Аннотация:We report on the results of the experimental simultaneous observation of all the parametric optical signals related to
terahertz-wave detection in a continuously pumped nonlinear-optical crystal [3]: signals due to up-conversion of the incident THz
radiation to the Stokes and anti-Stokes optical ranges, due to up-conversion of the thermal noise to the both ranges and due to
SPDC in the Stokes part of the optical signal spectrum (Fig.). To avoid the different influence of the idle absorption effects, we
have detected THz radiation in the frequency range 0.120-0.300 THz where Mg:LiNbO3 crystals used for parametric frequency
conversion can be considered as practically transparent. Nevertheless, it became clear that this is not enough for estimating the
SPDC-calibration as an absolute procedure in the THz range. In case of strongly non-degenerate SPDC one has to take special
care for the single-mode character of the parametric interaction. Transverse spatial limitation of the nonlinear interaction volume
in a crystal (mostly due to a pump beam focusing) leads to increase of a number of idle modes which can contribute to a single
mode of an output signal. This number grows considerably when the frequency relation / i s is going down. All the idle
modes are well-filled by classical and quantum fluctuations forming the background signals of the thermal up-conversion and
SPDC, but cannot be filled uniformly by an incident THz radiation. The filling factor has to be taken into account in the SPDC
calibration of any energy characteristic of the input radiation. Results of theoretical calculations of the filling factor, made for
differently focused Gaussian pump beams and different geometries of the parametric THz up-conversion schemes, will be
discussed.