A Michelson interferometer using a 5‐mW He–Ne laser can either be used in a photodiode detector mode to resolve transient fringe shifts ≲10−4fringe or in a streak mode to obtain entire nanosecond resolved density profiles in a single shot. Extreme sensitivity is achieved by avoiding most noise sources. The effects of plasma light and of ray deflection due to density gradients are compensated for by the use of a lens, stop, and filter combination. Vibration induced signals are nearly eliminated by the use of feedback stabilization. A differential light detector system which subtracts signals at opposite sides of a fringe protects against laser noise and other spurious signals while doubling the sensitivity. In the streak mode a cylindrical lens beam expander allows utilization of nearly all of the laser light.