Power and coherence

An integral part of a fibre optic sensor is the light source. Its primary task is to deliver an appropriate light, which possesses such features as an optical power suitable to interact with an analyte or an indicator from the optrode, a wavelength matched to the spectral properties of the sensors in order to obtain the highest sensitivity, and, in dependence on the construction of the sensor, polarisation, short pulse etc. There are many various light sources utilised in the fibre optic chemical sensors. They differ in spectral properties, generated optical power and coherence. 

Most of the vibrational and rotational spectra obtained before the second world war were measured using Raman methods. Interest in Raman then declined as infrared and microwave absorption instrumentation developed, but the introduction of visible lasers in the early 1960s has led to dramatic renaissance in Raman spectroscopy. As well as decreasing the acquisition time and increasing the sensitivity of conventional Raman spectra by orders of magnitude, the high power and coherence properties of laser radiation has spawned a host of new nonlinear Raman spectroscopies, some of which can be performed without a... 

X/Y coherence transfer steps as in the original HNCA experiment.41 As in the previous case, X nuclei are selectively irradiated by low power rectangular or shaped pulses, and coherence selection is accomplished by the matched pulsed field gradients Gb G2 and further assisted by a spoil gradient Gs. Owing to the need to avoid -pulses in the X-channel, the spectra are processed in magnitude mode in the fft dimension. 

Another revolutionary application of electronically excited molecular systems is in laser technology. Lasers are intense sources of monochromatic and coherent radiation. From their early development in 1960 they have found wide fields of application. They have provided powerful tools for the study of diverse phenomena ranging from moonquakes to picosecond processes of nonradiative decay of excitational energy in molecules. The intense and powerful beam of coherent radiation capable of concentra-... 

Laser Doppler velocimetry is a powerful technique for the in situ measurement of fluid velocities. The basic optical configuration for the measurement is shown in Figure 6.1. The velocity measurement is made at the intersection of two laser beams that are focused to a point in the flow. The use of laser radiation is essential since the light must be monochromatic and coherent. This is required since the intersection of the two beams must create an interference pattern within the fluid. Such a pattern is shown in Figure 6.2, where two plane waves intersect at an angle 2(J). The two waves will have the following form [55] ... 

The main features of the coherent multiple pulse technique are that it is self-calibrating in frequency that the signal strength is the same as that obtained with a single mode laser of the same average power and that frequency doubling efficiency can be good. 

All the powerful methods of magnetic resonance, from solid-state nuclear magnetic resonance (NMR) to medical magnetic resonance imaging, depend on measuring the time evolution of a spin system following the application of one or more radio frequency pulses. In the visible and ultraviolet, ultrafast optical pulse sequences have been used for many years to measure both population dynamics and coherence phenomena. At low... 

Even in the absence of relaxation, Hartmann-Hahn transfer depends on a large number of parameters pulse sequence parameters (multiple-pulse sequence, irradiation frequency, average rf power, etc.) and spin system parameters (size of the spin system, chemical shifts, /-coupling constants). For most multiple-pulse sequences, these parameters may be destilled into effective coupling tensors, which completely determine the transfer of polarization and coherence in the spin system. This provides a general classification scheme for homo- and heteronuclear Hartmann-Hahn experiments and allows one to characterize the transfer properties of related... 

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