Principle of path-matched white-light interferometry: (a) set-up with an unbalanced readout interferometer, using a beam splitter BS, and two mirrors M₁ and M₂; (b) interferogram as a function of the path unbalance ∆L_r, with the secondary peaks yielding a measurement of group birefringence B_g; this interferogram is the Fourier transform (FT) of the channeled spectrum; and the coherence length L_c of the original broadband source is equal to λ ²/∆λ_FWHM.