Gaussian-like Pulses Research Articles

Microwave fractional Hilbert transformer implemented in a dispersion-tailored few-mode fiber

We experimentally demonstrate, for the first time to our knowledge, a microwave fractional Hilbert transformer in a few-mode fiber using a transversal filtering approach. The filter taps are provided by a tunable true-time delay line that is realized by exploiting the spatial dimension of a dispersion-engineered double-clad step-index few-mode fiber. Both the fractional order and operational bandwidth of the fractional Hilbert transformer can be continuously tuned by adjusting the tap coefficients and varying the operational optical wavelength, respectively. The magnitude and phase response for different fractional orders, ranging from 0.17 to 1.00 that correspond to phase shifts of 15° to 90°, are measured. Operational bandwidths of 7.4 to 10.6 GHz are demonstrated for a classical Hilbert transformer. Real-time temporal fractional Hilbert transform of a Gaussian-like pulse is also performed. Our results are in good agreement with theory, validating the viability of our approach for implementation of microwave fractional Hilbert transformers.

Sub-gigahertz bandwidth all-optical tunable differentiator with high-energy efficiency based on a micro-ring resonator.

We propose and demonstrate a tunable fractional-order photonic differentiator (DIFF) that can process input pulses with a sub-gigahertz bandwidth. Our scheme utilizes the self-induced optical modulation effect observed in a silicon-on-insulator micro-ring resonator. Gaussian-like pulses with varying pulse widths between 7.5 and 20 ns are employed for differentiation, achieving an energy efficiency over 45%, to the best of our knowledge, which surpasses all previously reported schemes for input pulses with a sub-gigahertz bandwidth. We simulate the temporal dynamics of pulses to gain insight into the physical mechanisms underlying the differentiated outputs and provide a method for differentiation order adjustment, which is experimentally realized using an all-optical pump-probe technique.

Effect of wiggler magnetic field on wakefield excitation and electron energy gain in laser wakefield acceleration

Abstract Laser wakefield acceleration is a frequently utilised research methodology for enhancing the energy levels of lighter charged particles, specifically electrons, to relativistic magnitudes. In this investigation, we utilised a linear polarised Gaussian-like laser pulse that propagated along the z-axis through cold collisionless underdense plasma in weakly nonlinear regime. An external planer magnetic wiggler field is applied along the y-axis. The influence of various critical parameters, such as amplitude and propagation constant of wiggler magnetic field, amplitude of laser electric field and laser pulse length on the wakefield and electron energy gain has been studied. A wiggler-assisted laser wakefield accelerator, the electron energy and wakefield evolution can be tuned by the wiggler magnetic field strength. The numerical findings demonstrate that by varying the strength of wiggler magnetic field and laser electric field, the amplitude of the wakefield is affected significantly. Furthermore, the equality of the order of pulse length and plasma wavelength is essential to obtain energy efficient acceleration mechanism. By employing specific parameters, a maximum energy increase of 2.26 GeV is achieved. This research will aid in the development of an energy-efficient electron acceleration technology by choosing suitable laser and plasma parameters.

Systematic performance of the ASKAP fast radio burst search algorithm

ABSTRACT Detecting fast radio bursts (FRBs) requires software pipelines to search for dispersed single pulses of emission in radio telescope data. In order to enable an unbiased estimation of the underlying FRB population, it is important to understand the algorithm efficiency with respect to the search parameter space and thus the survey completeness. The Fast Real-time Engine for Dedispersing Amplitudes (fredda) search pipeline is a single pulse detection pipeline designed to identify radio pulses over a large range of dispersion measures (DM) with low latency. It is used on the Australian Square Kilometre Array Pathfinder (ASKAP) for the Commensal Real-time ASKAP Fast Transients (CRAFT) project. We utilize simulated single pulses in the low- and high-frequency observation bands of ASKAP to analyse the performance of the pipeline and infer the underlying FRB population. The simulation explores the signal-to-noise ratio (S/N) recovery as a function of DM and the temporal duration of FRB pulses in comparison to injected values. The effects of intrachannel broadening caused by dispersion are also carefully studied in this work using control data sets. Our results show that for Gaussian-like single pulses, >85 per cent of the injected signal is recovered by pipelines such as fredda at DM <3000 pc cm−3 using standard boxcar filters compared to an ideal incoherent dedispersion match filter. Further calculations with sensitivity implies at least ∼10 per cent of FRBs in a Euclidean universe at target sensitivity will be missed by fredda and heimdall, another common pipeline, in ideal radio environments at 1.1 GHz.

Generation of THz radiation by wakefield under a wiggler magnetic field: An analytical study

In this article, the generation of terahertz (THz) radiation owing to wakefields produced by Gaussian-like laser pulse propagating throughout magnetized plasma has been investigated analytically under a wiggler magnetic field. Non-linear equations and analytical formulations governing the laser pulse THz radiation generation at a mildly relativistic regime have been derived. Besides, the total electric and magnetic wakefields inside and behind the laser pulse by employing the relativistic equations of motion, Maxwell’s equations, and perturbation technique in the attendance of a planar magnetostatic wiggler have been obtained. It is shown that the maximum transverse electric wakefield amplitude has an increasing trend with enchanting the wiggler frequency for inside and behind the laser pulse and the peak of the maximum transverse electric wakefield amplitude behind of laser pulse is higher compared to the inside of laser pulse for different values of the wiggler magnetic field. Furthermore, it is observed that electric wakefield amplitude has a fluctuating behavior inside and behind the laser pulse due to the existence of the planar magnetostatic wiggler. Also, it is shown that by using a wiggler magnetic field and components of perturbed velocities, we can generate THz waves. Moreover, it is found that we can achieve the needed values of the wiggler magnetic field for THz radiation generation for different values of the laser strength parameters. Indeed, we can control the generation of THz radiation throughout magnetized plasma by adjusting the wiggler magnetic field and laser pulse intensity.

Mutual manipulation between a dark soliton and an Airy pulse at the optical event horizon

A fiber-optical analog of the event horizon is proposed via the collision dynamics between a dark soliton and an Airy pulse for the first time, under the effect of soliton-induced Kerr nonlinear effect. An Airy wave packet is more effective in manipulating the properties of a dark soliton than a Gaussian-like pulse in the regime of the optical event horizon since the Airy-soliton horizon can be regarded as the successively isolated horizon processes. Besides, the initial chirp and attenuation factor are two key regulators of the Airy pulse, which can not only adjust the flatness and width of the output spectrum during the collision process but also modify the properties of the output dark soliton. The interesting dynamic investigated in this work as a process of mutual manipulation can be considered as a promising candidate for frequency conversion and broadband supercontinuum generation, which proves the theory basis for further understanding of the light-by-light controlling in a more controllable manner.

Numerical study of converting beat-note signals of dual-frequency lasers to optical frequency combs by optical injection locking of semiconductor lasers

The optical injection locking of semiconductor lasers to dual-frequency lasers is studied by numerical simulations. The beat-note signals can be effectively transformed to optical frequency combs due to the effective four wave-mixing in the active semiconductor gain medium. The low-noise Gaussian-like pulse can be obtained by locking the relaxation oscillation and compensating the gain asymmetry. The simulations suggest that pulse trains of width below 30 ps and repetition rate in GHz frequency can be generated simply by the optical injection locking of semiconductor lasers. Since the optical injection locking can broaden the spectrum and amplify the optical power simultaneously, it can be a good initial stage for generating optical frequency combs from dual-frequency lasers by multi-stage of spectral broadening in nonlinear waveguides.

Spectral Engineering of Tamm Plasmon Resonances in Dielectric Nanoporous Photonic Crystal Sensors.

Model light-confining Tamm plasmon cavities based on gold-coated nanoporous anodic alumina photonic crystals (TMM-NAA-PCs) with spectrally tunable resonance bands were engineered. Laplacian and Lorentzian NAA-PCs produced by a modified Gaussian-like pulse anodization approach showed well-resolved, high-quality photonic stopbands, the position of which was precisely controlled across the visible spectrum by the periodicity in the input anodization profile. These PC structures were used as a platform material to develop highly reflective distributed Bragg mirrors, the top sides of which were coated with a thin gold film. The resulting nanoporous hybrid plasmonic-photonic crystals showed strong light-confining properties attributed to Tamm plasmon resonances at three specific positions of the visible spectrum. These structures achieved high sensitivity to changes in refractive index, with a sensitivity of ∼106 nm RIU-1. The optical sensitivity of TMM-NAA-PCs was assessed in real time, using a model chemically selective binding interaction between thiol-containing molecules and gold. The optical sensitivity was found to rely linearly on the spectral position of the Tamm resonance band, for both Laplacian and Lorentzian TMM-NAA-PCs. The density of self-assembled monolayers of thiol-containing analyte molecules formed on the surface of the metallic film directly contributes to the dependence of sensitivity on TMM resonance position in these optical transducers. Our findings provide opportunities to integrate TMM modes in NAA-based photonic crystal structures, with promising potential for optical technologies and applications requiring high-quality surface plasmon resonance bands.

Mathematical Model for Gaussian-Like Pulse Design for a UWB System Based on the Generalized Bessel Polynomials

Mathematical Model for Gaussian-Like Pulse Design for a UWB System Based on the Generalized Bessel Polynomials

Optical engineering of nanoporous photonic crystals by Gaussian-Like pulse anodization

Abstract A new class of nanoporous anodic alumina photonic crystals (NAA–PCs) are optically engineered by four distinct forms of Gaussian-like pulse anodization (GLPA) – Gaussian, Lorentzian, logarithmic normal, and Laplacian. NAA–PCs produced by GLPA under current density control conditions show well-resolved, spectrally tunable photonic stopbands (PSBs), the features of which – central wavelength, full width at half maximum, intensity, and quality factor – can be precisely tuned across the UV–visible spectrum by the input Gaussian-like current density pulses. Effects of type, width, and period of Gaussian-like current density pulses on the structural and optical properties of NAA–PCs are systematically assessed. Comprehensive electrochemical, structural, and optical characterizations allow us to elucidate the interplay of input GLPA profile and structural features in determining forbidden light propagation within these NAA–PCs. Sensitivity of these PC structures toward changes in their effective medium are determined by quantifying spectral shifts in their characteristic PSB upon infiltration of their nanoporous structure with analytical solutions of alcohol mixtures with varying refractive index, from 1.362 to 1.383 RIU. It is found that, at fixed anodization period, NAA–PCs produced by Laplacian current density pulses achieve the highest sensitivity (105 ± 4 nm RIU−1) and, for a given type of GLPA, NAA–PCs produced with longer anodization period achieve higher sensitivity (123 ± 10 nm RIU−1 for NAA–PCs fabricated by Gaussian pulses with a 1400 s-period). Our advances provide exciting new opportunities for future developments of high-quality NAA–PCs with broad applicability across various photonic technologies, including optical sensing, photocatalysis, lasing, and optical encoding.

Super Gaussian pulse generation in a passively mode-locked fiber laser

The generation of self-similar super Gaussian-like pulses from a passively mode-locked fiber laser is numerically demonstrated. The laser cavity utilizes a long period fiber grating as a pulse-shaping element. The model is based on the normalized complex Ginzburg-Landau equation and the nonlinear coupled mode equations of the grating. The numerical results show stable super Gaussian pulses with a linear frequency chirp, a 1.5 ps pulse duration and 2 nJ energy. The generated pulses are scalable and exhibits no bifurcation or instability as the gain is increased.

ASE narrow-band noise pulsing in erbium-doped fiber amplifier and its effect on self-phase modulation.

In this paper, we report a study of the features of polarized and unpolarized narrow-band amplified spontaneous emission (ASE) in a low-doped erbium fiber at 976-nm pumping. We demonstrate that ASE noise can be treated as a train of Gaussian-like pulses with random magnitudes, widths, and inter-pulse intervals. ASE noise can also provide a statistical analysis of these three parameters. We also present the data that reveal ASE noise's role in optical spectrum broadening through self-phase modulation of light propagating in a communication fiber. In particular, the data show that the ASE noise derivative defines the broadening's spectral shape.

Slowing 80-ns light pulses by four-wave mixing in potassium vapor

We experimentally and theoretically study propagation of 80-ns Gaussian-like probe pulses in hot potassium vapor under conditions of four-wave mixing (FWM). The atomic scheme for FWM is off-resonant, double-$\mathrm{\ensuremath{\Lambda}}$ atomic scheme, with pump and probe photons, mediated in the K vapor, generating new probe and conjugate photons. We define the subset of FWM parameters, one-photon pump detuning, two-photon pump-probe Raman detuning, vapor density, the pump Rabi frequencies, when slowed pulses exit the vapor are also Gaussian-like. When Gaussian-like pulses exit the cell we are able to compare theoretical and experimental results for fractional delays and broadening for the probe and conjugate. We have obtained fractional delays above 1. Results of the model are compared with the experiment, with and without the model of Doppler averaging, when the atom velocity distribution is divided into different number of groups. We analyze possible causes for pulse broadening and distortion of slowed probe pulses and show that they are the result of quite different behavior of the probe pulse in the FWM vapor. Besides presenting the first results of slowing 80-ns probe pulses, this work is a useful test of the numerical model and values of parameters taken in the model that are not known in experiments.

Reconfigurable microwave signal processor with a phase shift of π.

We propose and experimentally demonstrate a reconfigurable microwave signal processor, with a bandwidth up to tens of gigahertz. In this technique, any microwave signal processing function with a phase shift of π could be performed by shaping the input optical intensity spectrum. The phase shift of π is implemented by using a differential detection. Thanks to the broad bandwidth provided by the incoherent optical source and the high resolution of the user-defined optical filter, the frequency response of our approach could be in a great agreement with that of an ideal signal processing function. In the experiment, temporal intensity Hilbert transformations and temporal intensity differentiations of Gaussian-like pulses with widths of 125ps, 85ps and 68ps are accurately achieved.

All-optical temporal fractional order differentiator using an in-fiber ellipsoidal air-microcavity * * Project supported by the the National Natural Science Foundation of China (Nos. 61522509, 61377002, 61535012), the National High-Tech Research & Development Program of China (No. SS2015AA011002), and the Beijing Natural Science Foundation (No. 4152052). Ming Li was supported in part by the Thousand Young Talent Program.

An all-optical temporal fractional order differentiator with ultrabroad bandwidth (~1.6 THz) and extremely simple fabrication is proposed and experimentally demonstrated based on an in-fiber ellipsoidal air-microcavity. The ellipsoidal air-microcavity is fabricated by splicing a single mode fiber (SMF) and a photonic crystal fiber (PCF) together using a simple arc-discharging technology. By changing the arc-discharging times, the propagation loss can be adjusted and then the differentiation order is tuned. A nearly Gaussian-like optical pulse with 3 dB bandwidth of 8 nm is launched into the differentiator and a 0.65 order differentiation of the input pulse is achieved with a processing error of 2.55%.

Simultaneous emission of Gaussian-like and parabolic-like pulse waveforms in an erbium-doped dual-wavelength fiber laser

We report on the observation of different pulse formation dynamics in a nonlinear polarization evolution (NPE)-based broadband erbium-doped fiber laser when the net cavity group-velocity dispersion (GVD) is managed to be close to zero. The fiber laser can generate pulses with a single wavelength or dual wavelengths by adjusting the waveplates. When the laser operates in dual-wavelength emission, the output pulses corresponding to the two wavelengths exhibit Gaussian- and parabolic-like waveforms, respectively, indicating that the laser can simultaneously operate in stretched-pulse and self-similar regimes. The generation of dual-wavelength emission with different pulse waveforms can be attributed to an overdriven NPE switch acting on a chirped broadband pulse and different dispersion mechanisms. These findings provide a good foundation for comprehensively studying pulse formation dynamics in laser cavities.

Phase-sensitive OTDR probe pulse shapes robust against modulation-instability fading

Typical phase-sensitive optical time-domain reflectometry (ϕOTDR) schemes rely on the use of coherent rectangular-shaped probe pulses. In these systems, there is a trade-off between the signal-to-noise ratio (SNR), spatial resolution, and operating range of the ϕOTDR system. To increase any of these parameters, an increase in the pulse peak power is usually indispensable. However, as it is well known, there is a limit in the allowable increase in probe power due to the onset of undesired nonlinear effects such as modulation instability. In this Letter, we perform an analysis of the effect of the probe pulse shape on the visibility fading due to modulation instability. In particular, four different temporal profiles are chosen: rectangular, Gaussian, triangular, and super-Gaussian (order 2). Our numerical and experimental analyses reveal that the use of triangular or Gaussian-like pulses can significantly inhibit the visibility fading issues. As such, an increase in the range up to twofold for the same pulse energy (i.e., SNR) and nominal spatial resolution can be achieved, as compared with the results obtained when using rectangular pulses. This is due to a more robust behavior of the Gaussian and triangular pulses against the Fermi-Pasta-Ulam recurrence occurring in modulation instability.

Reconfigurable symmetric pulses generation using on-chip cascaded optical differentiators.

We report a type of programmable pulse shaping method based on cascaded frequency-detuned optical differentiators. By properly adjusting the central wavelength of each differentiator, a large variety of symmetric pulses can be generated from a transform-limited Gaussian-like pulse. We numerically and experimentally demonstrate the generation of flat-top, parabolic and triangular pulses with tunable pulse widths from a 20-ps Gaussian-like pulse, using no more than three cascaded differentiators. It can be found that as more differentiators are used, higher synthesized accuracy and larger tuning range of pulse widths can be obtained in general. Additionally, in our experiment, we design and fabricate thermally tunable delay interferometers on the silicon-on-insulator (SOI) platform to work as optical differentiators, which can help us realize the shaping system with small footprint (943μm × 395μm) and high stability.

Acoustic time-of-flight for proton range verification in water.

Measurement of the arrival times of thermoacoustic waves induced by pulsed proton dose depositions (protoacoustics) may provide a proton range verification method. The goal of this study is to characterize the required dose and protoacoustic proton range (distance) verification accuracy in a homogeneous water medium at a hospital-based clinical cyclotron. Gaussian-like proton pulses with 17 μs widths and instantaneous currents of 480 nA (5.6 × 10(7) protons/pulse, 3.4 cGy/pulse at the Bragg peak) were generated by modulating the cyclotron proton source with a function generator. After energy degradation, the 190 MeV proton pulses irradiated a water phantom, and the generated protoacoustic emissions were measured by a hydrophone. The detector position and proton pulse characteristics were varied. The experimental results were compared to simulations. Different arrival time metrics derived from acoustic waveforms were compared, and the accuracy of protoacoustic time-of-flight distance calculations was assessed. A 27 mPa noise level was observed in the treatment room during irradiation. At 5 cm from the proton beam, an average maximum pressure of 5.2 mPa/1 × 10(7) protons (6.1 mGy at the Bragg peak) was measured after irradiation with a proton pulse with 10%-90% rise time of 11 μs. Simulation and experiment arrival times agreed well, and the observed 2.4 μs delay between simulation and experiment is attributed to the difference between the hydrophone's acoustic and geometric centers. Based on protoacoustic arrival times, the beam axis position was measured to within (x, y) = (-2.0, 0.5) ± 1 mm. After deconvolution of the exciting proton pulse, the protoacoustic compression peak provided the most consistent measure of the distance to the Bragg peak, with an error distribution with mean = - 4.5 mm and standard deviation = 2.0 mm. Based on water tank measurements at a clinical hospital-based cyclotron, protoacoustics is a potential method for measuring the beam's position (x and y within 2.0 mm) and Bragg peak range (2.0 mm standard deviation), although range verification will require simulation or experimental calibration to remove systematic error. Based on extrapolation, a protoacoustic arrival time reproducibility of 1.5 μs (2.2 mm) is achievable with 2 Gy of total deposited dose. Of the compared methods, deconvolution of the excitation proton pulse is the best technique for extracting protoacoustic arrival times, particularly if there is variation in the proton pulse shape.

Widely Continuous Tunable ps Pulse Train Generation Based on SOA Nonlinear Dynamics

We propose a simple setup based on nonlinear dynamics in a semiconductor optical amplifier (SOA) and solitonic compression for the generation of stable and tunable repetition rate picosecond (ps) optical pulses. A quantum-well SOA is used to amplify electrical-optic-modulated rectangular optical signal, during which the nonlinear amplification process induces front-edge distortion. Due to the non-zero linewidth enhancement factor, the front-edge distortion can be isolated by the optical filtration of the chirped frequency component, which eventually becomes Gaussian-like pulse train of tens of ps FWHM pulsewidth. Following a stage of solitonic compression and a stage of pedestal suppression, we are able to obtain a train of 1-ps high-quality pulses for which the root mean square timing jitter is $C$ -band.