Pulse Distortion Research Articles

UWB System for Indoor Positioning and Tracking with Arbitrary Target Orientation, Optimal Anchor Location, and Adaptive NLOS Mitigation

The Ultra-wideband (UWB) system for indoor positioning and tracking with the characteristics of arbitrary target orientation, optimal anchor location, and adaptive non-line-of-sight (NLOS) mitigation characteristics is proposed and implemented by introducing the circularly polarized antenna, the genetic algorithm (GA), and the machine learning method. The time-domain characteristic of the UWB system using the proposed circularly polarized antennas with wide bandwidth and omnidirectional radiation is investigated by transient response. Contrary to UWB system using the conventional linearly polarized antenna, the pulse distortion is insignificant and is verified by the measured antenna performance with high signal fidelity (>0.98) and low standard deviation (STD) of time delay (<; 0.05 ns). By considering the NLOS electromagnetic wave propagation models, the locations of the anchors in the UWB system are effectively optimized by using the proposed GA to minimize the average root-mean-square error (RMSE) of each tag location in the dense multipath area. By optimizing the three anchor locations, the average RMSE of tag location is minimized to 36.72 cm for a 45 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area with concrete walls and pillars. The adaptive NLOS mitigation is investigated by using and optimizing machine learning models, including deep neural network (DNN), convolutional neural network (CNN) and long short-term memory (LSTM). The three-anchor UWB system for a 45 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area is established to track an autonomous vehicle in severe NLOS environment by using the proposed circularly polarized antenna combined with the optimized LSTM model, achieving the measured positioning error of 26.1 cm. Moreover, the measured result of 20-30 cm positioning error with concrete walls, pillars and walking humans is demonstrated and analyzed.

Spatiotemporal characterization of laser pulse amplification in double-pass active mirror geometry

Abstract We present a spatiotemporal model of pulse amplification in the double-pass active mirror (AM) geometry. Three types of overlap condition are studied, and the spatiotemporal scaling under the four-pulse overlapping (4PO) condition is fully characterized for the first time, by mapping the temporal and spatial segments of beam to the instantaneous gain windows. Furthermore, the influence of spatiotemporal overlaps on the amplified energy, pulse distortion and intensity profile is unraveled for both AM and zigzag configurations. The model, verified by excellent agreement between the predicted and measured results, can be a powerful tool for designing and optimizing high energy multi-pass solid-state laser amplifiers with AM, zigzag and other geometries.

Sporadic-Slot Photonic-Crystal Waveguide for All-Optical Buffers With Low-Dispersion, Distortion, and Insertion Loss

A novel structure is studied with implanting sporadic slots inside a photonic crystal waveguide (PCW) to form sporadic-slot PCW (SSPCW) for realizing compact, all-optical buffers with low-dispersion, distortion, and attenuation (DDA). We implement the first demonstration that, to the best of our knowledge, the SSPCW works for both TE- and TM-modes all-optical buffers and slow-light waveguides. High buffer performance and wider transmission bandwidth in telecommunication band are obtained, which are guaranteed through bit rate optimization that exceeded 2 Tb/s for both TE and TM modes in ultra-low dispersion region which is 20 times greater than that required for 5G mobile communications and it is also useful for all-optical signal processing. The storage length of 1 bit is obtained as $4.5202~\mu \text {m}$ for TE mode and $6.0525~\mu \text {m}$ for TM mode. Moreover, a low relative pulse distortion of 0.0801% $\mu \text{m}^{-1}$ along the propagation path per unit length is acquired for a 0.63 ps pulse. Besides, the attenuation coefficient of the optical power pulse between the input and output is $0.0170~\text {dB}/\mu \text {m}$ . Additionally, the deigned SSPCW is insensitive for small variations of waveguide parameters and the deviation is virtually negligible between the simulated results and that of the fabricated structures with totally acceptable tolerance below ±3 nm, which is the challenge in the fabrication process of conventional PCW and PCW-cavity.

Distortion of location pulses in high-frequency paths of overhead power lines

The work is devoted to the features of propagation of electromagnetic signals (20–1000 kHz) along multi-wire overhead transmission lines. For monitoring the status of overhead power lines, a location method can be used. For connection to power lines, the connection equipment is used, which forms the high-frequency path of the power line, which has a limited frequency bandwidth. To select the optimal signals of location probing, it is necessary to investigate the impact of high-frequency path on the pulsed signals. This paper investigates the distortion of pulsed location signals in high-frequency paths. The influence of elements of the high-frequency path is studied using a simulation model of the high-frequency path of an overhead transmission line developed in the PSCAD software environment with subsequent experimental verification. Elements of high-frequency path of the developed simulation model are described. The influence of duration of the probe pulses on the shape and spectrum of the reflected signals is analyzed. It was established that during the passage of microsecond pulses, their differentiation occurs, the reflected signal is a combination of responses from the rising and falling edges of the probe pulse. With this in mind, criteria are proposed for optimizing the duration of the location pulses. During formation of ice deposits on the overhead lines wires, additional distortion of the pulse signals’ shape occurs. Using the experimental data, the distortions of the reflected pulsed signals and their spectra are analyzed as ice deposits grow on the wires of overhead power lines. The established patterns of pulse shape distortion and the developed criteria for optimizing the pulse duration are used for location probing of overhead power lines to control ice deposits on the wires and to detect damage.

Unveiling the mechanism of wave-front distortion of superfluorescent pulses

We investigated the transverse properties of 420-nm yoked superfluorescence (YSF) emitted from the atomic vapor of rubidium by driving the $5S\text{\ensuremath{-}}5D$ two-photon transition with an ultrashort laser pulse. From the observed interferograms of the superfluorescent pulses, we derived both the amplitude and phase properties of their electric fields. By increasing the pump-pulse intensity, the spatial profile of the 420-nm YSF changed from a central bright spot into a ring-shaped radial profile, accompanied by a simultaneous distortion of the wave front. This behavior can be explained by considering the intensity-dependent phase shifts of the $5S\text{\ensuremath{-}}5D$ wave packets during the excitation process, which was further imprinted to the 420-nm YSF wave front. The single-shot experiments enabled us to clarify the locally coherent regions of the 420-nm YSF as well as their stochastic feature.

Theoretical analysis of bottom reflected broadband waveforms

Bottom reflection of broadband waveforms from explosive sources leads to pulse distortion, including the presence of a precursor to the main pulse, a topic that goes back to the 1950s [Arons and Yennie, JASA 22, 231–237 (1950)). A theoretical analysis of broadband bottom reflection can provide valuable insights into observed waveform distortion and its dependence on the grazing angle and sediment properties. When the attenuation in the bottom can be approximated as varying linearly with frequency, the magnitude of the reflection coefficient is independent of frequency, but with a common phase shift over positive frequencies (and with the opposite phase shift over negative frequencies). Combining these contributions leads to the pulse distortion . Variations of the sediment properties that cause the same phase shift at a given grazing angle produce the same pulse distortion. Conversely, inferring a phase shift from the pulse distortion can be used to place constraints on the inversion of sediment properties. Examples will be discussed. [Work supported by ONR.]

Influences of group velocity dispersion on ultrafast pulse shaping in time lens

Time-lens technology is of significant interest in signal processing and optical communication. The impacts of group velocity dispersion (GVD) on ultrafast pulse shaping in a time-lens system based on four-wave mixing are explored in this paper. The output signals of temporal magnification and time-to-frequency conversion under different GVDs are theoretically investigated in detail. The simulation results imply that the femtosecond pulse is sensitive to GVD in propagation. GVD has an important effect on nonlinear parametric processes, which results in output signals presenting different pulse shapes and different frequency profiles. Furthermore, a model of silicon nitride waveguide with flat dispersion is proposed by finite element method and signal processing with negligible pulse distortion is realized in near-infrared region.

Ultrasonic online monitoring of the ham salting process. Methods for signal analysis: Time of flight calculation

In the dry-cured ham industry, an accurate control of the dry-salting process is especially complex because of the great heterogeneity of the meat pieces and the effect of different operational variables. The main objective of this study was to evaluate the feasibility of using an ultrasound system and methodology, adapted to the industry requirements, for the online monitoring of the ham dry-salting process. For that purpose, hams were dry salted for different times (4, 10, 11, 14, 16 and 30 days) at 2 °C. The cushion zone of the ham was placed over the transducers during salting and ultrasonic signals were taken automatically (5 min interval by using pulse-echo mode. Several methods of signal analysis were considered in order to assess the time of flight (TOF). TOF estimations by means of the energy threshold and cross-correlation methods (between the initial ultrasonic signal and the remaining signals measured during salting and between consecutive signals 5 min apart without interpolation) were affected by the low signal-to-noise ratio and the pulse distortion and were discarded for the online monitoring of ham salting. Otherwise, the cross-correlation method between consecutive signals (5 min apart) with interpolation n = 3 (CCM-CS n = 3), between non-consecutive signals (1 h apart) (CCM-NCS) and the phase spectrum method (PSM), provided close estimations of the variation of the TOF, which correlated well with the ham salt gain (R2 = 0.83 for CCM-CS n = 3, 0.93 for CCM-NCS and 0.90 for PSM). Consequently, the use of ultrasonic pulse-echo TOF measurements could be considered as a simple, non-invasive, non-destructive and reliable technique for the industrial monitoring of the ham dry-salting process.

General framework for the frequency shifting of electromagnetic pulses using time-dependent surfaces

Metasurfaces allow for agile manipulation of incoming light using a single layer of resonators. Despite recent progress, it remains difficult to generate new spectral components using nonlinear surfaces, because of the limited interaction length of a pulse traveling through a single surface. Time-dependent surfaces offer an exciting alternative to overcome this limitation, but a self-consistent framework to describe this mechanism is lacking. Based on an analytical model and finite-difference time-domain numerical simulations, we obtain physical insight into the frequency-shifting process that occurs when broadband electromagnetic pulses interact with time-varying surfaces. In particular, we find that there is an intriguing relationship between the bandwidth of the incident pulse, the targeted frequency shift, and the number of Lorentzian resonators that need to be implemented. We also demonstrate that in certain parameter regimes, pulse distortion and a deviation of pulse amplitude cannot be avoided. These results are independent of the mechanism that generates the time dependence. They are also independent of the frequency, the geometry, size, and material of the unit cell, but they are rather a direct result of the subtle interplay between time-dependent Lorentzian resonances.

Analysis of pulse amplification characteristics in vertical cavity semiconductor optical amplifiers

Based on the transfer matrix method (TMM), a comprehensive dynamic numerical model describing the optical pulse amplification in vertical cavity semiconductor optical amplifiers (VCSOAs) is established, which treats the whole VCSOA as one multilayer dielectric structure, including the two Distributed Bragg Reflectors (DBRs) and the middle active region. In this model, the spatial dependence of the carriers and optical field in the longitudinal direction and the discontinuity of refractive index distribution in the resonant cavity are accounted for. Moreover, in this study the influence of the two photon absorption (TWP) is also taken into consideration. By employing this model, we investigate the signal distortion and the pulse energy gain characteristic in single pulse amplification of VCSOAs operated in reflection mode in detail. The numerical results show that the pulse distortion can be mitigated in a satisfactory manner via reducing the period of top DBR. It further shows that the energy gain during the pulse amplification can be increased by enhancing the pump power; larger saturation input pulse energy (SIPE) could be obtained by either lowering the period of top DBR or reducing the pump power; for the input pulse with duration ranging from several to tens of picoseconds, the energy gain characteristics of VCSOA is pulsewidth independent.

A Low-Impedance Transmission Line Transformer Based on the Multicore Coaxial Transmission Line

The design, construction, and operational characteristics of a low-impedance and low pulse drop transmission line transformer (TLT) are described in this paper. The multicore coaxial transmission line used for this transformer is different from traditional coaxial transmission line since its inner conductor consists of several high-voltage cables that are connected in parallel by using two metal disks placed at both ends of them. Simultaneously, the impedance of the coaxial transmission lines with ten cables designed in this paper is perfectly reduced from 60 to $13.5~\Omega $ . Moreover, the transmission lines can withstand a high voltage of several tens of kilovolts with 13.9 mm in diameter. The transmission lines were then used to construct TLTs. The transformers are wound inductively in separated cylinders made of polypropylene to enhance the isolation from the input to the output of the transformers. Consequently, the pulse drop is substantially reduced, minimizing pulse distortion in the transformer. A three-stage TLT, whose input and output impedances are 4.5 and $40.5~\Omega $ , respectively, was developed with the transmission lines. The experimental results show that the three-stage transformer based on the low-impedance and low pulse drop design transmission lines can achieve a voltage gain factor of almost 3.0.

Optical plasmon nanostrip probe as an effective ultrashort pulse delivery system.

In this paper, we analyze the ultrafast temporal and spectral responses of optical fields in tapered and metalized optical fibers (MOFs) and optical plasmon nanostrip probes (NPs). Computational experiment shows that output pulses of the NPs are virtually unchanged in shape and duration for input pulses with a duration of >1 fs and are not sensitive to changes in the parameters of the probe (such as convergence angle and taper length), while local enhancement of the electric field intensity reaches 300 times at the NP apex. Compared with the NPs, MOFs lead to significant output pulse distortions, even for input pulses with a duration of 10 fs. In addition, the temporal response at the MOF apex is critically sensitive to changes in MOF parameters and cannot provide any significant local enhancement of the electric field. These findings reveal the high potential of optical plasmon nanostrip probes as an ultrashort pulse delivery system to nanometer-size areas and indicate that its usage can be promising for a wide variety of techniques studying ultrafast processes in nanoscopic volumes.

Compact broadband frequency selective microstrip antenna and its application to indoor positioning systems for wireless networks

This study presents a low-profile broadband microstrip patch antenna with filtering response. The proposed antenna consists of a rectangular patch and four parasitic gap-coupled elements, two L- and two rectangular-shaped patches. A broadband quasi-elliptic boresight gain response is obtained without using any extra filtering circuits. The input impedance of each radiating element, i.e., driven patch and parasitic elements, is matched to its radiating quality factor and the couplings between patches are optimised for broadband impedance bandwidth with filtering response. Prototype hardware is designed and fabricated on Kappa 438 substrate with a relative permittivity of 4.4 and thickness of 3.2 mm. The antenna exhibits a total size of 25 × 23 × 3.2 mm3 with relative impedance bandwidth (voltage standing wave ratio<2) of 60% ranging from 4.4 to 7.8 GHz. The experimental results demonstrate good performance with nearly flat gain and good filtering response. The proposed filtering antenna exhibits low pulse distortion in time domain which makes it a good candidate for location-aware Internet-of-things applications employing the IEEE 802.15.4 ultra-wideband standard. Switchable sector base-station antenna system is studied to demonstrate the capability of this design to enhance the localisation and communication performance of the wireless network.

Simultaneous generation and compression of broadband terahertz pulses in aperiodically poled crystals.

We introduce a technique to generate compressed broadband terahertz pulses based on cascaded difference-frequency generation. The approach employs a non-uniform sequence of pump pulses in aperiodically poled crystals. The pump-pulse format and poling of crystals conceived are such that the emergent terahertz pulse is already compressed. The method circumvents pump-pulse distortions that result from non-collinear approaches and the need for external compression. While capable of generating even single-cycle pulses, it is particularly efficient for the generation of pulses with few to tens-of-cycles duration. For instance, calculations accounting for cascading effects predict conversion efficiencies in the few percent range for cryogenically-cooled lithium niobate. The focused electric fields are ≫ 100 MV/m in free space.

Contrast improvement of autofluorescent vitamins by using shaped laser pulses after a kagome fiber

An improved two-photon excited fluorescence contrast difference between autofluorescent vitamins is reported by using shaped laser pulses after a kagome fiber. Specifically phase-tailored laser pulses are applied for two-photon excited fluorescence of retinol and riboflavin in solution at the distal end of the fiber. The pulse distortions due to the fiber properties are precompensated to achieve predefined tailored pulses after the kagome fiber. A clear two-photon excited fluorescence contrast difference is received between vitamin A and B2 for various third order phase parameters. Moreover, selective excitation is observed for retinal versus retinol, although they differ only in a side chain. The presented approach of guiding shaped pulses through kagome fibers has large perspectives for endoscopic applications.

Low-level RF control of a klystron for medical linear accelerator applications

A klystron is an important vacuum device which is useful in medical linear accelerators because it amplifies the radio-frequency (RF) signals required to produce high-energy particle beams. The properties of the nonlinear input and output dynamics in the klystron circuit arouse distortion in various types of parameters pertaining to the beam voltage, beam current, RF power, and operating frequency from unpredictable external factors. Therefore, low-level RF (LLRF) control systems can facilitate the regulation of the output of the klystron to a desired level. Low-level RF control system of the klystron is designed to stabilize the applied voltage and control the resonance frequency of the cavity of the klystron. In this work, an infallible low-level RF control system, the most suitable for a klystron, is designed to use a beam monitoring and an interlock system with components that include an isolator and a phase shifter. Moreover, with regard to pulse distortion at the flap top at the RF output power, the stability of the RF power pulse at the flap top part was improved somewhat by a variable inductor which controlled the inside of the pulse modulator. In this LLRF control experiment, the pulse modulator generated 140kV-95A and succeeded in generating RF output power of 2.856GHz-6MW level of an S-band klystron. During this process, the efficiency of the RF power was approximately 42∼45% and the pulse repetition rate was 10 Hz. The flattop pulse width was measured and found to be 1 µs.

微环谐振器中各参数对光速控制输出脉冲畸变的影响仿真分析

微环谐振器中各参数对光速控制输出脉冲畸变的影响仿真分析

光纤中双泵浦低失真受激布里渊散射快光

光纤中双泵浦低失真受激布里渊散射快光

A stage-scanning two-photon microscope equipped with a temporal and a spatial pulse shaper: Enhance fluorescence signal by phase shaping.

Here, we present a stage-scanning two-photon microscope (2PM) equipped with a temporal pulse shaper and a spatial light modulator enabling full control over spectral and spatial phases of the exciting laser pulse. We demonstrate the capability of correcting wavefronts and temporal pulse distortions without cross-dependencies induced by optical elements at the same time enhancing the fluorescence signal. We implemented phase resolved interferometric spectral modulation for temporal pulse shaping and the iterative feedback adaptive compensation technique for spatial pulse modulation as iterative techniques. Sample distortions were simulated by cover glass plates in the optical path and by chirping the exciting laser pulses. Optimization of the spectral and spatial phases results in a signal increase of 30% and nearly complete recovery of the losses. Applying a measured spatial compensation phase within a real leaf sample shows the enhancement in contrast due to wavefront shaping with local fluorescence increase up to 75%. The setup allows full independent control over spatial and spectral phases keeping or improving the spatial resolution of our microscope and provides the optimal tool for sensitive non-linear and coherent control microscopy.

Five-Line Photonic Crystal Waveguide for Optical Buffering and Data Interconnection of Picosecond Pulse

A five-line (5L) photonic crystal waveguide (PCW) capable of realizing compact, optical buffering and low-distortion data interconnection of picosecond pulse is presented and studied through plane-wave expansion and finite-difference time-domain methods. By appropriately modifying the radii of defects in the 5L PCW, high buffering performance and low-distortion slow light transmission of picosecond pulses are achieved, which are guaranteed by an optimized value of normalized delay bandwidth product (NDBP) of 0.6811 for the proposed PCW. The largest buffering storage capacity and bandwidth obtained are about 219.73 bit and 19.55 nm, respectively. It is found that the distortion of pulse transmission in a PCW is greatly affected by the waveguide length. With these optimized parameters, a relative pulse distortion per unit length of 1.15×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is obtained for a 2.19-ps pulse, meaning that for a signal pulse with a dutyfactor of 0.5, the proposed structure can process optical digital pulse signals at the speed of 0.22831 Tb/s. Moreover, the NDBP value of 0.6811 obtained is much higher than that of all other structures based on waveguides reported in the literature previously so that high performance of the proposed PCW can be achieved. The waveguide proposed is suitable for data interconnections among chips or among processing units within a chip in optical signal processing, and computations where the size of the transmission line or interconnector is highly limited.