Abstract

The application of volumetric diffraction gratings in photochromic materials to the measurement of velocity gradients in fluid flows is presented. The process involves the writing of a crossbeams hologram by two coherent pulsed beams that intersect inside a fluid to which a photochromic dye has been added. The hologram is continuously read at the proper Bragg angle by a third beam of lower intensity and different wavelength. The effect of flow shearing is to change the main orientation of the plane of the fringes of the hologram, which in turn reflects in a change of the angular properties of the readout beam. This is carried out until the hologram fades out by diffusion or it moves out of the Bragg condition. We show the optimization of the writing of the cross-beam grating in an optically thick material (like the photochromies used) as well as the wavelength for the most efficient reading of such gratings. Velocity gradients in a carefully controlled flow are shown and compared to alternative established methods and theoretical profiles. Finally, we discuss several alternative geometries capable of measuring different components of the velocity gradient tensor as well as multichannel arrangements capable of measuring vorticity components (e.g., ω_y = ∂v_z/∂x− ∂v_z/∂z).

© 1988 Optical Society of America