A CO2‐laser interferometer with heterodyne detection has been built to measure electron line densities of plasmas in the range of 1018to 5×1022m−2. Heterodyne operation is accomplished by using two counteracting acousto‐optic modulators at a difference frequency of 500 kHz. A digital phase comparator is used to obtain direct read‐out of the phase shift. The time resolution is 3.5 &mgr;s.