Optical precursors with finite rise and fall time

Laser switching transients in the form of turnon and turnoff delays and finite rise and fall times limit the speed of high-bit-rate digital optical systems. The turnon delay can be reduced by adding a bias to the drive current so that the off-level is above threshold,1 with the constraint that the on-off ratio of the optical signal must be sufficiently large (~10) not to degrade system performance. Even if the off-level is above threshold, the turnon and turnoff delays are nonzero, and the rise and fall times are finite.

Acoustic emission (AE) signals are conventionally modelled as damped or decaying sinusoidal functions. A major drawback of this model is its negligible or zero rise time. This paper proposes an alternative model, which provides for the rising part of the signal without sacrificing the analytical tractability and simplicity of the conventional model. Signals obtained from the proposed model through computer programs are illustrated for demonstrating their parity with actual AE signals. Analytic expressions for the time-domain parameters, viz., peak amplitude and rise time used in conventional AE signal analysis, are also derived. The model is believed to be also of use in modelling the output signal of any transducer that has finite rise time and fall time.

Image artifacts caused by a temporally delayed response of a back illuminated photodiode deployed in a CT detector were studied. The temporal response pattern, characterized by a finite rise and fall time, is an intrinsic property of a photodiode. Generally, electron-hole pairs generated in the diode take time to diffuse to contacts where they get finally registered. In the case of backilluminated diodes, diffusion time is significantly prolonged, since photons hit the diode on the back. Electrons or holes only contribute to the signal, if they travel the full distance to the frontend, where contacts are located. To study the temporal behavior of a back illuminated photodiode a computer model for a standard third generation CT scanner was devised and simulations were carried out. Resulting image artifacts were quantified for various phantom and photodiode parameters. Simulations and theory demonstrate that for a given phantom, artifacts scale with rise/fall time and the angular speed of the scanner.

Passive mixers have become an essential component of SAW-less communication receivers in the last few years, this has encouraged many researchers to study their impedance characteristics. When observed from the input radio frequency (RF) port of the passive mixer, the impedance exhibits bandpass filtering characteristics centered around the frequency of the local oscillator (LO) signal. The theory of baseband impedance up-conversion is widely accepted in the analysis of passive mixer circuits as well as in describing the origin of this bandpass impedance characteristics. In this paper, we revisit this theory and show that it is not always accurate since it does not account for switching transients. We begin our work by showing that the most commonly used single-balanced mixer is not the right choice for a half-circuit used in analyzing the input impedance of a fully-balanced passive mixer. We propose an alternative half-circuit, and proceed with a novel analysis that accounts for switching transients. This switching-aware analysis allows capturing the switched-capacitor behavior of the passive mixer. We then derive closed-form equations for the mixer impedance using purely-capacitive loads as well as resistive-capacitive (RC) loads. We also discuss the impact of finite LO rise and fall times on the mixer impedance. Simulation results of quadrature passive mixers using ideal and MOSFET switches show very close match with the proposed theoretical analysis.

In this paper, the surface discharge current pulses of PEEK (polyether-ether-ketone) under positive repetitive square voltage are measured. The effects of voltage rise time and fall time on pulse parameters of discharge current, instantaneous voltage of discharge and time-lag of discharge are analyzed in detail. The results show that the amplitude of discharge current and the instantaneous voltage of discharge decrease, and the time-lag of discharge and the dispersion of time-lag increase with the increase of voltage rise time. With the increase of voltage fall time, the amplitude of discharge current decreases and the instantaneous voltage of discharge, the time-lag of discharge and the dispersion of time-lag increase. To analyze the influence mechanism of voltage rise time and fall time on surface discharge, the accumulation of surface charge under repetitive square voltage is regarded as a periodic process. Besides, the correlation between forward discharge and back discharge in the process of discharge is analyzed, and the development of discharge is analyzed by using the time-lag theory. Furthermore, the relationship between instantaneous discharge voltage and voltage slew rate is revealed, and the surface charge accumulation processes under different voltage rise times and fall times are compared to reveal the influence mechanism of rise time and fall time on the surface discharge under positive repetitive square voltage.

The carrier injection split‐ridge waveguide based optical modulator on SOI has been analyzed in terms of its transient performance. The device operating at both 1.3 and 1.55 μm was simulated and data were collected to compare the transient performance. The device performance is predicted using the 2‐D semiconductor package SILVACO (CAD) software under DC operation. The rise time, fall time and bandwidth for the device operating at both wavelengths were characterized. It is found that at 1.3 μm, the rise time and fall time are 36.5 ns and 13 ns respectively. Meanwhile at 1.55 μm, the rise time and fall time are 41 ns and 9 ns respectively. It is found that the 3 dB bandwidths for the device are 8.53 and 9.7 MHz for 1.55 and 1.3 μm respectively.

A comprehensive temporal analysis has been performed on the 319 brightest GRBs with T90 > 2s from the BATSE current catalog. The GRBs were denoised using wavelets and subjected to an automatic pulse selection algorithm as an objective way of identifying pulses and quantifying the eects of neighbouring pulses. The number of statistically signicant pulses selected from the sample was greater than 3000. The rise times, fall times, full-widths at half-maximum (FWHM), pulse amplitudes and pulse areas were measured and the frequency distributions are presented here. All are consistent with lognormal distributions provided the pulses are well separated. The distribution of time intervals between pulses is not random but compatible with a lognormal distribution when allowance was made for the 64 ms time resolution and a small excess (5%) of long duration intervals that is often referred to as a Pareto-L evy tail. The time intervals between pulses are most important because they may be an almost direct measure of the activity in the central engine. Lognormal distributions of time intervals also occur in pulsars and SGR sources and therefore provide indirect evidence that the time intervals between pulses in GRBs are also generated by rotation powered systems with super-strong magnetic elds. A range of correlations are presented on pulse and burst properties. The rise and fall times, FWHM and area of the pulses are highly correlated with each other. The pulse amplitudes are anticorrelated with the FWHM. The time intervals between pulses and pulse amplitudes of neighbouring pulses are correlated with each other. It was also found that the number of pulses, N, in GRBs is strongly correlated with the fluence and duration and that can explain the well known correlation between duration and fluence. The GRBs were sorted into three categories based on N i.e. 3 N 12, 13 N 24 and N 25. The properties of pulses before and after the strongest pulse were compared for three categories of bursts. No major dierences were found between the distributions of the pulse properties before and after the strongest pulse in the GRB. However there is a strong trend for pulses to have slower rise times and faster fall times in the rst half of the burst and this pattern is strongest for category 3 N 12. This analysis revealed that the GRBs with large numbers of pulses have narrower and faster pulses and also larger fluences, longer durations and higher hardness ratios than the GRBs with smaller numbers of pulses. These results may be explained by either homogeneous or inhomogeneous jet models of GRBs. The GRBs with larger number of pulses are closer to the axis if varies with the opening angle of the jet and the imprint of the jet is preserved in the pulse structure of the burst. The distribution of the number of pulses per GRB broadly reflects the beaming by the jet.

Temperature is an important factor affecting the dynamic processes of corona discharge, since the ionization, attachment, and mobility of charged particles are directly related to temperature. Most existing research has investigated the temperature effect on corona discharge through experimental works. However, they are hard to acquire the corona discharge microphysical processes, which are related to the formation of the current pulse and electromagnetic (EM) wave signals. This article proposes a numerical model with temperature-related transport parameters to study the temperature influence on microphysical processes and current pulses for negative corona. By comparing microphysical quantities such as the effective ionization coefficient, density, and velocity of space charges, the effect of temperature on the current pulse parameters including the rise and fall times, amplitude, and pulsewidth is systematically explained. It shows that as the temperature rises, the current pulse amplitude increases, while the other three parameters decrease. The rise time is dependent on the effective ionization coefficient. The fall time is affected by the velocities of negative and positive ions. The experimental results indicate the simulated current pulses are consistent with the measured ones, proving the effectiveness of the proposed numerical model with temperature variation.

Temporally resolved response of a natural type IIA diamond detector to single-particle excitation

Rotor position sensing is essential in providing stable operation for closed-loop control of variable reluctance (VR) motors over a wide speed range. Position sensing can be carried out by two different methods; either by using direct position sensors or by using an indirect sensing method. There are obvious disadvantages in using direct position sensors. In this paper further developments in indirect rotor position detection using the waveform detection (WD) technique for VR motors are described. In principle, rise and fall times of an active or inactive phase can be used for position detection. However, position detection by monitoring the rise or fall times of the active phase current has been found suitable only in a narrow operating range. In order to overcome this problem, the rise or fall times of a small magnitude diagnostic current injected into an inactive phase can be monitored for position detection. Theoretical analysis and simulation results reported elsewhere have revealed that the use of the inactive (diagnostic) phase chop current rise time leads to instability in the drive system and hence is not recommended. In this paper, the inactive (diagnostic) phase chop current fall time is used instead for position detection. A hard-wired digital closed-loop controller is designed and implemented to drive a 3-phase VR stepping motor. Experimental results obtained conclude that rotor position detection using the WD technique with diagnostic phase chop current fall time provides reliable position detection and stable operation for the drive system over a wide operating range. >

We experimentally investigate the frequency down-conversion through the four-wave mixing (FWM) process in a cold 85Rb atomic ensemble, with a diamond-level configuration. An atomic cloud with a high optical depth (OD) of 190 is prepared to achieve a high efficiency frequency conversion. Here, we convert a signal pulse field (795 nm) attenuated to a single-photon level, into a telecom light at 1529.3 nm within near C-band range and the frequency-conversion efficiency can reach up to 32%. We find that the OD is an essential factor affecting conversion efficiency and the efficiency may exceed 32% with an improvement in the OD. Moreover, we note the signal-to-noise ratio of the detected telecom field is higher than 10 while the mean signal count is larger than 0.2. Our work may be combined with quantum memories based on cold 85Rb ensemble at 795 nm and serve for long-distance quantum networks.

Ternary alloy CdS<sub><i>x</i></sub>Se<sub>1–<i>x</i></sub> has the physical properties of CdS and CdSe, and its band gap can be adjusted by changing the component ratio of the elements. The alloy has excellent photoelectric properties and has potential application in optoelectronic devices. Although one has made some research progress of the CdSSe-based photodetectors, their performances are still far from the commercial requirements, so how to improve the performance of the device is the focus of current research. In this work, a single crystal CdS<sub>0.42</sub>Se<sub>0.58</sub> nanobelt device is first prepared by thermal evaporation. Under 550 nm illumination and 1 V bias, the ratio of photocurrent to dark current of the device is 1.24×10<sup>3</sup>, the responsivity arrives at 60.1 A/W, and the external quantum efficiency reaches 1.36×10<sup>4</sup>%, and the detectivity is 2.16×10<sup>11</sup> Jones. Its rise time and fall time are about 41.1/41.5 ms, respectively. Secondly, after the CdSSe nanobelt is decorated by Au nanoislands, the optoelectronic performance of the device is significantly improved. Under 550 nm illumination and 1 V bias, the <i>I</i><sub>p</sub>/<i>I</i><sub>d</sub> ratio, responsivity, external quantum efficiency and detectivity of the device are increased by 5.4, 11.8, 11.8 and 10.6 times, respectively, and the rise time and fall time are both reduced to half of counterparts of single CdSSe nanobelt. Finally, the microscopic physical mechanism of the enhanced optoelectronic performance of the device is explained based on localized surface plasmon resonance of Au nanoislands. After the combination of gold nanoislands and CdSSe nanobelt, the difference in Fermi level between them results in the transfer of electrons from CdSSe nanobelt to Au nanoislands, thus forming an internal electric field at the interface, which is directed from CdSSe nanobelt to Au nanoislands. Under illumination, the electrons in the Au nanoislands acquire enough energy to jump over the Schottky barrier because of localized surface plasmon resonance. These photoexcited hot electrons are trapped and stored in extra energy levels above the conduction band minimum, and then are cooled down to the band edge, thus realizing the transfer of electrons from Au nanoislands to CdSSe nanobelt. Moreover, the internal electric field also greatly promotes the transfer of hot electrons from Au nanoislands to CdSSe nanobelt, and inhibits the recombination of carriers at the interface, resulting in large photocurrent. Our work provides an effective strategy for fabricating high-performance photodetectors without increasing the device area.

High-performance infrared emitters hold substantial importance in modern engineering and physics. Here, we introduce graphene/PZT (lead zirconate titanate) heterostructure as a new platform for the development of infrared source structure based on an electron–phonon coupling and emitting mechanism. A series of electrical characterizations including carrier mobility [11,361.55 cm2/(V·s)], pulse current (30 ms response time), and cycling stability (2000 cycles) modulated by polarized film was provided. Its maximum working temperature reaches ∼1041 K (∼768°C), and it was broken at 1173 K (∼900°C) within ∼1.2 s rise time and fall time. Based on Wien’s displacement law, the high temperature will lead to near–mid–far thermal infrared when the heterostructure is applied to external voltages, and obvious bright white light could be observed by the naked eye. The changing process has also been recorded by mobile phone. In subsequent infrared emitting applications, 11 V bias voltage was applied on the PZT/graphene structure to produce the temperature change of ∼299 to 445 K within ∼0.96 s rise time and ∼0.98 s fall time. To demonstrate its optical information transmission ability, we exhibited “N, U, C” letters by the time-frequency method at 3 mm×3 mm@20 m condition. Combining with spatial Morse code infrared units, alphabetic information could also be transmitted by infrared array images. Compared with the traditional infrared emitter, the electron–phonon enhancing mechanism and high-performance emission properties of the heterostructure demonstrated a novel and reliable platform for further infrared optical applications.

Lung cancer remains one of the most common malignant tumors. Identifying cervical lymph node metastases is important to stage lung cancer, evaluate patients’ prognosis and select appropriate treatments. This study evaluated the utility of conventional ultrasound combined with contrast-enhanced ultrasound (CEUS) quantitative analysis in diagnosing lung cancer metastases in the lymph nodes of the neck. The cervical lymph nodes of 39 patients with primary lung cancer who underwent CEUS and ultrasound-guided needle biopsy were assessed. Routine type B ultrasound was used to record the morphology of the lymph nodes and the presence of lymphatic hilum structure. The Vuebox quantitative analysis software was used to collect lymph node CEUS peak enhancements (PE), rise time (RT) and fall time (FT). The receiver operating characteristic (ROC) curve was used to evaluate the diagnostic value of various parameters and imaging characteristics. The cohort comprised 26 cervical lymph nodes with lung cancer metastasis and 13 cervical lymph nodes with reactive hyperplasia. Most hilum of lymph gland were absent in the metastasis group (92.30%), most lymph nodes showed a full shape (80.77%), and few were irregularly shaped (19.23%). Most hilum of lymph gland were present in the hyperplasia group (76.95%) and showed a full shape (61.54%). The PE of the metastasis group (4852.29 a.u) was significantly lower than the hyperplasia group (11,464.70 a.u), while the difference between RT and FT indicators was not significant. ROC analysis showed that combined PE of the quantitative ultrasound contrast analysis and the presence of the lymphatic phylum structure showed satisfactory diagnostic ability for detecting lung cancer lymph node metastasis in the neck (sensitivity 73.10%, specificity 76.90%, accuracy 48.73%, p = 0.004). Conventional ultrasound observation of lymph node structure combined with quantitative analysis of peak enhancement of contrast-enhanced ultrasound showed promising value for diagnosing lung cancer metastatic lymph nodes.

This thesis examines the Class D tuned amplifier, emphasizing the factors which govern its use as a Class D power oscillator. The relative merits of the voltage-switched and current-switched modes for use in an oscillator are discussed. The most desirable properties are exhibited by the current-switched mode, therefore the current-switched amplifier is examined as a basis for the oscillator circuit. A comparison of sinusoidal drive and quasi-voltage drive for the current-switched amplifier illustrates the improved collector current rise time capability of the quasi-voltage drive. Factors such as saturation voltage, base storage time, current rise time, and passive collector circuit components are investigated to determine their effect on the overall efficiency of the quasi-voltage driven amplifier. It is shown the effect of long storage time may be neutralized by delay time and that rise and fall times are short and relatively unimportant. Saturation voltage and tuned circuit losses are shown to present the major efficiency limitation both at higher powers and higher frequencies. It is demonstrated that the phase lag caused by storage time and rise time must be compensated if the Class D oscillator is to be efficient.