An apparatus for determining the velocity of a fluid flowing through a lumen comprises a first diffraction grating transducer (DGT) responsive to a continuous wave (CW) input and operable in a first mode for producing a first signal beam at a first frequency and first phase, and in a second mode for producing a second signal beam at the first frequency and a second phase; a second diffraction grating transducer (DGT) operating as a receiver and coupled to the first diffraction grating transducer at a predetermined angle, the second diffraction grating transducer producing a first beam which intersects the first DGT first beam for receiving a first reflected signal associated with the first signal beam, and for producing a second beam which intersects the first DGT second beam for receiving a second reflected signal associated with the second signal beam; the first beams adapted to intersect in a first predetermined region through which dynamic particles are undergoing velocity analysis, and the second beams adapted to intersect in a second predetermined region through which dynamic particles are undergoing velocity analysis; and electronic circuitry responsive to the first and second reflected signals for comparing with the first frequency and the first and second phase to determine the velocity of the fluid.