A system for measuring a velocity of tracer particle motion in a fluid for providing an estimate of fluid velocity, the system comprising: - at least one laser emitter (16, 24, 31, 32, 33, 100) configured to emit a continuous-wave laser beam, - a plurality of optical devices (1, 2, 3, 19, 106, 107), - a switching device (25, 101, 118) configured to receive the continuous-wave laser beam and switch the continuous-wave laser beam sequentially to each of the plurality of optical devices (1, 2, 3, 19, 106, 107), each of the plurality of optical devices (1, 2, 3, 19, 106, 107) being configured to focus the continuous-wave laser beam received from the switching device (25, 101, 118) onto a focus volume, and comprising - a focusing arrangement, - an optical output port (port 2) configured to emit the continuous-wave laser beam towards the focusing arrangement, the focusing arrangement being configured to focus the emitted continuous-wave laser beam onto a same probe volume comprising tracer particles, and an optical input port (port 2) configured to receive back-scattered light from the tracer particles, wherein the optical output port (port 2) and the optical input port (port 2) form a common optical input/output port (port 2) for transmitting the received continuous-wave laser beam and for receiving the backscattered radiation, the plurality of optical devices (1, 2, 3, 19, 106, 107) being provided to have mutual pointing angles so that each of the plurality of optical devices (1, 2, 3, 19, 106, 107) points at the probe volume under a different angle, and a processor (23, 50) configured to receive at least a part of the transmitted laser beam and the received backscattered radiation beam to calculate a Doppler shift, and thereby time sequentially determine a plurality of velocity components of the tracer particles using coherent detection. - at least one laser emitter (16, 24, 31, 32, 33, 100) configured to emit a continuous-wave laser beam, - a plurality of optical devices (1, 2, 3, 19, 106, 107), - a switching device (25, 101, 118) configured to receive the continuous-wave laser beam and switch the continuous-wave laser beam sequentially to each of the plurality of optical devices (1, 2, 3, 19, 106, 107), each of the plurality of optical devices (1, 2, 3, 19, 106, 107) being configured to focus the continuous-wave laser beam received from the switching device (25, 101, 118) onto a focus volume, and comprising - a focusing arrangement, - an optical output port (port 2) configured to emit the continuous-wave laser beam towards the focusing arrangement, the focusing arrangement being configured to focus the emitted continuous-wave laser beam onto a same probe volume comprising tracer particles, and an optical input port (port 2) configured to receive back-scattered light from the tracer particles, - a focusing arrangement, - an optical output port (port 2) configured to emit the continuous-wave laser beam towards the focusing arrangement, the focusing arrangement being configured to focus the emitted continuous-wave laser beam onto a same probe volume comprising tracer particles, and an optical input port (port 2) configured to receive back-scattered light from the tracer particles,