Fringe counting

Increase and decrease of (A/pm+A/p0) are discriminated from each other by the direction of the movement of the 0-1 transitions. Sudden disappearance of such a moving transition can be electronically monitored and can be used for fringe counting. 

Optical methods of spectrometer calibration include laser interferometric methods and Moire fringe counting techniques. Since such methods depend on the accurately known wavelength of light (for example, 6328.1983 A for a He-Ne laser at 293 K under standard conditions), they are independent of any assumptions made in the iron foil technique and, thus, are intrinsically more reliable. Moreover, such methods do not require the acquisition of57 Co Mdssbauer sources and reference materials for iron absorber studies and may thus be attractive... 

The exact measurement of the optical path in cells of low thickness is made by interferometry (interference pattern method). The transmittance of the empty cell is measured for an interval between two wavenumbers rq and (in cm ). Figure 10.22 shows that the beam S2 has undergone a double reflection from the internal walls of the cell, thus for a normal incidence, there would be, if 21 = kX, addition of both light intensities (the two beams Sj and S2 are in phase). As a function of the wavelength a modulation of the main beam Sj of a few percent is observed. After calculation, if N represents the number of interference fringes counted between rq and PJ (in cm ), then ... 

Modem displacement measuring interferometer systems (Badami and de Groot 2013) rarely rely on fringe counting. Generally,... 

FIGURE 1 Fringe-counting interferometer using a two-frequency iaser. [After J. N. Dukes and G. B. Gordon (1970). Hewlett-Packard J. 21(12), 2-8. Copyright 1986 Hewlett-Packard Company. Reproduced with permission.]... 

In the interferometric dilatometer, the change in length of the sample causes the movement of interference fringes. Knowing the laser wavelength and counting the moved fringes, it is possible to deduce the dilatation of the sample. Hereafter, we shall briefly describe a very simple interferometric dilatometer used for the measurement of the linear contraction coefficient of Torlon. For a more detailed description of this dilatometer, see ref. [53],... 

The pathlength of a cell for infrared spectroscopy can be measured by counting interference fringes (ripples in the transmission spectrum). The spectrum below shows 30 interference maxima between 1 906 and 698 cm-1 obtained by placing an empty KBr cell in a spectrophotometer. 

Individual photons appear to arrive at entirely random times and positions. As more photons are counted, a pattern starts to emerge, and eventually, when we have enough, the interference fringes predicted by the wave theory can be seen. We know that, when the experiment is done with massive numbers of photons, the light intensity can be predicted by the wave theory. Under these conditions, therefore, the number of photons arriving at any point must be proportional to the wave intensity. On an individual basis, however, each photon appears to be unpredictable. AH we can say is that the probability of a photon arriving at a particular point is proportional to the wave intensity. (Note that according to eqn 1.5 this is the square of the wave amplitude.)... 

The next big advance towards higher precision was the 1997 phase-coherent measurement of the frequency gap with an optical frequency interval divider chain [27]. The 2.1 THz gap was no longer measured by counting cavity fringes, but divided down to the radio frequency domain by a phase-locked chain of five optical frequency interval dividers [56] (see Fig. 5). The accuracy of this approach was limited by the secondary frequency standard to 3.4 parts in 1013, exceeding the accuracy of the best previous measurements by almost two orders of magnitude. 

Michelson interferometer, the detector was the human eye aided by a telescope. The fringes could be counted or measured through the telescope. 

To see more fringes we have to increase the coherence length and therefore decrease the velocity spread. For this purpose we employ a mechanical velocity selector, as shown after the oven in Fig. 1. It consists of four slotted disks that rotate around a common axis. The first disk chops the fullerene beam. Only those molecules are transmitted which traverse the distance from one disk to the next in the same time that the disks rotate from one open slot to the next. Although two disks would suffice for this purpose, the additional disks decrease the velocity spread even further and help to eliminate velocity sidebands. By varying the rotation frequency of the selector, the desired velocity class of the transmitted molecules can be adjusted. To measure the time of flight distribution we chopped the fullerene beam with the chopper right behind the source (see Fig. 1). The selection is of course accompanied by a significant loss in count rate, but we can still retain about 7% of the unselected molecules. 

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