A feedback stabilization technique is described for a fractional fringe interferometer measuring plasma electron densities. Using this technique, a CO2laser Michelson interferometer with a pyroelectric detector exhibited a sensitivity of 3.4×10−4fringe on a 1‐ms time scale and, due to acoustic pickup, 1.8×10−2fringe on a 10‐ms time scale. The rise time is 45 &mgr;s. Stabilization against slow drifts in mirror distances is achieved by an electromechanically translated mirror driven by a servo system having a 0.2‐s response time. A mechanical chopper in one of the two beam paths generates the signal which drives the servo system.