This invention is a device and method for the measurement of optical path length. A frequency chirped electromagnetic wave source, such as a laser beam, is split into two branches. A reference branch is projected directly onto a photosensor, while a probe branch is launched towards a target whose distance relative to the reference path is to be determined. A reflected wave from the target is collected and mixed with the reference onto a photosensor. Due to the unequal path lengths traveled by the reference and the reflected probe laser beams as well as the chirped nature of their frequencies, a certain optical frequency difference exists between the two beams. This frequency difference is linearly proportional to the relative optical path length difference between the two laser beams and the relative optical path length can be readily determined by using a photosensor that generates photocurrents linearly proportional to the relative optical frequency differences between the reference and reflected branches.