Matlab and used to design a pair of aspheric lenses with customized vertex radius of curvature and conic constant. The lenses were printed at maximum possible resolution in acrylonitrile butadiene styrene ( n = 1.6 ) and tested with a 102 GHz continuous-wave subterahertz source. The aspheric lens combination produced a flat-top profile from a low-quality incident Gaussian beam at the minimal cost of 3D printing substrate. The flat-top profile exhibited small ( < 14 % root-mean-square deviation over a flat region) intensity fluctuations and is expected to prove useful in future terahertz applications that require a high degree of beam uniformity.

Heuristic self-photography approach for flat-top beam generation with arbitrary beam intensity distribution

Flat-top beam (FTB) is a type of laser illumination with broad applications. To achieve an easily accessible and cost-effective flat-top intensity profile, a self-photography approach was developed in this study. Utilizing the linear region of the Hurter–Driffield (H&D) curve, we developed an analytical formulation and simulation approach for FTB power optimization with a Gaussian beam (GB) and other intensity profiles. Notably, when a GB profile was used, the maximum transmission efficiency of an FTB depended on the lowest photographic density within the linear region of the H&D curve. Various beam profiles and experimental conditions were evaluated through numerical simulations. We performed experiments to verify the simulation results, which exhibited power efficiency of up to 14.89 % when using a GB. Current beam reshaping techniques use phase modulation; our self-photography method provides an alternative approach to generating uniform illumination with arbitrary intensity profiles, regardless of beam coherence properties. This newly developed cost-effective film approach can be easily incorporated into systems without excessive additional components. This straightforward implementation can be adapted for various illumination needs, such as high-order beam modes and multiple beam sources.

Anisotropic two-photon absorbers measured by the Z-scan technique and its application in laser beam shaping

We investigate anisotropic two-photon absorption (TPA) effects in a cubic-symmetry crystal excited by scalar and vectorial optical fields. First, we present the anisotropic TPA coefficient, which depends on the anisotropy coefficient, the dichroism coefficient, the crystal orientation angle, and the ellipticity of the polarized ellipse. Second, we develop the elliptically polarized light Z-scan technique for characterizing anisotropic two-photon absorbers, which is demonstrated experimentally. Last, we present the laser beam shaping of vectorial optical fields with Gaussian intensity distribution into a flat-top profile through anisotropic TPA effects. It is shown that the anisotropic TPA offers a new avenue to manipulate the intensity distribution of the polarization-structured light field, which may find interesting applications in beam shaping, optical limiting, and photodetection.

Impact of laser stacking and photocathode materials on microbunching stability in photoinjectors

Microbunching instability is a well-known phenomenon that may deteriorate the performance of accelerators. The instability may be triggered by a shot-noise mechanism or by some initial intensity modulations at the generation of the electron bunch (or both) and can be amplified all along the machine. At SwissFEL, the free-electron laser (FEL) facility operating at the Paul Scherrer Institute (PSI), the initial design stipulated a shaping of the photocathode laser output to obtain a flat-top longitudinal profile. This scheme is attractive in terms of the uniformity of the beam properties along the bunch. The drawback of this approach is that some unavoidable modulations are generated along the laser pulse. We investigate, both experimentally and by numerical simulations, the longitudinal dynamics of a beam obtained illuminating a copper cathode with a laser profile shaped by the stacking technique. We repeat the analysis for several compression factors and initial laser profile modulations. We find that the microbunching instability gain renders the use of the stacking technique not efficient to run a free-electron laser facility using as photocathode a material with a short response time. We experimentally demonstrate that the use of a material with a longer response time efficiently damps the structures originating from the laser profile obtained with stacking, and helps to improve the performance of the facility. In general, this is an approach to minimize the microbunching instability at any FEL (also not using stacking) or at least reduce the use of other countermeasures, which, such as the laser heater, may degrade the final FEL performance.

Development of shock-dynamics study with synchrotron-based time-resolved X-ray diffraction using an Nd:glass laser system.

The combination of high-power laser and synchrotron X-ray pulses allows us to observe material responses under shock compression and release states at the crystal structure on a nanosecond time scale. A higher-power Nd:glass laser system for laser shock experiments was installed as a shock driving source at the NW14A beamline of PF-AR, KEK, Japan. It had a maximum pulse energy of 16 J, a pulse duration of 12 ns and a flat-top intensity profile on the target position. The shock-induced deformation dynamics of polycrystalline aluminium was investigated using synchrotron-based time-resolved X-ray diffraction (XRD) under laser-induced shock. The shock pressure reached up to about 17 GPa with a strain rate of at least 4.6 × 107 s-1 and remained there for nanoseconds. The plastic deformation caused by the shock-wave loading led to crystallite fragmentation. The preferred orientation of the polycrystalline aluminium remained essentially unchanged during the shock compression and release processes in this strain rate. The newly established time-resolved XRD experimental system can provide useful information for understanding the complex dynamic compression and release behaviors.

Investigating helium–deuterium synergies in plasma-exposed tungsten using laser ablation techniques

This article describes the application of laser ablation based techniques to quantify the depth-dependent concentrations of gaseous species below a tungsten surface. In this work, we use a Quantel QSMART Nd:YAG 60 mJ, 5 ns frequency doubled laser that operates at a 532 nm wavelength, which we couple to an MVAT energy modulator to decrease the laser energy to 1.8 mJ. We use this laser to ablate tungsten and then characterized the resulting laser-induced plasma via laser-induced breakdown spectroscopy (LIBS). The ablated gases are pumped into a quadropole mass spectrometer to perform laser ablation mass spectroscopy (LAMS). The coupled LIBS–LAMS technique is demonstrated using a polycrystalline tungsten specimen that was helium ion implanted with variable helium ion energies to create an approximately 200 atomic parts per million flat-top concentration profile. LIBS–LAMS measurements are then performed on polycrystalline tungsten exposed to helium only, deuterium only, or 90% deuterium-10% helium plasmas on the linear plasma device PISCES/A using either a 75 or 250 eV bias voltage. The LIBS–LAMS measurements clearly indicate that exposure to helium during mixed deuterium–helium plasma exposure leads to an increased near surface deuterium concentration but reduces deuterium permeation below the tungsten surface.

Twisted elliptical multi-Gaussian Schell-model beams and their propagation properties.

We introduce a new kind of partially coherent source whose cross-spectral density (CSD) function is described as the incoherent superposition of elliptical twisted Gaussian Schell-model sources with different beam widths and transverse coherence widths, named twisted elliptical multi-Gaussian Schell-model (TEMGSM) beams. Analytical expression for the CSD function propagating through a paraxial ABCD optical system is derived with the help of the generalized Collins formula. Our results show that the TEMGSM beam is capable of generating a flat-topped elliptical beam profile in the far field, and the beam spot during propagation exhibits clockwise/anti-clockwise rotation with respect to its propagation axis. In addition, the analytical expressions for the orbital angular momentum (OAM) and the propagation factor are also derived by means of the Wigner distribution function. The influences of the twisted factor and the beam index on the OAM and the propagation factor are studied and discussed in detail.

Integrated flat-top reflection filters operating near bound states in the continuum

We propose and theoretically and numerically investigate narrowband integrated filters consisting of identical resonant dielectric ridges on the surface of a single-mode dielectric slab waveguide. The proposed composite structures operate near a bound state in the continuum (BIC) and enable spectral filtering of transverse-electric-polarized guided modes propagating in the waveguide. We demonstrate that by proper choice of the distances between the ridges, flat-top reflectance profiles with steep slopes and virtually no sidelobes can be obtained using just a few ridges. In particular, the structure consisting of two ridges can optically implement the second-order Butterworth filter, whereas at a larger number of ridges, excellent approximations to higher-order Butterworth filters can be achieved. Owing to the BIC supported by the ridges constituting the composite structure, the flat-top reflection band can be made arbitrarily narrow without increasing structure size. In addition to the filtering properties, the investigated structures support another type of BIC—the Fabry–Perot BIC—arising when the distances between adjacent ridges meet the Fabry–Perot resonance condition. In the vicinity of the Fabry–Perot BIC, an effect similar to electromagnetically induced transparency is observed, namely, sharp transmittance peaks against the background of a wide transmittance dip.