Recent faculty/student research projects in Holography
(student names in italics)

The photo at left shows the hologram setup used to observe standing waves in a tube filled with air. The object beam from the HeNe laser is illuminating the interior of the resonance tube. The hologram, resulting from the interference pattern caused by the reflected object beam and a reference beam, is formed on a thermoplastic plate. A CCD camera is positioned to look through the hologram, enabling real time video capture of the hologram. The idea behind this experiment is that since sound waves involve change in pressure, and thus a change in the local gas density, there should be a resulting change in the index of refraction in the gas. A change in the refractive index would resulting in an effective change in the object path length, resulting in the formation of fringes on a hologram.

This photo shows a close-up of the resonance tube. The tube has a rectangular cross section and measures 6.4 x 7.0 x 61.0cm. The front of the tube was constructed from 3/4-inch plexiglass, allowing optical interrogation of the interior. The remaining three sides and the end capes are 1/2-inch tempered steel. The tube is driven by a horn loudspeaker at the right end. A ruler is fixed to the front of the tube to allow measurement of the wavelengths of the resulting standing sound wave patterns in the tube.

The photo at left shows a real-time hologram of a standing sound wave in the resonance tube. A hologram of the tube was made with the sound turned off. Carrier fringes were then laid down by moving the object beam point source (by inserting a glass wedge in front of the object beam pinhole). Then the hologram was viewed with the sound turned on. When the frequency of the sound matches a resonance frequency of the tube, standing waves result. At the nodes of the standing wave pattern, the pressure does not change, and the carrier fringes are still visible. At the antinodes of the standing wave pattern, the pressure (and thus the density and index of refraction) undergoes a maximum change and the carrier fringes are washed out.

The photo at left shows a time-averaged hologram of standing sound waves in the resonance tube. A hologram of the tube was made while the horn loudspeaker was turned on very loud at the same frequency as the real-time hologram above. The resulting pressure maxima and minima caused the index of refraction to change at antinodes of the standing wave pattern. This shift in the refractive index altered the effective path length of the object beam, resulting in the fringe pattern shown. In the figure shown the bright fringes correspond to nodes (no change in refractive index) and the dark fringes correspond to antinodes (maximum change in refractive index and effective path length). The wavelength of the standing wave may be determined by doubline the distance between two bright fringes.