The invention relates to an optical measurement device designed notably to evaluate the velocity of an aircraft by detection of a Doppler shift. The device for measurement by optical probe comprises at least two light beams of different wavelengths .lamda.1 , .lamda.2, denoted as first beam and second beam, the device illuminating a reference medium MILREF (60) by the first beam and receiving a light echo Sr reflected back by the reference medium, and the device comprising: an optical coupler COUPL (70) receiving, on the one hand, the light echo Sr reflected back by the reference medium MILREF and, on the other, the second beam, the coupler delivering a resultant wave Or comprising the second beam and the light echo Sr; an optical amplification unit AMPLI (80), of bandwidth B.sub.opt, receiving the resultant wave Or and delivering an amplified resultant wave Ora.
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.
