The results of the precise measurement of the refractive index of stable germanium isotopes Ge72, Ge73, Ge74, and Ge76 single crystals with high enrichment and a germanium single-crystal Genat of natural isotopic composition with the Fourier-transform interference refractometry method from 1.94 to 20 μm with 0.1 cm-1 resolution and accuracy of 2×10-5 to 1×10-4 are shown. The content of 72 impurities measured by the mass spectrometric method was below 10-5-10-6 wt. %. Oxygen and carbon concentrations measured by IR spectroscopy in all crystals are within 5×1015 cm-3. All germanium crystals have a hole conductivity with specific electric resistivity of 45-50 Ohm×cm. The coefficients of the generalized Cauchy dispersion function approximating the experimental refractive index values all over the measuring range are given. The transmission and Raman spectra are also given.