Pulse Dispersion Research Articles

Dual spectral phase and diffraction angle compensation of a broadband AOM 4-f pulse-shaper for ultrafast spectroscopy.

AOM-based pulse shaping as a method has been shown to provide many advantages in the field of ultrafast spectroscopy, in particular for the creation of phase matched pulse pairs for two-dimensional IR and electronic spectroscopy. In this paper we demonstrate the capabilities of a quartz-based AOM pulse-shaper to provide fine control over the phase and spatial dispersion of ultrafast supercontinuum pulses. We show that by using the Bragg condition, we can define a mask function for our AOM such that the angle of diffraction is constant for all frequencies. By summing all the contributions to spectral phase due to normal and anomalous dispersion of our optical components, and taking into account the intrinsic frequency dependent phase as a result of the acoustic sine wave propagating through the AOM, we can determine an optimal mask function that meets the Bragg condition for all frequencies, and generates compressed (∼50 fs) supercontinuum pulses.

Dispersion characterization and pulse prediction with machine learning

In this work we demonstrate the efficacy of neural networks in the characterization of dispersive media. We also develop a neural network to make predictions for input probe pulses which propagate through a nonlinear dispersive medium, which may be applied to predicting optimal pulse shapes for a desired output. The setup requires only a single pulse for the probe, providing considerable simplification of the current method of dispersion characterization that requires frequency scanning across the entirety of the gain and absorption features. We show that the trained networks are able to predict pulse profiles as well as dispersive features that are nearly identical to their experimental counterparts. We anticipate that the use of machine learning in conjunction with optical communication and sensing methods, both classical and quantum, can provide signal enhancement and experimental simplifications even in the face of highly complex, layered nonlinear light-matter interactions.

On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses

High-energy narrowband terahertz (THz) pulses, relevant for a plethora of applications, can be created from the interference of two chirped-pulse drive lasers. The presence of third order dispersion, an intrinsic feature of many high-energy drive lasers, however, can significantly reduce the optical-to-THz conversion efficiency and have other undesired effects. Here, we present a detailed description of the effect of third-order dispersion (TOD) in the pump pulse on the generation of THz radiation via phase-matching of broadband highly chirped pulse trains. Although the analysis is general, we focus specifically on parameters typical to a Ti:Sapphire chirped-pulse amplification laser system for quasi-phase-matching in periodically-poled lithium niobate (PPLN) in the range of THz frequencies around 0.5 THz. Our analysis provides the tools to optimize the THz generation process for applications requiring high energy and to control it to produce desired THz waveforms in a variety of scenarios.

Group velocity distribution and short-pulse dispersion in a disordered transverse Anderson localization optical waveguide

Abstract We investigate the group velocity distribution of waveguide modes in the presence of disorder. The results are based on extensive numerical simulations of disordered optical waveguides using statistical methods. We observe that the narrowest distribution of group velocities is obtained in the presence of a small amount of disorder; therefore, the modal dispersion of an optical pulse is minimized when there is only a slight disorder in the waveguide. The absence of disorder or the presence of a large amount of disorder can result in a large modal dispersion due to the broadening of the distribution of the group velocities. We devise a metric that can be applied to the mode group index probability-density-function and predict the optimal level of disorder that results in the lowest amount of modal dispersion for short pulse propagation. Our results are important for studying the propagation of optical pulses in the linear regime, e.g., for optical communications; and the nonlinear regime for high-power short-pulse propagation.

Resonant X-ray Sum-Frequency-Generation Spectroscopy of K-Edges in Acetyl Fluoride.

Resonant X-ray sum-frequency generation is calculated for excitations of the fluorine and the oxygen core K-edge in acetyl fluoride using real-time time-dependent density functional theory. The signal is generated by an extreme-ultraviolet pulse followed by an X-ray pulse with variable delay T. The X-ray pulse is tuned to different element-specific core excitations and used to probe the dynamics of a valence electronic wave packet. A two-dimensional signal is recorded depending on the dispersed X-ray pulse frequency and the frequency conjugated to T, revealing the couplings between core and valence excited states. Molecular orbital decomposition of the signal reveals which regions of the molecule contribute to the X-ray excitation.

Ultrashort pulse propagation through depressed-cladding channel waveguides in YAG crystal: Spatio-temporal characterization

Inscription of optical waveguides by direct femtosecond laser irradiation has become a very versatile tool for the development of integrated photonic devices, such as waveguide lasers, frequency converters or photonic lanterns, among many others. The potential application of such devices for the control and manipulation of ultrashort pulses requires the precise knowledge of the temporal distortions that may be induced in the pulse propagation. Currently, research in this topic is scarce, and to our knowledge there is no previous experimental study on the spatio-temporal characterization at the output of waveguides inscribed inside crystals. Here, we have firstly fabricated depressed-cladding waveguides with different modal behavior in YAG crystal by direct femtosecond laser irradiation. Then, we implemented an experimental method based on the fiber coupler assisted spectral interferometry technique, that allows obtaining: (1) the temporal dispersion of a pulse at the output of an inscribed waveguide and, (2) full spatio-spectral and spatio-temporal characterization of the output of single-mode and multi-mode waveguides. Our results suggest that the main contribution to the pulse dispersion is due to the material dispersion. Moreover, we found that multimodal waveguides may induce an appreciable inhomogeneity in the temporal features of the pulses that needs to be taken into account in the design of complex devices.

Broadband dispersive Ge/YbF3 mirrors for mid-infrared spectral range.

Broadband dispersive mirrors operating in the mid-infrared spectral range of 6.5-11.5μm are developed for the first time, to the best of our knowledge. The mirrors comprise Ge and YbF3 layers, which have not been used before for manufacturing of multilayer dispersive optics. The design and production processes are described; mechanical stresses of the coatings are estimated based on experimental data; and spectral and phase properties of the produced mirrors are measured. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse.

Temporal pulse broadening due to dispersion and weak turbulence in FSO communications

In this paper, a novel analytical expression for the temporal pulse broadening caused by atmospheric dispersion and weak turbulence is derived. We developed an on-axis two-frequency mutual coherence function (MCF) for dispersive pulse propagating in weak atmospheric turbulence. Based on the derived temporal pulse broadening expression we formulated the signal-to-noise ratio (SNR) for Gaussian pulse propagating along a horizontal path, and we deducted an analytical expression for the bit error rate (BER) caused by dispersion and weak atmospheric turbulence effect on terrestrial free-space optical transmission system. Specific examples are presented for the pulse broadening due to weak turbulence and atmospheric dispersion. The results show that the temporal pulse broadening increases with propagation distance and decreases with low outer scale turbulence. The analytical BER results validated by using Monte Carlo simulations for on-off keying (OOK) intensity modulation/direct detection (IM/DD) show that BER due to pulse broadening is very significant when the propagation distance is increased. However, longer propagation distance with minimum bit error rate can be achieved by increasing the input power.

Analytical analysis and simulation on fringe field effect of deflector plates applied in ultrafast electron microscopy

Deflector plates are one of the critical components in ultrafast electron microscope (UEM), which generates the short electron pulse. While the electron pulse can be stretched due to the velocity dispersion induced by the fringe field of the deflector plates, understanding the effect of the fringe field on the electron velocity and how to avoid the adverse effect of it are needed in the design of the deflector plates. In this paper, firstly, an analytical model of the fringe field of deflector plates with double field input is given. This analytical model can be adapted to other new designs with different double field inputs, and as an example, the velocity dispersion, stretched time, and energy gain of an electron pulse are calculated with a linear double field input. Secondly, three sets of particle tracing simulations on electron velocities have been studied in terms of different single field inputs. The simulated results show that the longitudinal velocity of the electron changes very differently as a result of the different voltage sign on the deflector plates. In our model, the longitudinal velocity is almost constant until the electron leaves the deflector plates. So the results suggest that the deflector plates with double field input are recommended not only in UEM setups but also in other charged particle beam setups where needs high deflection accuracy.

Photonic Time-Stretch Technology with Prismatic Pulse Dispersion towards Fast Real-Time Measurements

Photonic time-stretch (PTS) technology enables revolutionary technical breakthroughs in ultrafast electronic and optical systems. By means of employing large chromatic dispersion to map the spectrum of an ultrashort optical pulse into a stretched time-domain waveform (namely, using the dispersive Fourier transformation), PTS overcomes the fundamental speed limitations of conventional techniques. The chromatic dispersion utilized in PTS can be implemented using multiple optical prism arrays, which have the particular advantages of low loss in the extended spectrum outside of the specific telecommunication band, flexibility, and cost-effectiveness. In this article, we propose and demonstrate the PTS technology established for a pair of prisms, which works as a data acquisition approach in ultrafast digitizing, imaging, and measurement regimes.

Compensation of Dispersion in Sinuous Antennas for Polarimetric Ground Penetrating Radar Applications

In order to improve the accuracy of subsurface target classification with ground penetrating radar (GPR) systems, it is desired to transmit and receive ultra-wide band pulses with varying combinations of polarization (a technique referred to as polarimetry). The sinuous antenna exhibits such desirable properties as ultra-wide bandwidth, polarization diversity, and low-profile form factor, making it an excellent candidate for the radiating element of such systems. However, sinuous antennas are dispersive since the active region moves with frequency along the structure, resulting in the distortion of radiated pulses. This distortion may be compensated in signal processing with accurately simulated or measured antenna phase information. However, in a practical GPR, the antenna performance may deviate from that simulated, accurate measurements may be impractical, and/or the dielectric loading of the environment may cause deviations. In such cases, it may be desirable to employ a simple dispersion model based on antenna design parameters which may be optimized in situ. This paper explores the dispersive properties of the sinuous antenna and presents a simple, adjustable, model that may be used to correct dispersed pulses. The dispersion model is successfully applied to both simulated and measured scenarios, thereby enabling the use of sinuous antennas in polarimetric GPR applications.

Sensitivity of Mammalian Cone Photoreceptors to Infrared Light

Two-photon vision arises from the perception of pulsed infrared (IR) laser light as color corresponding to approximately half of the laser wavelength. The physical process responsible for two-photon vision in rods has been delineated and verified experimentally only recently. Here, we sought to determine whether IR light can also be perceived by mammalian cone photoreceptors via a similar activation mechanism. To investigate selectively mammalian cone signaling in mice, we used animals with disabled rod signal transduction. We found that, contrary to the expected progressive sensitivity decrease based on the one-photon cone visual pigment spectral template, the sensitivity of mouse cone photoreceptors decreases only up to 800 nm and then increases at 900 nm and 1000 nm. Similarly, in experiments with the parafoveal region of macaque retinas, we found that the spectral sensitivity of primate cones diverged above the predicted one-photon spectral sensitivity template beyond 800 nm. In both cases, efficient detection of IR light was dependent on minimizing the dispersion of the ultrashort light pulses, indicating a non-linear two-photon activation process. Together, our studies demonstrate that mammalian cones can be activated by near IR light by a nonlinear two-photon excitation. Our results pave the way for the creation of a two-photon IR-based ophthalmoscope for the simultaneous imaging and functional testing of human retinas as a novel tool for the diagnosis and treatment of a wide range of visual disorders.

Laser pulse dispersion in underdense plasma and associated ion acceleration by relativistic self-induced transparency

The propagation of laser pulses in the underdense plasma is a very crucial aspect of the laser-plasma interaction process. In this work, we explored the two regimes of laser propagation in the plasma, one with a0 < 1 and the other with a0≳10. For the a0 < 1 case, we used a cold relativistic fluid model, wherein apart from immobile ions no further approximations are made. The effects of laser pulse amplitude, pulse duration, and plasma density are studied using the fluid model and compared with the expected scaling laws and also with the particle-in-cell (PIC) simulations. The agreement between the fluid model and the PIC simulations are found to be excellent. Furthermore, for the a0≳10 case, we used the PIC simulations alone. The delicate interplay between the conversion from the electromagnetic field energy to the longitudinal electrostatic fields results in dispersion, and so the redshift of the pump laser pulse. The dispersed pulse is then allowed to be incident on the subwavelength two-layer composite target. The underdense plasma before the target regulates the dispersion of the pulse. We observed an optimum pretarget plasma density which results in the acceleration of the ions from the secondary layer to ∼170 MeV by a ∼8 fs linearly polarized Gaussian laser pulse with ∼8.5 × 1020 W/cm2.

Second-order Talbot self-imaging effect in the time domain

Different from classical Talbot self-imaging, second-order spatial Talbot effect is a kind of second-order imaging, in which the self-imaging of an object is observed through intensity correlation measurement. Here, in terms of space-time duality, we investigate the temporal counterpart of second-order Talbot self-imaging with temporally incoherent light. Our work shows that second-order Talbot effect in the time domain is related to the joint dispersion of light pulse in dispersive medium. Particularly, based on the discrete Fourier transformation, the fractional Talbot self-imaging via intensity correlation doesn't generally possess a definite fractional period, which contradicts some articles' results. Our results may be useful for implementing the repetition-rate-multiplication technique and remotely transmitting the periodic temporal signals robustly.

Corrections to “Toward On-Chip MEMS-Based Optical Autocorrelator” [Oct 18 5003-5009

We propose a compact MEMS-based optical autocorrelator based on a micromachined Michelson interferometer in silicon and the two-photon absorption non-linearity in a photodetector. The miniaturized autocorrelator has a scanning range of 1.2 ps and operates in the wavelength range of 1100-2000 nm. The device measures the interferometric autocorrelation due to its collinear nature, from which the intensity autocorrelation can be calculated. The field autocorrelation can also be measured, from which the optical pulse spectrum can be calculated. A theoretical model based on Gaussian beam propagation is developed to study the effect of optical beam divergence, pulse dispersion, tilt angle between the interferometer mirrors, and amplitude mismatch between the interfering pulses. This model explains many of the effects observed in experimental measurements due to the use of a MEMS interferometer. The experimental results of autocorrelation signals for several pulses in the order of 100 fs are compared to a commercial autocorrelator and a good match is found.

Evidence for low density holes in Jupiter\u2019s ionosphere

Intense electromagnetic impulses induced by Jupiter’s lightning have been recognised to produce both low-frequency dispersed whistler emissions and non-dispersed radio pulses. Here we report the discovery of electromagnetic pulses associated with Jovian lightning. Detected by the Juno Waves instrument during its polar perijove passes, the dispersed millisecond pulses called Jupiter dispersed pulses (JDPs) provide evidence of low density holes in Jupiter’s ionosphere. 445 of these JDP emissions have been observed in snapshots of electric field waveforms. Assuming that the maximum delay occurs in the vicinity of the free space ordinary mode cutoff frequency, we estimate the characteristic plasma densities (5.1 to 250 cm−3) and lengths (0.6 km to 1.3 × 105 km) of plasma irregularities along the line of propagation from lightning to Juno. These irregularities show a direct link to low plasma density holes with ≤250 cm−3 in the nightside ionosphere.

Real-time and high-precision interrogation of a linearly chirped fiber Bragg grating sensor array based on dispersive time delay and optical pulse compression.

A novel technique to achieve real-time and high-precision interrogation of a linearly chirped fiber Bragg (LCFBG) grating sensor array based on dispersive time delay and optical pulse compression is proposed and experimentally demonstrated. In the proposed system, an ultra-short optical pulse is launched into an LCFBG sensor array consisting of N identical LCFBGs, to produce N reflected pulses with their time delay changes corresponding to the strain or temperature changes. The reflected pulses are sent to another LCFBG having an opposite dispersion coefficient to achieve optical pulse compression to improve the measurement accuracy. A proof-of-concept experiment is demonstrated. A 2×3 LCFBG sensor array using six identical LCFBGs is implemented, and the performance is evaluated experimentally. The sensing speed can be as high as 48.6MHz, and the sensing accuracy can be as high as 0.26 με for strain and 0.045°C for temperature.

Modifying the second order dispersion of femtosecond laser pulses to crack silver nanoparticles and control their dimensions

The potential application of metallic nanoparticles had been attracting attention and interest from different areas of academia and industry. The nanoparticles properties and applications depend heavily on their dimension and shape, thus special interest is aimed to controlling the nanoparticles sizes. Ultrashort laser pulses are known to change metallic nanoparticles characteristics, but the interaction mechanism is still not completely understood. In this work we reduced the dimension of silver nanoparticles with ultrashort pulses and demonstrated that there is a dependence of the particles size on the second order dispersion introduced in the pulses, which became smaller as the dispersion becomes more negative. Based on the results, we propose that the Coulomb explosion that reduces the nanoparticles size is predominantly initiated by multiphotonic ionization for the intensities used (1014 W/cm2).

Chirped dispersive pulse propagation of arbitrary initial width: a saddle point perspective

The propagated field dynamics in chirped Gaussian pulse propagation of arbitrary initial width in a linear, causally dispersive, Lorentz-type dielectric are derived, validated, and elaborated. The performed asymptotic analysis relies on a two-term series expansion around the saddle points of the unified phase. As I show, the dynamics of each relevant saddle point are mapped into the characteristics of a respective pulse component contributing to the total propagated field. The accuracy of the description is verified upon comparison with depicted numerical results. This asymptotic approach provides unique insight that is accurate and valid from the quasi-monochromatic to the sub-cycle pulse regimes.

A survey of the Galactic plane for dispersed radio pulses with the Australian Square Kilometre Array Pathfinder

We report the results from a survey of the Galactic plane for dispersed single pulses using the Australian SKA Pathfinder (ASKAP). We searched for rare bright dispersed radio pulses comprising 160 pointings covering 4800 deg$^2$ of the Galactic plane within |b| $< 7 \deg$, each pointing with an exposure time of 10 hours. We detected one fast radio burst, FRB 180430, and single pulses from 11 pulsars. No rotating radio transients were detected. We detected FRB 180430 in the Galactic plane in the anticentre direction with a fluence of 216$\pm5$Jy ms a dispersion measure (DM) of 264.1 pc cm-3. We estimate the extragalactic DM of the object to be less than 86.7 $ \text{pc} \ \text{cm}^{-3} $ depending on the electron density model. One model suggests that this FRB may be a giant pulse within our galaxy; we discuss how this may not correctly represent the line-of-sight DM. Based on the single detection of FRB 180430 in 3.47 $\times 10^{4}$ deg$^2$ h we derive a FRB event rate in the Galactic plane at the 20 Jy ms threshold to be in the range 2-140 per sky per day at 95% confidence. Despite the necessarily large uncertainties from this single detection, this is consistent with the current ASKAP all-sky detection rate.