The present invention relates to a frequency shifter in an optical path containing a continuous laser source, and it comprises at least two frequency shift modules placed in parallel and each containing an optical propagation medium, the optical path length of which is modified according to the desired frequency shift, each of these modules being controlled separately, this shifter comprising a switch (14) which is connected to the outputs of these modules and is controlled so as to choose, at each instant, the module providing the desired frequency shift, the switch delivering, at its output, a continuous optical beam whose frequency is adjusted by contiguous sections.