The changes in frequency are by means of a Mach-Zehnder-interferometer converted into a time-series in intensity, whose frequency domain is accessible to further electronic processing. Within the interferometer the laser beam is split into a reference beam and a measurement beam. The light reflected from the probe is brought to interference with the reference beam. The intensity recorded in a photodetector contains, apart from the intensities of the reference beam Ic, and the reflected light Iv, a contribution which depends on the difference in optical path Δz,
I(t) = Ic + Iv + 2 (Ic Iv)1/2 cos (2 π Δz(t)/λ) .