Fast heterodyne interferometer with electronically stabilized initial phase

A heterodyne interferometer setup with initial‐phase stabilization is described, which allows the full range of ‘‘half a fringe’’ to be used without ambiguity, using only one phase comparator. This phase stabilization is achieved by the insertion of a voltage‐controlled delay line in the reference path of the phase comparator. The system is used to measure the electron line density of pulsed plasmas (with pulse lengths of 1 ms) in the range 5×101 9–2×102 1 m− 2 with a time resolution of 200 ns. The resolution of the electronic system is approximately (half a fringe)/200.