The impact of spatial limitation of the nonlinear interaction volume on the spontaneous parametric down-conversion spectra in a strongly nondegenerate regime is analyzed from the point of absolute calibration of the terahertz-wave nonlinear-optical detectors. We show that the idler wave angular resolution of the calibration method can be inherently low (about 1 to 10 degrees in lithium niobate crystals), and to fill all the input modes with external idler radiation, one has to take special care on geometry of the nonlinear interaction. Angular sensitivity distribution function is constructed, and a consecutive description is provided for both the parametric down-conversion and the external idler radiation detection processes.
Absolute Calibration of Analog Detectors by Using Parametric Down Conversion