Pulse shaping

As described in the section on nonlinear absorption, the transmission of a pulse which is short compared to the various molecular relaxation times is determined by its energy content. A measurement of the energy transmission ratio will then give the peak intensity of the pulse when its pulse shape is known 44 . In fact, the temporal and spatial pulse shape is of relatively little importance. Fig. 11 gives the energy transmission as a function of the peak intensity I [W/cm2] for the saturable dye Kodak 9860 with the pulse halfwidth as a parameter. It is seen that this method is useful in the intensity region between 10 and 1010 MW/cm2 for pulses with halfwidths greater than 5 to 10 psec. Since one can easily manipulate the cross-section and hence the intensity of a laser beam with a telescope, this method is almost universally applicable. 

Since the transmission of a nonlinear absorber is lower in the low-intensity wings of a laser pulse traveling through the absorber than in the peak of the pulse, the pulse shape is generally distorted in such a way that the pulse width is decreased. The front wing of the laser pulse is especially steepened since more energy is used up in the first portion of the pulse to excite dye molecules. 

This effect can be used to produce shorter pulses than are obtainable by modelocking alone. Using five transits through a saturable absorber cell (and 4 ampli- 

The pulse produced at the output of a radiation detector has to be modified or shaped for better performance of the counting system. There are three reasons that necessitate pulse shaping  

To prevent overlap. Each pulse should last for as short a period of time as possible, and then its effect should be abolished so that the system may be 

For special pulse manipulation. The detector pulse may, in certain applications, need special pulse shaping to satisfy the needs of certain units of the counting system. As an example, the signal at the output of the amplifier needs to be stretched before it is recorded in the memory of a multichannel analyzer (see Sec. 10.12). 

The pulse-shaping methods used today are based on combinations of RC circuits and delay lines. For example, the use of a CR-RC circuit combination produces the pulse shown in Fig. 10.15. The exact shape and size of the output pulse depends on the relative magnitudes of the time constants Cjf and C2f 2-The use of the CR-RC circuit combination provides, in addition to pulse shaping, a better signal-to-noise ratio by acting as high-pass and low-pass filter for undesired frequencies. 

If one adds more RC integrating circuits, the pulse will approach a Gaussian shape (Fig. 10.16). 

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Fig. 8 Pulse shape (top) and spectrum (bottom) for a 2 MHz immersion probe with PZT (left) and composite transducer (right)...

Fig.6 shows the pulse shape at lOMHz in each point in Fig 5. As for measurement dB is constant because the material of the incidence of ultrasonic wave. The pulse amplitude has changed according to the thickness of the insert metal. Moreover, the direction of the first part of the pulse waveis different according to the material of the incidence of ultrasonic wave. 

The pulse shape in each point in Fig.7 (a) are shown in Fig.8 respectively. The direction of the intial part of pulse shape and pulse amplitude has changed by the defect position when the defect area s ratio Sr/So is a constant value. 

Fig.6 shows the pulse shape on the contact surface in ( i ), ( ii) and (iii) of Fig.5 The intial part of the pulse shapes on ( 1 ) and (ii ) became the minus direction. On the other hand,the intial part of the pulse shape on (iii) became the plus direction So that the pulse shape cause the reverse of the phase.

The intial part of the pulse shapes are a direction of the minus as the incidence S45C side for the ultrasonic wave. 

On the other hand, the intial part of the pulse shapes on the Sa /So=20 ... 

So that the pulse shape cause the reverse of the phase. 

The next step, therefore, is to address the question how is it possible to take advantage of the many additional available parameters pulse shaping, multiple pulse sequences, etc—m general an E(t) with arbitrary complexity—to maximize and perhaps obtain perfect selectivity Posing the problem mathematically, one seeks to maximize... 

Figure Bl.10.2. Schematic diagram of a counting experiment. The detector intercepts signals from the source. The output of the detector is amplified by a preamplifier and then shaped and amplified friitlier by an amplifier. The discriminator has variable lower and upper level tliresholds. If a signal from the amplifier exceeds tlie lower tlireshold while remaming below the upper tlireshold, a pulse is produced that can be registered by a preprogrammed counter. The contents of the counter can be periodically transferred to an online storage device for fiirther processing and analysis. The pulse shapes produced by each of the devices are shown schematically above tlieni.

The integral describes the spatial amplitude modulation of the excited magnetization. It represents the excitation or slice profile, g(z), of the pulse in real space. As drops to zero for t outside the pulse, the integration limits can be extended to infinity whereupon it is seen that the excitation profile is the Fourier transfonn of the pulse shape envelope ... 

Narrow transitions (small a) yield large peak voltages and narrow pulse shapes (PIF q). The width at half height is given by equation 4 where... 

Acoustic Measurements. Measurement of the propagation of ultrasonic acoustic waves has been found useful for determining the viscoelastic properties of thin films of adhesives. In this method, the specimen is clamped between transmitting and receiving transducers. The change in pulse shape between successive reverberation of the pulse is dependent on the viscoelastic properties of the transmitting material. Modulus values can be calculated (267,268). 

Pulse shapes other than rectangular can be used to obtain the same result. Triangular or Gaussian pulses could be used, for example. The Umit must be taken as the pulse duration becomes infinitesimally short while the amount of injected tracer remains finite. Any of these limits will correspond to a delta function input. 

For studies in molecular physics, several characteristics of ultrafast laser pulses are of crucial importance. A fundamental consequence of the short duration of femtosecond laser pulses is that they are not truly monochromatic. This is usually considered one of the defining characteristics of laser radiation, but it is only true for laser radiation with pulse durations of a nanosecond (0.000 000 001s, or a million femtoseconds) or longer. Because the duration of a femtosecond pulse is so precisely known, the time-energy uncertainty principle of quantum mechanics imposes an inherent imprecision in its frequency, or colour. Femtosecond pulses must also be coherent, that is the peaks of the waves at different frequencies must come into periodic alignment to construct the overall pulse shape and intensity. The result is that femtosecond laser pulses are built from a range of frequencies the shorter the pulse, the greater the number of frequencies that it supports, and vice versa. 

Straightforward application of OCT as described above often results in a quite complicated pulse shapes and may especially introduce some high frequency components, which are difficult to realize experimentally, into the pulse. It is thus highly desirable to find an optimized pulse with spectral components within a predefined frequency range. With this end in view the projected search direction is subjected to a spectral filter... 

The rf transmitter amplifies an rf pulse signal of about 1 mW up to several W or up to several kW. The amplifier should work in a linear mode (class AB) because excitation pulse shape for slice selection must be reproduced. Class AB rf transmitters such as these with blanking gates are widely available commercially. 

Dazhu Y, Yongjun Z, Mobius S. 1991. Rapid method for alpha counting with extractive scintillator and pulse shape analysis. J Radioanal Nucl Chem 147(1) 177-189. 

The response of the overall structure may be determined by the same techniques described above. The difference between directly loaded elements and supporting elements is the force amplitude and pulse shape of the applied loading. The loading on the overall stmctural system is determined from the reaction force time histories from directly loaded elements. Note that the loads on supporting members, frames, or shear walls, in some cases, may comprise reaction forces from pressures acting on the front and back faces of the structure simultaneously, taking into account the different arrival times of the blast wave. 

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