Phase distortions

The additional delay causes a decrease in signal strength due to loss of magetization from transverse relaxation. Moreover, severe phase distortions, particularly for CH3 carbons, can produce anomalous results. A further modification of INEPT known as INEIH" incorporates an additional... 

It is well-known that the excitation profile by a periodic pulse also has a pattern of multiple bands in response to the multiple effective RF fields. The DANTE sequence,26 for instance, was one of the most frequently used periodic pulse in the past for selective excitation of a narrow centre band. It is constructed by a long train of hard pulses with a certain delay between two adjacent pulses. The advantage of using the DANTE sequence over the weak, soft RF pulses relies on that it is not necessary to change the RF power level in the pulse sequence. Consequently, phase distortions and certain delays accompanied by the abrupt changes of the RF power level are avoided. 

The simplest and most popular experimental method is the well known one-dimensional (ID) NOE difference procedure [3], which is very easily implemented in any spectrometer and which can be routinely set up even by novice spectrometer operators. However, this difference method is based on subtraction of the unperturbed spectrum from the NOE-containing one, both separately recorded, and therefore the required difference information contributes only a small part of the recorded signal. Furthermore, the difference spectrum is very sensitive to subtraction errors, as well as pulse imperfections or missettings, or other spectrometer instabilities, all of which often result in prominent phase distortions or other subtraction artifacts which prevent the accurate measurement of the desired NOE values. Therefore the reliable measurement (or even detection) of enhancements below 1 % is not generally available using this difference method. 

The ability to cancel all orders of phase distortion gives us an opportunity to evaluate the effect of partial dispersion correction on TPM. In particular, we focus on comparing SOD correction, which can be achieved with a prism pair arrangement, and correction of all orders of phase dispersion using MIIPS. For these measurements we used a pair of prisms in addition to our pulse shaper. With the aid of the pulse shaper, we found the condition for which SOD at the center wavelength was fully eliminated by the prism pair, and only higher-order dispersion was compensated by the pulse shaper. 

It is important to realize that dispersion compensation can eliminate the high-order phase distortions (in the spectral domain) introduced by the objective lens, as discnssed above, but it cannot eliminate the scattering (in the spatial domain) that occnrs in depth imaging. Here we explore the nse of laser pulses that are dispersion compensated only before the medinm. In principle, it is possible to compensate for dispersion at greater depths, bnt if the dispersion of tissues is similar to that of pure water, it should be insignificant. Finally, we could titrate the amount of laser power nsed, increasing the intensity as the focal plane moves deeper into the tissue. 

In summary, nonlinear optical imaging methods, such as TPM, allow for an order of improvement in the signal simply by utilization of 10 times shorter pulses. The enhancement is even more dramatic for higher-order nonlinearities, such as third harmonic generation where two orders of magnitude increase is expected. These improvements, however, rely on successful measurement and elimination of phase distortions. 

We first review the essentials of the phase distribution of the electric fields at the focus of a high numerical aperture lens in Section II. After discussing the phase properties of the emitted signal, in Section HI we zoom in on how the information carried by the emitted held can be detected with phase-sensitive detection methods. Interferometric CARS imaging is presented as a useful technique for background suppression and signal enhancement. In Section IV, the principles of spatial interferometry in coherent microscopy are laid out and applications are discussed. The influence of phase distortions in turbid samples on phase-sensitive nonlinear microscopy is considered in Section V. Finally, in Section VI, we conclude this chapter with a brief discussion on the utility of phase-sensitive approaches to coherent microscopy. 

Fig. 2. Theoretical 2H MAS NMR spectra calculated with quadrupolar coupling constant Cq = 200 kHz, asymmetry parameter r Q = 0.10, rotation frequency = 5.0 kHz (left) and uT = 10.0 kHz (right). The spectra represent (a) ideal RF irradiation conditions with RF field strength i/Rf = 100 kHz and optimum pulse length Tp = 2.25 fis, and (b, c) nonideal RF irradiation conditions with i rf = 25 kHz and tp = 4.25 fjbs. The phase distortion effects are illustrated in (b), while (c) demonstrates the result of performing a first-order phase correction. (Adapted from Kristensen et alP with permission.)...

Figure 2 " Sn 90° traasmitter pulse measurement. SnMe4 in CDCI3. (a) Signal 100 ppm off-resonance gives phase distortion, which can be severe, as pulse length varies, (b) Probe breakdown gives random intensity variations, (c) Correct...

Similar editing techniques may also be applied to more complex spin systems as outlined for an olefinic doublet in Figure 6. Figures 6(c) and 6(d) illustrate the phase distortion from pure absorptive lineshapes that result from a mismatch between the refocusing delay and the value of V(" Sn, H) on which the editing is based. 

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