Time-domain Pulse Research Articles

Thermal Dynamics Effects using Pulse-Shaping Laser Sintering of Printed Silver Inks

In recent years, additive manufacturing has been evolving towards flexible substrates for the fabrication of printable electronic devices and circuits. Generally polymer-based, these emerging substrates suffer from their heat sensitivity and low glass-transition temperatures. As such they require new highly-localized sintering processes to treat the electronic inks without damaging the polymer-based substrate. Laser-based sintering techniques have shown great promises to achieve high-quality sintering locally, while controlling the heat penetration to preserve the polymer substrates integrity. In this report, we explore new optimization pathways for dynamic laser-based sintering of conductive silver inks. Multiple passes of a pulsed laser are first performed while varying pulse train frequencies and pulse energies as an attempt to optimize the properties of the silver inks. Then, time-domain pulse shaping is performed to alter the properties of the conductive inks. Together, these pathways allow for the careful control of the time-domain laser energy distribution in order to achieve the best electronic performances while preserving the substrate’s integrity. Sheet resistance values as low as 0.024Ω/□ are achieved, which is comparable to conventional 1-hour oven annealing, with the processing time dramatically reduced to the milisecond range. These results are supported by finite element modeling of the laser-induced thermal dynamics.

On terahertz pulsed broadband Gauss-Bessel beam free-space propagation

Terahertz pulse time-domain holography is the ultimate technique allowing the evaluating a propagation of pulse broadband terahertz wavefronts and analyze their spatial, temporal and spectral evolution. We have numerically analyzed pulsed broadband terahertz Gauss-Bessel beam’s both spatio-temporal and spatio-spectral evolution in the non-paraxial approach. We have characterized two-dimensional spatio-temporal beam behavior and demonstrated all stages of pulse reshaping during the propagation, including X-shape pulse forming. The reshaping is also illustrated by the energy transfer dynamics, where the pulse energy flows from leading edge to trailing edge. This behavior illustrates strong spatio-temporal coupling effect when spatio-temporal distribution of Bessel beam’s wavefront depends on propagation distance. The spatio-temporal and spatio-spectral profiles for different spectral components clearly illustrate the model where the Bessel beam’s wavefront at the exit from the axicon can be divided into radial segments for which the wave vectors intersect. Phase velocity via propagation distance is estimated and compared with existing experimantal results. Results of the phase velocity calculation depend strongly on distance increment value, thus demonstrating superluminal or subluminal behavior.

Antenna Gain Impact on UWB Wind Turbine Blade Deflection Sensing

Antenna gain impact on ultra-wideband wind turbine blade deflection sensing is studied in this paper. Simulations are applied with a 4.5-m blade tip. The antennas with high gain (HG) and low gain (LG) in free space are simulated inside a blade. It is interesting to find that tip antennas with HG and LG in free space have similar realized gain when allocated inside blades, so that the emission power for the HG and LG antennas in blades can be the same. The antenna gain impacts on time-domain pulse waveforms and power distributions around a blade are carefully investigated (with the tip antenna inside a blade). Higher antenna gain enlarges both direct pulse and multipath but in different levels. To verify the simulations, time-domain measurements are performed with a full 37-m blade. Pulse waveforms and power delay profiles are measured. From all the studies, it follows that, with the similar effective (or equivalent) isotropic radiated power, an HG tip antenna inside a blade gives stronger direct pulse amplitudes and better pulse waveforms for accurate and reliable distance estimations than the LG. Moreover, the direct pulse with the HG antenna is also closer to the blade surface, which is important for the deflection sensing of a blade over 37 m.

Study on Pretreatment Methods of Terahertz Time Domain Spectral Image for Maize Seeds

Pretreatment is an important step before analyzing THz spectral image. In this paper, maize seed was taken as an example to focus on exploring pretreatment methods and steps for THz optical information extracting of different seeds tissue structures (embryo and endosperm) based on THz time-domain spectral images. Firstly, the double Gaussian filter was used to de-noise THz spectral image of seeds. Then, the image reconstruction methods based on Time Slice, Diff (Differential) Time Slice and Peak-Peak in Slice were compared to enhance the visual representation of different tissues of seeds. Finally, the Sober edge detection algorithm was used to extract the embryo and endosperm regions of the reconstructed image, and calculated mean value of the THz time-domain pulse in different regions as representative information of the embryo and endosperm. The experimental results show that the double Gaussian filter denoising, Diff Time Slice image reconstruction and Sober edge detection algorithm can provide a feasible technical basis and reference for pretreatment methods for the content and distribution of seed chemical components based on THz spectral image.

Time and Fourier domain jointly mode locked frequency comb swept fiber laser

Fourier domain mode locked (FDML) fiber lasers enable megahertz wavelength sweeping rate but suffer from the short coherent length. Discretization of the swept spectrum by a comb filter was demonstrated effective to enhance the coherent length. In this paper, we propose a novel discretization method of the FDML signal with an intracavity intensity modulator. We propose and successfully demonstrate a time and Fourier domain jointly mode locked fiber laser with a Fabry-Perot comb filter and an intensity modulator in the cavity. A 50 GHz free spectral range comb filter in the Fourier domain mode locked fiber swept laser slices the spectrum into a series of comb lines and chops the swept signal into short pulses in time domain. The temporal signal is detected by a photodetector to generate a series of ultrashort pulses to drive the intensity modulator to further polish the intracavity pulses. We experimentally realized the proposed time and Fourier domain jointly mode locked fiber laser. Discrete wavelength swept laser output with a wavelength spacing of ~0.4 nm in a 41 nm sweeping range has been achieved.

Coherence-length-gated distributed optical fiber sensing based on microwave-photonic interferometry.

This paper presents a new optical fiber distributed sensing concept based on coherent microwave-photonics interferometry (CMPI), which uses a microwave modulated coherent light source to interrogate cascaded interferometers for distributed measurement. By scanning the microwave frequencies, the complex microwave spectrum is obtained and converted to time domain signals at known locations by complex Fourier transform. The amplitudes of these time domain pulses are a function of the optical path differences (OPDs) of the distributed interferometers. Cascaded fiber Fabry-Perot interferometers (FPIs) fabricated by femtosecond laser micromachining were used to demonstrate the concept. The experimental results indicated that the strain measurement resolution can be better than 0.6 µε using a FPI with a cavity length of 1.5 cm. Further improvement of the strain resolution to the nε level is achievable by increasing the cavity length of the FPI to over 1m. The tradeoff between the sensitivity and dynamic range was also analyzed in detail. To minimize the optical power instability (either from the light source or the fiber loss) induced errors, a single reflector was added in front of an individual FPI as an optical power reference for the purpose of compensation.

Broadband Oscillator-Free THz Pulse Generation and Radiation Based on Direct Digital-to-Impulse Architecture

Broadband 0.03–1.1 THz signal generation and radiation are demonstrated based on an oscillator-free direct digital-to-impulse architecture with a 1.9-ps full width at half maximum and 130-GHz 3-dB bandwidth (BW) (200-GHz 10-dB BW) centered at 160 GHz. The radiated pulse achieves a peak pulse effective isotropic-radiated power of 19.2 dBm and peak pulse-radiated power of 2.6 mW. An ON/OFF impulse-shaping technique is introduced and implemented to suppress undesired ringing and to increase dc-to-radiated efficiency. The frequency-comb spectrum of the radiated pulse train with 5.2-GHz repetition rate is measured up to 1.1 THz. At a distance of 4 cm, the measured received SNR at 1 and 1.1 THz is 28 and 22 dB, respectively. A 1.1-THz tone is measured with a 10-dB spectral width of 2 Hz, demonstrating an extremely narrow spectral line width (two parts per trillion). Time-domain picosecond pulses are characterized using a custom femtosecond-laser-based terahertz time-domain spectroscopy system. Coherent spatial combining from two widely spaced chips is demonstrated. It is shown that the starting time of the radiated pulses is locked to the edge of the input digital trigger with a timing jitter of 270 fs. The chip is fabricated in a 130-nm SiGe BiCMOS process technology.

Graphene Characterization by Hilbert-Huang Transform-Based Terahertz Spectroscopy

We demonstrate a contact-less characterization method of monolayer graphene (MG) with all-fiber terahertz time-domain spectroscopy (THz-TDS) based on the Hilbert–Huang transform (HHT). The MG was grown on a Cu foil by chemical vapor deposition and then transferred to the poly (ethylene terephthalate) substrate by polymethyl methacrylate-assisted wet transfer. In our THz-TDS system, the time-domain pulse signal through MG is decomposed into eight components called the intrinsic mode functions (IMFs). It is found that the decomposed low-order modes are related to the original MG signal oscillations, whereas the other high-order modes correspond to the noise. The first IMF component can be used to extract the apparent graphene absorption peak near 0.7 THz, which could be more obvious with superposition of the second IMF due to the increasing spectral amplitude in the lower frequency range. The proposed HHT-based THz-TDS provides a more efficient way for graphene characterization.

Investigation of stimulus signals for a time domain microwave imaging system

Two different types of time domain signals, pulse and pseudo random noise (PN) sequence, were investigated for a time domain microwave imaging system in order to study the effect of stimulus signals on the system performance. The system's measurement accuracy was investigated when the two different stimulus signals were used, respectively. An imaging test was performed in order to show the effect of transmitted waveforms on the imaging quality. The results suggest that the PN sequence in general gives higher measurement accuracy than the pulse. Better imaging results, although not significant, were also obtained when the pseudo random sequence was employed.

Terahertz Pulse Reflection Imaging Using the Time-Domain Correlating Synthesis Method

The synthetic aperture method is an attractive technique for terahertz nondestructive imaging without strict restraints on the beam form or the Rayleigh length of focus. However, the limitation of the aperture of the receiver array results in distortion and restricted lateral resolution of the reconstructed image. This paper introduces a nonlinear imaging processing principle, the correlating synthesis method, which has been applied in ultrasonic measurements. Using a pulse reflection mode terahertz time-domain spectrometry system and pulse compression preprocessing, we reconstructed images of metal cylinders. The results of comparisons with a synthetic aperture method verify the efficiency of the correlating synthesis method for improving the lateral resolution and eliminating fake images.

Anderson localization in synthetic photonic lattices

Synthetic photonic lattices provide unique capabilities to realize theoretical concepts emerging in different fields of wave physics via the utilization of powerful photonic technologies. Here we observe experimentally Anderson localization for optical pulses in time domain, using a photonic mesh lattice composed of coupled fiber loops. We introduce a random potential through programmed electro-optic pulse phase modulation, and identify the localization features associated with varying degree of disorder. Furthermore, we present a practical approach to control the band-gap width in photonic lattices by varying the coupling between the fiber loops, and reveal that the strongest degree of localization is limited and increases in lattices with wider band-gaps. Importantly, this opens a possibility to enhance or reduce the effect of disorder and associated localization of optical pulses.

Evolution of optical force on two-level atom by ultrashort time-domain dark hollow Gaussian pulse

Based on the analytical expression of the ultrashort time-domain dark hollow Gaussian (TDHG) pulse, the optical force on two-level atoms induced by a TDHG pulse is calculated in this paper. The phenomena of focusing or defocusing of the light force is numerical analyzed for different detuning, various duration time, and different order of the ultrashort pulse. The transverse optical force can change from a focusing force to a defocusing force depending on the spatial-temporal coupling effect as the TDHG pulses propagating in free space. Our results also show that the initial phase of the TDHG pulse can significantly changes the envelope of the optical force.

Partition Study on Leakage Current Characteristics of Artificial Pollution Experiment on Catenary Insulator

In this paper, the catenary insulator FQX-25 is selected to do artificial pollution experiments. Firstly, study on the partition of experimental data by using the variation characteristics of the leakage current then re-determine the partition threshold of the safe zone, forecast zone and danger zone which are 10 mA and 30 mA. Secondly, the RMS and the pulse time domain entropy are obtained and the relationship between them and pollution level is fitted by Matlab. The research shows that the RMS of the leakage current and the pulse time domain entropy have very good exponential fitting relation with the equivalent salt deposit density (ESDD). The results of this study can provide a new way to evaluate the pollution level of insulators.

Application of the time-domain and frequency-domain radiative measurement signals in retrieving the spectral complex refractive index of microalgae

Time-domain (TD) and frequency-domain (FD) pulse lasers are employed as the measurement lasers to retrieve the spectral complex refractive indices of microalgae, respectively. The finite volume method and an improved quantum-behaved particle swarm optimization algorithm are employed as the direct and inverse problem algorithms. The sensitivity analyses of measurement signals with respect to spectral complex refractive index are studied firstly. Then, the spectral complex refractive indices of Chlamydomonas reinhardtii (C. reinhardtii CC125) are retrieved by TD and FD pulse lasers, respectively. Finally, fitness function values and radiative properties of C. reinhardtii CC125 calculated by retrieval results are also studied. Results show that TD sampling region ct ∈[L, L + ctp] and dimensionless FD sampling region ω⌢∗∈[5,10] are the effective sensitivity regions to improve the retrieval accuracy. The incident TD pulse laser can obtain more accurate results. Retrieval results of real part n show better accuracy and robustness than those of imaginary part k.

Enhanced Cherenkov phase matching terahertz wave generation via a magnesium oxide doped lithium niobate ridged waveguide crystal

When combined with a nonlinear waveguide crystal, Cherenkov phase matching allows for highly effective generation of high power and broadband terahertz (THz) waves. Using a ridged Lithium Niobate (LiNbO3) waveguide coupled with a specially designed silicon lens, we successfully generated THz waves with intensity of approximately three orders of magnitude stronger than those from conventional photoconductive antenna. The broadband spectrum was from 0.1 THz to 7 THz with a maximum dynamic range of 80 dB. The temporal shape of time domain pulse is a regular single cycle which could be used for high depth resolution time of flight tomography. The generated THz wave can also be easily monitored by compact room-temperature THz camera, enabling us to determine the spatial characteristics of the THz propagation.

Improving GFDM Symbol Error Rate Performance using “Better than Nyquist” Pulse Shaping Filters

Fourth generation (4G) cellular systems have been optimized to provide high data rates and reliable coverage to mobile users. New waveforms at the physical layer are needed. Generalized frequency division multiplexing (GFDM) is a candidate modulation for the fifth generation (5G) standard based on multi-branch multicarrier filter bank approach. A main characteristic of GFDM is its low out of band emission, achieved by means of a flexible time-domain pulse shaping of individual subcarriers. In this paper, the influence of the improved “Better than Nyquist” pulse shaping filters on symbol error rate (SER) performance of the GFDM system in the case of zero forcing (ZF) receiver is investigated. We considered their use in GFDM to evaluate the impact on SER performance in case of 16-QAM transmission over an additive white Gaussian noise channel. Moreover, we also considered the concept of the wavelet for better time-frequency localization of the pulse shaping filters by using the Meyer auxiliary function. Numerical results are reported to demonstrate the superior SER performance achieved.

Temporal cloak with large fractional hiding window at telecommunication data rate

We design and experimentally investigate a temporal cloak scheme using ultrashort pulse compression and time-domain Fraunhofer diffraction. An input continuous-wave probe beam is compressed to ultrashort pulse train based on self-phase modulation effect and chirp compensation using single mode fiber. Accordingly, wide temporal gaps appear to act as the cloaking windows. The train of ultrashort pulses can be converted to continuous wave after experiencing enough dispersion, indicating that the temporal gaps are closed. Thus, any time events will be hidden in the temporal gaps from observers. In our study, we demonstrated a temporal cloak system, which is able to hide 88% of the whole time period and cloak pseudorandom digital data at a bitrate of 10 Gbit/s. The relationships of cloaking window fraction versus pump power and the condition of cloaking off are also investigated. These results present a new feasible way towards obtaining a high-fractional cloaking window at telecommunication data rate and hiding real-world messages.

Time domain characteristics of wave motion in dispersive and anisotropic continuum acoustic metamaterials.

The authors study the wave propagation in continuum acoustic metamaterials whose all or not all of the principal elements of the mass tensor or the scalar compressibility can be negative due to wave dispersion. Their time-domain wave characteristics are particularly investigated by the finite-difference time-domain (FDTD) method, in which algorithms for the Drude and Lorentz dispersion pertinent to acoustic metamaterials are provided necessarily. Wave propagation nature of anisotropic acoustic metamaterials with all admissible material parameters are analyzed in a general manner. It is found that anomalous negative refraction phenomena can appear in several dispersion regimes, and their unique time-domain signatures have been discovered by the FDTD modeling. It is further proposed that two different metamaterial layers with specially assigned dispersions could comprise a conjugate pair that permits wave propagation only at specific points in the wave vector space. The time-domain pulse simulation verifies that acoustic directive radiation capable of modulating radiation angle with the wave frequency can be realized with this conjugate pair. The study provides the detailed analysis of wave propagation in anisotropic and dispersive acoustic mediums, which makes a further step toward dispersion engineering and transient wave control through acoustic metamaterials.

Investigation of a UWB Wind Turbine Blade Deflection Sensing System With a Tip Antenna Inside a Blade

An ultra-wideband (UWB) blade deflection sensing system with a tip antenna inside a blade is investigated in this paper. The lower UWB band of 3.1–5.3 GHz is utilized. This system composes of two UWB radio links between one antenna inside the blade tip and two antennas outside the blade root. Blade deflections are tracked via two radio links using delay-based distance estimation and triangulation. In order to build reliable radio links, time-domain pulse field distributions are simulated to optimize the in-blade tip antenna polarization and the locations of the two root antennas around the root surface. Full-blade time-domain measurements are proposed to verify the simulations and realize the blade deflection sensing with an in-blade tip antenna. With the optimized in-blade tip antenna polarization and two root antenna locations, an accuracy of 2 cm is achieved for the tip-root antenna distance estimation, and the sensing system can realize the deflection tracking with a maximum deviation of 0.21 m and a root mean squared error of 0.11 m.

Experimental Study on Sound Characteristics Produced by DC Corona and Pulsed Discharges

The audible noise (AN) production mechanism caused by corona discharge in ultrahigh voltage transmission lines is less understood. The study on sound pressure characteristic is necessary to obtain the inherent relation between AN and corona discharge. In this paper, the experimental research on single-point direct current (dc) corona discharge was evolved and the time-domain sound pressure waveform was obtained using capacitor microphone. The experiment results show that the dc corona discharge comprises multiple subsequent discharge pulses. The discharge voltage and the electrode polarity have a direct effect on the rising time and pulsed width of the single current pulse. The waveform of sound pressure produced by single discharge is a bipolar microsecond pulse, which lags behind the current pulse in time domain. The time delay is propagation time of sound signal from the discharge spot to the measurement spot. Based on the corona discharge characteristic in dc voltage, the experiment platform for nanosecond pulsed discharge was built to obtain the relation between discharge parameter and sound parameter. The characteristics of sound pressure in different discharge conditions were analyzed. The study results indicate that the sound pressure produced by pulsed discharge is similar with that produced by corona discharge. The experiment platform for pulsed discharge can be used to research the AN production mechanism by dc corona discharge.