HOLOGRAPHIC INTERFEROMETRY.   Doug Broege, John Noé, and Harold Metcalf, Laser Teaching Center, SUNY at Stony Brook.

A hologram is a way of recording and recreating the complex optical wavefront that comes from an illuminated object. When this wavefront is combined with another beam of light, known as the reference beam, an interference pattern is created which can be recorded photographically on a high-resolution plate or film. When the developed plate is re-illuminated with a beam similar to the reference beam it acts either as a diffraction grating or a series of mirrors to transform the reference wavefront into the original one. Stability is one of the most important factors when creating a hologram. If either the object or the plate moves even a quarter of a wavelength of light over the span of the one minute exposure, the interference pattern is destroyed. This sensitivity to movement can have a positive side as well, for example it can be used to study the vibrational modes of a certain object, such as musical instrument. When an object is moved during the exposure, dark fringes will appear on its image. The number of fringes that appear is directly proportional to the number of wavelengths that the object moved during the exposure. This is useful when measuring small changes in the surface or edge of something that can't been seen with the naked eye. Also, when an object is vibrating at a single mode, the nodal lines (which don't move) can be clearly seen while circular fringes appear between these lines. My experiments thus far have involved making double exposures of objects moved with a micrometer, and creating a viable setup that will make a clear image of a tuning fork. One important result is that a HeNe laser -- even one with less than one mW output power -- provides much clearer images and a more versatile setup than the 3 mW uncollimated diode laser originally used.

This study was supported by NSF grant No. PHY99-12312.