BA multi-channel reflectometer diagnostic probing the mid-plane plasma in the O-mode polarization is routinely used for detection of Alfvén eigenmodes (AEs) on the JET tokamak. The AEs are observed on the spectrogram of the homodyne signal reflected by the inner wall when the probing frequency is higher than the maximum of the plasma frequency (interferometry regime). In particular the time and frequency resolution of the Alfvén cascades (ACs) is better from this technique than from any other diagnostic. We show here that knowing the shape of the density fluctuations induced by an AC, the level of these density fluctuations can also be inferred from the phase fluctuations of the reflectometry signal.