Abstract: Particle velocity is determined optically through the use of crossed interference fringe patterns. The fringe patterns are produced by passing laser beams of distinct wavelength along a common optical fibre and through two crossed diffraction gratings which are bonded to the free end of the fibre. Each diffraction grating is constructed in the manner of an interference filter so as to produce interference fringes at one wavelength but to transmit the other wavelength substantially uniformly across the grating. Other interference elements can be used in place of diffraction gratings and the use of a single diffraction grating is envisaged in certain circumstances. An indication of the sense of particle movement can be achieved by creating a fringe pattern of steadily increasing fringe spacing.