Average Pulse Width Research Articles

Source and Qp parameters from pulse width inversion of microearthquake data in southeastern Sicily, Italy

Source and Qp parameters were estimated from the inversion of first arrival P waveform durations of about 300 microearthquakes recorded at a digital seismic network operating in southeastern Sicily. The average risetime and pulse width at each station do not show large differences, allowing us to exclude significant differential attenuation site effects. A first Qp estimate was obtained by applying the classical risetime method, under the assumption of a point‐like source time function. In order to investigate the effect of directivity due to the finiteness of seismic sources, new nonlinear relationships, based on a circular crack model rupturing at a constant velocity, were numerically built. These relationships were used to formulate a nonlinear inverse method for retrieving source (radius, dip, and strike of the circular crack) and Qp parameters from the inversion of risetime and pulse width data. The application of the method produced a better fit of the observed data and a Qp value higher than that obtained by applying the risetime method. The discrepancy between the different Q estimates may be due to a trade‐off among source dimension and Qp, as we inferred from a test on a subset of low‐magnitude events (Ml ≤ 2.5). A good agreement with independent estimates of fault plane solutions, as inferred from P polarities and S polarizations, was found. The estimated stress drops are generally very low (0.1–10 bars). This suggests that the background seismic activity in southeastern Sicily is related to fault segments and/or weakened zones where great stress accumulations are hindered.

Moment tensor inversion of microseisms from the B-sand propped hydrofracture, M-site, Colorado

We apply a moment-tensor inversion algorithm to microseismic events generated by hydrofracturing. Our dataset consists of microseismic events associated with a hydraulic fracturing experiment conducted at the GRI/DOE M-site near Rifle, Colorado. The event recordings were from a vertical string of 3-component accelerometer pods that are cemented in a borehole. The accelerograms clearly show P and S waves, which can be rotated into the SH and SV components.To invert the P and S amplitudes for moment tensors, we use just the first two peaks after the first arrival. Using this amplitude information and the average pulse width, we formed a symmetric, stylized pulse for integration and we inverted for the moment tensor.Resolution tests revealed that with the source (microseism) to receiver geometry we have at the M-site, where all receivers are in one borehole and at the same azimuth from the source, we can extract the five deviatoric components of the moment tensor using combinations of P, SH and SV amplitudes (for example, a minimum data set of 3 P-wave, 2 SV-wave, and 1 SH-wave amplitudes). To obtain the sixth component, which is the isotropic component, requires amplitude information from receivers at another azimuth.We studied seven high-quality events in detail and tested their results against two alternative source geometries. These microseismic events tend to have a major-double–couple nodal plane aligned parallel to the main hydrofracture plane, rather than have their tension (T) axes aligned parallel to the regional minimum principal-compressive-stress direction. The events have seismic moments (M0) on the order of 10−5Nm and moment magnitudes (Mw) of about −2.5.

Passively mode-locked Nd:YVO 4 laser with up to 13 GHz repetition rate

Summary form only given. We demonstrate passive modelocking of a Nd: vanadate (Nd:YVO/sub 4/) laser to repetition rates of up to 13 GHz. With Ti:sapphire pumping, we achieved modelocking at 13 GHz with 200 mW of average output power and pulse widths of 9.5 ps. With diode pumping, we achieved modelocking at a repetition rate of 12.6 GHz with 195 mW of average output power and a pulse duration of 8.8 ps. In all cases we operated the laser at its fundamental cavity repetition rate, i.e. the repetition rate of the laser equals the laser cavity free-spectral range, as opposed to harmonic modelocking where the repetition rate is an integer multiple of the cavity free-spectral range.

Taladrado de pizarra mediante láser de Nd:Yag

A serie of experiments to investigate the feasibility of using a Nd: YAG laser to process slate tiles and the influence of the processing parameters have been undertaken. The objective of this work is to investigate the capabilities of a Nd:YAG pulsed laser to drill tiny holes in slate tiles, in order to produce a better quality drilled slate tile in a reduced time. Part of the results obtained in a systematic study about the influence of the processing parameters, such as average power, pulse width, frequency, pressure and type of assisting gas is presented.

Generation of high-current (1kA) subpicosecond electron single pulse

An achromatic pulse compressor and a grid pulser system were designed for generation of a high-current single pulse in a subpicosecond time domain and were installed to the S-band (2856 MHz) linear accelerator of the University of Tokyo. 35 MeV electron pulses with the pulse width of 10 ps have been compressed by the achromatic magnetic pulse compression system. Generation of subpicosecond single-electron pulse with the charge of 1 nC has been succeeded. The shortest and average pulse widths in FWHM are 670 and 890 fs, respectively. The length of the pulse has been measured by a femtosecond streak camera with a time resolution of 200 fs. No satellite of the main pulse could be observed.

Feedback Control of Colliding-Pulse Mode-Locking Operations in a Nd:YAG Laser.

The pulse width of a flashlamp pumped Nd:YAG laser in 120cm long linear cavity was significantly shortened by joint operation of CPM (Colliding Pulse Mode-locking) and FCM (Feedback Controlled Mode-locking). First, only CPM was operated with a saturable absorber cell of 0.5mm thickness set at one quarter of the cavity length apart from the end mirror. The average and shortest pulse widths were 15.8ps and 8.0ps in Gaussian shape (14.4ps and 7.3ps in sech2), respectively, while about 40 pulses of 2ns interval were emitted out within a one-shot pulse train of about 80ns duration. The average energy per pulse was about 40μJ. Next, in addition of CPM, FCM was done by a piece of 0.45mm thick GaAs wafer near the coupling mirror. The average and shortest pulse widths were 5.4ps (4.9ps) and 5.1ps (4.6ps), respectively, while the number of pulses (≈10μJ) increased by several times within the elongated train of about 500ns duration. Last, the pulse shortening mechanisms were also discussed by observing the down-chirping effect with a grating pair.

Gamma-Ray Burst Peak Duration as a Function of Energy

Gamma-ray burst time histories often consist of many peaks. These peaks tend to be narrower at higher energy. If gamma-ray bursts are cosmological, the energy dependence of gamma-ray burst time scales must be understood in order to correct the time scale dependence due to the expansion of the universe. By using the average autocorrelation function and the average pulse width, we show that the narrowing with energy follows, quite well, a power law. The power law index is about -0.4. This is the first quantitative relationship between temporal and spectral structure in gamma-ray bursts. It is unclear what physics causes this relationship. The average autocorrelation has a universal shape such that one energy range scales linearly with time into all other energy ranges. This shape is approximately the sum of two exponential.

Dynamics of a mode-locked low-pressure photolytic I* laser

A longitudinally pumped photolytic iodine laser has been reliably mode-locked over a broad range of iodide pressures (1-30 Torr). Average mode-locked pulse widths ranging from 1.4-2.5 ns were observed as a function of CF/sub 3/I iodide pressure and added buffer gas (Ar). Low-pressure data indicate an increased gain bandwidth as a result of an initial nonthermal I* atom distribution following rapid (<20 ns) photolysis of the parent molecule, CF/sub 3/I. Nearly 1.75 eV of excess pump energy is available and is deposited into the internal vibrational and rotational degrees of freedom of the large radical and into the translation of the nascent iodine atom, allowing for high iodine atom velocities in the frame of observation. A diode laser system tuned to the 3-4 transition of spin-orbit excited iodine atoms was used to observe the time-dependent behavior of line center gain along the optical axis. Preliminary data indicate an I* atom velocity relaxation on the order of 1 /spl mu/s for an iodide pressure of 1 Torr, and approximately 300 ns for a pressure of 10 Torr. At 5 Torr of CF/sub 3/I, the laser mode build up time is 200 ns, allowing access to the increased gain bandwidth and resulting in mode-locked pulse widths less than the predicted transform-limited pulse width for an inhomogeneous gas laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mode locking with linear and nonlinear phase shifts

The mode locking of a cw Nd:YAG laser by translation of one of the cavity mirrors is reported. The mode-locking effect is greatly enhanced when an intracavity nonlinear KTP crystal is used. Stable pulse trains with a 60-ps average pulse width are obtained. Theoretical analysis shows that the combined effect of linear phase shift imposed by the moving mirror and the nonlinear phase shift in the Kerr medium gives rise to a saturable-absorber-like behavior in a homogeneously broadened laser. The theory agrees well with the experimental results as well as with results obtained with a Ti:sapphire laser.

Generation of Steady-state Ultrashort Laser Pulses Based on a Novel Nonlinear Michelson Interferometer

Abstract A pulsed neodymium-doped yttrium aluminium garnet laser mode locked by a novel nonlinear Michelson interferometer, where one branch contains a frequency-doubling potassium titanyl orthophosphate crystal, is reported for the first time. Under the optimal experimental condition, shorter than 10 ps average pulse width at 1·06 μm and up to 95% reproducibility of mode locking can be obtained, and the stability of output energy is about 7·5%. Finally, the reason why its mode-locking output characteristics have been significantly improved is also analysed in this paper.

Cyclic wavelength jitter in actively mode-locked semiconductor lasers

Large spectral broadening has been observed in an actively mode-locked semiconductor laser tuned for minimum average pulsewidth. Numerical simulations show that the spectral broadening was a result of cyclic wavelength variations associated with large amplitude and timing instabilities.

Stability and spectral behavior of grating-controlled actively mode-locked lasers

Amplitude, timing, and wavelength instabilities in a semiconductor mode-locked laser are studied experimentally and by using an accurate numerical model. It is shown that these instabilities can occur when the RF drive frequency is tuned to give a minimum average pulsewidth. It is shown that experimental measurement techniques, such as sampling and averaging, can mask these instabilities. Using this numerical model, it is shown that the wavelength instability is associated with the amplitude and timing instabilities and that the large broadening of the optical spectrum observed experimentally is caused by a cyclic wavelength instability. >

Steroid‐Tipped Leads Versus Porous Platinum Permanent Pacemaker Leads: A Controlled Study

There is little data directly comparing steroid-tipped permanent pacemaker leads to otherwise state-of-the-art porous platinum leads. Eighteen patients receiving unipolar generators capable of low voltage outputs were randomized at the time of implant to receive either steroid-tipped leads or porous platinum leads. All leads were unipolar, tined, passive fixation, and placed in the right ventricular apex or atrial appendage. This study is single center. At implant, threshold pulse width was determined at 3 voltages (2.5, 1.5, and 0.8 V). Follow-up thresholds were determined at weeks 1, 2, 3, and 4, and at 3 and 6 months. There was no difference in implant thresholds or amplitudes for sensing. By 2 weeks postimplant, lower thresholds were noted for the steroid leads, and this discrepancy grew more significant with time. There was no significant postimplant rise in threshold for steroid-tipped leads. At 6 months, the average threshold pulse width for ventricular steroid leads at 0.8 V was 0.3 +/- 0.1 msec. In contrast, five patients with standard leads did not capture at maximum pulse width at 0.8 V (P less than 0.0001). There was no significant difference in the amplitude of the chronic atrial electrogram. This study shows steroid-tipped leads to offer a significant advantage in reducing thresholds early postimplant and chronically.

Switched control of bilinear converters via pseudolinearization

A method is proposed for designing regulating feedback control laws in bilinear networks such as the boost and the buck-boost DC-DC power supplies. A pseudolinearization approach is taken to the average pulsewidth modulated controlled model with physically meaningful local state coordinate transformations. For such transformations, the perturbed average model is independent of the operating point and exhibits a linear structure in Brunovsky's canonical form. The feedback regulation scheme is thus considerably simplified from the conceptual viewpoint. >

パルスめっきによる亜鉛の電析形態

The morphology of pulse electrodeposited zinc from acidic zinc sulfate baths without the presence of additives has been studied by means of scanning electron microscopy and X-ray diffraction. The zinc deposit exhibited a bright smooth surface at an appropriate pulse condition. The current efficiency of this pulse plating increased beyond that of d-c plating with increasing the average current density at low duty cycles. The grain size of zinc deposit was also dependent on the average current density and pulse time. It decreases with increasing the average current density. At constant average current density and pulse width, the grain size decreased with decreasing the duty cycle. The reduction of grain size at appropriate pulse condition was explained in terms of an enhanced nucleation rate resulting from high overpotential attainable in pulse plating. The preferred orientation of the zinc deposit changed with increasing the average current density in the order [0001], [1120] and [1010], which is consistent with that of Pangarov's theory based on nucleation energetics.

Theory of light pulse propagation through thick clouds

In the previous paper, the diffusion equation was derived from the ordinary space-time transport equation; corresponding boundary conditions on a surface of medium discontinuity were also obtained. In this paper, the theory is applied to evaluate the reflected and transmitted wave intensities of a plane wave pulse incident upon a scattering slab and the pulse shapes are displayed for a wide range of parameters. In both cases, the pulses are found to decrease exponentially with time t in the asymptotic domain, even when the scatterers have no absorption. This forms a marked contrast to the case of semi-infinite or infinite scattering media where the intensities decrease by the factor t3/2. An expression for average pulse width is obtained analytically for the pulses transmitted through the scattering slab and is given as a function of the mean cosine of the scattering angle, the optical thickness, and the absorption cross section. The expression obtained for pulse width, with a reasonable value of absorption cross section, seems to show good agreement with the experimental results of Bucher and Lerner concerning optical pulse propagation through thick clouds.

The effect of pulse to pulse variation on ultrashort pulsewidth measurements

An alternate interpretation is presented for the exponential shapes of the second harmonic autocorrelation functions obtained from synchronously pumped mode locked dye laser systems. The shape is reinterpreted as being due to a weighted average of pulses of varying widths and smoothly varying pulse shapes in time. Such pulse width measurements in the past have averaged over a large number of pulses. Several pulse shapes and pulse width distributions are examined all leading to exponential shaped autocorrelation functions. This interpretation implies that the currently determined pulsewidths are shorter, possibly many times shorter than the actual average pulsewidths. A method for selectively descriminating the shortest pulses from the train to increase the time resolution of current laser systems is presented.