Pulse-duration

Secondly, a short pulse duration is required in order to achieve a good axial resolution, i.e. two signals close together should be detected without interference. The task can be, for example, to detect a small reflector close to the surface or back wall of the test object, as the inspection has to cover the total volume as complete as possible, including the near-surface regions. 

In next calculations normalized values of CCF are used. Pulse duration of reference q(l) signals is assumed to be 8 periods. 

CCF dependences on the -factor of loaded probe vibrators are shown in Fig.4. For s(l) pulses growth of 2 factor increase CCF maximum amplitude and selectivity. In this case the higher the Q, the longer the pulse duration and the more its periods contribute to the processing. F or q(t) pulses rising of g-factor decrease CCF maximum amplitudes and reduce the selectivity. As q(l) pulse consists of a few first periods only its maximum amplitude depends on Q. the higher the Q, the lower the final pulse amplitude, and therefore, CCF amplitude and selectivity. 

The quantities in this fomuila are defined as in equation Bl.5,32. but with the laser parameters translated into more convenient tenns is the average power at the indicated frequency is the laser pulse duration ... 

A sine-shape has side lobes which impair the excitation of a distinct slice. Other pulse envelopes are therefore more commonly used. Ideally, one would like a rectangular excitation profile which results from a sine-shaped pulse with an infinite number of side lobes. In practice, a finite pulse duration is required and therefore the pulse has to be truncated, which causes oscillations in the excitation profile. Another frequently used pulse envelope is a Gaussian frmction ... 

The main cost of this enlianced time resolution compared to fluorescence upconversion, however, is the aforementioned problem of time ordering of the photons that arrive from the pump and probe pulses. Wlien the probe pulse either precedes or trails the arrival of the pump pulse by a time interval that is significantly longer than the pulse duration, the action of the probe and pump pulses on the populations resident in the various resonant states is nnambiguous. When the pump and probe pulses temporally overlap in tlie sample, however, all possible time orderings of field-molecule interactions contribute to the response and complicate the interpretation. Double-sided Feymuan diagrams, which provide a pictorial view of the density matrix s time evolution under the action of the laser pulses, can be used to detenuine the various contributions to the sample response [125]. 

Table B2.5.2. Examples for pulsed lasers with different pulse durations and corresponding path lengths. For...

Pulse duration Laser Availability Optical path... 

Figure 9.42 Intensity of sodium atom fluorescence as a function of time following excitation of Nal to the V potential with a pump wavelength of 307 nm (pulse duration ca 50 fs) and a probe wavelength of (a) 575 nm, (b) 580 nm, (c) 589 nm, and (d) 615 nm. (Reproduced, with permission, from Rose, T. S., Rosker, M. J. and Zewail, A. H., J. Chem. Phys., 91, 7415, 1989)...

Temporal Characteristics. Laser operation may be characterized as either pulsed or continuous. There are a number of distinctive types of pulsed laser operation having widely different pulse durations. 

The transversely excited atmospheric-pressure (TEA) laser, inherently a pulsed device rather than a continuous laser, is another common variety of carbon dioxide laser (33,34). Carbon dioxide—TEA lasers are an important class of high-power pulsed lasers. Pulse durations are in the submicrosecond regime peak powers exceed 10 MW. 

Ruby lasers are frequently operated in the normal pulse mode, ie, pulse durations are around 1 ms and pulse energy up to tens of joules, or in the... 

Q-switched mode, ie, pulse duration are on the order of a few tens of nanoseconds and peak power in excess of 10 W. 

The availability of lasers having pulse durations in the picosecond or femtosecond range offers many possibiUties for investigation of chemical kinetics. Spectroscopy can be performed on an extremely short time scale, and transient events can be monitored. For example, the growth and decay of intermediate products in a fast chemical reaction can be followed (see Kinetic measurements). 

This reaction has been carried out with a carbon dioxide laser line tuned to the wavelength of 10.61 p.m, which corresponds to the spacing of the lowest few states of the SF ladder. The laser is a high power TEA laser with pulse duration around 100 ns, so that there is no time for energy transfer by coUisions. This example shows the potential for breakup of individual molecules by a tuned laser. As with other laser chemistry, there is interest in driving the dissociation reaction in selected directions, to produce breakup in specific controllable reaction channels. 

A typical example might involve use of a krypton fluoride excimer laser operating at 249 nm with a pulse duration around 100 nanoseconds and a pulse repetition rate which can be varied up to 200 Hz. For metal deposition, energy densities in the range from 0.1 to 1 J/cm per pulse are typical. 

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

Pulsed ft mode esr instmments have appeared beginning in the mid-1980s. These collect digitized time-domain spectra which may be processed into the frequency domain as are nmr data. Pulse durations are much shorter than in nmr with typical 90° times of 8—20 ns. 

The resolution in an ultrasound image is, among other things, related to the duration of the ultrasound pulse, ie, the shorter the pulse the better the resolution. Imaging may not be performed when the pulse duration is longer than the time to receive an echo. The shorter the ultrasound pulse the more difficult it is to discern it from noise, and the poorer the SNR of the image. As the pulse duration is decreased, the power of the ultrasound pulse is typically increased to compensate for the poorer SNR. 

When eompared to explosive teehniques, guns offer greater control of the shoek eonditions imposed on the speeimen, ineluding shoek-pressure magnitude, pulse duration, and unloading eonditions. Several gun types are... 

In this chapter, we will review the effects of shock-wave deform.ation on material response after the completion of the shock cycle. The techniques and design parameters necessary to implement successful shock-recovery experiments in metallic and brittle solids will be discussed. The influence of shock parameters, including peak pressure and pulse duration, loading-rate effects, and the Bauschinger effect (in some shock-loaded materials) on postshock structure/property material behavior will be detailed. 

To illustrate the effect of radial release interactions on the structure/ property relationships in shock-loaded materials, experiments were conducted on copper shock loaded using several shock-recovery designs that yielded differences in es but all having been subjected to a 10 GPa, 1 fis pulse duration, shock process [13]. Compression specimens were sectioned from these soft recovery samples to measure the reload yield behavior, and examined in the transmission electron microscope (TEM) to study the substructure evolution. The substructure and yield strength of the bulk shock-loaded copper samples were found to depend on the amount of e, in the shock-recovered sample at a constant peak pressure and pulse duration. In Fig. 6.8 the quasi-static reload yield strength of the 10 GPa shock-loaded copper is observed to increase with increasing residual sample strain. 

Figure 6.8. Plot of the quasi-static reloaded yield stress of shock-loaded copper versus the natural logarithm of residual strain for a 10 GPa symmetric shock with 1 /is pulse duration.

Increasing the driver plate velocity and driver plate thickness for a fixed sample assembly results in an increased peak shock pressure and pulse duration. As far back as the review of Appleton [32] in 1965 on metallurgical... 

---