A novel device is described which provides the possibility for accurate sensing of laser beam or interference fringe positions, utilising a pair of photoconductive elements that differentiates the light intensity with respect to the lateral coordinate. The device consists of a microstripline T structure with an InP:Fe chip incorporated in the middle. The chip has a metallic layer divided into three parts by two narrow photoconductive gaps, separated by a distance of 350 mu m. The central part is connected to a microstripline that constitutes the output branch, while the two side parts are connected to two similar microstriplines that are biased with DC voltages of equal magnitudes but opposite polarities. The differential mode of operation gives improved resolution as compared with ordinary photodetectors. In the case of gaussian beam position measurements a lateral resolution of 10 mu m could be demonstrated experimentally, and in measurements of interference fringe positions the resolution 2 pi /60 rad was achieved. In both cases these limits were set by the mechanical translation stage used. The temporal resolution was set by the carrier lifetime of the material, 200 ps.