  We propose classical interferometry with low-intensity thermal radiation for
the estimation of nonclassical independent Gaussian processes in material
samples. We generally determine the mean square error of the phase-independent
parameters of an unknown Gaussian process, considering a noisy source of
radiation the phase of which is not locked to the pump of the process. We
verify the sufficiency of passive optical elements in the interferometer,
active optical elements do not improve the quality of the estimation. We also
prove the robustness of the method against the noise and loss in both
interferometric channels and the sample. The proposed method is suitable even
for the case when a source of radiation sufficient for homodyne detection is
not available.
