Pulse Spreading Research Articles

Spatial and temporal thermal management of a spintronic terahertz emitter

This work presents methods for addressing undesirable thermal effects induced by the pump beam of an oscillator laser to improve the efficiency of a terahertz (THz) spintronic emitter. We explore two approaches: spatial distribution of pump energy using a 2D lens array and temporal modulation of the pump duty cycle. Optimizing the spatial distribution approximately doubles the THz signal by increasing local heat dissipation, delaying the saturation limit. Similarly, temporal spreading of pump pulses by adjusting the duty cycle allows greater thermal relaxation within the heterostructure, enhancing the overall efficiency of THz wave generation.

Long distance QKD propagation using optical single sideband scheme

Implementation of the passive radio over-fiber technique permits modulating the low-frequency sub-carrier onto an optical channel for dissemination through a light-wave fiber network. Single-sideband modulation for optical signals allows the impressive utilization of channel capacity in optical fiber. Dispersion reduction techniques limit the pulse spreading of a propagated signal in any photonic scheme. To control pulse-spreading effects, the optical single-sideband modulation technique at different phase shifts is modeled, analyzed, and compared to examine the performance of a sub-carrier multiplexing system. Hence, in this paper, a quantum key distribution network using a single sideband modulation technique based on a Li-Nb Mach-Zehnder modulator has been proposed at different electrical phase shifts. In this suggested model, the Optisystem 14.2 simulator is used to analyze the nonlinear characteristics. We have designed a single-sideband contour reduction and amplification with each couplet of 120 km by increasing the distance up to 720 km, and the phase between quantum states is determined. The system performance of the suggested model is investigated and compared based on output power (dBm), quality factor, eye diagram, bit error rate (BER), extinction ratio (ER), and optical spectrum of the received signal by varying link distance (km), channel spacing (nm), input power (dBm), and fiber dispersion (ps/ns/km).

Monte-Carlo based vertical underwater optical communication performance analysis with chlorophyll depth profiles

Although underwater wireless optical communication (UWOC) has the advantages of high speed, low latency, and high confidentiality, the transmission of light in water will be affected by the absorption and scattering of particles, which will lead to the aggravation of channel path loss as well as channel pulse spreading, finally causing false codes. Therefore, how to analyze the channel impulse response (CIR) effectively is a key task in channel modeling. In this paper, we consider a two-way underwater vertical line-of-sight (LOS) communication system model, based on the inherent optical property (IOP) model of chlorophyll, using the Kopelevich phase function containing water depth information, the CIR curves under different water types and transceiver configurations are plotted using the Monte-Carlo Simulation (MCS). The obtained simulation results are fitted with the double gamma function (DGF) model and the Gaussian model, respectively. The Gaussian model exhibits better properties than the DGF model in each water condition. Based on the closed-form expression of the CIR obtained from the Gaussian model, we solve for the bit error rate (BER) and 3-dB bandwidth of the system under different settings. The conclusions obtained can be used for the design and optimization of underwater vertical channels.

The role of the Cx43/Cx45 gap junction voltage gating on wave propagation and arrhythmogenic activity in cardiac tissue

Gap junctions (GJs) formed of connexin (Cx) protein are the main conduits of electrical signals in the heart. Studies indicate that the transitional zone of the atrioventricular (AV) node contains heterotypic Cx43/Cx45 GJ channels which are highly sensitive to transjunctional voltage (Vj). To investigate the putative role of Vj gating of Cx43/Cx45 channels, we performed electrophysiological recordings in cell cultures and developed a novel mathematical/computational model which, for the first time, combines GJ channel Vj gating with a model of membrane excitability to simulate a spread of electrical pulses in 2D. Our simulation and electrophysiological data show that Vj transients during the spread of cardiac excitation can significantly affect the junctional conductance (gj) of Cx43/Cx45 GJs in a direction- and frequency-dependent manner. Subsequent simulation data indicate that such pulse-rate-dependent regulation of gj may have a physiological role in delaying impulse propagation through the AV node. We have also considered the putative role of the Cx43/Cx45 channel gating during pathological impulse propagation. Our simulation data show that Vj gating-induced changes in gj can cause the drift and subsequent termination of spiral waves of excitation. As a result, the development of fibrillation-like processes was significantly reduced in 2D clusters, which contained Vj-sensitive Cx43/Cx45 channels.

A Low-Cost Measurement Methodology for LiDAR Receiver Integrated Circuits

This paper presents a test methodology to facilitate the measuring processes of LiDAR receiver ICs by avoiding the inherent walk error issue. In a typical LiDAR system, a costly laser diode driver emits narrow light pulses with fast rising edges, and the reflected pulses from targets enter an optical detector followed by an analog front-end (AFE) circuit. Then, the received signals pass through the cascaded amplifiers down to the time-to-digital converter (TDC) that can estimate the detection range. However, this relatively long signal journey leads to the significant decline of rising-edge slopes and the output pulse spreading, thus producing inherent walk errors in LiDAR receiver ICs. Compensation methods requiring complex algorithms and extra chip area have frequently been exploited to lessen the walk errors. In this paper, however, a simpler and lower-cost methodology is proposed to test LiDAR receiver ICs by employing a high-speed buffer and variable delay cells right before the TDC. With these circuits, both START and STOP pulses show very similar pulse shapes, thus effectively avoiding the walk error issue. Additionally, the time interval between two pulses is easily determined by varying the number of the delay cells. Test chips of the proposed receiver IC implemented in a 180-nm CMOS process successfully demonstrate easier and more accurate measurement results.

Tailoring Group Delay Dispersion of Spatial Phaser using Anisotropic Metasurface and Polarized Incident Waves

AbstractMetasurface‐based spatial phaser, exhibiting dispersive group delay responses, is able to perform real‐time analogue signal processing on the temporal spectrum of spatial electromagnetic waves, e.g., Fourier transformation, pulse chirping, pulse spread, and compression. The functionality of a spatial phaser is closely related to its group delay dispersion slope. For instance, for an up‐chirped incident pulse, the positive‐slope‐dispersion phaser spreads the pulse whereas the negative‐slope‐dispersion one compresses the pulse. Most reported spatial phasers exhibit fixed dispersion slopes and hence perform single functionality only, which cannot meet the demand of multifunctional systems in communication and sensing applications. In this paper, a new way is proposed to control and tune the dispersion slope of spatial phasers using polarization of incident waves, which is simpler and more efficient than conventional approaches based on tunable circuits. To verify the proposed method, a first‐order spatial phaser is designed and experimentally demonstrated within 9.4–10.6 GHz. It is able to generate negative‐slope, positive‐slope, and zero‐slope group delay dispersions, in case of x‐, y‐, and ±45°‐polarized waves. Such a reconfigurable phaser may find wide applications in integrated communication and sensing.

Dispersion Compensation in Optical Fiber: A Review

A cylindrical-shaped dielectric waveguide is what an optical fiber is. The core-cladding interface confines light, as electromagnetic (EM) energy, within its surface and guides light through several internal reflections if the angle of incidence is larger than the C-critical angle. Dispersion is the spreading of light pulses as they travel along a fiber, resulting in a degradation of optical signal quality. The scattering of optical signs contorts both computerized and simple transmission across optical strands. When optical fiber transmission is widely employed and some sort of digital modulation is applied, dispersion mechanisms inside the transmitted light pulses broaden as they pass through the fiber and move along the channel. The waveguide dispersion is calculated using a simple curve-fitting method. Dispersion must require compensating. Dispersion compensation techniques are essential for the successful transmission of data over optical fiber. Dispersion compensation techniques are used to reduce the effects of dispersion and improve SNR. Dispersion compensation is the process of reducing or eliminating chromatic dispersion in an optical fiber. There are two primary methods of dispersion compensation electronic and optical. In electronic dispersion compensation, the distorted signal is converted into the electrical domain, and digital signal processing techniques are used to correct the dispersion-induced distortion. While this method is effective, it adds complexity and cost to the communication system. Hence, most modern-day systems employ optical dispersion compensation, which is a more efficient and cost-effective method. Optical dispersion compensation involves using specialized optical components, such as dispersion compensating fibers (DCFs) and fiber Bragg gratings (FBGs), to adjust the different wavelengths' arrival time. DCFs are designed to have the opposite dispersion characteristics of the transmission fiber, which allows them to compensate for the dispersion as the signal passes through. FBGs are periodic structures inscribed in the fiber that reflect specific wavelengths of light, effectively delaying their arrival time and compensating for the overall dispersion.

X-ray ultrashort pulse emission characteristic of carbon nanotube cold cathode X-ray source by pulse driving mode

In the present work, we build a triode structure of carbon nanotube (CNT) cold cathode X-ray source driven by pulse voltage in dynamic vacuum system. The results show that the grid voltage and pulse width affect the amplitude, delay time and relative pulse spread of X-ray. The X-ray emission delay time decreases sharply from 102 to 75 ns when the grid voltage increases from 600 to 650 V. The relative pulse spread tends to be saturated with approximately 350 ns (±20 ns) when the voltage increases from 600 to 650 V. The theoretical limit frequency of this X-ray pulsed emission system is approximately 2.86 MHz (1/350 ns). This work reveals the ultrashort pulse emission characteristics and affecting factors of CNT cold cathode X-ray source, promoting the development of CNT cold cathode X-ray source in pulse application.

Статистическое уточнение импульса последействия маршевого двигателя разгонного блока «Фрегат» для повышения точности выведения космических аппаратов

As it is known from practice, the most important influence on the accuracy of the payload launching into the target orbit is exerted by measurement errors of the command instrumentation system sensors, as well as by the sustainer afteraction pulse spread. A method is considered to increase accuracy of launching into a given orbit using the Fregat upper stage by refining the sustainer afteraction pulse. According to the results of launches, actual errors in the realized orbit linear parameters were recalculated in the afteraction pulse deviations. Then, the afteraction pulse average value was refined by statistical analysis in several iterations, which made it possible to significantly reduce the launching errors and confirm correctness of the developed technique.

Secure OCDM Mode Division Multiplexed Short-Reach Optical Communication Based on Time-Frequency Joint Perturbation

A secure orthogonal chirp division multiplexing (OCDM) mode division multiplexed (MDM) short-reach optical communication based on time-frequency joint perturbation is proposed in this paper. Firstly, the discrete Fresnel transform (DFnT) was used to realize the digital realization of OCDM, and then the Lorenz chaos model and Logistic chaos model were combined to realize the time-frequency perturbation of the orthogonal chirped subcarriers and chirped symbols. The digital up-conversion (DUC) was used to transform the OCDM signal and transmit it in the MDM short-reach optical communication system. The chirped signal has good pulse compression and spread spectrum capability, the maximum chirped spread spectrum can be achieved by using OCDM based on DFnT. The time-frequency joint perturbation of the double chaos model can achieve a large keyspace of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1{\bf } \times {\bf }{10^{144}}$</tex-math></inline-formula> and improve the security of information transmission. DUC technology can transform OCDM signal from complex to a real number, which can meet the transmission requirements of the MDM IM/DD system and increase the transmission spectrum efficiency. Through experiments, 32Gb /s OCDM encrypted signal transmission is realized across 5km few-mode fiber, the OCDM has a maximum receiver sensitivity gain of 1.48 dB compared with OFDM at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1{\bf } \times {\bf }{10^{ - 2}}$</tex-math></inline-formula> bit error rate. The results show that the proposed secure OCDM-MDM short-reach optical communication scheme based on time-frequency joint perturbation has good anti-interference and security performance, as well as a wide range of applications in the future short-reach optical communication.

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

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

The exact solutions for the nonlinear variable-coefficient fifth-order Schrödinger equation

In the paper, the nonlinear variable-coefficient fifth-order Schrödinger (NLVS) equation is researched. The NLVS equation is an integrable equation, which can be described the spreading of ultrashort pulses in an inhomogeneous optical fiber. Firstly, by using the modified traveling wave transformation, the NLVS equation is changed into an ordinary equation. Secondly, by the Jacobian elliptic function expansion method for the ordinary equation, we obtain the exact solutions for the ordinary equation, and then, we obtain the exact solutions to the NLVS equation. These solutions mainly include three types: Jacobi elliptic function solutions, hyperbolic function solutions, and triangular function solutions. Finally, according to the special parameters, we show the figures of the exact solutions.

Influence of nonlinear absorption on the propagation of ultrashort optical pulses in an anisotropic optical medium with carbon nanotubes

We study the features of the evolution of a pulse in a dielectric medium (crystal) with optical anisotropic properties taking into account pumping. The medium contains semiconductor carbon nanotubes of a fixed orientation directed at an angle to one of the principal axes of the crystal. We analyze the influence of pumping and resistance to disturbances. Our study shows that the effects of pumping and nonlinear absorption suppress the dispersive pulse spreading caused by the optically anisotropic properties of the medium.

Deriving Snow Depth From ICESat-2 Lidar Multiple Scattering Measurements: Uncertainty Analyses

The application of diffusion theory and Monte Carlo lidar radiative transfer simulations presented in Part I of this series of study suggests that snow depth can be derived from the first-, second- and third-order moments of the lidar backscattering pathlength distribution. These methods are now applied to the satellite ICESat-2 lidar measurements over the Arctic sea ice and land surfaces of Northern Hemisphere. Over the Arctic sea ice, the ICESat-2 retrieved snow depths agree well with co-located IceBridge snow radar measured values with a root-mean-square (RMS) difference of 7.8 cm or 29.2% of the mean snow depth. The terrestrial snow depths derived from ICESat-2 show drastic spatial variation of the snowpack along ICESat-2 ground tracks over the Northern Hemisphere, which are consistent with the University of Arizona (UA) and Canadian Meteorological Centre (CMC) gridded daily snow products. The RMS difference in snow depths between ICESat-2 and UA gridded daily snow products is 14 cm, or 28% of the mean UA snow depth. To better understand these results, we also discuss the possible sources of errors in ICESat-2 derived snow depths, including surface roughness within the laser footprint, atmospheric forward scattering, solar background noise, and detector dark current. Simulation results indicate that the snow depth errors would be less than 5 cm if the standard deviation of pulse spreading due to surface roughness is within 50 cm. Our results demonstrate that the ICESat-2 lidar measurements can be used to reliably derive snow depth, which is a critical geophysical parameter for cryosphere studies including sea ice thickness estimation and also provides important constraints in the modeling of terrestrial hydrological processes.

Modeling multiple scattering transient of an ultrashort laser pulse by spherical particles

The multiple scattering of an ultrashort laser pulse by a turbid dispersive medium (namely a cloud of bubbles in water) is investigated by means of Monte Carlo simulations. The theory of Gouesbet and Gréhan (2000) [14] is used to derive an energetic model of the scattering transient. It is shown that the spreading and extinction of the pulse can be decoupled from the transient of scattering, which allows to describe each phenomenon individually. The transient of scattering is modeled with the Lorenz-Mie Theory and thus is also valid for a relative refractive index lower than one, contrary to the Debye series expansion which does not converge close to the critical angle. This is made possible after the introduction of a new physical object, the Scattering Impulse Response Function (SIRF) which allows to detect the different modes of scattering transient, in time and direction. The present approach is more generic, as it enables to simulate clouds of air bubbles in water, which was not possible previously. Two different approaches are proposed within the Monte Carlo framework. The first is a pure Monte Carlo approach where the delay due to the scattering is randomly drawn at each event, while the second is based on the transport of the whole scattering signal. They are both embedded in the Monte Carlo code Scatter3D (Jenny et al., 2007) [21]. Both models produce equivalent trends and are validated against published numerical results. They are then applied to the multiple scattering of ultra short pulse by a cloud of bubble in water in the forward direction. The pulse spread due to the propagation in water is computed for a wide range of traveled distances and pulse durations, and the optimal pulse duration is given to minimize the pulse spread at a given distance. The main result is that the scattered photons exit the turbid medium earlier than the ballistic photons and produce a double peak related to the refraction in the bubble. This demonstrates the possibility to develop new diagnostics to characterize dynamic bubbly flows.

Implementation of Multimode Fiber in Eight Channels Mimo Multiplexing System By Eliminating Dispersion with Optical Grating Methodologies

In recent years, with the swift development of the internet industry, people urgently necessitate extra capacity and speedy network systems. Under this circumstance, optical fibers are becoming the most favourable delivering media as it has a major important role in the information business, with their huge bandwidth and excellent transmission performance. Dispersion compensation is the main important attribute required in an optical fiber communication system because the presence of dispersion leads to pulse spreading that causes the output signal pulses to overlap. For a long-haul transmission system, an 8x1 Wavelength Division Multiplexing Multi Input Multi Output (WDM MIMO) channel with a novel dispersion compensation system has been designed. Various dispersion compensation techniques are explored in this work, including dispersion-compensating fibers, fiber Bragg grating, Ideal compensation Bragg grating, and opti-grating, with a 2.5 Gbit/s data rate for each channel. With a power of 20 dBm, the proposed model is simulated with 32768 samples. The length of an optical fibre can range from 5 to 100 kilometres. Throughout the simulation, the operation frequency is 193.1 THz. The software opti-system-17.0 was used to design and implement the system. The 8-channel device was used to simulate and calculate metrics such as Q-factor, Bit Error Rate (BER), and Signal to Noise Ratio (SNR). The proposed model enhances performance in terms of (BER) and quality factor at the receiver end of systems.

Performance Assessment of Dispersion Compensation Using Fiber Bragg Grating and Dispersion Compensation Fiber Technique

Dispersion should be flattering to achieve the most optimal output of the contact device. Since dispersion causes pulse spreading, which causes the output signal pulses to overlap, dispersion compensation is the most important attribute needed in an optical fiber communication device. The 8*1 Wavelength Division Multiplexing (WDM) Multiple Input Multiple Output (MIMO) channel with compensation using Fiber Bragg Grating (FBG) and Dispersion Compensation Fiber (DCF) has been designed with a 5Gbps data rate for each channel. The optical fiber's length can range from 50 to 200 kilometers. In this, primarily the proposed system is implemented without applying the grating technique and the outcomes are simulated and analyzed. Subsequently, the system is designed and implemented with FBG and DCF and the outcomes are simulated with opti-system-16.0 software. The parameters like Q-factor, Bit Error Rate (BER) and SNR were simulated and calculated by exploiting the 8-channel device.

Non-blocking programmable delay line with minimal dead time and tens of picoseconds jitter

We report a non-blocking high-resolution digital delay line based on an asynchronous circuit design. Field-programmable gate array logic primitives were used as a source of delay and optimally arranged using combinatorial optimization. This approach allows for an efficient trade-off of the resolution and a delay range together with a minimized dead time operation. We demonstrate the method by implementing the delay line adjustable from 23ns up to 1635ns with a resolution of 10ps. We present a detailed experimental characterization of the device focusing on thermal instability, timing jitter, and pulse spreading, which represent three main issues of the asynchronous design. We found a linear dependence of the delay on the temperature with the slope of 0.2 ps K-1 per logic primitive. We measured the timing jitter of the delay to be in the range of 7-165ps, linearly increasing over the dynamic range of the delay. We reduced the effect of pulse spreading by introducing pulse shrinking circuits and reached the overall dead time of 4-22.5ns within the dynamic range of the delay. The presented non-blocking delay line finds usage in applications where the dead time minimization is crucial, and tens of picoseconds of excess jitter is acceptable, such as in many advanced photonic networks.

Plate-height model of ion mobility-mass spectrometry: Part 2-Peak-to-peak resolution and peak capacity.

In a previous work, we explored zone broadening and the achievable plate numbers in linear drift tube ion mobility-mass spectrometry through developing a plate-height model [1]. On the basis of these findings, the present theoretical study extends the model by exploring peak-to-peak resolution and peak capacity in ion mobility separations. The first part provides a critical overview of chromatography-influenced resolution equations, including refinement of existing formulae. Furthermore, we present exact resolution equations for drift tube ion mobility spectrometry based on first principles. Upon implementing simple modifications, these exact formulae could be readily extended to traveling wave ion mobility separations and to cases when ion mobility spectrometry is coupled to mass spectrometry. The second part focuses on peak capacity. The well-known assumptions of constant plate number and constant peak width form the basis of existing approximate solutions. To overcome their limitations, an exact peak capacity equation is derived for drift tube ion mobility spectrometry. This exact solution is rooted in a suitable physical model of peak broadening, accounting for the finite injection pulse and subsequent diffusional spreading. By borrowing concepts from the theoretical toolbox of chromatography, we believe that the present study will help in integrating ion mobility spectrometry into the unified language of separation science.

Dielectric loaded directive Vivaldi antenna considering dispersion in near and far field

A “V” slot dielectric loaded antipodal Vivaldi antenna is proposed in this article, which provides significant improvement for directivity, front to back lobe ratio and low side lobe level over the operating frequency band from 1.76 GHz to 10 GHz. Initially, the 35° upward tilted parallel slots are placed along the outer curvy conductor edges of the antenna to maintain the desired radiation performances and impedance matching. Thereafter the V slot extended dielectric slab is integrated with this design for the improvement of radiation performances and pulse spreading effect while maintaining high directivity. The antenna prototype is fabricated and tested which shows the return loss is below the 10 dB, maximum directivity is achieved at 10 GHz which is 17.5 dBi and front-to-back lobe level ratio is 29.2 dB, almost triple as compare to conventional Vivaldi antenna at 9 GHz. The measured results depicts that the highest main lobe squint angle is 2.7° at 6 GHz. In addition to this, the measured system fidelity factor (SFF) is 0.86 and 0.91 at near and far field respectively which confirms that the V slot actually reduces the pulse spreading and dispersion effect while maintaining the high directivity.