5 Noise Reduction in Amplitude-Fluctuation Electronic Speckle-Pattern Interferometry 35 Fig. 5.9 Effect of fringe shifts and angular fluctuation fluctuation increases the fringe spacing the Fourier spectrum bulges to the low frequency side and as the angular fluctuation decreases the fringe spacing the spectrum bulges in the other direction. Figure 5.9 indicates neither the fringe shift nor angular fluctuation explains the scattering of the Fourier spectrum on the low frequency foot exhibited by the bottom of Fig. 5.8c. We have found that the change in the size of the combined (interfering) laser beams and its relative motion account for this effect. Figure 5.10a shows the actual image of the data shown in Fig. 5.8c at frame #1 and # 100. Apparently, the brightest spot of the beam is shifted toward left from frame #1 to #100. Figure 5.10b, c are numerical results in which we move the intensity profile (more specifically we shift the Gaussian profile) at 1 Hz back and forth with no fringe shift or angular fluctuation. The amplitude of this motion is set to be five times of the fringe shift used in Fig. 5.9b, c. Here Fig. 5.10b uses the same beam spot size as Figs. 5.9 and 5.10c uses 40% narrower beam size. It is clearly seen that when we move the actual beam with the same beam size, the numerical simulation does not reproduce the low frequency side scattering observed in Fig. 5.8c, whereas it clearly exhibits the effect if the beam size is reduced to 40%. This is reasonable as a smaller beam size has more influence on the Fourier spectrum of the intensity profile.
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