A description is given of a modified Michelson interferometer that is particularly useful for locally probing highly transient plasmas such as occur behind incident shock waves in high performance shock tubes. The interferometer has the following features: (a) It is of a modified Michelson form with spherical mirrors to minimize effects of spatial variations in the index of refraction normal to the beam, (b) the light source is a cw laser that provides a very narrow coherent beam to allow high spatial and temporal resolution of the probe, and (c) it is operated in a single fringe mode over the narrow beam (constant phase difference between the two interfering beams over the illuminated field) and fringe shifts are observed through the time response of single channel detectors. Because photon transit time is negligible in shock tube diagnostics, the response time of the interferometer is determined solely by detection capabilities. Rapid response is achieved because nearly the entire laser energy is available for detection and high contrast interference takes place, allowing high signal‐to‐noise ratios at wide bandwidths. The spatial resolution can be diffraction limited. Thus, one‐dimensional observations can be made of temporally and spatially varying events with a combined temporal and spatial resolution exceeding that available with other current interferometric techniques. Index of refraction profiles behind strong shock waves have been observed at two wavelengths simultaneously and verify the usefulness of the design. A further design modification is discussed which removes the uncertainty in the sign of the temporal gradient of index of refraction and would allow accurate fractional fringe interferometry.