An interferometer is a useful diagnostic tool for measuring line-averaged electron density but is limited in its use because it generally measures at a fixed location. We report here a spatially translatable fiber-coupled interferometer that measures the density of a high-speed MHD-driven plasma jet colliding with a target cloud. The interferometer uses a He-Ne laser coupled to a polarization-maintaining single mode optical fiber having a vacuum feedthrough. The interferometer provides a measure of the spatial-temporal profile of the line-averaged electron density from which the change in jet velocity as a result of its collision with the target cloud can be deduced.