Pulse mode dispersion

At/L is referred to as pulse mode dispersion (PMD). The length dependence for this dispersion is linear for short lengths and increases as the square root of length for long spans. 

Fig. 19. Pulse broadening due to mode dispersion in a fiber with near optimum refractive index profile23. (Reproduced by the permission of IEE)...

Figure 12.8 The two mtegories of detectors used for energy dispersive X-ray fluorescence spectrometry. (a) Proportional counter used in pulse mode (b) Cooled Si/Li diode detector using Peltier effect (XR detector by Amptek Inc.) (c) Functioning principle of a scintillation detector containing a large size reverse polarized semi-conductor crystal. Each incident photon generates a variable number of electron-hole pairs. The very high quantum yield enables the use of low power primary sources of X-rays (a few watts or radio-isotopic sources).

When shorter pulse lengths are necessary these can be generated by mode locking a dye laser as discussed in section 13.6. The very wide gain bandwidth of dye lasers should theoretically permit the generation of sub-picosecond pulses, but dispersion in the cavity optics and other problems have so far imposed a lower limit of about 1-5 ps on the attain-... 

Depending on how the secondary magnetic field is applied, there are two fundamentally different types of spectrometers, namely, continuous wave (CW) and pulse Fourier transform (PFT) spectrometers. The older continuous wave NMR spectrometers (the equivalent of dispersive spectrometry) were operated in one of two modes (i) fixed magnetic field strength and frequency (vi) sweeping of Bi irradiation or (ii) fixed irradiation frequency and variable field strength. In this way, when the resonance condition is reached for a particular type of nuclei (vi = vo), the energy is absorbed and... 

The commercially available laser source is a mode-locked argon-ion laser synchronously pumping a cavity-dumped dye laser. This laser system produces tunable light pulses, each pulse with a time duration of about 10 picoseconds, and with pulse repetition rates up to 80 million laser pulses/second. The laser pulses are used to excite the sample under study and the resulting sample fluorescence is spectrally dispersed through a monochromator and detected by a fast photomultiplier tube (or in some cases a streak camera (h.)) ... 

Chromatic aberrations do not arise in the acoustic microscope because in its usual mode of operation it may be considered essentially monochromatic. Even when it is necessary to take the spread of frequencies in the acoustic pulses into account, the media through which the waves pass are essentially non-dispersive in solids over the frequency range of interest the phonons are very near the centre of the first Brillouin zone where the dispersion relationship is linear, especially for sapphire. 

The real and imaginary spectra obtained by Fourier transformation of FID signals are usually mixtures of the absorption and dispersion modes as shown in Fig. 2.13 (a). These phase errors mainly arise from frequency-independent maladjustments of the phase sensitive detector and from frequency-dependent factors such as the finite length of rf pulses, delays in the start of data acquisition, and phase shifts induced by filtering frequencies outside the spectral width A. 

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