Figure 1.

Simplified schemes for Fourier transform spectroscopy and spectral interferometry with broadband light sources. (a) Fourier transform spectroscopy; (b) low coherence (or white light) interferometry; (c) linear spectral interferometry and (d) swept frequency interferometry. In Fourier transform spectroscopy (a), an interferogram is recorded as a function of the relative delay between the arms of an interferometer (a Mach–Zehnder in the figure). By Fourier processing the interferogram, it is possible to retrieve the optical power spectrum of the light source, $S\left(\omega \right)$. If the spectrum of the light source is known, this technique can be used to characterize, in amplitude and phase, a component under test, which is mathematically represented by a complex transfer function $\mathcal{H}\left(\omega \right)$. The interferogram can be recorded in different ways. In white light interferometry (b), the signal is recorded as function of the delay between arms. In linear spectral interferometry (c), the delay between arms is fixed, and the interference pattern is recorded in the spectral domain using an optical spectrometer. An alternative modality for spectral interferometry is based on a coherent source with a single optical frequency that is (ideally) linearly swept over time. An interference pattern over time is recorded with the aid of a photodetector and analog to digital converter unit.