Miniature laser doppler velocimetry systems

Two new classes of compact Laser Doppler Velocimeter (LDV) designs are presented which offer significant advantages in cost and performance over classical, fixed focus, fringe generation systems using laser beam interference. Both offer multi-axis capability and direction sensing. The first system [2] is the "Magnifier LDV". One or more laser beams are focussed at the target, and a highly magnified image of the illuminated target is projected onto optical masks which contain one (or more) groups of parallel opaque lines, each group having a different spatial frequency. Each mask is either viewed directly by a detector or indirectly via a lens-ended fiber optic link. Direction of target movement is determined by the first arrival of either a higher or lower frequency signal burst. The second system [3] is the "Projection LDV". An optical mask is placed into a diverging or converging laser beam. The optical mask contain one (or more) groups of parallel opaque lines, each group having a different spatial frequency. The normal edge diffraction is disturbed by the non-parallel beam, and a high quality image of the mask is projected as a parallel beam by transmission optics throughout the length of the laser beam. Moving the mask to and from the laser source changes the mask spacing, and hence the LDV velocity constant. The mask can be rotated into other orientations for axis matching or for multi-axis systems. Zemax optical simulations and CAD engineering designs exist for prototype systems.