Two-wavelength interferometry

As we have seen previously, an interferogram is periodic when we superimpose monochromatic waves. In order to widen the range of possible uses, it is possible to make waves which have very similar wavelengths interfere [33]. This technique can also be used with fiber interferometers, like Kersey proved it [34].

When a two-wave interferometer is illuminated by two monochromatic sources whose wavelengths are   and each one will make an interferogram with a definite visibility (lets' take one to simplify) and a definite average intensity I0 (identical for   and   , once again to simplify), so the final intensity is:

This equation can be also written as:

where V is the fringe contrast function and can be written as:

In this configuration the measure is equivalent to a one-wavelength interferometer's measure. On the other hand the range within which the measure is obtained without ambiguity is related to the period of V, so the measure range is increase by a factor .