[en] The calculations are made to find out the optimum length of the operation wave of interferometer designed for the 10¹³-10¹⁵ cm^-3 density plasma investigation. It is shown that for the investigation of plasma with the expected density of 10¹⁴ cm^-3 in the center of the plasma filament cross section symmetry and with the cross section diameter of about 100 cm the operation wave of interferometer should be within the range 50μm < lambda < 600 μm. Constructed are the Michelson, Mach-Rozhdestvenskij and three-mirrored interferometers on the basis of the HCN laser with the 337μm wave length, which is in an intermediate position within the above range. To measure the space distribution of electron density constructed is an interferometer with the beam scanning. Their designs and principles of operation are described, the limits of their application are shown. The minimum phase shift at calibration of interferometer is 2PI/36. The quick-action of three-mirror interferometer measured in the experimental way achieves 1 MHz. Interferometers have been used for measurings on the electronless induction discharge, on the moving plasmoid generated by the coaxial plasma accelerator and on the single-cut torsatron