Fringe dispersion

As indicated earlier, clear cellulose solutions are not necessarily molec-ularly dispersed they may contain aggregates of still ordered cellulose molecules [107]. The structure of these aggregates has been described in terms of a fringed micellar structure. Figure 2a shows a schematic possi-... 

The fringes contrasts are subject to degradation resulting from dissymmetry in the interferometer. The optical fields to be mixed are characterized by a broadband spectrum so that differential dispersion may induce a variation of the differential phase over the spectrum. Detectors are sensitive to the superposition of the different spectral contributions. If differential dispersion shifts the fringes patterns for the different frequency, the global interferogramme is blurred and the contrast decreases. Fig. 5 shows corresponding experimental results. 

Figure 5. Evolution of the fringes contrast C as a function of the differential dispersion (a.u.) The maximum of this function corresponds to the cancellation of differential dispersion between the fibre arms of the interferometer.

The main consequences are twice. First, it results in contrast degradations as a function of the differential dispersion. This feature can be calibrated in order to correct this bias. The only limit concerns the degradation of the signal to noise ratio associated with the fringe modulation decay. The second drawback is an error on the phase closure acquisition. It results from the superposition of the phasor corresponding to the spectral channels. The wrapping and the nonlinearity of this process lead to a phase shift that is not compensated in the phase closure process. This effect depends on the three differential dispersions and on the spectral distribution. These effects have been demonstrated for the first time in the ISTROG experiment (Huss et al., 2001) at IRCOM as shown in Fig. 14. 

The shift of fringes at any wave-length together with a knowledge of the concentration of atomic hydrogen and the index of refraction of molecular hydrogen serves to determine the dispersion of atomic hydrogen. 

If there are variations in the period (period dispersion) then there are in effect different periods within the superlattice. The zero-order peak for each of these must be the same— because it is simply the average mismatch of the alloy— but the distribution of intensity in the various interference fringes will be slightly different. This will tend to affect the higher-order satellite peaks more than the lower orders, and if measured with a low resolution instmment there will appear to be an increase in the width of the satelhte peaks with order of the satellite, having taken out any instramental functions. 

For TDL spectra we have in fact linearized the data by using simultaneously recorded Ge etalon fringes. For such spectra this is often an absolute necessity because of the dispersion variation in the recorded data. 

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